Aims. Hip arthroplasty aims to accurately recreate joint biomechanics. Considerable attention has been paid to vertical and horizontal offset, but femoral head centre in the anteroposterior (AP) plane has received little attention. This study investigates the accuracy of restoration of joint centre of rotation in the AP plane. Methods. Postoperative CT scans of 40 patients who underwent unilateral uncemented total hip arthroplasty were analyzed. Anteroposterior offset (APO) and femoral anteversion were measured on both the operated and non-operated sides. Sagittal tilt of the femoral stem was also measured. APO measured on axial slices was defined as the perpendicular distance between a line drawn from the anterior most point of the proximal femur (anterior reference line) to the centre of the femoral head. The anterior reference line was made parallel to the posterior condylar axis of the knee to correct for rotation. Results. Overall, 26/40 hips had a centre of rotation displaced posteriorly compared to the contralateral hip, increasing to 33/40 once corrected for sagittal tilt, with a mean posterior displacement of 7 mm. Linear regression analysis indicated that stem anteversion needed to be increased by 10.8° to recreate the head centre in the AP plane. Merely matching the native version would result in a 12 mm posterior displacement. Conclusion. This study demonstrates the significant incidence of posterior displacement of the head centre in uncemented hip arthroplasty. Effects of such displacement include a reduction in impingement free range of motion, potential alterations in muscle force vectors and lever arms, and impaired proprioception due to muscle fibre reorientation. Cite this article: Bone Joint Res 2022;11(3):180–188

Reverse total shoulder arthroplasty (RTSA) has had rapidly increasingly utilization since its approval for U.S. use in 2004. RTSA accounted for 11% of extremity market procedure growth in 201. Although RTSA is widely used, there remain significant challenges in determining the location and configuration of implants to achieve optimal clinical and functional results. The goal of this study was to measure the 3D position of the shoulder joint center, relative to the center of the native glenoid face, in 16 subjects with RTSA of three different implant designs, and in 12 healthy young shoulders. CT scans of 12 healthy and 16 pre-operative shoulders were segmented to create 3D models of the scapula and humerus. A standardized bone coordinate system was defined for each bone (Figure 1). For healthy shoulders, the location of the humeral head center was measured relative to the glenoid face center. For the RTSA shoulders, a two-step measurement was required. First, 3D models of the pre-operative bones were reconstructed and oriented in the same manner as for healthy shoulders. Second, 3D model-image registration was used to determine the post-operative implant positioning relative to the bones. The 3D position and orientation of the implants and bones were determined in a sequence of six fluoroscopic images of the arm during abduction, and the mean implant-to-bone relationships were used to determine the surgical positioning of the implants (Figure 2). The RTSA center of rotation was defined as the offset from the center of the implant glenosphere to the center of the native glenoid face. The center of rotation in RTSA shoulders varied over a much greater range than the native shoulders (Table 1 (Figure 3)). Lateral offset of the joint center in RTSA shoulders was at least 6 mm smaller than the smallest joint center offset in the healthy shoulders. The center of rotation in RTSA shoulders was significantly more inferior than in healthy shoulders. The range of anterior/posterior placement of the rotation center for RTSA shoulders was bounded by the range for normal shoulders. How to best position RTSA implants for optimal patient outcomes remains a topic of great debate and research interest. We found that the 3D joint center position can vary over a supraphysiologic range in shoulders with RTSA, and that this variation is primarily in the coronal plane. By relating these geometric variations to muscle, shoulder and clinical function, we hope to establish methods and strategies for predictably obtaining the best clinical and functional outcomes for RTSA patients on a per-subject basis

“How does the knee move?” is a question of fundamental importance for treatment of knee injuries and knee replacement design. Unfortunately, we lack unambiguous and comprehensive knee function data sets and/or consensus on how healthy knees move. One can just as easily find reports stating the natural knee has a center of axial rotation in the medial compartment of the knee as in the lateral. This is due to technical and practical issues: It is extremely difficult to accurately measure knee motions during ambulatory activities and, when that can be done, very few studies have examined a range of weightbearing activities in the same study cohort. The purpose of this study is to report knee kinematics in a cohort of healthy older subjects whose motions were examined during four different movements, three of them weightbearing ambulation, using a high-speed stereoradiographic system. Six healthy consenting subjects (age = 61 ± 5 years, body mass = 75 ± 8 kg, BMI = 27 ± 4) were observed using a high-speed stereoradiographic system while completing four tasks. Subjects were instructed to perform an unloaded, seated knee extension from high flexion to full extension; to walk at a self selected pace; to step down from a 7 inch platform; and to walk and perform a 90° direction change (pivoting). Stereoradiographic images (1080 × 1080 pixels) were acquired at 100 images/second using 40cm image intensifiers and pulsed x-ray exposures. The three-dimensional knee kinematics were measured using the XROMM software suite (. xromm.org. , Brown University). Post-processing of the kinematics was performed in custom Matlab programs, and included fitting spheres to the posterior condylar surfaces of each knee, and then tracking the motions of the sphere centers relative to a fixed tibial reference frame (Figure 1). The motions of these flexion-facet centers, were used to determine an average center of axial rotation (CoR) over each activity as previously reported by Banks and Hodge. Average CoRs for all four activities were in the posterior-medial quadrant of the knee, with the CoR for open-chain knee extension being the most medial and gait the most lateral (Table 1, Figure 2). One-way ANOVA showed average CoRs are different (p « 0.001). There was considerable variation in individual CoRs, for example, with two knees showing lateral CoRs for gait and the remaining knees having medial CoRs. It should not be surprising that natural knee motions vary with dynamic activity, yet knee kinematics often are presented as being one stereotypic, monolithic pattern of motion. Our data show that the same healthy subjects performing different dynamic activities manifest different knee motions, with open-chain knee extension having the most medial CoR and gait the most lateral. This finding is consistent with previous reports comparing stair climbing and gait in knees with various implant designs. Additional experimental data and, ultimately, validated numerical simulations should facilitate an increasingly accurate process for designing improved treatments for diseased and damaged knees

Purpose. To determine if clinical outcomes are correlated with center of rotation (COR) in patients implanted with a viscoelastic total disc replacement (VTDR). Methods. Fifty patients with single-level, symptomatic lumbar DDD between L4 and S1 were enrolled in a clinical trial of a VTDR across three surgical centres. A comprehensive, independent review and statistical analysis of both clinical and radiographic outcomes was performed and analyzed for correlations. Data from preoperative through 2 years were available. The COR was calculated for the index levels and compared to data for an asymptomatic population. Each COR coordinate was classified as abnormal if outside of the 95% confidence interval for an asymptomatic population. Results. At most recent follow-up, 69% of the patients had achieved at least a 15 point Oswestry Disability Index (ODI) improvement. 76% of the patients achieved at least a 10 point improvement. At most recent follow-up, 78% of cases had a normal COR-X and 92% had a normal COR-Y. Results from three statistical tests show the association between COR-X and outcomes. 1) Based on latest available time point for each patient, the improvement in ODI score was significantly better for patients with a normal anterior-posterior (AP) coordinate of the COR (P=0.03). 2) Anterior COR corresponded with anterior placement of the device in the disc space, and patients were almost 7 times less likely to achieve at least a 15 point improvement in the ODI score if the COR was positioned too anteriorly. 3) This effect was also seen in the average AP coordinate of the COR for patients who achieved a 15 point ODI improvement. Conclusions. A viscoelastic TDR can restore a normal COR. This is the first study to show that restoration of a normal COR correlates with a significant and clinically relevant improvement in patient disability

Purpose. To determine if clinical outcomes are correlated with center of rotation (COR) in patients implanted with a viscoelastic total disc replacement (VTDR). Methods. Fifty patients with single-level, symptomatic lumbar DDD between L4 and S1 were enrolled in a clinical trial of a VTDR across three surgical centres. A comprehensive, independent review and statistical analysis of both clinical and radiographic outcomes was performed and analyzed for correlations. Data from preoperative through 2 years were available. The COR was calculated for the index levels and compared to data for an asymptomatic population. Each COR coordinate was classified as abnormal if outside of the 95% confidence interval for an asymptomatic population. Results. At most recent follow-up, 69% of the patients had achieved at least a 15 point Oswestry Disability Index (ODI) improvement. 76% of the patients achieved at least a 10 point improvement. At most recent follow-up, 78% of cases had a normal COR-X and 92% had a normal COR-Y. Results from three statistical tests show the association between COR-X and outcomes. 1) Based on latest available time point for each patient, the improvement in ODI score was significantly better for patients with a normal anterior-posterior (AP) coordinate of the COR (P=0.03). 2) Anterior COR corresponded with anterior placement of the device in the disc space, and patients were almost 7 times less likely to achieve at least a 15 point improvement in the ODI score if the COR was positioned too anteriorly. 3) This effect was also seen in the average AP coordinate of the COR for patients who achieved a 15 point ODI improvement. Conclusions. A viscoelastic TDR can restore a normal COR. This is the first study to show that restoration of a normal COR correlates with a significant and clinically relevant improvement in patient disability

Introduction. Restoration of the femoral head centre during THR should theoretically improve muscle function and soft tissue tension. The aim of this study was to assess whether 3D planning and an accurately controlled neck osteotomy could help recreate hip anatomy. Methods. 100 consecutive THR patients received OPS. TM. 3D femoral planning. For each patient a 3D stem+head position was pre-operatively planned which restored the native head height, restored global offset after cup medialisation and reproduced anterior offset, in the superior-inferior, medial-lateral and anterior-posterior directions respectively. The femoral osteotomy was planned preoperatively and controlled intra-operatively with a patient specific guide. All procedures were performed through a posterior approach with a TriFit/Trinity uncemented implant combination. Post-op implant position was determined from CT. Results. The mean difference between planned and achieved head height was 0.9mm (−1.2mm to 4.6mm). The mean difference between planned and achieved medial offset was −0.9mm (−6.2mm to 3.1mm). The mean difference between planned and achieved anterior offset was 3.2mm (−0.4mm to 6.6mm). Resultant 3D change between the planned and achieved head centre was 4.4mm (0.6mm to 9.1mm). The change in anterior offset was strongly correlated (r=0.78) to the change in achieved stem anteversion in comparison to the plan; mean values of 16.3° and 10.5° respectively. Conclusions. In this single centre pilot study, femoral centre of rotation was accurately reproduced by using 3D templating and controlling the femoral neck osteotomy with a patient-specific guide

Purpose: A functional centre of rotation (CoR) is often required in biomechanical analysis of the hip or as a landmark in computer guided surgery. It was previously shown that circumduction motions predict a CoR that is inferior and lateral to the geometric centre of the hip bearing surfaces. It is therefore necessary to establish the best method for determining the CoR to improve surgical planning. The objective of this study was to compare the predicted CoR from circumduction and star motions, and to compare these to the geometric centre of the joint. Method: Eight cadaveric hips from four cadavers were tested. Prior to testing, CT scans of the cadavers were made from the iliac crest to the tibial plateau; the alpha angle for all hips was less than 50° so all hips were considered ‘normal’. Reflective marker arrays were rigidly mounted on the femoral diaphysis and iliac spine using 4mm Steinman pins. A five-camera Vicon system (Oxford, UK) was used to track the motions of the arrays during manipulation of the lower limb. To determine the functional hip centre, trials consisting of five cycles each of circumduction, flexion-extension and abduction-adduction were performed on each lower limb; three trials of each motion were performed. The range of motion was approximately 45° in the coronal and sagittal planes. For the ‘star’ motion, the flexion-extension and abduction-adduction trial data were combined. Following the trials the hip was dissected to expose the articular surfaces of the femoral head and acetabulum. These surfaces were traced using a pointer equipped with reflective markers to determine the geometric centre. To calculate the functional centre, the 3D coordinates of the markers were used to construct a local-to-global 3D transform for each frame throughout the trial. The geometric centre was calculated using a least-squares sphere fit (Gauss-Newton) of the trace data, calculated in the respective local coordinate systems. The coordinates of the functional centres were then transformed to an anatomic coordinate system, using the geometric centre as the origin. All calculations were performed using Matlab (Mathworks, Inc, MA, USA). A t-test was performed in each anatomic direction to detect differences in CoR predicted by the two motions. Results: Both the circumduction and star motions resulted in a similar CoR. Differences were 0.41±2.25mm in the anterior-posterior direction; 0.09±0.72mm in the superior-inferior direction; and 0.21±0.82mm in the medial-lateral direction, none of which were significant (p> 0.5). The overall mean distance between the CoR predicted by the two motions was 2.0±1.3mm. The functional centre was also found to be lateral and inferior to the geometric centre, and was consistent for each motion. Results for the acetabulum showed similar trends. Conclusion: This study has shown that circumduction and star motions are equivalent in predicting the hip functional CoR; differences were small compared to the dimensions involved in studies such as gait analyses. However, both motions predicted a CoR that was inferior and lateral to the spherical centre of the femoral head, suggesting that the hip does not act as a true ball-and-socket joint with congruent spherical bearing surfaces. This may have important consequences in studies at the scale of the hip joint, especially for pathological conditions such as femoroacetabular impingement

Purpopse. Few Cervical Total Disc Replacement (TDR) devices are engineered to address both the Center of Balance (COB) and the Center of Rotation (COR) of the cervical motion segments. The COB is the axis in the intervertebral disc through which the axial compressive load is transmitted. TDRs placed posterior of this point tend to fall into kyphosis while devices placed anterior of this point tend to fall into lordosis. Thus from a “balancing” point of view the ideal placement would be at the COB. However, the COR position has been shown to be posterior and inferior to the disc space. It has also been shown that constrained devices tend to lose motion when there is a mismatch between device and anatomic centers. Mobile core devices may be placed at the COB since their unconstrained rotations and translations allow for the device COR to follow the anatomic COR, but they rely heavily on the facet joints and other anatomic features to resist the paradoxiacal motion. The TriLobe cervical TDR (Figure 2) was engineered for both the COB and COR. The purpose of this study was to compare the 3D kinematic and biomechanical performance of the TriLobe to a ball and trough(BT) cervical TDR in an augmented pure moment cadaveric study to find the ideal AP implant placement. Materials and methods. Specimen were CT imaged for three-dimensional reconstruction. Visual, CT, and DEXA screening was utilized to verify that specimens are free from any defects. Specimens were prepared by resecting all nonligamentous soft tissue leaving the facet joint capsules and spinal ligaments intact. C2 and T1 were potted to facilitate mounting in the testing apparatus (7-axis Spine Tester, Univ. of Utah, Salt Lake City, UT). OptoTRAK motion tracking flags were attached to each vertebra including C2/C3 and T1 to track the 3D motion of each vertebra. •. Specimens C2–T1. •. Treatment Level C5–C6. •. Insertion of fixture pins under fluoro. •. Load Control Testing to 2.5Nm in FE, LB, AR at 0.5Hz. •. 15 Pre-cycles in load control in FE / LB / AR (2.5Nm). •. Test implants in load control in FE / LB / AR to 2.5Nm for 4 cycles with data recorded for all cycles. Results. [Results Table - Figure 1]. Discussion. This study showed that the TriLobe had better control of motion compared to the ball and trough both in ROM and varibility for FE, LB, and AR. The TriLobe had better control of limiting kyphosis over the ball and trough by 41% of the flexion motion. The neutral zone slope, an measure for device stability, showed that the TriLobe was 51% more stable than the BT. AP placement of devices showed there was a general trend of decreasing stability from anterior to posterior placement; however, statistical significance was not established

Background. The position of the hip-joint centre of rotation (HJC) within the pelvis is known to influence functional outcome of total hip replacement (THR). Superior, lateral and posterior relocations of the HJC from anatomical position have been shown to be associated with greater joint reaction forces and a higher incidence of aseptic loosening. In biomechanical models, the maximum force, moment-generating capacity and the range of motion of the major hip muscle groups have been shown to be sensitive to HJC displacement. This clinical study investigated the effect of HJC displacement and acetabular cup inclination angle on functional performance in patients undergoing primary THR. Methods. Retrospective study of primary THR patients at the RNOH. HJC displacement from anatomical position in horizontal and vertical planes was measured relative to radiological landmarks using post-operative, calibrated, anterior-posterior pelvic radiographs. Acetabular cup inclination angle was measured relative to the inter-teardrop line. Maximum range of passive hip flexion, abduction, adduction, external and internal rotation were measured in clinic. Patient reported functional outcome was assessed by Oxford Hip Score (OHS) and WOMAC questionnaires. Data analysed using a linear regression model. Results. 109 THRs were studied in 104 patients (69 Female). Mean age at THR=63 years (22-88). Mean follow-up=17 months (11-39 months). Median OHS=16, WOMAC=8. Increasing vertical HJC displacement (in either superior or inferior direction) from anatomical position was associated with worsening OHS (p<0.05) and WOMAC scores (p<0.05) and a reduced range of passive hip flexion (p<0.05). No relationship was found between either horizontal HJC displacement or acetabular cup inclination angle and patient functional outcome. Conclusion. A significant relationship was identified between increasing vertical displacement of the HJC and worsening patient functional outcome. This supports current opinion regarding the disadvantageous consequences of a superiorly displaced HJC in terms of survivorship and function. We therefore advocate an anatomical restoration of HJC position wherever possible

INTRODUCTION. Accurate knowledge of knee joint kinematics following total knee arthroplasty (TKA) is critical for evaluating the functional performance of specific implant designs. Biplane fluoroscopy is currently the most accurate method for measuring 3D knee joint kinematics in vivo during daily activities such as walking. However, the relatively small imaging field of these systems has limited measurement of knee kinematics to only a portion of the gait cycle. We developed a mobile biplane X-ray (MoBiX) fluoroscopy system that enables concurrent tracking and imaging of the knee joint for multiple cycles of overground gait. The primary aim of the present study was to measure 6-degree-of-freedom (6-DOF) knee joint kinematics for one complete cycle of overground walking. A secondary aim was to quantify the position of the knee joint centre of rotation (COR) in the transverse plane during TKA gait. METHODS. Ten unilateral posterior-stabilised TKA patients (5 females, 5 males) were recruited to the study. Each subject walked over ground at their self-selected speed (0.93±0.12 m/s). The MoBiX imaging system tracked and recorded biplane X-ray images of the knee, from which tibiofemoral kinematics were calculated using an image processing and pose-estimation pipeline created in MATLAB. Mean 6-DOF tibiofemoral joint kinematics were plotted against the mean knee flexion angle for one complete cycle of overground walking. The joint COR in the transverse plane was calculated as the least squares intersection of the femoral flexion axis projected onto the tibial tray during the stance and swing phases. The femoral and tibial axes and 6-DOF kinematics were defined in accordance with the convention defined by Grood and Suntay in 1983. RESULTS AND DISCUSSION. The offset in secondary joint motions at a given flexion angle was greater at larger knee flexion angles than at smaller flexion angles for abduction, anterior drawer, and lateral shift, whereas the opposite was true for external rotation. Significant variability was observed between subjects for the COR. The mean COR was on the lateral side during stance, consistent with results reported in the literature for the intact knee. Interestingly, the mean COR was on the medial side during swing. CONCLUSIONS. Our results suggest that secondary joint motions in the TKA knee, specifically, external rotation, abduction, anterior drawer and lateral shift, are determined not only by implant geometry and ligament anatomy but also by external loading, and are therefore task-dependent. The mean COR in the transverse plane shifted from the lateral to the medial side of the knee as the leg transitioned from stance to swing. Mobile dynamic X-ray imaging is a valuable tool for evaluating the functional performance of knee implants during locomotion over ground

The reverse ball and socket shoulder replacement, employing a humeral socket and glenosphere, has revolutionized the treatment of patients with arthritis and rotator cuff insufficiency. The RSP (DjO Surgical, Inc., Austin, Texas) is one such device, characterized by a lateral center of rotation and approved for use in the United States since 2004. Multiple studies by the implant design team have documented excellent outcomes and low revision rates for the RSP, but other published outcomes data are relatively sparse. The objective of this study is to report on the complications and early outcomes in the first consecutive 60 RSPs implanted in 57 patients by a single shoulder replacement surgeon between 2004 and 2010. Forty-four patients were female and mean age at the time of reverse shoulder arthroplasty was 75 years (range 54 to 92 years). The RSP was used as a primary arthroplasty in 42 shoulders and to revise a failed prosthetic shoulder arthroplasty in 18 shoulders. During the study period, 365 shoulder replacements were implanted so that the RSP was used selectively, accounting for only 17% of all shoulder arthroplasties (8.4% for 2004-2007, 24.2% for 2008-2010). Most patients had pseudoparalysis and profound shoulder dysfunction so that mean pre-operative active forward elevation was to 45°, active abduction to 43°, active internal rotation to the buttock, and the mean pre-operative Simple Shoulder Test (SST) score was 1 out of 12. At final follow-up, mean active forward elevation had improved to 101° (p<0.0001), active abduction to 91° (p<0.0001), active internal rotation to the lumbosacral junction (p<0.001), and the mean final SST score was 7 out of 12. There were 16 complications in 14 patients, including 7 reoperations in 6 patients (11%): 3 closed reductions for dislocation, 2 open revisions for instability and for a dissociated liner in the same patient, one evacuation of a hematoma, and one open reduction and internal fixation of a post-operative scapular spine fracture. Two additional scapular spine or acromion fractures and one acromioclavicular joint separation developed postoperatively that impacted outcome adversely but did not require re-operation. None of the glenoid baseplates or humeral stems has been revised and no deep infections have occurred. Experience with reverse shoulder arthroplasty appears to influence the reoperation rate, as 3 of the reoperations occurred following the first 15 reverse shoulder arthroplasties. Overall improvements in active motion and self-assessed shoulder function were comparable to those reported previously. Final active motion results were somewhat lower than those reported previously, which may relate to the selection of predominately pseudoparalytic patients for reverse shoulder arthroplasty in this series. Use of the RSP device for reverse shoulder arthroplasty leads to improved motion and function in carefully selected older patients with pseudoparalysis or a failed shoulder replacement. Re-operations and complications occur but the learning curve may not be as steep as previously reported. This may relate to specific features of the implant system used in this series, as well as to surgeon experience

Introduction: The foot is a very complex structure acting as the platform for all gait patterns. At present, little is known about the exact biomechanics of the foot due to the difficulties in modeling all of the components of the foot accurately. This has made it virtually impossible to develop a complete understanding of the aetiology of many diseases of the foot including hallux rigidus. We hypothesize that sagittal plane incongruency of the rotation of the 1. st. Metatarsophalangeal Joint (MTPJ), or an increase in the tension of the intrinsic plantar flexors is responsible for the development of hallux rigidus. Materials & Methods: Ground reaction forces and kinematic data from gait analysis together with anthropometric data from MRI scans of a 24 y.o. female were used to create a Mimics model of the articulation of a normal 1st MTPJ during a gait cycle. The centre of rotation was calculated by triangulating the articular surfaces. Finite element analysis was performed on the model and on similar models with the hypothesized;. joint incongruency,. an increased tension in the Flexor Hallicus Brevis and. an increased tension in the plantar fascia. Results: The results demonstrated a significant increase in the peak stresses, contact areas and stress distributions between the incongruent models compared to the congruent models. Discussion: To the best of our knowledge this is the most accurate FE model of the 1st MTPJ calculated. Hallux Rigidus is a very common forefoot disorder, with multiple etiologies and treatments advocated. This model demonstrates that an increased tension in the plantar flexors results in a reduced ROM with increased contact stresses on the joint surface. Conclusion: While it is known Hallux Rigidus has a multi-factorial etiology, the authors feel the above study demonstrates an important inherent etiology

Introduction. Lateralizing the center of rotation (COR) of reverse total shoulder arthroplasty (rTSA) has the potential to increase functional outcomes of the procedure, namely adduction range of motion (ROM). However, increased torque at the bone-implant interface as a result of lateralization may provoke early implant loosening, especially in situations where two, rather than four, fixation screws are used. The aim of this study was to utilize finite element (FE) models to investigate the effects of lateralization and the number of fixation screws on micromotion and adduction ROM. Methods. Four patient-specific scapular geometries were developed from CT data in 3D Slicer using a semi-automatic threshold technique. A generic glenoid component including the baseplate, a lateralization spacer, and four fixation screws was modelled as a monoblock. Screws were simplified as 4.5 mm diameter cylinders. The glenoid of each scapula was virtually reamed after which the glenoid component was placed. Models were meshed with quadratic tetrahedral elements with an edge length of 1.3 mm. The baseplate and lateralization spacer were assigned titanium material properties (E = 113.8 GPa and ν = 0.34). Screws were also assigned titanium material properties with a corrected elastic modulus (56.7 GPa) to account for omitted thread geometry. Cortical bone was assigned an elastic modulus of 17.5 GPa and Poisson's ratio of 0.3. Cancellous bone material properties in the region of the glenoid were assigned on an element-by-element basis using previously established equations to convert Hounsfield Units from the CT data to density and subsequently to elastic modulus [1]. Fixed displacement boundary conditions were applied to the medial border of each scapula. Contact was simulated as frictional (μ = 0.8) between bone and screws and frictionless between bone and baseplate/spacer. Compressive and superiorly-oriented shear loads of 686 N were applied to the baseplate/spacer. Lateralization of the COR up to 16 mm was simulated by applying the shear load further from the glenoid surface in 4 mm increments (Fig. 1A). All lateralization levels were simulated with four and two (superior and inferior) fixation screws. Absolute micromotion of the baseplate/spacer with respect to the glenoid surface was averaged across the back surface of the spacer and normalized to the baseline configuration considered to be 0 mm lateralization and four fixation screws. Adduction ROM was measured as the angle between the glenoid surface and the humeral stem when impingement of the humeral cup occurred (Fig. 1B). Results. Lateralization (p = 0.015) and reducing the number of fixation screws (p = 0.008) significantly increased micromotion (Fig. 2). Lateralization significantly increased adduction ROM (p = 0.001). Relationships between lateralization, the number of fixation screws, micromotion, and adduction ROM were shoulder-specific (Fig. 3). Conclusions. Lateralizing the COR of rTSA can improve functional outcomes of the procedure, however may compromise long-term survival of the implant by increasing micromotion. Our results indicate that the trade-offs of lateralizing should be considered on a patient-specific basis, taking into account factors such as quality and availability of bone stock

CT-based, customised femoral stem enables optimal reconstruction of hip mechanics and leg length. However, traditional planning and execution of cup insertion may jeopardise these biomechanical parameters. The aim of this study was to examine the agreement of the preoperative planning of cup position and the final position of the cup.

Thirty total hip replacements with an uncemented acetabular cup (Duraloc, DePuy) or a cemented cup (Elite-Plus, DePuy) were included. A customised femoral stem was used in all hips. On the preoperative X-rays the planned position and orientation of the cup had been marked prior to the surgery. The pre- and postoperative X-ray images were then digitised and scaled. The planned and final positions of the cup centre in the frontal plane was then measured relative to a horizontal line defined by the tear-drops and to a vertical line through the centre of the tear-drop on the operated side. In addition the concurrence between the planned and final cup size was examined.

In the horizontal direction the cups were positioned 1.4 (7.6) mm (median, ±2SD) more medial than planned on the preoperative X-rays. In the vertical direction the corresponding figures were 1.2 (6.6) mm (median, ±2SD) and the cups were usually placed more cranially than was planned. The maximum discrepancy between the planned and final position was 10,6 mm in the horizontal direction (medial) and 7.1 mm in the vertical direction (cranial). In 63% of the hips there was agreement between the size of the cup planned preoperatively and the cup that was finally inserted. In 25% of the hips the final cup was larger and in 12% the final cup was smaller.

In most cases the acetabular cups were inserted within a few millimetres of the planned position. The combination of a standard uncemented or cemented cup with a custom femoral stem enables the surgeon to restore hip mechanics and leg length.

Introduction: Several techniques have been described for revision of acetabulum associated with severe superior bony defects. An Oblong cup inserted without bone cement has the advantage of restoration of the centre of hip rotation and maintenance of bone stock. The aim of this study was to analyse the medium-term results of acetabular revision using Oblong cup for severe superior bony defects. Methods: Thirty-five acetabular revisions using porous coated Oblong cups (S-Rom, Depuy) in 34 patients were performed by the senior author between 1998–2001. All patients were followed-up clinically and thirty-one hips were analysed radiologically for a mean duration of 39.6 months (range 18 to 60). The clinical assessment was performed using Harris hip score and subjective patient’s satisfaction. The acetabular defects were classified according to the method described by Paprosky et al. The position of the acetabular implant, restoration of the centre of hip rotation and the extent of osseo-integration of the acetabular shell were assessed in the post-operative radiographs. Results: The mean Harris hip score has improved from a preoperative value of 40.6 to 69.4 post-operatively. According to the Leprosy’s method, two acetabula were classified as type 2B, 12 as type 3A and 17 as 3B. The post-operative radiographs showed a mean abduction angle of the Oblong cups of 54.2 degrees (range 40–80). Osseointegration was achieved in 29(94%) of cups. Three cups showed early migration, of which two eventually stabilised and osseointegrated by 12 months. The centre of rotation of hip improved from a mean lateral migration of 8.3 mm and superior migration of 23.5 mm in the pre-operative radiographs to 2.8 mm and 4.3 mm respectively post-operatively. Discussion: Acetabular revision using porous coated oblong cup for severe superior acetabular defects provides satisfactory medium-term results with predictable restoration of hip centre of rotation

Aims. Custom triflange acetabular components (CTACs) play an important role in reconstructive orthopaedic surgery, particularly in revision total hip arthroplasty (rTHA) and pelvic tumour resection procedures. Accurate CTAC positioning is essential to successful surgical outcomes. While prior studies have explored CTAC positioning in rTHA, research focusing on tumour cases and implant flange positioning precision remains limited. Additionally, the impact of intraoperative navigation on positioning accuracy warrants further investigation. This study assesses CTAC positioning accuracy in tumour resection and rTHA cases, focusing on the differences between preoperative planning and postoperative implant positions. Methods. A multicentre observational cohort study in Australia between February 2017 and March 2021 included consecutive patients undergoing acetabular reconstruction with CTACs in rTHA (Paprosky 3A/3B defects) or tumour resection (including Enneking P2 peri-acetabular area). Of 103 eligible patients (104 hips), 34 patients (35 hips) were analyzed. Results. CTAC positioning was generally accurate, with minor deviations in cup inclination (mean 2.7°; SD 2.84°), anteversion (mean 3.6°; SD 5.04°), and rotation (mean 2.1°; SD 2.47°). Deviation of the hip centre of rotation (COR) showed a mean vector length of 5.9 mm (SD 7.24). Flange positions showed small deviations, with the ischial flange exhibiting the largest deviation (mean vector length of 7.0 mm; SD 8.65). Overall, 83% of the implants were accurately positioned, with 17% exceeding malpositioning thresholds. CTACs used in tumour resections exhibited higher positioning accuracy than rTHA cases, with significant differences in inclination (1.5° for tumour vs 3.4° for rTHA) and rotation (1.3° for tumour vs 2.4° for rTHA). The use of intraoperative navigation appeared to enhance positioning accuracy, but this did not reach statistical significance. Conclusion. This study demonstrates favourable CTAC positioning accuracy, with potential for improved accuracy through intraoperative navigation. Further research is needed to understand the implications of positioning accuracy on implant performance and long-term survival. Cite this article: Bone Jt Open 2024;5(4):260–268

Aims. Achieving accurate implant positioning and restoring native hip biomechanics are key surgeon-controlled technical objectives in total hip arthroplasty (THA). The primary objective of this study was to compare the reproducibility of the planned preoperative centre of hip rotation (COR) in patients undergoing robotic arm-assisted THA versus conventional THA. Methods. This prospective randomized controlled trial (RCT) included 60 patients with symptomatic hip osteoarthritis undergoing conventional THA (CO THA) versus robotic arm-assisted THA (RO THA). Patients in both arms underwent pre- and postoperative CT scans, and a patient-specific plan was created using the robotic software. The COR, combined offset, acetabular orientation, and leg length discrepancy were measured on the pre- and postoperative CT scanogram at six weeks following surgery. Results. There were no significant differences for any of the baseline characteristics including spinopelvic mobility. The absolute error for achieving the planned horizontal COR was median 1.4 mm (interquartile range (IQR) 0.87 to 3.42) in RO THA versus 4.3 mm (IQR 3 to 6.8; p < 0.001); vertical COR mean 0.91 mm (SD 0.73) in RO THA versus 2.3 mm (SD 1.3; p < 0.001); and combined offset median 2 mm (IQR 0.97 to 5.45) in RO THA versus 3.9 mm (IQR 2 to 7.9; p = 0.019). Improved accuracy was observed with RO THA in achieving the desired acetabular component positioning (root mean square error for anteversion and inclination was 2.6 and 1.3 vs 8.9 and 5.3, repectively) and leg length (mean 0.6 mm vs 1.4 mm; p < 0.001). Patient-reported outcome measures were comparable between the two groups at baseline and one year. Participants in the RO THA group needed fewer physiotherapy sessions postoperatively (median six (IQR 4.5 to 8) vs eight (IQR 6 to 11; p = 0.005). Conclusion. This RCT suggested that robotic-arm assistance in THA was associated with improved accuracy in restoring the native COR, better preservation of the combined offset, leg length correction, and superior accuracy in achieving the desired acetabular component positioning. Further evaluation through long-term and registry data is necessary to assess whether these findings translate into improved implant survival and functional outcomes. Cite this article: Bone Joint J 2024;106-B(4):324–335

Aims

One goal of total hip arthroplasty is to restore normal hip anatomy. The aim of this study was to compare displacement of the centre of rotation (COR) using a standard reaming technique with a technique in which the acetabulum was reamed immediately peripherally and referenced off the rim.

Aims. Complex total hip arthroplasty (THA) with subtrochanteric shortening osteotomy is necessary in conditions other than developmental dysplasia of the hip (DDH) and septic arthritis sequelae with significant proximal femur migration. Our aim was to evaluate the hip centre restoration with THAs in these hips. Methods. In all, 27 THAs in 25 patients requiring THA with femoral shortening between 2012 and 2019 were assessed. Bilateral shortening was required in two patients. Subtrochanteric shortening was required in 14 out of 27 hips (51.9%) with aetiology other than DDH or septic arthritis. Vertical centre of rotation (VCOR), horizontal centre of rotation, offset, and functional outcome was calculated. The mean followup was 24.4 months (5 to 92 months). Results. The mean VCOR was 17.43 mm (9.5 to 27 mm) and horizontal centre of rotation (HCOR) was 24.79 mm (17.2 to 37.6 mm). Dislocation at three months following acetabulum reconstruction required femoral shortening for offset correction and hip centre restoration in one hip. Mean horizontal offset was 39.72 (32.7 to 48.2 mm) compared to 42.89 (26.7 to 50.6 mm) on the normal side. Mean Harris Hip Score (HHS) of 22.64 (14 to 35) improved to 79.43 (68 to 92). Mean pre-operative shortening was 3.95 cm (2 to 8 cm). Residual limb length discrepancy was 1.5 cm (0 to 2 cm). Sciatic neuropraxia in two patients recovered by six months, and femoral neuropraxia in one hip recovered by 12 months. Mean Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) was 13.92 (9 to 19). Mean 12-item short form survey (SF-12) physical scores of 50.6 and mental of 60.12 were obtained. Conclusion. THA with subtrochanteric shortening is valuable in complex hips with high dislocation. The restoration of the hip centre of rotation and offset is important in these hips. Level of evidence IV. Femoral shortening useful in conditions other than DDH and septic sequelae. Restoration of hip centre combined with offset to be planned and ensured

Aims. The aim of this study was to compare the biomechanical models of two frequently used techniques for reconstructing severe acetabular defects with pelvic discontinuity in revision total hip arthroplasty (THA) – the Trabecular Metal Acetabular Revision System (TMARS) and custom triflange acetabular components (CTACs) – using virtual modelling. Methods. Pre- and postoperative CT scans from ten patients who underwent revision with the TMARS for a Paprosky IIIB acetabular defect with pelvic discontinuity were retrospectively collated. Computer models of a CTAC implant were designed from the preoperative CT scans of these patients. Computer models of the TMARS reconstruction were segmented from postoperative CT scans using a semi-automated method. The amount of bone removed, the implant-bone apposition that was achieved, and the restoration of the centre of rotation of the hip were compared between all the actual TMARS and the virtual CTAC implants. Results. The median amount of bone removed for TMARS reconstructions was significantly greater than for CTAC implants (9.07 cm. 3. (interquartile range (IQR) 5.86 to 21.42) vs 1.16 cm. 3. (IQR 0.42 to 3.53) (p = 0.004). There was no significant difference between the median overall implant-bone apposition between TMARS reconstructions and CTAC implants (54.8 cm. 2. (IQR 28.2 to 82.3) vs 56.6 cm. 2. (IQR 40.6 to 69.7) (p = 0.683). However, there was significantly more implant-bone apposition within the residual acetabulum (45.2 cm. 2. (IQR 28.2 to 72.4) vs 25.5 cm. 2. (IQR 12.8 to 44.1) (p = 0.001) and conversely significantly less apposition with the outer cortex of the pelvis for TMARS implants compared with CTAC reconstructions (0 cm. 2. (IQR 0 to 13.1) vs 23.2 cm. 2. (IQR 16.4 to 30.6) (p = 0.009). The mean centre of rotation of the hip of TMARS reconstructions differed by a mean of 11.1 mm (3 to 28) compared with CTAC implants. Conclusion. In using TMARS, more bone is removed, thus achieving more implant-bone apposition within the residual acetabular bone. In CTAC implants, the amount of bone removed is minimal, while the implant-bone apposition is more evenly distributed between the residual acetabulum and the outer cortex of the pelvis. The differences suggest that these implants used to treat pelvic discontinuity might achieve short- and long-term stability through different biomechanical mechanisms. Cite this article: Bone Joint J 2024;106-B(5 Supple B):74–81

The best treatment method of large acetabular bone defects at revision THR remains controversial. Some of the factors that need consideration are the amount of residual pelvic bone removed during revision; the contact area between the residual pelvic bone and the new implant; and the influence of the new acetabular construct on the centre of rotation of the hip. The purpose of this study was to compare these variables in two of the most used surgical techniques used to reconstruct severe acetabular defects: the trabecular metal acetabular revision system (TMARS) and a custom triflanged acetabular component (CTAC). Pre- and post-operative CT-scans were acquired from 11 patients who underwent revision THR with a TMARS construct for a Paprosky IIIB defect, 10 with pelvic discontinuity, at Royal Adelaide Hospital. The CT scans were used to generate computer models to virtually compare the TMARS and CTAC constructs using a semi-automated method. The TMARS construct model was calculated using postoperative CT scans while the CTAC constructs using the preoperative CT scans. The bone contact, centre of rotation, inclination, anteversion and reamed bone differences were calculated for both models. There was a significant difference in the mean amount of bone reamed for the TMARS reconstructions (15,997 mm. 3. ) compared to the CTAC reconstructions (2292 mm. 3. , p>0.01). There was no significant difference between overall implant bone contact (TMARS 5760mm. 2. vs CTAC 5447mm. 2. , p=0.63). However, there was a significant difference for both cancellous (TMARS 4966mm. 2. vs CTAC 2887mm. 2. , p=0.008) and cortical bone contact (TMARS 795mm. 2. vs CTAC 2560mm. 2. , p=0.001). There was no difference in inclination and anteversion achieved. TMARS constructs resulted on average in a centre of rotations 7.4mm more lateral and 4.0mm more posterior. Modelling of two different reconstructions of Paprosky IIIB defects demonstrated potential important differences between all variables investigated

Aims. After failed acetabular fractures, total hip arthroplasty (THA) is a challenging procedure and considered the gold standard treatment. The complexity of the procedure depends on the fracture pattern and the initial fracture management. This study’s primary aim was to evaluate patient-reported outcome measures (PROMs) for patients who underwent delayed uncemented acetabular THA after acetabular fractures. The secondary aims were to assess the radiological outcome and the incidence of the associated complications in those patients. Methods. A total of 40 patients underwent cementless acetabular THA following failed treatment of acetabular fractures. The postoperative clinical and radiological outcomes were evaluated for all the cohort. Results. The median (interquartile range (IQR)) Oxford Hip Score (OHS) improved significantly from 9.5 (7 to 11.5), (95% confidence interval (CI) (8 to 10.6)) to 40 (39 to 44), (95% CI (40 to 43)) postoperatively at the latest follow-up (p < 0.001). It was worth noting that the initial acetabular fracture type (simple vs complex), previous acetabular treatment (ORIF vs conservative), fracture union, and restoration of anatomical centre of rotation (COR) did not affect the final OHS. The reconstructed centre of rotation (COR) was restored in 29 (72.5%) patients. The mean abduction angle in whom acetabular fractures were managed conservatively was statistically significantly higher than the surgically treated patients 42.6° (SD 7.4) vs 38° (SD 5.6)) (p = 0.032). We did not have any case of acetabular or femoral loosening at the time of the last follow-up. We had two patients with successful two-stage revision for infection with overall eight-year survival rate was 95.2% (95% CI 86.6% to 100%) with revision for any reason at a median (IQR) duration of follow-up 50 months (16 to 87) months following THA. Conclusion. Delayed cementless acetabular THA in patients with previous failed acetabular fracture treatments produces good clinical outcomes (PROMS) with excellent survivorship, despite the technically demanding nature of the procedure. The initial fracture treatment does not influence the outcome of delayed THA. In selected cases of acetabular fractures (either nondisplaced or with secondary congruency), the initial nonoperative treatment neither resulted in large acetabular defects nor required additional acetabular reconstruction at the time of THA. Cite this article: Bone Jt Open 2021;2(12):1067–1074

Aims. Morphological abnormalities are present in patients with developmental dysplasia of the hip (DDH). We studied and compared the pelvic anatomy and morphology between the affected hemipelvis with the unaffected side in patients with unilateral Crowe type IV DDH using 3D imaging and analysis. Methods. A total of 20 patients with unilateral Crowe-IV DDH were included in the study. The contralateral side was considered normal in all patients. A coordinate system based on the sacral base (SB) in a reconstructed pelvic model was established. The pelvic orientations (tilt, rotation, and obliquity) of the affected side were assessed by establishing a virtual anterior pelvic plane (APP). The bilateral coordinates of the anterior superior iliac spine (ASIS) and the centres of hip rotation were established, and parameters concerning size and volume were compared for both sides of the pelvis. Results. The ASIS on the dislocated side was located inferiorly and anteriorly compared to the healthy side (coordinates on the y-axis and z-axis; p = 0.001; p = 0.031). The centre of hip rotation on the dislocated side was located inferiorly and medially compared to the healthy side (coordinates on the x-axis and the y-axis; p < 0.001; p = 0.003). The affected hemipelvis tilted anteriorly in the sagittal plane (mean 8.05° (SD 3.57°)), anteriorly rotated in the transverse plane (mean 3.31° (SD 1.41°)), and tilted obliquely and caudally in the coronal plane (mean 2.04° (SD 0.81°)) relative to the healthy hemipelvis. The affected hemipelvis was significantly smaller in the length, width, height, and volume than the healthy counterpart. (p = 0.014; p = 0.009; p = 0.035; p = 0.002). Conclusion. Asymmetric abnormalities were identified on the affected hemipelvis in patients with the unilateral Crowe-IV DDH using 3D imaging techniques. Improved understanding of the morphological changes may influence the positioning of the acetabular component at THA. Acetabular component malpositioning errors caused by anterior tilt of the affected hemi pelvis and the abnormal position of the affected side centre of rotation should be considered by orthopaedic surgeons when undertaking THA in patients with Crowe-IV DDH. Cite this article: Bone Joint J 2020;102-B(10):1311–1318

The advent of modular porous metal augments has ushered in a new form of treatment for acetabular bone loss. The function of an augment can be seen as reducing the size of a defect or reconstituting the anterosuperior/posteroinferior columns and/or allowing supplementary fixation. Depending on the function of the augment, the surgeon can decide on the sequence of introduction of the hemispherical shell, before or after the augment. Augments should always, however, be used with cement to form a unit with the acetabular component. Given their versatility, augments also allow the use of a hemispherical shell in a position that restores the centre of rotation and biomechanics of the hip. Progressive shedding or the appearance of metal debris is a particular finding with augments and, with other radiological signs of failure, should be recognized on serial radiographs. Mid- to long-term outcomes in studies reporting the use of augments with hemispherical shells in revision total hip arthroplasty have shown rates of survival of > 90%. However, a higher risk of failure has been reported when augments have been used for patients with chronic pelvic discontinuity. Cite this article: Bone Joint J 2024;106-B(4):312–318

Aims. Dislocation of the hip remains a major complication after periacetabular tumour resection and endoprosthetic reconstruction. The position of the acetabular component is an important modifiable factor for surgeons in determining the risk of postoperative dislocation. We investigated the significance of horizontal, vertical, and sagittal displacement of the hip centre of rotation (COR) on postoperative dislocation using a CT-based 3D model, as well as other potential risk factors for dislocation. Methods. A total of 122 patients who underwent reconstruction following resection of periacetabular tumour between January 2011 and January 2020 were studied. The risk factors for dislocation were investigated with univariate and multivariate logistic regression analysis on patient-specific, resection-specific, and reconstruction-specific variables. Results. The dislocation rate was 13.9% (n = 17). The hip COR was found to be significantly shifted anteriorly and inferiorly in most patients in the dislocation group compared with the non-dislocation group. Three independent risk factors were found to be related to dislocation: resection of gluteus medius (odds ratio (OR) 3.68 (95% confidence interval (CI) 1.24 to 19.70); p = 0.039), vertical shift of COR > 18 mm (OR 24.8 (95% CI 6.23 to 128.00); p = 0.001), and sagittal shift of COR > 20 mm (OR 6.22 (95% CI 1.33 to 32.2); p = 0.026). Conclusion. Among the 17 patients who dislocated, 70.3% (n = 12) were anterior dislocations. Three independent risk factors were identified, suggesting the importance of proper restoration of the COR and the role of the gluteus medius in maintaining hip joint stability. Cite this article: Bone Joint J 2022;104-B(10):1180–1188

Objectives. The primary objective of this study was to compare accuracy in restoring the native centre of hip rotation in patients undergoing conventional manual total hip arthroplasty (THA) versus robotic-arm assisted THA. Secondary objectives were to determine differences between these treatment techniques for THA in achieving the planned combined offset, component inclination, component version, and leg-length correction. Materials and Methods. This prospective cohort study included 50 patients undergoing conventional manual THA and 25 patients receiving robotic-arm assisted THA. Patients undergoing conventional manual THA and robotic-arm assisted THA were well matched for age (mean age, 69.4 years (. sd. 5.2) vs 67.5 years (. sd. 5.8) (p = 0.25); body mass index (27.4 kg/m. 2. (. sd. 2.1) vs 26.9 kg/m. 2. (. sd. 2.2); p = 0.39); and laterality of surgery (right = 28, left = 22 vs right = 12, left = 13; p = 0.78). All operative procedures were undertaken by a single surgeon using the posterior approach. Two independent blinded observers recorded all radiological outcomes of interest using plain radiographs. Results. The correlation coefficient was 0.92 (95% confidence interval (CI) 0.88 to 0.95) for intraobserver agreement and 0.88 (95% CI 0.82 to 0.94) for interobserver agreement in all study outcomes. Robotic THA was associated with improved accuracy in restoring the native horizontal (p < 0.001) and vertical (p < 0.001) centres of rotation, and improved preservation of the patient’s native combined offset (p < 0.001) compared with conventional THA. Robotic THA improved accuracy in positioning of the acetabular component within the combined safe zones of inclination and anteversion described by Lewinnek et al (p = 0.02) and Callanan et al (p = 0.01) compared with conventional THA. There was no difference between the two treatment groups in achieving the planned leg-length correction (p = 0.10). Conclusion. Robotic-arm assisted THA was associated with improved accuracy in restoring the native centre of rotation, better preservation of the combined offset, and more precise acetabular component positioning within the safe zones of inclination and anteversion compared with conventional manual THA

Aims. The aim of this study was to examine the implant accuracy of custom-made partial pelvis replacements (PPRs) in revision total hip arthroplasty (rTHA). Custom-made implants offer an option to achieve a reconstruction in cases with severe acetabular bone loss. By analyzing implant deviation in CT and radiograph imaging and correlating early clinical complications, we aimed to optimize the usage of custom-made implants. Methods. A consecutive series of 45 (2014 to 2019) PPRs for Paprosky III defects at rTHA were analyzed comparing the preoperative planning CT scans used to manufacture the implants with postoperative CT scans and radiographs. The anteversion (AV), inclination (IC), deviation from the preoperatively planned implant position, and deviation of the centre of rotation (COR) were explored. Early postoperative complications were recorded, and factors for malpositioning were sought. The mean follow-up was 30 months (SD 19; 6 to 74), with four patients lost to follow-up. Results. Mean CT defined discrepancy (Δ) between planned and achieved AV and IC was 4.5° (SD 3°; 0° to 12°) and 4° (SD 3.5°; 1° to 12°), respectively. Malpositioning (Δ > 10°) occurred in five hips (10.6%). Native COR reconstruction was planned in 42 cases (93%), and the mean 3D deviation vector was 15.5 mm (SD 8.5; 4 to 35). There was no significant influence in malpositioning found for femoral stem retention, surgical approach, or fixation method. Conclusion. At short-term follow-up, we found that PPR offers a viable solution for rTHA in cases with massive acetabular bone loss, as highly accurate positioning can be accomplished with meticulous planning, achieving anatomical reconstruction. Accuracy of achieved placement contributed to reduced complications with no injury to vital structures by screw fixation. Cite this article: Bone Joint J 2022;104-B(10):1110–1117

Total elbow arthroplasty (TEA) usage is increasing owing to expanded surgical indications, better implant designs, and improved long-term survival. Correct humeral implant positioning has been shown to diminish stem loading in vitro, and radiographic loosening in in the long-term. Replication of the native elbow centre of rotation is thought to restore normal muscle moment arms and has been suggested to improve elbow strength and function. While much of the focus has been on humeral component positioning, little is known about the effect of positioning of the ulnar stem on post-operative range of motion and clinical outcomes. The purpose of this study is to determine the effect of the sagittal alignment and positioning of the humeral and ulnar components on the functional outcomes after TEA. Between 2003 and 2016, 173 semi-constrained TEAs (Wright-Tornier Latitude/Latitude EV, Memphis, TN, USA) were performed at our institution, and our preliminary analysis includes 46 elbows in 41 patients (39 female, 7 male). Patients were excluded if they had severe elbow deformity precluding reliable measurement, experienced a major complication related to an ipsilateral upper limb procedure, or underwent revision TEA. For each elbow, saggital alignment was compared pre- and post-operatively. A best fit circle of the trochlea and capitellum was drawn, with its centre representing the rotation axis. Ninety degree tangent lines from the intramedullary axes of the ulna and humerus, and from the olecranon tip to the centre of rotation were drawn and measured relative to the rotation axis, representing the ulna posterior offset, humerus offset, and ulna proximal offset, respectively. In addition, we measured the ulna stem angle (angle subtended by the implant and the intramedullary axis of the ulna), as well as radial neck offset (the length of a 90o tangent line from the intramedullary axis of the radial neck and the centre of rotation) in patients with retained or replaced radial heads. Our primary outcome measure was the quickDASH score recorded at the latest follow-up for each patient. Our secondary outcome measures were postoperative flexion, extension, pronation and supination measured at the same timepoints. Each variable was tested for linear correlation with the primary and secondary outcome measures using the Pearson two-tailed test. At an average follow-up of 6.8 years (range 2–14 years), there was a strong positive correlation between anterior radial neck offset and the quickDASH (r=0.60, p=0.001). There was also a weak negative correlation between the posterior offset of the ulnar component and the qDASH (r=0.39, p=0.031), and a moderate positive correlation between the change in humeral offset and elbow supination (r=0.41, p=0.044). The ulna proximal offset and ulna stem angle were not correlated with either the primary, or secondary outcome measures. When performing primary TEA with radial head retention, or replacement, care should be taken to ensure that the ulnar component is correctly positioned such that intramedullary axis of the radial neck lines up with the centre of elbow rotation, as this strongly correlates with better function and less pain after surgery

Background. External fixation for a fracture-dislocation to a joint like the elbow, while maintaining joint mobility is currently done after identifying the center of rotation under X-ray guidance, when applying either a mono-lateral or a circular fixator. Current treatment. using the galaxy fixation system by Orthofix, the surgeon needs to correctly identify the center of rotation of the elbow under X-ray guidance on lateral views. If the center of rotation of the fixator is not aligned with that of the elbow joint, the assembly will not work, i.e. the elbow will be disrupted on trying to achieve flexion or extension movements. Figures (A, B, C and D) summarize the critical steps in identifying the centre of rotation (Courtesy of Orthofix Orthopedics International). New design. This new idea aims to propagate the principle of sliding external fixation applied on the extensor side of a joint, with the limbs of the fixator being able to slide in and out during joint extension and flexion respectively, without hindering the joint movement. Taking the ulno-humeral joint as an example, it is enough to apply the sliding external fixator in line with the subcutaneous border of the ulna, and the pins in the sagital plane, without the need to use x-ray guidance to identify the center of rotation, which simplifies the procedure, and makes it less technically demanding. The sliding external fixator over the elbow involves two bars which accommodate half pins fixation with headless grip screws to hold the pins, identical to the Rancho cubes technique by Smith & Nephew, these slide snugly into sleeves, those sleeves linked together through a hinge behind the elbow, and the bars are spring loaded to the hinge through the inside of the sleeves, which means they will slid into the sleeves in extension and out in flexion. Length of the sleeve should prevent the bars from dislodgement, and the cross section of both the bars and the sleeves have to correspond to each other for the sleeves to accommodate the bars within them and to prevent rotational instability within the construct itself. Summary. Applying an external fixator on the extensor surface is an idea could lead to major changes in external fixation product design, the ulno-humeral joint is taken as an example, and other joints could also be addressed taking in consideration joint size and anatomical structures at risk. The sliding technique makes the application easier, without the need to X-ray guided identification of the center of rotation

Background. Failed ingrowth and subsequent separation of revision acetabular components from the inferior hemi-pelvis constitutes a primary mode of failure in revision total hip arthroplasty (THA). Few studies have highlighted other techniques than multiple screws and an ischial flange or hook of cages to reinforce the ischiopubic fixation of the acetabular components, nor did any authors report the use of porous metal augments in the ischium and/or pubis to reinforce ischiopubic fixation of the acetabular cup. The aims of this study were to introduce the concept of extended ischiopubic fixation into the ischium and/or pubis during revision total hip arthroplasty [Fig. 2], and to determine the early clinical outcomes and the radiographic outcomes of hips revised with inferior extended fixation. Methods. Patients who underwent revision THA utilizing the surgical technique of extended ischiopubic fixation with porous metal augments secured in the ischium and/or pubis in a single institution from 2014 to 2016 were reviewed. 16 patients were included based on the criteria of minimum 24 months clinical and radiographic follow-up. No patients were lost to follow-up. The median duration of follow-up for the overall population was 37.43 months. The patients' clinical results were assessed using the Harris Hip Score (HHS), Western Ontario and McMaster Universities Osteoarthritis (WOMAC) index and Short form (SF)-12 score and satisfaction level based on a scale with five levels at each office visit. All inpatient and outpatient records were examined for complications, including infection, intraoperative fracture, dislocation, postoperative nerve palsy, hematoma, wound complication and/or any subsequent reoperation(s). The vertical and horizontal distances of the center of rotation to the anatomic femoral head and the inclination and anteversion angle of the cup were measured on the preoperative and postoperative radiographs. All the postoperative plain radiographs were reviewed to assess the stability of the components. Results. At the most recent follow-up, 11 (68.8%) patients rated their satisfaction level as “very satisfied” and 4 (25.0%) were “satisfied.” The median HHS improved significantly and the WOMAC global score decreased significantly at the latest follow-up (? 0.001). No intraoperative or postoperative complications were identified. All constructs were considered to have obtained bone ingrowth fixation. The median vertical distance between the latest postoperative center of rotation to the anatomic center of the femoral head improved from 14.7±10.05 mm preoperatively to 6.77±9.14 mm at final follow-up (p=0.002). The median horizontal distance between the latest postoperative center of rotation to the anatomic center of femoral head improved from 6.3±12.07 mm laterally preoperatively to 2.18±6.98 mm medially at the most recent follow-up (p=0.013) postoperatively. The median acetabular cup abduction angle improved from 55.04°±10.11° preoperatively to 44.43°± 5.73° at the most recent follow-up postoperatively (p=0.001). However, there was no difference in the median cup anteversion angles preoperatively (9.15°±5.36°) to postoperatively (9.66°±3.97°) (P=0.535). Conclusions. Early follow-up of patients reconstructed with the technique of extended ischiopubic fixation with porous metal augments demonstrated satisfactory clinical outcomes, restoration of the center of rotation and adequate biological fixation. For any figures or tables, please contact the authors directly

Summary. The mathematical model has proven to be highly accurate in measuring leg length before and after surgery to determine how leg length effects hip joint mechanics. Introduction. Leg length discrepancy (LLD) has been proven to be one of the most concerning problems associated with total hip arthroplasty (THA). Long-term follow-up studies have documented the presence of LLD having direct correlation with patient dissatisfaction, dislocation, back pain, and early complications. Several researchers sought to minimize limb length discrepancy based on pre-operative radiological templating or intra-operative measurements. While often being a common occurrence in clinical practice to compensate for LLD intra-operatively, the center of rotation of the hip joint has often changes unintentionally due to excessive reaming. Therefore, the clinical importance of LLD is still difficult to solve and remains a concern for clinicians. Objective. The objective of this study is two-fold: (1) use a validated forward-solution hip model to theoretically analyze the effects of LLD, gaining better understanding of mechanisms leading to early complication of THA and poor patient satisfaction and (2) to investigate the effect of the altered center of rotation of the hip joint regardless LLD compensation. Methods. The theoretical mathematical model used in this study has been previously validated using fluoroscopic results from existing implant designs and telemetric devices. The model can be used to theoretically investigate various surgical alignments, approaches, and procedures. In this study, we analyzed LLD and the effects of the altered center of rotation regardless of LLD compensation surgeons made. The simulations were conducted in both swing and stance phase of gait. Results. During swing phase, leg shortening lead to loosening of the hip capsular ligaments and subsequently, variable kinematic patterns. The momentum of the lower leg increased to levels where the ligaments could not properly constrain the hip leading to the femoral head sliding from within the acetabular cup (Figure 1). This piston motion led to decreased contact area and increased contact stress within the cup. Leg lengthening did not yield femoral head sliding but increased joint tension and contact stress. A tight hip may be an influential factor leading to back pain and poor patient satisfaction. During stance phase, leg shortening caused femoral head sliding leading to decreased contact area and an increase in contact stress. Leg lengthening caused an increase in capsular ligaments tension leading to higher stress in the hip joint (Figure 2). Interestingly, when the acetabular cup was superiorized and the surgeon compensated for LLD, thus matching the pre-operative leg length by increasing the neck length of the femoral implant, the contact forces and stresses were marginally increased at heel strike (Figure 3). Conclusion and Discussion. Altering the leg length during surgery can lead to higher contact forces and contact stresses due to tightening the hip joint or increasing likelihood of hip joint separation. Leg shortening often lead to higher stress within the joint. Further assessment must be conducted to develop tools that surgeons can use to ensure post-operative leg length is similar to the pre-operative condition. For any figures or tables, please contact authors directly

INTRODUCTION. The purpose of TKA is to restore normal kinematics and functioning to diseased knees. The purpose of this study was to determine whether intraoperative kinematic data are correlated with minimum one-year outcomes following primary TKA. METHODS. We reviewed data on 185 consecutive primary TKAs in which sensor-embedded tibial trials were used to evaluate kinematic patterns following traditional ligament balancing. Procedures were performed by two board-certified arthroplasty surgeons. The same implant design and surgical approach was used for all knees. Contact locations on the medial and lateral condyles were recorded for each patient at 0°, 45° and 90° of flexion, and full flexion. Vector equations were created by contact locations on the medial and lateral sides and the vector intersections determined the center of rotation between each measurement position. Center of rotation was calculated as the average of vector intersections at 0 to 45°, 45 to 90°, and 90° to full flexion. If the average center of rotation was between 16 and 1000 mm of the contact location on the medial side it was considered a medial pivot knee. Knees were also classified as medial (16 to 200 mm on medial side), lateral (16 to 200 mm on lateral side), translating (> 200 mm medially or laterally), and other (< 16 mm on both medial and lateral sides). The new Knee Society Scoring System (KSSO objective score, KSSS satisfaction score, KSSF function score), the EQ-5D™ Health Status Index, and the University of California Los Angeles (UCLA) Activity Level Score were measured preoperatively and at minimum one-year follow-up (average 20.4 months). RESULTS. Thirty-three TKAs were excluded to eliminate potential bias due to sensor device malfunction, atypical hardware, unresurfaced patella, surgery at a non-study hospital, or early postoperative infection, revision due to aseptic loosening, ipsilateral hip disease, and subsequent neurologic disease or death unrelated to the index TKA, resulting in a final sample size of 152 knees. Twelve (7.9%) patients were lost to follow-up, and two were excluded due to outlier values for average center of rotation. Seventy-five percent of the final sample was female. Mean age and BMI were 63.6 years 33.9, respectively. Average center of rotation ranged from −1017 to 1562 mm with negative signifying the lateral side. Medial pivot knees comprised 40% (55) of the total sample. Sex, age, height, surgeon, implant side, and implant type were unrelated to pivot classification. Patient weight (100.2 vs. 90.9 kg; p = 0.012) and BMI (35.5 vs. 32.8; p = 0.044) were greater in medial pivot knees. Controlling for BMI, KSSO, KSSF, KSSS, EQ5D, UCLA, and pain scores at latest follow-up did not differ in medial and non-medial pivot knees (p ≥ 0.151). The amount of improvement in outcomes from preoperative baseline also did not differ in medial and non-medial pivot knees (p ≥ 0.161). Outcomes did not vary among knees with translating medial, lateral, and other pivots (p ≥ 0.065). DISCUSSION. Our results suggest that a medial pivot kinematic pattern may not be a substantial governor of clinical success

Aims. There is little information in the literature about the use of dual-mobility (DM) bearings in preventing re-dislocation in revision total hip arthroplasty (THA). The aim of this study was to compare the use of DM bearings, standard bearings, and constrained liners in revision THA for recurrent dislocation, and to identify risk factors for re-dislocation. Methods. We reviewed 86 consecutive revision THAs performed for dislocation between August 2012 and July 2019. A total of 38 revisions (44.2%) involved a DM bearing, while 39 (45.3%) and nine (10.5%) involved a standard bearing and a constrained liner, respectively. Rates of re-dislocation, re-revision for dislocation, and overall re-revision were compared. Radiographs were assessed for the positioning of the acetabular component, the restoration of the centre of rotation, leg length, and offset. Risk factors for re-dislocation were determined by Cox regression analysis. The modified Harris Hip Scores (mHHSs) were recorded. The mean age of the patients at the time of revision was 70 years (43 to 88); 54 were female (62.8%). The mean follow-up was 5.0 years (2.0 to 8.75). Results. DM bearings were used significantly more frequently in elderly patients (p = 0.003) and in hips with abductor deficiency (p < 0.001). The re-dislocation rate was 13.2% for DM bearings compared with 17.9% for standard bearings, and 22.2% for constrained liners (p = 0.432). Re-revision-free survival for DM bearings was 84% (95% confidence interval (CI) 0.77 to 0.91) compared with 74% (95% CI 0.67 to 0.81) for standard articulations, and 67% (95% CI 0.51 to 0.82) for constrained liners (p = 0.361). Younger age (hazard ratio (HR) 0.92 (95% CI 0.85 to 0.99); p = 0.031), lower comorbidity (HR 0.44 (95% CI 0.20 to 0.95); p = 0.037), smaller heads (HR 0.80 (95% CI 0.64 to 0.99); p = 0.046), and retention of the acetabular component (HR 8.26 (95% CI 1.37 to 49.96); p = 0.022) were significantly associated with re-dislocation. All DM bearings which re-dislocated were in patients with abductor muscle deficiency (HR 48.34 (95% CI 0.03 to 7,737.98); p = 0.303). The radiological analysis did not reveal a significant relationship between restoration of the geometry of the hip and re-dislocation. The mean mHHSs significantly improved from 43 points (0 to 88) to 67 points (20 to 91; p < 0.001) at the final follow-up, with no differences between the types of bearing. Conclusion. We found that the use of DM bearings reduced the rates of re-dislocation and re-revision in revision THA for recurrent dislocation, but did not guarantee stability. Abductor deficiency is an important predictor of persistent instability. Cite this article: Bone Joint J 2024;106-B(5 Supple B):89–97

Introduction. Recent literature has shown that RSAs successfully improve pain and functionality, however variability in range of motion and high complication rates persist. Biomechanical studies suggest that tensioning of the deltoid, resulting from deltoid lengthening, improves range of motion by increasing the moment arm. This study aims to provide clinical significance for deltoid tensioning by comparing postoperative range of motion measurements with deltoid length for 93 patients. Methods. Deltoid length measurements were performed radiographically for 93 patients. Measurements were performed on both preoperative and postoperative x-rays in order to assess deltoid lengthening. The deltoid length was measured as the distance from the infeolateral tip of the acromion to the deltoid tuberosity on the humerus for both pre- and post- x-rays. For preoperative center of rotation measurements, the distance extended from the center of humeral head (estimated as radius of best fit circle) to deltoid length line. For postoperative measurements, the distance was from the center of glenosphere implant to deltoid length line. Forward flexion and external rotation was measured for all patients. Results. The average preoperative deltoid length was 154.25 mm while the average postoperative deltoid measurements was 178.93 mm. The average preoperative center of rotation as 21.33 mm and the average postoperative center of rotation measurement was 46.75 mm. There was low correlation between deltoid length and center of rotation with either forward flexion or external rotation or outcome scores. Discussion. Our results suggest that deltoid lengthening does not significantly influence optimizing clinical outcomes for RSAs. Further research is required to determine design parameters and implants positioning to improve RSAs

Glenoid baseplate orientation in reverse shoulder arthroplasty (RSA) influences clinical outcomes, complications, and failure rates. Novel technologies have been produced to decrease performance heterogeneity of low and high-volume surgeons. This study aimed to determine novice and experienced shoulder surgeon's ability to accurately characterise glenoid component orientation in an intra-operative scenario. Glenoid baseplates were implanted in eight fresh frozen cadavers by novice surgical trainees. Glenoid baseplate version, inclination, augment rotation, and superior-inferior centre of rotation (COR) offset were then measured using in-person visual assessments by novice and experienced shoulder surgeons immediately after implantation. Glenoid orientation parameters were then measured using 3D CT scans with digitally reconstructed radiographs (DRRs) by two independent observers. Bland-Altman plots were produced to determine the accuracy of glenoid orientation using standard intraoperative assessment compared to postoperative 3D CT scan results. Visual assessment of glenoid baseplate orientation showed “poor” to “fair” correlation to 3D CT DRR measurements for both novice and experienced surgeon groups for all measured parameters. There was a clinically relevant, large discrepancy between intra-operative visual assessments and 3D CT DRR measurements for all parameters. Errors in visual assessment of up to 19.2 degrees of inclination and 8mm supero-inferior COR offset occurred. Experienced surgeons had greater measurement error than novices for all measured parameters. Intra-operative measurement errors in glenoid placement may reach unacceptable clinical limits. Kinesthetic input during implantation likely improves orientation understanding and has implications for hands-on learning

Introduction. While THA is associated with positive results and long-term improvement in patient quality of life, outcomes are nonetheless associated with adverse events and post-procedural deficits related to discrepancies in leg length (LLD), offset and cup placement. Post-THA errors in these parameters are associated with gait alteration, low back pain and patient dissatisfaction. Such discrepancies often necessitate revision and increasingly lead to medical malpractice litigation. Maintaining accuracy in post-surgical leg length, offset and cup placement during THA is difficult and subject to error. The sensitivity of these factors is highlighted in studies that have shown that a change of as little as 5 degrees of flexion or abduction can induce alterations in leg length of up to several millimeters. Similarly, positioning of implants can alter global and femoral offset, affecting abductor strength, range of motion and overall physical function. Compounding the biochemical issues associated with inaccurate leg length are the costs associated with these deficits. Traditional freehand techniques of managing intra-operative parameters rely on surgeon experience and tissue tensioning to manually place components accurately. These methods, however, are only able to assess leg length and are subject to inaccuracies associated with patient movement or orientation changes during surgery. Mechanical methods of minimizing post-surgical discrepancies have been developed, such as outrigger or caliper devices, although these methods also address leg length only and provide poor feedback regarding offset and center of rotation, therefore providing insufficient data to accurately achieve appropriate post-surgical leg length. Computer-assisted navigation methods provide more data regarding leg length, offset and center of rotation, but are limited by their cumbersome nature and the large capital costs associated with the systems. The Intellijoint HIP. ®. surgical smart tool (Intellijoint Surgical, Inc., Waterloo, ON) is an intra-operative guidance tool that provides surgeons with real time data on leg length, offset and center of rotation, thereby allowing for confident selection of the correct implant in order to ensure appropriate post-surgical biomechanics. The early clinical results from an initial cohort of patients indicate that Intellijoint HIP. ®. is safe and effective. No adverse events were reported in the initial cohort, and the smart tool was able to measure surgical parameters to within 1mm when compared to radiographic measurements. With training cases removed, 100% of cases had a post-procedure leg length discrepancy of less than 5mm. This paper describes the indications, procedural technique and early clinical results of the Intellijoint HIP. ®. smart tool, which offers a safe, accurate and easy-to-use option for hip surgeons to manage leg length, offset and cup position intra-operatively

Purpose: Impacted piecemeal allografts for nonce-mented hemispheric cups raises a problem of primary stability in the case of extensive bone defects. The high centre of rotation of the oversized cup further increases bone loss, requiring an extralong neck. The purpose of this study was to describe the use of impacted piecemeal grafts associated with a pressfit supporting ring with reposition of the centre of rotation. Material and methods: The piecemeal grafts were impacted into the acetabulum to fill the defect. The hydroxyapatite coated ring was pressfit for primary stability then stabilised with axial screws in the upper paste. A distal hook on the obturator foramen repositioned the centre of rotation. The study group included 103 cases of acetabular reconstruction, including 34 for aseptic loosening and type 2 and 3 acetabular substance loss. Clinical and radiographic assessment was performed at 5 and 12 years. Results: Mean patient age was 58 years, mean weight was 64kg. The Harris score improved from 53 points preoperatively to 88 points at last follow-up. Radiographically, there were no cases of cup migration according to the Massin classification, and the centre of rotation (Pierchon) was anatomic in 66% horizontally and in 44% vertically. There were two lucent lines in zone 2 and mean polyethylene wear was 0.015 mm per year. Graft integration (Conn) was identical to the host in 84% with disappearance of the interface in 67%. There were three dislocations treated without changing the implant and two revisions for infection. Discussion: Several theoretical and clinical studies have shown that the high centre of rotation increases stress on the implants and decreased abductor force. The results obtained in this study with a maximum 12 year follow-up show that indications for this pressfit technique associating reposition of the centre of rotation, fixation for stability, and restoration of bone stock can be widened. Limitations are bone destructions with rupture of the pelvic girdle

The history of knee mechanics studies and the evolution of knee arthroplasty design have been well reported through the last decade (e.g. [1],[2]). Through the early 2000's, there was near consensus on the dominant motions occurring in the healthy knee among much of the biomechanics and orthopaedic communities. However, the past decade has seen the application of improved measurement techniques to permit accurate measurement of natural knee motion during activities like walking and running. The results of these studies suggest healthy knee motion is more complex than previously thought, and therefore, design of suitable arthroplasty devices more difficult. The purpose of this paper is to briefly review the knee biomechanics literature before 2008, to present newer studies for walking and running, and to discuss the implications of these findings for the design of knee replacement implants that seek to replicate physiologic knee motions. Many surgeons point to Brantigan and Voshell [3], an anatomic study of over one hundred specimens focusing on the ligamentous and passive stabilizers of the knee, as being an important influence in their thinking about normal knee function. M.A.R. Freeman and colleagues in London claim particular influence from this work, which motivated their extensive series of MR-based knee studies reported in 2000 [4,5,6]. These papers, perhaps more than any others, are responsible for the common impression that knee kinematics are well and simply described as having a ‘medial pivot’ pattern, where the medial condyle remains stationary on the tibial plateau while the lateral condyle translates posteriorly with knee flexion. Indeed, subsequent studies in healthy and arthritic knees during squatting and kneeling [7,8,9] and healthy and ACL-deficient knees during deep knee bends [10,11] show patterns of motion quite similar to those reported by Freeman and coworkers. These studies make a convincing case for how the healthy knee moves during squatting, kneeling and lunging activities. However, these studies are essentially silent on knee motions during ambulatory activities like walking, running and stair-climbing; activities which most agree are critically important to a high-function lifestyle. In 2008 Koo and Andriacchi reported a motion laboratory study of walking in 46 young healthy individuals and found that the stance phase knee center of rotation was LATERAL in 100% of study participants [12]. One year later, Kozanek et al. published a bi-plane fluoroscopy study of healthy knees walking on a treadmill and corroborated the findings of Koo and Andriacchi, i.e. the center of rotation in healthy knees walking was lateral [13]. Isberg et al. published in 2011 a dynamic radiostereometric study of knee motions in healthy, ACL-deficient and ACL-reconstructed knees during a weight-bearing flexion-to-extension activity, and showed consistent anterior-to-posterior medial condylar translations with knee extension, accompanied by relatively little lateral condylar translation [14]. Hoshino and Tashman reported in 2012 another dynamic radiostereometric analysis of healthy knees during downhill running and concluded “While the location of the knee rotational axis may be dependent on the specific loading condition, during … walking and running … it is positioned primarily on the lateral side of the joint. ”[15] Finally, Claes et al. reported in late 2013 the detailed anatomy of the anterolateral ligament (ALL), another structure serving to stabilize the lateral knee compartment near extension, roughly in parallel with the anterior cruciate ligament (ACL) [16]. Studies since 2008 [9,12–16] show knee motions during walking, running and pivoting activities do not fit the “medial pivot” pattern of motion, but rather point to a “lateral pivot” pattern of knee motion consistent with the stabilizing roles of the ACL and ALL. Having a medial center of rotation in flexion and a lateral center of rotation in extension greatly complicates knee arthroplasty design if the goal is to reproduce kinematics approximating those observed in the natural knee. Consistent kinematics having a fixed center of rotation implies joint stabilizing structures or surfaces, not simply articular laxity allowing the knee to move as forces dictate. Thus, a total knee arthroplasty design seeking to reproduce physiologic motions may need to provide distinct means for controlling tibiofemoral motion in both extension and flexion. Recent studies of natural knee motions have made the implant designer's job more difficult!

Local anatomical abnormalities vary in congenital hip disease patients. Authors often present early to mid-term total hip arthroplasty clinical outcomes using different techniques and implants randomly on patients with different types of the disease, making same conclusions difficult. We report long term outcomes (13 to 23 years) of the treatment of low and high dislocation cases (separately) with total hip arthroplasty using TM technology acetabular cups (Implex initially and then Zimmer) and short fluted conical (Zimmer) femoral stems. From 2000 to 2010, 418 congenital hip disease hip joints were treated in our department with total hip arthroplasty. According to Hartofilakidis et al's classification, 230 hips had dysplasia, 101 low dislocation, (group A) and 87 high dislocation (group B). Pre-operative and post-operative values, at regular intervals, of HHS, SF-12, WOMAC, OHS and HOOS were available for all patients. Patient, surgeon and implant related failures and complications were recorded for all patients. In all cases an attempt was made to restore hip center of rotation. In group A the average lengthening was 2.8 cm (range: 1 to 4.2) and in group B 5.7 cm (range: 4.2 to 11). In both groups, no hips were revised due to aseptic loosening of either the acetabular cup or the femoral stem. In group A, a cumulative success rate of 95.6% (95% confidence interval, 92.7% – 97.4%) and in group B a cumulative success rate of 94.8% (95% confidence interval, 92.6%–96.9%) was recorded, at 20 years, with revision for any reason as an end point. No s.s. differences were found between groups when mean values of HHS, SF-12, WOMAC and OKS were compared. Satisfactory long-term clinical outcomes can be achieved in treating different types of congenital hip disease when appropriate surgical techniques combined with “game changing” implants are used

Lateralization of the reverse arthroplasty may be desirable to more effectively tension the remaining rotator cuff, decrease scapular notching, improve the cosmetic appearance of the shoulder, and improve stability as well as the arc of motion prior to impingement. There are two primary options to lateralise a reverse shoulder arthroplasty: bone graft with a long post (BIO-RSA) vs. using metal. The two metal options generally include a thicker glenosphere or a thicker glenoid baseplate. Potential benefits of a BIO-RSA include lateralization of the glenoid center of rotation but without placing the center of rotation lateral to the prosthetic-bone interface. By maintaining the position of the center of rotation, the shear forces at the prosthesis-bone interface are lessened and are converted to compressive forces which will minimise glenoid failure. Edwards et al. performed a prospective study on a bony increased offset reverse arthroplasty. Among the 18 shoulders in the BIO-RSA group, the incidence of notching was 78% compared to controls 70%. The graft completely incorporated in 12 (67%), partially incorporated in 4 (22%), and failed to incorporate in 2 (11%). Frankle et al. reported on the minimum 5-year follow-up of reverse arthroplasty with a central compression screw and a lateralised glenoid component. The survivorship was 94% at 5 years. There were seven (9%) cases of scapular notching and no patient had glenoid baseplate loosening or baseplate failure. The authors noted that the patients maintained their improved function and radiographic results at a minimum of five years. In summary, lateralisation of the glenosphere is an attractive option to improve the outcome of reverse arthroplasty. Benefits of lateralisation with metal rather than bone graft include elimination of concern over bone graft healing or resorption. In addition, the procedure has the potential to be more precise with the exact offset amount known pre-operatively as well as improved efficiency of the procedure. Preparing the graft takes additional OR time and there is variable quality of the bone graft

Lower back pain (LBP) is a worldwide clinical problem and a prominent area for research. Numerous in vitro biomechanical studies on spine specimens have been undertaken, attempting to understand spinal response to loading and possible factors contributing to LBP. However, despite employing similar testing protocols, there are challenges in replicating in vivo conditions and significant variations in published results. The aim of this study was to use the University of Bath (UoB) spine simulator to perform tests to highlight the major limitations associated with six degree of freedom (DOF) dynamic spine testing. A steel helical spring was used as a validation model and was potted in Wood's metal. Six porcine lumbar spinal motion segments were harvested and dissected to produce isolated spinal disc specimens. These were potted in Wood's metal, ensuring the midplane of the disc remained horizontal and then sprayed with 0.9% saline and wrapped in saline-soaked tissue and plastic wrap to prevent dehydration. A 400N axial preload was used for spinal specimens. Specimens were tested under the stiffness and flexibility protocols. Tests were performed using the UoB custom 6-axis spine simulator with coordinate axes. Tests comprised five cycles with data acquired at 100Hz. Stiffness and flexibility matrices were evaluated from the last three motion cycles using the linear least squares method. According to theory, inverted flexibility matrices should equal stiffness matrices. In the case of the spring, the matrices matched analytical solutions and inverted flexibility matrices were equivalent to stiffness matrices. Matrices from the spinal tests demonstrated some symmetry, with similarities between inverted flexibility- and stiffness matrices, though these were unequal overall. Matrix element values were significantly affected by displacements assumed to occur at disc centre. Spring tests proved that for linear, elastic specimens, the spine simulator functioned as expected. However, multiple factors limit the confidence in spine test results. Centre of rotation, displacement assumptions and rigid body transformations are known to impact the results from spinal testing, and these should be addressed going forward to improve the replication of in vivo conditions

Accurate cup placement in total hip arthroplasty (THA) for the patients with developmental dysplasia of the hip (DDH) is one of the challenges due to distinctive bone deformity. Robotic-arm assisted system have been developed to improve the accuracy of implant placement. This study aimed to compare the accuracy of robotic-arm assisted (Robo-THA), CT-based navigated (Navi-THA), and manual (M-THA) cup position and orientation in THA for DDH. A total of 285 patients (335 hips) including 202 M-THAs, 45 Navi-THAs, and 88 Robo-THA were analyzed. The choice of procedure followed the patient's preferences. Horizontal and vertical center of rotation (HCOR and VCOR) were measured for cup position, and radiographic inclination (RI) and anteversion (RA) were measured for cup orientation. The propensity score-matching was performed among three groups to compare the absolute error from the preoperative target position and angle. Navi-THA showed significantly smaller absolute errors than M-THA in RI (3.6° and 5.4°) and RA (3.8° and 6.0°), however, there were no significant differences between them in HCOR (2.5 mm and 3.0 mm) or VCOR (2.2 mm and 2.6 mm). In contrast, Robo-THA showed significantly smaller absolute errors of cup position than both M-THA and Navi-THA (HCOR: 1.7 mm and 2.9 mm, vs. M-THA, 1.6 mm and 2.5 mm vs. Navi-THA, VCOR:1.7 mm and 2.4 mm, vs. M-THA, 1.4 mm and 2.2 mm vs. Navi-THA). Robo-THA also showed significantly smaller absolute errors of cup orientation than both M-THA and Navi-THA (RI: 1.4° and 5.7°, vs. M-THA, 1.5° and 3.6°, vs. Navi-THA, RA: 1.9° and 5.8° vs. M-THA, 2.1° and 3.8° vs. Navi-THA). Robotic-arm assisted system showed more accurate cup position and orientation compared to manual and CT-based navigation in THA for DDH. CT-based navigation increased the accuracy of cup orientation compared to manual procedures, but not cup position

Aims. Only a small number of studies exist that report the results of EBM-produced porous coated trabecular titanium cups in primary total hip arthroplasty (THA). This study aims to investigate the patient satisfaction level, clinical function and radiographic outcomes of the patients who underwent THA using an EBM-produced porous coated titanium cup. Patients and Methods. A total of 32 patients who underwent primary THA with using an EBM-produced porous coated titanium cup from five hospitals between May and December, 2012 were retrospectively reviewed. Five patients were lost prior to the minimum 6-year follow-up. Clinical and radiographic outcomes were analyzed with an average follow-up of 81.48 (range: 77.00–87.00) months. Results. The median HHS and SF-36 scores improved significantly while the WOMAC Osteoarthritis Index decreased significantly at the latest follow-up (p<0.001). Eighteen (66.7%) patients rated their satisfaction level as very satisfied, 6 (22.2%) as satisfied, 2 (7.4%) as neutral and 1(3.7%) as dissatisfied. No intraoperative or postoperative complications were identified, including aseptic cup loosening, hip dislocation, periprosthetic joint infection, periprosthetic fracture, nerve palsy, hematoma. At the latest follow-up, all cups were considered to have achieved fixation via bone ingrowth with three or more of the five signs occurring in the most recent X-ray. However, three cups revealed radiolucent lines with a width of less than 1 mm. These radiolucent lines were distributed in the DeLee-Charnley zone 1 in 1 patient, zone 3 in another patient and the area between zone1 and zone 2 in an additional patient. The latest postoperative centers of rotation were restored nearly to the anatomic center of rotation both vertically and horizontally and the acetabular cups obtained satisfactory orientation postoperatively. Conclusions. The mid-term follow-up of patients who underwent primary THA using EBM-produced porous coated titanium cups demonstrated favorable patient satisfaction, clinical function and adequate biological fixation. For any figures or tables, please contact authors directly

Purpose of the study: The extramedullary anatomy of the femur must be reproduced during total hip arthroplasty in order to ensure correct tension on the gluteus muscles. This requires:. correct offset of the femur, measured as the distance between the center of the head and the anatomic axis of the shaft;. offset of the center of rotation, measured as the distance between the center of the head and the pubic symphesis. Addition of these two offsets gives the overall offset. The purpose of this work was to analyze postoperative offset after standard total hip arthroplasty as a function of the preoperative head-shaft angle. Material and methods: Prospective study of 150 files of patients who underwent first-intention total hip arthroplasty. A prosthesis with matched increasing head size was implanted. The head-shaft angle was 135°. Mean offset was 41.7 mm (range 33–47 mm) for the 0 head-neck. The preoperative neck-shaft angle was measured on the upright ap view (comparable rotation of the two hemipelvi). Pre- and postoperative femur and center of rotation offset were noted. Results: The preoperative neck-shaft angle varied from 118° to 1400. Mean preoperative femur offset was 40.2 mm (range 29–52 mm). Mean postoperative femur offset was 42.2 mm. This gave a 2 mm lateralization of the femur, which was apparently negligible, favorable, and therefore satisfactory. Mean offset was 90.5 mm preoperatively and 84.5 mm postoperatively, medializing the center of rotation 6°. Mean overall offset was thus displaced medially (6 mm minus 2 mm = 4 mm). This was considered acceptable. Among these 150 files, 24 were coxa vara hips with a neck-shaft angle 125°. For these 25 coxavara hips, the mean preoperative femur offset was 44.5 mm. The mean postoperative femur offset was 42.2 mm. This produced, inversely, a medial displacement of the postoperative femur offset of 2.3 mm. The center of rotation was displace medially 6 mm. Thus globally the medial displacement was 6 mm plus 2.3 mm = 8.3 mm. This appeared to be excessive. Discussion: The postoperative offset of the femur is prosthesis-dependent. The majority of implants currently marketed have a mean offset in the 40–45mm range. The offset of the center of rotation is operator-dependent: as the acetabular reaming is accentuated, the center of rotation is displaced medially. Acetabular reaming is necessary to reach the subchondral bone. The medial offset can be limited but at least some displacement is inevitable. Thus in the event of a coxavara hip, it is very difficult to limit excessive overall medial offset when using a standard prosthesis. If the goal is to mimic the anatomic femur offset, it would appear justified to use prostheses with a smaller neck-shaft angle for patients with coxavara. A 10° reduction, from 135° to 125° would increase the femur offset 5 mm and thus enable reproduction of the preoperative anatomy

Dual-mobility (DM) bearings are effective to mitigate dislocation in revision total hip arthroplasty (THA). However, data on its use for treating dislocation is scarce. Aim of this study was to compare DM bearings, standard bearings and constrained liner (CL) in revision THA for recurrent dislocation and to identify risk factors for re-dislocation. We reviewed 100 consecutive revision THAs performed for dislocation from 2012 and 2019. 45 hips (45%) received a DM construct, while 44 hips (44%) and 11 hips (11%) had a standard bearing and CL, respectively. Rates of re-dislocation, re-revision for dislocation and overall re-revision were compared. Radiographs were assessed for cup positioning, restoration of centre of rotation, leg length and offset. Risk factors for re-dislocation were determined by cox regression analysis. Modified Harris hip scores (mHHS) were calculated. Mean follow-up was 53 months (1 to 103). DM constructs were used more frequently in elderly patients (p=0.011) and hips with abductor deficiency (p< 0.001). The re-dislocation rate was 11.1% for DM bearings compared with 15.9% for standard bearings and 18.2% for CL (p=0.732). Revision-free survival for DM constructs was 83% (95% CI 0.77 – 0.90) compared to 75% (95% CI 0.68 – 0.82) for standard articulations and 71% (95% CI 0.56 – 0.85) for CL (p=0.455). Younger age (HR 0.91; p=0.020), lower comorbidity (HR 0.42; p=0.031), smaller heads (HR 0.80; p=0.041) and cup retention (HR 8.23; p=0.022) were associated with re-dislocation. Radiological analysis did not reveal a relationship between restoration of hip geometry and re-dislocation. mHHS significantly improved from 43.8 points to 65.7 points (p<0.001) with no differences among bearing types. Our findings suggest that DM bearings do not sufficiently prevent dislocation in revision THA for recurrent dislocation. Reconstruction of the abductor complex may play a key role to reduce the burden in these high-risk patients

Aims. There is an increased risk of dislocation of the hip after the resection of a periacetabular tumour and endoprosthetic reconstruction of the defect in the hemipelvis. The aim of this study was to determine the rate and timing of dislocation and to identify its risk factors. Methods. To determine the dislocation rate, we conducted a retrospective single-institution study of 441 patients with a periacetabular tumour who had undergone a standard modular hemipelvic endoprosthetic reconstruction between 2003 and 2019. After excluding ineligible patients, 420 patients were enrolled. Patient-specific, resection-specific, and reconstruction-specific variables were studied using univariate and multivariate analyses. Results. The dislocation rate was 9.3% (n = 41). Dislocation was most likely to occur in the first three months after surgery. Four independent risk factors were found, one of which was older age at operation (p = 0.039). The odds ratios (ORs) of those aged ≥ 60 years and 30 to 60 years were 8.50 and 4.64, respectively, compared with those aged < 30 years. The other three risk factors were resection of gluteus maximus (p = 0.010, OR = 5.8), vertical shift of the centre of rotation (COR) of the hip by ≥ 20 mm (p = 0.008, OR = 3.60), and a type I+II+III pelvic resection (p = 0.014, OR = 3.04). Conclusion. Hemipelvic endoprosthetic reconstruction after resection of a periacetabular tumour has a dislocation rate of 9.3% (n = 41). Patients are most likely to dislocate in the first three months after surgery. The risk is increased for older patients (especially those aged > 60 years) and for those with gluteus maximus resection, vertical shift of the COR ≥ 20 mm, and a type I+II+III pelvic resection. Cite this article: Bone Joint J 2021;103-B(2):382–390

Robotic assisted surgery aims to reduce surgical errors in implant positioning and better restore native hip biomechanics compared to conventional techniques for total hip arthroplasty (THA). The primary objective of this study was to compare accuracy in restoring the native centre of hip rotation in patients undergoing conventional manual THA versus robotic-arm assisted THA. Secondary objectives were to determine differences between these treatment techniques for THA in achieving the planned combined offset, cup inclination, cup version, and leg-length correction. This prospective cohort study included 50 patients undergoing conventional manual THA and 25 patients receiving robotic-arm assisted THA. All operative procedures were undertaken by a single surgeon using the minimally-invasive posterior approach. Two independent blinded observers recoded all radiological outcomes of interest using plain radiographs. Patients in both treatment groups were well-matched for age, gender, body mass index, laterality of surgery, and ASA scores. Interclass correlation coefficient was 0.92 (95% CI: 0.84 – 0.95) for intra-observer agreement and 0.88 (95% CI: 0.82–0.94) for inter-observer agreement in all study outcomes. Robotic THA was associated with improved accuracy in restoring the native horizontal (p<0.001) and vertical (p<0.001) centres of rotation, and improved preservation of the patient's native combined offset (P<0.001) compared to conventional THA. Robotic THA improved accuracy in positioning of the acetabular cup within the combined safe zones of inclination and anteversion described by Lewinnek et al (p=0.02) and Callanan et al (p=0.01) compared to conventional THA (figures 1–2). There was no difference between the two treatment groups in achieving the planned leg-length correction (p=0.10). Robotic-arm assisted THA was associated with improved accuracy in restoring the native centre of rotation, better preservation of the combined offset, and more precise acetabular cup positioning within the safe zones of inclination and anteversion compared to conventional manual THA. Robotic-arm assisted THA enables improved preservation of native hip biomechanics compared to conventional manual THA. For any figures or tables, please contact authors directly: . fsh@fareshaddad.net

INTRODUCTION. Useful feedback from a Total Knee Replacement (TKR) can be obtained from post-surgery in-vivo assessments. Dynamic Fluoroscopy and 3D model registration using the method of Banks and Hodge (1996) [1] can be used to measure TKR kinematics to within 1° of rotation and 0.5mm of translation, determine tibio-femoral contact locations and centre of rotation. This procedure also provides an accurate way of quantifying natural knee kinematics and involves registering 3D implant or bone models to a series of 2D fluoroscopic images of a dynamic movement. AIM. The aim of this study was to implement a methodology employing the registration methods of Banks and Hodge (1996) [1] to assess the function of different TKR design types and gain a greater understanding of non-pathological (NP) knee biomechanics. METHODS. Knee function was assessed for five subjects with NP knees (4 males and 1 female, 34.8 ± 10.28 years, BMI 25.59 ± 3.35 Kg/m. 2. ) and five subjects 13.2 (± 1.8) months following a TKR (2 males, 3 females, 68 ± 9.86 years, BMI 30 ± 3 Kg/m. 2. ). The TKR types studied included 1 cruciate retaining, 2 cruciate substituting, 1 mobile-bearing (high flex) and 1 medial pivot). Ethical approval was obtained from the South East Wales Local Research Ethics Committee. Each subject's knee was recorded whilst they performed a step up/down task, using dynamic fluoroscopy (Philips). 3D CAD models of each TKR were obtained for the TKR subjects. 3D bone models of the knee, tibia and femur were created for the 5 NP subjects by segmenting MRI scans (3T GE scanner, General Electric Company) using ScanIP (Simpleware, Ltd.). Using the program KneeTrack (S A Banks, USA), each TKR component and bone model was projected onto a series of fluoroscopic images and their 3D pose iteratively adjusted to match the contours on each image. Joint Kinematics were determined from the 3D pose of each 3D model using Cardan/Euler angles [2]. The contact points and centre of rotation of each TKR were also computed. RESULTS. The mean range of motion (ROM) in the sagittal plane was 61° for the NP cohort and 64° for the TKR cohort. The mean frontal plane ROM was 4° for NP knees and 3° for TKR. A greater axial ROM was achieved by the mobile-bearing (7.5°) and medial pivot TKR (7.0°), compared to the cruciate retaining (4.4°) and substituting (3.6°). The Medial Pivot TKR rotated around a medial centre of rotation, whereas the centre of rotation was located laterally for the other TKR types. This has also been found in other studies of stair climbing activities [3]. CONCLUSIONS. This study demonstrates how this method can be used to quantify and compare the kinematics, contact locations and centre of rotation for a range of TKR designs and NP knees in-vivo. Initial analyses have identified functional differences associated with different TKR designs

Two critical steps in achieving optimal results and minimizing complications (dislocation, lengthening, and intraoperative fracture) are careful preoperative planning and more recently, the option of intraoperative imaging in order to optimise accurate and reproducible total hip replacement. The important issues to ascertain are relative limb length, offset and center of rotation. It is important to start the case knowing the patient's perception of their limb length. Patient perception is equally important, if not more important, than the radiographic assessment. On the acetabular side, the teardrop should be identified and the amount of reaming necessary to place the inferior margin of the acetabular component adjacent to the tear drop should be noted. Superiorly the amount of exposed metal that is expected to be seen during surgery should be measured in millimeters. Once the key issues of limb length, offset, center of rotation, and acetabular component position relative to the native acetabulum have been confirmed along with the expected sizing of the acetabular and femoral components, it is critical that the operative plan is reproduced at the time of surgery and this can best be consistently performed with the use of intraoperative imaging. Advances in digital imaging now make efficient, cost-effective assessment of hip replacement possible. Embedded software allows accurate confirmation of the preoperative plan intraoperatively when correction of potential errors is easily possible. Such technology is now mature after years of clinical use and studies have confirmed its success in avoiding outliers and achieving optimal results. A pilot study at Washington University demonstrated that intraoperative imaging was able to eliminate outliers for acetabular inclination and anteversion. In addition, the ability to achieve accurate reproduction of femoral offset and limb length within 5mm was three times better with intraoperative imaging (P < 0.001)

Background. Aseptic loosening is the leading cause of total knee arthroplasty (TKA) failure in the long term, of which osteolysis from polyethylene wear debris remains a problem that can limit the lifetime of TKA past the second decade. To help speed up design innovations, our goal was to develop a computational framework that could efficiently predict the effect of many sources of variability on TKA wear—including design, surgical, and patient variability. Methods. We developed a computational framework for predicting TKA contact mechanics and wear. The framework accepts multiple forms of input data: patient-specific, population-specific, or standardized motions and forces. CAD models are used to create the FEA mesh. An analytical wear model, calibrated from materials testing (wheel-on-flat) experiments, is fully integrated into the FEA process. Isight execution engine runs a design of experiments (DOE) analysis with an outcome variable, such as volumetric wear, to guide statistical model output. We report two DOE applications to test the utility of the computational framework for performing large variable studies in an efficient manner: one to test the sensitivity of TKA wear to the femoral center of rotation, and the second to test the sensitivity of TKA wear to gait input perturbations. Results. Using this method, we demonstrated that choice of femoral center of rotation matters, and that although volumetric wear was most sensitive to variation in flexion/extension peaks, no one kinematic factor dominates TKA volumetric wear variability. Conclusion. The two DOE applications represent initial first attempts to study variability in component alignment and input waveforms across large solution spaces. The computational framework will be most useful if it can be used in a TKA design setting, where new innovations can be tested as soon as they are developed to see if they are worthy of further mechanical testing