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.

Abstract. Objectives. Accurate orientation of the acetabular component during a total hip replacement is critical for optimising patient function, increasing the longevity of components, and reducing the risk of complications. This study aimed to determine the validity of a novel VR platform (AescularVR) in assessing acetabular component orientation in a simulated model used in surgical training. Methods. The AescularVR platform was developed using the HTC Vive® VR system hardware, including wireless trackers attached to the surgical instruments and pelvic sawbone. Following calibration, data on the relative position of both trackers are used to determine the acetabular cup orientation (version and inclination). The acetabular cup was manually implanted across a range of orientations representative of those expected intra-operatively. Simultaneous readings from the Vicon® optical motion capture system were used as the ‘gold standard’ for comparison. Correlation and agreement between these two methods was determined using Bland-Altman plots, Pearson's correlation co-efficient, and linear regression modelling. Results. A total of 55 separate orientation readings were obtained. The mean average difference in acetabular cup version and inclination between the Vicon and VR systems was 3.4° (95% CI: −3–9.9°), and −0.005° (95% CI: −4.5–4.5°) respectively. Strong positive correlations were demonstrated between the Vicon and VR systems in both acetabular cup version (Pearson's R = 0.92, 99% CI: 0.84–0.96, p<0.001), and inclination (Pearson's R = 0.94, 99% CI: 0.88–0.97, p<0.001). Using linear regression modelling, the adjusted R. 2. for acetabular version was 0.84, and 0.88 for acetabular inclination. Conclusion. The results of this study indicate that the AescularVR platform is highly accurate and reliable in determining acetabular component orientation in a simulated environment. The AescularVR platform is an adaptable tracking system, which may be modified for use in a range of simulated surgical training and educational purposes, particularly in orthopaedic surgery. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Introduction. Excessive standing posterior pelvic tilt (PT), lumbar spine stiffness, low pelvic Incidence (PI), and severe sagittal spinal deformity (SSD) have been linked to increased dislocation rates. We aimed to compare the prevalence of these 4 parameters in unstable and stable primary Total Hip Arthroplasty (THA) patients. Methods. In this retrospective cohort study, 40 patients with instability following primary THA for osteoarthritis were referred for functional analysis. All patients received lateral X-rays in standing and flexed seated positions to assess functional pelvic tilt and lumbar lordosis (LL). Computed tomography scans were used to measure pelvic incidence and acetabular cup orientation. Literature thresholds for “at risk” spinopelvic parameters were standing pelvic tilt ≤ −10°, lumbar flexion (LL. stand. – LL. seated. ) ≤ 20°, PI ≤ 41°, and sagittal spinal deformity (PI – LL. stand. mismatch) ≥ 10°. The prevalence of each risk factor in the dislocation cohort was calculated and compared to a previously published cohort of 4042 stable THA patients. Results. Median supine cup inclination for the dislocating cohort was 43° (range, 26°- 58°). Median cup anteversion was 23° (range, 7° − 40°) for the dislocating cohort. 65% of the dislocating patients had socket positions within the Lewinnek safe zone (Figure 1). Standing PT (-10° v −1°), lumbar flexion (20° v 45°), and PI-LL mismatch (12° v −1°) were all significantly different (p < 0.001) in the dislocating group compared to the stable THA population (Figure 2). There was no difference in PI between the dislocating group and the stable THA population (58° v 56° respectively, p = 0.33), with the numbers available. 80% of the dislocating patients had one or more of the 3 statistically significant risk factors, compared to only 24% of the stable THA population. Conclusion. Excessive standing posterior pelvic tilt, low lumbar flexion and a severe SSD are highly prevalent in unstable THAs. Pre-op screening for these parameters may reduce the prevalence of dislocation. For any figures or tables, please contact the authors directly

Iliopsoas tendonitis after total hip arthroplasty (THA) can be a considerable cause of pain and patient dissatisfaction. The optimal cup position to avoid iliopsoas tendonitis has not been clearly established. Implant designs have also been developed with an anterior recess to avoid iliopsoas impingement. The purpose of this cadaveric study was to determine the effect of cup position and implant design on iliopsoas impingement.

Bilateral THA was performed on three fresh frozen cadavers using oversized (jumbo) offset head center revision acetabular cups with an anterior recess (60, 62 and 66 mm diameter) and tapered wedge primary stems through a posterior approach. A 2mm diameter flexible stainless steel cable was inserted into the psoas tendon sheath between the muscle and the surrounding membrane to identify the location of the psoas muscle radiographically. CT scans of each cadaver were imported in an imaging software. The acetabular shells, cables as well as pelvis were segmented to create separate solid models of each. The offset head center shell was virtually replaced with an equivalent diameter hemispherical shell by overlaying the outer shell surfaces of both designs and keeping the faces of shells parallel. The shortest distance between each shell and cable was measured. To determine the influence of cup inclination and anteversion on psoas impingement, we virtually varied the inclination (30°/40°/50°) and anteversion (10°/20°/30°) angles for both shell designs.

The CT analysis revealed that the original orientation (inclination/anteversion) of the shells implanted in 3 cadavers were as follows: Left1: 44.7°/23.3°, Right1: 41.7°/33.8°, Left2: 40/17, Right2: 31.7/23.5, Left3: 33/2908, Right3: 46.7/6.3. For the offset center shells, the shell to cable distance in all the above cases were positive indicating that there was clearance between the shells and psoas. For the hemispherical shells, in 3 out of 6 cases, the distance was negative indicating impingement of psoas. With the virtual implantation of both shell designs at orientations 40°/10°, 40°/20°, 40°/30° we found that greater anteversion helped decrease psoas impingement in both shell designs. When we analyzed the influence of inclination angle on psoas impingement by comparing wire distances for three orientations (30°/20°, 40°/20°, 50°/20°), we found that the effect was less pronounced. Further analysis comparing the offset head center shell to the conventional hemispherical shell revealed that the offset design was favored (greater clearance between the shell and the wire) in 17 out of 18 cases when the effect of anteversion was considered and in 15 out of 18 cases when the effect of inclinations was considered.

Our results indicate that psoas impingement is related to both cup position and implant geometry. For an oversized jumbo cup, psoas impingement is reduced by greater anteversion while cup inclination has little effect. An offset head center cup with an anterior recess was effective in reducing psoas impingement in comparison to a conventional hemispherical geometry. In conclusion, adequate anteversion is important to avoid psoas impingement with jumbo acetabular shells and an implant with an anterior recess may further mitigate the risk of psoas impingement.

Introduction & aims. Apparently well-orientated total hip replacements (THR) can still fail due to functional component malalignment. Previously defined “safe zones” are not appropriate for all patients as they do not consider an individual's spinopelvic mobility. The Optimized Positioning System, OPS. TM. (Corin, UK), comprises preoperative planning based on a patient-specific dynamic analysis, and patient-specific instrumentation for delivery of the target component alignment. The aim of this study was to determine the early revision rate from the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) for THRs implanted using OPS. TM. . Method. Between January 4. th. 2016 and December 20. st. 2017, a consecutive series of 841 OPS. TM. cementless total hip replacements were implanted using a Trinity acetabular cup (Corin, UK) with either a TriFit TS stem (98%) or a non-collared MetaFix stem (2%). 502 (59%) procedures were performed through a posterior approach, and 355 (41%) using the direct superior approach. Mean age was 64 (range; 27 to 92) and 51% were female. At a mean follow-up of 15 months (range; 3 to 27), the complete list of 857 patients was sent to the AOANJRR for analysis. Results. There were 5 revisions: . a periprosthetic femoral fracture at 1-month post-op in a 70F. a ceramic head fracture at 12-months post-op in a 59M. a femoral stem loosening at 7-months post-op in a 58M. a femoral stem loosening at 16-months post-op in a 64M. an anterior dislocation in a 53M, that was revised 9 days after the primary procedure. CT analysis, prior to revision surgery, revealed acetabular cup orientation of 46°/31° (inclination/anteversion) and femoral stem anteversion of 38°. Conclusions. These preliminary findings suggest the OPS. TM. dynamic planning and delivery system provides good early results, with a low rate of revision for dislocation. Limitations of the study will be discussed

Introduction. Most of studies on Total Hip Arthroplasty (THA) are focused on acetabular cup orientation. Even though the literature suggests that femoral anteversion and combined anteversion have a clinical impact on THA stability, there are not many reports on these parameters. Combined anteversion can be considered morphologically as the addition of anatomical acetabular and femoral anteversions (Anatomical Combined Anatomical Anteversion ACA). It is also possible to evaluate the Combined Functional Anteversion (CFA) generated by the relative functional position of femoral and acetabular implants while standing. This preliminary study is focused on the comparison of the anatomical and functional data in asymptomatic THA patients. Material and methods. 50 asymptomatic unilateral THA patients (21 short stems and 29 standard stems) have been enrolled. All patients underwent an EOS low dose evaluation in standing position. SterEOS software was used for the 3D measurements of cup and femur orientation. Cup anatomical anteversion (CAA) was computed as the cup anteversion in axial plane perpendicular to the Anterior Pelvic Plane. Femoral anatomical anteversion (FAA) was computed as the angle between the femoral neck axis and the posterior femoral condyles in a plane perpendicular to femoral mechanical axis. Functional anteversions for the cup (CFA) and femur (FFA) were measured in the horizontal axial patient plane in standing position. Both anatomical and functional cumulative anteversions were calculated as a sum. All 3D measures were evaluated and compared for the repeatability and reproducibility. Statistical analysis used Mann-Whitney U-test considering the non-normal distribution of data and the short number of patients (<30 for each group). Results. Functional cumulative anteversion was significantly higher than anatomical cumulative anteversion for all groups (p<0.05). No significant difference could be noted between the cases according to the use of short or standard stems. Conclusion. This study shows the difference of functional implant orientation as compared to the anatomical measurements. This preliminary study has limitations. First the limited sample of patients. Then this series only includes asymptomatic subjects. Nevertheless, this work focused on the feasibility of the measurements shows the potential interest of a functional analysis of cumulated anteversion. Standing position influences the relative position of THA implants according to the frontal and sagittal orientation of the pelvis. The relevance of these functional measurements in instability cases must be demonstrated, especially in patients with anterior subluxation in standing position which is potentially associated with pelvic adaptative extension. Further studies are needed for the feasibility of measurements on EOS images in sitting position and their analysis in case of instability. For any figures or tables, please contact authors directly

Impingement of total hip replacements (THRs) can cause rim damage of polyethylene liners, and lead to dislocation and/or mechanical failure of liner locking mechanisms[1]. Previous work has focussed on the influence of femoral neck profile on impingement without consideration of neck-shaft angle. This study assessed the occurrence of impingement with two different stem designs (Corail standard [135°] and coxa vara [125°]) under different activities with varying acetabular cup orientation (30° to 70° inclination; 0° to 50° anteversion) using a geometric modelling tool. The tool was created in a computer aided design software programme, and incorporated an individual's hemi-pelvis and femur geometry[3] with a THR (DePuy Synthes Pinnacle. ®. shell and neutral liner; size 12 Corail. ®. standard or coxa vara and 32mm head). Kinematic data of activities associated with dislocation[2], such as stooping to pick an object from the floor was applied and incidences of impingement were recorded. Predicted implant impingement was influenced by stem design. The coxa vara stem was predicted to cause implant impingement less frequently across the range of activities and cup orientations investigated, compared to the standard stem [Fig. 1]. The cup orientations predicted to cause impingement the least frequently were at lower inclination and anteversion angles, relative to the standard stem [Fig. 1]. The coxa vara stem included a collar, while the standard stem was collarless; additional analysis indicated that differences were due to neck angle and not the presence of a collar. This study demonstrated that stem neck-shaft angle is an important variable in prosthetic impingement in THR and surgeons should be aware of this when choosing implants. Future work will consider further implant design and bone geometry variables. This tool has the potential for use in optimising stem design and position and could assist with patient specific stem selection based on an individual's activity profile. For any figures or tables, please contact the authors directly

Background. Virtual Reality (VR) uses headsets and motion-tracked controllers so surgeons can perform simulated total hip arthroplasty (THA) in a fully-immersive, interactive 3D operating theatre. The aim of this study was to investigate the effect of laboratory-based VR training on the ability of surgical trainees to perform direct anterior approach THA on cadavers. Methods. Eighteen surgical trainees (CT1-ST4) with no prior experience of direct anterior approach (DAA) THA completed an intensive 1-day course (lectures, dry-bone workshops and technique demonstrations). They were randomised to either a 5-week protocol of VR simulator training or conventional preparation (operation manuals and observation of real surgery). Trainees performed DAA-THA on cadaveric hips, assisted by a passive scrub nurse and surgical assistant. Performance was measured on the Intercollegiate Surgical Curriculum Project (ISCP) procedure-based assessment (PBA), on a 9-point global summary score (Table 1). This was independently assessed by 2 hip surgeons blinded to group allocation. The secondary outcome measure was error in cup orientation from a predefined target (40° inclination and 20° anteversion). Results. Surgeons trained using VR performed a cadaveric DAA-THA significantly better than those using conventional preparation, as assessed by acetabular cup orientation (p<0.001) and using the ISCP-PBA. Two VR surgeons achieved Level 3b, 6 were graded at Level 3a, and 1 was graded at Level 2b. Six non-VR surgeons achieved Level 2a and 3 were graded at Level 1b. Discussion. These data demonstrate transfer of procedural knowledge and psychomotor skills learnt from VR to a real-world setting. Conventional preparation had limited value for novice surgeons learning arthroplasty. VR training advanced them further up the learning curve. Implications. Virtual reality can augment surgical training for open procedures in orthopaedics curve, so opportunities in real surgery can be maximised. This has implications for how surgical training is delivered for surgeons learning a new, complex procedure. For any figures or tables, please contact the authors directly

Introduction

Iliopsoas tendonitis after total hip arthroplasty (THA) can be a considerable cause of pain and patient dissatisfaction. The optimal cup position to avoid iliopsoas tendonitis has not been clearly established. Implant designs have also been developed with an anterior recess to avoid iliopsoas impingement. The purpose of this cadaveric study was to determine the effect of cup position and implant design on iliopsoas impingement.

Background. Complications such as dislocations, impingement and early wear following total hip arthroplasty (THA) increase with acetabular cup implant malorientation. These errors are more common with low-volume centres or in novice hands. Currently, this skill is most commonly taught during real surgery with an expert trainer, but simulated training may offer a safer and more accessible solution. This study investigated if a novel MicronTracker® enhanced Microsoft HoloLens® augmented reality (EAR) headset was as effective as one-on-one expert surgeon (ES) training for teaching novice surgeons hip cup orientation skill. Methods. Twenty-four medical students were randomly assigned to EAR or ES training groups. Participants used a modified sawbone/foam pelvis model for hip cup orientation simulation. A validated EAR headset measured the orientation of acetabular cup implants and displayed this in the participant”s field of view. The system calculated the difference between planned and achieved orientation as a solid-angle error. Six different inclination and anteversion combinations, related to hypothetical patient-specific anatomy, were used as target orientations. Learning curves were measured over four sessions, each one week apart. Error in orientations of non-taught angles and during a concealed pelvic tilt were measured to assess translation of skills. A post-test questionnaire was used for qualitative analysis of procedure understanding and participant experience. Results. Novice surgeons of similar experience in both groups performed with a similar error prior to training (ES: 15.7°±6.9°, EAR: 14.2°±7.1°, p>0.05). During training, EAR participants were guided to significantly better orientation errors than ES (ES: 6.0°±3.4°, EAR: 1.1°±0.9°, p<0.001). After four training sessions, the orientation error in both groups significantly reduced (ES: 15.7°±6.9° to 8.2°±4.6°, p<0.001; EAR: 14.2°±7.0° to 9.6°±5.7°, p<0.001). Participants in both groups achieved the same levels of orientation accuracy in non-taught angles and when the pelvis was tilted (p>0.05). In post-training evaluation, participants expressed a preference towards ES rather than EAR for learning orientation skills and related visuospatial and procedure-specific skills. 79% of participants indicated EAR simulator training and ES in combination would be their preferred training method. Discussion. A novel head-mounted EAR platform delivered training to novice surgeons more accurately than an expert surgeon. Both EAR and ES enabled novices to acquire and retain skills on a learning curve to orientate the implant. These skills were translated to non-taught orientations and in the presence of a pelvic tilt. Conclusions. Augmented-reality simulators may be a feasible and valid method for teaching novice surgeon”s visuospatial skills for THA on a learning curve, to compliment traditional intraoperative training

Background. Trust in the validity of a measurement tool is critical to its function in both clinical and educational settings. Acetabular cup malposition within total hip arthroplasty (THA) can lead to increased dislocation rates, impingement and increased wear as a result of edge loading. We have developed a THA simulator incorporating a foam/Sawbone pelvis model with a modified Microsoft HoloLens® augmented reality (AR) headset. We aimed to measure the trueness, precision, reliability and reproducibility of this platform for translating spatial measurements of acetabular cup orientation to angular values before developing it as a training tool. Methods. A MicronTracker® stereoscopic camera was integrated onto a HoloLens® AR system. Trueness and precision values were obtained through comparison of the AR system measurements to a gold-standard motion capture system”s (OptiTrack®) measurements for acetabular cup orientation on a benchtop trainer, in six clinically relevant pairs of anteversion and inclination angles. Four surgeons performed these six orientations, and repeated each orientation twice. Pearson”s coefficients and Bland-Altman plots were computed to assess correlation and agreement between the AR and Motion Capture systems. Intraclass correlation coefficients (ICC) were calculated to evaluate the degree of repeatability and reproducibility of the AR system by comparing repeated tasks and between surgeons, respectively. Results. The trueness of the AR system was 0.24° (95% CI limit 0.92°) for inclination and 0.90° (95% CI limit 1.8°) for anteversion. Precision was 0.46° for inclination and 0.91° for anteversion. There was significant correlation between the two methods for both inclination (r = 0.996, p<0.001) and anteversion (r = 0.974, p<0.001). Repeatability for the AR system was 0.995 for inclination and 0.989 for anteversion. Reproducibility for the AR system was 0.999 for inclination and 0.995 for anteversion. Conclusion. Measurements obtained from the enhanced HoloLens® AR system were accurate and precise in regards to determining angular measurements of acetabular cup orientation. They exceeded those of currently used methods of cup angle determination such as CT and computer-assisted navigation. Measurements obtained were also highly repeatable and reproducible, therefore this platform is accurately validated for use in a THA training simulator

INTRODUCTION. Acetabular cup malpositioning has been implicated in instability and wear-related complications after total hip arthroplasty. Although computer navigation and robotic assistance have been shown to improve the precision of implant placement, most surgeons use mechanical and visual guides to place acetabular components. Authors have shown that, when using a bean bag positioner, mechanical guides are misleading as they are unable to account for the variability in pelvic orientation during positioning and surgery. However, more rigid patient positioning devices may allow for more accurate free hand cup placement. To our knowledge, no study has assessed the ability of rigid devices to afford surgeons with ideal pelvic positioning throughout surgery. The purpose of this study is to utilize robotic-arm assisted computer navigation to assess the reliability of pelvic position in total hip arthroplasty performed on patients positioned with rigid positioning devices. METHODS. 100 hips (94 patients) prospectively underwent total hip Makoplasty in the lateral decubitus position from the posterior approach; 77 stabilized by universal lateral positioner, and 23 by peg board. After dislocation but prior to reaming, one fellowship trained arthroplasty surgeon manually placed the robotic arm parallel to both the longitudinal axis of the patient and the horizontal surface of the operating table, which, if the pelvis were oriented perfectly, would represent 0 degrees of anteversion and 0 degrees of inclination. The CT-templated computer software then generated true values of this perceived zero degrees of anteversion and inclination based on the position of the robot arm registered to a preoperative pelvic CT. Therefore, variations in pelvic positioning are represented by these robotic navigation generated values. To assure the accuracy of robotic measurements, cup anteversion and inclination at times of impaction were recorded and compared to those calculated via the trigonometric ellipse method of Lewinnek on standardized 3 months postoperative X-rays. RESULTS. Mean alteration in anteversion and inclination values were 1.7 degrees (absolute value 5.3 degrees, range −20 – 20 degrees) and 1.6 degrees (absolute value 2.6 degrees, range −8 – 10 degrees) respectively. 22% of anteversion values were altered by >10 degrees; 41% by > 5 degrees. There was no difference between positioners (p=0.36) and regression analysis revealed that anteversion differences were correlated with BMI (p=0.02). Robotic navigation acetabular cup anteversion (mean 21.8 degrees) was not different from postoperative X-ray anteversion (mean 21.9 degrees)(p=0.50), nor was robotic navigation acetabular cup inclination (mean 40.6 degrees) different from postoperative X-ray inclination (mean 40.5 degrees)(p=0.34). DISCUSSION AND CONCLUSION. Rigid pelvic positioning devices present 5 to 20 degrees of variability in acetabular cup orientation, particularly with regards to anteversion. Compounding this with 20 degree safe zones and prior author demonstrations that human error is prone to 10 degrees of anteversion inaccuracy in a fixed pelvis model, there is a clear need to pay particular attention to anatomic landmarks or computer assisted techniques to assure accurate acetabular cup positioning. Patient positioning by itself should not be trusted

INTRODUCTION. Femoral stem impingement can damage an acetabular liner, create polyethylene wear, and potentially lead to dislocation. To avoid component-to-component impingement, many surgeons aim to align acetabular cups based on the “Safe Zone” proposed by Lewinnek. However, a recent study indicates that the historical target values for cup inclination and anteversion defined by Lewinnek et al. may be useful but should not be considered a safe zone. The purpose of this study was to determine the effect of altering femoral head size on hip range-of-motion (ROM) to impingement. METHODS. Ten healthy subjects were instrumented and asked to perform six motions commonly associated with hip dislocation, including picking up an object, squatting, and low-chair rising. Femur-to-pelvis relative motions were recorded throughout for flexion/extension, abduction/adduction, and internal/external rotation. A previously reported custom, validated hip ROM three-dimensional simulator was utilized. The user imports implant models, and sets parameters for pelvic tilt, stem version, and specific motions as defined by the subjects. Acetabular cup orientations for abduction and anteversion combinations were chosen. The software was then used to compute minimum clearances or impingement between the components for any hip position. Graphs for acetabular cup abduction vs. anteversion were generated using a tapered wedge stem with a 132º neck angle, a stem version of 15°, and a pelvic tilt of 0°. The only variable changed was femoral head size. Head sizes reviewed were 32mm, 36mm, and a Dual-Mobility liner with an effective head size of 42mm. All femoral head sizes can be used with a 50mm acetabular cup. RESULTS. We found that the “Safe Zone” varies considerably depending on the size of the femoral head used for all subjects. A typical plot illustrating the ROM to impingement is presented in Figures 1a-1c. The area to the left of each curve represents an impingement zone for that motion, and to the right, a non-impingement zone. In all cases the non-impingement area is smaller than the Lewinnek safe zone. Motions like picking up an object and squatting had the greatest influence on impingement. CONCLUSIONS. The true acetabular target for impingement-avoidance motion is much smaller than previously believed and identifies the need to take into account the size of the femoral head to be used. This may explain why approximately 70% of dislocations have been reported to be found even when cups were placed within the Lewinnek safe zone. Certain activities, such as picking up an object and squatting reduce the size of the safe zone. This study supports the need for better patient planning and intraoperative execution for placement of the acetabular component

Introduction. Accurate acetabular cup orientation could lead to successful surgical results in total hip arthroplasty (THA). We introduce a novel CT-based three-dimensional (3D) planning system, HipCOMPASS (Fig.1) and TARGET (Fig.2), which enable to design suitable alignment not only cup also surgical devices calculatingly, according to each pelvic inclination. Patients and Methods. We performed THA in 45 hips in 43 patients (female 37 and 6 men) between April 2014 and October 2015. Average age were 68 years old. THA operation was based on each parameter of the cup and device, providing a preoperative planning by ZedView system. HipCOMPASS and TARGET is linked with ZedView software, which is simultaneously calibrated adjustable parameters on this devices. Cup alignment was assessed by ZedView as well. Results. The differences of component alignment from the preoperative planning were shown in Tables. Conclusion. HipCOMPASS and TARGET might be more accurate than conventional method and more accessible system than navigation system in THA

Positioning of the hip resurfacing is crucial for its long term survival and is critical in young patients for some reasons; manly increase the wear in the components and change the load distribution. THR have increased in the last years, mainly in young patients between 45 to 59 years old. The resurfacing solution is indicated for young patients with good bone quality. A long term solution is required for these patients to prevent hip revision. The resurfacing prosthesis Birmingham Hip Resurfacing (BHR) was analyzed in the present study by in vitro experimental studies. This gives indications for surgeons when placing the acetabular cup. One synthetic left model of composite femur (Sawbones®, model 3403), which replicates the cadaveric femur, and four composite pelvic bones (Sawbones®, model 3405), were used to fix the commercial models of Hip resurfacing (Birmingham model). The resurfacing size was chosen according to the head size of femurs with 48 mm head diameter and a cup with 58 mm. They were introduced by an experimented surgeon with instrumental of prosthesis. The cup is a press fit system and the hip component was cemented using bone cement Simplex, Stryker Corp. The acetabular cup was analyzed in 4 orientations; in anteverion with 15º and 20°; and in inclination 40 and 45°. Combinations of these were also considered. The experimental set-up was applied according to a system previously established by Ramos et al. (2013) in the anatomic position. The femur rotates distally and the Pelvic moves vertically as model changes, such that the same boundary conditions are satisfied. This system allows compensating motions of the acetabular cup orientation. A vertical load of 1700 N was applied on all cases, which have resulted in joint reaction force of 2.4 kN. The femur and iliac bone was instrumented with rosettes. 5 repetitions at each position were conducted. When the femur was instrumented with three rosettes in medial, anterior and posterior aspect, the maximum strain magnitude was observed in the medial aspect of femur with a minimum principal strain of −2070µε for 45° inclination and 20° of anterversion. The pubic region was found most critical region after instrumenting the Iliac bone with four rosettes, with a minimum principal strain around −2500µε (rosette 1), for the 45° inclination and 20° of anterversion. We have observed the great influence of the inclination on the strain distribution, changing its magnitude from compression to traction in different bone regions. The minimum principal strain is more critical in medial aspect of the femur and the influence of strain is about 7% when orientation and inclination change. The maximum influence was observed in the anterior aspect, where the anteversion presents a significant influence. The results show the interaction between inclination and anterversion in all aspects, being observed lower values in lower angles. The orientation of the acetabular cup significantly influences the strain distribution on the iliac surface. Besides, as anterversion increases, more strains are induced, mainly in the region of iliac body (rosette 3)

Introduction. Appropriate acetabular cup orientation is an important factor in reducing instability and maximising the performance of the bearing after Total Hip Arthroplasty (THA). However, postoperative analyses of two large cohorts in the US have shown that more than half of cups are malorientated. In addition, there is no consensus as to what inclination and anteversion angles should be targeted, with contemporary literature suggesting that the orientation should be customised for each individual patient. The aim of this study was to measure the accuracy of a novel patient specific instrumentation system in a consecutive series of 22 acetabular cups, each with a customised orientation. Methodology. Twenty-two consecutive total hip replacement patients were sent for Trinity Optimized Positioning System (OPS) acetabular planning (Optimized Ortho, Sydney). The Trinity OPS planning is a preoperative, dynamic analysis of each patient performing a deep flexion and full extension activity. The software calculates the dynamic force at the hip to be replaced and plots the bearing contact patch as it traces across the articulating surface. The software modelled multiple cup orientations and the alignment which best centralised the load was chosen by the surgeon from the preoperative reports. Once the target orientations had been determined, a unique patient specific guide was 3D printed and used intra-operatively with a laser guided system to achieve the planned alignment, Fig 1. All patients received a post-operative CT scan at 3 months and the radiographic cup inclination and anteversion was measured. The study was ethically approved by The Avenue Hospital Human Research Ethics Committee, Trial Number 176. Results. The mean planned radiographic inclination, reference to the Anterior Pelvic Plane (APP), was 42.8° (range 36.2° – 50.1°). The mean planned radiographic anteversion, reference to the APP, was 28.3° (range 19.4° – 37.0°). Only 23% of the planned orientations fell within Lewinnek's “safe zone”, taking into consideration that that this safe zone is not comparable to the coronal plane of radiographs. However, all 22 cups were planned within a range of 40° ± 10° of inclination and 25° ± 10° of anteversion, when referenced to the coronal plane when supine. The mean inclination difference between the planned and achieved orientations was −1.3° (range −7.6° – 9.2°). The mean anteversion difference was 1.2° (range −5.3° – 7.0°). The mean absolute difference was 4.2° for inclination (range 0.4° – 9.2°) and 3.6° for anteversion (range 0.6° – 7.0°). All 22 cups were within ±10° of their intended target orientation, Fig 2. All 22 cups were within the range of 40° ± 10° of inclination and 25° ± 10° of anteversion, when reference to the coronal plane when supine, Fig 3. Conclusions. These are the early results of a new technology for planning and delivering a customised acetabular cup orientation. We expect further improvements in accuracy with current developments. However, the results suggest that Trinity OPS is a simple way to achieve a patient-specific cup orientation, with accuracy comparable to imageless navigation

INTRODUCTION. Several papers have reported the efficacy of an imageless navigation system in acetabular cup orientation during total hip arthroplasty (THA). Also, an imageless navigation system is useful for recovering leg length discrepancy. However, no study has evaluated the accuracy of the stem antetorsion angle (SAA) with an imageless navigation system in THA. The purpose of this study was to evaluate the accuracy of the stem antetorsion angles, which were measured by CT with the CT-free navigation system. Also, we evaluate the factors that affect the inaccuracy. MATERIALS AND METHODS. CT evaluation was performed in 60 patients (60 joints) who underwent primary THA from December 2011 to March 2014. Fifty-nine patients were female. The mean age at surgery was 67 years. The mean BMI at surgery was 24.0 kg/m2. Fifty-four patients had osteoarthrosis, 5 patients had osteonecrosis, and 1 patient had femoral neck fracture. All surgeries were performed in the supine position with the direct anterior approach. The OrthoPilot imageless navigation system was used during surgery. An Excia stem was used in 47 patients and a Bicontact stem was used in the other 13. Evaluation of SAA was carried out. Instead of SAA, the navigation indicates the rasp antetorsion angle based on the hip-knee-ankle plane during surgery. SAA based on the posterior condylar plane was measured with CT by using 3D THA plannning software. The accuracy of the imageless navigation system was evaluated by comparison of the navigation values obtained during surgery with the CT measured values. Correlations were analyzed with Pearson correlation analysis. RESULTS. The rasp antetorsion angle was 28.1±9.6 degrees [range 7.7–49.2 degrees]. The mean SAA was 29.9±8.9 degrees [10.7–49.7 degrees] in CT. Strong correlation was found between the rasp antetorsion angle and SAA (p<0.001, r=0.858, Figure). The difference between SAA and the rasp antetorsion angle (SAA – rasp antetorsion angle) was 1.8±4.7 degrees [-6.7–11.9 degrees]. The mean absolute difference between the rasp antetorsion angle and SAA was 4.0±3.1 degrees [0.2–11.9 degrees]. Forty-five cases (75%) showed less than 5 degrees of difference between SAA and the rasp antetorsion angle. Five cases (8.3%) showed more than 10 degrees of difference. In these five cases, SAAs were greater than the rasp antetorsion angles. Patients’ age, height, weight, and BMI did not affect the difference. Deformity of the femoral head, osteoarthrosis of the hip with subluxation, and osteoarthrosis of the knee were found in the patients who showed more than 5 degrees of difference between SAA and the rasp antetorsion angle. DISCUSSION. This study demonstrated that imageless navigation showed good accuracy for determining SAA during operation. This helps surgeons to recognize combined anteversion during surgery, and to avoid impingement after THA. This navigation system determines rasp antetorsion angle based on the hip-knee-ankle plane. The hip center is defined by setting the trial cup on the acetabulum. The knee center and the ankle center are defined by pointing over the skin. Therefore, deformity of the acetabulum or femoral head may affect the accuracy of the rasp antetorsion angle

Introduction. There is increasing interest in the functional positions the pelvis assumes with activities of daily living and its effect on acetabular cup orientation. A number of systems are commercially available to assess these movements, and attempt to position the acetabular component of a total hip replacement in a patient specific safe zone. However, these functional positions are assessed pre operatively when the patient still has the arthritis which may affect the range of movement of the hips, and thus affect the functional position of the pelvis. Obviously the planned acetabular position must take into account any changes in the functional movement of the pelvis as a result of the surgery. Ishida et al showed that a pelvis with more than a 10° anterior tilt when standing can be expected to correct towards neutral by 12 months post-surgery. However many of Ishida's cases were dysplastic. Hip arthritis in the Caucasian population is far less likely due to dysplasia and this may affect these pelvic tilt changes post-operatively. Methods. 120 patients who underwent total hip replacement by two surgeons through a posterior approach had had their acetabular planning based on functional imaging according to the Optimized Ortho Protocol (Optimized Ortho, Sydney Australia). They were re-assessed at 12 months post-surgery to determine the changes in their functional pelvic tilts. The Optimized Ortho protocol includes lateral radiographs with the patient standing, sitting forward about to lift off a seat, stepping up with the contralateral leg and a limited supine CT. The functional views are designed to display common functional activities. Results. There was minimal change in pelvic position post-operatively when supine. Patients with a significant anterior tilt when standing tended to revert to a more neutral position. Moreover, patients with a significant posterior tilt also tended to correct towards neutral when standing. The seated forward positions changed from pre to post surgery. Those patients who has a large seated posterior pelvic rotation, corrected significantly towards neutral post operation. Conclusions. These predicted standing changes are similar to those seen by Ishida and are important for planning patient specific positioning of acetabular components. There are no previous reports on the changes in forward sitting position. Confounding contra-lateral issues need to be taken into account when planning these predicted changes

Introduction. Acetabular cup orientation has been shown to be a factor in edge-loading of a ceramic-on-ceramic THR bearing. Currently all recommended guidelines for cup orientation are defined from static measurements with the patient positioned supine. The objectives of this study are to investigate functional cup orientation and the incidence of edge-loading in ceramic hips using commercially available, dynamic musculoskeletal modelling software that simulates each patient performing activities associated with edge-loading. Methodology. Eighteen patients with reproducible squeaking in their ceramic-on-ceramic total hip arthroplasties were recruited from a previous study investigating the incidence of noise in large-diameter ceramic bearings. All 18 patients had a Delta Motion acetabular component, with head sizes ranging from 40 – 48mm. All had a reproducible squeak during a deep flexion activity. A control group of thirty-six patients with Delta Motion bearings who had never experienced a squeak were recruited from the silent cohort of the same original study. They were matched to the squeaking group for implant type, acetabular cup orientation, ligament laxity, maximum hip flexion and BMI. All 54 patients were modelled performing two functional activities using the Optimized Ortho Postoperative Kinematics Simulation software. The software uses standard medical imaging to produce a patient-specific rigid body dynamics analysis of the subject performing a sit-to-stand task and a step-up with the contralateral leg, Fig 1. The software calculates the dynamic force at the replaced hip throughout the two activities and plots the bearing contact patch, using a Hertzian contact algorithm, as it traces across the articulating surface, Fig 2. As all the squeaking hips did so during deep flexion, the minimum posterior Contact Patch to Rim Distance (CPRD) can then be determined by calculating the smallest distance between the edge of the contact patch and the true rim of the ceramic liner, Fig 2. A negative posterior CPRD indicates posterior edge-loading. Results. The mean CPRD was significantly less in the squeaking group than the control group, −2.5mm and 2.9mm respectively, (p < 0.001), Fig 3. The mean pelvic tilt in the flexed seated position was 12.6° (range −13.5° to 30.3°) for the squeaking group and 5.1° (−9.8° to 26.4°) for the control group. Consequently, the mean functional cup anteversion at seat-off was significantly less in the squeaking group than the control group, 8.1° (−10.5° to 36.0°) and 21.1° (−1.9° to 38.4°) respectively (p < 0.001), Fig 3. There were 67% (12) of patients in the squeaking group that showed posterior edge-loading in the simulation compared to only 28% (10) in the control group that exhibited posterior edge-loading in the simulation. Conclusions. Acetabular cup orientation during activities associated with edge-loading are likely very different from those measured when supine. Patients with large anterior pelvic tilts during deep flexion activities might be more susceptible to posterior edge-loading and squeaking in ceramic-on-ceramic bearings, as a consequence of a significant decrease in cup anteversion. If these patients can be identified preoperatively, cup orientation and bearing choice could be customised accordingly to accommodate these individual motion patterns

Introduction. Accurate and reproducible cup positioning is one the most important technical factors that affects outcomes of total hip arthroplasty (THA). Although Lewinnek's safe zone is the most accepted range for anteversion and abduction angles socket orientation, the effect of fixed lumbosacral spine on pelvic tilt and obliquity is not yet established. Questions:. What is the change in anteversion and abduction angle from standing to sitting in a consecutive cohort of patients undergoing THA?. What is the effect of fixed and flexible spinal deformities on acetabular cup orientation after THA?. Material and Methods. Between July 2011 and October 2011, 68 consecutive unilateral THAs were implanted in 68 patients with a mean age of 71 ± 6 years old. Radiographic evaluation included standing anteroposterior (AP) and lateral pelvic radiographs, and sitting lateral pelvic radiograph, measuring lumbosacral angle (LSA), sacral angle (SA), and sagittal pelvic tilt angle (SPTA). Computer generated 3D pelvis models were used to analyze the correlation between different pelvic tilts and acetabular cup orientation in abduction and anteversion. Results. The mean standing STPA was 3.7º of anterior tilt (range: −9º of anterior tilt to 25º of posterior tilt). The mean SPTA in sitting was −17.7º of posterior tilt (range: −38º of posterior tilt to 7º of anterior tilt). In a fixed spinal deformity (54%) the SPTA was significantly smaller compared to the flexible pelvis group (10º versus 30.9º, p=0.0001). Flexible pelvises (46%) had a posterior tilt from standing to sitting resulting in increased anteversion. Overall, mean change in LSA and SA from standing to sitting was 22.5º and 20.2º, respectively. The mean post-operative functional anteversion and abduction angles were 19.2º and 42.1º, respectively. In the virtual 3D pelvic models, when the abduction angle was between 40 and 45 degrees, anteversion changed of 0.75 degrees for 1 degree of change of pelvic tilt. Discussion. There is a significant change in pelvic tilt from standing to sitting, especially in patients with flexible spines, where the functional anteversion increases with sitting. Patients with a fixed pelvis have significantly less change in sagittal tilt and therefore less change in anteversion from standing to sitting position. Care should be taken to adjust cup positioning in fixed spinal deformity