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

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

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

The human acetabulofemoral joint is commonly modelled as a pure ball-and-socket joint, but there has been no quantitative assessment of this assumption in the literature. Our aim was to test the limits and validity of this hypothesis. We performed experiments on four adult cadavers. Cortical pins, each equipped with a marker cluster, were implanted in the pelvis and the femur. Movements were recorded using stereophotogrammetry while an operator rotated the cadaver’s acetabulofemoral joint, exploiting the widest possible range of movement. The functional consistency of the acetabulofemoral joint as a pure spherical joint was assessed by comparing the magnitude of the translations of the hip joint centre as obtained on cadavers, with the centre of rotation of two metal segments linked through a perfectly spherical hinge. The results showed that the radii of the spheres containing 95% of the positions of the estimated centres of rotation were separated by less than 1 mm for both the acetabulofemoral joint and the mechanical spherical hinge. Therefore, the acetabulofemoral joint can be modelled as a spherical joint within the considered range of movement (flexion/extension 20° to 70°; abduction/adduction 0° to 45°; internal/external rotation 0° to 30°)

Abstract. Objectives. Impingement in total hip replacements (THRs), including bone-on-bone impingement, can lead to complications such as dislocation and loosening. The aim of this study was to investigate how the location of the anterior inferior iliac spine (AIIS) affected the range of motion before impingement. Methods. A cohort of 25 CT scans (50 hips) were assessed and nine hips were selected with a range of AIIS locations relative to the hip joint centre. The selected CT Scans were converted to solid models (ScanIP) and THR components (DePuy Synthes) were virtually implanted (Solidworks). Flexion angles of 100⁰, 110⁰, and 120⁰ were applied to the femur, each followed by internal rotation to the point of impingement. The lateral, superior and anterior extent of the AIIS from the Centre of Rotation (CoR) of the hip was measured and its effect on the range of motion was recorded. Results. There was found to be a significant (p<0.05) inverse relationship between the ROM of the THR and the lateral measure of the AIIS. Of the three measures, the lateral AIIS measure showed the strongest relationship with ROM to impingement (R=0.73) with the anterior and superior measures resulting in R values of 0.41 and 0.56 respectively. For every millimetre lateral the AIIS location, there was typically a loss of 1.2° of range of motion. With increasing lateralisation, the AIIS was positioned more directly over the femur, thereby reducing the ROM in the THR during high flexion positions. No soft tissue was included in the models which would have affected the ROM. Conclusions. The results from this study have shown that the lateral measure of the AIIS could be a predictor for bone-on-bone impingement. To build confidence, wider study of AIIS location variation is needed, as well as analysis under impingement prone activities of daily living. 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

During revision THR, the surgery is often difficult and compromised due to lack of patient's bone especially in the pelvis. Any extra bone in the acetabulum is expected to be of advantage to the patient and the surgeon. The aim of this study was to see if preservation of medial acetabular osteophyte in uncemented total hip replacement had any adverse effect on the prosthesis survival or patient satisfaction. Conventional acetabular preparation involves reaming down to the true floor. This not only medialises the centre of rotation of the hip but also reduces the acetabular offset. In contrast the main surgeon preserved the acetabular offset by preserving some osteophytic bone between the true floor of the acetabulum and the acetabular cup. This is achieved by reaming the acetabular cavity conservatively while achieving secure primary fixation of the prosthesis. We report the outcome of a single surgeon series of such cases. The endpoint was assessed as the need for revision of the acetabular cup. A total of 106 consecutive patients were identified who underwent uncemented THR from 2005 to 2010. The medial osteophyte was measured on immediate post-operative x-rays, from the “teardrop” to the nearest point of the acetabular cup, by 3 surgeons (one consultant and 2 registrars). The patients were contacted for a telephone interview and their clinical notes, including x-rays, were reviewed. Outcome was available for 79 patients. 74 patients were available for follow-up and 5 patients died unrelated to THR. Average follow-up was for 8.3 years (range 5.5–10.8). Average age was 62 years. The average medial osteophyte was 1.98 mm (range 0–14mm). One patient had late infection and one had dislocation. There was not a single failure of the acetabular component. The patient satisfaction was high at 8.8 out of 10. Preservation of medial osteophyte in the acetabulum whilst doing uncemented THR has the advantage of retaining the patient's own bone stock which can be of great advantage to the surgeon as well as the patient should revision THR be required in future. Our study has shown that this can be achieved without compromising the survival of the prosthesis or the patient satisfaction. This technique may increase the range of motion of the hip by reducing the risk of bony or soft tissue impingement, and also reduce the risk of dislocation. Furthermore, not recreating the native centre of rotation of the hip does not seem to have any adverse effect for the patients, who are very happy with the outcome. We recommend that whilst doing uncemented THR, the acetabulum should not be reamed to the true floor as has been the conventional teaching, but attempt should be made to preserve some medial osteophyte where possible, at the same ensuring that good primary fixation of the cup is achieved. This is to give the patient and surgeon the advantage of extra available bone should revision surgery be required in the future

Tear pattern and tendon involvement are risk factors for the development of a pseudoparalytic shoulder. However, some patients have similar tendon involvement but significantly different active forward flexion. In these cases, it remains unclear why some patients suffer from pseudoparalysis and others with the same tear pattern show good active range of motion. Moment arms (MA) and force vectors of the RC and the deltoid muscle play an important role in the muscular equilibrium to stabilize the glenohumeral joint. Biomechanical and clinical analyses were conducted calculating different MA-ratios of the RC and the deltoid muscle using computer rigid body simulation and a retrospective radiographic investigation of two cohorts with and without pseudoparalysis and massive RC tears. Idealized MAs were represented by two spheres concentric to the joints centre of rotation either spanning to the humeral head or deltoid origin of the acromion. Individual ratios of the RC /deltoid MAs on antero-posterior radiographs using the newly introduced Shoulder Abduction Moment (SAM) Index was compared between the pseudoparalytic and non-pseudoparalytic patients. Decrease of RC activity and improved glenohumeral stability (+14%) was found in simulations for MA ratios with larger diameters of the humeral head which also were consequently beneficial for the (remaining) RC. Clinical investigation of the MA-ratio showed significant risk of having pseudoparalysis in patients with massive tears and a SAM Index <0.77 (OR=11). The SAM index, representing individual biomechanical characteristics of shoulder morphology has an impact on the presence or absence of pseudoparalysis in shoulders with massive RC tears

One of the main surgical goals when performing a total knee replacement (TKR) is to ensure the implants are properly aligned and correctly sized; however, understanding the effect of alignment and rotation on the biomechanics of the knee during functional activities is limited. Cardiff University has unique access to a group of local patients who have relatively high frequency of poor alignment, and early failure. This provides a rare insight into how malalignment of TKR's can affect patients from a clinical and biomechanical point of view to determine how to best align a TKR. This study aims to explore relationship clinical surgical measurements of Implant alignment with in-vivo joint kinematics. 28 patient volunteers (with 32 Kinemax (Stryker) TKR's were recruited. Patients undertook single plane video fluoroscopy of the knee during a step-up and step-down task to determine TKR in-vivo kinematics and centre of rotation (COR). Joint Track image registration software (University of Florida, USA) was used to match CAD models of the implant to the x-ray images. Hip-Knee-Ankle (HKA) was measured using long-leg radiographs to determine frontal plane alignment. Posterior tibial slope angle was calculated using radiographs. An independent sample t-test was used to explore differences between neutral (HKA:-2° to 2°), varus (≥2°) and valgus alignment (≤-2°) groups. Other measures were explored across the whole cohort using Pearson's correlations (SPSS V23). There was found to be no statistical difference between groups or correlations for HKA. The exploratory analysis found that tibial slope correlated with Superior/Inferior translation ROM during step up (r=−0.601, p<0.001) and step down (r=−.512, p=0.03) the position of the COR heading towards the lateral (r=−.479, p=0.006) during step down. Initial results suggest no relationship between frontal plane alignment and in-vivo. Exploratory analyses have found other relationships that are worthy of further research and may be important in optimizing function

Background. Finite element (FE) models have become a standard pre-clinical tool to study biomechanics of spine and are used to simulate and evaluate different strategies in scoliosis treatment: examine their efficacy as well as the effect of different implant design parameters. The goal of this study is to investigate, in a system of rods and laminar wires, the effect of the number of wires and their pre-stress on whole spine stiffness. Methods. A generic FE model was developed to represent a full human spine, including vertebrae, intervertebral discs, ligaments, facet and costovertebral joints, and ribcage. Intervertebral discs were modeled with 3D rebar elements with linear elastic material properties. Vertebrae, ribs, sternum, facet joints, cartilage and endplates were modeled with brick elements, and costal muscles with shell elements with linear elastic properties. Furthermore, ligaments were modeled with truss elements with nonlinear hypo-elastic properties. The spine model was instrumented from T7 to T12 with rods and wires modeled as titanium. Nonlinear contact properties were defined for rib neck-vertebra, transverse processes-rib and facet joint sets. The FE model was loaded in flexion and the whole spine instantaneous stiffness was calculated for different wire pre-stressing levels (0.1 to 2 MPa). Similar analyses were performed with changed numbers of wires and whole spine stiffness was calculated. Results. The results show that with increasing the pre-stress level the whole spine instantaneous stiffness increases by up to 6%. Reducing the number of wires decreases the whole spine stiffness almost linearly by 5%. These changes also alter center of rotation of the spine. The results suggest that pre-stressing and number of wires have an effect on whole spine stiffness. Conclusions. In summary, the develop FE model can be used to simulate different treatment strategies and to improve implant designs used in surgical treatment of scoliosis. Level of evidence. FEA study

Background. Many factors contribute to the occurrence of edge-loading conditions in hip replacement; soft tissue tension, surgical position, patient biomechanical variations and type of activities, hip design, etc. The aim of this study was to determine the effect of different levels of rotational and translational surgical positioning of hip replacement bearings on the occurrence and severity of edge-loading and the resultant wear rates. Method. The Leeds II Hip-Joint Simulator and 36mm diameter alumina matrix composite ceramic bearings (BIOLOX delta, DePuy Synthes, UK) were used in this study. Different levels of mismatch between the reconstructed rotational centres of the head and the cup were considered (2, 3 and 4mm) in the medial-lateral axis. Two cup inclination angles were investigated; an equivalent to 45 and 65 degrees in-vivo, thus six conditions (n=6 for each condition) were studied in total with three million cycles completed for each condition. The wear of the ceramic-on-ceramic bearings were determined using a microbalance (Mettler Toledo, XP205, UK) and the dynamic microseparation displacement was measured using a Liner Variable Differential Transformer. Results. When a translational joint centre mismatch was coupled with a higher cup inclination angle, the severity of edge-loading increased when compared with the effect of those variables applied individually. Increasing the medial-lateral joint centre mismatch from 2 to 3 to 4mm resulted in increased wear rates under both cup inclination angles, with the 65 degree cup inclination angle having significantly higher wear rate than the cup inclination angle of 45 degree (p=0.02, p=0.02, and p<0.01 respectively). Conclusion. The cups with a 45 degree inclination angle showed greater resistance to dynamic microseparation as a result of joint centre mismatch. This study demonstrated that optimal position should not only consider the rotational position of the acetabular cup but also the relative centres of rotation of the head and the cup. Disclosure. John Fisher is a paid consultant to DePuy Synthes. Jonathan Thompson and Graham H. Issac are employees at DePuy Synthes

Edge loading due to dynamic separation can occur due to variations in component positioning such as a steep cup inclination angle (rotational) or mismatch between the centres of rotation of the head and the cup (translational). The aim of this study was to determine the effect of variations in rotational and translational positioning of the cup on the magnitude of dynamic separation, wear and deformation of metal-on-polyethylene bearings. Eighteen 36mm diameter metal-on-polyethylene hip replacements were tested on an electromechanical hip simulator. Standard gait with concentric head and cup centres were applied for cups inclined at 45° (n=3) and 65° (n=3) for two million cycles. A further two tests with translational mismatch of 4mm applied between the head and cup bearing centres for cups inclined at 45° (n=6) and 65° (n=6) were run for three million cycles. Wear was determined using a microbalance and deformation by geometric analysis. Confidence intervals of 95% were calculated for mean values, and t-tests and ANOVA were used for statistical analysis (p<0.05). Under 4mm mismatch conditions, a steeper cup inclination angle of 65° resulted in larger dynamic separation (2.1±0.5mm) compared with cups inclined at 45° (0.9±0.2mm). This resulted in larger penetration at the rim under 65° (0.28±0.04mm) compared to 45° (0.10±0.09mm) cup inclination conditions (p<0.01). Wear rates under standard concentric conditions were 12.8±3.8 mm. 3. /million cycles and 15.4±5.0 mm. 3. /million cycles for cups inclined at 45° and 65° respectively. Higher wear rates were observed under 4mm of translational mismatch compared with standard concentric conditions at 45° (21.5±5.5 mm. 3. /million cycles, p<0.01) and 65° (23.0±5.7 mm. 3. /million cycles, p<0.01) cup inclination. Edge loading under dynamic separation conditions due to translational mismatch resulted in increased wear and deformation of the polyethylene liner. Minimising the occurrence and severity of edge loading through optimal component positioning may reduce the clinical failure rates of polyethylene

Edge loading due to dynamic separation can occur due to variations in component positioning such as a steep cup inclination angle (rotational) or mismatch between the centres of rotation of the head and the cup (translational). The aim of this study was to determine the effect of variations in rotational and translational positioning of the cup on the magnitude of dynamic separation, wear and deformation of metal-on-polyethylene bearings. Eighteen 36mm diameter metal-on-polyethylene hip replacements were tested on an electromechanical hip simulator. Standard gait with concentric head and cup centres were applied for cups inclined at 45° (n=3) and 65° (n=3) for two million cycles. A further two tests with translational mismatch of 4mm applied between the head and cup bearing centres for cups inclined at 45° (n=6) and 65° (n=6) were run for three million cycles. Wear was determined using a microbalance and deformation by geometric analysis. Confidence intervals of 95% were calculated for mean values, and t-tests and ANOVA were used for statistical analysis (p<0.05). Under 4mm mismatch conditions, a steeper cup inclination angle of 65° resulted in larger dynamic separation (2.1±0.5mm) compared with cups inclined at 45° (0.9±0.2mm). This resulted in larger penetration at the rim under 65° (0.28±0.04mm) compared to 45° (0.10±0.05mm) cup inclination conditions (p<0.01). Wear rates under standard concentric conditions were 12.8±3.8 mm. 3. /million cycles and 15.4±5.0 mm. 3. /million cycles for cups inclined at 45° and 65° respectively. Higher wear rates were observed under 4mm of translational mismatch compared with standard concentric conditions at 45° (21.5±5.5 mm. 3. /million cycles, p<0.01) and 65° (23.0±5.7 mm. 3. /million cycles, p<0.01) cup inclination. Edge loading under dynamic separation conditions due to translational mismatch resulted in increased wear and deformation of the polyethylene liner. Minimising the occurrence and severity of edge loading through optimal component positioning may reduce the clinical failure rates of polyethylene

In the congenital hip dysplasia, patients treated with total hip replacement (THR) often report persistent disability and pain, with unsatisfactory function and quality of life. A major challenge is to restore the center of rotation of the hip and a satisfactory abduction function [1]. The position of the acetabular cup during THR might be crucial, as it affects abduction moment and motor function. Recently, several software systems have been developed for surgical planning of endoprostheses. Previously developed software called HipOp [2], which is routinely used in clinics, allows surgeons to properly position the prosthetic components into the 3D space of CT data. However, this software did not allow to simulate the articular range of motion and the condition of the abductor muscles. Our aim is to present HipOpCT, an advanced version of the software that includes 3D musculoskeletal planning, through the application to hip dysplasia patients to add knowledge in the diagnosis and treatment of such patients who need THR. 40 hip dysplasia patients received pre-operative CT scanning of pelvis and thighs and had their THR surgery planned using HipOpCT. The base planning includes import of CT data, positioning of prosthetic components interactively through multimodal display, as well as geometrical measurements of the implant and the host bone. The advanced planning additionally includes evaluation of femoro-acetabular impingement and calculation of leg lengths, abductor muscle lengths and lever arms through the automatic creation of a musculoskeletal model. The musculoskeletal parameters in all patients were calculated during the surgical planning, and the data were processed to evaluate pre- and post-operative differences in leg length discrepancy, length and lever arm of the abductor muscles, and how these parameters correlated. The surgical planning led to an increase in the operated leg length of 7.6 ± 5.7 mm. The variation in abductors lever arm was −0.9% ± 4.8% and significantly correlated with the variation in the operated leg length (r = −0.49), pre-operative leg length discrepancy (r = 0.32) and variation in abductors length (r = −0.32). The variation in abductors length was 6.6% ± 5.5%, and significantly correlated with the variation in the operated leg length (r = 0.92), post-operative leg length discrepancy (r = 0.37), pre-operative abductors length (r = −0.37) and variation in abductors lever arm (r = −0.32). The increase in the operated leg length was strongly correlated to the increase in abductor muscle length. Conversely, abductor lever arms slightly decreased on average, and were inversely correlated to leg length variation and abductors lengths. This interactive technology for surgical planning represent a powerful tool for orthopaedic surgeons to consider the best muscle reconstruction, and for rehabilitation specialists to achieve the best functional recovery based on biomechanical outcomes. In a parallel study, we are investigating how these advanced planning is reflected onto the function, pain and biomechanical outcome after a rehabilitation protocol is completed

Conventional TKA surgery attempts to restore patients to a neutral alignment, and devices are designed with this in mind. Neutral alignment may not be natural for many patients, and may cause dissatisfaction. To solve this, kinematical alignment (KA) attempts to restore the native pre-arthritic joint-line of the knee, with the goal of improving knee kinematics and therefore patient's function and satisfaction. Proper prosthetic trochlea alignment is important to prevent patella complications such as instability or loosening. However, available TKA components have been designed for mechanical implantation, and concerns remain relating the orientation of the prosthetic trochlea when implants are kinematically positioned. The goal of this study is to investigate how a currently available femoral component restores the native trochlear geometry of healthy knees when virtually placed in kinematic alignment. The healthy knee OAI (Osteoarthritis Initiative) MRI dataset was used. 36 MRI scans of healthy knees were segmented to produce models of the bone and cartilage surfaces of the distal femur. A set of commercially available femoral components was laser scanned. Custom 3D planning software aligned these components with the anatomical models: distal and posterior condyle surfaces of implants were coincident with distal and posterior condyle surfaces of the cartilage; the anterior flange of the implant sat on the anterior cortex; the largest implant that fitted with minimal overhang was used, performing ‘virtual surgery’ on healthy subjects. Software developed in-house fitted circles to the deepest points in the trochlear grooves of the implant and the cartilage. The centre of the cartilage trochlear circle was found and planes, rotated from horizontal (0%, approximately cutting through the proximal trochlea) through to vertical (100%, cutting through the distal trochlea) rotated around this, with the axis of rotation parallel to the flexion facet axis. These planes cut through the trochlea allowing comparison of cartilage and implant surfaces at 1 degree increments. Trochlear groove geometry was quantified with (1) groove radial distance from centre of rotation cylinder (2) medial facet radial distance (3) lateral facet radial distance and (4) sulcus angle, along the length of the trochlea. Data were normalised to the mean trochlear radius. The orientation of the groove was measured in the coronal and axial plane relative to the flexion facet axis. Inter- and intra-observer reliability was measured. In the coronal plane, the implant trochlear groove was oriented a mean of 8.7° more valgus (p<0.001) than the normal trochlea. The lateral facet was understuffed most at the proximal groove between 0–60% by a mean of 5.3 mm (p<0.001). The medial facet was understuffed by a mean of 4.4 mm between 0–60% (p<0.001). Despite attempts to design femoral components with a more anatomical trochlea, there is significant understuffing of the trochlea, which could lead to reduced extensor moment of the quadriceps and contribute to patient dissatisfaction

Arthritis of the glenohumeral joint accompanied by an irreparable tear of the rotator cuff can cause severe pain, disability and loss of function, particularly in the elderly population. Anatomical shoulder arthroplasty requires a functioning rotator cuff, however, reverse shoulder arthroplasty is capable of addressing both rotator cuff disorders and glenohumeral deficiencies. The Aequalis Reversed Shoulder Prosthesis design is based on two bio-mechanical principles by Grammont; a medialized center of rotation located inside the glenoid bone surface and second, a 155 degree angle of inclination. Combined, they increase the deltoid lever arm by distalizing the humerus and make the prosthesis inherently stable. 24 consecutive primary reverse total shoulder arthroplasties were performed by a single surgeon for arthritis with rotator cuff compromise and 1 as a revision for a failed primary total shoulder replacement between December 2009 and October 2012. Patients were assessed postoperatively with the use of the DASH score, Oxford shoulder score, range of shoulder motion and plain radiography with Sirveaux score for scapular notching. Mean age at the time of surgery was 72.5 years (range 59 to 86). Average follow up time was 19.4 months (range 4 to 38). Functional outcome scores from our series were comparable with patients from other follow up studies of similar prosthesis design. All patients showed improvement in range of shoulder movement postoperatively. Complications included one dislocation, one acromion fracture and one humeral shaft fracture. No cases of deep infection were recorded. Overall, the short-term clinical results were promising for this series of patients and indicate reverse shoulder arthroplasty as an appropriate treatment for this group of patients

Lower limb mal-alignment due to deformity is a significant cause of early degenerative change and dysfunction. Standard techniques are available to determine the centre of rotation of angulation (CORA) and extent of the majority of deformities, however distal femoral deformity is difficult to assess because of the difference between anatomic and mechanical axes. We found the described technique involving constructing a line perpendicular to a line from the tip of the greater trochanter to the centre of the femoral head inaccurate, particularly if the trochanter is abnormal. We devised a novel technique which accurately determines the CORA and extent of distal femoral deformity, allowing accurate correction. Using standard leg alignment views of the normal femur, the distal femoral metaphysis and joint line are stylized as a block. A line bisecting the axis of the proximal femur is then extended distally to intersect the joint. The angle (Θ) between the joint and the proximal femoral axis and the position (p) where the extended proximal femoral axis intersects the joint line are calculated. These measurements can then be reproduced on the abnormal distal femur in order to calculate the CORA and extent of the deformity, permitting accurate correction. We examined the utility and reproducibility of the new method using 100 normal femora. Θ = 81 ± sd 2.5. As expected, Θ correlated with femoral length (r=0.74). P (expressed as the percentage of the distance from the lateral edge of the joint block to the intersection) = 61% ± sd 8%. P was not correlated with Θ. Intra-and inter-observer errors for these measurements are within acceptable limits and observations of 30-paired normal femora demonstrate similar values for Θ and p on the two sides. We have found this technique to be universally applicable and reliable in a variety of distal femoral deformities

Summary Statement. Reverse shoulder design philosophy can impact external rotation moment arms. Lateralizing the humerus can increase the external rotator moment arms relative to normal anatomy. Introduction. The design of reverse shoulders continues to evolve. These devices are unique in that they are not meant to reproduce the healthy anatomy. The reversal of the fulcurm in these devices impacts every muscle that surrounds the joint. This study is focused on analyzing the moment arms for the rotator cuff muscles involved in internal and external rotation for a number of reverse shoulder design philosophies. Methods. Four of the most common design philosophies were chosen. The first, a Grammont style prosthesis, with a center of rotation (COR) on the glenoid face and a humeral cup countersunk into the proximal humerus (MGMH). The second concept is the MGMH design lateralised by a 10mm bone graft (BIO). The third concept has a lateralised glenosphere COR and a humeral component inside the proximal humerus (LGMH). The fourth design has a medialised COR with a humeral component placed on top of the humerus (MGLH). This places the humerus further lateral than the previous designs. For each component set, a representative implant was modeled based on published specifications. Each design was implanted into the same digital bone models (consisting of a humerus, scapula, clavicle, and ribcage) following the manufacturer's recommended surgical technique. The muscles analyzed were the posterior-deltoid (PD), subscapularis (SSC), infraspinatus (IS), and teres minor (TM). These muscles were allowed to wrap around the bone of the scapula and proximal humerus through the range of motion. All muscle origin and insertion points were kept constant throughout the analysis. The assemblies were externally rotated from an initial position of 45° internal rotation to 45° of external rotation of the humerus with the arm at 0° of abduction. The moment arms for all muscles were compared to those calculated for the anatomic shoulder. Results. All the rotator cuff muscles displayed a similar trend with the reverse shoulder. The external rotators all had similar moment arm values at neutral (IS∼22mm, TM∼20mm), but increased at rates proportional to their humeral offsets with external rotation (IS-MGLH 32.3mm, LGMH 27.5mm, MGMH and BIO 26.25mm; TM-MGLH 31.3mm, LGMH 27.8mm, MGMH and BIO 26.5mm). The SSC internal rotation moment arm remains roughly constant at 20mm for the anatomic shoulder, but varies widely from 45° external to 45° internal rotation with the different designs (MGLH 31.4mm to 6.7mm; MGMH 25.1mm to 11.2mm; LGMH 26.2mm to 10.8mm; BIO 25.4mm to 4.8mm). The PD moment arm is increased relative to the anatomic shoulder during external rotation for the MGLH design (9.3mm vs. 7.4mm). The other designs exhibit a decrease in the moment arm of this muscle relative to the anatomic design (LGMH 7.3mm, MGMH 5.8mm, BIO 6.4mm). Discussion. The lateral offset between the center of humeral axis and the muscle insertion on the humerus dominates the external rotation moment arm value through this range of motion. This is evident by the increase in the moment arms with external rotation for the different reverse shoulder designs. The increase in external rotation efficiency for the external rotators and PD could play a critical role in post-operative external rotation strength and motion

Objectives

Sustained intra-articular delivery of pharmacological agents is an attractive modality but requires use of a safe carrier that would not induce cartilage damage or fibrosis. Collagen scaffolds are widely available and could be used intra-articularly, but no investigation has looked at the safety of collagen scaffolds within synovial joints. The aim of this study was to determine the safety of collagen scaffold implantation in a validated in vivo animal model of knee arthrofibrosis.

Dislocation remains a major concern after total hip replacement, and is often attributed to malposition of the components. The optimum position for placement of the components remains uncertain. We have attempted to identify a relatively safe zone in which movement of the hip will occur without impingement, even if one component is positioned incorrectly. A three-dimensional computer model was designed to simulate impingement and used to examine 125 combinations of positioning of the components in order to allow maximum movement without impingement. Increase in acetabular and/or femoral anteversion allowed greater internal rotation before impingement occurred, but decreases the amount of external rotation. A decrease in abduction of the acetabular components increased internal rotation while decreasing external rotation. Although some correction for malposition was allowable on the opposite side of the joint, extreme degrees could not be corrected because of bony impingement.

We introduce the concept of combined component position, in which anteversion and abduction of the acetabular component, along with femoral anteversion, are all defined as critical elements for stability.

The biomechanics of the patellofemoral joint can become disturbed during total knee replacement by alterations induced by the position and shape of the different prosthetic components. The role of the patella and femoral trochlea has been well studied. We have examined the effect of anterior or posterior positioning of the tibial component on the mechanisms of patellofemoral contact in total knee replacement. The hypothesis was that placing the tibial component more posteriorly would reduce patellofemoral contact stress while providing a more efficient lever arm during extension of the knee.

We studied five different positions of the tibial component using a six degrees of freedom dynamic knee simulator system based on the Oxford rig, while simulating an active knee squat under physiological loading conditions. The patellofemoral contact force decreased at a mean of 2.2% for every millimetre of posterior translation of the tibial component. Anterior positions of the tibial component were associated with elevation of the patellofemoral joint pressure, which was particularly marked in flexion > 90°.

From our results we believe that more posterior positioning of the tibial component in total knee replacement would be beneficial to the patellofemoral joint.