Abstract. Optical motion capture (OMC) is the current gold standard for motion analysis, however measuring patellofemoral kinematics is not possible using the technique. One approach to measuring in-vivo kinematics is to use biplane video X-ray (BVX) and 3D models generated from MRI to track the movement of the patellar. Understanding how the patellar is moving during different loaded dynamic activities can help with understanding the effects of different interventions when treating disease or injury. Objective. To develop a protocol and compare patellofemoral kinematics for different activities using biplane video X-ray (BVX). Methods. Two healthy volunteers performed level walk, lunge, and stair ascent activities while simultaneous capturing BVX and synchronised OMC. Participants undertook MR imaging (Magnetom 3T Prisma, Siemens) which was manually segmented into 3D bone models (Simpleware Scan IP, Synopsis). Bone position and orientation for the patellar and femur were calculated by manual matching of 3D Bone models to X-Rays (DSX Suite, C-Motion, Inc.). Patellofemoral kinematics were calculated using Visual 3D (C-Motion, Inc.). Results. Initial results show that patellar flexion(+) (PF) was greatest during lunge (52.1º) compared with stair ascent (49.4º) and stance phase of gait (5.4º), however stair ascent had the largest PF range of motion (ROM) of 48.8º. The lunge activity had the greatest ROM for patellar lateral rotation (12.8º) compared with stair ascent (8.7º) and gait (3.7º). Patellar lateral (+) tilt was found to be greatest during gait (8.4º) compared with stair ascent (6.7º) and lunge (6.8º). Conclusions. These results highlight the variability of patellofemoral kinematics between different loaded dynamic activities. When considering the influence and efficacy of patellofemoral interventions it is important to investigate different activities to fully understand their effects. Future work will look at more dynamic activities and to investigate further the effect of different activities on patellofemoral tracking. 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

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

Malrotation of the femoral component is a cause of patellofemoral maltracking after total knee arthroplasty. Its precise effect on the patellofemoral mechanics has not been well quantified. We have developed an in vitro method to measure the influence of patellar maltracking on contact. Maltracking was induced by progressively rotating the femoral component either internally or externally. The contact mechanics were analysed using Tekscan. The results showed that excessive malrotation of the femoral component, both internally and externally, had a significant influence on the mechanics of contact. The contact area decreased with progressive maltracking, with a concomitant increase in contact pressure. The amount of contact area that carries more than the yield stress of ultra-high molecular weight polyethylene significantly increases with progressive maltracking. It is likely that the elevated pressures noted in malrotation could cause accelerated and excessive wear of the patellar button

Introduction. The medial patellofemoral ligament (MPFL) is the main stabilizer of the patella and therefore mostly reconstructed in the surgical correction of patellofemoral dislocation. Various biomechanical and clinical studies have been conducted on MPFL reconstruction, while the patellofemoral contact pressure (PFCP) which is indicated as one of the predictors of retropatellar osteoarthritis was neglected. Therefore, the aim of this study was to investigate how different MPFL reconstruction approaches affect PFCP. Material & Methods. After radiographic examination and preparation six human cadaveric knee joints (52.1 ± 8.4yrs) were placed in a 6-DOF knee simulator. Three flexion-extension cycles (0–90°) were applied, while the extensor muscles (175N) and an axial joint load (200N) were simulated. PFCP was measured in knee flexion of 0°, 30° and 90° using a calibrated pressure measurement system (K-Scan, Tekscan Inc., USA). The following MPFL conditions were examined: native (P. nat. ), anatomical reconstruction (P. a. ), proximal and distal patellar single-bundle reconstruction (P. p. , P. d. ), proximal and ventral femoral reconstruction (F. p. , F. v. ). The cohesive gracillis graft of each knee was used for MPFL reconstruction. Further, the effect of three different graft pre-tensioning levels (2N, 10N, 20N) on the PFCP were compared. Nonparametric statistical analysis was performed using SPSS (IBM Inc., USA). Results. In 0° knee flexion median PFCP of the native state (P. nat. =0.46MPa) was significantly higher (p=0.04) compared to the ventral femoral fixation state (F. v. =0.24MPa). No significant differences were observed in 30° knee flexion. In 90° knee flexion PCFP of both femoral reconstructions (F. p. =1.26MPa, F. v. =1.12MPa) were significantly higher (p<0.04) compared to the native state (P. nat. =0.43MPa). Graft pre-tensioning had no significant impact (p>0.27) on the PFCP in 0°, 30° and 90° knee flexion for all pre-tensioning levels. Discussion. We investigated the PFCP of different MPFL reconstructions and compared them during continuous joint motion from 0° to 90° knee flexion. While a non-anatomical graft fixation on the femoral side leads to an excessive increase of PFCP (293%), a non-anatomical positioning on the patellar side only showed minor impact on the PFCP. An anatomical MPFL reconstruction showed comparable PFCP to the native joint. In contrast to the literature, we did not find a significant influence of graft pre-tensioning from 2N up to 20N on the PFCP. With respect to all study findings we would recommend to use the anatomical footprints for MPFL reconstruction and a moderate graft pre-tensioning of 2N

Summary Statement. This project proves that Patellofemoral (PF) joint degeneration is not a contraindication to medial unicompartmental knee replacement. Introduction. Unicompartmental knee arthroplasty (UKA) is a recognised procedure for treatment of medial compartment osteoarthritis. Patellofemoral (PF) joint degeneration is widely considered to be a contraindication to medial unicompartmental knee replacement. We examined the validity of this preconception using information gathered prospectively on consecutive patients who underwent UKA using the Repicci II® UKA prosthesis for medial compartment osteoarthritis. Methods. We prospectively collected data on 147 consecutive patients who underwent the Repicci II® UKA for medial compartment osteoarthritis. All operations were performed between July 1999 and September 2000 by the same surgeon. The status of the PF joint was assessed intra-operatively in all patients, and accordingly patients were divided into two groups, one group with a normal PF joint, and the second group with degenerative changes of the PF joint. Variables measured for outcome included the International Knee Society (IKS) score, limb alignment, and range of motion. Radiographs were assessed for progression of disease or failure of implant. The mean follow-up was 9.4 years (range: 5–10.7 years). Patients were reviewed initially at 2 weeks, and then at 6 months post-operatively. They were subsequently reviewed on an annual basis.. All patients completed an IKS score preoperatively and at last follow-up. Age, gender, BMI, length of hospital stay, perioperative complications, all subsequent surgery, including revision of the prosthesis, and survivorship at 10 years was recorded, and results of the 2 groups compared. Results. A total of 147 patients were included in the study. None were lost to follow-up. Sixty nine had associated PF osteoarthritis (group A) while 78 patients had a normal PF compartment when assessed intra-operatively (group B). Post-operative outcomes of the two groups were compared using the ANCOVA analysis with adjustment for pre-operation values firstly, and adjusting also for gender, age, BMI and follow-up secondly. There were no significant differences in terms of IKS, alignment, and flexion between the two groups. However, patients in group B had significantly better extension post-operatively than patients in group A (p<0.05). Conclusion. We concluded that damage to the articular cartilage of the patellofemoral joint to the extent of full-thickness cartilage loss is not a contraindication to the Repicci II® unicondylar knee arthroplasty for medial compartment osteoarthritis. However, extension is significantly improved post-operatively in those patients with minimal or no PF joint degenerative disease

Using a new, non-invasive method, we measured the patellofemoral force (PFF) in cadaver knees mounted in a rig to simulate weight-bearing. The PFF was measured from 20° to 120° of flexion before and after implanting three designs of knee prosthesis. Medial unicompartmental arthroplasty with a meniscal-bearing prosthesis and with retention of both cruciate ligaments caused no significant change in the PFF. After arthroplasty with a posterior-cruciate-retaining prosthesis and division of the anterior cruciate ligament, the PFF decreased in extension and increased by 20% in flexion. Implantation of a posterior stabilised prosthesis and division of both cruciate ligaments produced a decrease in the PFF in extension but maintained normal load in flexion. There was a direct relationship between the PFF and the angle made with the patellar tendon and the long axis of the tibia. The abnormalities of the patellar tendon angle which resulted from implantation of the two total prostheses explain the observed changes in the PFF and show how the mechanics of the patellofemoral joint depend upon the kinematics of the tibiofemoral articulation

Patellofemoral osteoarthritis (PFOA) affects 32% men and 36% women over the age of 60years and is associated with anterior knee pain, stiffness, and poor mobility. Patellofemoral arthroplasty (PFA) is a bone-sparing treatment for isolated PFOA. This study set out to investigate the relationship between patient-related outcome measures (PROMs) and measurements obtained from gait analysis before and after PFA. There are currently no studies relating to gait analysis and PFA available in the literature

A prospective cohort study was conducted of ten patients known to have isolated PFOA who had undergone PFA compared to a gender and age matched control group. The patients were also asked to complete questionnaires (Oxford knee score (OKS), EQ-5D-5L) before surgery and one year after surgery. Gait analysis was done on an instrumented treadmill comparing Ground reaction force parameters between the control and pre and post-operative PFA patients

The average age 60 (49–69) years with a female to male ratio of 9:1. Patient and healthy subjects were matched for age and gender, with no significant difference in BMI. Post-op PFA improvement in gait seen in ground reaction force at 6.5km/h. Base support difference was statistically significant both on the flat P=0.0001 and uphill P=0.429 (5% inclination) and P=0.0062 (10% inclination). PROMS response rate was 70%(7/10) pre-operative and 60%(6/10) post-operative. EQ-5D-5L scores reflected patient health state was better post-operatively.

This study found that gait analysis provides an objective measure of functional gait and reflected by significant quality-of-life improvement of patients post PFA. Literature lacks studies relating to gait-analysis and PFA. Valuable information provided by this study highlights that PFA has a beneficial outcome reflected by PROMs and improvement in vertical ground reaction force and gait

Further research is needed to assess how care-providers may use gait-analysis as part of patient care plans for PFOA patients.

Introduction and Objective. TKA have shown both excellent long-term survival rate and symptoms and knee function improvement. Despite the good results, the literature reports dissatisfaction rates around 20%. This rate of dissatisfaction could be due to the overstuff that mechanically aligned prostheses could produce during the range of motion. Either size discrepancy between bone resection and prosthetic component and constitutional mechanical tibiofemoral alignment (MTFA) alteration might increase soft tissue tension within the joint, inducing pain and functional limitation. Materials and Methods. Total knee arthroplasties performed between July 2019 and September 2020 were examined and then divided into two groups based on the presence (Group A) or absence (Group B) of patellofemoral overstuff, defined as a thickness difference of more than 2 mm between chosen component and bone resection performed, taking into account at least one of the following: femoral medial and lateral condyle, medial or lateral trochlea and patella. Based on pre and post-operative MTFA measurements, Group A was further divided into two subgroups whether the considered alignment was modified or not. Patients were assessed pre-operatively and at 6 months post-op using the Knee Society Score (KSS), Oxford Knee Score (OKS), Forgotten Joint Score (FJS), Visual Analogue Scale (VAS) and Range of Motion (ROM). Results. One hundred total knee arthroplasties were included in the present study, 69 in Group A and 31 in group B. Mean age and BMI of patients was respectively 71 and 29.2. The greatest percentage of Patellofemoral Overstuff was found at the distal lateral femoral condyle. OKS, KSS functional score, and FJS were statistically significant higher in patients without Patellofemoral Overstuff. Therefore, Group A patients with a non-modified MTFA demonstrated statistically significant better KSS, ROM and FJS. Conclusions. Patellofemoral Overstuff decrease post-operative clinical scores in patients treated with TKA. The conventional mechanically aligned positioning of TKA components might be the primary cause of prosthetic overstuffing leading to worsened clinical results. Level of evidence: III; Prospective Cohort Observational study;

Abstract. Objectives. There is renewed interest in bi-unicondylar arthroplasty (Bi-UKA) for patients with medial and lateral tibiofemoral osteoarthritis, but a spared patellofemoral compartment and functional cruciate ligaments. The bone island between the two tibial components may be at risk of tibial eminence avulsion fracture, compromising function. This finite element analysis compared intraoperative tibial strains for Bi-UKA to isolated medial unicompartmental arthroplasty (UKA-M) to assess the risk of avulsion. Methods. A validated model of a large, high bone-quality tibia was prepared for both UKA-M and Bi-UKA. Load totalling 450N was distributed between the two ACL bundles, implant components and collateral ligaments based on experimental and intraoperative measurements with the knee extended and appropriately sized bearings used. 95th percentile maximum principal elastic strain was predicted in the proximal tibia. The effect of overcuts/positioning for the medial implant were studied; the magnitude of these variations was double the standard deviation associated with conventional technique. Results. For all simulations, strains were an order of magnitude lower than that associated with bone fracture. Highest strain occurred in the spine, under the anteromedial ACL attachment, adjacent to transverse overcut of the medial component. Consequently, Bi-UKA had little effect on strain: <10% increases were predicted when compared to UKA-M with equivalent medial cuts/positioning. However, surgical overcutting/positional variation that resulted in loss of anteromedial bone in the spine increased strain. The biggest increase was for lateral translation of the medial component: 44% and 42% for UKA-M and Bi-UKA, respectively. Conclusions. For a large tibia with high bone quality, Bi-UKA with a well-positioned lateral implant had no tangible effect on the risk of tibial eminence avulsion fracture compared to UKA-M. Malpositioning of the medial component that removes bone from the anterior spine could prove problematic for smaller tibiae. Declaration of Interest. (a) fully declare any financial or other potential conflict of interest

Abstract. Background. Recurrent patellar dislocation in combination with cartilage injures are difficult injuries to treat with confounding pathways of treatment. The aim of this study is to compare the clinical and functional outcomes of patients operated for patellofemoral instability with and without cartilage defects. Methods. 82 patients (mean age-28.8 years) with recurrent patellar dislocations, who underwent soft-tissue or bony procedures, were divided into 2 matched groups (age, sex, follow-up and type of procedure) of 41 each based on the presence or absence of cartilage defects in patella. Chondroplasty, microfracture, osteochondral fixation or AMIC-type procedures were done depending on the nature of cartilage injury. Lysholm, Kujala, Tegner and Subjective Knee scores of both groups were compared and analysed. Complications and return to theatre were noted. Results. With a mean follow-up of 8 years (2 years-12.3 years), there was a significant improvement observed in all the mean post-operative Patient Reported Outcome Measures (p<0.05) of both the groups, as compared to the pre-operative scores. Comparing the 2 groups, post-operative Lysholm, Kujala and Subjective knee scores were significantly higher in patients operated without cartilage defects (p<0.05). 3 patients operated for PFJ instability with cartilage defects had to undergo patellofemoral replacement in the long term. Odds ratio for developing complications is 2.6 for patients operated with cartilage defects. Conclusion. Although there is a significant improvement in the long term outcome scores of patients operated for recurrent patellar dislocation with cartilage defects, the results are significantly inferior as compared to those without cartilage defects, along with a higher risk of developing complications and returning to theatre. 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

Abstract. Objectives. Patella resurfacing in primary total knee arthroplasty (TKA) remains a contentious issue. Australian rates of patellar resurfacing are 66.6%, significantly higher compared to UK rates of 8–15% and Swedish rates of 2%. Resurfacing has gained popularity in Australia since registry data has shown decreased revision rates with no increase in patellar component related complications. We present for discussion an analysis of 113,694 total knee arthroplasties using commonly implanted prostheses in the UK. Methods. We included all TKA's since the Australian register's conception on 01/09/1999 for a primary diagnosis of osteoarthritis involving the use of either the Triathlon or Duracon implant with and without patellar components. The primary outcome of the study was time to revision for Triathlon's resurfaced and non-resurfaced prosthesis compared to the Duracon's equivalent data. We also analysed the reasons for revision between the 4 groups, type of revision and complication rates. We then compared minimally stabilised and posterior stabilised prostheses. Results. The cumulative revision rate for Triathlon prostheses with resurfacing after 12 years was 3.2% (95% CI, 2.9% to 3.6%) compared to 5.6% (95% CI, 5.0% to 6.2%) without resurfacing. Duracon's equivalent data was 6.3% with resurfacing and 5.9% without resurfacing. Triathlon prosthesis with resurfacing have much lower rates of revision due to loosening, patellofemoral pain, patellar erosion compared to unresurfaced Triathlon prostheses. Conclusion. Triathlon with re-surfacing has lower revision rates regardless of age or BMI. Previous concerns regarding patellofemoral loosening, tibial wear, maltracking relate to Duracon only, indicating the importance of implant specific studies. 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

Background. Recurrent patellar dislocation in combination with cartilage injures are difficult injuries to treat with confounding pathways of treatment. The aim of this study is to compare the clinical and functional outcomes of patients operated for patellofemoral instability with and without cartilage defects. Methods. 82 patients (mean age-28.8 years) with recurrent patellar dislocations, who underwent soft-tissue or bony procedures, were divided into 2 matched groups (age, sex, follow-up and type of procedure) of 41 each based on the presence or absence of cartilage defects in patella. Chondroplasty, microfracture, osteochondral fixation or Autologous Matrix-Induced Chondrogenesis(AMIC)-type procedures were done depending on the nature of cartilage injury. Lysholm, Kujala, Tegner and Subjective Knee scores of both groups were compared and analysed. Complications and return to theatre were noted. Results. With a mean follow-up of 8 years (2 years-12.3 years), there was a significant improvement observed in all the mean post-operative Patient Reported Outcome Measures (p<0.05) of both the groups, as compared to the pre-operative scores. Comparing the 2 groups, post-operative Lysholm, Kujala and Subjective knee scores were significantly higher in patients operated without cartilage defects (p<0.05). 3 patients operated for patellofemoral instability with cartilage defects had to undergo patellofemoral replacement in the long term. Odds ratio for developing complications is 2.6 for patients operated with cartilage defects. Conclusion. Although there is a significant improvement in the long term outcome scores of patients operated for recurrent patellar dislocation with cartilage defects, the results are significantly inferior as compared to those without cartilage defects, along with a higher risk of developing complications and returning to theatre

Abstract. Objectives. The need for gender specific knee arthroplasty is debated. This research aimed to establish whether gender differences in patellar tendon moment arm (PTMA), a composite measure that characterises function of both the patellofemoral and tibiofemoral joints, are a consequence of knee size or other variation. Methods. PTMA about the instantaneous helical axis was calculated from positional data acquired using optical tracking. First, data post-processing was optimised, comparing four smoothing techniques (raw, Butterworth filtered, generalised cross-validation cubic spline interpolated and combined filtered/interpolated) using a fabricated knee. Then PTMA was measured during open-chain extension for N=24 (11 female) fresh-frozen cadaveric knees, with physiologically based loading and extension rates (420°/s) applied. Gender differences in PTMA were assessed before and after accounting for knee size with epicondylar width. Results. Combined smoothing enabled sub-mm accuracy (root-mean-squared (RMS) error 0.16mm, max error 0.47mm), whereas large errors were measured for raw (RMS 3.61mm, max 23.71mm), filtered-only (RMS 1.19mm, max 7.38mm) and interpolated-only (RMS 0.68mm, max 1.80mm) techniques. Before scaling, average PTMA throughout knee flexion was 46mm and mean, maximum, and minimum absolute values of PTMA were larger in males (mean differences >8mm, p<0.001), as were the PTMAs at terminal extension and flexion, and the change in PTMA from peak to terminal extension (differences >4mm, p<0.05). After scaling, the PTMA in deep flexion and the change in PTMA from peak to terminal extension were still larger in male knees (differences >2mm, p<0.05). The flexion angle of peak PTMA, unaffected by scaling, was closer to terminal extension for female knee (female 15°, male 29°, p<0.05). Conclusion. Gender differences in PTMA were identified both before and after accounting for knee size, with implications for gender-specific arthroplasty and musculoskeletal models. The developed measurement framework could also be applied in vivo for accurate measurement of the PTMA. 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

The Medial Patellofemoral Ligament (MPFL) is the main restraining force against lateral patellar displacement. It is often disrupted following patellar subluxation or dislocation. MPFL reconstruction is frequently performed when conservative management fails and the patient experiences recurrent patellar dislocation. Various MPFL reconstruction procedures have been described in the literature and reported outcomes are encouraging. This study analyses the radiographic outcomes following MPFL reconstruction. From January 2006 to January 2011, 76 consecutive patients (80 knees) with patellar recurrent dislocation underwent medial patellofemoral ligament reconstruction in three large teaching hospitals. Mean follow-up was 31.8 months (range, 13–72). Semitendinosus and gracilis autografts were used for the reconstruction and all procedures were carried out by the senior authors (WL, BR, CW, MB). Plain radiographs (Anteroposterior (AP), Lateral and Skyline) performed preoperatively and postoperatively were used to compare the sulcus angle, congruence angle, lateral patellofemoral angle, trochlear dysplasia (Dejour classification), trochlear boss height and patellar height (Caton-Deschamps ratio). Plain radiographs (Lateral) performed postoperatively were used to evaluate the femoral tunnel placement used for MPFL reconstruction. The sulcus angle improved from 143.2° (122.9–157.7) to 139.3° (115.7–154.6) and the congruence angle improved from 26.7° (−17.5–82.6) to 0.26° (−35.3–7.8). The lateral patellofemoral angle was 7.99° (3.2–19.2) preoperatively and 9.02° (3.2–18.2) postoperatively. The Caton-Deschamps ratio was 1.2 (1.0–1.5) preoperatively and 1.0 (0.8–1.1) postoperatively. Using the Dejour classification of trochlear dysplasia, all preoperative radiographs were considered to be grade C or D and all postoperative radiographs were considered to be grade A or B. Trochlear boss height was 5.9mm (1.8–11.6) preoperatively and 4.7mm (1.6–6.9) postoperatively. 59% of the femoral tunnels were considered to be in a good position on postoperative radiographs. This study displayed a significant improvement in postoperative radiographic parameters, demonstrating the importance of anatomic restoration when performing MPFL reconstruction

Introduction. Management of the patellofemoral surface in total knee arthroplasty (TKA) remains a topic of debate. Incidence of anterior knee pain and incidence of repeat operation have been the focus of several recent meta-analyses, however there is little recent data regarding patients” subjective ability to kneel effectively after TKA. The purpose of this study was to compare patient reported outcomes, including reported ability to kneel, after total knee arthroplasty with and without patellar resurfacing. Methods. Retrospective chart review of 84 consecutive patients who underwent primary TKA with patella resurfacing (56 knees) or without patella resurfacing (28 knees) having a minimum of 2.5 year follow up was performed. Oxford knee scores (OKS), visual analog pain scores (VAS), and questionnaires regarding ability to kneel were evaluated from both groups. Inability to kneel was defined as patients reporting inability or extreme difficulty with kneeling. Shapiro-Wilk test was used to determine normality of data. Mann Whitney U test was used to compare the OKS and VAS between groups. Chi square test was used to compare kneeling ability between groups. Statistical analysis was performed with SPSS version 23 (IBM, Aramonk, NY). Results. The 84 patients included 26 males and 58 females with average age 66.5 (range 46–91). Average follow up was 51 months (range 30–85). There was no significant difference in the percentage of female patients (64% vs 79%), age (67.8 vs 63.8), or reoperation rate (4% vs 7%) between the resurfaced and non-resurfaced groups. There was significantly longer follow up in the non-resurfaced group (57 vs 48 months). There was no statistically significant difference between the resurfacing and non-resurfacing group in terms of OKS (39 vs 38) or VAS (2.5 vs 3.0). However, those patients who did not have their patellofemoral joint resurfaced were more likely to report ability to kneel when compared to the resurfacing group (64% vs 39%, p=0.035). Kneeling ability was not correlated with duration of follow up, patient age or VAS. Kneeling ability was higher in female patients (57%) than males (27%), p=0.017. Discussion. There is concern for increased anterior knee pain and reoperation in patients whose patellae are not resurfaced. However, their failure to imnprove after revision to a resurfaced patella has left some room for depate as to whether or not the lack of resurfacing is the cause of their problems. This study did not show any increase in knee pain or reoperation between groups. There was an increased subjective ability to kneel in paients whose patellae were not resurfaced. This may have implications for the subset of paeitnts whose work or hobbies may require kneeling. There have been previous reports that subjective ability to kneel and actual ability may differ, and also that kneeling can be taught by a therapist. Our data also shows that female gender had a higher reported rate of kneeling

Background. Biomechanical joint contact pressure distribution measurements have proven to be a very valuable tool in orthopaedic research to investigate the influence of surgical techniques such as total knee arthroplasty (TKA) on the human knee joint. Quantification of the in vitro tibiofemoral and patellofemoral contact pressure distribution before and after the intervention are an important measure to evaluate the impact of the surgery. The K scan pressure sensor from Tekscan (South Boston USA) is a commonly reported device for these in vitro pressure measurements. Despite the large interest in the sensor, the effective measurement accuracy for in vitro biomechanical joint contact measurement still remains a big question and therefore the reliability of these measurements should be questioned. Methods. Reliable contact pressure measurements can only be done if the sensor behaviour is fully understood. Therefore, a tailored multi-axial testing machine has been designed to profoundly investigate and characterise the sensor behaviour. This test setup is unique through its ability to apply a predefined tangential force or sliding velocity to the sensor's interface next to a normal force. Dynamic effects occurring in knee joint motion can thus be simulated while evaluating the effect on the contact pressure measurements. Results. The change in contact friction coefficient by insertion of the sensor in the joint is quantified. Different interface conditions (dry, lubricated with PTFE spray, lubricated with surgical lubricant) have been evaluated to obtain the best sliding conditions and to minimise the undesired sensor accuracy deteriorating effects. Conclusion. An optimal calibration procedure is put forward and side-effects that deteriorate the measurement accuracy are quantified. The provided knowledge facilitates orthopaedic biomechanics researchers to optimally perform joint contact measurements and to estimate the effective measurement accuracy. In addition, the presented rig provides the opportunity to study the inherent knee kinematics and prosthesis shape optimisation through the inherent degrees of freedom in the rig

Distal femur resection for correction of flexion contractures in total knee arthroplasty (TKA) can lead to joint line elevation, abnormal knee kinematics and patellofemoral problems. The aim of this retrospective study was to establish the contribution of soft tissue releases and bony cuts in the change in maximum knee extension in TKA. Data were available for 211 TKAs performed by a single surgeon using a medial approach. Intra-operatively pre- and post-implant extension angles and the size of bone resection were collected using a commercial navigation system. The thickness of polyethylene insert and the extent of soft tissue release performed (no release, moderate and extensive release) were collected from the patient record. A linear model was used to predict change in maximum extension from pre- to post-implant. The analysis showed that bone cuts (p<0.001), soft tissue release (p=0.001) and insert thickness (p=0.010) were all significant terms in the model (r. 2. adj. =0.170). This model predicted that carrying out a TKA with 19 mm bone cuts, 10 mm insert and no soft tissue release would give 4.2° increase in extension. It predicted that a moderate release would give a further 2.8° increase in extension with an extensive release giving 3.9°. For each mm increase in bone cuts the model predicted an 0.8° increase in extension and for each mm increase in insert size a decrease extension by 1.1°. The modelling results show that in general to increase maximum extension by the same as an extensive soft tissue release that bone cuts would have to be increased by 4–5 mm. However this model only accounted for 17% of the variation in change in extension pre- to post-implant so may not be accurate at predicting outcomes for specific patients

Summary Statement. Computational models are the primary tools for efficient design-phase exploration of knee replacement concepts before in vitro testing. To improve design-phase efficiency, a subject-specific computational platform was developed that allows designers to assess devices in realistic conditions by directly integrating subject-specific experimental data in these models. Introduction. Early in the design-phase of new implant design, numerous in vitro tests would be desirable to assess the influence of design parameters or component alignment on the performance of the device. However, cadaveric testing of knee replacement devices is a costly and time-consuming procedure, requiring manufacture of parts, preparation of cadaveric specimens, and personnel to carry of the experiments. Validated computational models are ideally suited for pre-clinical, high-volume design evaluation. Initial development of these models requires substantial time and expertise; once developed, however, computational simulations may be applied for comparative evaluation of devices in an extremely efficient manner [Baldwin et al. 2012]. Still, computational models are complementary of experimental testing and for this reason, computational models tuned with subject-specific experimental data, e.g. soft tissue parameters, could bring even more efficiency in the design phase. The objective of the current study was to develop a platform of tools that easily allows for subject-specific knee simulations. The system integrates with commercially available medical imaging and finite element software to allow for direct, efficient comparison of designs and surgical alignment under a host of different boundary conditions. Patients & Methods. MRI image was acquired, and 3D bone models were generated using the Mimics Innovation Suite® (Materialise NV, Leuven, Belgium). The two models (1) tibiofemoral (TF) joint laxity including ligamentous constraint and (2) whole joint (TF and patellofemoral (PF)) mechanics during dynamic activities of daily living (e.g. gait, squat, chair-rise), developed in Abaqus/Explicit (SIMULIA, Providence, RI), were then be adapted with integrated subject-specific attachment sites. Results. The suite of tools provides a platform for baseline evaluation of design factors, comparison of new implant designs with predicate devices, and assessment of robustness to surgical alignment. This platform is currently capable of taking into account subject-specific factors in order to provide realistic results in relation with experimental data. Implant material properties, ligament properties and initial conditions can be varied, and results compared, to evaluate the influence of a host of design and surgical factors on implant performance. The interface allows users without complex finite element expertise to setup, analyze and compare devices and interpret results. Discussion/Conclusion. A platform which allows implant designers to evaluate their design ideas in realistic conditions integrating subject-specific parameters and to compare with predicate devices has the potential to substantially decrease the development time for new devices. Designers can perform iterative modification to their devices to focus on an optimal design solution prior to in vitro testing, reducing the number of pre-clinical cadaveric experiments that may be required, and ultimately improving TKR mechanics in the patient population

Summary Statement. A large proportion of knee arthroplasty patients are dissatisfied with their replacement. Significant differences exist between preoperative, postoperative and normal kinematics. A better understanding of the inter-relationships between kinematics, shape and prosthesis placement could lead to improved quality of life. Introduction. Knee kinematics are altered by total knee arthroplasty (TKA) both intentionally and unintentionally. Knowledge of how and why kinematics change may improve patient outcome and satisfaction through improved implant design, implant placement or rehabilitation. Comparing preoperative to postoperative kinematics and shape of the natural and replaced joint will allow an investigation of the inter-relationships between knee shape, prosthesis placement, knee kinematics and quality of life. Patients & Methods. Using a sequential-biplanar radiographic protocol that allowed imaging the preoperative and postoperative patellofemoral (PF) and tibiofemoral (TF) joints under weightbearing throughout the range of motion, we imaged and compared the 6 degree-of-freedom PF and TF kinematics of 9 pre-TKA subjects to those of 15 post-TKA subjects (Zimmer NexGen Legacy Posterior Stabilised Gender Solutions (GS) components). Using a novel computed tomography (CT) protocol, we obtained the femoral, tibial and patellar knee shapes, plus component placement after TKA. The same 9 pre-TKA subjects have now been re-imaged a minimum of one year postoperatively (DePuy Sigma Mobile Bearing cruciate-sacrificing components) to determine their changes in knee geometry and kinematics; full analysis is in progress. Results. Clear, statistically significant differences were seen between the kinematics of the pre-TKA and post-TKA groups. For the TF joint, the tibia was more posterior and inferior in the post-TKA group compared to the pre-TKA group (max 20 mm and 15 mm, respectively) (p<0.001). Subjects had neutral alignment in the post-TKA group compared to varus alignment (max 9°) in the pre-TKA group (p<0.001). For the PF joint, the patella was shifted more posteriorly and less laterally postoperatively and was tilted neutrally compared to laterally (p<0.001). Our preliminary analysis of the matched preop-postop subjects likewise shows a more posterior and inferior tibia and neutral versus valgus alignment. Greater tibial rotations were seen postoperatively due to the mobile bearing. The patella was more posterior and less lateral postoperatively, as seen with the two groups. Discussion/Conclusion. The kinematic differences seen are likely due to a combination of surgical, implant and patient factors. Both groups showed differences from normal kinematics, based on previous studies in the literature. In the future, by comparing the preoperative and postoperative kinematics, shape and quality of life for the same subjects (i.e. the 9 pre-TKA subjects in this study), and analyzing the interrelationships amongst these, we aim to determine if a different implant shape or different component positioning could create more normal kinematics, resulting in a better clinical outcome

Summary. Both endogenous lubricin and injectable hyaluronic acid reduced cartilage friction coefficients, but by distinct mechanisms. Lubricin operated in boundary mode and hyaluronic acid shifted lubrication to mixed or hydrodynamic mode. Introduction. Intra-articular injections of viscous agents and boundary lubricants have been presented as options to mitigate the progression of articular cartilage damage after the onset of osteoarthritis. 1,2. Mechanically, these injections are predicted to lower the friction coefficient within a load bearing joint and consequently slow the propagation of damage at the articular surface. Tribologically, boundary lubricants and viscous agents are hypothesised to be effective through different mechanisms affecting boundary-mode lubrication and transition to mixed-mode lubrication, respectively. By normalizing sliding speeds on a Stribeck curve, this study evaluated the efficacy of injectable hyaluronic acid (HA) supplements and endogenous lubricin to alter tribological properties. Methods. Cartilage samples were extracted from the patellofemoral groove of neonatal bovine. A custom-built tribometer was used to measure friction coefficients of cartilage sliding against polished glass while in a lubricant solution. Cartilage samples were compressed to 20% strain and the normal load was allowed to reach a steady-state value before sliding at speeds from 0.1 to 10mm/s. For some samples, endogenous lubricin was removed from the surface as described previously. 3. via incubation in 1.5M NaCl in PBS for 20 minutes followed by re-equilibration in PBS for 1hr. Samples were tested in bathing solutions of PBS (control), equine synovial fluid (ESF), 10mg/ml HA, and a hydrophobic HA derivative (HYADD). Results. Friction coefficients as a function of sliding speed for some lubricants are presented. Comparisons show that lubricin removal from the tissue surface increases friction coefficients when PBS is used as a lubricant (p<0.05). At slow sliding speeds there was no significant difference between PBS and 10mg/ml HA, but at higher speeds HA transitions to a reduced friction coefficient. The hydrophobic HA (HYADD) provides significantly reduced friction coefficients compared to regular HA for speeds up to 7mm/s. These trends can be explained mechanically by normalizing data to a Hersey number (sliding speed∗viscosity/normal load). The data curves are similar to Stribeck curves which are characterised by different lubrication modes: boundary, mixed, and hydrodynamic. The PBS data appear to be in boundary mode (characteristic of highest friction coefficients), and a transition to mixed mode lubrication appears to occur within the ESF and HA solutions. The most viscous forms of HA (HYADD) lowered the friction coefficient to ≤ 0.05, with an apparent minimum at a Hersey number of ∼10. −6. m, suggesting that this lubricant may have enabled hydrodynamic lubrication, a phenomenon not noted previously in this system. Lubricin removal increased friction coefficient from 0.16 to 0.25, occurring at slow sliding speeds and Hersey numbers of 10. −12. m. Conclusions. Endogenous lubricin and injectable hyaluronan both effectively lower friction coefficients, but do so by distinct mechanism. At the same operating conditions (normal loads and sliding speed), lubricin lowers boundary mode friction coefficient, while hyaluronan shifts behavior to mixed mode (HA) or hydrodynamic mode (HYADD). The combination of the presence of lubricin on cartilage surface in the most viscous formulation of HA (HYADD) lowered the friction coefficient of articular cartilage from 0.26 to 0.05