Abstract. Background. The primary objective of the study is to determine the function outcome and survivorship of patellofemoral replacement. The secondary aim is to find the determinants of successful/poor outcome. Methods. This retrospective study involved 45 patients who underwent AVON patellofemoral replacement between January 2015 to December 2020 with the minimal follow-up off for 12 months. The functional outcome was measured using Oxford Knee score (OKS), EuroQol five dimension (EQ-5D). IWANO and Kellgren-Lawrence classification was used to analyse radiographs. To identify determinants of outcome, the following subgroups the presence or absence of normal alignment, tibiofemoral arthritis, trochlear dysplasia and previous surgery. Complications and revision rates were also recorded. Results. The mean follow-up period was 41.7 +/− 8.3 months with no patients lost in follow-up. Patellofemoral replacement significantly improved the Oxford Knee score (OKS), EuroQol five dimension (EQ-5D) (p<0.001). Four out of 45(8.9%) patients underwent revision surgery. Patients with normal alignment preoperatively did worse than those with abnormal alignment with patellar instability. Patients with grade two tibiofemoral arthritis, history of previous surgery did significantly worse with poorer functional outcome. Conclusion. Patellofemoral arthroplasty is reliable treatment option which improves patient function and quality of life with good survivorship in isolated patellofemoral arthritis in mid-term follow-up

Aims

Debate remains whether the patella should be resurfaced during total knee replacement (TKR). For non-resurfaced TKRs, we estimated what the revision rate would have been if the patella had been resurfaced, and examined the risk of re-revision following secondary patellar resurfacing.

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

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

Aims

There has been a recent resurgence in interest in combined partial knee arthroplasty (PKA) as an alternative to total knee arthroplasty (TKA). The varied terminology used to describe these procedures leads to confusion and ambiguity in communication between surgeons, allied health professionals, and patients. A standardized classification system is required for patient safety, accurate clinical record-keeping, clear communication, correct coding for appropriate remuneration, and joint registry data collection.

The primary aim of this study was to compare the knee specific functional outcome of partial compared with total knee replacement (TKR) for the management of patellofemoral osteoarthritis. Fifty-four consecutive Avon patellofemoral replacements were identified and propensity score matched to a group of 54 patients undergoing a TKR with patella resurfacing for patellofemoral osteoarthritis. The Oxford knee score (OKS), the Short Form (SF-) 12 and patient satisfaction were collected (mean follow up 9.2 years). Survival was defined by revision or intention to revise. There was no significant difference in the OKS (p>0.60) or SF-12 (p>0.28) between the groups. The TKR group was significantly less likely to be satisfied with their knee (95.1% versus 78.3%, OR 0.18, p=0.03). Length of stay was significantly (p=0.008) shorter for the Avon group (difference 1.8 days, 95% CI 0.4 to 3.2). The 10 year survival for the Avon group was 92.3% (95% CI 87.1 to 97.5) and for the TKR group was 100% (95% CI 93.8 to 100). There was no statistical difference in the survival rate (Log Rank p=0.10). The Avon patellofemoral replacement have a shorter length of stay with a functional outcome and satisfaction rate that is equal to that of TKR. The benefits of the Avon need to be balanced against the increased rate of revision when compared with TKR

Aims. This study reports on the medium- to long-term implant survivorship and patient-reported outcomes for the Avon patellofemoral joint (PFJ) arthroplasty. Patients and Methods. A total of 558 Avon PFJ arthroplasties in 431 patients, with minimum two-year follow-up, were identified from a prospective database. Patient-reported outcomes and implant survivorship were analyzed, with follow-up of up to 18 years. Results. Outcomes were recorded for 483 implants (368 patients), representing an 86% follow-up rate. The median postoperative Oxford Knee Score (0 to 48 scale) was 35 (interquartile range (IQR) 25.5 to 43) and the median Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC, 0 to 100 scale) was 35 (IQR 25 to 53) at two years. There were 105 revisions, 61 (58%) for progression of osteoarthritis. All documented revisions were to primary knee systems without augmentation. The implant survival rate was 77.3% (95% confidence interval (CI) 72.4 to 81.7, number at risk 204) at ten years and 67.4% (95% CI 72.4 to 81.7 number at risk 45) at 15 years. Regression analysis of explanatory data variable showed that cases performed in the last nine years had improved survival compared with the first nine years of the cohort, but the individual operating surgeon had the strongest effect on survivorship. Conclusion. Satisfactory long-term results can be obtained with the Avon PFJ arthroplasty, with maintenance of patient-reported outcome measures (PROMs), satisfactory survival, and low rates of loosening and wear. Cite this article: Bone Joint J 2018;100-B:1162–7

Despite the successful, durable results, concern remains for using TKA in patients with isolated patello-femoral OA, as it requires an extensive surgical exposure and bone resection, a long recovery time, and a potentially more complex revision than that required for a patient with a failed patello-femoral arthroplasty (PFA).

PFA was introduced in the late 1970s. While PFA was successful in providing pain relief, the procedure did not gain widespread use because of initial design limitations that predisposed to PF maltracking, catching, and subluxation. The mechanical complications associated with first-generation PFA offset the potential advantages of maintaining the knee's native soft tissues and spurred efforts to improve implant design, and to refine surgical techniques and patient selection.

Over the past two decades, second generation PFAs incorporated changes in implant design and instrumentation and have shown promising results when used in the properly selected patient population. In addition, with improved instrumentation and robotics, adequate implant alignment and rotation can be achieved in the majority of patients, including those with severe patellofemoral dysplasia.

Our meta-analysis of TKA and PFA for the treatment of isolated patello-femoral OA showed that the rate of complications of patients undergoing PFA was 30% after a median follow up of 5.3 years, which is significantly higher than the 7% rate of complications in patients who underwent TKA. The most frequent type of complication associated with PFA was mechanical (including loosening and instability), which is consistent with the malalignment and maltracking-related failures previously reported. The incidence of re-operation after PFA (21%) was significantly higher than that seen after TKA (2%). The most frequent indication for re-operation after PFA was mechanical failure (7%), followed by progression of OA (6%), and persistent pain or stiffness (5%). The most common re-operations after PFA were conversion to TKA, revision of PFA components, lateral releases, open or arthroscopic debridement, manipulations under anesthesia, and bony and/or soft tissue extensor mechanism re-alignment procedures. In our study, 11% of patients treated with PFA underwent a revision arthroplasty, with 4% undergoing revision PFA and 7% undergoing conversion to TKA.

Our comparison of patients who were treated with second-generation PFA designs versus TKA showed no difference in the rate of complications, re-operation, or revision arthroplasty. Additionally, length of follow-up did not significantly influence any of these outcomes when comparing second-generation PFA and TKA.

These observations provide support for the use of current PFA designs. The mechanical complications and subsequent re-operations that affected first-generation PFA designs appear to be of less concern with proper patient selection, meticulous surgical technique, current implant designs and peri-operative care. While it is difficult to predict the survivorship of current PFA designs, it is our expectation that patient selection will continue to be a critical component in determining long-term results. The potential benefit of providing pain relief while preserving the tibiofemoral articulations makes PFA a promising treatment option.

Introduction

Epidemiologic studies indicate that isolated patellofemoral (PF) arthritis affects nearly 10% of the population over 40 years of age, with a predilection for females. A small percentage of patients with PF arthritis may require surgical intervention. Surgical options include non-arthroplasty procedures (arthroscopic debridement, tibial tubercle unloading procedures, cartilage restoration, and patellectomy), and patellofemoral or total knee arthroplasty (PFA or TKA). Historically, non-arthroplasty surgical treatment has provided inconsistent results, with short-term success rates of 60–70%, especially in patients with advanced arthritis. Although TKA provides reproducible results in patients with isolated PF arthritis, it may be undesirable for those interested in a more conservative, kinematic-preserving approach, particularly in younger patients, who may account for nearly 50% of patients undergoing surgery for PF arthritis. Due to these limitations, patellofemoral arthroplasty (PFA) has become utilised more frequently over the past two decades.

Introduction. The trochlea of a typical patellofemoral replacement or anterior flange of a total knee replacement usually extends past the natural trochlea and continues onto the femoral anterior cortex. One reason for this is that it allows a simple patella button to be permanently engaged in the trochlea groove in an attempt to ensure stability. On the natural patella, the apex helps to guide it into the trochlea groove as the knee moves from full extension into flexion. The aim is to study whether a generalised patella can be created that is close in form to a healthy patella. Method. MRI scans were taken of 30 patellae. Characteristics of these patellae (height, width, thickness, apex angle) were measured. The apex angle was found to be similar between patellae (mean=126 degrees, sd = 8.8), as were the ratios between height and width (mean width/height = 1.05, sd = 0.07) and between thickness and width (mean width/thickness = 1.8, sd = 0.19). These patellae were then segmented to create a surface including cartilage, resulting in 30 STL (stereolithography) files in which the surfaces are represented by triangle meshes. To design the average patella the individual patellae were aligned to a standard frame of reference by placing a set of landmarks on the proximal/distal, medial/lateral and anterior/posterior extents of each (fig.1). The vertical axis was defined as passing parallel to the proximal/distal points and the horizontal as passing parallel to the medial/lateral points when looking along the computed vertical axis. The origin centre of the frame of reference was chosen to be mid-way between these points. The mean width was then computed and each patella scaled linearly around the origin to give them all equal width. All the aligned patellae were then averaged together to provide a composite cartilaginous patella. The averaging process was achieved by taking one patella as a seed. The patella chosen for seed was that whose parameters were closest to the average width, height and thickness. An approximately normal vector was passed a point ‘P’ on the seeds, and the points at which these intersected the other models were then determined. The closest intersection point to ‘P’ on each model was chosen and these averaged together. ‘P’ is then replaced in the model with this average point. The averaging process then continues with all the remaining points on the seed model in the same manner to build the average models. Results and Discussion. The mean patella was compared with individual patellae. This comparison was performed by taking each point on the mean patella and finding the closest point on individual patellae - a colour coded map of differences was obtained (fig.2) along with a mean of the absolute difference for each patella. The absolute mean difference ranged from 0.56mm to 1.33mm, averaging at 0.85mm. This shows a reasonable fit between the average patella and each individual example, raising the possibility of using the average shape in future research to develop anatomical patellofemoral replacements and for planning patella resurfacing

To progress to a same day surgery program for arthroplasty, it is important that we examine and resolve the issues of why patients stay in the hospital. The number one reason is fear and anxiety for the unknown and for surgical pain. The need for hospital stay is also related to risk arising from comorbidities and medical complications. Patients also need an extended stay to manage the side effects of our treatment, including after-effects of narcotics and anesthesia, blood loss, and surgical trauma. The process begins pre-operatively with an appropriate orthopaedic assessment of the patient and determination of the need for surgery. The orthopaedic team must motivate the patient, and ensure that the expectations of the patient, family and surgeon are aligned. In conjunction with our affiliated hospitalist group that performs almost all pre-admission testing, we have established guidelines for patient selection for outpatient arthroplasty. The outpatient surgical candidate must have failed conservative measures, must have appropriate insurance coverage, and must be functionally independent. Previous or ongoing comorbidities that contraindicate the outpatient setting include: cardiac – prior revascularization, congestive heart failure, or valve disease; pulmonary – chronic obstructive pulmonary disease, or home use of supplemental oxygen; untreated obstructive sleep apnea – BMI >40 kg/m2; renal disease – hemodialysis or severely elevated serum creatinine; gastrointestinal – history or post-operative ileus or chronic hepatic disease; genitourinary – history of urinary retention or severe benign prostatic hyperplasia; hematologic – chronic Coumadin use, coagulopathy, anemia with hemoglobin <13.0 g/dl, or thrombophilia; neurological – history of cerebrovascular accident or history of delirium or dementia; solid organ transplant. Pre-arthroplasty rehabilitation prepares the patient for peri-operative protocols. Patients meet with a physical therapist and are provided with extensive educational materials before surgery to learn the exercises they will need for functional recovery. Enhancement of our peri-operative pain management protocols has resulted in accelerated rehabilitation. The operative intervention must be smooth and efficient, but not hurried. Less invasive approaches and techniques have been shown to decrease pain, reduce length of stay, and improve outcomes, especially in the short term. In 2014, 385 primary partial knee arthroplasty procedures (7 patellofemoral replacement, 13 lateral, and 365 medial) were performed by the author and his 3 associates at an outpatient surgery center. Of those, 348 (95%) went home the same day while 17 (5%) required an overnight stay, with 11 for convenience related to travel distance or later operative time and 6 for medical issues. Outpatient arthroplasty is safe, it's better for us and our patients, and it is here now. In an outpatient environment the surgeon actually spends more time with the patients and family in a friendly environment. Patients feel safe and well cared for, and are highly satisfied with their arthroplasty experience

It is important to remember that osteoarthritis is a noninflammatory condition that can affect 1, 2 or all 3 compartments of the knee. Moreover, this disease is a continuum from very mild to very severe involvement of the soft tissue, articular cartilage and bone. For this reason, a variety of nonsurgical and surgical options are indicated. The rheumatologist and/or orthopedist must understand the stage of the disease and fit that both to the pathology, age, activity level, and functional needs of the patient. For that reason, each of the options discussed today have an indication. The important issue about tricompartmental replacement is that we have improved technology and technique and the indications of today are broader than those of 20 years ago. Hopefully, they will continue to evolve both in terms of materials and instruments. The American Rheumatologic Association (ARA) has stated that joint replacement has been the major improvement in the care of the arthritic patient. The tricompartmental solution is the treatment of choice in patients with inflammatory arthritis such as rheumatoid arthritis as well as the solution in osteoarthritic patients with tricompartmental disease. There is an indication for osteotomy, unicompartmental replacement and perhaps patellofemoral replacement. I think the next frontier will be to find disease modifying osteoarthritic drugs (DMOADS) that will provide disease intervention as the DMARDs have done in rheumatoid arthritis. Moreover, cartilage repair combined with osteotomy will hopefully allow us to prevent progression of this disease

Isolated patellofemoral arthritis is not an uncommon problem, with no clear consensus on treatment. Nonoperative and many forms of operative treatments have failed to demonstrate long-term effectiveness in the setting of advanced arthritis. Total knee arthroplasty (TKA) has produced excellent results, but many surgeons are hesitant to perform TKA in younger patients with isolated patellofemoral arthritis. In properly selected patients, patellofemoral arthroplasty (PFA) is an effective procedure with good long-term results. Contemporary PFA prostheses have eliminated many of the patellar maltracking problems associated with older designs, and short-term results, as described here, are encouraging. Long-term outcome and prospective trials comparing TKA to PFA are needed.

Isolated patellofemoral arthritis is a common condition and there are varying opinions on the most effective treatments. Non-operative and operative treatments have failed to demonstrate effective long-term treatment for those in an advanced stage of the condition. Newer designs and increased technology in patellofemoral replacement (PFR) have produced more consistent outcomes. This has led to a renewed enthusiasm for this procedure. Newer PFR prostheses have addressed the patellar maltracking issues plaguing some of the older designs. Short-term results with contemporary prostheses and new technology are described here.

Cite this article: Bone Joint J 2013;95-B, Supple A:124–8.

The aim of this study was to review the early outcome of the Femoro-Patella Vialla (FPV) joint replacement. A total of 48 consecutive FPVs were implanted between December 2007 and June 2011. Case-note analysis was performed to evaluate the indications, operative histology, operative findings, post-operative complications and reasons for revision. The mean age of the patients was 63.3 years (48.2 to 81.0) and the mean follow-up was 25.0 months (6.1 to 48.9). Revision was performed in seven (14.6%) at a mean of 21.7 months, and there was one re-revision. Persistent pain was observed in three further patients who remain unrevised. The reasons for revision were pain due to progressive tibiofemoral disease in five, inflammatory arthritis in one, and patellar fracture following trauma in one. No failures were related to the implant or the technique. Trochlear dysplasia was associated with a significantly lower rate of revision (5.9% vs 35.7%, p = 0.017) and a lower incidence of revision or persistent pain (11.8% vs 42.9%, p = 0.045). . Focal patellofemoral osteoarthritis secondary to trochlear dysplasia should be considered the best indication for patellofemoral replacement. Standardised radiological imaging, with MRI to exclude overt tibiofemoral disease should be part of the pre-operative assessment, especially for the non-dysplastic knee. Cite this article: Bone Joint J 2013;95-B:793–7

Introduction

The optimal treatment for isolated patello-femoral arthritis is unclear. Patello-femoral arthroplasty (PFA) may offer superior knee function in isolated patello-femoral osteoarthritis compare to TKA. The literature is controversial for patient outcomes in PFA. Some reports showed improved outcomes while others were disappointing. We assessed our outcomes to try to identify causes for poor outcomes.

Ensuring correct rotation of the femoral component is a challenging aspect of patellofemoral replacement surgery. Rotation equal to the epicondylar axis or marginally more external rotation is acceptable. Internal rotation is associated with poor outcomes. This paper comprises two studies evaluating the use of the medial malleolus as a landmark to guide rotation. We used 100 lower-leg anteroposterior radiographs to evaluate the reliability of the medial malleolus as a landmark. Assessment was made of the angle between the tibial shaft and a line from the intramedullary rod entry site to the medial malleolus. The femoral cut was made in ten cadaver knees using the inferior tip of the medial malleolus as a landmark for rotation. Rotation of the cut relative to the anatomical epicondylar axis was assessed using CT. The study of radiographs found the position of the medial malleolus relative to the tibial axis is consistent. Using the inferior tip of the medial malleolus in the cadaver study produced a mean external rotation of 1.6° (0.1° to 3.7°) from the anatomical epicondylar axis. Using the inferior tip of the medial malleolus to guide the femoral cutting jig avoids internal rotation and introduces an acceptable amount of external rotation of the femoral component

Introduction. Patellar tracking in total knee replacements has been extensively studied, but little is known about patellar tracking in isolated patellofemoral replacements. We compared patellar tracking and the position of the patellar groove in the natural knee, followed by implantation of the femoral component of a PFR (patella unresurfaced) and after implantation of the femoral & patellar component of the PFR. Methods. Computer navigation was used to track the patella in eight whole lower extremities of four cadavers in the natural knee, in the same knee with the femoral component of the PFR (PFR-P) and with the femoral and patellar component of the PFR (PFR+P, patella resurfaced) (Depuy Sigma PFR). The form and position of the trochlea in the natural knee and the patellar groove of the femoral component was also analysed. Values are means+/−SD, two tailed Student's t-test for paired samples. Results. With a PFR-P the patella had a slightly more lateral tilt (0.8+/−0.8° to 2.8+/−2.5° at 40–100° of flexion, p<0.05 vs. Nat), this was more pronounced with the PFR+P (2.0+/−0.7° to 4.9+/−1.8° at 20–90° flexion, p<0.05 vs. Nat., p<0.05 vs. PFR-P at 20–80° flexion). No differences in patella rotation were seen between the three groups. In the PFR-P group the patella tracked a little more medially compared to the natural knee (0.6+/− 0.7mm to 1.3+/−2.6mm, p<0.05 at 20°,80°,90° flexion). The difference was more pronounced after patella resurfacing (PFR+P) (2.1+/−2.0mm to 3.0+/−2.2mm, p<0.05 vs. Nat. at 10°–100°, p<0.05 vs. PFR-P from 10–100°). When analysed relative to the patellar groove of the trochlea/femoral component the patella in the natural knee tracked slightly lateral to the groove (2.0+/−1.7mm to 2.9+/−2.0mm at 50–100° p<0.05), so did the patella of PFR-P (2.0+/−2.3mm to 2.3+/−2.3mm at 60–90° flexion, p<0.05), whilst the PFR+P tracked right on the groove (0.6+/−3.7mm medially to 0.6+/−2.9mm laterally, p<0.05 vs Nat at 10–30° & 70–100°, p<0.05 vs. PFR-P at 10–100°). Distance from the patellatot the epicondylar axis was slightly larger in the PFR-P group (0.6+/− 0.7mm to 1.3+/−1.4mm, p<0.05 vs. Natu at 20,80 & 90°. This was more pronounced with patellar resurfacing (2.1+/−2.0 to 3.0+/−2.2mm, p<0.05 vs. Nat at 10–100°, p<0.05 vs. PFR-P at 20–100°) The patella groove on the natural knee and the implanted femoral component of the implanted PFR had the same radius, inclination relative to the femoral mechanical axis, antero-posterior position and medio-lateral orientation. As intended by the designers the groove of the patellar component extended about 13mm further superiorly and 0.5mm more inferiorly. Discussion. The patella groove on the femoral component of the PFR reproduces the natural trochlear anatomy well. Patella tracking in the PFR-P shows only minor differences compared to the natural knee. Resurfacing of the patella in the PFR+P group causes the patella to tilt a little more laterally and track a little more medially, the distance to the epicondylar axis is slightly larger but this allows the patella to engage better in the patellar groove of the femoral component

INTRODUCTION. Patellofemoral joint (PFJ) replacement is a successful treatment option for isolated patellofemoral osteoarthritis. With this approach only the involved joint compartment is replaced and the femoro-tibial joint remains intact. Minimizing periprosthetic bone loss, which may occur due to the stress shielding effect of the femoral component, is important to insure long-term outcomes. The objective of this study was to investigate, using finite element analyses, the effects of patellofemoral replacement on the expected stress distribution of the distal femur eventually leading to changes in bone density. METHODS. MRI images of a healthy knee were acquired, segmented and reconstructed into a 3D physiological model of the bony and cartilaginous geometries of distal femur and patella with patellar tendon and insertion of the quadriceps tendon. This model was modified to include PFJ replacements with either a Journey PFJ or a Richards II PFJ prosthesis, and a Genesis II TKA (Smith&Nephew, Memphis, TN). The prosthetic components were incorporated in the intact model based on the manufacturer's instructions or previously described surgical techniques (Figure 1). Cortical bone was modeled with orthotropic properties, while homogeneous linear isotropic elasticity was assumed for trabecular bone, cartilage, cement and femoral components materials. The patellar tendon was given Neo-Hookean behavior. UHMWPE patellar buttons for all designs were assigned non-linear elasto-plastic material. The simulated motion consisted of a 10 second loaded squat, starting from 0° until a flexion angle of 120° matching experimental kinematics tests performed in previous in-vitro analysis on physiological cadaveric legs [1-2]. The patella model was constrained fixing the distal part of the patellar ligament and applying a quadriceps force distributed on the quadriceps insertion on the proximal surface of the patella. During the dynamic simulation the average Von Mises stress was calculated in two regions of interest (ROI) defined in the femoral bone: one anterior and one proximal. The location of the ROIs was defined to fit the same regions as used in a previous bone mineral density analysis following patellofemoral arthroplasty (height 1cm, length 1cm). RESULTS AND DISCUSSION. Overall, the average bone stresses in both ROIs increased with flexion angle. Maximal stresses during squat were reached at 90° flexion angle, (2.8–3.8 MPa for the anterior ROI and 1.4–1.6 MPa for the proximal ROI). Mean stresses in the proximal ROI were similar for both PFJ designs and the physiological model, and slightly lower for the TKA. Between 80° and 120°, anterior ROI bone stresses for Journey PFJ design were comparable to the physiological knee, while reduced by almost 25% for the other designs (Figure 1). These results suggest a different stress-shielding behavior depending on design geometry and material properties. CONCLUSIONS. This study evaluated periprosthetic bone stress distributions of different patellofemoral replacements. The numerical analyses of physiological and replaced knee models predicted a decrease in stress behind the anterior flange of the femoral component for some designs. This reduction was dependent on prosthesis design geometry and materials properties