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Patellofemoral Arthroplasty Designs and Osteoarthritis Progression

Patellofemoral Arthroplasty Designs and Osteoarthritis Progression


This study aimed to evaluate clinical and functional outcomes, complication rates, implant survivorship, and the progression of tibiofemoral osteoarthritis (OA) in patients with isolated patellofemoral OA undergoing new inlay or onlay patellofemoral arthroplasty (PFA). The research also sought to compare different implant types and models where possible. A systematic literature search following PRISMA guidelines yielded 404 articles, with 29 meeting inclusion criteria.

The selected studies, encompassing randomized control trials, case series, case control studies, and cohort studies, were assessed for quality using the Methodological Index for Non-Randomized Studies (MINORS). Clinical and functional outcomes, patient-reported outcomes (PROMs), radiographic OA progression, complication rates, implant survival rates, pain, and conversion to total knee arthroplasty (TKA) rates were considered.

Results indicated that both inlay and onlay PFA designs demonstrated similar clinical and functional outcomes at short, medium, and long-term follow-ups. Both designs effectively improved postoperative pain, with no significant difference in postoperative visual analog scale (VAS) scores. Notably, the onlay design group exhibited a higher preoperative VAS, but no disparity in outcomes emerged postoperatively. In terms of radiographic progression of OA, the inlay trochlea design showed a lower rate compared to the onlay trochlea design.

In conclusion, this study revealed no discernible difference in functional or clinical outcomes between the new inlay and onlay PFA designs. Both designs exhibited improvements in various scores, although the onlay group demonstrated a higher rate of OA progression. These findings contribute valuable insights for clinicians considering PFA procedures in patients with isolated patellofemoral OA.


This study addresses the ongoing controversy surrounding patellofemoral arthroplasty (PFA) as a treatment for isolated patellofemoral osteoarthritis (OA), emphasizing the impact of patient selection, surgical technique, and implant choice on clinical outcomes. The historical evolution of PFA includes the 1955 introduction of the vitallium cell patella cap by McKeever, and contemporary PFA designs can be broadly categorized into inlay and onlay types.

First-generation inlay designs, exemplified by the Richard and Lubinus prosthesis in 1979, replaced only the worn cartilage without addressing the subchondral bone, yielding less satisfactory short-term outcomes and a high conversion rate to total knee arthroplasty (TKA). The second-generation onlay design, emerging in the 1990s, involved a trochlea prosthesis that fully replaced the anterior compartment of the knee, offering corrective possibilities for trochlea rotation or dysplasia.

Complications associated with onlay designs, such as patellar catching, anterior notching, overstuing, and increased bone loss compared to inlay designs, prompted the development of new-generation inlay trochlea implants. These advanced implants aim to replicate patellofemoral joint kinematics with reduced mechanical and patellofemoral complications, enhanced implant stability, and no alterations to soft tissue tension or the extensor mechanism.

The study’s primary objective is to present a systematic review reporting clinical and functional outcomes, complication rates, implant survivorship, and tibiofemoral OA progression following inlay or onlay PFA. Additionally, the research aims to compare different trochlea implant types and models where possible. The authors stress the limited availability of up-to-date studies comparing these outcomes across various trochlea designs and emphasize the potential contribution of their systematic review to enhancing patient management, functional and clinical outcomes, and overall patient satisfaction.


The authors conducted a systematic literature search following PRISMA guidelines, covering databases such as PubMed, Scopus, Embase, and Cochrane, to identify relevant studies published until November 11, 2022. The study protocol received registration and approval from Prospero (CRD42022330285). The inclusion criteria encompassed randomized control trials (RCTs), case series, case control studies, and cohort studies in English or German, published in peer-reviewed journals after 2010. Excluded were non-original studies, case reports, simulation studies, systematic reviews, or studies involving patients undergoing total knee arthroplasty (TKA) or unicompartmental arthroplasty (UKA) of the medial or lateral knee compartment.

In the title and abstract screening, the authors excluded surgical technique studies, abstract-only studies, those reporting outcomes after PFA with additional UKA, robotic PFA, or outcomes of the patellar components of TKA. They also excluded studies comparing PFA with TKA and those lacking preoperative data. The subsequent full-text analysis, performed by two authors, focused on articles presenting results in numerical data form. The eligibility criteria were maintained, and discrepancies were resolved by consulting a third author.

For quality assessment, the authors utilized the Methodological Index for Non-Randomized Studies (MINORS) for both non-comparative and comparative clinical intervention studies. The global ideal scores were 16 for non-comparative studies and 24 for comparative studies. Studies not meeting a score of at least 11 for non-comparative studies or at least 16 for comparative studies according to MINORS were excluded to enhance the systematic review’s quality.

Data extraction involved collecting information such as title, author names, study design, publication details, abstract, level of evidence, follow-up time, trochlea implant design, clinical and functional outcomes, revision rates, complication rates, conversion to TKA rates, progression of osteoarthritis (OA), and reported pain levels and patient-reported outcomes (PROMs). This data was organized into a Microsoft Excel spreadsheet.

Statistical Analysis

Quantitative data, expressed as continuous variables, were presented using means and standard deviations or medians and ranges, while categorical variables were conveyed through absolute and relative frequencies. Significance in statistical analysis was determined with a threshold of p < 0.05.


The comprehensive review involved a meticulous analysis of 29 selected studies encompassing 1,761 patients who underwent patellofemoral arthroplasty (PFA). The median age at the time of surgery was 53 years, reflecting a broad demographic range from 22 to 92 years. The diverse studies reported a myriad of clinical and functional outcomes using a range of scoring systems, such as OKS, KSS, KOOS, WOMAC, VAS, IKDC, AKP, HKS, IKS, HSS-PF, UCLA, among others.

The primary outcome measures revealed consistent improvement in postoperative scores for OKS and WOMAC across studies, demonstrating the efficacy of PFA in addressing patellofemoral osteoarthritis (OA). The analysis of range of motion (ROM) indicated an overall increase postoperatively, except for isolated instances reported by Al-Hadithy et al. and Ajnin et al., emphasizing the general positive impact of PFA on joint mobility.

Evaluation of Knee Society Score (KSS) across nine studies consistently demonstrated enhanced postoperative clinical and functional scores, corroborating the effectiveness of PFA in improving knee function and overall clinical outcomes. It’s noteworthy that different variants of KSS (1989 and 2011) were employed across studies, providing a comprehensive perspective on patient outcomes.

Various Patient-Reported Outcome Measures (PROMs) such as AKSS, Tegner score, Kujala score, Lysholm score, KOOS, SF-12, SF-36, UCLA, MKS, HKS, IKS, IKDC, and HSS-PF were reported in several studies. These PROMs collectively contributed to a holistic understanding of patient-reported experiences, consistently reflecting statistically significant improvements postoperatively.

The reduction in perceived pain, a critical aspect of the patients’ postoperative experience, was uniformly observed across nine out of ten studies. Feucht et al.’s comparison between onlay and inlay groups highlighted the impact of preoperative VAS values on postoperative outcomes, emphasizing the importance of thorough preoperative assessment.

Complication rates and implant survivorship demonstrated substantial heterogeneity across the studies, with patellar maltracking and anterior knee pain emerging as prevalent complications. Revision rates exhibited variability, ranging from low percentages to relatively higher figures, indicating the need for standardized reporting and further research in this aspect.

The progression of osteoarthritis (OA) and conversion to total knee arthroplasty (TKA) showcased considerable diversity in reported rates. Feucht et al.’s observation of a significant difference in OA progression between inlay and onlay designs underscores the importance of implant selection and merits further investigation.

In summary, the review provides a nuanced and multifaceted overview of PFA outcomes, emphasizing the need for standardized reporting, comparative research, and a comprehensive understanding of patient experiences to enhance the management and satisfaction of individuals undergoing PFA.


The primary conclusion drawn from this comprehensive review is that both onlay and inlay patellofemoral arthroplasty (PFA) exhibit satisfactory clinical and functional outcomes across short, medium, and long-term follow-ups. Notably, no discernible differences between these designs were identified, except for a single study by Feucht et al., which reported a minor and statistically insignificant advantage for the inlay design based on WOMAC and Lysholm scores. Both onlay and inlay designs effectively alleviated postoperative pain, with no significant variation in postoperative Visual Analog Scale (VAS) scores, despite the onlay group presenting higher preoperative VAS values.

The review highlighted substantial heterogeneity among studies concerning complication rates, implant survivorship, and revision rates. The predominant complications observed were patellar maltracking and anterior knee pain. Of particular interest was the observation regarding the progression of osteoarthritis (OA) in the tibiofemoral compartment. A statistically significant difference favoring the inlay group was noted in a comparison of inlay and onlay trochlea designs, emphasizing a potential advantage in OA management with the inlay approach.

While previous systematic reviews have explored patient-reported outcome measures (PROMs) and survivorship in patellofemoral arthroplasty, this review stands out for its comprehensive analysis of postoperative outcomes, including both onlay and new inlay designs. Prior reviews, such as Pisanu et al. and Lonner et al., primarily focused on first-generation inlay designs, potentially limiting the generalizability of their findings. Lonner’s analysis, based on the Australian National Joint Registry, underscored a higher cumulative revision rate for inlay compared to onlay designs over a five-year period.

However, this review acknowledges several limitations, notably the scarcity of studies directly comparing the new inlay trochlea prosthesis with the onlay design. Additionally, the retrospective nature of the majority of included studies introduces potential selection bias. The lack of mid- and long-term follow-ups for the new inlay prosthesis also underscores the need for cautious interpretation of its clinical and functional outcomes. Furthermore, the potential conflict of interest, with many authors having consultancy ties to the companies designing the prostheses, adds a layer of consideration.

In conclusion, this systematic review consolidates valuable insights for physicians, aiding in enhanced patient management, improved functional and clinical outcomes, and heightened patient satisfaction. The overarching finding is the absence of significant disparities in functional or clinical outcomes between the new inlay and onlay PFA designs, despite a slightly higher rate of osteoarthritis progression in the onlay design group.

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