Unicompartmental knee arthroplasty (UKA), a widely accepted surgical treatment for isolated compartmental osteoarthritis (OA), provides more natural knee kinematics and proprioception than total knee arthroplasty by retaining the cruciate ligament and meniscus, preserving bone stock, and avoiding disruption of the extensor mechanism.[1-3] Since Kozinn and Scott first developed UKA guidelines in 1989, patellofemoral joint (PFJ) degenerative changes have traditionally been regarded as a contraindication to UKA.
PFJ degeneration is a relatively common disease, influencing up to 24% and 11% of women and men >55 years of age, respectively, with symptomatic knee OA.[5-7] Ordinarily, PFJ degeneration is radiographically or intra-operatively diagnosed by osteophytes, joint space narrowing, and cartilage loss. Most studies use the above criterion and stratify PFJ degenerative changes into medial, lateral, and trochlear facets and show that patients with lateral facet degeneration have worse knee function than patients without it.[6,8] However, multiple studies from the Oxford Knee Group have demonstrated that PFJ degeneration does not affect outcome scores or survivorship of Oxford UKA, which is attributed to the dynamic nature of mobile-bearing articulations.[9-12] Because of the contradictory evidence, until today, whether PFJ degeneration represents a contraindication for UKA remains highly controversial.
This systematic review and meta-analysis aimed to assess the impact of PFJ degeneration on knee function and revision rates after medial UKA, which was stratified by PFJ degenerative facets (medial, lateral, trochlear, and unspecified), severe PFJ degeneration (bone exposed or not), and bearing type for UKA (mobile or fixed), providing a comprehensive synthesis of effect estimates and quality of evidence.
This systematic review and meta-analysis were conducted in accordance with the Assessing the Methodological Quality of Systematic Reviews guidelines [Supplementary Table 1, https://links.lww.com/CM9/B305] and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement [Supplementary Table 2, https://links.lww.com/CM9/B305], and they were registered in the International Prospective Register of Systematic Reviews (PROSPERO 2020: CRD42021229241).
Search strategy and selection criteria
PubMed, Embase, Web of Science, Cochrane, Ovid, MEDLINE, CNKI, and CSTJ (Chinese) were systematically searched (up to October 4, 2020) to obtain all literature focusing on the impact of PFJ degeneration on clinical outcomes after UKA using the following keywords and their synonyms: “patellofemoral,” “knee,” “unicompartmental,” and “arthroplasty” [Supplementary Table 3, https://links.lww.com/CM9/B305]. There was no limitation on publication dates or language. The references of the included studies and previous systematic reviews were also reviewed to identify other published or unpublished studies. Two investigators (L. W. and Q. C.) independently screened the titles and/or abstracts to decide the suitability of the studies for inclusion, using the following inclusion criteria: (1) involving patients who presented with preoperative radiological evidence of PFJ OA (osteophytes, joint space narrowing, subchondral sclerosis, and bone destruction) or intra-operative evidence of PFJ chondromalacia (at least a partial-thickness defect with fissures on the surface); (2) evaluating the impact of PFJ degeneration on clinical outcomes following primary medial UKA; (3) fully reporting the numbers of patients and involved knees and time of follow-up; and (4) cohort or case-control studies. Studies not reporting knee function scores, biomechanical or cadaveric studies, letters to the editor, reviews, meta-analyses, case series, or studies on non-PFJ degeneration were excluded. A third investigator (M. L.) was asked to review the articles where consensus on suitability was debated. All investigators then reviewed and confirmed the included articles.
Two investigators (L. W. and H. S.) independently extracted data onto a preformatted Microsoft Excel spreadsheet (2016 Microsoft Corporation 32-bit edition). The general study data were extracted in [Supplementary Table 4, https://links.lww.com/CM9/B305] (first author name, publication year, study design, sample size, PFJ degenerative facets, UKA types, time of follow-up, clinical outcome measures, and conclusions), and details regarding PFJ data were extracted in [Supplementary Table 5, https://links.lww.com/CM9/B305] (PFJ degenerative assessment approach, severity grade, score range, and PFJ degeneration excluding criteria). Outcome data for the Oxford knee score, Knee society score (KSS), and incidence of revision at each follow-up time point were extracted. When data were only presented in figure form (line chart, bar graph, or box-and-whisker plot), the Engauge Digitizer (v9.7) was utilized to extrapolate the mean value.
Quality of assessment
The quality of the included articles was assessed using the Newcastle-Ottawa scale (a maximum score of nine stars as points) by two investigators (L. W. and Q. C.) independently in Review Manager software (RevMan 5.3, Cochrane Collaboration), including the following eight items in three domains: selection (representativeness of the exposed, selection of the non-exposed cohort, and ascertainment of exposure), comparability (outcome of interest was not present at study and comparability of groups), and outcomes (assessment of outcome, length, and adequacy of follow-up). A maximum of two points can be allotted for comparability, one for baseline and the other for multiple regression analysis. Each item was finally rated as low risk, unclear risk, or high risk of bias. To evaluate the certainty/quality of evidence contributing to the pooled effect estimate for each outcome, the Grades for Recommendation, Assessment, Development and Evaluation (GRADE) approach was used as an objective measure of risk of bias, inconsistency, indirectness, imprecision, and other domains. Each piece of evidence was categorized on four quality ratings: high, moderate, low, and very low. Disagreements were resolved through discussion with the authors (B. S. and H. S).
Outcomes measures and data analysis
The meta-analysis was conducted using RevMan 5.3.5 and Stata software (15.1 Version 08 MAC). The primary outcomes were the OKS and KSS after UKA. The secondary outcome was revision rates. Then, the results were summarized using mean differences (MDs) with 95% confidence intervals (CIs) for continuous data and risk ratios with 95% CIs for dichotomous data. Stratified meta-analyses were performed according to the PFJ degenerative facets (medial, lateral, trochlear, and unspecified), severity (bone exposed or not), and type of UKA (medial/lateral and fixed-/mobile-bearing). In addition, to avoid one cohort being accorded more weight than others, the inclusion of the same cohort several times in the same analysis was avoided and the data closest to maximum follow-up time was used for analysis. A random-effects meta-analysis approach was used to give a more conservative estimate of the impact of PFJ degeneration on knee function after UKA, and heterogeneity was assessed by the Cochran Q test statistic and chi-squared tests with the I-squared (I2) statistic. Visual inspection of funnel plot asymmetry was used to address possible small study effects, and Egger test was used to evaluate publication bias. Sensitivity analysis was conducted to demonstrate: (1) whether a single study altered the outcomes for the OKS, KSS, or revision rate analyses; (2) whether random effects demonstrate a different magnitude of significance compared with the fixed-effects meta-analysis; and (3) whether PFJ degenerative severity altered the outcomes for the effects of degenerative facets on the OKS and KSS. Two-sided P values < 0.05 were considered statistically significant.
The initial systematic search retrieved 1599 articles. After the removal of duplicated studies and screening of titles and abstracts, 45 studies qualified for full-text extraction. Of these, 34 studies were included in this study based on our inclusion-exclusion criteria.[6,8,9,16-46] The PRISMA flow diagram summarizing the search results is presented in [Figure 1].
Quality of articles
Full details of the study quality assessment for clinical trials are provided in [Figure 2]. Among the 34 trials, the main issue was the high risk of bias due to lack of blinding of outcome assessment and follow-up not being long enough (>2 years). However, >50% of trials still had a low bias for these two items. Of the 34 studies, two studies[35,40] were not included in the meta-analysis due to poor quality. Similar results were observed in all the sensitivity analyses when compared with the original analysis without excluding lower quality studies.
Characteristics of included studies
The study characteristics are summarized and shown in [Supplementary Table 4, https://links.lww.com/CM9/B305]. All included studies were cohort studies (17 prospective and 17 retrospective), comprising 7007 knees (2267 with PFJ degeneration) of 6906 patients who underwent primary medial UKA (5762 mobile-bearing and 1145 fixed-bearing and 100 unspecified). A total of 17 studies excluded patients with severe PFJ degeneration (severe bone exposed, grooving, or bone on bone contact in the PFJ), 1 study excluded lateral PFJ degeneration, seven studies included patients with all PFJ degeneration types, and the remaining nine studies did not report PFJ degeneration exclusion criteria [Supplementary Table 5, https://links.lww.com/CM9/B305].
PFJ degenerative facets
A total of 19 studies assessed the effect of PFJ degenerative facets on the post-operative OKS in medial UKA. Of the 19 studies, eight studies excluded patients with severe PFJ degeneration, four studies included patients with all PFJ degeneration types, and the remaining seven studies did not report the including criteria about PFJ degeneration. Overall, PFJ degeneration significantly decreased the OKS in patients who underwent UKA (MD = −0.90, 95% CI: −1.37 to −0.44, I2= 40%, P < 0.01). After stratification by degenerative facets, only lateral PFJ degeneration significantly decreased the post-operative OKS (MD = −2.18, 95% CI: −2.86 to −1.50, I2= 2%, P < 0.01), and none of the other PFJ degenerative facets (medial and trochlear) decreased the OKS after UKA [Figure 3]. A total of 16 studies assessed the effect of PFJ degenerative facets on the post-operative KSS in medial UKA. Of the 16 studies, seven studies excluded patients with severe PFJ degeneration, four studies included patients with all PFJ degeneration types, and the remaining five studies did not report the including criteria about PFJ degeneration. Overall, PFJ degeneration significantly decreased the KSS in patients who underwent medial UKA (MD = −0.80, 95% CI: −1.57 to −0.04, I2 = 5%, P = 0.04). After stratification by degenerative facets, only lateral PFJ degeneration significantly decreased the post-operative KSS (MD = −2.61, 95% CI: −4.09 to −1.12, I2 = 0%, P < 0.01), and none of the other PFJ degenerativefacets (medialandtrochlear) decreasedtheKSS after UKA [Figure 4]. There is no study available on the association between revision rates and PFJ degenerative facets.
Severe PFJ degeneration
A total of four studies assessed the effect of severe PFJ degeneration (bone exposed) on the post-operative OKS, KSS, and revision rates; and no adverse effects were seen
[Figure 5]. Only one study excluded patients with severe lateral PFJ degeneration (lateral facet Iwano grade 3 or 4 wear and lateral patellar subluxation), and the remaining three studies included all patients with severe PFJ degeneration.
Bearing type for UKA
In medial mobile-bearing UKA, a total of 15 studies assessed the effect of PFJ degeneration on the postoperative OKS, KSS, and revision rates. Overall, PFJ degeneration significantly decreased the OKS (MD = −0.92, 95% CI: −1.43 to −0.41, I2 = 47%, P < 0.01) in patients who underwent medial mobile-bearing UKA, but no significant association was observed between PFJ degeneration and the KSS/revision rates. After further stratification by the PFJ degenerative facet, only lateral PFJ degeneration significantly decreased the OKS (MD = −2.21, 95% CI: −2.89 to −1.53, I2 = 1%, P < 0.01) and KSS (MD = −2.44, 95% CI: −3.85 to −0.93, I2 = 0%, P < 0.01), and no adverse effect of medial/trochlear PFJ degeneration on the OKS or KSS was observed [Figures 6 and 7].
In medial fixed-bearing UKA, a total of six studies assessed the effect of PFJ degeneration on the post-operative OKS, KSS, and revision rates. Overall, no adverse effect of PFJ degeneration was seen in the OKS, KSS, or revision rates; and subgroup analysis also did not yield statistically significant results [Figures 7 and 8].
Publication bias, sensitivity analyses, and grade of evidence
The funnel plots of meta-analyses did not show clear asymmetry [Supplementary Figure 1, https://links.lww.com/CM9/B305], and no evidence of a small study effect was observed by Egger test [Supplementary Table 6, https://links.lww.com/CM9/B305], implying no publication bias. In addition, sensitivity analysis showed that no single study altered the outcomes for the OKS, KSS, or revision rate analyses; the random-effects meta-analysis did not demonstrate a different magnitude of significance compared with the fixed-effects meta-analysis; and excluding studies with severe PFJ degeneration did not alter the outcomes for the effects of degenerative facets on the OKS and KSS [Supplementary Figures 1–4, https://links.lww.com/CM9/B305]. The rating of quality for the pooled data on the OKS, KSS, and revision rates was graded as very low to low, and effect estimates were downgraded mainly due to the observational studies included in this analysis [Supplementary Table 7, https://links.lww.com/CM9/B305].
The main finding of this meta-analysis is that slight to moderate degenerative changes in the PFJ, with the exception of the lateral facet, did not affect the clinical outcomes in UKA.
PFJ degenerative facets
Current data suggest that slight to moderate degenerative changes in the medial and trochlear facets of the PFJ did not affect the overall functional outcome after surgery. Notably, lateral PFJ degeneration was associated with decreased knee function, although it is not a highly prevalent condition, comprising approximately 6% of UKAs. Recent studies have suggested a plausible explanation of this finding by analyzing anatomical changes following UKA.[36,47] The correction of varus malalignment with medial UKA will restore PFJ congruence angles, which may offload the medial and trochlear PFJ, recover joint forces and mitigate symptoms related to PFJ degeneration. In contrast, this correction might load the lateral PFJ, especially during knee extension-flexion, and the impingement between the lateral femoral condyle and the lateral PFJ articular surface will aggravate the degeneration, leading to knee pain and functional decline. For this reason, the surgical technique and implant design used for lateral PFJ degeneration are important factors that need to be further considered.
Severe PFJ degeneration
Although counterintuitive, our results suggest that preoperative severe PFJ degeneration (bone exposed or not) did not impact overall functional outcomes and revision rates following UKA (three studies for medial mobile-bearing UKA and one study for medial fixed-bearing UKA), and there was no evidence to suggest that these revisions were related to PFJ degeneration, where only the study by Berger et al excluded patients with lateral PFJ degeneration, and the remaining four studies[30,41,45] enrolled patients with all PFJ degeneration types. Hamilton et al reported survivorship on 1000 mobile-bearing UKAs was similar between groups with (93.4%) and without (92.1%) PFJ bone exposed at a minimum 10-year follow-up. Thein et al also demonstrated that preoperative patellofemoral degeneration severity does not impact 2-year post-operative outcomes after robotically assisted fixed-bearing medial UKA. Therefore, it seems that bone exposed in the PFJ probably does not lead to failure, but the location of PFJ degeneration needs careful consideration. This surprising finding should be interpreted with caution due to the limited quantifiable data (only four studies were included).
Bearing type for medial UKA
To date, most published studies have assessed the impact of PFJ degeneration on clinical outcomes after mobile-bearing UKA. These studies demonstrated that PFJ degeneration has no significant impact on outcomes following UKA,[22,31] and because there was no impingement between the meniscus prosthesis and patella during knee extension-flexion, which reduced the wear on the polyethylene component, mobile-bearing UKA is theoretically believed to be more patella friendly and more suitable for patients with PFJ degeneration than fixed-bearing UKA.[48,49] However, other studies, with smaller numbers, have confirmed that fixed-bearing UKA also provides good long-term survival rates and clinical outcomes regardless of the presence of PFJ degeneration on radiographs or at surgery.[22,31,33] Fixed-bearing UKA could contribute to patients not being exposed to the additional risk of bearing dislocation.[10,50]
This review suggests that the clinical outcomes of both medial mobile- and fixed-bearing UKAs are not adversely affected by the presence of medial and trochlear PFJ degeneration (slight to moderate), and the status of the PFJ was not associated with higher revision rates in either mobile- or fixed-bearing implants. Therefore, fixed-bearing implants might follow the same logic that the correction of varus malalignment is sufficient to improve the symptoms and survivorship in patients with medial and trochlear PFJ degeneration.[47,51] However, subject to the insufficient number of studies on medial fixed-bearing UKA, while these initial results are encouraging, lateral PFJ degeneration does not have an adverse effect on knee function in medial fixed-bearing UKA and may change as a consequence of long-term, high-quality, and prospective studies. Given the success of mobile-bearing UKA, even in the presence of medial and trochlear PFJ degeneration (slight to moderate), it is important to further elucidate whether lateral PFJ degeneration has a similar negative effect on clinical outcomes in medial fixed-bearing UKA and whether indications for lateral UKA can also be expanded to include patients with PFJ degeneration involving the above facets.
Taken together, the historical literature, along with pooling of the current data, suggests that patients with medial and trochlear PFJ degenerative changes (slight to moderate) can safely receive medial UKA. These refined criteria might offer an opportunity to nearly double the number of patients fitted for medial UKA.
We acknowledge some of the limitations of this study. First, the retrospective literature included in our analysis causes some potential confounding factors that are difficult to eliminate. Second, six of the 34 studies included in this meta-analysis were written by the same authors. Although the research subjects did not overlap during analysis, there is still a need for data from more diverse research groups. Third, in the analysis of the adverse effects of PFJ degenerative facets, most studies excluded patients who had severe degenerative changes with excess bone loss and grooving in the PFJ. Therefore, this analysis only permits conservative conclusions to be drawn on knees with slight to moderate degenerative changes in the PFJ. Accordingly, the trend toward the results being worse with lateral PFJ degeneration still recommends that these patients need a more cautious approach.
For medial mobile-bearing UKA, patients with slight to moderate PFJ degenerative changes in the medial and trochlear facets could be considered candidates, as functional outcomes and revision rates were ideal. A cautious approach is needed in patients with lateral PFJdegeneration. For medial fixed-bearing UKA, although it might not be conclusive enough, functional outcomes or revision rates were not adversely affected by PFJ degeneration (regardless of the facet). It is important to further elucidate whether lateral PFJ degeneration has a similar adverse effect on clinical outcomes in medial fixed-bearing UKA.
This study was supported through grants from the National Natural Science Foundation of China (81802210 and 81672219), the Key Project of Sichuan Science & Technology Department (2018SZ0223 and 2018SZ0250), and the National Clinical Research Center for Geriatrics, West China Hospital, Sichuan University (Z20191008 and Z2018B20).
Conflicts of interest
All authors declare that they have no conflicts of interest with other people or organizations that could inappropriately influence this work.
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