Systematic review and meta-analysis of implant-specific outcomes in inlay and onlay patellofemoral arthroplasty for isolated patellofemoral osteoarthritis

Purpose: The purpose of this systematic review was to compare clinical outcomes and complication rates between inlay and onlay patellofemoral arthroplasty (PFA) for isolated patellofemoral osteoarthritis (PFOA). A secondary objective was to evaluate implant-specific performance among various prosthetic designs. Methods: A comprehensive literature search was conducted using PubMed/MEDLINE, EMBASE and the Cochrane Database to identify studies published between January 1980 and 20 January 2025. Studies were included if they reported functional outcomes and complication rates for inlay or onlay PFA with a mean follow-up of 5 years. The majority of included studies were Level III or IV observational studies; therefore, the strength of the evidence is limited and conclusions should be interpreted with caution. Results: Seventy-six studies met the inclusion criteria, encompassing 4484 patients and 5084 implants. Onlay prostheses were associated with significantly higher rates of good to excellent outcomes (88% vs. 76%, p < 0.001), lower complication rates (7% vs. 19%, p < 0.001), and lower revision rates (4% vs. 8% TKA conversions, p = 0.02) compared to inlay implants. Among onlay designs, the Avon (Stryker) and Gender Solutions (Zimmer) prostheses showed the best results, with 90% and 87% success rates, respectively. The Lubinus (Link) inlay prosthesis demonstrated the poorest performance, with high complication and revision rates. Patellar maltracking was more frequent in the inlay group (4% vs. 1%), though not statistically significant. Infection and aseptic loosening rates were negligible in both groups. Conclusion: Onlay PFA appears to offer clinical advantages over inlay designs in the treatment of isolated PFOA, with trends toward superior functional outcomes, lower complication and revision rates, and more consistent performance across models. However, these results should be interpreted in light of the predominance of non-randomized studies and the heterogeneity of the available literature. The findings support the preferential use of onlay implants, particularly the Avon and Gender Solutions designs. Implant selection should be guided by evidence-based performance and patient-specific anatomical considerations. Level of Evidence: Level IV.