Patella alta and patellar subluxation might lead to early failure with inlay patello-femoral joint arthroplasty.

PURPOSE: With the growing interest in resurfacing procedures, several new implants have been recently introduced for isolated patello-femoral joint arthroplasty (PFA). However, not much data are available for these new techniques or about the right indications for each type of implant. METHODS: Out of a retrospective cohort of 20 inlay PFA, 11 PFA with an elevated Insall-Salvati index and an increased patello-femoral congruence angle showed an initial satisfactory result, but presented thereafter with recurrent pain and "clunk" phenomena. They were all revised after a median time of 25 months (range 8-28 months) into an onlay technique PFA and analyzed for their failure mode and revision technique. RESULTS: Clinical symptoms such as clunking, as well as abraded areas craniolateral of the inlay implant found intraoperatively, were the main observations of this study. The modified Insall-Salvati index (mISI) was significantly higher in the revised knees compared to the unrevised (median 1.8 versus 1.6; p = 0.041). VAS and KSS significantly improved after revision (median VAS reduction in pain of 4.0 points, median KSS improvement of 20.0 points; p < 0.05). CONCLUSION: Patients with high-normal patellar height index or patella alta, as well as a craniolateral type of arthritis with additional lateralization, should be considered contra-indicated for an inlay technique PFA. They could be considered for a PFA system reaching further proximal into the distal femur. An onlay PFA can be an option for early revision of failed inlay implants. The clinical relevance of this study is that patella alta and patellar subluxation are more difficult to adjust for with an inlay PFJ component. LEVEL OF EVIDENCE: Level IV.