Low local curvature index and history of previous surgery are risk factors for revision in focal metallic inlay implants

Background: Focal chondral defects are debilitating lesions with poor healing potential. Focal metallic inlay implants were developed as a salvage procedure, whose reoperation causes and risk factors for revision are still debatable. The aim of this study is to analyze the local subchondral curvature matching of focal metallic inlay implants and its effects on survival and clinical results.

Methods: Patients operated with a knee focal metallic inlay resurfacing implant between 2014 and 2017 were eligible. Surgery was indicated for painful, focal, full-thickness cartilage lesions that had failed alternative treatments. Inclusion criteria were patients treated for a lesion ≤ 5 cm² in the femoral condyle, aged 40-65 years, with complete surgical records and a knee CT scan. The curvature index (K_index) was calculated as the ratio of the mean curvature of the implant (K₁) to the mean curvature of the subchondral bone (K₂).

Results: Sixty-nine patients were included, of which 60.9% were female. Mean age was 54.8 ± 6.0. Seven patients (10.1%) underwent revision surgery. When adjusted for age and sex, lesion size was not significantly correlated to revision in a multivariate regression model, while previous surgery and smaller K index were. A positive history for previous surgery was significantly correlated with worse clinical outcomes in surviving patients.

Conclusion: A positive history of previous knee surgery and a low local curvature index are risk factors for revision after focal metallic inlay implant resurfacing. Patients with a history of knee surgery should be counseled on the advantages and disadvantages before undergoing a focal resurfacing procedure.