Femoral Component Rotation in Patellofemoral Joint Replacement: A Study Protocol for a Prospective Observational Study

Patellofemoral joint replacement is a successful treatment option for isolated patellofemoral osteoarthritis. The short and mid-term outcomes are related to malposition and unexplained pain. Whether external rotation of the femoral component in isolated patellofemoral joint replacement is required is unclear. The primary aim of this study is to determine the CT-measured femoral component rotation of patellofemoral joint replacement relative to the transepicondylar axis. The secondary aim is to correlate the CT-measured femoral component rotation with the clinical outcomes at 1-year follow-up as assessed with the KOOS questionnaire. We designed a prospective observational study with medical research ethics committee and institutional review board approval. A total of 40 patients who will be treated with patellofemoral joint replacement for isolated patellofemoral osteoarthritis will be included. Intra-operatively, rotation of the femoral component will be assessed using anatomical landmarks including the epicondylar axis, Whiteside's line, and lower leg axis. The aim is to insert the femoral component between 3 and 6 degrees external rotation relative to the transepicondylar axis. Two experienced musculoskeletal radiologists will measure the angle between the transepicondylar axis and the femoral component, two to three days after surgery. The primary outcome is the CT-based femoral component rotation of the prosthesis relative to the transepicondylar axis. The secondary outcome is the patient reported KOOS questionnaire at 1-year follow-up. Successful completion of this study will provide data on the actual amount of femoral component rotation in patellofemoral joint replacement, and its relationship with clinical results. (Netherlands Trial Register NTR4175).