Femoral Component Position Can Affect Patellar Congruency After Medial Unicompartmental Knee Arthroplasty

Purpose The aim of this study was to clarify whether femoral component positions can affect the postoperative patellar congruency. Methods The preoperative and postoperative radiographic data at 1, 3, 6, and 12 months for all medial unicompartmental knee arthroplasties (41 knees in 35 patients) performed from November 2013 to December 2016 were analyzed. Standardized knee radiographs included standing anteroposterior and Merchant's views. Component positions were evaluated according to the lateral shift of the medial condyle (LSMC) and femoral valgus angle (FVA). Congruency was evaluated according to the patellar congruence angle (PCA), lateral patellar displacement (LPD), and lateral patellar tilting angle (LPTA). Changes in congruence after the operation and correlations between the component positions and congruence were assessed. Results PCAs in 22 knees, LPD in 27, and the LPTAs in 20 were greater at one year compared with the respective preoperative angles. The mean PCA, LPD, and LPTA for these knees were significantly greater at one month. Changes in the two parameters for the component position (LSMC and FVA) and changes in the three parameters for the patellar congruency (PCA, LPD, and LPTA) were significantly positively correlated. Conclusions Lateral and distal femoral placement of the femoral component is likely to worsen patellar congruency. The anatomical reconstruction of the medial femoral condyle with an appropriate placement is needed to avoid postoperative deterioration of patellar congruency due to overstuffing of the patellofemoral joint.