Reasons for Early Failure in Medial Unicondylar Arthroplasty. Radiographic Analysis on the Importance of Joint Line Restoration

Background: Survivorship of Unicompartimental Knee Arthroplasty (UKA) remains a drawback, especially compared to the outcome of Total knee Arthroplasty (TKA). However, this could be improved by identifying and correcting failure mechanisms. To this purpose, this study aims at exploring failure modalities of UKA, with particular focus on the role of Joint Line (JL) position and alignment as variable to be optimized for a successful outcome Material & methods: This study explores modes of failure in 266 medial UKAs, by analyzing the correlation between changes in the obtained alignment and the ideal JL position. In detail, a radiological comparison was performed between 24 failures and 24 matched controls, to determine the importance of UKA positioning in terms of femoro-tibial angle (FTA), tibial plateau angle (TPA), and posterior tibial slope (PTS). Results: Failure occurred for subsidence of the tibial component in two knees, unexplained pain in seven patients, aseptic loosening of the tibial component in eight, aseptic loosening of the femoral component in three, medial tibial fracture in one, and overall osteoarthritis progression in three. The radiographic analysis showed that statistically significant differences could be found in the failure group in terms of higher variation of FTA, PTS, and JL height with respect to the control group. Conclusion: A successful outcome after UKA is determined by a correct alignment in all planes, as demonstrated by the failures analyzed in our series: not appropriate coronal alignment, distal JL line positioning, and abnormal PTS were observed and correlated with the failed cases. Thus, based on the results of this study, it could be recommended that the JL position should be carefully controlled while implanting a UKA not only with regard to the coronal plane: in fact, attention should be paid on the implant component positioning in all planes.