Unicompartmental knee arthroplasty (UKA) is becoming an increasingly popular option in single compartment osteoarthritis. As a result, diverse second operations including revisions to total knee arthroplasty (TKA) will also increase. The objective of this study is to investigate the distribution of causes of second operations after UKA. We retrospectively reviewed 695 UKAs performed on 597 patients between January 2003 and December 2011. Except in one case, all UKAs were replaced at the medial compartment of the knee. The UKAs were performed on 559 (80.4%) women's knees and 136 (19.6%) men's knees. The mean age at the time of UKA was 61.5 years. The mobile-bearing designs were those that were predominantly implanted (n = 628 mobile, 90.2%; n = 67 fixed). The mean interval between UKA and second operation was 14.1 months.Introduction:
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