Failure mechanisms after unicompartmental and tricompartmental primary knee replacement with cement

Background: Concern exists regarding the durability of unicompartmental knee replacements. The purpose of the present study was to compare the early failure rates and failure mechanisms of primary cemented unicompartmental knee replacements with those of primary cemented tricompartmental total knee replacements. Methods: The rates of failure of primary cemented unicompartmental knee replacements (n = 2288) and tricompartmental total knee replacements (n = 3032) as reported to the Norwegian Arthroplasty Register from January 1994 through December 2004 were compared with use of Kaplan-Meier estimated survival rates and Cox multiple regression. Results: The ten-year survival probability was 80.1% (95% confidence interval, 76.0% to 84.2%) for unicompartmental knee replacements, compared with 92.0% (95% confidence interval, 90.4 to 93.6%) for total knee replacements, with a relative risk of revision of 2.0 (95% confidence interval, 1.6 to 2.5) (p < 0.001). This increased risk of revision following unicompartmental knee replacement was seen in all age-categories. Unicompartmental knee replacement was associated with an increased risk of revision due to pain (relative risk, 11.3 [95% confidence interval, 4.8 to 26.8]; p < 0.001), aseptic loosening of the tibial component (relative risk, 1.9 [95% confidence interval, 1.2 to 3.0]; p = 0.01) and of the femoral component (relative risk, 4.8 [95% confidence interval, 2.3 to 10.3]; p < 0.001), and periprosthetic fracture (relative risk, 3.2 [95% confidence interval, 1.2 to 8.9]; p = 0.02) as compared with total knee replacement. Unicompartmental knee replacement was associated with a lower risk of infection compared with total knee replacement (relative risk, 0.28 [95% confidence interval, 0.10 to 0.74]; p = 0.01). Conclusions: The survival of cemented unicompartmental knee replacements is inferior to that of cemented tricompartmental total knee replacements in all age-categories. Level of Evidence: Therapeutic Level II. See Instructions to Authors for a complete description of levels of evidence.