134 Table 3 Angles measured on the medial side with respect to the anteroposterior diaphyseal axis for the femoral (a) and tibial (b) components; posterior angles between the axis of the implant and the J Orthopaed Traumatol (2011) 12:131–136 femoral (c) and tibial (d) diaphyseal axis according to the Knee Society Roentgenographic Evaluation System [11] a angle b angle c angle d angle All implants 94.4 (95% CI 93.9 –94.8 ) 89.1 (95% CI 88.7 –89.5 ) 3.6 (IC 95%:3.6 –3.9 ) 88.6 (95% CI 88.2 –88.9 ) Revisioned implants 93.5 (95% CI 91.5 –95.5 ) 87.9 (95% CI 86.2 –89.6 ) 3.7 (IC 95%:2.6 –4.8 ) 89.9 (95% CI 87.7 –90.1 ) Nonrevisioned implants 95.5 (95% CI 94.0 –95.1 ) 89.3 (95% CI 88.8 –89.8 ) 3.6 (IC 95%:3.1 –4.1 ) 88.8 (95% CI 88.4 –89.4 ) Fig. 1 Kaplan–Meier curve of survival rate with failure with implant revision as the endpoint. Time reported in months Failures and survivorship analysis Prosthesis failure requiring revision surgery was necessary in 20 cases (11.1%): 16 aseptic loosenings, two septic loosenings, one ligamentous severe instability and one supracondylic fracture. Cumulative average implant sur- vival rate at 15 years was calculated according to the Kaplan–Meier method. Taking failure with knee arthro- plasty revision as the endpoint, the cumulative average survival rate at 15 years was 90.6% ± 2% SD (Fig. 1). Taking an HSS score \60 points (corresponding to the limit between satisﬁed patients and unsatisﬁed patients) as the endpoint, the cumulative aerage survival rate at 15 years was 94.1% ± 2.4% SD (Fig. 2). The two different groups of patella substitution and conservation were com- pared for revision and patient satisfaction. The cumulative average survivorship at 15 years was calculated: taking implant revision for any reason as the endpoint, it was 81.3% ± 6.6% SD for TKA with patellar replacement and 83.8% ± 8.4% SD for TKA without patella replacement, even if the resurfaced patellar knee showed better survival rate at midterm (Fig. 3). Using a total score of B60 points as the endpoint, it was 93.6% ± 7.4% SD and 94.7% 123 Fig. 2 Kaplan–Meier curve of survival rate with Hospital for Special Surgery (HSS) total knee score of B60 as the endpoint (limit between satisﬁed and dissatisﬁed patients). Time reported in months Fig. 3 Comparison between resurfaced patella (group 1, continuous line) and unresurfaced patella (group 2, dotted line) total knee arthroplasties. Kaplan–Meier curve of survival rate with failure with revision as the endpoint. Time reported in months ± 8.7% SD, respectively (Fig. 4). In all these cases, there was no statistically relevant difference in terms of survival and patient satisfaction between the two methods (respec- tively, P = 0.916 and P = 0.210). Rates of failure and reintervention on the patella for patellar problems were