Acta Orthopaedica 2011; 82 (6): 685–691 689 A B Figure 4. Kaplan-Meier survival plots. A. With any reoperation as endpoint, the PFC group had a survival of 97% (95% CI: 92–100) after 8 years and the CKS group had a survival of 79% (66–92) (p = 0.02). B. With aseptic reoperation as endpoint, the PFC group had a survival of 97% (92–100) after 8 years and the CKS group had a survival of 85% (73–97) (p = 0.08). 10-year survival rates for the PFC prosthesis: 97% survival for aseptic loosening (Santini and Raut 2008) and 97% survival for revision with any reason other than infection as endpoint (Dalury et al. 2008). The functional results of the PFC pros- thesis in our study were also comparable to those in previous reports (Dalury et al. 2008, Harrington et al. 2009, Hanusch et al. 2010). The excellent clinical scores of the PFC prosthesis do not leave a lot of room for improvement, which is probably why recent RCTs have failed to show a superior design (Harrington et al. 2009, Choi et al. 2010, Rahman et al. 2010). Subtle dif- ferences in outcome after TKA require more sensitive instru- ments. It has been reported that patient-based questionnaires such as WOMAC and the KSS are subjective and largely influenced by pain (Terwee et al. 2006, Stratford and Kennedy 2006). Objective, functional tests may be a valuable additional tool in comparing TKA systems. We have previously shown that monitoring of both knee extension velocity and loading symmetry during sit-to-stand movements is objective and has good discriminative capacity (Boonstra et al. 2008). Similar performance-based measurements to quantify functionality in TKA patients have been reported by others (Podsiadlo and Richardson 1991, Su et al. 1998, Mizner and Snyder-Mackler 2005). In addition, we have to realize that the outcome after TKA not only depends on the type of implant; Fortin et al. (1999, 2002) stated that the preoperative status of the patient is the strongest determinant of functional outcomes after hip and knee surgery, and Noble et al. (2006) and Mahomed et al. (2002) emphasized the importance of the expectations of the patients. Nevertheless, our study showed an inferior outcome with the CKS design. Although different results have been published about the CKS prosthesis in the limited amount of literature that is available (Martucci et al. 1996, Gobel and Schultz 2008), a 79% survival rate for any reoperation after 8 years in our study is unacceptably low—which made us decide to stop further using the CKS implant system. The question remains as to why we found such a differ- ence between the PFC and the CKS prostheses, as the designs appear to be quite similar. Concerning the articular part of the prosthesis, the most prominent difference is the orientation of the patella groove. Although patellofemoral complaints are one of the complications after TKA, with the highest inci- dence (1–24%) and an important reason for revision surgery (Boyd et al. 1993, Healy et al. 1995, Harwin 1998), it seems to be illogical that only a more anatomical trochlea orientation in the CKS design would be responsible for a worse outcome. An important issue is the observation of bad fixation strength of the prosthetic components to the bone. It appeared that all femoral components of the failed CKS group were easy to remove. Only 2 revisions could be attributed to positive bacte- rial cultures; the other 3 revisions were defined as aseptic loos- ening after 13, 16, and 51 months. This high rate of aseptic loosening is uncommon, especially at this early stage (Bozic et al. 2005, Vessely et al. 2006, Schwartz et al. 2010). More- over, during the removal of the CKS prostheses there was an intact cement layer on the bones, indicating inadequate fixa- tion between the prosthesis and cement. Thus, we believe that an important problem of the CKS design is limited cement- metal interfacial strength of the femoral components. We therefore wondered what the reason could be for a weak cement-metal interface of the CKS components. We analyzed the differences in the backside of both designs. The femoral component of the PFC has fixation pegs in both condyles, whereas the CKS design uses only 1 central peg. 2 pegs might enhance the fixation relative to 1 central peg, but to