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the Femoral Component?*Jorrit Zelle - Radboud University Nijmegen Medical Centre - Nijmegen, The NetherlandsMaarten De Waal Malefijt - Radboud University Nijmegen Medical Centre - Nijmegen, TheNetherlandsNico Verdonschot - Radboud University Nijmegen Medical Centre - Nijmegen, NetherlandsIntroduction*Email: j.zelle@orthop.umcn.nlHigh-flexion knee implants have been developed to accommodate a large range of motion(ROM > 120°) after total knee arthroplasty (TKA). In a recent follow-up study, Han et al. [1]reported a disturbingly high incidence of femoral loosening for high-flexion TKA. The femoralcomponent loosened particularly at the implant-cement interface. Highly flexed knee implantsmay be more sensitive to femoral loosening as the knee load is high during deep knee flexion[2], which may result in increased tensile and/or shear stresses at the femoral implant fixation.The objective of this study was to analyse the load-transfer mechanism at the femoral implantcementinterface during deep knee flexion (ROM ≤ 155°). For this purpose, a three-dimensionalfinite element (FE) knee model was developed including high-flexion TKA components. Zerothicknesscohesive elements were used to model the femoral implant-cement interface. Theresearch questions addressed in this study were whether high-flexion leads to an increasedtensile and/or shear stress at the femoral implant-cement interface and whether this would leadto an increased risk of femoral loosening.Materials & methodsThe FE knee model utilized in this study has been described previously [3] and consisted of aproximal tibia and fibula, TKA components, a quadriceps and patella tendon and a nonresurfacedpatella. For use in this study, the distal femur was integrated in the FE modelincluding cohesive interface elements and a 1 mm bone cement layer. High-flexion TKAcomponents of the posterior-stabilised PFC Sigma RP-F (DePuy, J&J, USA) were incorporatedin the FE knee model following the surgical procedure provided by the manufacturer. A fullweight-bearing squatting cycle was simulated (ROM = 50°-155°). The interface stressescalculated by the FE knee model were decomposed into tension, compression and shearcomponents. The strength of the femoral implant-cement interface was determinedexperimentally using interface specimens to predict whether a local interface stress-statecalculated by the FE knee model would lead to interface debonding.ResultsDuring deep knee flexion, tensile stress concentrations were found at the femoral implantcementinterface particularly beneath the anterior flange. Shear stress concentrations wereobserved at the interface beneath the anterior flange and the posterior femoral condyles. Thepeak tensile interface stress increased from 1.6 MPa at 120° of flexion to 5.5 MPa during deepknee flexion at the interface beneath the anterior flange. The peak shear stress was even higherat this interface location and increased from 4.1 MPa at 120° of flexion to 11.0 MPa at maximalflexion (155°). Based on the interface strength experiments, 5.8% of the interface beneath theanterior flange was predicted to debond at 120° of flexion, which increased to 10.8% duringdeep knee flexion.DiscussionObviously, the FE knee model utilized in this study contains limitations which may havefile:///E|/ISTA2010-Abstracts.htm[12/7/2011 3:15:47 PM]