Does high-flexion total knee arthroplasty promote early loosening of the femoral component?

High-flexion knee replacements have been developed to accommodate a large range of motion (RoM > 120 degrees ). Knee implants that allow for higher flexion may be more sensitive to femoral loosening as the knee load is relatively high during deep knee flexion, which could result in an increased failure potential at the implant-cement interface of the femoral component. A 3D finite element knee model was developed including a posterior-stabilized high-flexion knee replacement to analyze the stress state at the femoral implant-cement interface during a full squatting movement (RoM </= 155 degrees ). During deep flexion (RoM > 120 degrees ), tensile and shear stress concentrations were found at the implant-cement interface beneath the proximal part of the anterior flange. Particularly, the shear stresses at this interface location increased during high flexion, from a peak stress of 4.03 MPa at 90 degrees to 6.89 MPa at 140 degrees of flexion. Tensile stresses were substantially lower, having a peak stress of 0.72 MPa at 100 degrees of flexion. Using data from earlier interface strength experiments, none of the interface beneath the anterior flange was predicted to fail in the normal flexion range (RoM </= 120 degrees ), whereas the prediction increased to 2.2% of the interface during deeper knee flexion. Thigh-calf contact reduced the knee forces, interface load, and failure risk beyond 140-145 degrees of flexion. Based on the more critical stresses at the femoral fixation site between 120 degrees and 145 degrees of flexion, we conclude that the femoral component has a higher risk of loosening at high-flexion angles