The Influence of Implant Design

Traditional knee replacement designs utilize multiple discrete radii to define the sagittal plane curvature of the femoral component (Fig. 1). This geometry was derived to replicate the function of the natural knee joint. The broader distal radius articulating against a relatively congruent tibial insert provides increased constraint and reduced contact stress in the range of flexion used for high demand, high frequency activities such as gait. By transitioning to a smaller posterior radius in knee flexion, the conformity of the implant is reduced and the femur becomes more mobile on the tibial insert. This enables the necessary knee rotation and rollback to facilitate deep knee flexion. Many factors contribute to providing a stable knee, including both surgical technique and implant design. Balancing flexion and extension gaps, setting proper femoral rotation, and restoring posterior condylar offset are critical steps to provide knee stability. In addition, the design of TKR geometry should enhance the stability of the joint provided by the soft tissue envelope, particularly in midflexion. Recent fluoroscopic evidence has elucidated the relationship between implant design and knee stability, highlighting the need to address “paradoxical anterior femoral translation” in TKR design.