Total knee arthroplasty (TKA) is an effective treatment for arthritic knee pain, but patients may not return to their desired recreational or functional activity level post-TKA. The difference in functional outcome between patients who underwent TKA and patients with normal (nonarthritic) knees may be related to differences in kinematics between the normal and replaced knee. Posterior cruciate-retaining TKAs are associated with paradoxical motion in which the tibia is subluxed anteriorly in extension and the femur translates anteriorly during knee flexion. However, unicompartmental and patellofemoral arthroplasties, which retain the anterior cruciate ligament (ACL), however provide relatively normal knee kinematics and favorable knee function compared with conventional TKAs, which sacrifice the ACL. Early results with a bicruciate-substituting TKA suggest that ACL function is necessary to achieve more normal kinematics after TKA.