Prosthetic Kinematics: Cruciate Retaining Versus Posterior Stabilized Versus Medial Pivot

Abstract

There are a number of different total knee arthroplasty (TKA) implant options on the market today with different kinematic properties. Posterior-stabilized total knee systems sacrifice the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). The PS system relies on a cam-post mechanism to engage during flexion, forcing posterior femoral translation. This allows for improved deep flexion by both providing stability and decreasing posterior femoral impingement on the tibial component. Cruciate-retaining total knee arthroplasty follows the tenets of retaining an intact, appropriately tensioned PCL. Cruciate-retaining (CR) systems rely on the knee’s ligamentous balance and PCL to provide those same goals as described above. The medial pivot knee is cruciate sacrificing and available with or without posterior-stabilization (PS). The medial rotation (MR) design focuses on achieving both stability and the native knee’s “screw home mechanism” of internal tibial rotation in comparison to the femoral condyles through a highly concentric tibial polyethylene insert. The spherical medial femoral condyle and complimentary medial tibial tray with a slotted lateral compartment encourage both femoral rollback and the medial pivot axis of the normal knee. The differences and/or benefits of one type of system over the other have not been clearly and definitively born out in the literature. As long-term success of total knee arthroplasty has continued to improve since its inception and first implantation in the 1970s, the progression over the past decades has been with the end goal of achieving native knee motion through the weight-bearing flexion arc. PS implant designs may more reliably create posterior femoral rollback. However, none of the described TKA systems completely restore normal knee kinematics. Outcomes for the described implant categories have been good with survivorship reported as high as 90–95 % at 15–20 years. Despite this, further research is ongoing to better replicate the normal knee and restore the stability and flexion that both patients and orthopedic surgeons seek in a total knee arthroplasty.