Total Knee Arthroplasty (TKA), commonly known as total knee replacement, is a highly effective surgical procedure performed to alleviate pain and restore function in a joint severely damaged by conditions like advanced osteoarthritis. The procedure involves removing the damaged cartilage and bone ends and replacing the joint surfaces with artificial components, typically made of metal and a high-density plastic insert. Implant manufacturers have developed several distinct designs to address the complex biomechanics of the knee joint. One primary design philosophy is the Cruciate Retaining (CR) knee implant, which aims to preserve a significant structure of the patient’s original anatomy.
Understanding the Cruciate Retaining Design
The defining characteristic of the Cruciate Retaining design is the surgical preservation of the Posterior Cruciate Ligament (PCL). The PCL is a strong band of tissue connecting the femur (thigh bone) to the tibia (shin bone). Its function is to prevent the tibia from sliding too far backward in relation to the femur.
In a CR TKA, the surgeon leaves the PCL intact to continue its stabilizing role in the joint replacement. This retention is intended to provide a more natural feel and function post-operatively by leveraging the patient’s existing biological stabilizer. The Anterior Cruciate Ligament (ACL), which prevents the tibia from sliding forward, is typically removed in almost all total knee replacement designs, including the CR type. The prosthetic components compensate for the loss of the ACL.
The CR implant’s femoral component, which resurfaces the end of the thigh bone, features a smooth surface on its posterior aspect. This allows the retained PCL to interact directly with the new joint surfaces. By retaining this ligament, the implant system is considered less constrained than other options, relying partially on the body’s natural soft tissues for stability. The success of this design hinges on the PCL being healthy, functional, and properly tensioned after the artificial joint components are placed.
Mechanical Function and Post-Operative Kinematics
The preserved PCL controls the kinematics of the knee after a CR TKA. As the knee bends, the PCL tightens, pulling the femoral component backward on the tibial insert. This motion, known as “femoral rollback,” increases the effective range of motion by preventing the femur from running off the back of the tibia at deep flexion angles.
In a CR implant, the retained PCL facilitates this rollback, which is thought to provide a more natural feeling joint, especially during activities like climbing stairs. However, studies on CR TKA show that the knee’s movement pattern is not fully identical to a healthy, native knee. Some CR designs can exhibit “paradoxical anterior femoral movement” during mid-flexion, where the femur slides slightly forward instead of rolling back.
Despite this, the PCL provides mid-flexion stability and contributes to the overall dynamic stability of the new joint. This function relies heavily on achieving precise tensioning of the PCL during surgery. If the ligament is too tight or too loose, it negatively affects the post-operative movement and stability. The surgeon must balance the ligament tension to ensure the joint operates correctly throughout the full range of motion.
Comparison to Posterior Stabilized Implants
The primary alternative to the Cruciate Retaining design is the Posterior Stabilized (PS) implant, which operates on a different principle. The PS design requires the intentional removal of the patient’s PCL during surgery. The PS implant then substitutes the PCL’s function with an integrated mechanical component.
This mechanical stabilizer consists of a raised polyethylene post on the tibial insert that engages with a cam on the underside of the femoral component. As the knee flexes, the cam mechanism engages the post, forcing the femur to roll back on the tibia, mechanically replicating the action of the lost PCL. The trade-off between the two designs is clear: the CR design relies on biological ligament health, while the PS design relies entirely on the durability of its mechanical components.
The box cutout in the femoral component for the PS cam mechanism requires the surgeon to remove more bone from the distal end of the femur compared to a CR procedure. While the PS design may offer slightly greater maximum flexion and stability in cases of severe pre-existing knee deformity, the CR design is considered more bone-sparing. The choice often comes down to the surgeon’s preference, the health of the patient’s PCL, and the severity of the pre-operative joint deformity.
Candidacy and Long-Term Considerations
Candidacy for a CR implant depends on the integrity and health of the patient’s PCL. The ligament must be intact, free of significant disease or damage, and able to function effectively to stabilize the joint. Patients with severe knee deformities, such as substantial varus or valgus alignment, may not be suitable for a CR design because the PCL may be too contracted or stretched to be balanced correctly.
CR implants are associated with a lower risk of aseptic loosening compared to PS designs, which is a factor in implant survival. Because the CR design is less constrained, it places less stress on the cement-bone interface, potentially contributing to improved longevity. CR procedures typically involve shorter operating times and may carry a lower risk of complications, such as peri-prosthetic fractures. While the CR design is less invasive in terms of bone removal, the overall recovery time is generally similar to that of a PS implant.