Total knee replacement (TKR) is a surgical procedure designed to alleviate pain and restore function in a severely damaged knee joint, most often due to advanced arthritis. The operation involves resurfacing the worn-out ends of the femur and tibia, replacing them with metal components and a plastic spacer. This process requires significant modification of the bone structure to accommodate the new implant. A frequent question concerns the fate of the native stabilizing structures: the Anterior Cruciate Ligament (ACL) and the Posterior Cruciate Ligament (PCL). Understanding how the TKR addresses these ligaments is key to grasping how the artificial joint achieves stability and motion.
The Crucial Role of the ACL and PCL in Knee Stability
The knee joint relies on ligaments to maintain stability. The ACL and PCL form a cross shape, known as the cruciates, within the center of the joint. The Anterior Cruciate Ligament prevents the tibia (shin bone) from sliding too far forward relative to the femur (thigh bone). It also provides important rotational stability to the knee. The Posterior Cruciate Ligament is the stronger of the two and is positioned at the back of the joint. Its main role is to prevent the tibia from excessive posterior displacement, stopping the shin bone from slipping too far backward underneath the femur. Both ligaments work together to ensure that the femur and tibia track correctly during flexion and extension.
Universal Removal of the Anterior Cruciate Ligament (ACL)
In nearly all standard Total Knee Replacement procedures, the Anterior Cruciate Ligament is removed, regardless of its original condition. This removal is dictated by the biomechanics and design of the prosthetic components. The surgical cuts required to prepare the femur and the tibia for the new implant inevitably disrupt the ligament’s attachment points. The artificial tibial tray is designed to cover the area where the ACL naturally inserts. Even if the ligament were intact, its tension would become unpredictable after the bone cuts, negatively affecting the joint’s new mechanics. Therefore, the removal of the ACL is standard practice, and the stability it once provided is functionally replaced by the geometry and soft tissue balancing of the new joint components.
Management of the Posterior Cruciate Ligament (PCL)
The management of the Posterior Cruciate Ligament is the main difference between the two primary types of total knee replacement implants. Unlike the ACL, the PCL is often retained because its attachment points are typically preserved during the standard bone cuts. The decision to keep or remove the PCL determines whether a Cruciate-Retaining (CR) or a Posterior-Stabilized (PS) implant is used.
Cruciate-Retaining (CR) TKR
In the PCL-retaining approach, the PCL is left intact to maintain its function of preventing posterior tibial translation. This is done when the ligament is healthy and correctly tensioned, theoretically offering kinematics closer to the natural knee motion. The implant components are designed to work harmoniously with the native ligament.
Posterior-Stabilized (PS) TKR
The alternative, the PCL-sacrificing or Posterior-Stabilized (PS) TKR, involves the deliberate removal of the PCL. This approach is often chosen if the PCL is already damaged, too stiff, or otherwise unsuitable for retention. When the PCL is removed, the implant itself must incorporate a feature to substitute for the lost stability.
How the Knee Prosthesis Restores Function
The stability and function once provided by the removed cruciate ligaments are restored through the mechanical design of the total knee components. In the PCL-sacrificing (PS) implant, a specific feature known as the “cam and post” mechanism is integrated into the prosthesis. The tibial component has a raised polyethylene structure called the post, and the femoral component has a curved surface known as the cam.
As the knee bends, the cam contacts the post. This interaction serves two primary purposes: it forces the femoral component to roll backward (femoral rollback), which is necessary for deep knee flexion, and it acts as a mechanical stop. This mechanism prevents the tibia from shifting too far backward, substituting for the function of the lost PCL. Even in Cruciate-Retaining (CR) designs, the overall geometry of the components provides inherent stability, limiting excessive motion and ensuring the new joint operates smoothly and securely.