A total knee replacement (TKR) is a common surgical procedure where the damaged surfaces of the knee joint are resurfaced with metal and plastic components. This complex resurfacing is designed to mimic the natural joint’s movement. The primary goals are to significantly reduce chronic pain caused by severe arthritis and restore functional mobility. Achieving these outcomes requires careful management of the surrounding soft tissues, including the ligaments.
Anatomy of the Knee and Its Key Ligaments
The knee’s stability and function rely on four major ligaments that connect the thigh bone (femur) to the shin bone (tibia). Two of these, the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL), are located deep within the joint and are known as the cruciates because they cross over each other. The ACL prevents the tibia from sliding too far forward beneath the femur, while the PCL prevents the tibia from sliding too far backward.
The other two major stabilizers are the medial collateral ligament (MCL) on the inner side of the knee and the lateral collateral ligament (LCL) on the outer side. These collateral ligaments are positioned on the sides of the joint. They prevent excessive side-to-side movement, keeping the joint stable during walking and standing.
Ligaments Always Removed During Standard TKR
In the vast majority of TKR procedures, the Anterior Cruciate Ligament (ACL) is routinely removed, even if it is healthy. Removal is necessary because the prosthetic components, particularly the metal covering for the femur, physically occupy the space where the ACL naturally attaches and functions.
Removing the ACL also provides the surgeon with necessary access to prepare the bone surfaces and properly position the implant components. This clear access allows for the precise bone cuts and component alignment necessary for a successful outcome. Furthermore, the ACL is often already degraded or non-functional in patients undergoing TKR for severe arthritis. Modern knee implants are specifically engineered to compensate for the absence of this ligament.
Ligaments That May Be Retained or Released
The management of the other three major ligaments—the PCL, MCL, and LCL—is determined by the type of implant selected and the need to achieve proper joint balance. The fate of the Posterior Cruciate Ligament (PCL) is the main factor differentiating TKR implant types. In a cruciate-retaining design, the PCL is preserved, relying on the healthy ligament to provide backward stability and assist with joint motion.
If the PCL is damaged, the patient has a significant knee deformity, or the surgeon prefers a different approach, a posterior-stabilized implant is used, and the PCL is removed. This approach, often called cruciate-sacrificing, replaces the PCL’s function with a specialized mechanism built into the implant.
In contrast, the Medial and Lateral Collateral Ligaments (MCL and LCL) are almost always preserved. These collateral ligaments are the primary restraints against side-to-side instability and are only removed if severely damaged. Instead, the surgery involves soft-tissue balancing, where the surgeon may release or tighten these ligaments to ensure the knee is stable and tracks correctly. For instance, a tight MCL in a patient with a bowed-leg (varus) deformity may be partially released to create equal tension.
How the Artificial Joint Provides Stability
When ligaments are removed or released, stability in the new joint is achieved through surgical technique and implant engineering. A major element is Soft-Tissue Balancing, where the surgeon adjusts the tension of the retained collateral ligaments and surrounding muscles. This balancing ensures the joint space is equal in extension and flexion, preventing side-to-side wobble and promoting implant longevity by loading the plastic bearing insert evenly.
The second mechanism involves the Implant Design, especially when the PCL is sacrificed. Posterior-stabilized implants feature a built-in stability system, often referred to as a “post and cam” mechanism. The post is a raised section on the plastic tibial insert, and the cam is a corresponding feature on the femoral component. As the knee bends, the cam engages the post, providing the necessary front-to-back stability. This engineered constraint effectively replaces the function of the removed ACL and PCL.