TKR is a procedure designed to resurface the ends of the thigh bone and shin bone, typically to treat joint damage caused by arthritis. The knee is a complex hinge joint that relies on four major ligaments to maintain alignment and function. The fate of these ligaments during surgery varies significantly, depending on the ligament’s purpose and the design of the artificial components being implanted. Understanding which structures are preserved or removed clarifies how the new knee achieves stability.
Ligaments Sacrificed During Surgery
The Anterior Cruciate Ligament (ACL) is almost universally removed during total knee replacement. This decision is technical, driven by the geometry of the prosthetic implants. The ACL’s attachment points interfere directly with the placement of the new metal and plastic components. For most patients, the ACL is already weakened by osteoarthritis. Even if intact, its removal allows the surgeon to make necessary bone cuts and properly align the new joint. Modern implant designs are engineered to provide the anterior-posterior stability that the ACL previously supplied.
Ligament Retention and Implant Design
The Posterior Cruciate Ligament (PCL) introduces the most variability in total knee replacement surgery. The decision regarding the PCL determines the type of implant used. Surgeons choose between two primary implant designs based on the health of the PCL.
One option is the Cruciate-Retaining (CR) design, where the PCL is kept intact. The PCL continues to function as a stabilizer, controlling the backward rolling motion of the thigh bone on the shin bone during bending. The CR implant is typically used when the PCL is healthy and the knee deformity is mild, utilizing the natural ligament for stability.
The alternative is the Posterior-Stabilized (PS) design, which involves removing the PCL. When the PCL is sacrificed, the implant must compensate for its absence. The PS implant features a built-in mechanism, often described as a post-and-cam, which engages as the knee bends. This mechanical interaction prevents the thigh bone from sliding too far forward, substituting the function of the removed PCL. This design is often necessary in cases of severe deformity or when the PCL is too damaged to function reliably.
The Role of Remaining Stabilizers
The Medial Collateral Ligament (MCL) and the Lateral Collateral Ligament (LCL) are the joint’s remaining static stabilizers and are always preserved in total knee replacement. These ligaments run along the sides of the knee and provide varus and valgus stability, preventing the joint from buckling inward or outward. Their health and proper tension are necessary for the success of the new joint.
The surgeon must achieve “soft tissue balancing” during the operation, which is directly related to the condition of the MCL and LCL. This balancing involves ensuring that the space, or gap, between the femoral and tibial components is rectangular in both extension and flexion. If the ligaments are too tight, the surgeon may perform a controlled release of the fibers. If the ligaments are too loose, the surgeon may use a thicker plastic insert or a more constrained implant design to achieve tension.
Post-Surgical Tension and Long-Term Stability
Long-term stability relies on the implant’s design and the proper healing of the retained ligaments and surrounding soft tissues. The MCL and LCL must heal at the correct length and tension established during the surgical balancing phase. If balancing was imperfect, the knee may feel either too loose or too tight, affecting function and potentially leading to long-term issues.
Physical therapy enhances dynamic stability following surgery. While the implant and ligaments provide static stability, the surrounding leg muscles must be strengthened to support the new joint during movement. Strengthening the quadriceps and hamstrings provides active support that complements the passive stability offered by the balanced MCL and LCL. A stable, well-balanced knee is less prone to excessive wear and component loosening, contributing to the replacement’s longevity.