Total Knee Replacement: Understanding Ligament Removal and Preservation

What ligaments are cut during total knee replacement?

During total knee replacement (TKR), the anterior cruciate ligament (ACL) is almost universally removed, and the posterior cruciate ligament (PCL) is often sacrificed depending on the implant design, while the medial and lateral collateral ligaments (MCL and LCL) are typically preserved to maintain knee stability.

Understanding Total Knee Replacement (TKR)

Total Knee Replacement, or Total Knee Arthroplasty (TKA), is a major surgical procedure performed to alleviate severe knee pain and disability, most commonly due to advanced osteoarthritis. During this procedure, the damaged bone and cartilage surfaces of the femur (thigh bone), tibia (shin bone), and sometimes the patella (kneecap) are removed and replaced with artificial components made of metal alloys, high-grade plastics, and polymers. The goal is to restore function, reduce pain, and improve the patient's quality of life.

The Knee's Crucial Ligamentous Architecture

To understand which ligaments are affected during TKR, it's essential to first grasp the role of the knee's primary ligamentous structures in maintaining stability and guiding motion. The knee joint relies on four main ligaments:

• Anterior Cruciate Ligament (ACL): Located in the center of the knee, the ACL primarily prevents the tibia from sliding too far forward relative to the femur and limits hyperextension.
• Posterior Cruciate Ligament (PCL): Also located centrally, the PCL is stronger than the ACL and prevents the tibia from sliding too far backward relative to the femur.
• Medial Collateral Ligament (MCL): Situated on the inner side of the knee, the MCL resists valgus (inward) forces, preventing the knee from bending excessively inward.
• Lateral Collateral Ligament (LCL): Located on the outer side of the knee, the LCL resists varus (outward) forces, preventing the knee from bending excessively outward.

Together, these ligaments provide crucial static stability to the knee, guiding its complex rolling and gliding motions during activities.

Ligaments Typically Sacrificed in Total Knee Arthroplasty

The decision to cut or preserve specific ligaments during TKR is largely dictated by the design of the prosthetic components and the surgeon's strategy to achieve optimal joint mechanics and stability.

• Anterior Cruciate Ligament (ACL):

  • Almost Universally Removed: The ACL is nearly always cut during conventional total knee replacement. The design of the femoral and tibial prosthetic components typically occupies the space where the ACL would attach and function. Its presence would interfere with the proper seating and kinematic function of the artificial joint. The stability once provided by the ACL is then managed by the geometry of the prosthetic components, the preserved collateral ligaments, and, in some cases, a retained PCL or a PCL-substituting mechanism within the implant.

• Posterior Cruciate Ligament (PCL):

  • Variable Removal: The fate of the PCL depends significantly on the type of knee implant chosen by the surgeon:
    • Posterior-Stabilized (PS) Knee Designs: In these common designs, the PCL is removed. The implant itself incorporates a "cam" on the femoral component that engages with a "post" on the tibial component. This cam-and-post mechanism replicates the function of the PCL, preventing posterior translation of the tibia and promoting femoral rollback during knee flexion, which is crucial for a full range of motion.
    • Cruciate-Retaining (CR) Knee Designs: Some implant designs are specifically engineered to preserve the PCL. In these cases, the PCL continues to function, contributing to the knee's stability and kinematics. Surgeons may opt for a CR design if the patient's PCL is healthy, intact, and capable of functioning effectively after the replacement. The decision often hinges on the surgeon's preference, patient anatomy, and the quality of the PCL.

• Medial Collateral Ligament (MCL) and Lateral Collateral Ligament (LCL):

  • Rarely Cut (Typically Preserved): The MCL and LCL are fundamental for mediolateral stability of the knee and are almost always preserved during TKR. Their integrity is paramount for the stability of the new joint.
  • Exceptions for Release/Lengthening: In specific circumstances, such as severe pre-existing knee deformities (e.g., severe valgus or varus deformity), the surgeon may need to carefully release (cut or lengthen) or augment one of these ligaments to achieve proper alignment and balance of the joint. This is a delicate procedure, as excessive release can lead to instability. If a collateral ligament is severely damaged or incompetent prior to surgery, a more constrained type of implant may be necessary to provide the required stability.

Ligaments Preserved in Total Knee Arthroplasty

As highlighted, the preservation of certain ligaments is critical for the success of TKR:

• Medial Collateral Ligament (MCL): Essential for maintaining stability against inward forces.
• Lateral Collateral Ligament (LCL): Crucial for maintaining stability against outward forces.
• Posterior Cruciate Ligament (PCL): Preserved in Cruciate-Retaining (CR) implant designs, contributing to the knee's natural kinematics and proprioception.

Surgical Considerations and Ligament Balancing

A key objective of TKR is to achieve a stable, well-aligned, and pain-free knee with an excellent range of motion. This is heavily reliant on a process called soft tissue balancing. The surgeon meticulously adjusts the tension of the remaining ligaments (MCL, LCL, and potentially the PCL) and the joint capsule to ensure that the new prosthetic joint functions optimally throughout its entire range of motion, providing symmetric flexion and extension gaps. The choice of implant design, particularly concerning PCL retention or sacrifice, directly impacts this balancing act and the overall biomechanics of the reconstructed knee.

Post-Surgical Recovery and Rehabilitation

Following TKR, the body adapts to the altered ligamentous structure. The stability previously provided by the sacrificed ligaments is now primarily maintained by the new prosthetic components, the preserved collateral ligaments, and the strength of the surrounding musculature.

Post-operative rehabilitation is crucial. A structured physical therapy program focuses on:

• Strengthening: Building strength in the quadriceps, hamstrings, and other leg muscles to provide dynamic stability.
• Range of Motion: Restoring and maximizing the knee's flexibility.
• Proprioception: Re-educating the body's sense of joint position and movement, which can be affected by the removal of ligaments and altered joint mechanics.

Conclusion

Total knee replacement is a sophisticated surgical procedure that meticulously addresses the damaged components of the knee joint. While the anterior cruciate ligament is routinely removed, and the posterior cruciate ligament may or may not be, the integrity of the medial and lateral collateral ligaments is almost always maintained. This selective approach, combined with advanced prosthetic design and precise surgical technique, ensures that the reconstructed knee achieves optimal stability, function, and pain relief, allowing patients to regain mobility and improve their quality of life.