Total Knee Replacement: Which Ligaments Are Cut, Preserved, and Why

Which ligaments are cut during TKR?

During Total Knee Replacement (TKR), the Anterior Cruciate Ligament (ACL) is almost always removed. The Posterior Cruciate Ligament (PCL) is often removed depending on the implant design, while the Medial Collateral Ligament (MCL) and Lateral Collateral Ligament (LCL) are typically preserved but may be released or lengthened to achieve proper knee balance.

Understanding Total Knee Replacement (TKR)

Total Knee Replacement, or knee arthroplasty, is a surgical procedure to resurface a knee damaged by arthritis or injury. It involves removing damaged bone and cartilage from the thigh bone (femur), shin bone (tibia), and kneecap (patella) and replacing them with prosthetic components made of metal alloys, high-grade plastics, and polymers. The primary goals are to alleviate pain, correct deformity, and restore knee function, allowing individuals to resume daily activities with improved mobility.

The Role of Ligaments in Knee Stability

Ligaments are strong, fibrous bands of connective tissue that connect bones to other bones, providing stability to joints. In the knee, four primary ligaments are crucial for maintaining stability and guiding motion:

• Anterior Cruciate Ligament (ACL): Prevents the tibia from sliding too far forward relative to the femur and limits rotational movements.
• Posterior Cruciate Ligament (PCL): Prevents the tibia from sliding too far backward relative to the femur.
• Medial Collateral Ligament (MCL): Provides stability to the inner (medial) side of the knee, preventing excessive valgus (knock-knee) stress.
• Lateral Collateral Ligament (LCL): Provides stability to the outer (lateral) side of the knee, preventing excessive varus (bow-legged) stress.

These ligaments work in conjunction with muscles and the joint capsule to ensure controlled, stable movement throughout the knee's range of motion.

Ligaments Typically Sacrificed in TKR

During a standard Total Knee Replacement, the following ligaments are commonly addressed:

• Anterior Cruciate Ligament (ACL): The ACL is almost universally removed during TKR. This is primarily because its presence would interfere with the placement and function of the prosthetic components. Modern knee implant designs are engineered to provide stability without the need for an intact ACL, relying instead on the geometry of the implant components and the preserved collateral ligaments.
• Posterior Cruciate Ligament (PCL): The fate of the PCL depends on the specific type of knee implant chosen by the surgeon.
  • PCL-Sacrificing Designs: Many TKR implants are designed to remove the PCL. This approach allows for greater flexibility in implant design, can simplify the surgical technique, and may be preferred in cases where the PCL is already diseased, stiff, or calcified due to arthritis. Stability is then provided by a cam-and-post mechanism within the prosthetic components.
  • PCL-Retaining Designs: Some TKR implants are designed to preserve the PCL. The rationale for PCL retention includes potentially better proprioception (sense of joint position), preservation of bone stock, and a more "normal" kinematic feel for some patients. However, the retained PCL must be healthy and balanced correctly, which can add complexity to the surgery. The debate between PCL-sacrificing and PCL-retaining designs continues within the orthopedic community, with both offering excellent long-term outcomes.

Ligaments Managed Based on Knee Stability and Deformity

While the cruciate ligaments are often removed, the collateral ligaments (MCL and LCL) are almost always preserved due to their critical role in mediolateral stability. However, their tension may be surgically adjusted:

• Medial Collateral Ligament (MCL): The MCL is usually preserved. In cases of severe valgus deformity (knock-knee), where the MCL is contracted and tight, it may need to be carefully released (e.g., through a technique called "pie-crusting" or partial release) to balance the knee and ensure proper alignment of the new joint. Complete cutting of the MCL is rare and generally avoided as it can lead to significant instability.
• Lateral Collateral Ligament (LCL): Like the MCL, the LCL is typically preserved. In cases of severe varus deformity (bow-legged), where the LCL (and often the popliteus tendon or posterolateral capsule) is contracted, these structures may be released to achieve proper alignment and balance. Complete cutting of the LCL is also rare due to the risk of instability.
• Other Soft Tissue Releases: Beyond the primary ligaments, surgeons may also release portions of the joint capsule, patellar retinaculum, or remove osteophytes (bone spurs) to optimize knee motion and balance.

The Concept of Ligament Balancing in TKR

Ligament balancing is a critical step during TKR. It involves ensuring that the soft tissues around the knee, including the collateral ligaments, are tensioned appropriately and equally in both flexion (bending) and extension (straightening). This precise balancing is essential for:

• Optimal Implant Function: Ensures the prosthetic components track correctly and wear evenly, prolonging the life of the implant.
• Knee Stability: Prevents the knee from feeling loose or unstable after surgery.
• Improved Range of Motion: Allows for comfortable and full movement of the new knee joint.
• Patient Satisfaction: Directly impacts the patient's perception of their new knee's feel and function.

The decision to release or adjust any ligament or soft tissue is made intraoperatively by the surgeon based on the specific anatomical presentation of the patient's knee and the degree of deformity.

Post-TKR Knee Stability and Function

Despite the removal of the ACL and often the PCL, the stability of the knee after TKR is maintained by several factors:

• Implant Design: The geometry and conformity of the artificial knee components themselves provide inherent stability.
• Preserved Collateral Ligaments: The MCL and LCL, when properly balanced, are crucial for mediolateral stability.
• Surrounding Musculature: The strength and coordination of the quadriceps, hamstrings, and calf muscles play a vital role in dynamic knee stability and function.
• Physical Therapy: Post-surgical rehabilitation focuses on strengthening these muscles, improving range of motion, and re-educating movement patterns to optimize the new knee's performance.

Conclusion: A Carefully Orchestrated Procedure

The management of ligaments during Total Knee Replacement is a precise and critical aspect of the surgery. While the ACL is routinely removed, and the PCL is often sacrificed depending on the implant design, the collateral ligaments are carefully preserved and balanced. This meticulous approach, combined with advanced implant technology and comprehensive rehabilitation, allows individuals undergoing TKR to achieve significant pain relief and restore functional mobility, ultimately enhancing their quality of life.