Total Knee Replacement: Ligaments Removed, Preserved, and Surgical Considerations

What ligament is removed during total knee replacement?

During a total knee replacement (TKR), the Anterior Cruciate Ligament (ACL) is routinely removed. The fate of the Posterior Cruciate Ligament (PCL) depends on the specific design of the prosthetic implant chosen by the surgeon.

Understanding Total Knee Replacement (TKR)

Total Knee Replacement, or total knee arthroplasty, is a surgical procedure designed to alleviate pain and restore function in severely damaged knee joints. It involves resurfacing the ends of the bones (femur, tibia, and sometimes the patella) with prosthetic components made of metal alloys and high-grade plastics. The primary goal is to replace the worn-out articular cartilage and damaged bone, not necessarily to remove all ligaments. However, certain anatomical structures, including specific ligaments, must be removed to accommodate the prosthetic components and ensure optimal function of the artificial joint.

Key Ligaments of the Knee Joint

The knee is a complex hinge joint stabilized by a network of ligaments, which are strong bands of fibrous connective tissue. The four primary ligaments providing stability are:

• 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 side of the knee, resisting valgus (knock-knee) forces.
• Lateral Collateral Ligament (LCL): Provides stability to the outer side of the knee, resisting varus (bow-leg) forces.

Additionally, the menisci (medial and lateral) are crescent-shaped fibrocartilage discs that act as shock absorbers and help distribute weight across the joint, though they are not ligaments.

The Primary Ligament Removed: The Anterior Cruciate Ligament (ACL)

In nearly all total knee replacement procedures, the Anterior Cruciate Ligament (ACL) is removed. There are several biomechanical and practical reasons for this:

• Space Requirements: The prosthetic components, particularly the femoral and tibial implants, require significant space within the knee joint. The ACL lies directly in the path where these components are placed. Retaining it would obstruct the proper seating and alignment of the prosthesis.
• Functional Redundancy: The design of modern total knee implants inherently provides stability that mimics or replaces the function of the ACL. The interlocking surfaces and congruent design of the femoral and tibial components, along with the preserved collateral ligaments, provide the necessary anterior-posterior stability.
• Impediment to Range of Motion: An intact ACL could potentially impinge or become entrapped between the prosthetic components, limiting the knee's range of motion or causing pain after surgery.

Other Ligaments and Structures Affected

While the ACL is consistently removed, the status of other knee structures during TKR can vary:

Posterior Cruciate Ligament (PCL)

The fate of the Posterior Cruciate Ligament (PCL) is a key differentiator in total knee replacement surgical techniques and implant designs:

• Cruciate-Retaining (CR) Implants: In this design, the PCL is preserved. The prosthetic components are designed to work in conjunction with the PCL, allowing it to continue its role in providing posterior stability and influencing the rollback motion of the femur on the tibia during flexion. This design aims to replicate more closely the natural knee kinematics.
• Posterior-Stabilized (PS) Implants: In this more common design, the PCL is removed. To compensate for the loss of the PCL's function, posterior-stabilized implants incorporate a "post-and-cam" mechanism. A post on the tibial component engages with a cam on the femoral component as the knee flexes, providing the necessary posterior stability and promoting femoral rollback. This design is often chosen in cases of severe PCL damage or deformity, or when maximum correction of deformity is required.

Medial and Lateral Collateral Ligaments (MCL & LCL)

The Medial Collateral Ligament (MCL) and Lateral Collateral Ligament (LCL) are crucial for side-to-side (varus-valgus) stability of the knee. These ligaments are almost always preserved during total knee replacement. Their integrity is vital for the stability and proper function of the artificial joint. Surgeons carefully balance these ligaments during the procedure, sometimes releasing specific fibers or making minor adjustments to the bone cuts to ensure the knee is stable throughout its range of motion.

Menisci

The menisci (medial and lateral) are entirely removed during a total knee replacement. As they are damaged and degenerate in arthritic knees, and because the prosthetic components are designed to articulate directly with each other, the menisci no longer serve a purpose and would interfere with the new joint mechanics.

Surgical Rationale and Implant Design Considerations

The decision to remove or retain specific ligaments, particularly the PCL, is based on:

• Patient Anatomy and Pathology: The extent of arthritis, bone deformity, and pre-existing ligamentous damage.
• Surgeon Preference and Experience: Surgeons often have a preferred implant design based on their training and successful outcomes.
• Implant Design: The specific engineering of the prosthetic components dictates which ligaments can be preserved or must be removed.
• Goal of Surgery: Achieving optimal stability, range of motion, and pain relief while ensuring the longevity of the implant.

The precise balancing of the remaining ligaments and the appropriate selection of implant design are critical for the long-term success and functional outcome of a total knee replacement.

Post-Surgical Recovery and Rehabilitation

Following TKR, regardless of which ligaments were removed or retained, a structured rehabilitation program is essential. This program focuses on restoring range of motion, strengthening the surrounding musculature (quadriceps, hamstrings, calves), and improving proprioception and balance. The body adapts to the new joint mechanics, and the preserved collateral ligaments play a crucial role in the stability and function of the newly implanted knee.

Conclusion

While the Anterior Cruciate Ligament (ACL) is consistently removed during total knee replacement to facilitate implant placement and function, the Posterior Cruciate Ligament (PCL) may either be preserved or sacrificed depending on the prosthetic design (cruciate-retaining vs. posterior-stabilized). The Medial and Lateral Collateral Ligaments (MCL and LCL) are typically preserved and meticulously balanced to ensure stability of the new joint. Understanding these surgical considerations provides valuable insight into the intricate nature of total knee arthroplasty and the biomechanical principles guiding its success.