Total Knee Replacement: ACL's Fate, Prosthetic Design, and Stability

What is the ACL in TKR?

The Anterior Cruciate Ligament (ACL) is a vital stabilizing ligament within the knee joint; however, in the vast majority of Total Knee Replacement (TKR) surgeries, the ACL is removed because it is often diseased or compromised and its function is superseded by the design of the prosthetic implants.

Understanding the Anterior Cruciate Ligament (ACL)

The Anterior Cruciate Ligament (ACL) is one of the four major ligaments in the knee, connecting the femur (thigh bone) to the tibia (shin bone). Its primary role is to prevent the tibia from sliding too far forward relative to the femur and to limit excessive rotation of the knee joint. Together with the Posterior Cruciate Ligament (PCL), it forms a crucial "cross" within the knee, providing significant stability during movement.

While often associated with sports injuries (e.g., tears from pivoting or hyperextension), the ACL is also a critical component of the knee's natural kinematics and stability in everyday activities.

Total Knee Replacement (TKR): An Overview

Total Knee Replacement (TKR), also known as total knee arthroplasty, is a surgical procedure performed to relieve pain and disability caused by severe knee arthritis (most commonly osteoarthritis). During a TKR, the damaged cartilage and bone surfaces of the femur and tibia are removed, and the patella (kneecap) may also be resurfaced. These damaged surfaces are then replaced with metal and plastic components designed to recreate the knee's natural movement and function.

The primary goals of TKR are to:

• Alleviate chronic knee pain.
• Restore range of motion.
• Improve functional mobility and quality of life.

The Fate of the ACL During Total Knee Replacement

A common question arises regarding the fate of the ACL during a total knee replacement. In nearly all standard total knee replacement procedures, the Anterior Cruciate Ligament (ACL) is removed. There are several compelling reasons for this:

• Degenerative Changes: By the time a patient requires a TKR, the knee joint has typically undergone significant arthritic degeneration. The ACL itself is often compromised, stretched, or fibrotic due to the long-standing inflammatory process and mechanical changes within the arthritic joint. It may no longer be functionally intact or capable of providing meaningful stability.
• Prosthetic Design: The design of modern TKR implants is generally not conducive to ACL preservation. The femoral and tibial components are shaped to replace the articulating surfaces and often incorporate features (like a PCL-retaining or posterior-stabilized design) that effectively take over the ACL's stabilizing role. Attempting to preserve a compromised ACL could lead to impingement with the new prosthetic components or continued instability.
• Surgical Access: Removing the ACL provides better surgical access to prepare the bone surfaces for the implants, ensuring precise placement and alignment of the prosthetic components.

While the ACL is routinely removed, the Posterior Cruciate Ligament (PCL) may or may not be preserved, depending on the type of knee replacement implant used.

Types of Total Knee Replacement and Ligament Preservation

There are two primary categories of TKR implants based on how they manage the cruciate ligaments:

• Cruciate-Retaining (CR) Implants: These prostheses are designed to retain the Posterior Cruciate Ligament (PCL). The PCL's function is preserved to help provide stability and guide the femoral rollback during flexion, which can contribute to a more natural feel and potentially better proprioception. However, even in CR designs, the ACL is still typically removed.
• Posterior-Stabilized (PS) Implants: In these designs, both the ACL and the PCL are removed. To compensate for the loss of the PCL's stabilizing function, PS implants incorporate a "cam and post" mechanism. A post on the tibial component engages with a cam on the femoral component during flexion, preventing posterior subluxation of the tibia and providing stability. PS implants are often chosen when the PCL is severely damaged, diseased, or when the surgeon prefers this specific design.

It's important to note that ACL-sparing TKR is a niche and less common approach, typically reserved for highly specific cases or experimental designs. It is not the standard of care for most patients undergoing TKR.

Why ACL Removal is Standard in TKR

The standard practice of ACL removal in TKR is well-established due to:

• Pre-existing Pathology: The ligament is often already functionally compromised by the severe arthritis that necessitates the surgery.
• Mechanical Interference: The presence of the ACL can interfere with the proper seating and kinematics of the prosthetic components, potentially leading to impingement, abnormal wear, or instability.
• Redundancy of Function: The design of modern knee implants, particularly the articular surfaces and the PCL-retaining or PS mechanisms, effectively replaces the stabilizing function of the native ACL. The new joint surfaces are designed to articulate smoothly and provide inherent stability.
• Surgical Efficiency: Removing the ACL simplifies the surgical approach and allows for better visualization and preparation of the bone cuts.

Post-TKR Knee Stability and Function

Despite the removal of the ACL, a properly performed total knee replacement provides excellent stability and allows for a significant return of function. The stability of the new knee joint is primarily achieved through:

• The precise fit and alignment of the prosthetic components.
• The integrity of the remaining collateral ligaments (Medial Collateral Ligament - MCL, and Lateral Collateral Ligament - LCL), which are crucial for medial-lateral stability.
• The retained PCL (in CR designs) or the cam-post mechanism (in PS designs).
• The strength and coordination of the surrounding musculature, particularly the quadriceps and hamstrings, which are vital for dynamic stability and functional movement.

Post-operative rehabilitation focuses on strengthening these muscles, restoring range of motion, and improving gait mechanics to ensure the best possible long-term outcomes after TKR.

Conclusion

The Anterior Cruciate Ligament (ACL) is a crucial stabilizer in the native knee. However, in the context of Total Knee Replacement (TKR), the ACL is almost universally removed. This decision is based on the ligament's often compromised state due to arthritis, the design of modern knee implants that negate the need for the ACL, and the practicalities of surgical access. The success and stability of a TKR rely on the proper implantation of prosthetic components, the integrity of other ligaments (like the PCL or collateral ligaments), and the strength of the surrounding musculature, not on the preservation of the ACL.