PCL in Total Knee Replacement: Understanding its Role and Implant Designs

What is PCL in total knee?

In the context of a total knee replacement (Total Knee Arthroplasty or TKA), the PCL refers to the Posterior Cruciate Ligament, a critical stabilizing ligament in the knee joint whose fate (retention or sacrifice) is a major design consideration for prosthetic knee implants.

Understanding the PCL: Anatomy and Function

The knee joint is a complex hinge joint stabilized by a network of ligaments. Among these, the cruciate ligaments – the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) – are paramount for maintaining joint integrity and controlling movement.

• Anatomical Position: The PCL is located deep within the knee joint, connecting the posterior part of the tibia (shin bone) to the anterior part of the femur (thigh bone).
• Primary Function: The PCL is the primary restraint to posterior translation of the tibia relative to the femur. In simpler terms, it prevents the shin bone from sliding too far backward under the thigh bone. It also contributes to rotational stability of the knee.
• Importance in Healthy Knee: A healthy PCL is crucial for normal knee kinematics, allowing for smooth gliding and rolling motions during activities like walking, squatting, and climbing stairs, while preventing excessive stress on other joint structures.

The PCL's Role in Total Knee Arthroplasty (TKA)

Total Knee Arthroplasty (TKA) is a surgical procedure to replace a damaged knee joint with artificial components, typically made of metal and plastic. When designing these prosthetic components, orthopedic surgeons and engineers must consider how the new joint will interact with or replace the knee's natural ligaments, particularly the PCL.

Unlike the ACL, which is almost universally sacrificed during TKA due to its complex biomechanical role and the difficulty of replicating its function with standard implants, the PCL's role in TKA is a subject of ongoing debate and design variations. The decision to retain or sacrifice the PCL significantly influences the design of the prosthetic components and the biomechanics of the new joint.

PCL-Retaining vs. PCL-Sacrificing TKA Designs

There are two primary categories of TKA implant designs based on their interaction with the PCL:

• PCL-Retaining Designs (Cruciate-Retaining or CR-TKA): These designs are engineered to preserve the patient's native PCL. The femoral component is typically designed with a groove that accommodates the PCL, allowing it to continue functioning as a stabilizer. The tibial component often has a flat or slightly dished articulating surface.
• PCL-Sacrificing Designs (Posterior-Stabilized or PS-TKA): In these designs, the native PCL is removed during surgery. To compensate for the loss of the PCL's posterior stability, the implant incorporates a cam-and-post mechanism. A "post" on the polyethylene insert of the tibial component engages with a "cam" on the femoral component during knee flexion, preventing posterior translation of the tibia and providing stability.

Advantages and Disadvantages of PCL-Retaining Designs

Advantages:

• Preservation of Natural Kinematics: Proponents argue that retaining the PCL allows for more natural knee motion and proprioception (the body's sense of joint position).
• Improved Femoral Rollback: The PCL helps pull the femur backward on the tibia during flexion, which can lead to increased range of motion, especially at higher degrees of flexion.
• Bone Preservation: Some designs may require less bone removal from the femur.
• Potentially Reduced Polyethylene Wear: By distributing forces more naturally, some theorize reduced stress on the plastic insert.

Disadvantages:

• PCL Integrity: The preserved PCL must be healthy and balanced. If it's too tight or too loose, it can lead to abnormal kinematics, pain, or early implant failure.
• Surgical Complexity: Requires more precise surgical technique to ensure proper tensioning of the retained PCL.
• Limited Correction: May be less suitable for knees with significant deformity or severe arthritis, where balancing the PCL can be challenging.
• Potential for PCL Failure: The retained PCL can still stretch, rupture, or become inflamed post-surgery.

Considerations for PCL-Sacrificing Designs

Advantages:

• Predictable Stability: The cam-and-post mechanism provides consistent posterior stability, independent of the patient's native PCL.
• Greater Deformity Correction: Easier to correct severe knee deformities as the PCL's tension is not a factor.
• Simplified Surgical Technique: Generally considered a more straightforward surgical procedure.
• Reliable Performance: Widely used and has a long track record of successful outcomes.

Disadvantages:

• Non-Physiological Kinematics: The cam-and-post mechanism provides an artificial form of stability that does not fully replicate the natural PCL function.
• "Jumping" or "Clunking": In some cases, patients may experience a sensation of the cam engaging the post, leading to a "clunk" or "jump."
• Polyethylene Wear: The cam-and-post mechanism can be a site of increased stress and potential polyethylene wear over time.
• Increased Bone Resection: Typically requires more bone removal from the distal femur to accommodate the cam box.

Impact on Rehabilitation and Function

While both PCL-retaining and PCL-sacrificing designs generally lead to excellent outcomes in terms of pain relief and functional improvement, there can be subtle differences in rehabilitation and long-term function:

• Early Rehabilitation: Both approaches follow similar early post-operative rehabilitation protocols focused on pain management, range of motion, and muscle strengthening.
• Long-Term Function: Studies comparing long-term outcomes often show comparable results in terms of patient satisfaction, pain scores, and functional abilities for most activities of daily living. Some patients with PCL-retaining designs report feeling a "more natural" knee, while others find the stability of PS designs reassuring.
• High-Demand Activities: For individuals aiming to return to higher-impact activities, the choice of implant design, along with surgical precision and diligent rehabilitation, plays a role in optimizing outcomes. However, the decision between CR and PS is often based more on surgeon preference, patient anatomy, and the extent of knee pathology rather than a definitive superior functional outcome for all patients.

Conclusion

The PCL in total knee replacement refers to the Posterior Cruciate Ligament, a key stabilizer whose fate dictates the design of the prosthetic components. Whether a surgeon opts for a PCL-retaining (CR) or PCL-sacrificing (PS) implant depends on various factors, including the patient's anatomy, the severity of arthritis, the presence of deformity, and the surgeon's experience and preference. Both approaches have proven successful in restoring function and relieving pain for millions of patients, underscoring the sophisticated biomechanical considerations inherent in modern total knee arthroplasty. Understanding these distinctions is crucial for anyone involved in the care, education, or personal journey of knee replacement.