PS Knee: Understanding Posterior Stabilized Knee Replacement

What is a PS knee?

A "PS knee" refers to a Posterior Stabilized (PS) total knee arthroplasty (TKA) design, which is a specific type of artificial knee joint used in knee replacement surgery, primarily characterized by its mechanism to compensate for a resected or dysfunctional posterior cruciate ligament (PCL).

Understanding Total Knee Arthroplasty (TKA)

Total knee arthroplasty, commonly known as knee replacement surgery, is a procedure to resurface a knee damaged by arthritis or injury. The diseased bone and cartilage are removed and replaced with prosthetic components made of metal alloys, high-grade plastics, and polymers. These components are designed to replicate the natural movement of a healthy knee joint.

Modern knee replacement prostheses come in various designs, each tailored to different patient needs, anatomical considerations, and surgical approaches. The choice of prosthesis often depends on the integrity of the patient's ligaments, particularly the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), as well as the severity of the joint damage.

What Does "PS" Stand For? (Posterior Stabilized)

The "PS" in PS knee stands for Posterior Stabilized. This design is distinct from other common TKA designs, most notably the Cruciate Retaining (CR) knee.

• Cruciate Retaining (CR) Knee: In a CR knee, the surgeon preserves both the patient's native ACL and PCL. The design of the prosthetic components is then intended to work in conjunction with these ligaments to provide stability and natural kinematics. This approach is favored when the patient's PCL is healthy and intact, as it allows for more natural rollback of the femur on the tibia during flexion.
• Posterior Stabilized (PS) Knee: In contrast, a PS knee design is used when the patient's PCL is either resected during surgery (which is common to facilitate surgical access and correct deformities) or is already compromised and unable to function effectively. The PS design incorporates an internal mechanism within the prosthetic components to provide the stability and control of femoral rollback that the PCL would normally provide.

The Anatomy and Mechanics of a PS Knee

The key distinguishing feature of a PS knee lies in its unique "cam and post" mechanism.

• Role of the PCL: The posterior cruciate ligament (PCL) is a crucial ligament within the knee that prevents the tibia (shin bone) from sliding too far backward relative to the femur (thigh bone). It plays a vital role in knee stability and guiding the natural "rollback" motion of the femur during knee flexion.
• The Cam and Post Mechanism: In a PS knee, the prosthetic components are engineered to replace the function of the PCL. The femoral component (the part that caps the end of the femur) features a cam (a raised, curved surface). The tibial insert (the plastic spacer placed on top of the tibia) has a corresponding post (a raised, central spine). As the knee flexes, the femoral cam engages with the tibial post. This interaction prevents excessive posterior translation of the tibia and guides the femoral rollback, mimicking the action of a healthy PCL.

This design allows the surgeon to remove the PCL, which can be advantageous in cases where the ligament is diseased, damaged, or prevents adequate correction of a severe knee deformity.

Advantages of a PS Knee Design

The Posterior Stabilized knee offers several benefits, making it a preferred choice in specific clinical scenarios:

• Enhanced Stability: The cam and post mechanism provides robust anterior-posterior stability, compensating effectively for a deficient or resected PCL. This can be particularly beneficial for patients with significant pre-operative instability.
• Correction of Severe Deformities: By removing the PCL, surgeons gain more flexibility in correcting severe angular deformities (like significant bow-legs or knock-knees) or contractures of the knee joint. This allows for better alignment and balancing of the soft tissues around the knee.
• Improved Range of Motion: In some cases, especially those with pre-existing stiffness or significant arthritis, a PS design can facilitate a more predictable and often greater range of flexion, as the PCL, if tight or scarred, would otherwise restrict this motion.
• Reliability: The PS design is a tried and tested system, offering predictable outcomes in a wide range of patients, particularly when PCL integrity is questionable.

Disadvantages and Considerations

While offering significant advantages, PS knees also come with certain considerations:

• More Bone Resection: The design of the PS knee typically requires slightly more bone to be removed from the femur and tibia compared to a CR design to accommodate the cam and post mechanism.
• Potential for "Clunk" or "Thud" Sensation: In some patients, the engagement of the cam and post can occasionally lead to a perceptible "clunk" or "thud" sensation, particularly during deep flexion or extension. While often benign, it can be disconcerting for some individuals.
• Slightly Higher Wear Rates (Historically): Early PS designs sometimes showed higher wear rates of the polyethylene tibial insert due to the concentrated stress at the cam-post interface. However, advancements in materials and design have significantly mitigated this concern in modern prostheses.
• More Complex Surgical Technique: The implantation of a PS knee may involve a slightly more intricate surgical technique compared to a CR knee due to the precise positioning required for the cam and post engagement.

Who is a PS Knee For?

The decision to use a PS knee prosthesis is made by the orthopedic surgeon based on a thorough evaluation of the patient's knee condition, including:

• PCL Insufficiency or Damage: Patients whose PCL is already compromised, diseased, or significantly damaged by arthritis or injury are ideal candidates.
• Severe Arthritis and Deformity: When the knee joint has severe degeneration, significant angular deformities, or substantial bone loss, a PS design provides the necessary stability and allows for better correction.
• Limited Pre-operative Range of Motion: In cases where the patient has a very stiff knee or a significant flexion contracture, removing the PCL and using a PS design can help achieve a better post-operative range of motion.
• Revision Surgery: PS knees are frequently used in revision total knee arthroplasty (re-doing a previous knee replacement) where the original ligaments may have been removed or are no longer functional.

Rehabilitation and Functional Implications

Rehabilitation after a PS knee replacement is similar to other TKA types, focusing on:

• Early Mobilization: Initiating movement soon after surgery to prevent stiffness and promote healing.
• Pain Management: Controlling post-operative pain to facilitate participation in physical therapy.
• Range of Motion Exercises: Restoring the knee's ability to bend and straighten fully.
• Strengthening Exercises: Building strength in the quadriceps, hamstrings, and other supporting muscles.
• Gait Training: Re-learning how to walk safely and efficiently.

While the fundamental principles of rehab remain consistent, a PS knee's inherent stability from the cam and post mechanism can sometimes lead to a quicker progression in certain weight-bearing or range-of-motion exercises, as the reliance on the native PCL is eliminated. Patients can typically expect to return to most daily activities and low-impact exercises within a few months, with continued improvement over a year or more.

Conclusion: The Role of PS Knees in Modern Orthopedics

The Posterior Stabilized (PS) knee is a cornerstone design in total knee arthroplasty, offering a robust and reliable solution for patients requiring knee replacement, particularly when the native posterior cruciate ligament is compromised or needs to be resected. Its unique cam and post mechanism effectively mimics the PCL's function, providing crucial stability and facilitating optimal kinematics. While surgical advancements continue to refine all TKA designs, the PS knee remains a vital option, allowing orthopedic surgeons to achieve excellent long-term outcomes for a wide spectrum of patients with knee joint degeneration.