Posterior Stabilized (PS) Knee: Design, Indications, and Advantages in Total Knee Arthroplasty

What is the full form of PS knee?

The full form of "PS knee" refers to a Posterior Stabilized knee, a specific design used in Total Knee Arthroplasty (TKA) or total knee replacement surgery. This design is characterized by its unique mechanism that compensates for the absence or removal of the posterior cruciate ligament (PCL).

Understanding the PS Knee: A Deeper Dive into Knee Arthroplasty

When discussing a "PS knee," we are specifically referring to a type of implant used in total knee replacement surgery. In the context of knee arthroplasty, the primary goal is to replace damaged articular cartilage and bone with prosthetic components, restoring function and alleviating pain. There are several design philosophies for these implants, with the posterior stabilized (PS) design being one of the most common and versatile.

The PS knee design is engineered to provide stability to the knee joint, particularly when the patient's own posterior cruciate ligament (PCL) is either deficient prior to surgery or is resected (removed) during the surgical procedure. Unlike cruciate-retaining (CR) designs that preserve the PCL, the PS design incorporates an internal mechanism to replicate the PCL's critical role in preventing posterior translation of the tibia relative to the femur.

Anatomy and Biomechanics: How the PS Knee Works

The PS knee system fundamentally replaces the native PCL's function through a mechanical interaction between the femoral and tibial components of the prosthetic joint.

• Femoral Component (Cam): The femoral component, typically made of a cobalt-chromium alloy, features a raised, intercondylar cam on its posterior aspect. This cam is designed to articulate with a corresponding structure on the tibial insert.
• Tibial Component (Post): The tibial component consists of a metal tray (often titanium) that is fixated to the tibia, and a polyethylene (plastic) insert that articulates with the femoral component. The polyethylene insert in a PS design has a raised central post or spine that engages with the femoral cam.

The biomechanical principle behind the PS knee lies in this cam-post mechanism. As the knee flexes, particularly beyond 45-60 degrees, the femoral cam engages with the tibial post. This engagement guides the femur to "roll back" on the tibia, a movement crucial for achieving greater knee flexion without impingement and for maintaining joint stability. This controlled rollback helps to prevent paradoxical anterior translation of the femur during flexion, which can occur in PCL-deficient knees and lead to anterior knee pain or reduced range of motion. By replicating the PCL's role, the PS design ensures proper knee kinematics throughout the range of motion.

Indications for a PS Knee Design

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

• Posterior Cruciate Ligament (PCL) Deficiency: This is the primary indication. If the PCL is severely damaged, absent, or if its integrity cannot be reliably maintained during surgery, a PS design is often preferred. The PCL may be removed during surgery to facilitate implant placement or to correct severe deformities.
• Severe Deformities: In cases of significant varus (bow-legged) or valgus (knock-kneed) deformities, the soft tissue releases required to correct alignment may compromise the PCL. The PS design provides inherent stability that can compensate for these soft tissue imbalances.
• Significant Bone Loss or Instability: When there is substantial bone loss from the distal femur or proximal tibia, or if the knee exhibits significant instability, the PS design offers enhanced constraint and stability.
• Revision Total Knee Arthroplasty: In revision surgeries (replacing a previous knee replacement), the PS design is frequently used, as the PCL may have been sacrificed in the initial surgery or compromised due to implant loosening or wear.
• Limited Range of Motion: While not a direct indication, in some cases, the controlled rollback provided by a PS design can help achieve a greater range of flexion compared to some cruciate-retaining designs, particularly for patients with pre-existing flexion contractures.

Advantages and Considerations of the PS Knee

The PS knee design offers several distinct advantages but also comes with specific considerations.

Advantages

• Enhanced Stability: The cam-post mechanism provides excellent anteroposterior stability, compensating for a deficient or resected PCL.
• Improved Kinematics: The guided femoral rollback helps to restore more natural knee motion, potentially leading to better deep flexion and reduced impingement.
• Reliable Solution: It is a well-established and highly successful design, particularly for complex knee presentations where PCL preservation is not feasible or desirable.
• Reduced Paradoxical Rollback: By forcing posterior femoral translation, it helps prevent the "paradoxical anterior slide" of the femur during flexion, which can occur in PCL-deficient knees and lead to less efficient quadriceps function.

Considerations/Potential Disadvantages

• Increased Bone Resection: The design of the femoral component with its intercondylar box may require slightly more bone removal from the distal femur compared to cruciate-retaining designs.
• Potential for "Clunk" or "Thud": In some instances, patients may report a palpable or audible "clunk" or "thud" sensation, particularly during deep flexion and extension, as the cam engages and disengages with the post. While often benign, it can be a source of concern for some patients.
• Polyethylene Wear: Historically, concerns existed about higher wear rates of the polyethylene post due to the concentrated forces of the cam-post engagement. However, advances in polyethylene materials and manufacturing have significantly mitigated this risk in modern implants.
• Slightly More Complex Surgical Technique: While routine for experienced surgeons, the precise alignment and soft tissue balancing required for a PS knee can be more intricate than for some CR designs.

Rehabilitation and Recovery Post-PS Knee Arthroplasty

Post-operative rehabilitation following a PS knee replacement is crucial for optimal outcomes. The rehabilitation protocol will be tailored to the individual but generally focuses on:

• Early Mobilization: Initiating movement soon after surgery to prevent stiffness and promote circulation.
• Range of Motion Exercises: Gradually increasing knee flexion and extension to regain full functional range.
• Strengthening Exercises: Targeting the quadriceps, hamstrings, glutes, and calf muscles to restore muscle strength and support the new joint.
• Proprioception and Balance Training: Re-educating the joint's sense of position and improving balance to reduce fall risk.
• Gait Training: Re-learning to walk with a natural and efficient stride.

Patients are guided through progressive exercises, initially with the assistance of a physical therapist, and then independently at home. Adherence to the rehabilitation program is paramount for achieving the full benefits of a PS knee replacement.

Conclusion: The Role of PS Knee in Modern Orthopedics

The Posterior Stabilized (PS) knee design represents a cornerstone in modern total knee arthroplasty. Its ability to reliably restore stability and kinematics in knees with PCL deficiency or severe deformities makes it an invaluable option for orthopedic surgeons. While the decision between a PS and other knee designs is complex and patient-specific, understanding the mechanics and indications of the PS knee is fundamental for anyone involved in exercise science, kinesiology, or fitness education, as it directly impacts post-surgical rehabilitation and long-term patient outcomes.