Total knee replacement (TKR) is a surgical procedure that alleviates severe pain and restores function in a knee joint damaged by conditions like arthritis. During TKR, injured bone and cartilage are replaced with artificial components made of metal and plastic. This procedure aims to improve mobility, reduce pain, and enhance overall quality of life. Ligaments, strong bands of tissue connecting bones, are crucial for knee stability and movement.
Understanding Knee Ligaments
The knee joint contains four major ligaments that provide stability and control movement. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are inside the knee, crossing to form an “X” shape. The ACL prevents the shinbone from sliding too far forward and controls rotational movements, while the PCL prevents backward shinbone movement. On the sides, the medial collateral ligament (MCL) and lateral collateral ligament (LCL) provide stability against sideways forces. The MCL runs along the inner knee, connecting the thighbone to the shinbone, while the LCL is on the outer side, connecting the thighbone to the smaller lower leg bone. These four ligaments work together to ensure the knee moves in a controlled and stable manner.
Ligament Management During Total Knee Replacement
During total knee replacement, ligament management is a key aspect. The anterior cruciate ligament (ACL) is almost always removed to accommodate prosthetic components and simplify surgery. Often, in severe arthritis, the ACL is already damaged or absent.
The posterior cruciate ligament (PCL) management varies by implant design. Some procedures, called cruciate-retaining (CR) total knee replacements, preserve the PCL. Proponents suggest this can help maintain more natural knee movement and proprioception. Another common approach removes the PCL, typical with posterior-stabilized (PS) implants.
When the PCL is removed, PS implants incorporate a post-and-cam mechanism to replicate its function. This design prevents the thighbone from sliding too far forward during knee bending. The decision to retain or sacrifice the PCL depends on its condition, the implant type, and surgeon preference, with studies showing mixed results regarding definitive superiority of one method over the other.
The medial collateral ligament (MCL) and lateral collateral ligament (LCL) are almost always preserved. These collateral ligaments are essential for maintaining side-to-side knee stability. Surgeons meticulously balance their tension during surgery to ensure proper knee alignment and function. If damaged or compromised, specific techniques or constrained implant designs may be used for stability.
How Knee Stability is Achieved After Replacement
After total knee replacement, stability comes from prosthetic components, preserved ligaments, and surrounding muscles. The design of the artificial knee joint plays a primary role, as its metal and plastic components are shaped to articulate smoothly and provide inherent stability. For implants where the PCL is removed, an internal post-and-cam mechanism effectively substitutes for the ligament’s function, controlling motion between the thigh and shin components. Similarly, ACL-deficient implants have a tibial component designed with features like a central post or lip to manage rotational forces.
The preserved collateral ligaments (MCL and LCL) continue to provide crucial side-to-side stability to the knee. Surgeons carefully adjust their tension to ensure the joint is neither too tight nor too loose, allowing for balanced movement. Beyond the hardware and preserved ligaments, the strength of surrounding muscles—including quadriceps, hamstrings, and hip muscles—is fundamental for dynamic stability. These muscles and other soft tissues support and control the new joint, compensating for altered natural mechanisms.
Rehabilitation and Ligament Adaptation
Rehabilitation is a fundamental part of recovery after total knee replacement, beginning very soon after surgery. Patients are often encouraged to start moving their knee and walking with assistance on the same day or the day after the procedure. The primary goals of physical therapy are to improve the knee’s range of motion, strengthen the surrounding muscles, manage pain and swelling, and restore the ability to perform daily activities like walking and climbing stairs.
While some original ligaments may be removed or their function replaced by the implant, the body adapts to these changes over time. Physical therapy focuses on strengthening the muscles around the knee, which learn to work with the new joint mechanics to provide dynamic support and control. This process of adaptation, guided by consistent exercise and professional support, allows individuals to regain function and lead active lives, with full recovery typically taking several months.