Total knee replacement (TKR) is a widely performed orthopedic procedure to relieve severe joint pain and restore functional mobility. When non-surgical treatments no longer provide adequate relief, replacing the damaged joint surfaces becomes necessary. This process involves substituting the worn-out ends of the thigh and shin bones with artificial components, creating a smooth, new joint. Understanding TKR involves visualizing the joint’s condition before surgery, examining the prosthetic parts, and reviewing the surgical installation steps.
Visualizing Pre-Surgical Damage
Before a surgeon intervenes, images of a knee requiring replacement often display clear signs of severe deterioration within the joint space. Pre-operative X-rays typically reveal a significant narrowing of the space between the femur (thigh bone) and the tibia (shin bone). This diminished gap confirms the destruction or complete absence of the smooth, protective articular cartilage that normally cushions the bones.
The most severe finding is “bone-on-bone” contact, which appears on the images as two bony surfaces rubbing directly against each other. The body’s response to this constant friction is a hardening of the bone tissue just beneath the joint surface, a process called subchondral sclerosis.
Chronic inflammation and joint instability also lead to the formation of osteophytes (bone spurs), which are irregular growths extending from the edges of the joint. These visual markers collectively confirm the extent of the joint’s pathology and the necessity of surgical intervention.
Understanding the Replacement Components
The artificial joint is comprised of three primary components, each designed to replicate a part of the natural knee structure. The femoral component is a highly polished metal cap that fits over the shaped end of the thigh bone. This component is manufactured from a biocompatible metal alloy, such as cobalt-chromium, offering resilience and a scratch-resistant surface for smooth movement.
The tibial tray is a flat, metal base secured to the top surface of the shin bone. Made of titanium or a similar metal, it provides a stable foundation for the prosthetic joint. The surgeon affixes the tray by cementing it firmly into place or by using a porous surface that encourages bone growth directly into the implant, known as press-fitting.
The third component is the polyethylene spacer, which functions as the joint’s new cartilage and facilitates gliding motion. This specialized plastic insert snaps into the metal tibial tray, creating a low-friction surface that articulates against the femoral component. This polyethylene is engineered to resist wear, allowing the metal components to move without causing damage.
The Surgical Installation Process
The installation process begins with an incision to access the joint capsule and expose the damaged bone ends. The surgeon uses specialized instruments and alignment guides to remove the diseased cartilage and the minimal amount of underlying bone necessary from both the femur and the tibia. This preparation is performed precisely to ensure the joint’s correct mechanical alignment and stability.
Once the bone surfaces are prepared, the metal components are secured to their respective bones. The femoral component is cemented or press-fit onto the lower end of the femur, while the tibial tray is fixed to the top of the tibia. The surgeon then inserts the polyethylene spacer between the two metal surfaces, confirming that it seats correctly within the tibial tray.
After all parts are in place, the surgical team checks the knee’s range of motion, ligament tension, and overall stability across the full arc of movement. Achieving perfect alignment is necessary for the long-term success and function of the implant. The procedure concludes with the layered closure of the tissues and skin.