Total Knee Replacement: Bones Involved, Resection Process, and Implants

How Many Bones Are Cut in a Total Knee Replacement?

In a total knee replacement (total knee arthroplasty), the affected surfaces of three bones that form the knee joint—the femur (thigh bone), tibia (shin bone), and often the patella (kneecap)—are precisely resected or "trimmed" to prepare them for prosthetic components.

Understanding Total Knee Arthroplasty (TKA)

Total Knee Arthroplasty (TKA), commonly known as total knee replacement, is a surgical procedure designed to alleviate pain and restore function in severely damaged knee joints. This damage is most frequently caused by osteoarthritis, but can also result from rheumatoid arthritis, post-traumatic arthritis, or other degenerative conditions. The procedure involves removing the diseased or damaged bone and cartilage and replacing them with prosthetic components made of metal alloys, high-grade plastics, and polymers.

The Bones Involved in the Knee Joint

The knee is a complex hinge joint primarily formed by the articulation of three bones:

• Femur (Thigh Bone): The distal (lower) end of the femur forms the upper part of the knee joint.
• Tibia (Shin Bone): The proximal (upper) end of the tibia forms the lower part of the knee joint.
• Patella (Kneecap): This small, triangular bone sits in front of the joint, within the quadriceps tendon, and glides over the end of the femur.

These three bones, along with their articular cartilage, ligaments, and menisci, work in concert to facilitate knee movement and stability.

What "Cutting" Entails in TKA

The term "cutting" in the context of knee replacement can be misleading. While bone is indeed removed, it is typically a precise resection or trimming of the damaged articular surfaces, rather than a full severance of the bones. The goal is to remove only the diseased bone and cartilage, creating perfectly flat and angled surfaces to precisely fit the prosthetic implants. Surgeons utilize specialized cutting guides and instruments, often guided by computer navigation or robotic assistance, to ensure accuracy down to fractions of a millimeter.

The Surgical Process: Bone Resection Details

During a total knee replacement, the following bone surfaces are typically addressed:

• Distal Femur Resection: The damaged cartilage and a small amount of underlying bone from the end of the femur are precisely removed. This often involves creating several flat surfaces to accommodate the femoral component of the prosthetic knee, which typically mimics the natural curve of the femur. The amount of bone removed is minimal, usually just enough to remove the damaged area and prepare for the implant.
• Proximal Tibia Resection: Similarly, the damaged cartilage and a thin slice of bone from the top surface of the tibia are resected. This creates a flat platform onto which the tibial component, consisting of a metal tray and a polyethylene (plastic) insert, will be cemented or pressed into place.
• Patella Resurfacing: In most total knee replacements, the posterior (back) surface of the patella is also resurfaced. The damaged cartilage is removed, and a small, dome-shaped polyethylene "button" is cemented to the prepared bone surface. This new surface then articulates smoothly with the new femoral component. In some cases, if the patellar cartilage is healthy or if the surgeon deems it unnecessary, the patella may not be resurfaced.

Therefore, in a standard total knee replacement, three bones (femur, tibia, and patella) have their articulating surfaces precisely resected or trimmed.

Why Bone Resection is Necessary

The precise removal of bone is critical for several reasons:

• Removal of Damaged Tissue: It eliminates the arthritic, painful, and irregular surfaces of the joint.
• Creation of Precision Surfaces: It provides clean, flat, and appropriately angled surfaces that allow for the secure and stable attachment of the prosthetic components.
• Restoration of Alignment and Mechanics: By removing specific amounts of bone and inserting components of precise thickness, the surgeon can correct any deformities (like bow-legged or knock-kneed alignment) and restore the joint's natural mechanical axis and range of motion.

The Implants: What Replaces the Resected Bone

After the bone surfaces are prepared, the prosthetic components are implanted:

• Femoral Component: A curved metal component (often cobalt-chromium or titanium alloy) that caps the end of the femur.
• Tibial Component: A flat metal tray (often titanium alloy) that fits onto the resected tibia, topped with a durable polyethylene insert that acts as the new cartilage surface.
• Patellar Component: A dome-shaped polyethylene button that replaces the back surface of the kneecap.

Post-Operative Considerations and Rehabilitation

Following a total knee replacement, patients undergo a structured rehabilitation program. This typically involves physical therapy to regain strength, flexibility, and range of motion. While the surgical procedure involves precise bone removal, the ultimate goal is to restore a pain-free, functional joint, allowing individuals to return to daily activities and often, low-impact exercise.