Total Knee Replacement: Bone Resection, Resurfacing, and Implant Specifics

How much bone is removed during total knee replacement?

During a total knee replacement (TKR), only a minimal amount of damaged bone and cartilage is precisely resected from the ends of the femur, tibia, and sometimes the patella, typically just a few millimeters to prepare the surfaces for the prosthetic implants.

Understanding Total Knee Replacement (TKR)

Total Knee Replacement, or knee arthroplasty, is a highly effective surgical procedure designed to alleviate severe knee pain and restore function in individuals suffering from conditions like osteoarthritis, rheumatoid arthritis, or post-traumatic arthritis. Contrary to a common misconception, the procedure does not involve removing the entire knee joint or a large portion of the bones. Instead, it is more accurately described as a "resurfacing" procedure.

The Rationale for Bone Resection

The primary goal of TKR is to remove the diseased and damaged articular cartilage and a thin layer of the underlying bone, which are the sources of pain and limited mobility. This removal creates precise, flat surfaces to which the artificial knee components (prostheses) can be securely attached. The amount of bone removed is carefully calculated and executed to achieve several critical objectives:

• Eliminate Damaged Surfaces: Remove the worn-out cartilage and eroded bone that cause friction and pain.
• Correct Deformity: Address any existing angular deformities (e.g., bow-legged or knock-kneed) to restore proper mechanical alignment of the limb.
• Optimize Joint Mechanics: Create an optimal foundation for the prosthetic components to ensure smooth, stable, and pain-free movement.
• Ensure Proper Fit: Prepare the bone ends to perfectly accommodate the specific dimensions of the chosen implants.

Specifics of Bone Removal: The Tibia, Femur, and Patella

The exact amount of bone removed varies slightly depending on the individual's anatomy, the extent of joint damage, the type of implant used, and the surgeon's technique. However, the principles remain consistent across the three main bones involved:

• Femur (Thigh Bone):

  • The distal end of the femur, specifically the condyles (the rounded ends that form the top part of the knee joint), is prepared.
  • A thin section, typically 5 to 9 millimeters (approximately 0.2 to 0.35 inches), is removed from the weight-bearing surfaces of the femoral condyles.
  • Precise cuts are made to shape the end of the femur to fit the metal femoral component, which is designed to mimic the natural curve of the condyles. This often involves shaping the bone to allow for the component's groove for the patella (kneecap).

• Tibia (Shin Bone):

  • The proximal end of the tibia, specifically the tibial plateau (the flat top surface of the shin bone), is prepared.
  • A thin section, generally 5 to 10 millimeters (approximately 0.2 to 0.4 inches), is removed from the top surface of the tibia.
  • This cut is crucial for establishing a flat, stable platform for the polyethylene (plastic) insert, which acts as the new bearing surface. The cut is often made at a slight angle to correct alignment.

• Patella (Kneecap):

  • The decision to resurface the patella is made by the surgeon and depends on the condition of its articular cartilage. It is not always performed in every TKR.
  • If resurfaced, a thin layer, typically a few millimeters (e.g., 3-5 mm), is removed from the posterior (underside) surface of the patella.
  • A small plastic button or dome-shaped component is then cemented to this prepared surface.

The Goal: Achieving Proper Alignment and Stability

The precision of bone resection is paramount. Orthopedic surgeons utilize specialized instruments, jigs, and computer-assisted navigation systems to make highly accurate cuts. The aim is not just to remove bone, but to remove just enough bone to:

• Correct Varus or Valgus Deformity: Straighten a "bow-legged" (varus) or "knock-kneed" (valgus) alignment.
• Balance Ligaments: Ensure that the surrounding ligaments (cruciate and collateral ligaments) are appropriately tensioned, providing stability throughout the knee's range of motion.
• Optimize Patellar Tracking: Ensure the kneecap glides smoothly in the femoral groove, preventing pain and instability.

The Prosthetic Components

Once the bone surfaces are prepared, the artificial components are implanted. These typically consist of:

• Femoral Component: A metal alloy (e.g., cobalt-chrome or titanium) component that caps the end of the femur.
• Tibial Component: A flat metal tray that is fixed to the top of the tibia, often with a polyethylene (medical-grade plastic) insert snapped or locked into it. This plastic insert acts as the new cartilage surface, allowing smooth gliding against the femoral component.
• Patellar Component (if used): A polyethylene "button" that resurfaces the back of the kneecap.

These components are typically secured to the bone using bone cement, though some designs allow for "press-fit" fixation where the bone grows onto the implant surface.

Minimally Invasive Techniques and Bone Preservation

Modern advances in TKR have led to the development of minimally invasive techniques and implant designs aimed at preserving as much natural bone and soft tissue as possible. While the principle of resurfacing remains, these techniques may involve smaller incisions and more refined instrumentation to achieve the desired outcomes with less disruption to surrounding structures. However, the fundamental amount of bone that needs to be removed to accommodate the implants and correct alignment remains largely consistent.

What This Means for Patients and Recovery

Understanding that TKR is a resurfacing procedure, rather than a radical bone removal, can help demystify the surgery for patients. The minimal bone removal, coupled with precise prosthetic fitting, contributes to the procedure's high success rates in restoring function and significantly reducing pain. Recovery involves physical therapy to regain strength, flexibility, and range of motion, allowing the patient to return to many daily activities with a new, functional knee joint.

Conclusion

In summary, total knee replacement involves the precise removal of only the damaged articular cartilage and a thin layer of subchondral bone from the ends of the femur, tibia, and sometimes the patella. This carefully calculated resection, typically measured in millimeters, prepares the bone surfaces for the durable prosthetic components. The goal is not to remove large sections of bone, but to resurface the joint, correct alignment, and create a stable, pain-free articulating surface that can serve the patient for many years.