Total Knee Replacement: Bone Removal, Implants, and Benefits

How Much Bone Is Removed in a Total Knee Replacement?

In a total knee replacement (TKR), the procedure primarily involves resurfacing the damaged bone and cartilage, not removing large sections of bone. Surgeons typically remove only a few millimeters of bone from the ends of the femur, tibia, and sometimes the back of the patella to prepare the surfaces for prosthetic implants.

Understanding Total Knee Replacement (TKR)

Total knee replacement, or total knee arthroplasty, is a highly effective surgical procedure designed to alleviate pain and restore function in knees severely damaged by arthritis or injury. Contrary to what the term "replacement" might suggest, it is not an amputation or the removal of the entire knee joint. Instead, it is a resurfacing procedure where the damaged cartilage and a minimal amount of underlying bone are precisely removed and replaced with artificial components.

The Surgical Goal: Reshaping, Not Removing Entirely

The primary objective of a TKR is to remove the diseased or damaged articulating surfaces of the knee joint and reshape the remaining bone to allow for the precise fit of prosthetic components. This process aims to eliminate bone-on-bone friction, correct deformities, and restore smooth, pain-free movement. The amount of bone removed is carefully calculated to achieve optimal alignment, stability, and range of motion for the new artificial joint.

Specific Bone Resection Amounts

The precise amount of bone removed varies slightly depending on the patient's anatomy, the extent of joint damage, the specific type of implant used, and the surgeon's technique. However, general guidelines apply to the three main bones involved:

• Femur (Thigh Bone): The distal end of the femur, where it meets the tibia, is prepared. This involves removing the damaged cartilage and a thin layer of underlying bone, typically 8 to 10 millimeters (mm) from the weight-bearing surfaces. Precise cuts are made to accommodate the femoral component, which often involves a combination of distal cuts and anterior/posterior chamfer cuts to match the implant's contours.
• Tibia (Shin Bone): The proximal (top) end of the tibia, known as the tibial plateau, is also resurfaced. The damaged cartilage and a thin layer of underlying bone are removed, usually 6 to 10 mm from the top surface. This cut is typically flat to provide a stable base for the tibial component.
• Patella (Kneecap): The decision to resurface the patella is made on a case-by-case basis. If deemed necessary, the posterior (underside) surface of the patella is trimmed, removing approximately 5 to 8 mm of bone to accommodate a plastic patellar component. If the patella is not resurfaced, no bone is removed from it.

In essence, the cumulative amount of bone removed from the primary articulating surfaces is measured in millimeters, not centimeters or inches. The goal is to remove only what is necessary to create a smooth, stable, and well-aligned surface for the implants.

Why is Bone Removed? The Rationale

Bone is removed for several critical reasons:

• To Remove Damaged Tissue: The primary reason is to eliminate the arthritic, eroded cartilage and the underlying sclerotic (hardened) or cystic bone that causes pain and limits function.
• To Create a Flat/Reshaped Surface: The natural contours of an arthritic joint are irregular. Removing a precise amount of bone creates flat or uniformly shaped surfaces that allow the artificial components to fit snugly and track correctly.
• To Correct Deformity: Many patients undergoing TKR have significant knee deformities (e.g., bow-legged or knock-kneed). Precise bone removal helps correct these deformities, restoring the limb to proper alignment.
• To Accommodate Implants: The dimensions and shapes of the prosthetic components dictate the precise amount and angle of bone that must be removed for a secure and functional fit.

The Role of Implants

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

• Femoral Component: A metal (e.g., cobalt-chromium or titanium alloy) cap that covers the end of the thigh bone.
• Tibial Component: A flat metal platform (e.g., titanium) that covers the top of the shin bone, often with a highly durable plastic (polyethylene) insert that acts as the new cartilage surface.
• Patellar Component: A dome-shaped plastic component that replaces the underside of the kneecap (if resurfaced).

These implants are designed to mimic the natural motion of a healthy knee joint, allowing for smooth gliding and rotation.

Factors Influencing Bone Resection

While general amounts are consistent, several factors can influence the exact volume of bone removed:

• Severity of Arthritis: More severe cartilage loss and bone erosion might necessitate slightly more bone removal to reach healthy bone.
• Bone Quality: Patients with osteoporosis or very dense bone may require different surgical considerations.
• Pre-existing Deformity: Significant varus (bowleg) or valgus (knock-knee) deformities require precise bone cuts to realign the limb axis.
• Implant Design: Different manufacturers' implant designs have slightly varying dimensions, which can influence the necessary bone cuts.
• Surgeon's Expertise and Technique: An experienced surgeon utilizes precise instruments and surgical navigation techniques to ensure minimal yet adequate bone removal.

Benefits and Considerations of TKR

The precise bone removal and subsequent implantation of prosthetic components are crucial steps that lead to the significant benefits of TKR, including:

• Pain Relief: Eliminating the painful bone-on-bone friction.
• Improved Mobility: Restoring the ability to walk, bend, and extend the knee.
• Correction of Deformity: Straightening a bowed or knocked knee.
• Enhanced Quality of Life: Enabling participation in daily activities and light recreational pursuits.

While the amount of bone removed is minimal and calculated for optimal outcomes, TKR is a major surgical procedure requiring significant rehabilitation. Understanding the precise nature of the procedure, including the careful and limited bone removal, helps demystify the process and highlights the advanced surgical techniques employed to restore knee function.