Total Knee Replacement (TKR): Understanding Femoral, Tibial, and Patellar Bone Cuts

What are the bone cuts in the TKR?

Total Knee Replacement (TKR), also known as total knee arthroplasty, involves precise surgical resections of the damaged bone surfaces of the femur, tibia, and sometimes the patella, to prepare them for the implantation of prosthetic components.

Introduction to Total Knee Replacement (TKR)

Total Knee Replacement (TKR) is a common orthopedic surgical procedure performed to alleviate pain and restore function in individuals suffering from severe knee arthritis or injury. Contrary to popular misconception, TKR is not an amputation; rather, it is a resurfacing procedure where the damaged articular cartilage and underlying bone are removed from the ends of the thigh bone (femur) and shin bone (tibia), and often the kneecap (patella), and replaced with artificial components made of metal alloys and high-grade plastics. The success of a TKR heavily relies on the precise and accurate bone cuts made during the operation, which are critical for proper implant fit, limb alignment, joint stability, and long-term function.

The Primary Bone Cuts in TKR

The goal of the bone cuts is to create precisely shaped surfaces that will perfectly accommodate the prosthetic components, restore the mechanical axis of the limb, and balance the surrounding ligaments. These cuts are typically performed using specialized jigs, guides, and oscillating saws.

Femoral Bone Cuts

The distal end of the femur undergoes several critical resections to prepare it for the femoral component, which often resembles the natural shape of the condyles.

• Distal Femoral Resection: This is the initial cut made across the bottom of the femur, determining the overall length of the leg and the component's position. This cut is made perpendicular to the mechanical axis of the femur (or at a slight valgus angle, typically 5-7 degrees, to match the natural anatomical alignment of the knee). This ensures proper alignment of the new joint relative to the hip and ankle.
• Posterior Condylar Resection: Cuts are made along the posterior aspects of the femoral condyles. The depth of these cuts, along with the distal cut, helps establish the flexion gap (the space between the femur and tibia when the knee is bent).
• Anterior and Posterior Chamfer Cuts: These angled cuts connect the distal and posterior resections, creating the characteristic "box" or "house" shape on the end of the femur. These chamfer cuts are crucial for accommodating the anterior and posterior sections of the femoral component and ensuring a snug fit that allows for smooth articulation and range of motion.
• Intercondylar Notch Cut (Box Cut): For certain implant designs, a central box-shaped cut may be made between the condyles to accommodate the central spine or post of the tibial component, particularly in posterior-stabilized (PS) knee designs.

Tibial Bone Cuts

The proximal end of the tibia is prepared to receive the tibial baseplate, which supports the polyethylene insert.

• Proximal Tibial Resection: A single, flat, horizontal cut is made across the top of the tibia. This cut is typically made perpendicular to the mechanical axis of the tibia, often with a slight posterior slope (usually 3-7 degrees) to mimic the natural posterior slope of the tibial plateau and facilitate knee flexion. The depth of this cut is minimized to preserve as much bone stock as possible while removing the damaged articular surface. This cut creates a stable, level platform for the tibial component.

Patellar Bone Cuts (Patellar Resurfacing)

While not always performed, the patella (kneecap) may also be resurfaced if its articular cartilage is significantly damaged.

• Posterior Patellar Resection: A thin slice of bone is removed from the posterior (underside) surface of the patella. This creates a flat surface for the patellar button, a high-density polyethylene component that articulates with the femoral component. This cut aims to relieve patellofemoral pain and improve tracking.

The Precision of Surgical Instrumentation

The accuracy of these bone cuts is paramount for the long-term success of a TKR. Surgeons utilize advanced instrumentation, including:

• Intramedullary (IM) and Extramedullary (EM) Guides: These guides help the surgeon align the cutting blocks precisely relative to the anatomical and mechanical axes of the bones.
• Cutting Jigs and Blocks: These metal templates are secured to the bone and provide a precise path for the oscillating saw blade.
• Computer-Assisted Navigation Systems: In some cases, computer navigation or robotic assistance is used to further enhance the precision of bone cuts and implant placement, providing real-time feedback on alignment and balance.

Anatomical and Biomechanical Considerations

The specific angles and depths of the bone cuts are meticulously planned pre-operatively using X-rays and other imaging, and confirmed intra-operatively. The goal is to:

• Restore Mechanical Alignment: Ensure the new knee joint aligns correctly with the hip and ankle, distributing weight evenly across the joint.
• Balance Ligaments: Create appropriate flexion and extension gaps to allow the collateral and cruciate ligaments (if retained) to function optimally, providing stability throughout the range of motion.
• Maximize Range of Motion: Allow for full and pain-free flexion and extension of the knee.
• Optimize Implant Fit: Ensure the prosthetic components fit securely and precisely to the prepared bone surfaces, promoting osseointegration and preventing loosening.

Post-Surgical Recovery and Rehabilitation

Understanding the nature of these bone cuts underscores the importance of the post-surgical rehabilitation process. The new joint surfaces require controlled movement and strengthening to adapt and integrate. Physical therapy plays a crucial role in restoring range of motion, muscle strength, and functional mobility, building upon the stable foundation created by the precise surgical preparation.

Conclusion

The bone cuts performed during a Total Knee Replacement are highly precise and critical steps in the procedure. By meticulously reshaping the ends of the femur, tibia, and sometimes the patella, orthopedic surgeons create the ideal foundation for the prosthetic components, aiming to restore proper alignment, stability, and function to the knee joint, ultimately leading to significant pain relief and improved quality of life for the patient.