The meniscus is a C-shaped piece of fibrocartilage that sits between the thigh bone (femur) and the shin bone (tibia) in the knee joint.
A Total Knee Replacement (TKR) is a surgical procedure designed to resurface the damaged ends of these bones with metallic and plastic components to restore smooth movement.
When joint surfaces become severely damaged, often due to conditions like osteoarthritis, a TKR is performed to eliminate pain and improve mobility. Understanding what happens to the meniscus is a frequent concern for patients preparing for surgery.
The Essential Role of the Meniscus in Knee Function
The menisci, which are two in each knee—the medial and the lateral—perform several biomechanical functions within the joint. They act as shock absorbers, helping to attenuate the intermittent forces and impacts generated during activities like walking or running. This cushioning capability protects the smooth articular cartilage covering the ends of the bones.
The menisci are also responsible for distributing the axial load across the knee joint, which prevents the concentration of pressure on a small area of the bone. Furthermore, their wedge shape helps to stabilize the curved femoral condyle as it articulates with the flatter tibial plateau, while also assisting in joint lubrication. The deterioration or tearing of this tissue is a major factor that contributes to the progression of osteoarthritis, ultimately leading many patients to require a total knee replacement.
Why the Meniscus is Removed During Total Knee Replacement
In a standard total knee replacement procedure, the entire natural meniscus is removed by the orthopedic surgeon. This removal is a necessary step to ensure the long-term success and proper function of the prosthetic joint. The surgical technique involves cutting and resurfacing the ends of the femur and tibia to create precise, flat surfaces where the metal implants will be cemented or secured.
The presence of the natural meniscus, even if partially preserved, would interfere with the precise seating and stability of the metallic components. The prosthetic joint is designed to operate with a specific, manufactured spacer in the place of the meniscus, and any remaining natural tissue would disrupt the intended mechanical alignment. The meniscus tissue needing replacement is typically already damaged, torn, or severely degenerated, which is often a source of the patient’s original pain and inflammation. Removing this damaged material eliminates the source of discomfort and ensures a uniform foundation for the artificial joint surfaces.
The Implant Component That Replaces Meniscus Function
The functions of the natural meniscus are taken over by a specialized component of the knee implant, referred to as the tibial insert or plastic spacer. This component is typically made from ultra-high molecular weight polyethylene (UHMWPE), a medical-grade plastic known for its high wear resistance and low-friction properties. The insert is engineered to snap securely into the metal tibial component, which sits on the resurfaced shin bone.
The UHMWPE spacer acts as the smooth, gliding surface that articulates with the metal component attached to the femur. While it does not replicate the complex anatomical shape of the natural menisci, the polyethylene insert provides the necessary cushioning and stability, distributing the forces across the joint surfaces and acting as the new shock absorber. Advances in material science, such as highly crosslinked polyethylene, continue to improve the durability and longevity of these inserts by reducing wear and the resulting debris that can cause implant loosening over time.