Knee replacement, medically known as arthroplasty, is a surgical procedure designed to alleviate pain and restore function in a damaged knee joint. The surgery involves removing the diseased or damaged bone and cartilage surfaces and replacing them with artificial components, called prostheses. This intervention is most often performed to treat severe arthritis or significant joint injury when non-surgical treatments are no longer effective. The surgical approach and the specific implant used vary widely, leading to a classification of different knee replacement types.
Classification by Scope: Total vs. Partial Replacement
The most immediate way to categorize a knee replacement is by the extent of the joint that is surgically replaced. The knee joint consists of three main compartments: the medial (inner) compartment, the lateral (outer) compartment, and the patellofemoral (kneecap) compartment.
Total Knee Arthroplasty (TKA) is the most common procedure and involves replacing all three compartments (medial, lateral, and patellofemoral) with prostheses. This comprehensive approach is recommended when arthritis or damage affects multiple areas of the joint or when there is significant deformity. TKA offers a reliable solution for widespread joint disease, reducing the likelihood of needing further surgery.
Partial Knee Arthroplasty (PKA), also referred to as unicompartmental knee arthroplasty (UKA), is a less invasive option. This procedure replaces only the single damaged compartment, usually the medial compartment. Patients are candidates for PKA only if their arthritis is strictly confined to one area and the surrounding ligaments are stable and intact. This selective approach preserves healthy bone, cartilage, and ligaments, often leading to a smaller incision and a quicker recovery time.
Classification by Implant Fixation
Another primary distinction between knee replacements is the method used to attach the metal and plastic components of the implant to the patient’s existing bone. The two main strategies for fixation are cemented and cementless, though a hybrid approach is also utilized.
Cemented fixation uses polymethylmethacrylate, an acrylic bone cement, to secure the implant components to the bone immediately during surgery. This method is the historical standard and is preferred for older patients or those with diminished bone quality, such as patients with osteoporosis, as it provides instant stability. The cement can sometimes break down or loosen over many years.
Cementless, or biologic, fixation relies on the patient’s bone to grow directly onto the implant’s porous surface, a process known as osseointegration. These implants are used in younger, more active patients who have healthy bone stock capable of supporting bone ingrowth. This fixation aims for a more durable bond that lasts longer than cement, though it may be associated with initial pain until the bone fully integrates.
Hybrid fixation combines cemented and cementless techniques. This is most commonly done by cementing the tibial (shinbone) component while leaving the femoral (thighbone) component cementless. This strategy leverages the benefits of immediate stability on the tibial side, where bone ingrowth is challenging, with the durable biologic fixation on the femoral side.
Classification by Prosthesis Design
Beyond the scope of replacement and the method of attachment, knee implants are also classified by their internal mechanical design, particularly how they interact with the knee’s remaining ligaments, specifically the posterior cruciate ligament (PCL).
Cruciate-Retaining (CR) designs are engineered to preserve the patient’s posterior cruciate ligament (PCL). This allows the natural ligament to guide the knee’s movement and provide stability. The goal of a CR implant is to replicate the knee’s natural motion, which requires the PCL to be healthy and functional prior to the operation.
Posterior-Stabilized (PS) designs require the removal of the PCL during total knee arthroplasty. To compensate for the loss of this ligament, the implant incorporates a specific mechanism: a post on the tibial insert fits into a cam on the femoral component. This mechanism provides stability and prevents the thighbone from sliding too far forward on the shinbone. This feature helps mimic the normal “roll-back” motion of the femur, offering stability in patients whose PCL is damaged or insufficient.
Highly Constrained designs represent the most stable, and least mobile, category of knee replacement. These implants are reserved for complex cases where the patient’s surrounding ligaments, including the medial and lateral collateral ligaments, are severely compromised or absent. The components are physically locked together, often using a hinge mechanism or long stems that extend deep into the bone. This provides maximal stability, though often at the expense of a reduced range of motion.
Primary vs. Revision Knee Replacement
Knee replacement procedures are also classified based on whether they are the patient’s initial surgery or a subsequent operation to address a failed implant. This distinction affects the complexity of the surgery and the type of implant used.
A primary arthroplasty is the initial knee replacement procedure performed to address severe arthritis or joint destruction. This is the most common type, involving the removal of damaged joint surfaces and the implantation of standard prosthetic components. Primary procedures are less complex and have a more predictable recovery course than subsequent surgeries.
A revision arthroplasty is a more complex operation undertaken to replace components from a previous knee replacement that has failed. Failure can occur due to factors including implant loosening, wear of the plastic spacer, infection, or joint instability. The procedure requires removing the old components, which is challenging due to scar tissue and bone loss, and then implanting specialized revision components. Revision implants often feature longer stems and more constraint to provide necessary stability in a compromised bone structure.