Knee replacement surgery (arthroplasty) alleviates pain and restores mobility by replacing damaged portions of the knee joint with artificial components (prosthetics). The procedure involves removing worn cartilage and some bone from the ends of the thigh bone (femur) and shin bone (tibia), and sometimes the kneecap (patella). These damaged surfaces are resurfaced with metal and plastic implants shaped for smooth, continued motion. Advancements in technology allow surgeons to tailor the replacement to the unique needs and joint damage pattern of each patient.
Scope of the Procedure
The distinction between knee replacement procedures is based on the extent of the joint replaced. The knee has three compartments: the medial (inner), the lateral (outer), and the patellofemoral (under the kneecap).
Total Knee Arthroplasty (TKA) is the most common procedure, addressing damage across all three compartments. TKA involves resurfacing the end of the femur, the top of the tibia, and often the back surface of the patella with prosthetic components. This comprehensive approach is necessary when conditions like osteoarthritis have caused widespread damage, creating a new, smooth bearing surface for stable movement.
Partial Knee Arthroplasty (PKA), or Unicompartmental Knee Arthroplasty (UKA), is a less invasive option. This procedure replaces only one damaged compartment, usually the medial side, preserving healthy cartilage and bone in the other two areas. Candidates for PKA must have healthy ligaments and undamaged cartilage remaining.
PKA offers advantages like less bone removal and reduced soft tissue disruption, potentially leading to faster recovery and a more natural feeling knee. However, PKA is not suitable for patients with inflammatory arthritis or damage spanning multiple compartments. The choice between total and partial replacement depends on the location and severity of the joint disease.
Implant Design and Function
The internal mechanism of the prosthetic is engineered for articulation and movement. One design feature involves how the plastic spacer interacts with the metal tibial tray.
Fixed-bearing implants use a polyethylene insert securely locked or cemented onto the metal base plate of the tibia. This design offers maximum stability because the plastic component does not move relative to the tibial component, making it a reliable option for most patients.
Mobile-bearing implants utilize a plastic insert that can rotate or move slightly within the tibial tray, potentially mimicking more natural knee motion. This movement may help distribute forces more evenly, theoretically reducing wear on the polyethylene. However, this design requires greater stability from the patient’s ligaments to prevent dislocation.
A major design consideration is the management of the posterior cruciate ligament (PCL), which prevents the shin bone from slipping backward. Posterior Cruciate Retaining implants preserve the patient’s PCL, relying on the natural ligament for stability and a more normal gait pattern when the PCL is healthy and functional.
If the PCL is damaged or removed during surgery, a Posterior Stabilized implant is used. This design incorporates a built-in mechanical feature: a central post on the tibial component that fits into a cam mechanism on the femoral component. This post-and-cam system replaces the function of the PCL, providing necessary stability.
Materials Used in Prosthetics
The composition of artificial joint components is a major factor in the implant’s longevity and biocompatibility. The metal components (the femoral component and tibial tray) are typically made from durable metal alloys. Cobalt-Chromium and Titanium alloys are common choices due to their strength, corrosion resistance, and ability to be securely fixed to the bone.
The bearing surface separating the metal components is a plastic spacer made of polyethylene. This material acts as artificial cartilage, providing a smooth surface for gliding motion. Modern prosthetics often use highly cross-linked polyethylene, which is modified to improve its resistance to wear over time.
Ceramic materials are sometimes used to form joint surfaces, particularly for patients with sensitivities or allergies to the nickel content found in standard metal alloys. Selecting materials that minimize friction and maximize lifespan is the primary goal.
Addressing Previous Replacements
When a primary knee replacement fails due to infection, instability, or mechanical loosening, a specialized procedure called Revision Knee Arthroplasty is performed. This complex surgery requires a distinct set of implants designed to address the challenges of a failed joint. The hardware used in revisions is structurally different from primary components.
Revision implants are typically larger and more modular, consisting of multiple interchangeable pieces assembled to fit the patient’s needs. These components often feature longer stems that extend further into the femur and tibia to gain better purchase in bone that may have been weakened or lost.
They are engineered to compensate for significant bone loss and provide greater stability when surrounding ligaments and soft tissues are compromised. The modularity allows the surgeon to customize the implant to manage instability and bone deficiency encountered during the procedure, ensuring stability and long-term function.