Total knee replacement, also known as total knee arthroplasty, is a surgical procedure that alleviates pain and restores function in a knee joint damaged by conditions like arthritis or severe injury. This intervention resurfaces affected areas of the knee with artificial components, allowing for smoother movement and improved mobility. Understanding the hardware involved is helpful for individuals considering or undergoing this procedure. This article explores the components, materials, design variations, and factors influencing implant longevity.
Understanding the Components
A total knee replacement system typically consists of three main parts that mimic the natural knee joint’s movement. The femoral component resurfaces the end of the thigh bone (femur), fitting over its lower end. This metallic cap replicates the natural shape of the femoral condyles and includes a groove for the kneecap.
The tibial component addresses the top of the shin bone (tibia). It features a metal tray that attaches directly to the bone, topped with a durable plastic spacer. This spacer, often made of polyethylene, provides the bearing surface for the femoral component, facilitating smooth articulation. The patellar component resurfaces the underside of the kneecap (patella), allowing it to glide smoothly within the femoral groove. This part is typically a dome-shaped plastic button, sometimes metal-backed. Together, these components replace damaged cartilage and bone surfaces, recreating a functional joint.
Materials Used in Implants
Material selection for knee replacement hardware prioritizes strength, wear resistance, and integration with the body. Common metal alloys include cobalt-chromium and titanium. Cobalt-chromium alloys are widely used for femoral and tibial components due to their high strength, corrosion resistance, and wear characteristics. Titanium and its alloys offer excellent strength, biocompatibility, and corrosion resistance, valued for their lightweight nature and ability to bond with bone.
High-grade plastics, specifically ultra-high molecular weight polyethylene (UHMWPE), are used for the tibial spacer and patellar component. UHMWPE is chosen for its wear resistance and low friction, allowing for smooth movement between metal components. Advancements in polyethylene technology have helped reduce wear-related issues. Some implants may also incorporate ceramic materials, either as a coating or for entire components, offering scratch resistance and reduced wear. Zirconium, a newer material, shows potential to reduce wear and extend implant longevity.
Different Design Approaches
Total knee replacement systems employ various design philosophies to optimize patient outcomes. One distinction lies in fixation: cemented versus cementless. Cemented fixation uses polymethylmethacrylate (PMMA) cement to secure components to the bone, providing immediate stability. Cementless fixation relies on a porous implant surface that encourages bone ingrowth over time, creating a biological bond.
Another design choice concerns the management of the posterior cruciate ligament (PCL). Cruciate-retaining (CR) designs preserve the patient’s PCL, aiming to maintain natural knee motion and proprioception. Posterior-stabilized (PS) designs involve removing the PCL and incorporating a mechanism within the implant to provide stability and replicate the PCL’s function. Both CR and PS designs have demonstrated similar functional outcomes and survival rates.
Implants also vary between fixed-bearing and mobile-bearing inserts. Fixed-bearing designs have the polyethylene tibial insert rigidly attached to the metal tibial tray. Mobile-bearing designs allow the polyethylene insert to rotate or translate slightly relative to the tibial tray, theoretically reducing stress and wear on the polyethylene and improving kinematics. While mobile-bearing designs aim to improve longevity by minimizing polyethylene wear and matching natural knee movement, clinical studies have not consistently shown a significant advantage over fixed-bearing designs in long-term outcomes.
Factors Affecting Hardware Lifespan
Several factors influence the longevity of total knee replacement hardware, with most implants expected to last at least 15 to 20 years. Patient activity levels impact wear and tear on the prosthetic components. Individuals engaging in high-impact activities may experience faster wear of the polyethylene insert and shorten the implant’s lifespan. Body weight plays a role, as increased weight places greater stress on the knee joint, accelerating implant wear and loosening.
Adherence to post-operative instructions, including rehabilitation and activity restrictions, helps protect the implant and promote healing. Surgical technique, including precise alignment and implantation, determines long-term success. Misalignment can lead to uneven loading and accelerated wear. The quality and wear characteristics of the implant materials directly contribute to the hardware’s durability.
Common Hardware-Related Concerns
Total knee replacements are successful, but certain hardware-related concerns can arise. Implant loosening is a common issue where prosthetic components detach from the bone, causing pain. This can result from natural wear and tear, infection, poor bone quality, or excessive activity. Polyethylene insert wear is another concern, where the plastic spacer degrades. This wear can produce particles that can trigger an inflammatory response, leading to bone loss and loosening.
Periprosthetic fracture, a break in the bone around the implant, is a serious but rare complication that often requires additional surgery. These fractures can occur in the femur, tibia, or patella, often associated with falls, high-energy injuries, or loose components. Allergic reactions to implant materials, though uncommon, can occur. Metals like nickel, chromium, and cobalt, found in some alloys, can cause hypersensitivity in susceptible individuals, leading to pain, swelling, rashes, or implant loosening. Modern implant designs and careful patient screening help minimize these issues.