In knee replacement surgery, orthopedic surgeons use artificial components, known as implants, to resurface damaged knees. These implants come in various types, and the choice depends on several factors, including the patient’s needs, the surgeon’s expertise, and the performance record of the implant. This article will provide a detailed overview of total knee replacement implants and their different designs.
Implants are typically made of metal alloys, ceramic materials, or strong plastic parts. During total knee replacement, up to three bone surfaces are replaced:
- The lower end of the femur: This component curves around the end of the thighbone and is grooved to allow smooth movement of the kneecap.
- The top surface of the tibia: The tibial component is a flat metal platform with a cushion of durable plastic. Some designs attach the plastic directly to the bone for additional stability.
- The back surface of the patella: The patellar component is a dome-shaped piece of polyethylene that replicates the shape of the kneecap.
These components are designed to facilitate smoother movement and reduce wear on the implant by ensuring that metal always borders with plastic.
Current implant designs aim to mimic the complexity of a healthy knee joint. They take into account the role of ligaments in providing stability and offer options to preserve or substitute for these ligaments. There are over 150 designs available today, including:
One commonly used implant design is the posterior-stabilized component. In this design, parts of the implant replace the posterior cruciate ligament (PCL) to prevent the thighbone from sliding too far forward during knee flexion.
These designs preserve the posterior cruciate ligament while removing the anterior cruciate ligament. They are suitable for patients with a healthy posterior cruciate ligament that continues to stabilize the knee joint.
Bicruciate-retaining designs preserve both the anterior and posterior cruciate ligaments. By saving both ligaments, these implants aim to provide a more natural knee function and sensation.
In cases where only one side of the knee joint is damaged, a partial knee replacement may be performed using smaller implants. This approach, known as unicompartmental knee replacement, resurfaces only the damaged part of the knee.
Types of Tibial Components
The tibial component, which forms the top surface of the tibia, can be classified into different types:
Most patients receive fixed-bearing implants, where the polyethylene is securely attached to the metal implant beneath. The femoral component then rolls on this cushioned surface.
Mobile-bearing implants allow slight rotation of the polyethylene insert within the metal tibial tray. This design aims to provide a few degrees of greater rotation to the medial and lateral sides of the knee.
Fixed- vs. Mobile-Bearing Implants
While mobile-bearing implants offer potential benefits in terms of wear and function, research has not yet definitively proven their improved durability over fixed-bearing implants. Mobile-bearing implants rely more on soft tissue support and may be costlier.
The metal parts of knee implants are typically made of titanium or cobalt-chromium alloys, while the plastic parts consist of ultra high molecular weight polyethylene. Ceramic or ceramic/metal mixtures are also used in some cases. The choice of materials depends on their biocompatibility, ability to replicate knee structures, and long-term durability.
Different types of fixation methods are used to secure knee implants to the bone:
- Cemented fixation: The most common method, where implants are held in place with bone cement.
- Cementless fixation: Implants can be press-fit onto the bone, relying on new bone growth into the implant’s surface.
- Hybrid fixation: In total knee replacement, the femoral component may be inserted without cement, while the tibial and patellar components are inserted with cement.
The choice of fixation method depends on several factors, which your surgeon will evaluate and discuss with you.
Revision components are used in cases where the original implant wears out or causes complications. These components have longer stems for better attachment to the bones and may include metal pieces to replace missing bone. Revision components often have a larger cam to provide greater stability, and in severe cases, a metal “hinge” may be used to connect the femur and tibia.
Total knee replacement implants offer a solution to patients with severe knee damage. The choice of implants depends on various factors, including patient needs, surgeon expertise, and implant performance. Understanding the different implant components, designs, materials, fixation methods, and revision options will empower patients and encourage informed discussions with their healthcare providers.
Source: AAOS: Total Knee Replacement