Implant designs vary in important ways to meet specific patient needs. Restoration of normal knee joint function is the goal of knee replacement surgery.
Some implants are designed for patients undergoing total knee surgery for the first time. This is called “primary” knee replacement.
Other implants are designed specially for people undergoing a second operation, called “revision surgery.” This is where it becomes necessary to remove the primary implant and occurs in a small percentage of cases.
Another variable is whether the implant is cemented or cementless. Most knee implants are affixed using a special bone cement similar to dental cement. Certain implants have been approved by the FDA to be implanted without bone cement and are secured biologically as the patient’s tissues grow and attach to a special porous texture that coats the implant. These are called cementless. Both types have advantages in different patient situations that your surgeon will assess. In many cases, both types are used in combination.
The most common knee implant consists of a femoral (thigh) component, a tibial (shin) and bearing components; and a patella (kneecap):
The femoral, or thighbone, component is made of metal (chromium-cobalt) and covers the lower end of the thighbone. It may be cemented to the bone or, for some implants, inserted without cement so that the patient’s tissues grow into the porous coating of the device. This natural bond between the patient’s tissue and the implant is called “biological fixation.”
The tibia, or shin bone, component is often called a “tray” and is typically made of metal (titanium or chromium-cobalt), and a plastic cushion, or bearing. The tibial component may be secured with cement or by biological fixation. The metal forms the base of this component, while the plastic (ultra-high molecular weight polyethylene) is attached to the top of the metal to serve as a bearing. This bearing creates a smooth gliding surface between the metal of the thigh and shin components.
The patella, or knee cap, component is made of either of plastic (polyethylene), or of a combination of plastic and metal. Again, this component may be fixed with or without cement.