There are two types of revision implants:
- Cemented prosthesis
- Uncemented prosthesis
A cemented prosthesis is carried out by using a type of epoxy cement that attaches the metal to the bone while an uncemented prosthesis has a fine mesh of holes on the surface that allows the bone to grow into the mesh and attach the prosthesis to the bone.
Both are widely used procedures. In many cases a combination of the two types are used. The patellar (Kneecap) portion of the prosthesis is commonly cemented when replaced. The decision on what method to be used is usually made by the surgeon based on patient’s age, bone quality and lifestyle.
Each prosthesis is made up of three main parts.
The bottom portion (Tibial Component) replaces the top surface of the lower bone. The stem for tibia used in the revision surgery is comparatively longer than the type used for primary knee replacements. This is due to the fact that bone of the tibia is usually not the same as when the initial replacement was done. The bone may be weaker, or there may be areas inside the tibia where the bone is missing.
The longer stem reaches further down the tibial canal and distributes the body weight in a balanced way. It also gives the body a greater surface area for healing. This helps to improve fixation of the implant to the bone inside the tibia. The top portion (Femoral Component) replaces the bottom surface of the upper bone (The Femur) and the groove where the patella fits. Like the tibial component used in revision, the femoral component may also need a long stem.
The kneecap portion (Patellar Component) replaces the surface of the patella where it glides in the groove on the femur.
The tibial component is usually made of two parts: a metal tray that is attached directly to the bone, and a plastic spacer that provides the slick surface. The femoral component is made out of metal. In some types of knee implants, the patellar component is made by a combination of metal and plastic.
The surgeon makes an incision down the front of the knee to allow access to the joint. Your surgeon always tries to minimize the damage to the soft tissues (muscles and the ligaments), while opening the knee joint.
In the second step, if the primary artificial joint was cemented then the cement has to be removed from inside the canal of femur and the tibia . As the bone is often fragile and the cement is hard while removing it can cause fracture to the femur or tibia during the operation. In most cases the surgeon will simply continue with the operation and fix the fracture as well.
During the surgery bone and tissue samples are removed and sent to the laboratory to check for the presence of infection. Normally, a new artificial joint will not be put in if infection is present.
Revision joint replacement is totally different from a primary joint replacement because, bone loss around the primary prosthesis makes the revision procedure more difficult.
The surgeon deals with this problem by placing a bone graft or some other material around the artificial joint to reinforce the bone. This bone graft may come from your own body, such as bone taken from the pelvis during the same operation and this type of graft is called an autograft.
If the amount of bone needed is too large to take from your body, your surgeon may choose to use bone graft from the bone bank. This type of bone graft that has been taken from someone else is called an allograft.
After application of bone and other materials to rebuild the tibia and femur, a new prosthesis is implanted. It is quite challenging to imitate or replace the natural shape of the bones after rebuilding the bone, so a specially designed prosthesis is usually needed.
To get the best and secure fixation between the tibial and femoral components, the surgeon adjusts and balances the soft tissues that surround the knee joint. This may require cutting or tightening the ligaments on each side of the knee. Afterward, the surgeon checks the fit of the new knee components with the knee bent and then with the knee straightened. Further adjustment can be made by changing the tibial plastic spacer (insert). In the end, the surgeon tries to get the best fit so that the knee is stable through a full range of movement.
When the tibial and femoral components are in place and the soft tissues have been balanced, the surgeon will address the patella. In some cases, the patella need not be revised, especially when the surgeon sees good fixation of the original patellar implant. Sometimes the old patella component is simply removed, allowing the bone on the back of the patella to glide against the smooth surface on the front of the revision femoral component. In either case, the surgeon checks to see that the patella is lined up correctly and that it rides normally within the groove in the front of the femur.
In some cases if an artificial joint fails, it may not be possible to put another artificial joint back in. This can occur if the primary joint has failed because of an infection that cannot be controlled, if the bone has been destroyed so much that it will not support an artificial joint, or if your medical condition will not tolerate a major operation.
Finally, the soft tissues of the knee are sewn back together, and metal staples or stitches are used to hold the skin incision together.