The steps involved in replacing the knee begin with making an incision on the front of the knee to allow access to the knee joint. There are several different approaches used to make the incision, usually based on the surgeon’s training and preferences.
Shaping the Distal Femoral Bone: Once the knee joint is entered, a special cutting jig is placed on the end of the femur. This jig is used to make sure that the bone is cut in the proper alignment to the leg’s original angles, even if the arthritis has made you bowlegged or knock-kneed. The jig is used to cut several pieces of bone from the distal femur so that the artificial knee can replace the worn surfaces with a metal surface.
Preparing the Tibial Bone: Attention is then turned toward the lower bone, the tibia. The top of the tibia is cut using another jig that ensures the alignment is satisfactory.
Preparing the Patella: The undersurface of the patella is removed.
Placing the Femoral Component: The metal femoral component is then placed on the femur. When using an uncemented femoral component, the prosthesis is held on the end of the bone through a taper on the end of the bone. In addition, the metal prosthesis is cut so that it matches the taper almost exactly. Driving the metal component onto the end of the bone holds the component in place by friction. The stable implant will allow bone tissue to grow into the porous surface, providing long-term stability. With a cemented femoral component, an epoxy cement is used to attach the metal prosthesis to the bone.
Placing the Tibial Components: The metal tray that will hold the polyethylene spacer is attached to the top of the tibia. The metal tray is either cemented into place, or may be held with screws if the component is uncemented. The screws are primarily used to hold the tibial tray in place until the bone grows into the porous coating. (The screws remain in place and are not removed.)
The plastic spacer is then attached to the metal tray of the tibial component. If this component wears out while the rest of the artificial knee is sound, it can be replaced.
Placing the Patellar Component: The patella button is usually cemented into place behind the patella.
The artificial knee replacement is now complete.
Closing the Incision: There are several ways that orthopaedic surgeons can close the incision after performing an artificial joint replacement. Stainless steel staples are popular with many orthopaedic surgeons because they are easy to put in and easy to take out. This can reduce time in the operating room. The stainless steel staples are one of the most inert types of sutures, meaning they have a very low risk of allergic reaction by the patient.
Some surgeons prefer using sutures that dissolve on their own after several weeks. These stitches are normally put in just under the skin. The advantage of this type of closure is that you don’t have to have your stitches taken out! Usually there are special tape closures (sometimes called “butterfly” tapes or “steri-strips”) that are used to hold the edges of the skin closed for the first few days. If you see strips of tape across the incision, this is probably the type of closure that was done. This type of incision closure takes a bit more time in the operating room. There is also a small chance that you may have an allergic reaction to the stitch material that delays the healing of the incision, but this risk is pretty small.
Finally, many surgeons still use the old “tried and true” nylon stitches one at a time. Nylon has withstood the test of time and is nearly as inert as stainless steel. It is strong and holds well until it is removed (somewhere between 10 to 14 days after surgery).