The most common reason for kneecap to dislocate after primary knee replacement surgery is because the femoral implant is not aligned correctly in the right rotation on the thigh bone. To clarify, if the femoral implant is rotated internally during the surgery, then the kneecap dislocates to the outside because of the forces pulling the kneecap to the outside of the knee joint. The way to resolve it is, unfortunately, revision knee surgery in which the femoral component has to be removed and replaced correctly in correct rotation.
When the surgery is done, in addition to rotating the component and placing it in the correct position, the stem typically has to be placed into the thigh bone to provide additional stability. Depending upon the implant that was used for the primary knee replacement surgery, the tibial component (component going into the shin) also may need to be revised, because the revision knee replacement femoral component will not articulate with the distal component that is placed into the shin bone.
In my experience, most knee revisions typically need replacement of all three components of the knee joint, the femoral component, the tibial component, as well as the plastic that is between the femoral and the tibial component. It is rarely needed to replace the component under the kneecap even when the kneecap is dislocating. Under the circumstance where the kneecap dislocates in the postoperative period after a primary knee replacement surgery, its important to check if the kneecap was dislocating before the knee replacement surgery and there was arthritis associated with subluxation or dislocation of the kneecap.
Under those circumstances, there are two options available: To do a femoral osteotomy to rotate the femur along with doing a knee replacement surgery, which I feel is extensive procedure. The other option, which is a much lesser procedure, is realigning the tibial tubercle when the revision knee replacement surgery is performed. I get a CT scan If I see any rotational malalignment.