Joint replacements are wonderful but they aren’t 100 per cent perfect yet. In the case of a serious fracture with bone loss, it may be impossible to restore the normal length of the humerus (upper arm bone).
A change in the length of the bone alters the way the muscles pull to move the arm. Sometimes during the operation, muscle tension is changed: the muscle tensioning can be too much or not enough.
These kind of problems are more likely with a humeral fracture or tumor. It may be referred to as an inferior instability. The best way to find out what to do is go back to your orthopedic surgeon. An X-ray and examination are needed to find out what’s causing the problem. You may be a good candidate for a rehab program but further surgery may also be the answer.