Instability is one of the most common complications after shoulder arthroplasty. The literature cites subluxation or luxation to occure between 0% and 38% in various studies. Instabilities may present either as subluxation or frank dislocation, and may be directed in an anterior, posterior, inferior or, depending on the state of the rotator cuff, cranial direction. The stability of any shoulder joint is given by the balance of the muscles directing the forces around the shoulder joint in association with the passive stabilizers of the shoulder joint capsule as well as the bony contours between glenoid and humeral head. Any disturbance of this delicate balance will lead the shoulder into instability, particular so if bony errosion patterns such as posterior glenoid wear in osteoarthritics will develop subluxation early on. Therefore implantation of any prosthesis is required to be done in the appropriate version as to avoid secondary instability through the prosthetic components. In the study undertaken here instability was found to be the most common complication in 44 shoulder revision surgeries. The result with an avarage Score of 41.9 recorded after Constant demonstrates that the excellent and good results obtained with primary arthroplasties can not be expected in revision surgery. Posterior instability may be present just as well as the more easily observed anterior instability. Separate to frank luxation or instability is the late cranialisation of the rotator cuff deficient shoulder which, although resulting in many cases in a superior anterior subluxation, will mostly be seen as a late complication after arthroplasty.
