[Radio-cinematographic study of active elevation of the prosthetic shoulder]

In order to verify whether it is possible to reproduce the physiologic anterior elevation of the shoulder with a non constrained Neer type prosthesis, the authors have examined the dynamic comportement of 21 total shoulder arthroplasties. Recording of anterior active elevation with video-fluoroscopy and evaluation glenohumeral motion and scapulothoracic motion allowed to distinguish 3 types of biomechanics after total shoulder replacement. A first group of prosthesis (4 cases) had normal biomechanics with conservation of a normal scapulohumeral rythm. This concerned arthroplasties performed for necrosis or osteoarthritis with no cuff tear or loose bone stock and no technical error. A second group of prosthesis (10 cases) had an abnormal biomechanics. There was a superior excursion of the humeral head prosthesis without real glenohumeral motion. A more severe pathology (cuff tear arthropathy, rheumatoid arthritis, old traumas) with bone and/or muscular destruction was sufficient to explain this elevation possible only because of the scapulothoracic motion. Finally, there was a third group of prosthesis (7 cases) which had a reversed scapulohumeral rythm: there was less motion between the prosthetic components but the scapulothoracic motion was unchanged. Three factors can be responsible for this reversed scapulohumeral rythm: 1) The initial pathology, because of the difference in the cuff trophicity. 2) The surgical technic because of the difficulties to respect the position of the joint line and the lever arms. 3) The prosthesis itself, because of the higher degree of conformity and stability between the prosthetic components.