An anatomic study of 65 cadaveric humeri allowed determination of the main features of the superior part of the humerus and design of a modular and adaptable humeral prosthesis. This cemented prosthesis is made in 3 different stem sizes, 7 humeral head sizes, and a variable humeral neck component. The 4 different humeral necks (125 degrees, 130 degrees, 135 degrees, and 140 degrees) allow the head to be cut at the level of the anatomic neck and adaptation of the prosthesis to the patient's anatomy with respect to the variable inclination and retroversion. An eccentric dial is located on the undersurface of the head; 8 positions are possible reproducing the posterior and medial offset of the humeral head with regard to the diaphysis. The large variety of components used for the first 101 implanted prostheses confirm the extreme variability of the anatomy and the need for a flexible shoulder prosthesis system. The glenoid component is made of polyethylene and is available in 3 different sizes to be used with a cemented keel. Eighty-six shoulders, which were monitored from 24 to 65 months, were evaluated with the Constant score. Primary glenohumeral arthritis (46 cases) and avascular necrosis (11 cases) yielded the best results, with scores of 95% and 86%. Rheumatoid arthritis (20 cases) scored 78%, although post-traumatic arthritis (11 cases) and cuff tear arthropathy (13 cases) had a less satisfactory result: 66% and 61%, respectively. Radiologic results showed 2 glenoids loosening and 48% of glenoids with radiolucent lines, with half of them being partial. An 8% complication rate has been observed, leading to 6 revisions (6%), in 2 cases for infection, 2 cases for secondary rupture of the subscapularis, and 2 cases for glenoid loosening.