The displacement of the rotation centre of the humeral head in relation to the axis of the shaft varies strongly between individuals. The Affinis shoulder endoprosthesis has a double excentric adjustment possibility that permits us to adapt the head to the medial and dorsal offset. So far, such examinations have taken place exclusively on anatomic preparations. This raises the question of whether the need for such a prosthetic system can be derived from the anatomic variation of the pivot points.
In 49 patients with an implanted Affinis shoulder prostheses, we calculated the individual rotation centres of the head from the position of the relocatable prosthetic cone and the excentric position of the head. In addition, we used the Constant score to record the clinical function.
In the examined case material, we needed the entire setting range of 12 mm mediolateral and 6 mm dorsoventral of the prosthesis, as it presented a great variation of the rotational centres of the head. The majority of the head centres were displaced in a posterior-lateral direction. The examination showed that none of the found anatomic head centres could have been reconstructed exactly with a conventional prosthesis. Prostheses with a single excentricity would allow correct adjustment in only 9 cases, as the adjustable rotational centres of the head are situated in an orbit, which limits the setting possibilities. In 20 of the 49 patients humeral head prosthesis were implanted in cases of primary osteoarthritis. The Constant score of this group improved from 18 points preoperatively to 76 points after 24.1 months in the mean.
The high variation of the head centres indicates the necessity of a freely adjustable system, such as in a double excentric bearing. This is a way that permits an optimal reconstruction of the anatomic conditions. Therefore all technical chances for bony balancing should be used to adapt the prosthesis to the bone and not the bone to the prosthesis.