Problem: The displacement of the rotation centre of the humeral head in relation to the shaft axis strongly varies individually. In order to measure the position of the pivot points of the head, the Affinis shoulder prosthesis has a double eccentric adjustment possibility that permits to adapt the head to the medial and dorsal offset. So far, such examinations took place exclusively on the anatomical preparation. This raises the question of whether the need for such a prosthetic system can be derived from the anatomical variation of the pivot points.

Method: In 126 patients with an Affinis shoulder prosthesis, we calculated the individual rotation centres of the head from the position of the adjustable prosthetic cone and the eccentric position of the head. In addition, we used the Constant Score to record the clinical function.

Results: We found great variation of the rotational centres. Therefore we needed the entire setting range of 12mm mediolateral and 6mm dorsoventral. The examination showed that none of the found anatomic head centres could have been reconstructed exactly with a conventional prosthesis. Prosthesis with a single eccentricity would allow correct adjustment in only 22 cases, as the adjustable rotational centres of the head are situated on a circle, which limits the setting possibilities. The Constant Score of the total group improved from 29% preoperatively to 94% after 2 years.

Conclusions: The high variation of the head centres indicates the necessity of a freely adjustable system, such as in a double eccentric bearing. This is the only way that permits an optimal reconstruction of the anatomical conditions – it prevents an increased tension of the rotator cuff, reduces the eccentric loading of the glenoid and creates the prerequisites for the anatomical kinematics. The good clinical results speak for the need to observe bony balancing.

The aim of our study was to evaluate if PTH is able to increase the trabecular density of osteoporotic bone at the site of an implant and whether the anabolic effect of PTH at this side is stronger then the effect of an osteoclast inhibitor like alendronate.

48 cement rod was inserted in the tibia of 48 female rats, of which 36 had been ovariectomized. The cement rods, which served as implants, were made of Palacos R bone cement. After implantation, the 36 ovariectomized rats were divided in 3 groups. One was injected subcutaneusly with PTH (1–34) at a dose of 60 g/kg BW. The second was injected with alendronate at a dose of 205 g/kg BW. The third with vehicle only. The remaining 12 sham operated rats were also injected with vehicle only. All injections were given three times a week and the rats were killed 2 weeks after implantation.

The tibial segments around the hole of the rods were prepared histologically. Thus the surfaces which had been in contact with the rod appeared as straight lines and could be analyzed histomorphometricly. The trabecular density of the bone closest to the implant was measured. One femur of all animals was used for measurement by DEXA.

There was a substantial increase in the trabecular density close to the rods with PTH treatment (Anova p=0.002). PTH lead to a trabecular density of 89%, where as the ovariectomized animals revealed a trabecular density of 58% and the sham operated control of 68%. No significant increase of implant related trabecular density could be found in the alendronate treated group. In this group a density of 72% was established. DEXA showed the expected differences in bone mineral content (Anova p=0.001).

In this study, intermittent PTH treatment increased implant-related trabecular density in osteoporotic bone after 2 weeks. No such positive effect could be found with alendronate treatment at such a short period of time. We think the reason for this phenomenon could be the early onset of the anabolic PTH effect on regenerating bone, whereas alendronate is thought to only inhibit bone resorption, which might lead to a later effect.

The early onset of PTH effects even in osteoporotic bone suggests that intermittent PTH treatment might lead to an increased micro-interlock between implant and bone and might therefore be considered as a possible drug to enhance incorporation of orthopedic implants.