Introduction

Modern prostheses of the 3rd and 4th generation facilitate a precise adjustment to various humeral anatomies. This provides major advantages regarding soft tissue balancing and the reconstruction of the rotational center. Thus, high expectations are linked to the use of modern shoulder prostheses compared to conventional designs.

Introduction: In most of the reported series, scapular notching in inverse shoulder arthroplasty has been identified as a major problem. Therefore, a novel concept has been developed in order to minimize the incidence and the evolution (pathophysiology) of scapular notching. The current cohort study is now large enough to examine the results with special attention on notching.

Methods: A dual peg design of the metaglene with CaP coating provides high primary and secondary stability. “Geometrical” notching is reduced by inferior (eccentric) fixation of the glenosphere on the metaglene, beveling of the medial part of the humeral inlay, and by the choice of three different sizes of the glenosphere (36, 39 and 42). “Biological” notching is addressed by inversion of the components: the epiphysis – as the mobile part – is metallic. Its contact to the scapula, should this occur, cannot lead to polyethylene wear. This study is a prospective multicentric study on Affinis Inverse and Affinis Fracture Inverse shoulder prosthesis (Mathys Ltd Bettlach, Switzerland), which is running in 7 European hospitals since December 2007. All cases but two (lost to follow-up) are included.

Preoperative and all postoperative radiographs were reviewed. Notching has been graded 0 to 4, on a scale adapted after Sirveaux.

Results: At submission deadline for the abstract, 163 cases were included. Grade 1 notching was detected in 8 cases (4.9%), and grade 2 notching in one. In those cases, notching developed early, but was not progressive over time. There were no cases of grade 3 or 4. In 17 cases, the X-rays were not assessable and therefore it was impossible to definitively rule out a possible grade 1 notching. None of the Affinis Fracture Inverse prostheses produced any notching. New bone apposition on the inferior aspect of the scapula was detected in 15 cases. We postulate this to be a metaplasia of the long head of the triceps due to local periosteal stimulation.

Conclusions: The present design leads to a very low rate of scapular notching. Even in the 9 cases where notching was present, it appears that the epiphysis only created the space it needed, without any ongoing osteolytic process beyond this. Specific prosthetic design improves both quantity and quality of scapular notching.

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.