Conversion of hemi into reverse shoulder arthroplasty: implant design limitations

Purpose Cranial migration of shoulder hemiarthroplasties due to rotator cuff insufficiency typically requires conversion into a reverse total shoulder arthroplasty. This study was conducted to analyze differences between the height and offset of six implants designed to enable conversion of a hemiarthroplasty into a reverse system. Methods Anteroposterior radiographs of 40 shoulders were taken. An image analyzing software was used to simulate the implantation of the hemiprostheses. Then the implant was dissembled, leaving on the stem within the humeral shaft. Finally, the implantation of a reverse system was simulated using the stem in the same position. Values are reported as Delta-height and Delta-offset +/- standard deviation. Significance was assumed for P < 0.05. Results The least decrease in height was determined for Implantcast with 11.6 +/- 3.3 mm, followed by DePuy (16 +/- 5.7 mm) and the greatest for Tornier with 33 +/- 5.3 mm. No significant differences were found among Exactech, Mathys and Zimmer. The largest offset-deviation was calculated for DePuy (-21.7 +/- 3.7 mm) and the smallest for Implantcast (-3.3 +/- 2.8 mm) and Tornier (1.5 +/- 5.7 mm). Conclusions Due to the modular stem, the system of Implantcast can be converted in a reverse system with the least changes in height and offset. For the other manufacturers it does not seem possible to convert a hemiprosthesis to a reversed prosthesis without accepting additional tension of the deltoid muscle. Further experimental studies have to analyze the changes in deltoid abduction moments after conversion of a hemi- into a reversed prosthesis.