Shoulder arthroplasty is the treatment of choice for a range of degenerative diseases. However, long term follow-up suggests almost half of patients graded their treatment as unsatisfactory. Component malalignment is thought the most likely cause. The anterior anatomical neck is used as a reference for the osteotomy. The objective of the study was to analyse the cartilage/metaphyseal interface to identify reference points that may recover version accurately.

Twenty-four humeri were scanned using a Microscribe digitiser and surface laser scanner. Modelling software was used to analyse the Cartilage/metaphyseal interface. The retroversion angle was calculated for the normal geometry and for the standard osteotomy. An ideal osteotomy plane was then created for each specimen and the distance from the cartilage/metaphyseal interface determined, identifying points of least deviation. The reference points were used to simulate a new osteotomy for which retroversion was calculated. The novel osteotomy and traditional osteotomy were compared to the normal geometry.

The mean retroversion for the normal geometry was 18.5±9.0 degrees. The mean retroversion for the traditional osteotomy technique was 29.5±10.7 degrees, significantly different from the original (p<0.001). The mean retroversion using the novel osteotomy was 18.9±8.9 degrees and similar to the normal geometry (p=0.528).

The traditional osteotomy resulted in a mean increase in retroversion of 38%. The increase in version may result in eccentric loading at the glenoid and alter rotator cuff balance. The novel osteotomy resulted in more accurate recovery of head geometry and may improve clinical outcome.