A NOVEL OSTEOTOMY TECHNIQUE IMPROVES RESTORATION OF HUMERAL HEAD GEOMETRY IN SHOULDER ARTHROPLASTY

Abstract

Introduction: The osteotomy in shoulder arthroplasty is based on the assumption that the resected articular segment corresponds to a segment of a sphere oriented, identically, in inclination and retroversion to the original humeral head(1). A previous report has suggested that the traditional osteotomy technique performed along the antero-superior part of the anatomical neck does not accurately replicate inclination and retroversion of the humeral head(2). We hypothesize that a simulated osteotomy performed along the antero-inferior anatomical neck resects a portion of the humeral head similarly oriented to the original head in terms of inclination and retroversion, and, more closely matches head diameter and radius of curvature when compared to the traditional osteotomy approach.

Methods: Twenty-eight fresh frozen cadaveric full arms were dissected free of soft tissue. Lines, points and surfaces were identified on each specimen. A Microscribe digitizer was used to digitize the points and lines. Data were imported into Rhinoceros NURBS modelling software and graphically modelled. The following parameters were used to describe the humeral head geometry: the longitudinal and axial radii of curvature (RoC) of the articular surface; the inclination angle (ƒÑ) and retroversion angle (ƒÒ). To simulate the traditional osteotomy, a plane was constructed using points at the anterior portion of the anatomical neck. The new osteotomy plane was formed using points at the antero-inferior anatomical neck. Paired Student’s t-test was used to compare techniques.

Results: No differences were found between the axial RoC of the resected segment for the new technique (22.5mm) when compared to the original head (22.5mm); a difference was found for the old osteotomy technique (23.0mm). In the coronal plane, no differences were found for the RoC of both the new and traditional techniques when compared to the original head. The axial and coronal diameters of the osteotomized surface were significantly different for both techniques. However, the mean difference between the axial and coronal diameters for the new technique was 2.4mm and, for the traditional technique, 3.2mm. Significant differences in retroversion of the resected surface were found when the new osteotomy technique (24.5deg) and traditional technique (40.5deg) were compared to the original head (29.0deg). Further, significant differences in inclination were found, when the new osteotomy technique (129.5deg) and traditional technique (132.1deg) were compared to the original head inclination (136.9deg).

Discussion: This study found that an osteotomy performed along the anteroinferior part of the anatomical neck removes an articular segment that is more spherical than a segment removed by the traditional osteotomy approach. Although significantly different from the original head, the retroversion associated with the new technique more closely matches the anatomy when compared to the traditional technique. The new osteotomy decreased the inclination angle by 7 degrees. This finding is unlikely to be clinically relevant. Cadaveric studies will reveal the accuracy of an anatomical reconstruction using the novel osteotomy approach.