Geometry of the proximal humerus and implications for prosthetic design.

The purpose of this study was to add critical information to the data already available on anthropometry of the proximal humerus. Two hundred macerated humeri were examined. Measurements were taken either directly on the bones or on standardized radiographic projections. The methodology was validated and showed a mean interobserver correlation of 0.94 +/- 0.067. Results were expressed in mean values, first SD, and minimum and maximum values, as well as the 10th and 90th percentiles. The frontal radius of the head ranged between 21 and 26.5 mm (10th respectively 90th percentile). The frontal diameter of the base of the head ranged between 39.4 and 50 mm. The head height ranged between 14.4 and 18.8 mm. The frontal radius-head height ratio ranged between 0.64 and 0.77. The inclination of the head ranged between 132 degrees and 142 degrees. The medial offset ranged between 3.9 and 8.6 mm. The posterior offset ranged between -0.4 and 3.2 mm. The greater tuberosity offset (distance between the axis of the proximal humerus and the most medial insertion point of the supraspinatus tendon) ranged between 2.5 and 9.2 mm. Retrotorsion ranged between 7 degrees and 38.5 degrees. The distance from the bicipital groove to the head equator ranged between 6 and 10.5 mm. The anatomy of the proximal humerus showed a wide range for variables such as the medial offset and the greater tuberosity offset but was surprisingly constant for the inclination and relative dimensions of the head. The implications for prosthetic design are as follows: stem design and insertion should respect the insertion facet of the supraspinatus, a constant head inclination is an adequate approximation, only one head height per radius is required, and the capability for adjustment of medial offset is mandatory.