Anthropometric study of normal glenohumeral relationships

Ninety-three cadaveric proximal humeri and matching scapulae with no evidence of shoulder disease were obtained. High-resolution roentgenograms were made. They were the digitized, and a custom computer program was used to obtain periosteal and endosteal dimensions (humeral canal width, shaft width, tuberosity offset, head offset, radius of curvature, head diameter, canal flare index, glenoid height and depth, arc of enclosure, radius of curvature, and depth of cancellous bone, among others). Statistical analyses included correlations among pairs of dimensions, regression analysis, and confidence intervals to estimate each geometric parameter. To represent the degree of conformity and constraint between the humeral head and glenoid, a conformity index (radius of head/radius glenoid) and constraint index (arc of enclosure/360) were calculated for each shoulder. Most of the parameters measured approximated a Gaussian distribution. Pairwise correlations of the geometric parameters showed that many were significantly associated at the 5% level (p < 0.05) with a high correlation coefficient (r > 0.4), meaning that the variations between the parameters were not purely random but rather they were related and that this relationship was useful in a predictive sense. With these key parameters identified, one can design components with an optimum fit or match an existing design to a patient's anatomy with a high degree of accuracy. No correlation was found between the radii of curvature for the humeral head and glenoid in either the coronal or sagittal plane or between the humeral head diameter and arc of enclosure. The mean conformity index was 0.72 in the coronal and 0.63 in the sagittal plane. Only 16% of the specimens had a conformity index greater than 0.9, indicating the radii of curvature matched. The vast majority had a more curved humeral head and flatter glenoid. Also, there was more constraint to the glenoid in the coronal versus sagittal plane (constraint index = 0.18 vs 0.13). These anatomic features help prevent superior-inferior translation of the humeral head but allow translation in the sagittal plane. This database of basic anatomic geometry defines the anatomic relationships of the proximal humerus and glenoid cavity that allow for a precise bone-implant fit and assesses the match between the shape of existing components and the patient's anatomy.