The size of the supraspinatus outlet during elevation of the arm in the frontal and sagittal plane: A 3-D model study

Objective. To quantify the size of the supraspinatus outlet as it is dictated by both the three-dimensional geometry of the shoulder and the relative orientation of the humerus with respect to the scapula during motions of the arm. Design. Previously obtained data of shoulder kinematics were brought into a geometrical model of the shoulder, derived from a cadaver study. Background. Knowledge of the parameters dictating the size of the supraspinatus outlet is essential for a better understanding of the impingement syndrome of the shoulder. Methods. A geometrical model, based on fitting spheres to various anatomical items of the shoulder was derived from three-dimensional position data of the gleno-humeral joint and coraco-acromial arch of 32 cadaver shoulders. Kinematical data were collected from 10 healthy volunteers. The geometrical and kinematical data were combined to study the supraspinatus outlet during elevation of the humerus in the frontal and sagittal plane. Results. No single geometry parameter correlated significantly with the initial size of the outlet. During arm elevation, the greater tuberosity was moved away from the coraco-acromial arch quite effectively resulting in narrowing of the outlet during elevation in the frontal plane from 60° to 120° only. Deviations from the average were quite substantial. This was caused by kinematical and especially geometrical variability. Conclusion. The size of the outlet is dictated by both the geometry and kinematics of the gleno-humeral joint. Assessment of the individual susceptibility to impingement requires three-dimensional viewing techniques including three-dimensional movements of both the scapula and humerus. Relevance. Little is known about etiology and pathogenesis of various shoulder disorders such as the impingement syndrome. The supraspinatus outlet plays probably a key role. More knowledge on the architecture of the outlet is required for a better understanding