Determination of in-vivo Glenohumeral Translation at Zero Position using Model-based 3D-to-2D Registration

Abstract

Joint damage in overhand athletes is believed to result from inadequate control of glenohumeral translation. It is assumed that normal shoulder stability limits glenohumeral translation, but posterosuperior or anterior humeral translation has been reported during abduction and external rotation in the pathological throwing shoulder. The purpose of this study was to measure glenohumeral translation in-vivo during active internal/external rotation at the zero position using model-based 3D-to-2D registration. 10 healthy shoulders (average 31.1 years old) were studied. 3D models of the scapula and humerus were created from CT images and fluoroscopic images were recorded during active rotation (0∼140°) at zero position. 3D motions of the scapula and humerus were determined using model-based 3D-to-2D registration to obtain 6 degrees-of-freedom shoulder kinematics. Glenohumeral translation was determined in each data frame by finding the location on the humeral head with the smallest separation from the plane of the glenoid. Humeral translation was referenced in the superior/inferior direction to the midpoint of the long axis. Motion data were grouped into 10° intervals of active rotation.The humerus moved an average of 1.7mm, from an inferior location to the glenoid center, during active arm rotation at zero position. The humeral head was centered within 1mm from the glenoid center below 80° ER. Humeral translations were consistent between shoulders, with average 0.6mm standard deviation across the range of rotation. Glenohumeral translation in healthy shoulders during a slow simulated throwing motion is small-approximately 2mm, and the humerus remains centered on the glenoid where the articulation is maximally congruent. Additional 3D fluoroscopic analysis of shoulder kinematics in throwing athletes, and individuals with injured shoulders, likely will provide useful information to better prevent and diagnose shoulder injuries, enhance rehabilitation, and improve surgical treatments.