Glenohumeral mechanics: a study of articular geometry, contact, and kinematics.

Stereophotogrammetry was used to investigate the functional relations between the articular surface geometry, contact patterns, and kinematics of the glenohumeral joint. Nine normal shoulder specimens were elevated in the scapular plane by using simulated muscle forces in neutral rotation (NR) and starting rotation (SR). Motion was quantified by analyzing the translations of the geometric centers of the humeral head cartilage and bone surfaces relative to the glenoid surface. In both NR and SR, the ranges of translations of the center of the humeral head cartilage surface were greatest in the inferior-superior direction (NR 2.0 +/- 0.7 mm, SR 2.9 +/- 1.2 mm). Results of this study also show that joints with less congruence of the articular surfaces exhibit larger translations, and elevation in SR yields greater translations than in NR. Kinematic analyses with the humeral head bone surface data yielded larger values of translation than analyses that used the cartilage surface data, suggesting that similar overestimations may occur in radiographic motion studies. Results of this study demonstrate that small translations of the humeral head center occurred in both SR and NR. The proximity of the origin of the helical axes to the geometric center of the humeral head articular surface confirmed that glenohumeral elevation is mainly rotation about this geometric center with small translations.