Glenohumeral mechanics: A study of articular geometry, contact, and kinematics

Stereophotogrammetry was used to investigate the functional relations betwe en the articular surface geometry, contact patterns, and kinematics of the glenohumeral joint. Nine normal shoulder specimens were elevated in the sca pular plane by using simulated muscle forces in neutral rotation (NR) and s tarting rotation (SR). Motion was quantified by analyzing the translations of the geometric centers of the humeral head cartilage and bone surfaces re lative to the glenoid surface. In both NR and SR, the ranges of translation s 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 +/- 7.2 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 tr anslations than in NR. Kinematic analyses with the humeral head bone surfac e data yielded larger values of translation than analyses that used the car tilage surface data, suggesting that similar overestimations may occur in r adiographic motion studies. Results of this study demonstrate that smalt tr anslations of the humeral head center occurred in both SR and NR. The proxi mity of the origin of the helical axes to the geometric center of the humer al head articular surface confirmed that glenohumeral elevation is mainly r otation about this geometric center with small translations.