Anteroposterior Centering of the Humeral Head on the Glenoid In Vivo

Background: The capsule and ligaments are gener-ally viewed as the primary stabilizers of the glenohu-meral joint, but many important activities are performed in midrange positions in which these structures are lax. Hypothesis: In vivo, the humeral head can be cen-tered in the glenoid, even when the shoulder is in positions in which the capsule is lax and even when the shoulder is passively positioned. Study Design: Controlled laboratory study. Methods: We documented the centering of the humeral head in the relaxed shoulders of six subjects using open-magnet magnetic resonance imaging scans. Results: While these shoulders were passively placed in midrange positions (those not at the extremes of motion)