RESUMO
BACKGROUND: The capsule and ligaments are generally viewed as the primary stabilizers of the glenohumeral joint, but many important activities are performed in midrange positions in which these structures are lax. HYPOTHESIS: In vivo, the humeral head can be centered 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), the humeral head center was never more than 2.2 mm from the glenoid center (mean + 0.1 +/- 1.2 mm). CONCLUSIONS: The results suggest that mechanisms other than ligamentous restraint, such as the compressive effect of resting muscle tone into the conforming concavity of the glenoid, may be sufficient to maintain centering of the glenohumeral joint. Further exploration of these mechanisms may lead to methods other than ligament tightening or capsular shrinkage for restoration of stability to joints that are unstable in the midrange of motion. CLINICAL RELEVANCE: In that many patients with unstable shoulders demonstrate instability in midrange positions, it is hoped that further study of living shoulders will lead to a more effective understanding of the nonligament mechanisms of shoulder stability and the ways in which these stabilizing mechanisms can be restored.