Mechanics of the Scapula in Shoulder Function and Dysfunction

Through the sternoclavicular (SC) and acromioclavicular (AC) joints, scapular position and motion play a critical role in overall shoulder function and dysfunction. The predominant SC joint motion during arm elevation is posterior long-axis rotation. Simultaneously, the SC joint retracts and undergoes a small amount of elevation. Synchronous to these SC joint motions, during arm elevation, the AC joint undergoes upward rotation and posterior tilt. In addition, the AC joint allows the scapula to remain stable on the curved thoracic surface by internally rotating during arm elevation. These SC and AC joint motions couple to produce overall scapular upward rotation, posterior tilt, and internal or external rotation motions on the thorax as the arm is elevated. From these normal motion patterns, a wide variety of position and motion abnormalities have been identified. These SC, AC, and scapular motion abnormalities have also been associated with a broad range of shoulder pathologies. Research studies are further beginning to assess the mechanical impact of scapular motion deviations.