Three-dimensional acromioclavicular joint motions during elevation of the arm

STUDY DESIGN: Descriptive laboratory study. OBJECTIVES: To determine the 3-dimensional motions occurring between the scapula relative to the clavicle at the acromioclavicular joint during humeral elevation in the scapular plane. BACKGROUND: Shoulder pathology is commonly treated through exercise programs aimed at correcting scapular motion abnormalities. However, little is known regarding how acromioclavicular joint motions contribute to normal and abnormal scapulothoracic motion. METHODS AND MEASURES: Thirty subjects (16 males, 14 females) participated. Subjects with positive symptoms on clinical exam or past history of shoulder pathology, trauma, or surgery were excluded. Electromagnetic surface motion analysis was performed tracking the thorax, clavicle, scapula, and humerus. Subjects performed 3 repetitions of scapular plane abduction, Passive motion data were, also collected for scapular plane abduction, from cadaver specimens, Data were analyzed using within-session reliability and descriptive statistics as well as repeated-measures analyses of variance(ANOVAs) to determine the effect of elevation angle from rest to 90 degrees humeral elevation. Reliability was determined from repeated trials in the same session without removing sensors or redigitizing landmarks. RESULTS: Angular values were highly repeatable within session (ICC > 0.94; SEM, < 2.3 degrees). During active scapular plane abduction from rest to 90 degrees, average acromioclavicular joint angular values demonstrated increased internal rotation (approximately 4.3 degrees), increased upward rotation (approximately 14.6 degrees), and increased posterior tilting (approximately 6.7 degrees) (P <.05). Passive motions on cadavers demonstrated similar kinematic patterns. CONCLUSIONS: Significant motion occurs at the acrornioclavicular joint during active humeral elevation, contributing to scapular motion on the thorax. This information provides a foundation for understanding normal acromioclavicular joint motion as a basis for further investigation of pathology and rehabilitation approaches.