Hypothesis: We hypothesized that the clavicle overrides the acromion during certain shoulder motions for individuals with acromioclavicular (AC) joint separation producing clinical symptoms. We measured 3-dimensional clavicular and scapular motions in AC joint separation models during humerothoracic motions, which should be impacted by the loss of AC joint continuity. Materials and methods: Ten shoulders from 6 whole cadavers were used. The scapular and clavicular motions were measured in intact and AC joint separation models using an electromagnetic tracking device. The measurement was performed during shoulder abduction with humerothoracic neutral rotation. It was also measured during shoulder abduction with humerothoracic internal rotation, which could cause clavicular overriding. The kinematic changes caused by ligament sectioning were evaluated in these 2 arm motions. Results: The clavicle completely overrode the acromion in all AC separation models during abduction with internal rotation, but not in any shoulders during abduction with neutral rotation. Upward clavicular rotation increased, posterior clavicular rotation decreased, and external scapular rotation decreased with ligament sectioning. These kinematic changes were common for both of the measured arm motions. Scapular upward rotation and posterior tilt did not change because of ligament sectioning during abduction with neutral rotation. However, these scapular rotations significantly decreased with ligament sectioning during shoulder abduction with internal rotation. Conclusion: Scapular and clavicular kinematics were affected in AC separation models. Abduction with humeral internal rotation resulted in a decrease in scapular posterior tilt and upward rotation in AC separation models, and thereby could lead to AC joint articulation dysfunction.
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