Kinematic analysis of the sternoclavicular, acromioclavicular and scapulothoracic joint demonstrates significant multiplanar alterations in acromioclavicular injuries with each consecutive ligamentous injury during movements of the shoulder girdle: A whole-cadaver study.

Purpose: In acromioclavicular (AC) joint injuries, the kinematical interplay between the AC ligament, coracoclavicular (CC) ligaments and deltotrapezial fascia (DTF) during motions of the shoulder complex is disturbed. This study assessed kinematic alterations of sternoclavicular (SC), scapulothoracic (ST) and AC joint motion during humerothoracic and ST movements in AC injuries.

Methods: Shoulder girdle motion was evaluated in 14 cadaveric shoulders in 4 conditions, consisting of an intact state and AC injuries of increasing severity by sequentially sectioning the AC and CC ligaments and DTF. Joint motions were registered during humerothoracic elevation and protraction. An optical navigation system measured three-dimensional rotations and translations in the SC, ST and AC joints.

Results: Sectioning of the AC ligament increased inferior and anterior AC translation with a concomitant increase of scapular protraction. The clavicle rotated to an overall more posteriorly rotated position. Sectioning of the CC ligaments increased lateral rotation and protraction of the scapula relative to the clavicle with a concomitant inferior translation of the acromion. Also, manifest overriding of the clavicle is noted due to instability in the superoinferior and anteroposterior axes. The clavicle rotated back to an overall more anteriorly rotated position, similar to the native condition. Sectioning of the DTF further increased protraction of the scapula relative to the clavicle, while a further medial translation of the acromion under the clavicle is observed.

Conclusion: The AC ligament affects anteroposterior stability, while the CC ligaments disturb stability in a superoinferior and mediolateral direction. All ligaments influence clavicular axial rotation. The DTF exacerbates alterations caused by the CC ligaments. Multiplanar decoupling results in overriding of the clavicle observed after sectioning the CC ligaments. Each sectioned ligament significantly increases scapular protraction. These findings guide further advancements in (non)surgical treatment of AC injuries to restore optimal function.

Level of evidence: N/A.