Acromioclavicular (AC) joint separation is a common injury that occurs following direct impact to the shoulder. Anterior-posterior stability of the AC joint is provided by the AC ligaments, and superior-inferior stability is provided by the coracoclavicular (CC) ligaments. Injury to the 2 CC ligaments (the medial conoid and the lateral trapezoid) can cause superior migration of the clavicle. The Rockwood classification system categorizes AC joint injuries into 6 types. Type I injuries are characterized by sprains of the AC ligaments without superior migration of the clavicle. Type II injuries cause tearing of the AC ligaments but not of the CC ligaments. Type III injuries cause tears in both the AC ligaments and the CC ligaments, resulting in 25% to 100% superior displacement of the clavicle.
Types IV, V, and VI injuries are rare. Type IV injuries result in posterior displacement of the clavicle into the fascia of the trapezius. Type V injuries are similar to Type III injuries with >100% superior displacement of the clavicle. Type VI injuries displace inferior to the coracoid process. With injuries categorized as type III to V, ≤18% of patients will have concomitant pathology such as SLAP lesions, rotator cuff injuries, and fractures.1,2
This patient has a grade III or borderline grade V AC joint separation which indicates that the AC and CC ligaments have been injured.
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
1. Li X, Ma R, Bedi A, Dines DM, Altchek DW, Dines JS. Management of acromioclavicular joint injuries. J Bone Joint Surg Am. 2014;96(1):73-84.
2. Spencer EE Jr. Treatment of grade III acromioclavicular joint injuries: a systematic review.Clin Orthop Relat Res. 2007;455:38-44.