Review Question - QID 219923

The patient in Figure A has a Type IIB distal clavicle fracture that disrupts only the more medial conoid ligament. In contrast, the patient in Figure B has an AC joint injury that involves injury to both the conoid and trapezoid ligaments, as measured by the increased coracoclavicular distance.

An acromioclavicular (AC) joint injury, colloquially known as a "shoulder separation," represents a traumatic injury to the AC joint that disrupts the coracoclavicular (CC) ligaments. While the AC ligaments control the horizontal or anterior-posterior stability of the joint, the CC ligaments are responsible for maintaining the vertical or superior-inferior stability. There are two CC ligaments, the conoid and trapezoid, with the former inserting 4.5cm medial to the lateral edge of the clavicle, and the latter 3.0cm medial to the lateral edge. Type I AC joint injuries are sprains and involve no vertical instability of the CC ligaments, while Type III AC joint injuries involve injury to both ligaments as measured by >25-100% increase in CC ligament distance on an AP radiograph that includes both shoulders.

Lee et al. provide a biomechanical study that elucidates the ligamentous and capsular restraints to anterior-posterior and superior-inferior laxity of the acromioclavicular joint. The authors included 20 fresh-frozen human cadavers and found that the conoid ligament was the primary stabilizer of superior displacement, the ACJ capsular ligament was the primary stabilizer of inferior displacement, and the capsular ligament and conoid ligament contributed equally to anterior stability, while the capsular ligament was the primary contributor to posterior stability. They concluded that the conoid ligament is the primary stabilizer of superior displacement of the clavicle at the ACJ and also contributes significantly to AP stability.

Saccomanno et al. reviewed the anatomy and biomechanics of pathologic AC joint instability. The authors note that in type I injuries the AC ligaments are sprained, but the joint is intact, while in type II injuries the AC ligaments are torn but the CC ligaments are intact, and in type III injuries both the AC and the CC ligaments are torn. They conclude that proper radiographic evaluation of the AC joint is necessary, with the Zanca view being the most accurate view for examining the AC joint and the axial view of the shoulder being important in differentiating a type III AC joint injury from a type IV injury in which there is complete dislocation with posterior displacement of the distal clavicle into or through the fascia of the trapezius.

Figure A is an AP radiograph of a Type IIB distal clavicle fracture that occurs between the conoid and trapezoid ligaments, as evidenced by the normal CC distance between the distal fragment and the coracoid compared to the elevated medial clavicle. Figure B is an AP radiograph of an AC separation showing between 50- 100% increase in the CC distance from normal.

Incorrect Answers:
Answers 2-5: A Type IIB distal clavicle fracture occurs between the conoid and trapezoid ligaments, leaving a distal fragment with an intact CC distance, while an AC joint separation involves an injury to both ligaments that allows for >25-100% increase in the normal CC distance.