Authors: Koval,
Kenneth J.; Zuckerman, Joseph D.
Title: Handbook
of Fractures, 3rd Edition
Copyright ©2006 Lippincott Williams & Wilkins
Epidemiology
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Most common in the second decade of life, associated with contact athletic activities
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More common in males (5 to 10:1)
Anatomy (Fig. 12.1)
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The AC joint is a diarthrodial joint, with fibrocartilage-covered articular surfaces, located between the lateral end of the clavicle and the medial acromion.
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Inclination of the plane of the joint may be vertical or inclined medially 50 degrees.
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The AC ligaments (anterior, posterior, superior, inferior) strengthen the thin capsule. Fibers of the deltoid and trapezius muscles blend with the superior AC ligament to strengthen the joint.
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The AC joint has minimal mobility through a meniscoid, intraarticular disc that demonstrates an age-dependent degeneration until it is essentially nonfunctional beyond the fourth decade.
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The horizontal stability of the AC joint is conferred by the AC ligaments, whereas the vertical stability is maintained by the coracoclavicular ligaments.
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The average coracoclavicular distance is 1.1 to 1.3 cm.
Mechanism of Injury
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Direct: This is the most common mechanism, resulting from a fall onto the shoulder with the arm adducted, driving the acromion medial and inferior.
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Indirect: This is caused by a fall onto an outstretched hand with force transmission through the humeral head and into the AC articulation (Fig. 12.2).
