Traumatic Acromioclavicular Separations
The most frequent cause of an AC separation is the application of a direct force, which frequently occurs during a fall onto the shoulder when the arm is adducted (Fig. 26-2). This commonly occurs in contact sports such as football and hockey or in recreational sports such as cycling when an individual goes over the handlebars of a bike. In these situations, the energy from the fall forces the acromion downward and medially, causing the AC ligaments to be damaged first, followed by a sequential disruption of CC ligaments in higher energy injuries.
AC injuries resulting from indirect trauma are less common. Typically, these injuries occur when landing on an outstretched arm. The force thrusts the humeral head superiorly into the acromion process, causing disruption of the AC ligaments. In these cases, it is rare to have a high-grade AC separation because the CC ligaments are not injured because this force decreases the CC space, thereby decreasing tension on the CC ligaments.
Deterioration of the fibrocartilaginous disk that separates the two articular surfaces of the AC joint is thought to occur as early as the second decade of life.2,3 The degeneration of the disk may be a natural process of aging. In elderly patients, radiographic changes of degeneration may occur in more than 50% of the population.2,3 However, all patients with joint space narrowing may not be symptomatic. It is therefore critical to determine whether these changes are associated with any symptoms. Patients with a symptomatic AC joint may describe pain localized to the joint or may also experience pain that radiates into the neck with more strenuous activities.
The onset of these problems is often related to overhead activities such as throwing or weight lifting. The throwing
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