The acromioclavicular joint is susceptible to injury from acute trauma such as falls directly onto the shoulder as well as repetitive microtrauma from activities that require repeated raising of the arm across the body such as throwing and painting on ladders. Let untreated, the acute inflammation associated with the injury may result in arthritis with its associated pain and functional disability. Patients suffering from acromioclavicular joint dysfunction or inflammation will complain of a marked exacerbation of pain when they perform activities that require raising their arm and reaching across their chest. A grating or grinding sensation with joint movement is often noted, and the patient frequently is unable to sleep on the affected shoulder. Patients with acromioclavicular joint dysfunction and inflammation will exhibit pain on downward traction or passive adduction of the affected shoulder. The chin adduction test is also frequently positive. Palpation of the acromioclavicular joint often reveals swelling or enlargement of the joint secondary to joint effusion (Fig. 36.1). If there is disruption of the ligaments that surround and support the acromioclavicular joint, joint instability may be evident on physical examination. Plain radiographs are indicated in patients suffering from acromioclavicular joint pain. They may reveal narrowing or sclerosis of the joint consistent with osteoarthritis or widening of the joint consistent with ligamentous injury (Fig. 36.2). They may also reveal occult fractures. If joint instability is suspected or detected on physical examination, magnetic resonance imaging and/or ultrasound scanning is a reasonable
next step (Figs. 36.3 and 36.4). Ultrasound-guided acromioclavicular joint injection can aid the clinician in both the diagnosis and treatment of acromioclavicular joint pain and dysfunction.

The acromioclavicular joint is the junction of the distal end of the clavicle and the anterior and medial aspects of the acromion of the scapula (Fig. 36.5). In many patients, the space between the distal end of the clavicle and the acromion is filled with an intra-articular disk. The dense coracoclavicular ligament provides the majority of strength of the joint. Additionally, strength is provided by the articular capsule, which completely surrounds the joint. The superior portion of the joint is covered by the superior acromioclavicular ligament, which attaches the distal clavicle to the upper surface of the acromion. The inferior portion of the joint is covered by the inferior acromioclavicular ligament, which attaches the inferior portion of the distal clavicle to the acromion. Both of these ligaments provide further joint stability. On palpation of the joint, a small indentation can be felt where the clavicle abuts the acromion. The acromioclavicular joint may or may not contain an articular disk. The volume of the acromioclavicular joint space is small, and care must be taken not to disrupt the joint by forcefully injecting large volumes of
local anesthetic and corticosteroid into the intra-articular space when performing this injection technique.