ANATOMY AND PATHOPHYSIOLOGY

The acromioclavicular joint is subcutaneous and exposed to substantial biomechanical forces during shoulder motion. It is therefore susceptible to degenerative processes and acute trauma. Located between the distal end of the clavicle and the anteromedial aspect of the acromion, the AC joint is a diarthrodial joint. The joint is initially composed of hyaline cartilage articulations incompletely separated by a fibrocartilaginous disk¹; however, it undergoes degeneration to fibrocartilage by 40 years.¹

Although the exact dimensions are debated, the average size of the adult acromioclavicular joint is approximately 9 mm in the coronal plane and 19 mm in the sagittal plane.² There is tremendous variability in the coronal orientation of the joint.^1,3 Three coronal orientations have been described: an overriding clavicle (the clavicle overrides the acromion), a neutral AC joint, and an underriding clavicle (the acromion overrides the clavicle).⁴ The overriding clavicle is the most common variant, occurring in almost 50% of individuals.⁵

Stability of the AC joint is conferred by dynamic and static constraints. Dynamic stabilization arises from the contraction of muscles spanning the clavicle and acromion or humerus. The anterior head of the deltoid, originating off the lateral third of the clavicle and inserting into the deltoid tuberosity of the humerus, acts as a suspensory support for the shoulder girdle. The trapezius adds additional dynamic stability. The acromioclavicular and coracoclavicular (CC) ligaments work in concert to provide static constraints for the AC joint.

The CC ligaments include the medial conoid and lateral trapezoid ligaments; however, up to 1% of patients have a coracoclavicular bar.⁶ The average distance between the clavicle and coracoid is from 11 to 13 mm.^2,7 The CC ligaments are extracapsular and the average distance from the most lateral extent of the trapezoid ligament fibers to the joint is 15.3 mm (range, 11 to 22 mm).⁸ The CC ligaments are the primary constraints to superior and inferior translation, whereas the AC ligaments and capsule are the primary restraints to anteroposterior instability. However, the AC and CC ligaments have a dynamic interplay, whose contributions to stability vary with the directions and forces applied.⁹

Given its anatomy, the potential motion of the AC joint far exceeds that actually demonstrated during active motion of the shoulder girdle.¹⁰ Actually, very little motion occurs at the AC joint in comparison to that of the sternoclavicular joint; less than 10 degrees of rotation occur at the AC joint with full elevation of the arm.¹¹

MECHANISM OF INJURY

Acromioclavicular joint pathology can be stratified into two broad categories, instability and arthritic conditions. Either condition may be the sequel of direct or indirect forces applied to the upper extremity. Direct forces are the most common mechanism of injury, resulting from a fall on the point of the shoulder, usually with the arm adducted. Indirect forces may be transmitted through the upper extremity and dissipated within the acromioclavicular articulation, leading to ligamentous and cartilaginous injury. These forces may lead to an acute injury or, with chronic degeneration, an arthritic condition. An isolated subset of the arthritic population is the younger athlete or weightlifter whose repetitive exercises have induced a stress reaction in the distal clavicle, known as distal clavicular osteolysis. Instability may also be iatrogenic, caused by an overzealous previous distal clavicle resection.

HISTORY AND PHYSICAL EXAMINATION

The history of a patient with AC joint pathology typically reflects the mechanism of injury. In a patient with AC instability, the patient will report a traumatic injury with resultant pain, deformity, and mechanical symptoms dominating their clinical picture. Patients with arthritic conditions typically present with an insidious onset of pain, leading to disability with activities of daily life and night pain.

Whereas pain arising in the AC joint is often localized to the anterosuperior aspect of the shoulder, referred pain may exist, given the rich innervation of the joint by the suprascapular, lateral pectoral, and axillary nerves. With injection studies, Gerber and colleagues¹² have demonstrated that patients may complain of pain distributed over the anterolateral neck, posterior trapezius, and anterolateral deltoid. Painful motions will often include cross-body adduction, overhead activities, internal rotation, and extension of the arm behind the plane of the scapula. Common complaints are pain while putting on a coat, strapping a bra, or washing the contralateral axilla. Weightlifters and athletes with distal clavicular osteolysis describe soreness and pain with flat and inclined bench presses, shoulder presses, and dips. Special attention should be paid to patients in whom a previous distal clavicular resection has been performed. These patients may present for inadequate resection, instability, or a missed diagnosis.

Physical examination of the shoulder always begins with the cervical spine. The examination then progresses to inspection, palpation, and a neurovascular examination. Scars about the shoulder should suggest prior surgical interventions to the physician. The portion of the examination focused on the AC joint should include direct palpation of the joint to elicit tenderness and instability. The AC joint is located just anterior to the Neviaser triangle, the soft spot between the distal clavicle and scapular spine. Often, an overriding distal clavicle will manifest a bony prominence immediately medial to the AC joint. The shoulder should be put through a range of motion. Painful movements may include cross-body adduction, internal rotation and extension of the shoulder, and overhead forward elevation. Shoulder extension, adduction, and internal rotation increase contact pressures across the posterior AC facet, and may be useful in identifying patients with an incomplete resection of the distal clavicle.

Many of these signs and symptoms may overlap with those of other shoulder pathologies, including rotator cuff and superior labrum anteroposterior (SLAP) tears. A complete shoulder examination should be completed on all patients with complex pathology. If confusion remains regarding the source of the pain, injections of local anesthetic into the AC joint or subacromial space may assist the surgeon in differentiation of the diagnoses. Relief of pain after an AC joint injection has an excellent prognostic value for resolution of symptoms after arthroscopic distal clavicle excision. However, relief with an AC injection should be interpreted with caution if more than 1 to 2 mL of fluid is accepted without palpable distention and resistance to further insufflations. In these cases, there may be a defect in the inferior AC joint capsule (seen especially after prior surgery or with significant rotator cuff tears) such that injected anesthetic may freely fill the subacromial bursa, also relieving pain from rotator cuff and bursal conditions.

DIAGNOSTIC IMAGING OF ACROMIOCLAVICULAR JOINT PATHOLOGY

Radiographic evaluation of the acromioclavicular joint should be used to confirm rather than diagnose most AC joint pathologies, so a standard shoulder series of four x-rays should be obtained. An anteroposterior (AP) of the shoulder, an AP of the glenohumeral joint (orthogonal to the scapular plane), a scapular Y, and an axillary x-ray are the minimum views necessary to evaluate a shoulder thoroughly. Unfortunately, the AC joint only requires half the x-ray penetration used for standard shoulder views and thus may be overpenetrated. Depending on its exact plane and morphology in the coronal plane, the AC joint space can usually be best visualized on the AP view of the shoulder. The axillary view can be very useful in assessing the AP position, and the most commonly missed area of prior distal clavicular resections, the posterior AC joint. Additional dedicated AC joint views, such as the Zanca view (x-ray beam is centered over the joint and angled 10 to 15 degrees cephalad to isolate the clavicle from the scapular shadow), have been described; however, we have found them of limited usefulness.

If AC joint stability is in question, an AP of bilateral shoulders including the AC joints should be taken to compare the coracoclavicular distances with the identical tangential view. Stress views may also be obtained, but are not usually required.¹³

Magnetic resonance imaging is not typically necessary for diagnosing acromioclavicular pathology; however, it is useful to evaluate for masses or cysts about the AC joint and concomitant shoulder pathology that may need to be addressed. Unfortunately, findings of increased T2 signal intensity can be nonspecific for symptomatic pathology¹⁴ and therefore must be correlated with physical findings that support the diagnosis of AC pathology.

TREATMENT OPTIONS