A detailed history including the mechanism of injury, location and duration of pain, and associated symptoms is vital to diagnosing an AC joint injury.

An accurate clinical evaluation may be difficult and painful in the acute setting. It is important to examine the patient in a sitting or standing position, allowing the weight of the injured arm to exaggerate any deformity. Pain may be variable in nature, given the AC joints’ dual innervation from the suprascapular nerve and the lateral pectoral nerve [13]; however, the diagnosis is often clinched with a visible or palpable defect noted at the AC joint. Tenderness directly at the ACJ is the main symptom with visually evident step formation between the acromion and the distal clavicle end in complete ACJ dislocations. A comparison to the unaffected contralateral ACJ should be drawn due to a sometimes physiological prominent distal clavicle end on both shoulders. A key part of the clinical assessment represents the testing of horizontal instability. Hereby, the distal clavicle is shifted posteriorly with the acromion fixed by the other hand. An increased posterior translation in comparison to the unaffected side indicates a horizontal component of ACJ instability. Discomfort is often exacerbated with range of motion of the shoulder and with loading of the joint with the crossarm adduction test, which is performed by forward elevating the arm to 90° with arm adduction. Assessment of the AC joint for stability after an acute injury may be difficult secondary to guarding; however, for subacute and chronic injuries, this should be attempted. The Paxinos test (thumb pressure at the posterior AC joint) combined with a positive bone scan has been found to predict AC joint pathology with a high degree of confidence. Pain localized to the acromioclavicular joint or “on top” is diagnostic of acromioclavicular joint abnormality, whereas pain or painful clicking described as “inside” the shoulder is considered indicative of labral abnormality. However, the sensitivity of this test for AC pathology is only 41 %, with a specificity of 94 % [14]. A simple shoulder shrug may be helpful in determining if the deltotrapezial fascia has been separated from the clavicle [5]. Reduction of the AC joint can also be tested by stabilizing the clavicle in one hand and with the other hand placing an upward force on the ipsilateral elbow and assessing the joint for visible or palpable incongruity. Additionally, a thorough neurovascular assessment of the upper extremity including the cervical spine should be performed. Suspicion for other associated injuries, such as clavicle, coracoid, and rib fractures, should be raised with higher injury mechanisms [15].

They were initially graded I through III based on radiographic displacement and the degree of ligamentous damage [16, 17]. Rockwood later added types IV through VI to the classification system. The rising type correlates with greater displacement and higher levels of ligamentous injury [5].

Confirmation of AC joint injury involves a complete radiographic shoulder series, which is essential in the analysis and classification of these injuries. Anteroposterior, scapular Y, and axillary views are obtained. These radiographs serve to provide information regarding the nearby glenohumeral joint and can rule out coexisting pathology. As standard anteroposterior (a.p.) projection, the Zanca view [18] (Fig. 23.4) has been established with the x-ray beam tilted for 10° in a caudocranial direction completed by a panorama stress view with a weight of 10 kg hanging on both wrists. Different methods have been employed for weight bearing. Detection of an increased AC and/or CC distance is indicative of ACJ dislocation. The normal ACJ width in the frontal plane (Zanca view) measures 1–3 mm and decreases with age. An ACJ width >7 mm in men and 6 mm in women is found to be pathologic (Fig. 23.2). The introduction of Rockwood’s classification necessitated a second plane to detect posterior dislocation of the distal clavicle. Routinely, at most radiologic departments, an axillary view with the patient in a sitting, supine, or standing position is performed [19].

Magnetic resonance (MR) imaging represents a sensitive diagnostic tool in evaluation of ACJ disorders. Assessment of the stabilizing soft tissue structures involving the AC ligaments, CC ligaments, and delto-trapezoidal fascia is possible in a reliable manner, and its results can change the clinical grading of dislocation [20, 21].

MR imaging is also useful if surgery is considered to identify accompanying pathologies and again to identify underestimated injuries.

T1-weighted imaging best demonstrates the CC ligaments, and fat-suppressed proton density-weighted or T2-weighted imaging best demonstrates the ligamentous disruption, when surrounded by blood or fluid [20].

In the author’s clinical practice, a panorama stress view and axillary dynamic radiological evaluation represent the basic imaging tools, on which a therapeutic decision can be made in almost all cases. MRI is used only in selected cases, where associated glenohumeral soft tissue injuries are assumed.

The goals of treatment for AC injuries are achieving painless range of motion of the shoulder, obtaining full strength, and exhibiting no limitation in activity. The treatment strategy varies according to the classification of the lesion.