The AC joint connects the lateral aspect of the clavicle to the acromion. Via the sternoclavicular joint, it provides the linkage between the shoulder girdle and the axial skeleton. It is a diarthrodial joint with a fibrocartilaginous disk and hyaline articular cartilage, which with age is gradually replaced by fibrocartilage [11]. Despite the existence of wide variation, the joint space is approximately 9 mm by 19 mm in adults [12]. Also the joint inclination varies, ranging from flat, through oblique to curved one [13]. The most common inclination is superolateral to inferomedial with the articular surface of the clavicle overhanging the acromial surface [14]. Sensory innervation is provided by the branches of the axillary, suprascapular, and lateral pectoral nerves.

The fibrocartilaginous disk contains stabilizing fibers that are contiguous with the superior joint capsule. Recently, the AC joints were classified into three major types depending on the presence or absence of the articular disk [15]. In this study, histological observation revealed that the upper part of the articular disk of the ACJ comprised fibrocartilage, while the lower part comprised dense connective tissue. In cases where the ACJ appears to be an ellipsoid joint, its limited axial rotation restricts posterior tilting of the scapula during arm elevation, which could contribute to shoulder impingement syndrome. While the function of the disk is still a matter of discussion, it is well known that it is implicated in degenerative conditions of the AC joint because this structure undergoes significant degeneration during the third and fourth decades of life [3, 11, 14]. The AC joint is surrounded by a thin synovial capsule reinforced by the AC ligaments that are located anteriorly, posteriorly, superiorly, and inferiorly. These ligaments are the stabilizers of the joint; the acromioclavicular ligaments provide the primary restraint to the horizontal clavicular translation, while the coracoclavicular one represents the primary restraint to vertical translation [16]. Global stability of the AC joint relies on the cooperation of AC and CC ligaments, but it is difficult to describe the individual contribution of each structure. There is surprisingly little motion at the AC joint, despite the significant degrees of freedom of the upper extremity. As Codman reported, during abduction or forward flexion, the AC joint has only 5 to 8 degrees of motion, while the clavicle and scapula have approximately 45°. Studies have demonstrated that stabilization of this joint has little effects on shoulder range of motion, and it may help to understand why arthrosis of this joint is often asymptomatic [17]. On the other side, disruption of the joint stability by a Rockwood type III AC joint dislocation alters significantly the kinesis of the shoulder girdle [18].

The AC joint commonly has degenerative changes within the third decade of life. This is due to the degeneration of the intra-articular disk and fibrocartilage replacement of the hyaline articular cartilage [3, 11, 14, 19]. In order to classify the degenerative changes of the joint, a MRI grading classification is used which is based on joint space narrowing, capsular distension, and subacromial irregularity. There are four grades altogether: grade 1 means that AC joint is normal (no degenerative changes); grade 2 is characterized by mild joint space narrowing and possible capsular distension; grade 3 has capsular distension, joint space narrowing, subacromial fat/bursal effacement, and marginal osteophyte formation; finally, grade 4 involves severe degeneration with joint irregularity, joint space obliteration, and large inferior osteophyte formation. Again, despite the large occurrence of degenerative changes, few patients have a correlation between radiographic and clinical examination of this joint (Fig. 2).