Although the clavicle is the first bone to ossify (fifth gestational week), the medial clavicular physis typically does not close until at least 25 years of age. In fact, some studies have shown that physeal closure may not actually occur until 31 years of age in some patients [1, 2]. Therefore, patients younger than 31 years of age who present with possible SC joint dislocations should be evaluated for both ligamentous integrity and patency of the medial clavicular physis.

The SC joint is a diarthrodial joint lined with synovium that primarily functions to connect the shoulder girdle to the axial skeleton (Fig. 8.1). Because the articular surface of the medial clavicle is highly incongruent with the manubrium, joint stability is maintained primarily by strong ligamentous attachments. Specifically, the articular surface of the medial clavicle is much larger than that of the manubrium and has been described as having a “saddle-like” configuration (i.e., concave in the axial plane and convex in the coronal plane) which biomechanically functions similarly to a ball-and-socket joint (Fig. 8.2) [4, 5].

Both the manubrium and the medial clavicle can have a variety of different anatomic configurations which may vary across populations, between genders and, potentially, within the same patient [6, 7]. Tuscano et al. [7] quantified this asymmetry in a series of 104 patients who had previously undergone a computed tomography (CT) scan of the chest for other reasons. In their study, the investigators measured joint spaces and the maximum diameter of the medial clavicular head in each patient. Overall, joint spaces ranged from 0.2 to 1.37 cm across the study population and medial clavicular head diameters ranged from 1.2 to 3.7 cm. Interestingly, some patients displayed differences in medial clavicular head diameters between their right and left clavicles (range, 0.0–1.0 cm difference). Other authors have found that the manubrium may also be subject to significant anatomic variation [8]. As a result, osseous asymmetry of the SC joint should be expected in the clinical setting to prevent misdiagnoses and unnecessary surgery.

The articular surfaces of the medial clavicle and the manubrium are covered with hyaline cartilage that eventually become fibrocartilage with increasing age [9]. Recent dissections performed at this institution revealed that only approximately two-thirds of the medial clavicle was covered with articular cartilage: the majority of this cartilage was found anteriorly and inferiorly where the medial clavicle was devoid of capsuloligamentous attachments (Fig. 8.3). This finding has also been confirmed by others [10]. We also identified a previously undescribed ridge that traveled along the superior aspect of the clavicular head of the pectoralis major insertion site [11]. This “pectoralis ridge” may prove to be an important landmark for orientation during arthroscopy or rotational alignment during open reconstructive procedures (see Figs. 8.3 and 8.4).

As mentioned above, the articular surfaces of the SC joint are highly incongruent and rely primarily on the patency of static capsuloligamentous structures to maintain stability. These structures include the capsular ligaments, costoclavicular ligament, interclavicular ligament, and the intra-articular disc ligament.

Traumatic injuries to the SC joint can result in severe complications due to the proximity of the mediastinal vessels, especially in those with posterior SC joint dislocations. These mediastinal vessels include the subclavian vessels, the right and left brachiocephalic veins, the brachiocephalic artery and the left carotid artery (Fig. 8.5). According to Ponce et al. [6], the closest vessel lies approximately 6.6 mm deep to the posterior SC joint capsule (left or right brachiocephalic vein). In some cases, the subclavian vein may actually cross over the first rib along the posterolateral aspect of the costoclavicular ligament before merging with the brachiocephalic vein [14].

In our cadaveric dissections, the fascial raphe between the sternohyoid and sternothyroid muscles was found to provide some protection to the mediastinal vessels since it forms a physical barrier between the medial clavicle and the mediastinal vessels (Fig. 8.6). We also noted a “safe zone” for posterior dissection in the interval between the sternothyroid muscle and area directly posterior to the manubrium and the medial clavicle [11]. However, in the event of posterior dislocation, the sternohyoid and sternothyroid muscle bellies may become disrupted, placing the posterior vessels at an increased risk for iatrogenic injury during posterior surgical dissection of the SC joint. Disruption of the sternohyoid and sternothyroid muscle bellies can easily be identified on diagnostic axial MRI scans.

For every 10° of humeral elevation, the SC joint contributes approximately 4° of rotation (Fig. 8.7) [19]. The SC joint also contributes approximately 50° of posterior rotation with every 35° of elevation, flexion and extension of the humerus (Fig. 8.8) [20, 21]. Therefore, surgical procedures involving rigid fixation of the SC joint have largely been abandoned due to the potential for poor functional outcomes in addition to the high reported incidence of hardware failure. Large resections of the medial clavicle have also been reported; however, this can result in uncontrollable scapulothoracic motion and, therefore, may require subsequent scapulothoracic fusion [22].