Sternoclavicular Joint Dislocations

Relevant Anatomy

The sternoclavicular joint (SCJ) is a saddle-shaped, diarthrodial synovial joint that provides the only bony articulation between the axial skeleton and upper extremity. It is an important fulcrum for the movement of the shoulder girdle, as translation and rotation of the clavicle through the SCJ and acromioclavicular joint (ACJ) dictate the position of scapula as it moves against the posterior chest wall. ^, The joint is formed by the articulation of the medial end of the clavicle, the clavicular notch of the sternum, and the cartilage of the first rib. Developmentally, the clavicle is the first bone to ossify in utero; however, the physis of the medial clavicle is the last to fuse in adulthood, doing so at a mean age of 23–25 years. The medial clavicle articulates with both the reciprocal notch of the sternum and the superior surface of the first costal cartilage. The articulating surfaces of the clavicle and sternum are not fully congruent, with less than half of the medial clavicular surface articulating with the corresponding articular facet on the sternum. Due to this incongruent osseous geometry, the SCJ is inherently unstable and has been described as the least constrained joint in the human body ( Fig. 1.1 ). Functionally, however, the SCJ is remarkably stable, owing to the robust surrounding fibrocartilaginous, ligamentous, and muscular structures.

Positioned between the clavicle and the sternum is a well-developed fibrocartilaginous intra-articular disc, which is closely associated with the medial end of the clavicle and is analogous to the menisci of the knee or the intra-articular disc of the ACJ ( Fig. 1.2 ). The SCJ disc has robust insertions on the posterosuperior aspect of the clavicle, with peripheral attachments to the anterior and posterior sternoclavicular ligaments and surrounding joint capsule. The thickness of the disc tapers centrally, causing a central concavity on both the sternal and clavicular sides. This disc undergoes physiologic degenerative changes with aging and is often incomplete by the seventh decade of life.

The stabilizing ligamentous structures of the SCJ include the costoclavicular ligament, anterior and posterior sternoclavicular ligaments, the interclavicular ligament, and the intra-articular discoligamentous complex ^{, ,} ( Fig. 1.1 ). The costoclavicular ligament is a short, robust ligament connecting the first-rib costal cartilage to the medial end of the clavicle. It consists of an anterior and posterior fasciculus, separated by a bursa. From its origin on the first rib, the costoclavicular ligament extends from medial to lateral to insert on the anteroinferior aspect of the clavicle. The costoclavicular ligament has the largest footprint of the SCJ-associated ligaments, with its insertion on the clavicle reported to average 182.8 mm. ^, Several anatomic studies have sought to establish the distance between the inferior articular surface of the medial clavicle and the most medial aspect of the clavicular insertion of the costoclavicular ligament, reporting average distances ranging from 1.0 to 1.3 cm. ^{, ,} The anterior sternoclavicular ligament is an oblique capsular thickening that courses from inferomedial to superolateral across the anterior aspect of the SCJ and serves as the anterior attachment of the intra-articular disc. The posterior sternoclavicular ligament is a diffuse thickening of the posterior joint capsule with broad attachments on both the posterosuperior sternum and posteroinferior clavicle. The interclavicular ligament is a discrete ligament that extends between the medial ends of the clavicle and is continuous with deep cervical fascia superiorly. ^,

The SCJ is enveloped by a closely applied muscular layer both anteriorly and posteriorly. Anteriorly, the sternal insertion of sternocleidomastoid muscle (SCM) is a tendinous extension of the muscle that covers the medial aspect of the SCJ ( Fig. 1.3 ). The clavicular head of the SCM has a broad, linear insertion on the superior surface of the clavicle, with the most medial extent of the tendon reported to be an average of 9.9 mm lateral to the superior articular margin of the clavicle. The subclavius originates on the inferomedial aspect of the clavicle and courses obliquely to insert on the lateral aspect of the first-rib costal cartilage. Posteriorly, the sternohyoid is a thin strap muscle that inserts onto the posterior aspect of the SCJ joint capsule and clavicle. The sternothyroid runs deep to the sternohyoid, with a more inferior insertion on the posterior aspect of the sternum and costal cartilage. A fascial attachment is present between the right and left muscle bellies of both strap muscles, with a midline raphe that inserts onto the posterior aspect of the sternum. No vascular structures are present anterior to this fascial layer.

Biomechanics

The SCJ is one of five articulations of the shoulder girdle and plays a critical role in maintaining normal glenohumeral and scapulothoracic motion. The normally functioning SCJ achieves 35 degrees of movement in both the horizontal and coronal planes, as well 45 degrees of rotation along its longitudinal axis. The majority of scapulothoracic movement occurs through the SCJ and relies on both translation and rotation between the sternum and clavicle.

In regard to stability, several authors have proposed the costoclavicular ligament to be the primary stabilizer of the SCJ. ^{, ,} Abbott and Lucas reported that medial clavicle resections that included any portion of bone lateral to the costoclavicular ligament resulted in proximal displacement and instability of the remaining clavicle. These findings were further supported by an investigation of medial clavicle resections performed by Rockwood et al., in which they reported superior clinical outcomes in patients whose resection spared the costoclavicular ligament as compared with those in which the ligament was violated. A close review of this literature, however, provides little biomechanical evidence for the importance of the costoclavicular ligament in the normally functioning and intact SCJ joint.

The most rigorous biomechanical investigation of the SCJ ligamentous structures was performed by Spencer and Kuhn, in which they established the contribution of each individual ligament to the translational stability of the joint. In their study, the anterior capsule, posterior capsule, costoclavicular ligament, and interclavicular ligament were singularly divided, and the resulting change in anterior and posterior translation of the SCJ was recorded using a custom biomechanical testing fixture. The results of this study clearly showed the posterior capsule to be the most important stabilizing structure to both anterior and posterior translation of the SCJ. The anterior capsule was shown to be a significant, but less important stabilizer to anterior translation. Interestingly, neither the costoclavicular nor interclavicular ligaments were not found to be important primary stabilizers of the SCJ.

Pathoanatomy

Sternoclavicular injuries are rare, accounting for 1% of all dislocations and 3% of all shoulder girdle injuries. Owing to the strong supporting ligamentous structures, traumatic dislocations of the SCJ generally only occur after a tremendous, high-energy force has been applied through the shoulder, such as those seen in collisions during contact sports or motor vehicle collisions. ^{, , ,} Several epidemiological studies on traumatic SCJ dislocations have reported motor vehicle collisions to be the most common cause of injury (40%), while sports injuries are also common (25%). The remaining SCJ injuries resulted from a combination of falls and other miscellaneous low-energy traumatic mechanisms. ^{, ,}

Traumatic dislocations can result from a force applied directly to the SCJ or from an indirect compressive force applied to the lateral aspect of the shoulder, which is transmitted along the long axis of the clavicle. When an SCJ dislocation occurs, the direction is classified as either anterior or posterior based on the direction of displacement of the medial clavicle, with anterior dislocations occurring much more commonly than posterior. ^{, ,} The direction of displacement of the clavicle is dependent on the position of ipsilateral shoulder at the moment of impact. If the shoulder is rolled backward (i.e., the acromion is posterior to the sternum) during lateral compression, the force applied to the clavicle produces an anterior dislocation of the SCJ. When the shoulder is at rest and in a neutral anatomic orientation, the acromion is positioned posterior to the sternum. It is this spatial relationship, as well the fact that the posterior capsular ligaments are the strongest of the sternoclavicular ligaments, that likely explains why anterior dislocations are much more common. ^{, ,} Conversely, if the shoulder is rolled forward (i.e., the acromion is anterior to the sternum), a laterally applied compressive force will produce a posterior dislocation of the SCJ. Posterior dislocations can also result from a direct posterior force applied to the anteromedial aspect of the clavicle. ^,

Although posterior SCJ dislocations represent a small subset of a rare injury that only accounts for 1% of dislocations overall, the consequences of this injury can be devastating and life-threatening due to the close proximity of the critical cardiopulmonary structures located in retrosternal mediastinal space. The critical mediastinal structures include the great vessels of the neck, subclavian vessels, brachial plexus, brachiocephalic vein, trachea, and esophagus ( Fig. 1.4 ). Serious sequelae of posterior SCJ dislocations reported in the literature include brachial plexus compression, pneumothorax, respiratory distress, dysphagia, vascular compromise, and death. Worman and Leagus performed a retrospective review of patients with posterior SCJ dislocations and reported that 16 of 60 patients (27%) had a serious injury of the trachea, esophagus, or great vessels. Other studies have reported compression of critical mediastinal structures in as many as 30% of posterior SCJ dislocations with a 3%–4% mortality rate. Ponce et al. investigated the proximity of critical mediastinal structures to the SCJ by reviewing computed tomography (CT) angiograms of the neck of 49 patients and reported that the nearest critical structure is, on average, only 6.6 mm deep to the posterior aspect of the SCJ. The closest structure in most patients was the brachiocephalic vein, but in >20% of patients the closest structure was an artery.