Anatomy, Shoulder and Upper Limb, Clavicle

Structure and Function

Although small, the clavicle supports optimal upper extremity function and protects the limb by dispersing forces transmitted through direct contact. Clavicular positioning maintains sufficient distance between the upper extremity and thorax, allowing unimpeded shoulder range of motion. Strut-like mechanics allow the scapula to glide smoothly along the posterior thoracic wall, which is critical for full upper extremity motion.[5] The anatomical location also protects the brachial plexus, subclavian artery, and subclavian vein, disruption of which substantially increases morbidity.[6]

Embryology

The clavicle is the first bone to begin ossification during embryologic development and is derived primarily from the lateral mesoderm. The medial and lateral ends undergo endochondral ossification, unlike the shaft.[7] Endochondral ossification begins with chondrocyte formation of a cartilaginous model that subsequently undergoes mineralization and ossification. In contrast, the shaft forms through intramembranous ossification, producing woven bone directly without a cartilaginous precursor. Subsequent remodeling produces lamellar bone in both processes. Despite its early ossification, the clavicle is one of the last to complete the process, and growth plates may not close until age 20 to 25 years.[8][9]

Blood Supply and Lymphatics

The clavicle has a relatively small or poorly defined medullary cavity compared with typical long bones. Previous studies have demonstrated a periosteal arterial blood supply but no central nutrient artery. The suprascapular, thoracoacromial, and internal thoracic arteries supply the clavicle.[10]

Nerves

The primary sensory innervation of the clavicle remains controversial. Anesthesiology studies following clavicular fractures suggest contributions from the supraclavicular, subclavian, long thoracic, and suprascapular nerves, in isolation or in combination.[11] A common anatomical variation is a perforating branch of the supraclavicular nerve that traverses the superior surface of the clavicle. Postmortem studies have demonstrated passage of this nerve through bony tunnels or grooves, creating susceptibility to injury and potentially explaining entrapment neuropathy following clavicular fracture.[12]

Muscles

The clavicle has multiple muscular and ligamentous attachments of anatomical importance. The anterior deltoid originates from the anterior aspect of the superior surface and assists in shoulder flexion, whereas the trapezius inserts onto the posterior aspect. The trapezius primarily stabilizes the scapula.[13] The subclavius occupies the subclavian groove on the inferior surface and depresses the shoulder while pulling the clavicle anteroinferiorly. Laterally, the coracoclavicular ligament supports the clavicle from the underlying coracoid process. The conoid ligament, which forms the medial component of the coracoclavicular ligament, inserts onto the conoid tubercle, whereas the trapezoid ligament, which forms the lateral component, inserts onto the trapezoid line.

Anteriorly, the clavicular head of the pectoralis major originates from the medial clavicle and contributes to flexion, horizontal adduction, and internal rotation of the humerus. Posteriorly, the trapezius inserts onto the posterosuperior clavicle. The clavicular head of the SCM occupies a similar location along the medial 1/3 of the clavicle. Unilateral SCM contraction rotates the head to the contralateral side and laterally flexes the head ipsilaterally. Bilateral SCM contraction produces head flexion. The sternohyoid originates from the inferior aspect of the posterior surface of the clavicle, the manubrium, and the posterior portion of the sternoclavicular ligament. Sternohyoid contraction depresses the hyoid bone.

Physiologic Variants

The clavicle exhibits several distinct features compared with other long bones. Thickness and length vary by sex, with men having longer and thicker clavicles than women. Men also exhibit greater clavicular curvature than women. Cadaveric studies have also shown that left clavicles are substantially longer than contralateral clavicles.[14] A rare but clinically relevant genetic disorder, cleidocranial dysplasia, can present with bilateral absence or partial absence of the clavicles. Other features include dental abnormalities, delayed fontanel closure, and failure of cranial suture fusion.[15]

Surgical Considerations

Clavicular fractures are among the most common fractures and typically involve the middle 1/3 of the bone. Most stable medial and lateral fractures can be managed nonoperatively, whereas midshaft fractures may exhibit greater displacement and a higher incidence of malunion or nonunion. Surgical intervention may be warranted, depending on fracture displacement, fragment shortening, and whether neurovascular compromise is present. Pediatric clavicular injuries typically involve the physes and, given the substantial healing potential of the immature skeleton, is often managed nonoperatively.[16]

Open reduction and internal fixation with plates and screws, as well as intramedullary nailing, have been used to manage these fractures.[17] Operative management has been shown to improve short-term functional outcomes, though long-term functional prognosis is comparable to that following nonoperative management. Surgical management has also been associated with increased patient satisfaction, earlier return to physical activity, and improved cost-effectiveness compared with nonoperative treatment. Still, decisions regarding operative intervention should be individualized according to current recommendations.[18]

Clinical Significance

The midclavicular line is a key surface landmark that serves as a reference point for several anatomical structures. The left midclavicular line approximates the location of the cardiac apex beat, while its right counterpart aids in assessing liver size. The intersection of the right midclavicular line and the transpyloric plane approximates the location of the gallbladder fundus, though localization accuracy varies among examiners.[19]

Clavicular fractures account for approximately 10% of all fractures and are the most common acute clavicular injury. The degree of comminution, displacement, and shortening determines the need for surgical intervention. Superior displacement of the medial fragment in midshaft fractures may result from SCM tension and contribute to further instability. Clavicular fractures typically result from trauma, with direct lateral impact to the shoulder accounting for 87% of cases. Clavicular fractures may also result from a fall onto an outstretched hand or direct medial impact to the clavicle.[20]

Acromioclavicular joint dislocation is common in contact sports and accounts for 9% of all traumatic shoulder girdle injuries.[21] Radiographic assessment classifies acromioclavicular joint injuries into 6 types. Injury severity increases progressively from types I through VI and depends on the degree of separation at the acromioclavicular articulation.[22]

Type I and II injuries are managed nonoperatively. Type I injuries manifest as acromioclavicular joint tenderness without instability. Type II injuries exhibit horizontal instability only because of disruption of the acromioclavicular joint, with an increase in coracoclavicular distance of less than 25% compared with the contralateral side.

Type III injuries are often managed nonoperatively, although this approach remains somewhat controversial. For example, a survey of 28 Major League Baseball team orthopedic surgeons found that 72% (20/28) preferred nonoperative treatment despite advances in surgical techniques.[23] Interestingly, this finding closely mirrored the classic report by McFarland et al, in which 69% of team clinicians favored nonoperative management of type III acromioclavicular separations.[24] Notably, surgical repair of acute, complete acromioclavicular joint separation was strongly favored 30 years earlier. A survey by Powers and Bach of 163 orthopedic program chairpersons in the US found that 92% advocated surgical treatment.[25]

Types IV through VI injuries are typically managed surgically. Type IV injuries consist of posterior displacement of the lateral clavicle through the trapezial fascia. Type V injuries represent an increase in coracoclavicular distance greater than 100% compared with the contralateral side. Type VI injuries are characterized by inferior dislocation of the lateral clavicle into the subacromial or subcoracoid position.[26]

Acromioclavicular joint osteoarthritis has multiple etiologies, including degenerative, posttraumatic, septic, and inflammatory causes. As the most common disorder of the acromioclavicular joint, this condition can impair activities of daily living substantially, particularly overhead activities. Management may include anti-inflammatory medication use, intraarticular injections, and physical therapy. Persistent symptoms may warrant acromioclavicular joint resection in selected patients.[27]

Sternoclavicular joint injuries can also occur but are less common. Anterior dislocations can result from anterolateral loading of the distal clavicle, whereas posterior dislocations occur with posterolateral loading. A much less common mechanism of posterior sternoclavicular joint dislocation involves substantial posteriorly directed force applied to the medial end of the clavicle. Women with ligamentous laxity have a higher incidence of sternoclavicular joint injuries, which may also be associated with trapezius nerve palsy.[28]