Clavicle Anatomy | Bone & Muscles

Clavicle anatomy consists of medial end, middle and lateral end. There are many ligaments and muscles connecting to these parts of clavicle.

Clavicle bone is the First bone in the body to ossify (at 5 weeks’ gestation) and last to fuse (medial epiphysis at 25 years of age).

See Also: Clavicle Fractures

Clavicle Anatomy

Clavicle Bone Anatomy

Serving as a strut between the sternum and scapula, the clavicle bone elevates and rotates to maintain the alignment of the scapula, allowing for additional motion when the arm is raised and preventing excessive anterior displacement of the scapula.

The proximal two-thirds of the clavicle bone is characterized by an anteriorly convex bend. The distal one-third begins to flatten while curving concavely to meet with the scapula. The point at which the clavicle begins to transition from a convex to a concave bend, approximately two-thirds of the way along its shaft, is relatively weak and is a common site for fractures.

Thin and triangular, the scapula’s anterior costal surface is concave, forming the subscapular fossa. The vertebral (medial) border is marked by the inferior and superior angles. The posterior surface is distinguished by the horizontal scapular spine, which divides the scapula into the large infraspinous fossa below and the smaller supraspinous fossa above.

On the lateral end of the scapular spine is the anteriorly projecting acromion process, which articulates with the clavicle. Projecting inferiorly and anteriorly to the acromion is the beak-shaped coracoid process. The infraspinous, supraspinous, and subscapular fossae merge on the axial border to form the glenoid fossa. Located below the acromion, this fossa articulates with the humeral head.

Ligamentous Anatomy of the clavicle:

Medial Clavicle:

Medial Clavicle articulates with sternal bone forming the sternoclavicular joint, there is relatively little motion at the sternoclavicular joint. Medially the clavicle is secured to the sternum by the sternoclavicular capsule. The thickening of the posterior capsule has been determined to be the single most important soft tissue constraint to anterior or posterior translation of the medial clavicle.

There is also an interclavicular ligament which runs from the medial end of one clavicle, gains purchase from the superior aspect of the sternum at the sternal notch, and attaches to the medial end of the contralateral clavicle. Acting as a tension wire at the base of the clavicle, this ligament helps prevent inferior angulation or translation of the clavicle.

There are extremely stout ligaments that originate on the first rib and insert on the undersurface or the inferior aspect of the clavicle.

Lateral Clavicle:

Laterally the clavicle articulates with the acromion bone.

The coracoclavicular ligaments are stout ligaments that arise from the base of the coracoid:

1. The trapezoid (more lateral): inserts onto the small osseous ridge of the inferior clavicle.
2. Conoid (more medial): inserts onto the clavicular conoid tubercle.

These ligaments are very strong and provide the primary resistance to superior displacement of the clavicle. Their integrity, or lack thereof, plays an important role in the decision making and fixation selection in the treatment of displaced lateral third clavicle fractures.

Clavicle fractures in this location will often have an avulsed inferior fragment to which these ligaments are attached, especially in younger individuals.

The capsule of the AC joint is thickened superiorly and is primarily responsible for resisting AP displacement of the joint.

If one is inserting a hook plate for fixation of a very distal fracture, a small defect can be made in the posterolateral aspect of the capsule for insertion of the hook portion into the posterior subacromial space.

Clavicle Muscle Attachments

Medially: the pectoralis major muscle originates from the clavicular shaft anteroinferiorly, and the sternocleidomastoid originates superiorly.

Laterally: the pectoralis origin merges with the origin of the anterior deltoid, while the trapezius insertion blends superiorly with the deltoid origin at the lateral margin.

The medial clavicular fragment is elevated by the unopposed pull of the sternocleidomastoid muscle, while the distal fragment is held inferiorly by the deltoid and medially by the pectoralis major.

The platysma or “shaving muscle” is variable in terms of thickness and extent, but usually envelopes the anterior and superior aspects of the clavicle and runs in the subcutaneous tissues, extending superiorly to the mandible and the deeper facial muscles. It is divided during the surgical approach, and is typically included in the closure of the superficial, or skin/subcutaneous layer.

Neurovascular Anatomy of the Clavicle

The supraclavicular nerves originate from cervical roots C3 and C4 and exit from a common trunk behind the posterior border of the sternocleidomastoid muscle.

There are typically three major branches (anterior, middle, and posterior) that cross the clavicle superficially from medial to lateral, and are risk during surgical approaches. If they are divided, an area of numbness is typically felt inferior to the surgical incision, although this tends to improve with time.

See Also: Brachial Plexus Anatomy

The subclavian vein runs directly below the subclavius muscle and above the first rib, where it is readily accessible (for central venous access) and vulnerable (to inadvertent injury).

More posteriorly lie the subclavian artery and the brachial plexus, separated from the vein and clavicle by the additional layer of the scalenus anterior muscle medially. subclavian vessels were closest at the medial end, with the vein directly apposed to the posterior cortex of the medial clavicle in some cases.

In the middle third, the artery and vein were a mean of 17 and 13 mm from the clavicle, respectively, at an approximate angle of 60 degrees to the horizontal (i.e., the vessels were posterior-inferior to the clavicle)

Laterally, the distances were greater, with the artery and vein a mean of 63 and 76 mm, respectively form the clavicle.

Sternoclavicular joint

The manubrium of the sternum serves as the site of attachment for each clavicle. Projecting above the body of the sternum, the superior surface of the manubrium is indented by the jugular (suprasternal) notch. Projecting off each side of the jugular notch is the clavicular notch, which accepts the medial head of the clavicle

At the Sternoclavicular joint, the proximal portion of the clavicle meets the manubrium of the sternum and a portion of the first costal cartilage to form a gliding joint that allows three degrees of freedom of motion:

1. Elevation and depression,
2. Protraction and retraction,
3. Anterior and posterior rotation.

The SC joint’s functional axis lies lateral to the joint itself. Elevation and depression and protraction and retraction describe the movement at the lateral clavicle. Posterior rotation (where the inferior surface of the clavicle moves anteriorly) occurs on the long axis of the clavicle, with anterior rotation used to describe return to neutral from a posteriorly rotated position. The articulation between the manubrium and clavicle is inherently incongruent because the proximal end of the clavicle extends one-half of its width above the manubrium.

Although the overall stability of the joint is enhanced by the presence of a fibrocartilaginous disc, the SC joint has the poorest bony stability of any of the major joints. Its strong ligamentous structure and protected location, however, makes it one of the least frequently dislocated joints. Surrounded by a synovial membrane, the SC joint is supported by the anterior and posterior SC ligaments, the costoclavicular ligament, and the interclavicular ligament.

The sternoclavicular disc, which has qualities similar to the menisci found in the knee, functions as a shock absorber. The upper portion of the disc is attached to the clavicle, and its lower portion is attached to the manubrium and first costal cartilage. This disc divides the joint into two articular cavities, one between the disc and the clavicle and a second between the disc and the manubrium.

The synovial membrane is reinforced by the anterior and posterior sternoclavicular ligaments. Whereas the anterior fibers resist posterior displacement of the clavicle on the manubrium, the posterior fibers resist anterior displacement. The costoclavicular ligament serves as an axis of clavicular elevation and depression and protraction and retraction. The SC joints are joined to each other by the interclavicular ligament. Attaching to the superior proximal ends of the left and right clavicles, the ligament has a common connection on the superior border of the sternum.

The interclavicular ligament resists downward movement of the clavicle and assists in dissipating force across the entire upper extremity. The costoclavicular ligament (rhomboid ligament) arises from the superior aspect of the first rib and connects to the inferior aspect of the clavicle. Likewise, the posterior fibers limit elevation and medial movement of the clavicle. The anterior fibers resist clavicular elevation from the superior pull of the sternomastoid and sternohyoid muscles and limit medial translation of the clavicle.

Acromioclavicular AC joint

The distal end of the clavicle meets the acromion process of the scapula to form the Acromioclavicular joint. A plane synovial joint, the AC joint allows a gliding articulation between the acromion and the clavicle, capable of 3 degrees of freedom of movement, each around an oblique axis:

1. Internal and external rotation around a vertical axis,
2. Upward and downward rotation around an axis perpendicular to the plane of the scapula,
3. Anterior and posterior scapular tilting around a horizontal axis.

This articulation allows for the motion necessary to maintain the relationship between the scapula and the clavicle in the early and late stages of the GH joint’s ROM.

Surrounded by a synovial membrane, the AC joint is supported by the AC ligament and the coracoclavicular ligament, which suspend the scapula from the clavicle. A synovial disc is present between the clavicle and the acromion that disappears by the fourth decade of life. Divided into two separate bands, the superior and inferior portions of the AC ligament function to maintain continuity between the articulating surfaces of the acromion and clavicle. With much of its restraint in the horizontal plane, this ligament maintains stability by preventing the clavicle from riding up and over the acromion process.

Most of the AC joint’s intrinsic stability arises from the coracoclavicular ligament, a structure extrinsic to the joint. Because of its direct connection to the scapula, the coracoclavicular ligament influences scapulohumeral motion. This ligament is divided into two distinct portions:

1. The lateral quadrilateral-shaped trapezoid ligament.
2. The medial triangular-shaped conoid ligament.

Separated by a bursa, the trapezoid ligament limits lateral movement of the clavicle over the acromion. The conoid ligament restricts superior movement of the clavicle. Acting jointly, these ligaments limit rotation of the scapula and provide some degree of horizontal stability. The conoid portion of the ligament is critical for the passive posterior rotation of the clavicle that occurs during shoulder elevation. A horizontal dislocation of the AC joint can occur with the coracoclavicular ligament remaining intact.

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References

1. Millers Review of Orthopaedics -7th Edition Book.
2. Rockwood and Green’s Fractures in Adults 8th Edition book.
3. Hyland S, Charlick M, Varacallo M. Anatomy, Shoulder and Upper Limb, Clavicle. [Updated 2021 Jul 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: NBK525990.