Clavicle bone

The clavicle, commonly known as the collarbone, is a long, slender bone that serves as a strut between the sternum and the scapula. It plays a critical role in the structural integrity and mobility of the shoulder girdle. The clavicle also serves as an important site for muscle and ligament attachment, contributing to upper limb function.

Anatomy of the Clavicle

Gross Anatomy

The clavicle is an S-shaped bone, flattened from top to bottom and curved along its length. It extends horizontally across the anterior part of the thorax, connecting the axial skeleton to the upper limb. The bone is divided into two ends and a shaft:

• Medial (Sternal) End: The rounded, thicker end that articulates with the manubrium of the sternum at the sternoclavicular joint.
• Lateral (Acromial) End: The flattened end that articulates with the acromion of the scapula at the acromioclavicular joint.
• Shaft: The slender middle portion connecting the two ends, featuring slight concave and convex curvatures along its length.

Surfaces and Borders

The clavicle has two main surfaces and two borders, each providing attachment for muscles and ligaments:

• Superior Surface: Smooth and subcutaneous, easily palpable along its length.
• Inferior Surface: Rough and irregular, providing attachment sites for ligaments such as the costoclavicular ligament.
• Anterior Border: Offers attachment to muscles like the pectoralis major and deltoid.
• Posterior Border: Serves as the attachment for the trapezius and subclavius muscles.

Landmarks

The clavicle features several prominent landmarks that serve as attachment points for muscles and ligaments:

• Sternal End: Contains a concave articular facet for articulation with the sternum.
• Acromial End: Flattened with an articular facet for the acromion of the scapula.
• Conoid Tubercle: A small prominence on the inferior surface near the lateral end, attachment for the conoid ligament.
• Trapezoid Line: A ridge extending laterally from the conoid tubercle, attachment for the trapezoid ligament.
• Impression for Costoclavicular Ligament: A roughened area near the sternal end where the costoclavicular ligament attaches.

Development and Ossification

The clavicle is unique among long bones as it is the first to begin ossification during fetal development and exhibits both intramembranous and endochondral ossification. Its development is important for understanding congenital anomalies and growth patterns.

• Embryological Origin: The clavicle arises from mesenchymal tissue in the mesoderm of the developing limb bud.
• Primary Ossification Centers: Two primary centers appear near the sternal end and midshaft around the fifth to sixth week of gestation.
• Secondary Ossification Centers: Appear at the sternal end during adolescence, contributing to the growth and eventual fusion of the bone.
• Timeline of Ossification and Fusion: The clavicle typically fuses completely by the age of 25 years, with the medial epiphysis being the last to unite.

Articulations

The clavicle forms essential joints that stabilize the shoulder girdle and facilitate upper limb mobility.

• Sternoclavicular Joint: A saddle-type synovial joint between the sternal end of the clavicle and the manubrium of the sternum. This joint allows elevation, depression, protraction, retraction, and limited rotation of the clavicle.
• Acromioclavicular Joint: A plane-type synovial joint formed by the lateral end of the clavicle and the acromion of the scapula. It permits gliding movements that contribute to shoulder rotation and scapular mobility.

Muscle and Ligament Attachments

Muscles

The clavicle serves as an important attachment site for several muscles that stabilize and move the shoulder and upper limb.

• Sternocleidomastoid: Attaches to the medial superior surface, contributing to head rotation and flexion.
• Trapezius: Inserts on the lateral superior surface, aiding in scapular elevation and rotation.
• Pectoralis Major: Originates from the anterior surface of the medial clavicle, facilitating arm adduction and medial rotation.
• Deltoid: Attaches to the lateral anterior surface, responsible for arm abduction, flexion, and extension.
• Subclavius: Originates from the inferior surface near the lateral end, stabilizing the clavicle and protecting underlying structures.

Ligaments

Several ligaments attach to the clavicle, providing stability to the sternoclavicular and acromioclavicular joints.

• Coracoclavicular Ligament: Composed of the conoid and trapezoid ligaments, attaching the clavicle to the coracoid process of the scapula.
• Sternoclavicular Ligaments: Strengthen the sternoclavicular joint and limit excessive movement.
• Acromioclavicular Ligament: Connects the clavicle to the acromion, maintaining joint alignment.
• Costoclavicular Ligament: Anchors the clavicle to the first rib, limiting elevation and providing stability.

Blood Supply and Innervation

The clavicle receives a rich vascular supply and nerve innervation, supporting bone metabolism and the muscles attached to it.

• Arterial Supply: Primarily from branches of the suprascapular artery and thoracoacromial artery, supplying both the bone and adjacent soft tissues.
• Venous Drainage: Through accompanying veins that drain into the subclavian and external jugular veins.
• Nerve Supply: Mainly from the supraclavicular nerves derived from the cervical plexus, providing sensory innervation to the periosteum and overlying skin.

Function

The clavicle serves several essential functions in the musculoskeletal system, particularly in relation to the shoulder and upper limb.

• Support and Stabilization: Acts as a strut that maintains the lateral position of the scapula, preventing collapse of the shoulder toward the thorax.
• Transmission of Forces: Transfers mechanical forces from the upper limb to the axial skeleton during lifting, pushing, and pulling movements.
• Contribution to Mobility: Allows a wide range of movements at the shoulder by maintaining the scapula at an optimal distance from the thorax, facilitating flexion, abduction, and rotation of the upper limb.

Clinical Significance

Fractures

Clavicular fractures are common injuries due to direct trauma or falls onto the shoulder. The midshaft is the most frequently fractured region.

• Common Fracture Sites: Midshaft (most common), distal end, proximal end.
• Mechanism of Injury: Falls onto the shoulder, outstretched hand, or direct impact to the clavicle.
• Clinical Presentation: Pain, swelling, visible deformity, limited shoulder movement, and possible tenting of the skin.
• Management and Treatment Options: Nonoperative management with slings for undisplaced fractures; surgical fixation for displaced, comminuted, or high-risk fractures.

Congenital Anomalies

• Clavicular Hypoplasia or Aplasia: Partial or complete absence of the clavicle, which may affect shoulder stability.
• Cleidocranial Dysplasia: A genetic disorder characterized by absent or underdeveloped clavicles, dental anomalies, and delayed ossification of other bones.

Degenerative and Other Conditions

• Osteolysis of the Distal Clavicle: A condition commonly seen in athletes, characterized by resorption of the lateral clavicle leading to pain and reduced shoulder function.
• Acromioclavicular Joint Arthritis: Degenerative changes in the acromioclavicular joint that may cause shoulder pain, decreased range of motion, and tenderness over the joint.

Imaging and Diagnostic Considerations

Imaging of the clavicle is essential for diagnosing fractures, congenital anomalies, and degenerative conditions.

• X-ray Views: Standard anteroposterior (AP), lateral, and cephalad/caudal angled views to assess bone alignment, fractures, and joint integrity.
• CT Scan: Provides detailed evaluation of complex fractures, malunions, and associated bony abnormalities.
• MRI: Useful for assessing soft tissue structures including ligaments, muscles, and the acromioclavicular joint capsule.

References

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