The clavicle, commonly known as the collarbone, is a long, slender bone that connects the shoulder girdle to the axial skeleton. It is S-shaped and acts as a strut to support the shoulder and facilitate arm movements. The clavicle is composed of a shaft with medial and lateral ends, each forming important articulations. It serves as a site for muscle and ligament attachments, contributing to the stability and mobility of the upper limb.
Location
The clavicle is located in the upper thorax, extending horizontally between the sternum and the scapula.[2] Medially, it articulates with the manubrium of the sternum at the sternoclavicular joint, and laterally, it connects to the acromion of the scapula at the acromioclavicular joint. It is positioned above the first rib and lies subcutaneously, making it easily palpable along its length.
Structure and Anatomy
The clavicle, or collarbone, is a long, slender, S-shaped bone that plays a crucial role in the shoulder girdle. It is divided into a shaft and two distinct ends: the medial (sternal) end and the lateral (acromial) end. Its structure allows it to serve as a strut between the sternum and scapula while providing support and mobility for the upper limb.[3]
Shaft of the Clavicle
a. Shape
- The shaft is S-shaped when viewed from above, with a convex curve medially and a concave curve laterally. This shape contributes to its strength and flexibility.
b. Surfaces
- Superior Surface: Smooth and subcutaneous, making it easily palpable.
- Inferior Surface: Rough and marked by grooves and ridges for ligament and muscle attachments.
c. Borders
- Anterior Border: Provides attachment for muscles like the pectoralis major (medially) and deltoid (laterally).
- Posterior Border: Gives attachment to the trapezius muscle.[5]
d. Notable Features
- Subclavian Groove: A longitudinal groove on the inferior surface, providing space for the subclavius muscle.
- Conoid Tubercle: Located on the lateral end of the inferior surface, where the conoid ligament attaches.
- Trapezoid Line: A ridge lateral to the conoid tubercle for the trapezoid ligament attachment.
Medial (Sternal) End
- Shape and Articulations:
- The medial end is rounded and articulates with the manubrium of the sternum, forming the sternoclavicular joint.
- Ligament Attachments:
- The costoclavicular ligament attaches to a rough area on the inferior side of the medial end, stabilizing the joint.
Lateral (Acromial) End
- Shape and Articulations:
- The lateral end is flattened and articulates with the acromion of the scapula, forming the acromioclavicular joint.[7]
- Ligament Attachments:
- Provides attachment for the acromioclavicular ligament and contributes to the integrity of the shoulder girdle.
Articulations
a. Sternoclavicular Joint
- A synovial saddle joint between the sternal end of the clavicle and the manubrium of the sternum.
- Allows a wide range of movements, including elevation, depression, and rotation.
b. Acromioclavicular Joint
- A planar synovial joint between the acromial end of the clavicle and the acromion of the scapula.
- Supports shoulder movements and transmits forces from the upper limb to the axial skeleton.
Blood Supply
- Nutrient Foramina:
- Located on the inferior surface of the shaft, allowing entry for the nutrient arteries.
- Arterial Supply:
- Branches from the suprascapular artery and thoracoacromial artery supply blood to the clavicle.[8]
Ossification
- Primary Ossification:
- The clavicle is the first bone in the body to ossify, beginning during the fifth to sixth week of fetal development.
- Secondary Ossification:
- Occurs at the sternal end and completes by the age of 25.
Muscular Attachments
a. Medial Half
- Pectoralis Major: Attaches to the anterior surface.
- Sternocleidomastoid: Attaches to the superior surface.
- Subclavius: Attaches to the inferior surface in the subclavian groove.
b. Lateral Half
- Deltoid: Attaches to the anterior border.
- Trapezius: Attaches to the posterior border.[1]
Function
Support for the Shoulder Girdle
The clavicle acts as a strut between the sternum and scapula, holding the shoulder away from the thorax and allowing free movement of the upper limb.
Force Transmission
It transmits mechanical forces from the upper limb to the axial skeleton, helping distribute loads during activities like lifting, pushing, and pulling.
Protection of Vital Structures
The clavicle protects underlying neurovascular structures, including the brachial plexus, subclavian artery, and vein, from external trauma.
Facilitation of Arm Movement
By articulating with the scapula at the acromioclavicular joint, the clavicle enhances the range of motion of the shoulder, enabling activities such as lifting and rotating the arm.[4]
Attachment Site for Muscles and Ligaments
The clavicle serves as an anchor point for muscles like the pectoralis major, deltoid, trapezius, and sternocleidomastoid, as well as ligaments that stabilize the sternoclavicular and acromioclavicular joints.
Clinical Significance
The clavicle is clinically significant due to its structural and functional roles, making it prone to various injuries and conditions:
- Fractures:
- Clavicle fractures are common, particularly from falls onto the shoulder or outstretched hand. The midshaft is the most frequently fractured region due to its thin structure and lack of reinforcement.
- Acromioclavicular Joint Injuries:
- Dislocations or separations at the acromioclavicular joint, often caused by trauma, result in shoulder instability and pain.
- Sternoclavicular Joint Disorders:
- Dislocations, arthritis, or infections in this joint can impair shoulder function and require medical intervention.
- Congenital Abnormalities:
- Conditions like cleidocranial dysplasia result in partial or complete absence of the clavicles, leading to hypermobility of the shoulders.
- Osteolysis:
- Repetitive stress, often seen in athletes, can cause resorption of the clavicle’s distal end, leading to pain and weakness.[6]