The clavicle is an elongated, S-shaped bone that rests horizontally at the sternum across the upper part of the ribcage, and the acromial end of the scapula. This bone plays an essential role in everyday functional movement, serving as the connection between the axial skeleton and the pectoral girdle. As a result, the clavicle is able to act as a brace for the shoulder, allowing weight to be transferred from the upper limbs to the axial skeleton.
Due to the clavicle’s structure, there are only two planar diarthrosis articulations that can be found. This type of articulation is also known as a ‘double plane joint’ – where two joint cavities are separated by a layer of articular cartilage.
The first is the acromioclavicular joint, which is formed by the acromial end of the clavicle and the acromion of the scapula respectively. It enables slight gliding movement about the shoulder region. The synovial joint is surrounded by a capsule of articular cartilage filled with intra-articular synovium.
From infancy, the articular cartilage starts off as hyaline cartilage, but soon develops into fibrocartilage (at the scapula acromion and the clavicle acromial end at ages 17 and 24, respectively).The acromioclavicular ligament forms a strong connection between the clavicle and the scapula acromion, which restricts movement about the clavicle at its acromial end.
The other is the sternoclavicular joint, which is formed by the sternal end of the clavicle and the manubrium of the sternum. This synovial joint is important as it anchors the clavicle and scapula to the axial skeleton. However the joint enables a variety of limited movements of the arm, including:
- protraction and retraction
- depression and elevation
- slight rotation
Like the acromioclavicular joint, the sternoclavicular joint is surrounded by an articular cartilage capsule, but with a fibrocartilage articular disk inside that creates a clavicular and a sternal synovial cavity. Sternoclavicular joint ligaments stabilise the joint on its anterior and posterior surfaces. The joint is also reinforced by two accessory ligaments:
- The anterior interclavicular ligament, which covers the superior surface of the joint. This ligament is responsible for preventing dislocation of the clavicle upon shoulder depression.
- The posterior costoclavicular ligament, which runs from the clavicle costal tuberosity to the superior and medial surface of the first rib. In contrast, this ligament prevents clavicle dislocation upon shoulder elevation.
The orientation of the clavicle can be distinguished by its ends: a broad, flat acromial end (referred to as the lateral third); and a round pyramidal-like sternal end (referred to as the medial two-thirds). Each end has unique bony landmarks, depending whether the superior or inferior surface of the bone is viewed.
The superior surface of the clavicle has a smooth appearance. The acromial facet can be seen at the far poster edge of the acromial end. It appears as a small flattened oval surface and enables the clavicle to articulate about the acromion of the scapula in the acromioclavicular joint.
In contrast, the inferior surface of the clavicle is quite rough and has many prominent lines, indicating sites where muscle and ligaments may attach to the bone. These can be divided into three bony landmarks:
- The sternal facet, found far at the edge of the sternal end. It is shaped like a triangle, with a posterior tip and an anterior base, and forms the sternoclavicular joint.
- The costal tuberosity, located at the sternal end of the bone. It is distinguished as a broad rough surface over 2 cm in length, and is the site where costoclavicular ligament attaches.
- The conoid tubercle, which is found more laterally towards the acromial end. It appears as a rough eminence found where the flat portion of the clavicle meets the tip of the pyramidal sternal end. This feature acts as the site where the conoid ligament attaches.
A total of five muscles are attached to the clavicle, found distributed at either the lateral third or medial two thirds of the bone.
Two muscles are attached to the lateral third of the clavicle:
- The trapezius muscle, which is attached along the posterior surface of the bone.
- The deltoid muscle, where the anterior portion of the muscle is attached to the periosteum at the anterior surface of the bone.
Three muscles are attached to the medial third of the clavicle:
- The sternocleidomastoid muscle, where the muscular clavicular head of the muscle is attached to the superior surface of the bone.
- The pectoralis major muscle, which is attached to the anterior surface of the bone.
- The subclavius muscle, which is attached to a groove found in the middle of the bone’s posterior surface. From there, the muscle extends into both lateral and medial areas of the clavicle.