Acromion anatomy

The role of the acromion in the anatomy of the pectoral girdle

The acromion is one of the most important processes of the scapula. The scapula itself, together with the clavicle, forms the bony basis of the pectoral girdle. The acromial process articulates with the lateral end of the clavicle, forming the acromioclavicular joint.

This joint has high stability due to its powerful ligamentous apparatus. Strong ligaments take part in its reinforcement:

• Acromioclavicular ligament.
• Coracoclavicular ligament.

Together with muscles, ligaments, and scapulothoracic articulation, the structures of the shoulder girdle (including the acromion) serve as a reliable support for the functioning of the free upper limb and act as a shock absorber during sudden movements or mechanical injuries.

Formation of the shoulder joint vault and restriction of movement

The glenohumeral joint is a typical multiaxial ball-and-socket articulation connecting the head of the humerus and the shallow glenoid cavity of the scapula. It has weak fixation and lacks true ligaments, which provides the human arm with the greatest freedom of movement among all joints of the body.

However, movements in the joint have their strict anatomical limits. Flexion and abduction of the upper limb are possible exclusively up to shoulder level (horizontal). Further movement is inhibited by the tension of the joint capsule and the hard abutment of the upper end of the humerus against the anatomical vault. This vault is jointly formed by the acromion of the scapula and the coracoacromial ligament (lig. coracoacromiale). To raise the arm above the horizontal line, the movement must occur together with the pectoral girdle — while the scapula rotates, and its inferior angle shifts anteriorly and laterally.

Injury mechanisms and acromial fractures

Scapular fractures as a whole are quite rare (accounting for only 1–2.5% of all musculoskeletal system fractures) because this bone is mobile and well protected by the surrounding muscle mass.

Isolated fractures of the acromial process occur with a specific injury mechanism: the action of a force directed from top to bottom, when the point of application is located directly above the process.

In addition, the acromion plays the role of a mechanical energy transmitter in indirect clavicle injuries. When falling on an outstretched hand or the outer surface of the shoulder, a sharp compression of the shoulder joints from the sides occurs. The load is transmitted through the acromial process along the S-shaped clavicle, which ultimately leads to its fracture at the site of greatest curvature (in the middle third).

Associated injuries and treatment tactics

In fractures of the lateral end of the clavicle, the peripheral bone fragment often remains anatomically connected to the acromial and coracoid processes of the scapula. At this time, the central fragment is displaced upward under the action of the sternocleidomastoid muscle (m. sternocleidomastoideus). Such displacement can clinically mimic a dislocation of the acromial end of the clavicle. Some incomplete intra-articular fractures in this zone are accompanied by rupture of the superior acromioclavicular ligament.

Treatment of acromial process fractures without displacement is conservative: the patient is prescribed immobilization on a special abduction splint lasting 4–5 weeks. If the fracture is accompanied by displacement of fragments, surgical treatment becomes the method of choice — open reduction and internal fixation. During rehabilitation, active movements in the shoulder joint are allowed after 4 weeks, and full recovery of working capacity occurs on average in 1.5–2 months.

Class and exam questions

What else to study on this topic

• Anatomy and biomechanics of the sternoclavicular joint.
• Classification of scapular fractures by localization.
• Typical fragment displacement in middle-third clavicle fractures.
• Structure and functions of the glenoid labrum of the shoulder joint.
• AO classification of metaepiphyseal fractures.

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.