Acromioclavicular Joint

Introduction

Introduction to the Acromioclavicular Joint

The acromioclavicular (AC) joint is a plane synovial joint between the lateral end of the clavicle and the acromion of the scapula. It forms the roof of the shoulder and contributes to the scapulothoracic rhythm by allowing the scapula to glide and rotate on the clavicle. Although small, this joint is essential for full overhead arm mobility, and its superficial location makes it susceptible to traumatic dislocation and degenerative conditions. The stability of the AC joint depends on the acromioclavicular ligament, the strong coracoclavicular ligament complex, and the surrounding muscles.

This module details the articular surfaces, intra-articular disc, ligamentous support, vascular and nerve supply, biomechanics, and clinical conditions of the acromioclavicular joint.

Study

1. Articular Surfaces and Classification

The acromioclavicular joint is a plane (gliding) synovial joint. The articular surface on the clavicle faces laterally and slightly inferiorly; the acromial surface faces medially and slightly superiorly. The joint surfaces are covered with fibrocartilage rather than hyaline cartilage. An incomplete wedge-shaped fibrocartilaginous articular disc is often present within the joint cavity, projecting downwards from the superior capsule. This disc may partially or completely divide the joint.

The AC joint is approximately 1–3 cm lateral to the sternoclavicular joint and lies just beneath the skin at the point of the shoulder. Its palpable prominence makes it a useful surface landmark.

2. Ligamentous Support

The AC joint is stabilised by two sets of ligaments:

Acromioclavicular ligament: This is a thickening of the joint capsule that surrounds the articulation. It consists of superior, inferior, anterior, and posterior bands, with the superior band being the strongest. It provides horizontal stability by resisting anteroposterior translation of the distal clavicle.

Coracoclavicular ligament: The primary suspensory ligament of the upper limb, this strong ligament runs from the coracoid process to the undersurface of the clavicle. It consists of two parts: the trapezoid ligament (anterolateral, attaching to the trapezoid line of the clavicle) and the conoid ligament (posteromedial, attaching to the conoid tubercle). The coracoclavicular ligament provides vertical stability, preventing superior displacement of the clavicle relative to the acromion.

Additional dynamic stabilisers include the deltoid and trapezius muscles, whose aponeuroses blend with the superior acromioclavicular ligament.

3. Blood Supply

The AC joint receives its arterial supply from the suprascapular artery and the acromial branch of the thoracoacromial artery. Venous drainage follows the corresponding veins into the cephalic and external jugular systems.

4. Nerve Supply

The nerve supply is provided by the suprascapular nerve (C5, C6) and the lateral pectoral nerve (C5, C6, C7). The articular branches arise from these nerves and are responsible for proprioceptive and pain sensation.

5. Movements and Biomechanics

The AC joint permits three types of movement: (1) gliding of the acromion on the clavicle in anteroposterior and superoinferior directions; (2) rotation of the scapula relative to the clavicle during overhead arm elevation, with up to 20–30° of rotation occurring at the AC joint during full abduction; and (3) tilting of the scapula. These movements are essential for the coordinated scapulothoracic rhythm that allows full shoulder flexion and abduction.

6. Clinical Significance

AC joint separation (shoulder separation): One of the most common shoulder injuries, typically caused by a fall onto the point of the shoulder. The Rockwood classification (types I–VI) grades the severity based on the extent of ligamentous injury and clavicular displacement. Type I and II are managed conservatively; type III is controversial (often non-operative); types IV–VI require surgical reconstruction.

Osteoarthritis of the AC joint: Common with age and in weightlifters, presenting with localised pain, crepitus, and a positive cross-body adduction test.

Distal clavicle osteolysis: Often seen in heavy weightlifters, due to repetitive microtrauma and resorption of the distal clavicle.

AC joint arthritis: May be post-traumatic or part of generalised inflammatory arthritis.

Summary

Summary of the Acromioclavicular Joint

The acromioclavicular joint is a plane synovial articulation between the acromion and clavicle, stabilised by the acromioclavicular ligament (horizontal stability) and the coracoclavicular ligament (vertical stability). It permits gliding, rotation, and tilting of the scapula, essential for full shoulder motion. AC joint injuries, particularly separations, are graded by the Rockwood classification. Osteoarthritis and osteolysis are common chronic conditions affecting this joint.