Learning Objectives

By the end of this topic, you should be able to:

1. Describe the articulating surfaces and type of the acromioclavicular joint.
2. Identify the capsule and ligamentous supports of the joint.
3. Explain the movements, blood supply, and innervation of the AC joint.
4. Recognize the mechanism, features, and consequences of acromioclavicular joint dislocation.

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The acromioclavicular (AC) joint is a synovial articulation in the shoulder region between the lateral end of the clavicle and the acromion of the scapula. It is classified as a plane-type synovial joint and plays a key role in transmitting forces from the upper limb to the axial skeleton.

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Articulating Surfaces

The joint is formed by:

• The lateral end of the clavicle
• The medial surface of the acromion

The AC joint has two distinctive features:

• The articular surfaces are covered by fibrocartilage (not hyaline cartilage).
• The joint cavity is partially divided by an articular disc, a wedge of fibrocartilage suspended from the superior part of the capsule.

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Joint Capsule

The joint is enclosed by a fibrous capsule lined internally with synovial membrane. The posterior aspect of the capsule is reinforced by fibres from the trapezius muscle.

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Ligaments

Three main ligaments stabilize the AC joint:

1. Acromioclavicular ligament
   1. Runs from the acromion to the lateral clavicle
   2. Reinforces the superior aspect of the capsule
2. Conoid ligament
   1. Extends from the coracoid process to the conoid tubercle of the clavicle
3. Trapezoid ligament
   1. Extends from the coracoid process to the trapezoid line of the clavicle

The conoid and trapezoid ligaments together form the coracoclavicular ligament, a very strong structure that suspends the upper limb from the clavicle.

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Movements

The AC joint permits:

• Gliding movements (superior–inferior and anteroposterior)
• A small amount of axial rotation

Because no muscles act directly on this joint, all movements are passive and occur secondary to motion at the shoulder girdle.

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Blood Supply

The joint is supplied by:

• Suprascapular artery (from the thyrocervical trunk of the subclavian artery)
• Thoracoacromial artery (from the axillary artery)

Venous drainage follows the arteries.

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Innervation

The AC joint is innervated by articular branches of:

• Suprascapular nerve
• Lateral pectoral nerve

Both arise from the brachial plexus.

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Clinical Relevance

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Acromioclavicular Joint Dislocation (“Separated Shoulder”)

An AC joint dislocation occurs when the clavicle and acromion are forcibly separated.

Common causes:

• Direct blow to the shoulder
• Fall on an outstretched hand

Severity increases if the ligaments are torn:

• AC ligament rupture → joint instability
• Coracoclavicular ligament rupture → the weight of the limb is no longer supported, causing the shoulder to drop inferiorly

Management depends on severity and functional limitation and may include:

• Ice, rest, and immobilization
• Physiotherapy
• Surgical ligament reconstruction in severe cases

⚠️ This injury is not the same as a shoulder (glenohumeral) dislocation.

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Activity