Acromioclavicular ligament

At a glance

English

Acromioclavicular ligament

Latin (TA2)

Ligamentum acromioclaviculare

System

Ligament

Region

Upper limb

Side

Paired, left and right

About the acromioclavicular ligament

The acromioclavicular joint is the small plane synovial joint at the point of the shoulder, where the flattened outer end of the clavicle meets the medial edge of the acromion. Its fibrous capsule is reinforced above and below by the acromioclavicular ligament, which is usually described as two bands, superior and inferior. Strength for the joint as a whole comes from this ligament together with the more distant coracoclavicular ligament.

The superior band is the stronger. It is a four-sided sheet of parallel fibres laid over the top of the joint, passing from the top of the clavicle's outer end to the adjacent upper surface of the acromion. Its superficial fibres interweave with the tendinous expansions of trapezius and deltoid, which arise from these same bones, so muscle tone adds to its grip. Beneath it, when present, lies an incomplete wedge-shaped articular disc. The inferior band is thinner and covers the undersurface of the joint, joining the lower surfaces of the two bones. Below it runs the tendon of supraspinatus on its way to the humerus.

The ligament's main job is to stop the clavicle sliding forwards and backwards on the acromion, so it controls horizontal stability. Vertical stability, which resists the shoulder dropping away from the clavicle, depends chiefly on the conoid and trapezoid parts of the coracoclavicular ligament.

Injury is common in contact sports and falls onto the point of the shoulder. In a mild acromioclavicular sprain only this ligament is stretched or torn. When the coracoclavicular ligament also fails, the outer clavicle rides up and a visible step appears at the shoulder, the classic shoulder separation. Degenerative arthritis of the joint, often with thickening of the ligament and capsule, can narrow the space beneath the acromion and add to impingement of the supraspinatus tendon.

Attachments and function

Attaches from

The upper and lower surfaces of the lateral (acromial) end of the clavicle.

Attaches to

The adjoining upper and lower surfaces of the acromion of the scapula.

Function

Reinforces the capsule of the acromioclavicular joint and limits forward and backward gliding of the clavicle on the acromion.

Nerve supply

Branches of the suprascapular and lateral pectoral nerves, with twigs from the supraclavicular nerves.

Blood supply

The acromial branches of the suprascapular and thoraco-acromial arteries.