The acromioclavicular joints are of the synovial and plane type between the lateral end of the clavicle and the acromion of the scapula. It is atypical in that its articular surfaces are covered by fibrocartilage. The articular surfaces slope inferiorly and medially so that the clavicle tends to override the acromion.
A fibrous capsule is attached to the margins of the articular surfaces. Externally, it is reinforced by fibres from trapezius and capsular thickenings - the superior and inferior acromioclavicular ligaments. Stronger support is provided inferiorly by the coracoclavicular ligaments which are intimately involved with movement. Also, some support is gained superiorly from trapezius and deltoid as their fibres cross the joint.
Internally, the joint capsule is lined by a synovial membrane. The joint cavity is partially divided by a wedge of fibrocartilage that is suspended from the upper part of the capsule.
It is mainly innervated by the lateral supraclavicular nerves. It obtains its vascular supply from the suprascapular and thoracacromial arteries.
The acromioclavicular joint line can be palpated at the lateral end of the clavicle. It is susceptible to subluxation and dislocation injuries with extremes of force applied to the tip of the shoulder.
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