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The acromioclavicular joint is between the small facet of the convex distal clavicle and flat anteromedial acromion. The articular surfaces are lined with fibrocartilage (like the sternoclavicular joint, it is an atypical synovial joint).
A fibrocartilaginous wedge-shaped articular disc, measuring between 1.5-4.0 mm 8, separates the two articular surfaces, but its function is unknown 2,6. There are three types of disc 8-11:
- complete disc (rare)
- meniscoid-like disc
- superior and inferior wedge-shaped meniscoid disc
A weak, synovium-lined joint capsule is attached to the articular margins and is reinforced superiorly by blending fibers of the trapezius muscle 6.
The acromioclavicular joint space measures 1-6 mm (females) and 1-7 mm (males), decreasing with age 12.
The acromioclavicular joint is one of the letter joints 13.
No muscles act directly on this joint. In turn, it permits passive movements only. Instead, the basic scapular movements of protraction/retraction, rotation and elevation/depression transmit to corresponding movements to AC joint.
Static stabilization is provided by:
- coracoclavicular ligament: main stabilizer 2,5
- coracoacromial ligament
- superior and inferior acromioclavicular ligaments: reinforce the relatively weak joint capsule 5
Forces transmitted from the upper limb to the glenoid are transmitted to the clavicle via the trapezoid ligament, largely bypassing the acromioclavicular joint. This means that a fall onto an outstretched hand or elbow can spare the ACJ (but fracture the mid-shaft of the clavicle) while falls onto the shoulder itself may dislocate the ACJ by forcing the acromion to sublux under the clavicle and tear the coracoclavicular ligament. 7
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