Acromioclavicular joint

The acromioclavicular (AC) joint is a plane synovial joint of the pectoral girdle.

The acromioclavicular joint is between the small facets of the convex distal clavicle and flat medial acromion. The articular surfaces are lined with hyaline cartilage 4. A fibrocartilaginous wedge-shaped articular disc separates the two articular surfaces, but its function is unknown 2, 6

A weak, synovium-lined joint capsule is attached to the articular margins and is reinforced superiorly by blending fibres of the trapezius muscle 6.

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 stabilisation is provided by: 

  • coracoclavicular ligament: main stabliser 2, 5
  • superior and inferior acromioclavicular ligaments: reinforce the relatively weak joint capsule 5

Dynamic stabilisation is also provided by the deltoid and trapezius muscles, with the superior acromioclavicular ligament blending with these muscles' aponeuroses. 

Forces transmitted from the upper limb to the glenoid are transmitted to the clavicle via the trapezoid ligament, largely bypassing the AC joint. This means that during a fall onto an outstretched hand or elbow can spare the AC joint (but fracture the mid-shaft of the clavicle) while falls onto the shoulder itself may dislocate the AC joint by forcing the acromion to sublux under the clavicle and tear the coracoclavicular ligament7.

Upper limb anatomy
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Article information

rID: 10208
Section: Anatomy
Tag: cases
Synonyms or Alternate Spellings:
  • ACJ
  • Acromioclavicular joint (ACJ)
  • Acromio-clavicular joint
  • AC joint

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Cases and figures

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    Bones and ligamen...
    Figure 1
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    Case 1: normal acromioclavicular joint
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    Supraspinatus tear
    Case 2: AC joint osteoarthritis
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