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Acromioclavicular Joint

The AC joint is a diarthrodial joint and the only articulation between the clavicle and scapula. Variable inclinations exist, with being nearly vertical to angled downward and medially accounting for up to 50 degrees. The AC joint has an incomplete fibrocartilagenous intra-articular disc, potentially predisposing it to degenerative changes.

Degenerative changes have been shown to occur as early as the second decade of life, with loss of articular cartilage, eburnation of subchondral bone, and formation of marginal osteophytes; however, these changes do not necessarily produce symptoms.

The motion of a normal AC joint is minimal, involving small translations and, mainly, rotation between the clavicle and the acromion with arm movement. The AC ligaments prevent anteroposterior instability. Biomechanical studies have shown that the inferior AC capsular ligaments represent the major restraint against anterior joint translations, whereas the superior and posterior ligaments provide the most stability against posterior translations (56% and 25%, respectively).

Therefore, the posterior and superior capsular ligaments should be spared during distal clavicle excision to prevent excessive posterior clavicle translations after resection. The coracoclavicular ligaments (the trapezoid and the conoid) together resist superior displacement of the joint, but with larger amounts of displacement and induced loads, the conoid ligament contributes the major share.

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