Complete tear of the supraspinatus and infraspinatus tendons leads to proximal migration of the humeral head due to pull by the deltoid muscle. The humeral head strikes the undersurface of the acromion, acromioclavicular (AC) joint. This intially tears the inferior acromioclavicular ligament and later tears the superior ligament. Glenohumeral joint fluid escapes through AC joint and reaches the subcutaneous tissues overlying AC joint.

This is known as Geyser sign.

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