The condition occurs when calcium crystals deposit within the substance of the rotator cuff, most commonly the supraspinatus tendon. The reason this happens is not entirely understood, but it tends to affect people in their 30s to 50s and can occur without any preceding injury.
The condition tends to progress through several phases: a formative phase (during which deposits form, often with relatively mild symptoms), a resting phase, and a resorptive phase, during which the calcium becomes fluid-like and can cause intensely severe pain as it irritates the surrounding tissue.