ACROMIOCLAVICULAR SPRAIN - A shoulder injury where the ligaments which stabilize the articulation of the acromion process and the clavicle become torn (sprain). A classic "lump" deformity is noted on the top contour of the shoulder in more serious sprains. Acromioclavicular (AC) separations occur most often after a direct fall onto the shoulder. Severe sprains may result in acromioclavicular dislocation.