injuries such as a dislocation of the shoulder or separation of the acromioclavicular joint. In the case of a tendon with pre-existing degeneration, the force may be more modest, such as with a sudden lift, particularly with the arm above the horizontal position. The type of loading involved with injury is usually eccentric, such as when two people are carrying a load and one lets go, forcing the other to maintain force while the muscle elongates. Chronic tears Chronic tears are indicative of extended use in conjunction with other factors such as poor biomechanics or muscular imbalance. Ultimately, most are the result of wear that occurs slowly over time as a natural part of aging. They are more common in the dominant arm, but a tear in one shoulder signals an increased risk of a tear in the opposing shoulder. Several factors contribute to degenerative, or chronic, rotator cuff tears of which repetitive stress is the most significant. This stress consists of repeating the same shoulder motions frequently, such as overhead throwing, rowing, and weightlifting. Many jobs that require frequent shoulder movement such as lifting and overhead movements also contribute. In older populations impairment of blood supply can also be an issue. With age, circulation to the rotator cuff tendons decreases, impairing natural ability to repair, increasing risk for tear. Another potential contributing cause is impingement syndrome, the most common non-sports related injury and which occurs when the tendons of the rotator cuff muscles become irritated and inflamed while passing through the subacromial space beneath the acromion. This relatively small space becomes even smaller when the arm is raised in a forward or upward position. Repetitive impingement can inflame the tendons and bursa, resulting in the syndrome. Extrinsic factors Well-documented anatomic factors include the morphologic characteristics of the acromion, a bony projection from the scapula that curves over the shoulder joint. Hooked, curved, and laterally sloping acromia are strongly associated with cuff tears and may cause damage through direct traction on the tendon. Conversely, flat acromia may have an insignificant involvement in cuff disease and consequently may be best treated conservatively. The development of these different acromial shapes is likely both genetic and acquired. In the latter case, there can be a progression from flat to curved or hooked with increasing age. Repetitive mechanical activities such as sports and exercise may contribute to flattening and hooking of the acromion. Cricket