Impingement syndrome is a term used to describe a disorder in which one or more soft tissues (rotator cuff tendons, bursa and biceps tendons) surrounding the ball of the shoulder joint get pinched, or impinged, on the bony surface under a portion of the shoulder blade (the acromion).
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The shoulder is made up of three bones (humerus, scapula, clavicle). The tendons that attach four muscles in the shoulder area to the humerus fuse together to form the rotator cuff. The lateral extension of the shoulder blade is the acromion, and the area underneath it is the subacromial space. A lubricating sac of tissue protects the tendons as they move between the acromion and the glenohumeral joint (subacromial bursa). Usually there is enough room between the acromion/acromioclavicular joint and the glenohumeral joint for the rotator cuff and biceps tendons to slide easily underneath the acromion as the arm is raised. However, when the space is made smaller, either by changes in the shape (hypertrophy) of the acromion and/or acromioclavicular joint or by bone spurs, the rotator cuff is forced to rub against the arch of the acromion. Impingement is thought to be a precursor to a rotator cuff tear. Impingement can also cause the bursa or tendons to become inflamed, resulting in bursitis and tendinitis, respectively.
Age, repetitive overhead activity, shoulder looseness (laxity), sleeping with the shoulder abducted, previous injury, osteoarthritis with resulting bone spurs (osteophytes), and anatomical abnormalities are all causative factors. Shoulder laxity that allows the head of the upper arm bone (humerus) to ride high in the shoulder joint will force the rotator cuff against the arch, resulting in impingement. Instability simulating impingement is usually seen in young athletes with a history of significant shoulder injury. As people age, some structures slowly enlarge. Thickening of the acromion and the acromioclavicular joint that occur with age, and the development of osteophytes (spurs) on the underside of the acromion and acromioclavicular joint, are the most common causes of shoulder impingement.
Usually, the acromion has a flat inferior or lower surface (Type I acromion). Some individuals have a congenital anomaly in which the underside of the acromion is hooked or curved (Type II or III), meaning there is less room for the rotator cuff because of a congenital difference in the space available for the cuff tendons.
Risk: Excluding impingement that results from sports that stress the shoulder (competitive swimming, throwing, etc.), impingement syndrome more commonly develops after the age of 30. Individuals who are at risk of developing impingement syndrome include athletes (e.g., baseball players), assembly-line workers, warehouse workers, and others who perform repetitive work with the arms raised above shoulder height.
Conditions that predispose individuals to impingement syndrome include bone spurs, osteoarthritis, and shoulder injuries or degenerative disease.
Incidence and Prevalence: No documented information on the occurrence of shoulder impingement syndrome exists (DeBerardino).