When the human arm is raised or rotated, the ball of the humerus bone must turn smoothly in the socket of the acromioclavicular (AC) joint. The supportive bones above this area define the amount of space for turning, or the impingement interval.
If this space narrows due to obstructions or inflammation, impingement of the rotator cuff muscles, shoulder pain and a loss of range of motion result. The types of shoulder impingement and their treatments are classified by cause.
Arthritis
According to the American Academy of Family Physicians (AAFP), osteoarthritis is the most frequent cause of shoulder impingement. Rotation of the shoulder joint and attached muscles and bones requires a normal impingement interval, and arthritis reduces that interval.
Periodic inflammation will cause periodic pain and restricted movement. Bone spurs on the acromion or spurs on the AC joint can cause long-term impingement of the rotator cuff and chronic shoulder pain. Monoarthritis and rheumatoid arthritis are also sources of impingement.
Calcification
Calcific deposits in shoulder bursa, ligament or tendon tissue represent forms of bursitis and tendinitis that may be secondary to another injury. Calcification may occur spontaneously or in response to tissue damage. Shoulder pain can be severe, and is accompanied by rigidity and significant inflammation, bringing on impingement.
The AAFP notes that, due to the body's inflammatory response, any condition causing pain in the rotator cuff area is likely to reduce the impingement interval. Calcification of soft tissue may be diagnosed effectively using x-rays, but may resolve without treatment.
Bursitis/Tendinitis
Tendinitis of the rotator cuff or biceps muscle causes impingement and is characterized by shoulder pain at night, in sleep movement and in overhead reaching. The National Institutes of Health (NIH) relate that these shoulder injuries are caused by overuse strains, and tears of weakened muscle and tendon tissue.
Irritation of the bursa reduces the impingement interval by creating inflammation just beneath the shoulder muscles. Both conditions evolve with time until they forcibly restrict shoulder mobility. The NIH notes that most cases of bursitis and tendinitis do not require surgical treatment to reverse shoulder impingement.
Dislocation
Shoulder instability may develop when overuse movements cause degeneration of cartilage, bone or other tissue surrounding the AC joint. The AAFP reports that the abnormal movement of the unstable humerus in the AC joint alone can create shoulder impingement.
A shoulder dislocation that arises from instability, or a traumatic blow or twist exaggerates the alteration to the impingement interval. This very painful condition will definitively restrict or eliminate shoulder mobility.
