Shoulder impingement occurs when obstructions develop in the space normally inhabited by the rotator cuff muscles. With less room for dynamic movement, the arm and shoulder do not enjoy their full range of motion. A breakdown in muscle, bursa and tendon tissue brought on by aging or overuse often is behind impingement. Bone and joint degeneration due to arthritis is another instigator. According to the American Academy of Family Physicians (AAFP), any injury or disease that produces shoulder pain can impinge the muscles.
Inflammation
Inflammation is the most common cause of shoulder impingement. A soft-tissue injury such as a bursa, muscle or tendon strain or tear ignites the body's inflammatory response in order to fight off the damage. Swelling will occur. This encroachment on the space used for muscle movement compresses the rotator cuff tendons, the American Orthopaedic Society for Sports Medicine (AOSSM) reports.
Shoulder pain and limited mobility are the result. Conditions that gradually or immediately inflame soft tissue include acute and chronic overuse injuries such as muscle strains, tendinitis and bursitis.
Tissue Calcification
The body sometimes responds to a mineral imbalance brought on by soft-tissue injury with calcification. It transports calcium to the site of an injury and deposits the mineral in muscles, tendons or bursae. Calcified tissue becomes enlarged and hardens. Calcifications within the impingement interval, or rotator cuff space, restrict movement and may be accompanied by extreme pain. The National Institutes of Health note that calcific tendinitis and bursitis are more common in older adults following shoulder injury or surgery.
Irregular Joint Rotation
Another effect of aging and overuse that can cause shoulder impingement is degeneration of the acromioclavicular (AC) joint. As the joint socket becomes wider or the ball of the humerus wears down, the shoulder becomes unstable and turns in an irregular motion. This breaks up the uniform space of the impingement interval, causing shoulder pain during rotation.
Shoulder instability also raises the risk for dislocation, or disconnection of the humerus and AC joint. Intense pain and swelling are associated with shoulder dislocation, which, can be additional contributors to rotator cuff impingement, the AAFP relates.
Bone Spurs
An arthritic condition in the AC joint may create further restrictions for the shoulder muscle group over time. The body's reaction to the periodic swelling and shoulder pain of osteoarthritis, rheumatoid arthritis or monoarthritis may be to form bone spurs. The AAFP notes that spurs that form beneath the acromion bone or on the AC joint will cause shoulder impingement.
