When raising an arm causes shoulder pain, the rotator cuff muscles are having difficulty sliding beneath the shoulder's supportive bones. This condition is termed shoulder impingement and results in limited mobility. The space reserved for function of the shoulder muscles is called the impingement interval.
Normally, the muscles glide unobstructed, allowing the shoulder's characteristic wide range of motion. Abnormal obstructions inhibit movement and may be caused by injury, aging tissue or progressive arthritis. Problems related to impingement are diagnosed by pain location and through imaging tests.
Bursitis
The bursa absorbs shock and creates a smooth surface for the shoulder joint when in motion. As noted by the University of Maryland Medical Center, bursitis causes shoulder impingement via tissue inflammation. A bursa damaged through overuse of the shoulder enlarges when inflamed and protrudes into the impingement interval, giving the rotator cuff muscles less room to move.
Sometimes bursitis is compounded by the accumulation of calcium minerals in the bursa, according to the National Institutes of Health (NIH). Calcification induces hardening of the bursa, which further restricts muscles and increases shoulder pain.
Tendinitis
Shoulder tendinitis is brought on by chronic muscle strain in repetitive motion or stressful posture. Damaged muscles and tendons receive the body's inflammatory response and cause impingement. Evolving shoulder pain symptoms of tendinitis will be dispersed over a greater area than in bursitis.
Because blood flow to the rotator cuff is not abundant, muscles and connective tissue strength may decline over time. The Mayo Clinic notes that advanced age increases the risk of developing tendinitis. Older adults also experience a greater incidence of calcification, which exacerbates impingement and pain.
Unstable Shoulder
Instability of the acromioclavicular (AC) joint causes shoulder impingement when the ball of the humerus turns unevenly in a degenerated AC socket. Shoulder instability may not be diagnosed until a dislocation occurs.
Dislocation symptoms include acute shoulder pain and visible musculoskeletal displacement. The American Orthopaedic Society for Sports Medicine (AOSSM) reports that this trauma can affect adjacent rotator cuff tissue, creating inflammation that further reduces the impingement interval.
Arthritis
The pain and swelling of arthritis are primary causes of shoulder impingement, restricting mobility by pinching the shoulder muscles. Pain that comes and goes may be traced to flare-ups of osteoarthritis or rheumatoid arthritis in the AC joint.
The AOSSM notes that bone spur formations related to arthritis are secondary causes of impingement, as they occupy the space normally reserved for muscle movement. Pain in this condition may be sharp and more constant.
