Causes / typical injury mechanism: Shoulder impingement syndrome, or subacromial impingement, occurs when soft tissue inside the shoulder joint is ensnared and irritated. It is often a result of rotator cuff degeneration due to age, overuse, microtrauma, and hypoxia. However, it can also be caused by subacromial bursitis, tendinosis, calcific tendonitis, os acromiale, coracoacromial ligament (CAL) ossification, anterior acromial osteophytes, acromioclavicular (AC) joint arthritis with inferior osteophytes, or a type III acromion morphology.

Classic history and presentation: Patients with the syndrome usually report experiencing pain when raising the arm between 70 and 120 degrees, when lying on the injured shoulder, and with forced movement of the arm overhead. Patients typically present with lateral and posterior shoulder pain.

Prevalence:

• Age – More common in those older than 40 years.
• Sex / gender – Equal prevalence.
• A reported 44%-65% of shoulder pain complaints are attributed to shoulder impingement syndrome.

Risk factors:

• Older age
• Sports participation
• Occupations with repetitive overhead arm movement
• Individual shoulder anatomy variants (eg, hooked acromion, excessive coverage of the shoulder joint by the acromion)

Pathophysiology: Shoulder impingement syndrome is caused by a conflict between the mechanics of the different structures of the shoulder joint causing soft tissue to be irritated in and above the joint.

Related topic: internal impingement syndrome