This narrative review synthesizes the anatomical and biomechanical mechanisms behind subacromial impingement syndrome (SAIS), the most common shoulder disorder. It frames SAIS as multifactorial and reviews how kinematics, muscle function, posture, and bony anatomy each narrow the subacromial space. It distinguishes intrinsic tendon degeneration from extrinsic mechanical compression as competing causal theories.
Treat SAIS as a multifactorial problem, not a single structural lesion. This paper's central lesson is that the subacromial space is tiny (1.0–1.5 cm), so small kinematic errors matter clinically. When you see an impingement patient, look beyond the acromion. Assess scapular posterior tilt and upward rotation, rotator cuff strength, posterior capsule tightness, and thoracic/cervical posture.
The painful arc in mid-range abduction is not random. That is where subacromial pressure peaks and cuff contact with the coracoacromial arch is greatest. The intrinsic-versus-extrinsic debate is unsettled: tendon degeneration and mechanical compression coexist by the time patients present, and the paper states which comes first cannot be determined.
The finding that non-operative care matches decompression surgery supports starting with rehab that restores scapular mechanics and cuff strength before considering acromioplasty.
This narrative review synthesizes the anatomical and biomechanical mechanisms behind subacromial impingement syndrome (SAIS), the most common shoulder disorder. It frames SAIS as multifactorial and reviews how kinematics, muscle function, posture, and bony anatomy each narrow the subacromial space. It distinguishes intrinsic tendon degeneration from extrinsic mechanical compression as competing causal theories.
Treat SAIS as a multifactorial problem, not a single structural lesion. This paper's central lesson is that the subacromial space is tiny (1.0–1.5 cm), so small kinematic errors matter clinically. When you see an impingement patient, look beyond the acromion. Assess scapular posterior tilt and upward rotation, rotator cuff strength, posterior capsule tightness, and thoracic/cervical posture.
The painful arc in mid-range abduction is not random. That is where subacromial pressure peaks and cuff contact with the coracoacromial arch is greatest. The intrinsic-versus-extrinsic debate is unsettled: tendon degeneration and mechanical compression coexist by the time patients present, and the paper states which comes first cannot be determined.
The finding that non-operative care matches decompression surgery supports starting with rehab that restores scapular mechanics and cuff strength before considering acromioplasty.