- 9–11 mm: Acromiohumeral distance (normal subacromial space)
- 5–6 mm: Supraspinatus thickness (healthy tendon caliber)
- Lubricates: Subacromial bursa role (reduces tendon-bone friction)
- None: Painful arc (full pain-free abduction)
Acromion and supraspinatus outlet
Acromion arches over the supraspinatus outlet tendon.
Subacromial bursa cushions the tendon against bone.
Rotator cuff force couple
Four tendons center the humeral head in the socket.
Normal shoulder biomechanics
Scapular rotation keeps the outlet open overhead.
- 800+: Overhead reps/day (swimmers and throwers)
- ~2 mm: Space reduction (with fatigue and swelling)
- Mild: Bursal irritation (early inflammatory response)
- Painters: Risk occupations (overhead laborers)
Mechanical impingement mechanics
Tendon repeatedly rubs the underside of the acromion.
Painful arc appears between sixty and one-twenty degrees.
Scapular dyskinesis
Poor scapular rhythm further narrows the outlet.
Cumulative microtrauma
Small repeated injuries outpace tendon healing.
- 7–8 mm: Tendon thickening (reactive hypertrophy)
- Present: Collagen disorganization (disordered fiber alignment)
- Increased: Neovascularization (new pain-signaling vessels)
- 6–7 mm: Space narrowing (further crowding under acromion)
Tendinopathy pathology
Collagen fibers disorganize under chronic mechanical stress.
Thickened tendon further crowds an already-tight space.
Neovascularization and pain
New nerve-accompanied vessels sensitize the tendon.
Failed healing response
Reactive thickening outpaces the tendon's repair capacity.
- ~75–80%: Neer test sensitivity (passive forced flexion)
- ~70–80%: Hawkins-Kennedy sensitivity (internal rotation at 90°)
- Moderate: Combined specificity (best used together)
- 60–120°: Painful arc range (classic impingement zone)
Neer test mechanics
Examiner passively flexes the arm to compress tissue.
Positive test reproduces sharp pain near full flexion.
Hawkins-Kennedy test mechanics
Ninety-degree flexion plus internal rotation compresses the tendon.
Clinical interpretation
Positive tests support impingement, not confirm tearing.
- 25–50%: Tear depth (of tendon thickness)
- Articular: Common tear side (undersurface torn more often)
- ~40%: Progression risk (may enlarge over years)
- 60–80%: Nonsurgical success (physical therapy responders)
Partial-thickness tear formation
Repeated compression frays and splits tendon fibers.
Tear gap widens as the space keeps narrowing.
Nonsurgical management
Physical therapy strengthens the rotator cuff force couple.
Monitoring progression
Imaging tracks tear depth through conservative treatment.