Overview & Pathophysiology
Subacromial impingement syndrome (SIS) is the most common cause of shoulder pain in adults, accounting for approximately 40–65% of all shoulder pain presentations. It refers to a spectrum of pathology caused by mechanical compression of the subacromial structures — primarily the supraspinatus tendon, subacromial bursa, and long head of biceps — between the superior humeral head and the coracoacromial arch (acromion + coracoacromial ligament + coracoid). It exists on a continuum ranging from bursitis and tendinopathy through partial thickness rotator cuff tears to full-thickness cuff tears. Two complementary classification systems guide its understanding: the Neer staging system (1983 — based on the progressive pathological changes in the subacromial space) and the Bigliani acromial morphology classification (1991 — based on the shape of the acromion on outlet view X-ray).
- Coracoacromial arch: the bony and ligamentous ceiling of the subacromial space; formed by: (1) the acromion superiorly and posteriorly; (2) the coracoacromial ligament (CAL) — running from the anterolateral acromion to the coracoid process; (3) the coracoid process anteriorly; the subacromial space normally measures 9–10 mm between the superior humeral head and the undersurface of the acromion on AP shoulder radiograph; a space of <6–7 mm indicates superior humeral head migration (from significant rotator cuff tearing or cuff arthropathy) and is associated with a massive tear until proven otherwise
- Intrinsic vs extrinsic impingement: extrinsic (outlet) impingement — compression of the cuff from outside by the coracoacromial arch (the classic Neer impingement — subacromial compression); intrinsic impingement — degeneration within the tendon substance (intrinsic tendinopathy) from repetitive loading without external compression; internal impingement — contact between the posterior-superior cuff and the posterosuperior glenoid rim during abduction and external rotation (common in overhead athletes)
Neer Classification — Stages of Impingement
| Stage | Pathology | Typical Age | Reversibility | Management |
|---|---|---|---|---|
| Stage I — Oedema & Haemorrhage | Reversible oedema and haemorrhage within the supraspinatus tendon and subacromial bursa; no structural damage to the tendon; caused by repetitive overhead activity or acute overuse; the subacromial bursa is inflamed and congested; the tendon is oedematous but architecturally intact | <25 years; young athletes in overhead sports | FULLY REVERSIBLE with conservative management | Activity modification; rest; NSAIDs; physiotherapy (rotator cuff strengthening, scapular stabilisation, posture correction); return to sport after symptom resolution |
| Stage II — Fibrosis & Tendinitis | Progression to tendon fibrosis and irreversible tendinitis (tendinopathy); the supraspinatus tendon shows fibrotic thickening and early degenerative changes; the subacromial bursa becomes fibrotic and thickened; repeated bouts of inflammation have led to scar tissue formation; the `critical zone` of the supraspinatus tendon (1 cm from the insertion — the avascular watershed zone) is the primary site of degenerative change; partial thickness tears of the supraspinatus may begin in this stage | 25–40 years | Partially reversible — the fibrosis will not fully resolve but symptoms can be adequately controlled; progressive without treatment | Physiotherapy (primary treatment); subacromial corticosteroid injection (short-term symptom relief — max 2–3 injections); ESWT for calcific tendinitis; surgical decompression (arthroscopic subacromial decompression — ASD) for persistent Stage II despite 3–6 months of adequate conservative management |
| Stage III — Bone Changes & Tendon Rupture | Full-thickness rotator cuff tears (supraspinatus most commonly; may extend to infraspinatus); acromial osteophyte formation on the undersurface of the acromion (the `subacromial spur` or `enthesophyte` at the coracoacromial ligament insertion); biceps tendon pathology (rupture, SLAP tears); AC joint degenerative changes; the structural damage is irreversible — the cuff tear will not heal spontaneously; progressive superior humeral head migration occurs as the cuff becomes incompetent | >40 years | IRREVERSIBLE structural damage; conservative management for pain control and function; surgical repair of the torn cuff if symptomatic and technically feasible | Rotator cuff repair (open, mini-open, or arthroscopic) if tear is repairable; subacromial decompression concurrently; reverse shoulder arthroplasty for massive irreparable tears with cuff tear arthropathy and superior humeral head migration; physiotherapy for those unfit for surgery or with asymptomatic tears |
Bigliani Acromial Morphology Classification
The Bigliani classification (Bigliani LU et al., 1991) grades the shape of the undersurface of the acromion on the supraspinatus outlet view X-ray (a Y-view of the shoulder — the X-ray beam is directed along the plane of the scapula spine, profiling the acromion). The acromial shape determines the degree of mechanical impingement on the underlying supraspinatus.
| Bigliani Type | Shape | Impingement Risk | Prevalence | Association with Tears |
|---|---|---|---|---|
| Type I — Flat | The undersurface of the acromion is flat (horizontal); the coracoacromial arch is wide; the supraspinatus has maximal clearance | LOWEST impingement risk | ~17% of shoulders | Lowest association with rotator cuff tears |
| Type II — Curved | The undersurface of the acromion has a gentle concave curve; moderate reduction in the subacromial space anteriorly | MODERATE impingement risk | ~43% of shoulders (most common) | Moderate association with rotator cuff tears |
| Type III — Hooked | A hook-shaped anterior downward projection of the acromion; the hook directly contacts the supraspinatus tendon during shoulder elevation (the greater tuberosity passes under the acromial hook during abduction); the most impinging morphology | HIGHEST impingement risk; the anterior acromial hook creates direct mechanical impingement on the supraspinatus during abduction in the 60–120° painful arc | ~40% of shoulders | Strongest association with full-thickness supraspinatus tears; Bigliani originally reported that 70% of full-thickness cuff tears occurred under Type III acromions |
| Type IV — Convex | A convex (upward-curved) undersurface; sometimes added to the classification; less clinically significant | Variable | Rare | Less data |
- Note on acromial morphology causality: the Bigliani classification describes a predisposing factor, not the sole cause of rotator cuff tears; intrinsic tendon degeneration (the `critical zone` ischaemia theory — Rathbun and Macnab), age-related changes, and repetitive microtrauma are equally important causes of cuff tearing; a Type III acromion may be an acromial enthesophyte (formed by traction from the coracoacromial ligament) rather than a truly congenital hook shape — this is the basis for acromioplasty (smoothing the undersurface of the acromion) during shoulder surgery
Clinical Tests for Impingement
| Test | Technique | Positive Finding | Sensitivity / Specificity |
|---|---|---|---|
| Neer impingement sign | Examiner stabilises the scapula and passively forward-flexes the arm in internal rotation (thumb down) — drives the greater tuberosity under the acromion | Anterior shoulder pain (subacromial) | Sensitivity ~72%, specificity ~66% |
| Hawkins-Kennedy test | Arm at 90° forward flexion, elbow at 90°; examiner forcibly internally rotates the arm — drives the greater tuberosity under the coracoacromial ligament | Anterior shoulder pain | Sensitivity ~79%, specificity ~59% — the most sensitive test for impingement |
| Painful arc | Active shoulder abduction — pain occurring specifically between 60–120° | Pain in the 60–120° arc (where the greater tuberosity passes under the acromion) | Sensitivity ~74%, specificity ~81%; the most specific individual sign for subacromial impingement; pain >150° = AC joint arthritis; full arc pain = GH joint pathology |
| Neer impingement test (injection test) | 10 mL 1% lignocaine injected into the subacromial bursa; repeat impingement tests 5–10 min later; positive if ≥50% pain reduction | ≥50% pain reduction | Most specific test for subacromial impingement; confirms the subacromial origin of pain; if no relief after injection → intra-articular GH pathology (OA, labral tear, instability) |
Management
- Conservative management (first-line, 3–6 months): physiotherapy is the cornerstone; rotator cuff strengthening (particularly the inferior rotator cuff — infraspinatus, teres minor, subscapularis — to improve dynamic stabilisation of the humeral head and reduce impingement); scapular stabiliser strengthening (lower trapezius, serratus anterior) to correct scapular dyskinesis; posture correction (thoracic extension exercises); NSAIDs; subacromial corticosteroid injection (up to 3 injections; excellent short-term relief but may not alter the natural history); Autologous Blood Injection (ABI) for calcific tendinitis
- Arthroscopic subacromial decompression (ASD — acromioplasty): the anterior and inferior surface of the acromion is smoothed using an arthroscopic burr to increase the subacromial space; the coracoacromial ligament is released; the bursal surface of the rotator cuff is inspected; the CSAW trial (UK, 2018 — Beard et al.) compared ASD vs sham arthroscopy vs physiotherapy for shoulder impingement; at 12 months, there was NO significant difference between ASD and sham surgery; there was a small advantage of shoulder-specific physiotherapy over no treatment; this landmark RCT has significantly reduced the use of isolated ASD for impingement without rotator cuff tear; however, ASD remains appropriate when performed concurrently with rotator cuff repair for Stage III disease
- CSAW trial implications (Beard et al., BMJ 2018): 313 patients; 3 arms — ASD vs sham vs physiotherapy; no significant difference at 12 months in Oxford Shoulder Score between ASD and sham; ASD should NOT be performed as a standalone procedure for subacromial impingement in the absence of a proven rotator cuff tear or significant structural abnormality; the placebo effect of surgery is substantial in this condition
Exam Pearls
- Neer stages: I (oedema/haemorrhage — reversible, <25 years); II (fibrosis/tendinitis — partially reversible, 25–40 years); III (full-thickness tear/bone changes — irreversible, >40 years)
- Bigliani types: I (flat, 17%, lowest risk); II (curved, 43%, most common); III (hooked, 40%, highest risk, most associated with full-thickness cuff tears); Type III → most impingement → most cuff tearing
- Hawkins-Kennedy: most sensitive test for impingement (sensitivity ~79%); painful arc 60–120° = most specific sign for subacromial impingement (specificity ~81%); painful arc >150° = AC joint pathology
- CSAW trial (2018): ASD = sham surgery = no significant difference at 12 months; ASD alone is NOT recommended for isolated impingement without structural cuff tear; physio is the appropriate first-line
- Subacromial injection test: the most specific test for confirming subacromial origin of pain; ≥50% relief after subacromial LA injection = positive; if no relief → look for GH joint pathology (OA, instability, labral tear); important before planning subacromial surgery
- Critical zone: the watershed avascular zone 1 cm proximal to the supraspinatus insertion; most vulnerable to ischaemia under tensile loading; the site of most supraspinatus partial and full-thickness tears; the basis of the intrinsic impingement theory