# Shoulder Impingement and Rotator Cuff Pathology

## Anatomy of the Shoulder Complex

### Osseous and Articular Structures
The glenohumeral joint is an inherently unstable ball-and-socket joint with minimal bony constraint. The acromion forms the roof of the subacromial space; morphology classified by Bigliani (Type I flat, Type II curved, Type III hooked). Type III acromion is associated with increased risk of impingement and rotator cuff tears. The coracoacromial arch consists of the acromion, coracoacromial ligament, and coracoid process. The subacromial-subdeltoid bursa lies between the rotator cuff tendons and the coracoacromial arch.

### Rotator Cuff Muscles (SITS)
**Supraspinatus**: abduction initiation (first 15-30 degrees), most commonly injured tendon. **Infraspinatus**: primary external rotator, second most commonly torn. **Teres Minor**: external rotation, especially in adducted position. **Subscapularis**: primary internal rotator, largest rotator cuff muscle, forms anterior restraint.

| Muscle | Primary Action | Innervation | Clinical Relevance |
|--------|---------------|-------------|-------------------|
| Supraspinatus | Abduction initiation (0-15°) | Suprascapular nerve (C5-C6) | Most commonly injured tendon |
| Infraspinatus | External rotation | Suprascapular nerve (C5-C6) | Second most commonly torn |
| Teres Minor | External rotation (adducted) | Axillary nerve (C5-C6) | Hornblower sign if torn |
| Subscapularis | Internal rotation | Upper/lower subscapular nerves (C5-C6) | Largest cuff muscle, anterior restraint |

All four tendons merge to form a continuous cuff insertion on the greater and lesser tuberosities. The "critical zone" of the supraspinatus tendon (1 cm proximal to insertion) is a relative hypovascular watershed area.

### Other Key Structures
Long head of biceps tendon runs through the bicipital groove and is closely associated with the rotator cuff (pulley system). Superior labrum anchors the long head of biceps (SLAP complex). Scapulothoracic articulation is a functional joint critical for overhead function.

## Pathophysiology of Subacromial Impingement

### Extrinsic (Structural) Factors
Subacromial spur formation (enthesophytes at the coracoacromial ligament insertion). Acromial morphology (Type III hooked acromion). Acromioclavicular joint osteophytes encroaching on the subacromial space. Thickened coracoacromial ligament. Os acromiale (unfused acromial apophysis).

### Intrinsic (Tendon) Factors
Age-related tendon degeneration (mucoid changes, chondroid metaplasia). Hypovascular critical zone predisposes to degenerative tearing. Repetitive microtrauma from overhead activities. Oxidative stress and matrix metalloproteinase-mediated degradation.

### Dynamic/Functional Factors
Scapular dyskinesis leading to loss of subacromial space during elevation. Rotator cuff weakness creating superior humeral head migration. Posterior capsule tightness causing anterosuperior migration (glenohumeral internal rotation deficit -- GIRD). Thoracic kyphosis altering scapular resting position.

### Neer Staging of Impingement
**Stage I**: Edema and hemorrhage (age <25, reversible). **Stage II**: Fibrosis and tendinitis (age 25-40, may require surgery). **Stage III**: Tendon degeneration, bony changes, tendon tears (age >40).

| Stage | Pathology | Typical Age | Reversibility | Management |
|-------|-----------|-------------|---------------|------------|
| I | Edema and hemorrhage | <25 | Reversible | Conservative |
| II | Fibrosis and tendinitis | 25-40 | Partially reversible | Conservative ± surgery |
| III | Tendon degeneration, bony changes, tears | >40 | Irreversible | Often surgical |

## Rotator Cuff Tear Continuum

### Classification
**Tendinopathy/Tendinosis**: intratendinous degeneration without macroscopic tear. **Partial-thickness tears**: articular-sided (more common), bursal-sided, or intratendinous. **Full-thickness tears**: small (<1 cm), medium (1-3 cm), large (3-5 cm), massive (>5 cm). **Irreparable/Chronic massive tears**: retraction, fatty infiltration (Goutallier classification), muscle atrophy.

### Risk Factors for Rotator Cuff Tears
Age (prevalence increases significantly after age 60). Dominant arm. Smoking. Hyperlipidemia and metabolic syndrome. Diabetes mellitus. Genetic predisposition. Overhead occupational or sports exposure.

## Clinical Evaluation

### History
Insidious onset of anterolateral shoulder pain. Pain with overhead activities, reaching behind back. Night pain (often the presenting complaint in rotator cuff tears). Weakness with elevation or rotation suggests a significant tear. Acute onset after trauma in older patients suggests acute-on-chronic tear.

### Physical Examination Maneuvers

#### Impingement Tests
**Neer test**: passive forward flexion with scapula stabilized (sensitivity ~79%, specificity ~53%). **Hawkins-Kennedy test**: forward flexion to 90 degrees with passive internal rotation (sensitivity ~79%, specificity ~59%). **Neer impingement test (injection test)**: relief of pain after subacromial lidocaine injection confirms impingement as the pain source.

| Test | Maneuver | Sensitivity | Specificity | Target Structure |
|------|----------|-------------|-------------|-----------------|
| Neer | Passive forward flexion, scapula stabilized | ~79% | ~53% | Subacromial space |
| Hawkins-Kennedy | 90° flexion + passive IR | ~79% | ~59% | Subacromial space |
| Empty Can (Jobe) | Resisted elevation in scaption + IR | ~69% | ~62% | Supraspinatus |
| External Rotation Lag | Maintain passive ER position | ~46% | ~94% | Infraspinatus/Teres minor |
| Belly-Press | Press abdomen with wrist straight | ~40% | ~98% | Subscapularis |
| Lift-Off (Gerber) | Lift hand off lower back | ~18% | ~100% | Subscapularis |

#### Rotator Cuff Tests
**Empty can (Jobe) test**: resisted elevation in scaption with internal rotation (supraspinatus). **Full can test**: resisted elevation in scaption with external rotation (supraspinatus; may be better tolerated). **External rotation lag sign**: inability to maintain passively externally rotated position (infraspinatus/teres minor). **Hornblower sign**: inability to externally rotate in 90 degrees of abduction (teres minor).

**Belly-press test / Bear-hug test / Lift-off test (Gerber)**: subscapularis integrity. **Drop arm test**: inability to slowly lower arm from 90 degrees abduction (large supraspinatus tear).

#### Other Tests
**Speed test / Yergason test**: biceps tendon pathology. **Cross-body adduction test**: acromioclavicular joint pathology. **Scapular dyskinesis assessment**: observe scapular winging, asymmetry during arm elevation.

### Imaging
**Radiographs**: AP, scapular Y, axillary lateral -- assess acromion morphology, superior humeral head migration, AC joint pathology. **Ultrasound**: high sensitivity and specificity for full-thickness tears (>90%), operator-dependent, dynamic assessment possible. **MRI**: gold standard for comprehensive evaluation; assess tear size, retraction, muscle atrophy, fatty infiltration. **MR arthrography**: superior for partial-thickness articular-sided tears and labral pathology.

## Conservative Management

### Phase-Based Rehabilitation Protocol
**Phase I (Acute/Pain dominant, 0-6 weeks)**: Activity modification, avoidance of provocative positions. Pain control: acetaminophen, NSAIDs, cryotherapy. Gentle PROM and AAROM as tolerated. Scapular stabilizer activation (low-load isometrics). Postural correction.

**Phase II (Recovery/Motion restoration, 6-12 weeks)**: Progressive AROM. Rotator cuff strengthening: isometrics progressing to isotonic. Closed kinetic chain exercises (wall push-ups, weight-bearing). Scapular strengthening: serratus anterior, lower trapezius emphasis.

**Phase III (Strengthening, 12+ weeks)**: Progressive resistance training for rotator cuff and scapular stabilizers. Eccentric exercise program for tendinopathy. Sport- or occupation-specific training. Plyometric progression if returning to overhead sport.

### Pharmacologic Management
Oral NSAIDs for acute flares (short course, 2-4 weeks). Topical NSAIDs as adjunct. Subacromial corticosteroid injection: effective short-term pain relief (4-8 weeks), limit to 3-4 per year; concern for tendon weakening with repeated injections. Avoid corticosteroid injection within 6 weeks before planned surgery (infection risk concern).

## Surgical Referral Criteria
Full-thickness rotator cuff tear with functional weakness not responding to 3-6 months conservative care. Acute traumatic rotator cuff tear in active patient (earlier surgical referral). Young patient (<60) with full-thickness tear (better surgical outcomes with early repair). Massive tears with progressive fatty infiltration on serial imaging.

Failed conservative management after 3-6 months with persistent functional limitation. Failure of non-operative management does not worsen surgical outcomes if addressed within a reasonable timeframe.

## Controversy: Subacromial Decompression

The CSAW trial (2018) and FIMPACT trial (2018) demonstrated that arthroscopic subacromial decompression was no more effective than sham surgery or exercise therapy. These findings challenge the traditional Neer model of structural impingement. Current best evidence supports structured rehabilitation as first-line treatment. Subacromial decompression may still have a role in cases with clear structural impingement (large acromial spur) failing prolonged conservative care.

<image>Anterior view of the shoulder showing the subacromial space with the acromion forming the roof, the coracoacromial ligament spanning from coracoid to acromion, and the rotator cuff tendons (supraspinatus, infraspinatus, subscapularis) passing through this space beneath the subacromial-subdeltoid bursa. Label the critical zone of the supraspinatus tendon. Show Bigliani acromion types I, II, and III in small inset diagrams.</image>

<image>Clinical examination illustration showing a clinician performing the Neer impingement test (passive forward flexion with one hand stabilizing the scapula), the Hawkins-Kennedy test (90 degrees forward flexion with passive internal rotation), and the Jobe empty can test (resisted elevation in scaption with thumbs pointing down). Show arrows indicating the direction of force application for each test.</image>

<image>Coronal cross-section of the shoulder depicting the spectrum of rotator cuff pathology: normal tendon, tendinopathy with thickening and intratendinous signal change, partial-thickness articular-sided tear, partial-thickness bursal-sided tear, and full-thickness tear with retraction. Label the subacromial bursa, deltoid muscle, and humeral head for each stage.</image>

## Clinical Pearls

A cluster of positive impingement tests (Neer + Hawkins) combined with rotator cuff weakness tests has higher diagnostic accuracy than any single test. Night pain is often the most bothersome symptom and may be the primary driver for seeking care -- it correlates with bursal inflammation. Always assess the cervical spine in patients presenting with shoulder pain; C5 radiculopathy mimics supraspinatus pathology. Scapular dyskinesis is present in the majority of patients with rotator cuff disease and must be addressed in rehabilitation.

Partial-thickness tears on the articular side are more common and more symptomatic than bursal-sided tears, but are harder to detect on ultrasound. In patients over 60, asymptomatic full-thickness rotator cuff tears are found in up to 25% on imaging -- clinical correlation is essential. Eccentric exercise programs (Alfredson-style protocols adapted to the shoulder) have the strongest evidence for tendinopathy. The "subacromial impingement" paradigm is shifting toward a more nuanced understanding of rotator cuff disease as a multifactorial degenerative process.

## References

- Neer CS. Impingement lesions. *Clin Orthop Relat Res*. 1983;(173):70-77.
- Beard DJ, et al. Arthroscopic subacromial decompression for subacromial shoulder pain (CSAW): a multicentre, pragmatic, parallel group, placebo-controlled, three-group, randomised surgical trial. *Lancet*. 2018;391(10118):329-338.
- Paavola M, et al. Subacromial decompression versus diagnostic arthroscopy for shoulder impingement: randomised, placebo surgery controlled clinical trial (FIMPACT). *BMJ*. 2018;362:k2860.
- Yamamoto A, et al. Prevalence and risk factors of a rotator cuff tear in the general population. *J Shoulder Elbow Surg*. 2010;19(1):116-120.
- Hegedus EJ, et al. Physical examination tests of the shoulder: a systematic review with meta-analysis of individual tests. *Br J Sports Med*. 2012;46(14):964-978.
- Kuhn JE. Exercise in the treatment of rotator cuff impingement: a systematic review and a synthesized evidence-based rehabilitation protocol. *J Shoulder Elbow Surg*. 2009;18(1):138-160.