Subacromial Impingement

Populations and Risk Factors

• Prevalence increases with age, though all age groups are affected; most common shoulder pathology across clinical settings
• Overhead athletes: swimmers ("swimmer's shoulder"), baseball pitchers, tennis players, volleyball players — repetitive overhead motion compresses the supraspinatus during the late cocking and deceleration phases of throwing
• Occupational risk: painters, electricians, carpenters, mechanics — any work requiring repetitive overhead reaching or sustained arm elevation above 90 degrees
• Acromial morphology: Type III hooked acromion has the highest association with impingement (Neer classification); Type II curved acromion carries moderate risk; Type I flat acromion is protective
• Scapular dyskinesis (abnormal scapular movement patterns) — insufficient upward rotation during arm elevation fails to clear the acromion, reducing subacromial space
• Thoracic kyphosis and forward head posture narrow the subacromial outlet by anteriorly tilting the scapula
• Rotator cuff weakness, particularly supraspinatus and external rotators — loss of the humeral head depressor mechanism allows superior migration during deltoid contraction
• Posterior glenohumeral capsule tightness — shifts the humeral head anterosuperiorly during flexion, narrowing the subacromial space from below
• Age-related tendon degeneration (intrinsic factor) — reduced cellularity, vascularity, and collagen organization after age 40

Causes and Pathophysiology

The Subacromial Space

• The subacromial space (normally 7-14 mm) lies between the acromion and coracoacromial ligament superiorly and the humeral head inferiorly. The supraspinatus tendon, subacromial bursa, and long head of biceps tendon occupy this space.
• The coracoacromial arch forms a rigid roof — unlike most anatomical spaces, the subacromial space cannot expand superiorly. Any factor that raises the floor (superior humeral head migration) or lowers the roof (hooked acromion, osteophytes, thickened coracoacromial ligament) reduces the available space.
• The subacromial bursa provides frictionless gliding between the supraspinatus tendon and the acromion. When compressed, the bursa becomes inflamed and thickens (subacromial bursitis), which further reduces the available space — creating a self-reinforcing impingement cycle.

Primary vs. Secondary Impingement

• Primary (structural) impingement: narrowing of the subacromial space from above due to fixed anatomical factors — Type II or III acromial morphology, acromial osteophytes (age-related bone spurs on the undersurface of the acromion), thickened coracoacromial ligament, or os acromiale (unfused acromial apophysis). These are non-modifiable factors; surgical acromioplasty (shaving the undersurface of the acromion) may be required if conservative management fails.
• Secondary (functional) impingement: relative narrowing from superior migration of the humeral head due to modifiable factors — rotator cuff weakness or fatigue (loss of the humeral head depressor mechanism), scapular dyskinesis (failure of scapular upward rotation to clear the acromion), glenohumeral instability (excessive humeral head translation), and posterior capsule tightness (anterosuperior humeral head migration). This type is correctable with rehabilitation targeting the root cause.
• The clinical distinction matters because primary impingement may require surgery whereas secondary impingement is a biomechanical problem that responds to conservative management — RC strengthening and scapular stabilization address the root cause.

Neer Classification (3 Stages)

• Stage I — Edema and hemorrhage (age < 25): Repetitive overhead activity produces mechanical compression of the supraspinatus tendon in the subacromial space. The tendon responds with edema and microhemorrhage. The bursa becomes swollen and irritated. This stage is fully reversible with activity modification and conservative treatment. Clinically: positive impingement signs (Neer's, Hawkins-Kennedy), painful arc, but no weakness on resisted testing and no structural tear.
• Stage II — Fibrosis and tendinopathy (age 25-40): Repeated episodes of compression cause cumulative tendon damage. The tendon develops fibrotic changes, thickening, and early partial tearing. The subacromial bursa thickens (fibrotic bursitis), further narrowing the space and perpetuating the cycle. Clinically: persistent pain, positive impingement signs, pain and possibly mild weakness on resisted testing (Empty Can), but no full-thickness disruption. Partially reversible with prolonged rehabilitation; structural changes in the tendon may persist.
• Stage III — Bone spurs and tendon tears (age > 40): Continued mechanical compression combined with intrinsic age-related tendon degeneration leads to acromial osteophyte formation, partial-thickness and eventually full-thickness rotator cuff tears, and involvement of the long head of biceps tendon. The critical zone of hypovascularity in the supraspinatus (1 cm proximal to the greater tuberosity insertion) makes this tendon particularly vulnerable because the watershed area cannot mount an adequate healing response. Clinically: significant AROM loss, weakness on resisted testing, possible positive Drop Arm test. Irreversible structural changes; may require surgical acromioplasty or rotator cuff repair.

Acromial Morphology Types

• Type I (flat): flat undersurface of the acromion; lowest impingement risk; adequate subacromial clearance during arm elevation
• Type II (curved): curved undersurface with a concave contour; moderate impingement risk; the curvature reduces outlet space at higher angles of elevation
• Type III (hooked): anterior hook on the undersurface of the acromion; highest impingement risk; the hook directly impinges on the supraspinatus tendon during forward flexion and abduction, particularly in the 60-120 degree arc
• Acromial morphology is identified on supraspinatus outlet view radiograph and is a key factor in determining whether impingement is primary (structural) or secondary (functional)

Scapular Dyskinesis and the Impingement Mechanism

• During normal arm elevation, the scapula must upwardly rotate, posteriorly tilt, and externally rotate to clear the acromion from the path of the elevating humerus. This scapulohumeral rhythm (approximately 2:1 glenohumeral to scapulothoracic motion after 30 degrees) ensures the subacromial space remains adequate throughout the arc.
• Scapular dyskinesis — abnormal scapular movement — disrupts this rhythm. Common patterns include: insufficient upward rotation (serratus anterior weakness), excessive elevation (upper trapezius dominance), anterior tilt (pectoralis minor shortening), and downward rotation (lower trapezius weakness). Each pattern narrows the subacromial outlet during arm elevation.
• Posterior capsule tightness compounds the problem by shifting the humeral head anterosuperiorly during flexion, further reducing the subacromial space from below.
• This is why subacromial impingement is fundamentally a biomechanical problem of the entire shoulder complex — not just a local tendon issue. Treatment that addresses only the supraspinatus without correcting scapular mechanics will provide temporary relief but not resolve the underlying impingement mechanism.

Signs and Symptoms

By Neer Stage

General Presentation

• Painful arc: Pain between 60-120 degrees of active abduction — pain-free below 60 degrees (the greater tuberosity has not yet entered the subacromial space) and often above 120 degrees (the greater tuberosity has cleared the acromion). This arc corresponds precisely to the range where the supraspinatus tendon and bursa are maximally compressed.
• Anterolateral shoulder pain: Often radiating to the lateral deltoid insertion (C5 referral zone); patients frequently cannot localize the pain precisely, pointing to the general deltoid area rather than a specific point
• Night pain: Especially when lying on the affected side — body weight compresses the subacromial space; arm position during sleep can maintain impingement. Patients often report waking when they roll onto the affected shoulder.
• Pain with overhead activities: reaching into high shelves, combing hair, putting on a jacket, washing hair
• Catching or clicking during arm elevation — from the inflamed bursa or irregular tendon surface moving through the compressed space
• Weakness in arm elevation and external rotation (secondary to pain inhibition in early stages; structural in late stages)
• No significant loss of passive ROM — this is the critical distinction from adhesive capsulitis, where PROM equals AROM in a capsular pattern

Assessment Profile

Subjective Presentation

• Chief complaint: "My shoulder hurts when I reach overhead"; "I get a sharp catch in my shoulder when I raise my arm"; "I can't sleep on that side"; pain described in the general deltoid area rather than a precise point
• Pain quality: Sharp, catching pain during the mid-range of arm elevation (the impingement arc); dull ache at rest and at night; anterolateral shoulder distribution; may radiate to the lateral deltoid insertion
• Onset: Gradual onset related to overhead activities — typically weeks to months of worsening shoulder pain with reaching, lifting, or sport-specific movements. May report a recent increase in overhead work or training volume. Acute-on-chronic pattern is common: a long history of intermittent catching evolves into constant pain after a specific episode.
• Aggravating factors: Overhead reaching (especially above 90 degrees), throwing, lying on the affected side, reaching behind the back, sustained arm elevation (painting, shelving), repetitive arm movements at or above shoulder height
• Easing factors: Rest with arm supported at the side (reduces subacromial compression); avoiding overhead positions; NSAIDs (temporary); ice after activity
• Red flags: Progressive weakness with inability to raise the arm despite minimal pain suggests full-thickness RC tear — refer for imaging and orthopedic evaluation. Acute traumatic onset with immediate inability to elevate suggests acute tear.

Observation

• Local inspection: Typically unremarkable in Stages I-II (pathology is deep); in Stage III, atrophy may be visible in the supraspinatus fossa (above the scapular spine) and/or infraspinatus fossa — compare bilateral. Mild swelling is uncommon.
• Posture: Forward head posture with protracted scapulae and increased thoracic kyphosis (upper crossed syndrome posture) — this narrowing posture perpetuates impingement by anteriorly tilting the scapulae. The affected shoulder may be held slightly lower (pain inhibition of upper trapezius) or higher (compensatory shoulder hiking). Scapular winging or asymmetry during bilateral arm elevation — observe from behind during active flexion and abduction for dyskinesis patterns (early scapular elevation, insufficient upward rotation, scapular winging).
• Gait: Not directly affected; arm swing may be reduced on the affected side due to pain avoidance

Palpation

• Tone: Upper trapezius and levator scapulae hypertonicity from the compensatory shoulder hiking pattern. Pectoralis minor shortened with increased resting tone. Infraspinatus and teres minor guarding. Serratus anterior may feel inhibited or atrophied compared to the contralateral side.
• Tenderness: Supraspinatus tendon insertion at the greater tuberosity — the primary landmark. Subacromial space tenderness palpated just inferior to the anterior acromion with the arm in slight extension. Bicipital groove (long head of biceps, frequently co-involved). Acromioclavicular joint (rule out AC pathology). Trigger points in upper trapezius, infraspinatus (TrPs refer into the anterior/lateral shoulder), and pectoralis minor.
• Temperature: Usually normal; mild warmth over the anterolateral shoulder suggests active bursitis (Stage I) or acute tendon inflammation
• Tissue quality: Subacromial bursa may feel boggy or thickened in chronic cases; infraspinatus and upper trapezius often have ropy, fibrotic texture with trigger points; pectoralis minor feels shortened and taut with restricted fascial glide; posterior capsule may feel thickened and inelastic (contributes to anterosuperior humeral head migration)

Motion Assessment

• AROM: Painful arc between 60-120 degrees of active abduction — the hallmark finding. Pain is absent or minimal below 60 degrees (greater tuberosity has not entered the subacromial space) and typically resolves above 120 degrees (greater tuberosity clears the acromion). Active elevation may show compensatory shoulder hiking (upper trapezius substitution for compromised supraspinatus). Compare bilateral: note asymmetry in scapulohumeral rhythm (the scapula should begin significant upward rotation around 60 degrees — early or excessive scapular motion indicates dyskinesis).
• PROM / end-feel: PROM is near-normal — this is the critical finding that distinguishes impingement (where passive structures are intact) from adhesive capsulitis (where PROM = AROM in a capsular pattern ER > ABD > IR). End-feel for passive elevation and rotation is normal (tissue stretch/capsular) unless posterior capsular tightness is present (firm end-feel in horizontal adduction and IR). Pain at end-range of passive flexion indicates subacromial compression as the examiner drives the greater tuberosity into the coracoacromial arch (this is the mechanism of Neer's test).
• Resisted testing: Isometric testing isolates contractile tissues. Supraspinatus: resisted abduction or Empty Can position — pain indicates tendinopathy (Stage I-II), weakness indicates tendon compromise (Stage II-III). Infraspinatus: resisted ER — pain and/or weakness if co-involved. The pattern of pain versus weakness across Neer stages is diagnostically significant: Stage I = strong and painful (intact tendon, inflamed); Stage II = strong and painful to mildly weak (fibrotic tendon); Stage III = weak and painful or weak and painless (structural failure).

Special Test Cluster

Impingement cluster diagnostic accuracy: The combination of Neer's + Hawkins-Kennedy + Painful Arc provides the highest diagnostic accuracy for subacromial impingement. All 3 positive = strong clinical evidence of subacromial pathology. 2 of 3 positive = probable impingement. 0-1 positive = impingement unlikely, investigate alternatives.

Differential Diagnoses