Anatomy of the Shoulder and the Subacromial Space

The shoulder is a complex region formed by the head of the upper arm bone (humeral head), the shallow socket on the shoulder blade (glenoid), the roof of bone above it (acromion) and the collar bone (clavicle). The narrow space between the acromion and the humeral head is called the subacromial space.

This space contains the supraspinatus tendon (the uppermost tendon of the rotator cuff), the subacromial bursa (a lubricating fluid sac), the long head of biceps tendon and the coracoacromial ligament. In a healthy shoulder these structures move freely; narrowing of the space (impingement) or thickening of the soft tissues (bursitis) causes compression.

Structural (Bony) Causes

• Acromial shape (Bigliani classification): Type I (flat), Type II (curved), Type III (hooked) — Type III is the most prone to impingement
• Acromial osteophyte (bone spur): typically develops on the anterior undersurface
• Osteophytes or arthritis beneath the acromioclavicular joint
• Os acromiale: incomplete fusion of the acromion during development, which increases the risk of impingement

Functional Causes

• Rotator cuff weakness: the humeral head rides upwards and the subacromial space narrows
• Scapular dyskinesis: abnormal positioning of the shoulder blade increases impingement
• Posterior capsular tightness: pushes the humeral head forwards and upwards, causing impingement
• Poor posture: rounded shoulders and a forward head position

Soft-tissue Causes

• Subacromial bursitis: thickening and inflammation of the bursa
• Supraspinatus tendinopathy / tendinitis: degenerative change or inflammatory thickening within the tendon
• Calcific tendinitis: deposition of calcium crystals within the tendon (severe pain during acute flare-ups)
• Partial rotator cuff tear: impingement and tearing create a self-perpetuating cycle

Neer Staging

• Stage I: oedema and haemorrhage (younger patients, reversible) — responds well to non-operative treatment
• Stage II: fibrosis and tendinitis (ages 25–40)
• Stage III: bony change and tendon tearing (over 40) — surgery is often required

Symptoms

• Pain over the outer upper shoulder: worse at night and when lying on the affected shoulder
• Pain with overhead activity: combing hair, hanging washing, taking items from a shelf
• Painful arc: pain between 60° and 120° when raising the arm to the side
• Sensation of impingement: catching or locking during movement
• Loss of strength: particularly with elevation and external rotation
• Crepitus: crackling or catching during movement
• Disturbed sleep: night pain is the last symptom to settle
• Decline in sport-specific performance: with throwing, serving and overhead movements

Diagnosis

Diagnosis is based on a detailed history, physical examination and imaging. The patient's occupation (particularly overhead work), sporting activity, duration and character of pain, night pain, previous shoulder injuries and comorbidities (diabetes, rheumatological disease, smoking) are reviewed.

Examination assesses active and passive range of motion, painful arc, impingement tests (Neer, Hawkins–Kennedy), rotator cuff strength tests (Jobe/empty can for supraspinatus; external rotation for infraspinatus; lift-off for subscapularis), acromioclavicular joint tenderness, scapular dyskinesis and neurovascular status. A subacromial lidocaine injection test may be used to confirm the diagnosis.

Plain radiographs are the first-line investigation; anteroposterior, supraspinatus outlet and axillary views are standard. Ultrasound is a practical, dynamic method for assessing the rotator cuff and subacromial bursa. MRI is the gold standard for assessing supraspinatus tendinopathy, partial or full-thickness tears, subacromial bursitis, acromioclavicular joint arthritis and associated glenohumeral pathology.

Non-operative (Conservative) Treatment

The great majority of patients (around 60%) respond to non-operative treatment. Typical candidates are those with recent or short-lived symptoms, those without a full-thickness rotator cuff tear, and patients at high surgical risk.

• Activity modification: avoiding overhead activity, adjusting work practices
• Non-steroidal anti-inflammatory drugs (NSAIDs)
• Cold therapy / ice
• Physiotherapy: rotator cuff strengthening, scapular stabilisation, posterior capsule stretching, posture correction — this is the most important element
• Subacromial corticosteroid injection: both diagnostic and therapeutic; repeated at most 2–3 times
• PRP (platelet-rich plasma) injection — in selected cases
• For calcific tendinitis: ultrasound-guided aspiration / needle fenestration
• Posture and ergonomic adjustment

Surgical Treatment

Surgery is planned for patients who have not responded to at least 3–6 months of non-operative treatment, whose symptoms significantly affect quality of life, or in whom associated pathology is identified (a prominent osteophyte, acromioclavicular arthritis, a partial cuff tear).

The basis of surgical treatment is arthroscopic subacromial decompression (ASD) and acromioplasty (the Neer procedure). Concomitant procedures (bursectomy, distal clavicle excision, removal of calcific deposits, rotator cuff repair) are added where required.

Advantages

The principal advantage is widening of the subacromial space and permanent relief of compression on the tendons. Chronic pain that has not responded to non-operative treatment improves, functional range of motion is regained, and the risk of progression to a rotator cuff tear is reduced.

• The arthroscopic technique is minimally invasive: small incisions (3–4 portals, each about 5 mm), less pain, lower infection risk and faster recovery
• Diagnosis under direct vision: arthroscopy allows partial rotator cuff tears and glenohumeral pathology not visible on MRI to be seen and treated in the same sitting
• Early motion: passive range-of-motion exercises usually begin the day after surgery, reducing joint stiffness
• Operative time: 30–60 minutes for arthroscopic subacromial decompression; 60–90 minutes with concomitant procedures
• Most patients are treated as day cases
• Less scarring, less pain and faster recovery than open acromioplasty

Disadvantages

• There are surgery-specific risks: wound infection, deep infection, nerve injury (particularly the axillary nerve) and rare anaesthetic complications
• A sling is recommended for 4–6 weeks; active use of the arm is limited during this period
• Frozen shoulder (adhesive capsulitis): may develop despite or after surgery; it is common in patients with diabetes
• Heterotopic ossification (bone formation in soft tissue; rare)
• Inadequate decompression: symptoms may persist if insufficient bone is removed
• Excessive decompression: removing too much of the acromion risks weakness at the deltoid attachment or anterosuperior escape

Why Surgery?

If subacromial impingement syndrome is left untreated, permanent degeneration of the rotator cuff tendons, partial tears and progression to full-thickness tears follow. Once a full-thickness tear develops the tendon retracts and muscle atrophy and fatty infiltration set in, making tendon reconstruction more difficult and the results poorer.

Early surgical decompression eliminates the impingement, improves the healing environment of the tendon and prevents progression of tendon damage in the long term. Timely surgery is preferred in patients who do not respond to non-operative treatment.

Who is a Candidate?

Surgery is offered to patients who have not responded to at least 3–6 months of non-operative treatment (physiotherapy, NSAIDs and at least one to two subacromial injections).

• People in overhead occupations (painters, carpenters) with night pain and functional limitation
• Overhead athletes (swimmers, tennis players, baseball players, volleyball players)
• Patients in whom MRI shows a structural abnormality (Type III acromion, prominent osteophyte, acromioclavicular osteophyte, partial rotator cuff tear, calcific tendinitis)
• Those with a temporary but marked response to subacromial corticosteroid injection whose symptoms recur
• Surgery is not preferred in younger patients before functional causes (scapular dyskinesis, posture) have been corrected

Expected Outcomes

With appropriate indications and compliant rehabilitation, the results of arthroscopic subacromial decompression are good. Between 85% and 90% of patients report a marked reduction in pain and functional improvement. Most return to daily activity within 6–12 weeks and to sport within 4–6 months. Recovery is longer where a rotator cuff repair is also performed (6–12 months).

Factors that adversely affect outcome include smoking, diabetes, advanced age, an associated large rotator cuff tear, previous failed surgery and coexisting frozen shoulder.

Risks Specific to Non-operative Treatment

• NSAID side effects: gastric bleeding, impaired kidney or liver function, cardiovascular risk
• Repeated subacromial corticosteroid injection: tendon weakening, local skin changes, raised blood glucose (in patients with diabetes), infection (very rare)
• Delayed diagnosis or treatment: progression of a partial rotator cuff tear to a full-thickness tear
• Development of frozen shoulder (adhesive capsulitis): restricted movement and prolonged pain
• Prolonged reduction in quality of life

Risks Specific to Surgery — Common (Under 5%)

• Superficial wound infection (usually treated with antibiotics)
• Postoperative pain, swelling and temporary bruising
• Temporary numbness or tenderness at the portal sites
• Transient symptoms related to anaesthesia such as nausea and dizziness
• Temporary numbness after an interscalene block; very rarely, diaphragmatic paralysis
• Temporary shoulder stiffness — resolves with regular physiotherapy

Risks Specific to Surgery — Less Common

• Deep infection: 0.5–2%; may require revision, irrigation and prolonged antibiotics; the most common organism is Cutibacterium acnes
• Nerve injury: the axillary nerve is the nerve most at risk in shoulder surgery; numbness over the outer upper arm or deltoid weakness may develop
• Temporary or permanent injury to elements of the brachial plexus (particularly from traction in the lateral position)
• Vascular injury (very rare)
• Fluid extravasation: leakage of high-pressure fluid into the subacromial space during arthroscopy, causing soft-tissue swelling and, rarely, airway compression
• Deep vein thrombosis (DVT) — very rare in the upper limb

Complications Specific to the Procedure

• Inadequate decompression: persistent symptoms because insufficient bone was removed; revision may be required
• Excessive decompression: weakness at the deltoid attachment from removing too much of the acromion
• Anterosuperior escape: superior migration of the humeral head due to insufficiency of the acromion and coracoacromial ligament (particularly where there is an associated cuff tear)
• Acromial stress fracture (very rare)
• Frozen shoulder (adhesive capsulitis): develops in 5–10% of patients after surgery, particularly in those with diabetes. Treatment: intensive physiotherapy, intra-articular injection and, in advanced cases, manipulation or arthroscopic capsular release
• Heterotopic ossification: bone formation in soft tissue, which may restrict movement; rare

Persistent or Recurrent Symptoms

• Incorrect diagnosis: surgery will fail if the true cause is scapular dyskinesis, cervical radiculopathy or another problem
• Failure to treat associated pathology (for example a missed rotator cuff tear)
• Excessive traumatic or occupational loading (inadequate activity modification)

Factors that Increase Risk

• Smoking (increases the risk of wound healing problems, frozen shoulder and infection)
• Poorly controlled diabetes (risk of frozen shoulder and infection)
• Immunosuppressive therapy and rheumatological disease
• Advanced age; previous shoulder surgery
• Coexisting cervical (neck) problems
• Active or recently treated infection (dental, urinary, skin)
• Non-compliance with the rehabilitation programme

Contact Your Doctor Immediately If You Have

• Sudden increase in pain or swelling
• Fever above 38 °C
• Discharge, malodour or separation of the wound
• Numbness, tingling or loss of strength in the arm, shoulder or hand
• Bruising, coldness or pallor of the hands or fingers
• Shortness of breath or chest pain
• Marked swelling of the neck or face (suspected fluid extravasation)

Long-term Outcomes

With appropriate indications and compliant rehabilitation, the results of arthroscopic subacromial decompression are good. Between 75% and 90% of patients report a marked reduction in pain and functional improvement, and patient satisfaction is high at 5–10 year follow-up.

Factors that adversely affect outcome include coexisting cervical pathology or scapular dyskinesis, an associated large rotator cuff tear, smoking, diabetes, advanced age and non-compliance with rehabilitation. Where a rotator cuff repair has also been performed, the long-term result depends largely on healing of the cuff. Postoperative physiotherapy, posture and occupational ergonomics are among the mainstays of success.