Shoulder Impingement Syndrome

Shoulder impingement syndrome (subacromial pain syndrome) is pain from compression of the supraspinatus tendon, subacromial bursa and long head of biceps beneath the coracoacromial arch during arm elevation. It accounts for 44 to 65% of shoulder pain presentations, and the lifetime prevalence of shoulder pain is 7 to 26%.

Mechanisms:

• Primary (structural): hooked acromion, acromioclavicular spurs, os acromiale, thickened coracoacromial ligament
• Secondary (functional): scapular dyskinesis, rotator cuff weakness, posterior capsule tightness, glenohumeral instability

Internal impingement occurs in throwers, where the articular surface of the cuff pinches against the posterosuperior glenoid. The pathology progresses from bursal inflammation and edema, to tendinopathy and fibrosis, to partial and full thickness tears. Most patients improve over 6 to 12 months with exercise. Complications are cuff tear, adhesive capsulitis and persistent pain.

• Lateral shoulder pain with overhead activity, reaching and lifting, pain at night on the affected side
• Painful arc of abduction between 60 and 120 degrees
• Pain with passive forced forward flexion, and with internal rotation at 90 degrees of forward flexion
• Pain and weakness on resisted elevation with the thumbs down (supraspinatus test), and tenderness at the greater tuberosity
• Red flags: weakness with inability to hold the arm abducted (cuff tear), a history of trauma, progressive night pain, fever, neck pain with radicular signs, and age under 35 with instability

• Clinical diagnosis from the painful arc and provocative tests, with exclusion of other causes.
• Subacromial lidocaine test: injection of lidocaine 1% 5 to 10 mL into the subacromial space abolishes pain on repeat testing.
• Radiographs (anteroposterior, scapular Y and axillary): acromial morphology, acromiohumeral distance under 7 mm (suggests cuff tear), calcification and acromioclavicular arthropathy.
• Ultrasound: bursal thickening, tendon thickening or tear, and dynamic impingement on abduction.
• MRI if a tear is suspected or symptoms continue beyond 3 to 6 months.

Differentials: rotator cuff tear, calcific tendinitis, adhesive capsulitis, acromioclavicular arthritis, biceps tendinopathy, glenohumeral instability, cervical radiculopathy, suprascapular neuropathy.

• Activity modification: avoid overhead tasks and sleeping on the shoulder for 2 to 4 weeks, with posture correction.
• Analgesia: paracetamol 1 g orally every 6 hours (maximum 4 g/day), naproxen 500 mg orally twice daily with food for up to 2 weeks (avoid in peptic ulcer, renal impairment, heart failure), or diclofenac 1% gel 4 g to the shoulder four times daily for up to 2 weeks.
• Physiotherapy for 12 weeks: scapular stabilization, rotator cuff strengthening (external rotation with elastic band, 3 sets of 10 to 15 repetitions daily), posterior capsule stretching (cross body adduction), and thoracic mobility.
• Subacromial corticosteroid injection: triamcinolone acetonide 40 mg with lidocaine 1% 5 mL by the posterior or lateral approach, under ultrasound guidance. Maximum 2 to 3 injections at least 3 months apart, because repeated injections weaken the tendon. Glucose rises transiently in diabetics.
• Shockwave therapy for calcific deposits.
• Surgery after 6 months of failed treatment: arthroscopic subacromial decompression with bursectomy and acromioplasty, with debridement of a partial cuff tear or distal clavicle excision for acromioclavicular arthritis. The benefit over structured exercise is small in most patients, so surgery is selective. Cefazolin 2 g IV before incision. Postoperative sling for 1 to 2 weeks, physiotherapy from day 1, and return to overhead work at 3 to 4 months.
• Complications: stiffness, infection, deltoid detachment and persistent pain.

Follow up at 6 and 12 weeks. Counsel on a graded return to overhead activity and on the high chance of recovery with exercise.