External Impingement of the Shoulder

External (subacromial) impingement is one of the most common causes of shoulder pain. The name implies a mechanism, the acromion compressing the cuff, and the label is shifting to the broader, mechanism-neutral term subacromial pain syndrome (SAPS).

Who. Symptoms may occur across adult age groups. Repetitive or sudden changes in overhead load may provoke pain, and tendon capacity, sleep, work, sport and psychosocial context influence the disability that follows.

Associated findings. Several structural and functional findings may sit alongside the pain:

• Rotator-cuff tendinopathy or tear
• Subacromial-subdeltoid bursal thickening or fluid
• AC-joint or os acromiale pathology
• Scapular movement variation or glenohumeral stiffness
• Acromial shape, which may coexist but is not a stand-alone diagnosis

External versus internal. SAPS concerns pain arising from the subacromial tissues during elevation. Posterior-superior internal impingement is a different mechanism, seen particularly in throwers in abduction–external rotation.

The coracoacromial arch. The supraspinatus tendon and the subacromial-subdeltoid bursa lie beneath the arch, which forms the roof of the space. The acromion gives anterior-superior coverage, the coracoid process lies anteriorly, and the coracoacromial ligament connects the two.

The impingement zone. It takes in the critical zone of supraspinatus, 1cm from its insertion, and the watershed area, which is relatively avascular.

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Extrinsic and intrinsic. Neer's extrinsic compression model is now balanced against intrinsic tendinopathy (age, vascularity, load), and most contemporary models are multifactorial. The causes of subacromial narrowing divide the same way.

Structural contributors. Acromial or AC-joint morphology, os acromiale and cuff disease may coexist with symptoms. Association does not prove that a spur is the pain generator.

Functional contributors. Cuff capacity, scapular movement, stiffness, instability, and work and sport load can alter symptoms and motion without a fixed outlet lesion.

Neer's stages. Neer described impingement in three stages:

• Stage I - oedema and haemorrhage
• Stage II - fibrosis and tendinopathy
• Stage III - rotator-cuff tear

The classification emphasises a progressive syndrome, though progression is not inevitable. It is a classic model, not a validated age-based natural history: tendinopathy, bursal pain and cuff tears overlap and do not inevitably progress through three stages. Do not use the stage as an age-based treatment rule. Treat the clinical syndrome and any defined pathology, and assess a tear separately.

Bigliani acromial morphology. Three undersurface shapes, which may be reported on outlet imaging:

• Type 1 - flat
• Type 2 - curved
• Type 3 - hooked

The classification is descriptive, with poor inter-observer reliability, and causal inference from it is limited. A Type 3 acromion is associated with cuff disease and cuff tears, but causation is unproven: the hook may be a traction enthesophyte, an effect rather than a primary cause. Shape alone should not be used to diagnose SAPS, and is not an indication for acromioplasty or any other bone resection.

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History. Anterolateral shoulder pain, classically at night, made worse by overhead activity. There is weakness if the cuff is involved.

Examination. Examine the local shoulder structures, and look for cervical and neurological mimics. The named tests:

• Neer test - forward flexion with the scapula stabilised
• Hawkins test - 90° of flexion, then internal rotation
• Painful arc - pain between 60° and 120° of abduction
• Jobe (empty can) test - supraspinatus strength; not impingement-specific
• Injection test - relief with subacromial local anaesthetic

Clinical assessment comes first. Screen for trauma, infection, tumour, inflammatory disease, stiffness, instability, AC-joint pain and cervical or neurological referral. No imaging sign substitutes for this.

Radiographs, when indicated. AP, axillary and scapular-Y/outlet views identify arthritis, calcific deposits, superior migration, os acromiale and other bone pathology.

Ultrasound or MRI, for problem solving. Use them when a cuff tear, bursal disorder or alternative pathology would change management. Dynamic ultrasound can assess tendon and bursal motion.

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MRI defines the extent of a tear and the quality of the muscle.

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First line. Non-operative treatment is first-line for SAPS, built from these components:

• Activity modification - avoid aggravating overhead activities; relative rest, not complete immobilisation
• Physiotherapy - rotator-cuff strengthening, scapular stabilisation, posterior capsule stretching and posture correction
• NSAIDs - short-term, for pain relief; they help reduce inflammation
• Subacromial corticosteroid injection - temporary relief that aids participation in physiotherapy; a maximum of 3 injections per year, with diminishing returns on repeated use

Why rehabilitation rather than bone removal. Rehabilitation targets load tolerance, cuff and scapular capacity and the individual movement limitation. Routine bone removal does not address these factors, and placebo-controlled trials show no important benefit for isolated SAPS.

What exercise achieves. No success rate for non-operative treatment is established, and what the evidence supports is narrower. Kuhn's synthesis of eleven randomised trials found that exercise produces statistically and clinically significant improvement in pain and function but no significant effect on range of motion or strength, and that a home programme performed as well as supervised therapy. Manual therapy augmented the effect of exercise. Set expectations around pain and function, and do not promise a percentage.

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When symptoms persist. Reassess the diagnosis. Treat a defined cuff tear, instability, AC-joint disorder, stiffness or neurological source on its own merits rather than labelling failure of exercise as proof of impingement.

The evidence on decompression. CSAW (UK, 2018; primary endpoint at 6 months, with concordant 1-year data) and FIMPACT (Finland, 24-month follow-up) both found no clinically important benefit of arthroscopic subacromial decompression (ASD) over placebo arthroscopy for impingement with an intact cuff. The 2019 Cochrane review (1062 patients) and the BMJ Rapid Recommendation graded this as high-certainty evidence and recommended against routine ASD. The trials undermine the assumption that acromial contact is the necessary pain generator; the remaining debate is patient selection, and whether a structural cuff tear changes the calculus.

Where surgery still has a place. Surgery is considered for:

• Failure of 3-6 months of conservative treatment
• Stage III disease, a rotator-cuff tear
• Significant functional impairment
• A young patient with mechanical symptoms

Read that list against the trials. Failure of exercise alone is a reason to reassess the diagnosis, not proof of impingement. Surgery may still help a genuine structural lesion, such as a large anterior spur or a symptomatic os acromiale, but not pain alone. Decompression as an adjunct during rotator-cuff repair remains accepted practice. Patients should be told of the trial evidence when they are consented.

Planning. Review the imaging first: confirm acromial morphology, assess for a cuff tear (MRI if needed), check the AC joint, evaluate for os acromiale and exclude other pathology. Plan the extent of the bursectomy and the acromioplasty targets, and make sure the consent includes the limited evidence.

Beyond the acromion. Assess the entire cuff and do not ignore its pathology: if a tear is present, cuff repair is combined with the decompression. Address the AC joint if it is arthritic, and check for an os acromiale. Do not operate without failed conservative care, and do not promise cure.

The coracoacromial ligament. The arch is the passive roof of the subacromial space. With an intact rotator cuff it is only a minor buttress, but in a cuff-deficient shoulder it becomes the last static restraint against anterosuperior escape of the humeral head.

Over-decompression. Aggressive acromioplasty with coracoacromial-ligament release, in a shoulder that has or later develops a large or massive cuff tear, can precipitate anterosuperior escape: the head subluxes up and forward through the deficient roof. It is a difficult, often effectively irreparable complication.

The corollary. Decompression should be conservative and cuff-status-aware. Release the coracoacromial ligament sparingly, or preserve it, when the cuff is deficient, and remove only enough acromial undersurface to relieve a genuine mechanical spur. Alongside CSAW and FIMPACT, it is a further argument against routine, aggressive bony decompression.

After isolated decompression. Recovery is relatively quick compared with cuff repair.

After combined cuff repair. The cuff repair dictates rehabilitation, not the decompression, and its protocol is more restrictive:

• Sling for 4-6 weeks
• No active elevation for 6 weeks
• Full recovery at 4-6 months

Reading the success rates. Improvement after surgery is largely placebo plus rehabilitation, which is how decompression can post the success rate below while giving no clinically important benefit over placebo surgery.

Global epidemiology. Shoulder pain is among the three most common musculoskeletal presentations worldwide; subacromial pain syndrome (the contemporary umbrella term for external impingement with an intact cuff) accounts for the majority of these consultations. Prevalence rises with age and overhead occupational or sporting load and is broadly similar across high- and limited-resource settings.