Subacromial Impingement Syndrome

Introduction

Shoulder pain is a common complaint, with an estimated point prevalence of 6.9-26%.¹ Shoulder impingement refers to when the soft tissue structures around the shoulder joint become trapped, causing damage and pain.

Subacromial impingement syndrome (SIS) is by far the most common variety of shoulder impingement and accounts for approximately 36% of shoulder pain diagnoses.²

SIS refers to a spectrum of pathology associated with the compression of and damage to the rotator cuff tendons as they pass through the subacromial space.³

Aetiology

Anatomy

The rotator cuff stabilises the humeral head against the glenoid to form the glenohumeral joint and controls many of the movements possible at the shoulder. It is comprised of four muscles: the subscapularis, teres minor, infraspinatus, and supraspinatus.³

The rotator cuff tendons pass through the subacromial space (along with the long head of the biceps) prior to their attachment to the proximal humerus. Their passage through this space is lubricated by the subacromial bursa, which lies between the tendons and the overlying acromion.⁵

The subacromial space is bounded by:⁶

• The coracoacromial ligaments and acromioclavicular joint
• The anterior edge and under-surface of the acromion
• The humeral head

The supraspinatus tendon is commonly implicated in the pathology of SIS as it runs directly beneath the overhanging acromion, and so is especially predisposed to damage.

For more information, see the Geeky Medics guide to the intrinsic muscles of the shoulder. 

Pathophysiology

Two key mechanisms have been proposed as to the underlying mechanism behind SIS development: extrinsic compression and intrinsic degeneration. The relationship between the two concepts is still a matter of debate.

Extrinsic compression^5,8,9

This relates to direct compression of the rotator cuff tendons against the surrounding structures, primarily due to established anatomic narrowing of the subacromial space, or by the superior translation of the humeral head on abduction of the shoulder.

The subacromial space can narrow due to the physical shape of the acromion, thickening of the coracoacromial ligament, or osteophytic change on the underside of the acromioclavicular joint.

Additionally, aberrant biomechanics from weakened rotator cuff musculature can result in superior translation of the humeral head and a dynamic narrowing of the subacromial space, impinging on the rotator cuff tendons.

Intrinsic degeneration^10,11

Intrinsic factors are those specific to the rotator cuff tendons themselves, causing degeneration of the tendons (or impairing recovery from insult).

The poor vascularity of the supraspinatus tendon can predispose it to degenerative tears through impaired repair following microtrauma. This can lead to inflammation and pain independently, as well as to a narrowing of the subacromial space from superior humeral head translation, as outlined above.

SIS progression¹²

The combination of extrinsic compression and intrinsic degeneration contributes to the spectrum of clinical findings associated with SIS.

As SIS represents a spectrum of pathology associated with damage to the rotator cuff tendons, it can progress with time. The progression of this spectrum is shown below:

• Stage 1: haemorrhage and oedema surrounding the cuff tendons.
• Stage 2: rotator cuff tendinopathy: fibrosis and inflammation of the tendons.
• Stage 3: rotator cuff tears (varying degrees of severity). May have corresponding arthritic changes, or a coexistent long head of biceps tear.

Risk factors

Impingement typically starts in young patients (<25 years) but is initially reversible. Rotator cuff pathologies are not typically present until >40 years of age with progression of the condition.¹²

Patients with high (often occupational) levels of above-shoulder activity, especially throwing athletes, are at higher risk of developing SIS.

Different risk factors are involved in the development of extrinsic compression and intrinsic degeneration.

Table 1. An overview of risk factors for SIS. 

Clinical features

History

The primary symptom is shoulder pain.

Typical characteristics of SIS pain include:

• Gradual in onset, but progressive (unless post-traumatic)
• Localised over deltoid region
• Worse during overhead activity, such as combing hair, or during the throwing motion in athletes
• Worse at night
• Improves with rest
• May radiate down the upper arm

True shoulder weakness is typically not present in SIS unless the patient has progressed to having a significant rotator cuff tear. However, significant pain may cause symptoms similar to weakness.

Stiffness is also typically not present unless the patient has progressed to rotator cuff tendinopathy and fibrosis. However, movements may be limited by pain.

Clinical examination

All patients with shoulder pain require a thorough examination of the shoulder. 

Typical clinical findings of SIS on shoulder examination include:

• Normal strength: although pain can simulate weakness on examination
• No restriction on passive movement (active abduction may be pain-limited)
• Pain on palpation of subacromial space may suggest subacromial bursitis

Special tests

There are many special tests to assess shoulder pain. No single test can exclude or confirm the diagnosis.

However, there are four tests that are good indicators of the presence of subacromial impingement (Table 2).

Table 2. An overview of special tests for subacromial impingement. 

Differential diagnoses

Table 3. An overview of differential diagnoses to consider and differentiating characteristics from SIS.

*These can be long-term sequelae from SIS, but can also present independently, and so should be differentiated from SIS where possible.

Investigations

The diagnosis of SIS is usually made from the clinical examination. The diagnostic sensitivity of clinical examination in the diagnosis of SIS is up to 90%.³

However, diagnostic imaging may be considered to exclude alternative diagnoses.^9,18

Imaging

Relevant imaging investigations include:

• Ultrasound: can be used diagnostically at the bedside to detect superficial tendon damage and established bursitis. Ultrasound can also assess any rotator cuff tears.
• X-ray (anteroposterior and lateral views): useful to assess for arthritis of the glenohumeral or acromioclavicular joints.
• MRI: expensive, but excellent at imaging the soft tissue structures of the shoulder.

Management

SIS is typically managed conservatively in the first instance. Debate exists surrounding the role of surgery in SIS management, but it is typically reserved for patients with symptoms refractory to conservative measures.^3,11

Conservative management

Conservative management of SIS includes:

• Analgesia: NSAIDs
• Physiotherapy
• Corticosteroid injections into the subacromial space
• Activity modification: avoiding overhead activities and excess load-bearing where possible

Surgical management

Surgical management of SIS may include:

• Arthroscopic subacromial decompression
• Bursectomy: often employed alongside decompression in cases with subacromial bursitis

Co-existent rotator cuff tears (sometimes seen in late-stage SIS progression) often require separate surgical management, employing constructs such as suture anchors to unite the torn muscle ends.

Complications

The complications of SIS can be avoided with prompt diagnosis and management. However, complications may include:

• Progression to rotator cuff tears
• Rotator cuff arthropathy: a pattern of glenohumeral arthritis secondary to rotator cuff degeneration
• Adhesive capsulitis

Key points

• SIS refers to a spectrum of pathology associated with the compression of and damage to the rotator cuff tendons as they pass through the subacromial space.
• Extrinsic compression of rotator cuff tendons is typically due to glenohumeral instability, repetitive above-shoulder compression, or osteophytic change of the ACJ joint, or acromion shape.
• Intrinsic degeneration of the rotator cuff tendons is typically due to ageing, smoking, or direct trauma.
• Typical features in the history of SIS relate to gradual onset pain that is worse at night and improves with rest. Weakness and stiffness are less common unless SIS has already progressed to rotator cuff tears.
• On examination of the shoulder, specific tests (including the ‘painful arc’ test, Neer’s test with diagnostic injection, the Hawkins test, and Jobe’s test) are often useful.
• Shoulder ultrasound is useful for visualising tendon injury and bursitis.
• MRI is reserved for cases refractory to conservative treatment, or where the diagnosis is uncertain, or if a rotator cuff deficit is suspected.
• Conservative measures (rest, activity modification, physiotherapy, analgesia and corticosteroid injections) are employed in the first instance.
• Arthroscopic subacromial decompression is the surgical technique most used.
• If untreated, SIS can progress to rotator cuff tears, rotator cuff arthropathy, or adhesive capsulitis.

Reviewer

Katie Hughes

ST1 Trauma & Orthopaedics

Editor

Dr Chris Jefferies

References

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