Shoulder Impingement Syndrome: Causes, Controversies, and Modern Management

What Is Shoulder Impingement?

Shoulder impingement syndrome (SIS) — also called subacromial pain syndrome — describes pain arising from the structures within the subacromial space: the bursa between the rotator cuff and the acromion, and the tendons of the rotator cuff, particularly supraspinatus. During shoulder elevation, the humeral head rises slightly and the subacromial space narrows. If the supraspinatus tendon, biceps long head tendon, or subacromial bursa are irritated, thickened, or if the subacromial space is structurally narrowed by acromial morphology or acromioclavicular joint osteophytes, this narrowing produces compressive pain between approximately 60 and 120 degrees of arm elevation — the classic painful arc. SIS is the most common cause of shoulder pain in clinical practice, accounting for 44–65% of all shoulder complaints. However, the diagnosis has undergone significant reconceptualisation in recent years, moving away from a purely structural impingement model toward a broader understanding of pain neuroscience and rotator cuff function.

What Actually Causes the Pain

The traditional explanation attributed subacromial pain entirely to mechanical compression of supraspinatus between the humeral head and acromion. Contemporary research has complicated this picture significantly. High-quality studies demonstrate that structural measures — acromial shape, subacromial space width — correlate poorly with pain and disability. Subacromial decompression surgery (acromioplasty) produces outcomes no better than sham surgery or structured physiotherapy in randomised controlled trials, casting serious doubt on the purely structural impingement model. The current understanding is that subacromial pain is multifactorial: rotator cuff weakness and fatigue impairs the inferior translation of the humeral head during elevation; scapular dyskinesis reduces acromial elevation and subacromial space; posterior capsular tightness alters glenohumeral kinematics; and central sensitisation in chronic cases amplifies pain independent of structural loading. This understanding fundamentally supports rehabilitation over surgery as the primary treatment.

Evidence-Based Rehabilitation

Structured rehabilitation for subacromial pain syndrome addresses the modifiable contributors systematically. Rotator cuff strengthening — emphasising the posterior cuff (infraspinatus and teres minor) and subscapularis for dynamic joint centring — reduces superior humeral head translation and subacromial compression during elevation. Scapular stabilisation — lower trapezius, serratus anterior, and middle trapezius — restores scapular upward rotation and acromial elevation during arm raising. Posterior capsule flexibility — the sleeper stretch and cross-body stretch — addresses the glenohumeral internal rotation deficit (GIRD) that alters cuff mechanics in overhead athletes. Progressive loading through overhead range, initially in pain-free arcs and progressively into the painful range as capacity increases, is introduced over 8–12 weeks. Manual therapy — glenohumeral posterior capsule stretching, thoracic mobilisation, and cervical assessment — improves biomechanics and reduces pain acutely. Corticosteroid injection into the subacromial bursa provides short-term pain relief that facilitates exercise participation, but does not alter the underlying mechanical deficit.