Supraspinatus — Anatomy & Clinical Tests

The supraspinatus muscle is the most clinically significant muscle of the rotator cuff — it is the most commonly injured, the most commonly operated upon, and the most extensively studied. Understanding its precise anatomy, neurovascular supply, the anatomy of the subacromial space, and the biomechanics of its function is essential for the orthopaedic management of rotator cuff pathology, shoulder impingement, and shoulder arthroplasty.

• Origin and insertion: the supraspinatus muscle originates from the supraspinous fossa of the scapula (the area above the spine of the scapula); it passes beneath the acromion (and the coracoacromial arch) and inserts onto the superior facet of the greater tuberosity of the humerus; the insertion is the most superior of the four rotator cuff insertions on the greater tuberosity; from superior to inferior, the greater tuberosity facets are: supraspinatus (superior/most superior facet), infraspinatus (middle facet), teres minor (inferior facet); the subscapularis inserts onto the lesser tuberosity (anteriorly); the combined rotator cuff footprint provides a broad attachment of the cuff tendons to the humeral head — this broad `cable and crescent` anatomy is critical for understanding partial tears
• The cable and crescent (Burkhart`s model): the supraspinatus and infraspinatus do not have a simple uniform sheet of insertion; histologically, there is a thick anterior `cable` of collagen fibres (the rotator cable — a thickening that runs perpendicular to the cuff fibres from the bicipital groove to the inferior border of the infraspinatus) and a thinner, more vulnerable `crescent` of tissue posterolateral to the cable; most partial and full-thickness tears begin in the crescent zone (the `critical zone`); because the cable spans the tear, large crescent tears can be paradoxically functional — the intact cable maintains force transmission even when the crescent is torn
• Neurovascular supply: the supraspinatus is innervated by the suprascapular nerve (C5, C6 — branches from the upper trunk of the brachial plexus); the nerve passes through the suprascapular notch (under the superior transverse scapular ligament — a common entrapment site in overhead athletes and from space-occupying lesions such as ganglion cysts in the spinoglenoid notch); the blood supply is from the suprascapular artery (branch of the thyrocervical trunk — a branch of the subclavian artery); the `critical zone` (the area 1 cm proximal to the insertion of the supraspinatus tendon) is a relatively avascular watershed zone where the intratendinous vessels from the musculotendinous junction meet the periosteal vessels from the greater tuberosity — this zone is most vulnerable to ischaemia under tensile loading and is where most supraspinatus tears begin

• The subacromial space: the supraspinatus tendon passes between the superior surface of the humeral head (below) and the coracoacromial arch (above) to reach its insertion on the greater tuberosity; the coracoacromial arch is formed by: (1) the acromion (posteriorly and superiorly); (2) the coracoacromial ligament (connecting the coracoid process to the acromion); (3) the coracoid process (anteriorly); the subacromial bursa (a large synovial bursa lying between the supraspinatus and the coracoacromial arch) lubricates the gliding of the cuff beneath the arch; the subacromial space normally measures approximately 9–10 mm on standard AP shoulder radiograph; a space <6–7 mm = pathological supraspinatus compression = evidence of massive cuff tear or superior humeral head migration
• Acromial morphology (Bigliani classification): the shape of the undersurface of the acromion directly affects supraspinatus compression; Type I (flat — 17%): least impingement risk; Type II (curved — 43%): moderate risk; Type III (hooked — 40%): highest risk of outlet impingement; the anterior hook contacts the supraspinatus tendon in elevation and internal rotation; Type III acromion is most commonly associated with supraspinatus tears; Type IV (convex) — sometimes added; the acromial morphology is a PREDISPOSING factor but NOT the sole cause of cuff tearing — intrinsic tendon degeneration is equally important in the pathogenesis of rotator cuff tears

• Primary function of supraspinatus: the supraspinatus initiates and assists abduction of the shoulder; it is responsible for the first 0–30° of glenohumeral abduction (along with the deltoid); however, this is an oversimplification — EMG studies show the supraspinatus is active throughout the full range of abduction and is not solely responsible for any specific arc; the supraspinatus generates approximately 50% of the total rotator cuff compression force on the glenohumeral joint; the critical function of ALL four rotator cuff muscles is to compress the humeral head into the glenoid fossa (concavity compression) — this centralises the humeral head and prevents it from migrating superiorly when the deltoid contracts
• Force couple: the rotator cuff and deltoid work as two force couples to produce efficient shoulder elevation; in the coronal plane, the deltoid provides a superior-directed force (which would translate the humeral head superiorly into the coracoacromial arch if unopposed) and the inferior rotator cuff (subscapularis anteriorly + infraspinatus + teres minor posteriorly) provides a corresponding inferior-directed force — together they produce a net resultant force directed into the glenoid (compression) with rotation rather than translation; in the axial plane, the subscapularis (anterior) and infraspinatus/teres minor (posterior) provide an anterior-posterior balanced force couple that centralises the humeral head; a massive supraspinatus tear alone does not necessarily cause superior humeral head migration if the inferior cuff force couple is intact
• Painful arc syndrome: pain during shoulder abduction specifically between 60–120° of abduction (where the greater tuberosity passes beneath the acromion — the maximum impingement zone); pain is absent from 0–60° (the greater tuberosity has not yet reached the acromion) and above 120° (the greater tuberosity has passed beyond the acromion and the impingement is relieved); this 60–120° arc of pain is the clinical hallmark of subacromial impingement/supraspinatus pathology; compare with AC joint pain — pain in the high arc (>150°) where the acromion contacts the distal clavicle

• Classification by depth: (1) Partial thickness tears — do not extend through the full thickness of the tendon; classified by location (articular surface — PASTA tears, bursal surface, or intratendinous); graded by depth (Grade I: <3 mm; Grade II: 3–6 mm; Grade III: >6 mm or >50% of tendon thickness); articular surface partial tears are more common than bursal surface tears (3:1 ratio); (2) Full thickness tears — extend through the entire tendon from articular to bursal surface; classified by size: small (<1 cm), medium (1–3 cm), large (3–5 cm), massive (>5 cm or involving two or more tendons)
• MRI: gold standard investigation for rotator cuff tears; sagittal oblique T2 or proton density fat-suppressed sequences demonstrate the supraspinatus tendon; full thickness tear = complete tendon discontinuity + fluid signal through the tear; partial thickness tear = focal increased signal within the tendon without full-thickness discontinuity; fatty infiltration of the supraspinatus muscle (Goutallier classification — Grade 0 to Grade 4 — proportion of fat within the muscle belly on parasagittal MRI) predicts surgical repair outcome — Grade 3 or 4 fatty infiltration = irreparable tear, poor muscle function after repair
• Ultrasound: as accurate as MRI for full-thickness supraspinatus tears in experienced hands; more accessible and cheaper; dynamic examination (real-time assessment during shoulder movement); however: operator-dependent; poor for labral and articular pathology

• Supraspinatus: origin — supraspinous fossa; inserts — superior facet of greater tuberosity; nerve — suprascapular nerve (C5, C6) through suprascapular notch; artery — suprascapular artery (thyrocervical trunk); critical zone = 1 cm proximal to insertion = watershed avascular zone = most tears begin here
• Greater tuberosity facets (superior to inferior): supraspinatus (superior), infraspinatus (middle), teres minor (inferior); subscapularis → lesser tuberosity
• Painful arc: 60–120° of abduction = subacromial/supraspinatus impingement; >150° = AC joint arthritis; full arc pain = GH joint OA or glenohumeral pathology
• Empty can test: arm 90° abduction, 30° forward flexion, internal rotation (thumb down); downward force resisted; weakness = supraspinatus tear; pain only = impingement; weakness more significant than pain; sensitivity ~75%, specificity ~68%
• Drop arm test: low sensitivity (27%) but very high specificity (88–98%) for large/massive tear; positive = arm drops suddenly at 90°; most cuff tears compensated by deltoid — drop arm = tear too large to compensate
• Bigliani acromial types: Type I (flat) — least impingement risk; Type II (curved) — moderate; Type III (hooked) — highest risk, most associated with supraspinatus tears; subacromial space <6–7 mm on X-ray = superior humeral head migration = significant cuff tear
• Goutallier fatty infiltration: Grade 0 (no fat); Grade 1 (fatty streaks); Grade 2 (fat < muscle); Grade 3 (fat = muscle); Grade 4 (fat > muscle); Grade 3–4 = irreparable tear + poor surgical outcome; assess on parasagittal MRI through the scapular Y plane
• Suprascapular nerve: C5, C6; passes through suprascapular notch under superior transverse scapular ligament (STSL); compressed in overhead athletes (traction injury — volleyball, tennis); spinoglenoid notch compression from ganglion cysts → isolated infraspinatus atrophy (nerve to infraspinatus branches at spinoglenoid notch); EMG confirms denervation; surgical decompression or ganglion aspiration