What is the board answer for subacromial impingement biomechanics?

The supraspinatus tendon is impinged between the acromion superiorly and the greater tuberosity of the humerus inferolaterally during the painful arc of 60 to 120 degrees of abduction.

Which test is positive in supraspinatus impingement?

The empty-can test (Jobe test) isolates the supraspinatus. Pain and weakness with the arm at 90 degrees abduction, 30 degrees forward flexion, and full internal rotation indicates supraspinatus tendon pathology.

What does the drop arm test indicate?

A positive drop arm test, where the patient cannot sustain arm position at 90 degrees after passive placement, indicates a full-thickness supraspinatus tear rather than impingement.

MSK deep dive · shoulder impingement

Rotator cuff disease

The mnemonic is SItS. Four muscles stabilize the humeral head in the glenoid. One of them passes through a bony corridor that narrows during a specific arc of motion. That corridor is the board answer.

From the Attending

The question about subacromial impingement is always asking about anatomy. You need one sentence: the supraspinatus tendon is compressed between the acromion above and the greater tuberosity of the humerus below during 60 to 120 degrees of abduction. Know that sentence and you answer every painful arc question in the block.

S

Supraspinatus Abducts the first 0-15 degrees before deltoid takes over. Most common rotator cuff injury. Empty-can (Jobe) test.

I

Infraspinatus External rotation. Common pitching injury. External rotation lag sign. Suprascapular nerve.

t

teres minor (lowercase t) Adducts and externally rotates. Axillary nerve. Part of the external rotation test set.

S

Subscapularis Internal rotation. Lift-off test and belly-press test. Upper and lower subscapular nerves.

Supraspinatus

Suprascapular nerve (C5-C6)

Initiates abduction from 0 to 15 degrees. Passes through the subacromial space above the humeral head and below the acromion and coracoacromial ligament. Tendon impingement here is the most common cause of shoulder pain in adults.

Empty-can test (Jobe) Hawkins-Kennedy test

Infraspinatus

Suprascapular nerve (C5-C6)

Externally rotates the humerus. Pitching injuries and posterior impingement. The same suprascapular nerve supplies both supraspinatus and infraspinatus, so a nerve lesion at the suprascapular notch weakens both abduction and external rotation.

External rotation lag sign

teres minor

Axillary nerve (C5-C6)

Adducts and externally rotates the humerus. The lowercase t in SItS is intentional: the muscle is smaller. Supplied by the axillary nerve alongside the deltoid. A quadrilateral space syndrome can compress the axillary nerve and weaken teres minor and deltoid together.

External rotation testing

Subscapularis

Upper and lower subscapular nerves (C5-C7)

Internal rotation and adduction. The only anterior rotator cuff muscle. Sits on the anterior surface of the scapula. Tears occur with forced external rotation. The lift-off test isolates internal rotation strength; a positive result is inability to hold the hand off the lumbar spine.

Lift-off test Belly-press test

Special tests · localize before naming the diagnosis

The test tells you which tendon

Each special test isolates one muscle by placing the arm in a position where only that muscle can produce the tested motion. Tap each card and flip it to reveal which muscle it targets and what a positive result means on boards.

A patient has shoulder pain that starts when abducting the arm, peaks around 90 degrees, then resolves as the arm approaches full elevation. Which specific arc range defines the classic painful arc of supraspinatus impingement?

A 62-year-old man has both a positive empty-can test and a positive drop arm test. Which finding upgrades the diagnosis from impingement to a complete tear?

Test	Muscle	Nerve	Positive means
Empty-can (Jobe)	Supraspinatus	Suprascapular (C5-C6)	Pain or weakness during resisted abduction in the scapular plane with internal rotation
Hawkins-Kennedy	Subacromial space	N/A (impingement test)	Pain with passive IR at 90 degrees forward flexion; drives GT under coracoacromial arch
Drop arm	Supraspinatus	Suprascapular (C5-C6)	Arm falls uncontrollably from 90 degrees = full-thickness tear (not just impingement)
External rotation lag	Infraspinatus	Suprascapular (C5-C6)	Arm falls back to internal rotation when released at max ER = infraspinatus tear
Lift-off	Subscapularis	Upper and lower subscapular	Cannot hold hand off lumbar spine = subscapularis tear
Belly-press	Subscapularis	Upper and lower subscapular	Elbow drops behind torso when pressing abdomen = subscapularis tear (alt to lift-off)

Painful arc 60-120 degrees plus positive empty-can equals supraspinatus tendon impingement between the acromion and the greater tuberosity of the humerus. That phrase, in full, is the board answer.

Mechanism theater · make the compression visible

The corridor closes at 90 degrees

The supraspinatus tendon runs through the subacromial space, bounded above by the acromion and coracoacromial ligament and below by the greater tuberosity of the humerus. As the arm abducts, the greater tuberosity arcs upward and medially, narrowing that corridor. Tap the states to watch the tendon get trapped, then freed.

At 90 degrees abduction the greater tuberosity rises toward the acromion and the supraspinatus tendon is trapped between them. That is why the painful arc is 60-120 degrees and not the full range. At 150 degrees and above the greater tuberosity rotates posteriorly and the corridor reopens.

From the Attending

If the arc is 60-120 degrees and the empty-can is positive, name the structure and name the corridor. The answer is not just "impingement." The answer is: supraspinatus tendon impingement between the acromion and the greater tuberosity of the humerus. The boards want the anatomy. Give it to them.

Anatomy strip · connect the visual to the mechanism

See the structures the stem is describing

The subacromial space, rotator cuff muscles, and clinical exam positions all translate into the same anatomical relationship. Tap any image to open the lightbox.

Rotator cuff muscles from Gray anatomy showing supraspinatus, infraspinatus, teres minor, and subscapularis — SItS: four tendons blend into a cuff around the humeral head. Supraspinatus runs most superior, directly under the acromion.

Shoulder joint anatomy showing glenohumeral joint, acromion, coracoid process, and subacromial space — The subacromial space: acromion above, greater tuberosity below. The gap is roughly 7-14 mm in a healthy shoulder.

MRI of shoulder in coronal plane showing supraspinatus tendon and subacromial space — Coronal MRI: the supraspinatus tendon appears as a low-signal band running between the acromion and the greater tuberosity. Edema or tear disrupts signal here.

Glenohumeral joint anatomy showing glenoid, labrum, capsule, and ligamentous structures — The glenoid and labrum are a different structure from the subacromial space. Labral pathology produces a different clinical pattern with instability and clicking, not a focal painful arc.

Mimic lab · flip before you commit

Painful shoulder is not always supraspinatus

Board stems plant deliberate traps by using similar presentations. Each card below presents the trap scenario on the front and the distinguishing anatomy on the back. Flip before naming the diagnosis.

Discriminator · split the look-alikes

Comparisons that boards test

Same Tendon, Different Severity

Subacromial Impingement vs Full-Thickness Tear

Impingement: Painful arc 60-120 degrees. Positive empty-can and Hawkins-Kennedy. Negative drop arm. The tendon is intact but compressed. Treatment: NSAIDs, PT, subacromial injection.

Full-thickness tear: Positive drop arm (arm falls when released at 90 degrees). Cannot sustain abduction. May use trapezius shrug to compensate. Treatment: surgical repair if acute/young, conservative if chronic/elderly.

The split: Drop arm positive = tear. Drop arm negative = impingement. That one test changes the entire management.

Different Structures Entirely

Labral Tear vs Rotator Cuff Pathology

Labral tear: Instability, clicking, catching. Positive apprehension and relocation tests. Deep, variable pain across arcs. Often in young athletes with dislocation history.

Rotator cuff: Focal painful arc (60-120 degrees for supraspinatus). Positive empty-can, Hawkins-Kennedy. No instability or catching. Muscle-specific weakness pattern.

The split: Instability + clicking = labrum. Focal arc + specific muscle weakness = cuff. The apprehension test and the empty-can test do not overlap.

Different Arc, Different Tenderness

AC Joint Pathology vs Subacromial Impingement

AC joint: Pain at the very top of the arc (above 120 degrees). Point tenderness directly over the distal clavicle. Positive cross-arm adduction test. Step deformity in separation.

Impingement: Pain in the mid-arc (60-120 degrees). Tenderness in the subacromial space, not the AC joint. Positive Hawkins-Kennedy and empty-can.

The split: Pain above 120 degrees + distal clavicle tenderness = AC joint. Pain at 60-120 degrees + subacromial tenderness = impingement. The arc range tells you the structure.

Memory Hooks

Tap each card to reveal the association.

SItS Muscles

Supraspinatus (abduction 0-15, suprascapular n.), Infraspinatus (external rotation, suprascapular n.), teres minor (ER + adduction, axillary n.), Subscapularis (internal rotation, upper + lower subscapular n.).

Painful Arc

60-120 degrees. Supraspinatus tendon impinged between the acromion (above) and the greater tuberosity (below). Below 60: corridor open. Above 120: greater tuberosity rotates posteriorly, corridor reopens.

Drop Arm = Full Tear

Drop arm positive = full-thickness supraspinatus tear (tendon discontinuity, no eccentric brake). Drop arm negative = impingement (tendon intact, just compressed). This one test upgrades the diagnosis.

Subscapularis Isolation

Lift-off test: hand dorsum against lumbar spine, push away. Belly-press: elbow dropout = subscapularis tear. Only anterior cuff muscle. Inserts on lesser tuberosity. Upper and lower subscapular nerves.

Suprascapular Nerve

Supplies BOTH supraspinatus and infraspinatus. Lesion at suprascapular notch = weakness in abduction (supraspinatus) AND external rotation (infraspinatus). Both supplied, both affected.

Impingement Board Sentence

Supraspinatus tendon impingement between the acromion and the greater tuberosity of the humerus during 60-120 degrees of abduction. That sentence, in full, is the board answer.

Board walkthrough · one case at a time

Commit, then reveal the chain

Right-click or long-press to cross out an answer choice. Double-click or double-tap to highlight. Each case shuffles answer choices and each restart remixes order without repeating until the set is exhausted.

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