Shoulder Anatomy: Glenohumeral Stability, Impingement, and Exam

The shoulder is the most mobile joint in the body and pays for it in stability. The glenohumeral joint articulates a large humeral head with a small, shallow glenoid fossa — about 25% of humeral head surface contacts the glenoid at any time. Stability comes from three layers: STATIC stabilizers (labrum, glenohumeral ligaments, joint capsule, negative intra-articular pressure), DYNAMIC stabilizers (rotator cuff, deltoid, scapular stabilizers acting in coordination), and SCAPULOTHORACIC contributions (the scapula must rotate up about 60° during full overhead motion through serratus anterior and trapezius action). When any layer fails, the others can compensate partially, which is why a torn supraspinatus may not produce dramatic weakness if deltoid and remaining cuff compensate. Common pathologies — impingement, rotator cuff tear, labral tear, anterior instability, adhesive capsulitis, AC joint pathology — each load specific structures and produce specific exam signs.

The rotator cuff is supraspinatus (abducts and steers the humeral head into the glenoid; suprascapular nerve), infraspinatus (external rotation; suprascapular nerve), teres minor (external rotation; axillary nerve), and subscapularis (internal rotation; upper and lower subscapular nerves). Together they DEPRESS and COMPRESS the humeral head against the glenoid during deltoid abduction — without cuff coupling, deltoid alone would migrate the humeral head superiorly and impinge under the acromion. Supraspinatus is the most commonly torn cuff tendon because it passes through the supraspinatus outlet under the coracoacromial arch — a tight space prone to repetitive impingement. Tears are described as partial-thickness (articular or bursal side) or full-thickness (with anteroposterior dimension on MRI). Massive rotator cuff tears (involving multiple tendons) can cause cuff tear arthropathy, where humeral head migrates superiorly and produces secondary glenohumeral arthritis.

The glenoid labrum is a fibrocartilaginous ring around the glenoid that deepens the socket by about 50% and serves as an attachment for the glenohumeral ligaments and the long head of biceps tendon (which inserts at the superior labrum). Two pathologic patterns dominate. ANTERIOR labral tear (Bankart lesion) typically occurs from anterior shoulder dislocation, where the humeral head shears the anteroinferior labrum off the glenoid rim. The bony equivalent — a Bankart fracture — involves the anterior glenoid rim. SUPERIOR labral tear from anterior to posterior (SLAP lesion) involves the biceps anchor and is common in overhead throwing athletes from peel-back mechanism. Symptoms include deep-seated pain, clicking, and catching, often without instability. MRI arthrography is the imaging gold standard; the O’Brien test is the most useful screening physical exam maneuver.

NEER\u2019S TEST: passively flex the shoulder forward with the arm internally rotated; pain over the lateral acromion suggests impingement. HAWKINS-KENNEDY: flex the shoulder to 90° and the elbow to 90°, then internally rotate; pain suggests subacromial impingement, with higher specificity than Neer. JOBE\u2019S (empty can) TEST: arm abducted 90° in the scapular plane with thumb pointing down (internal rotation); resist downward pressure; weakness or pain localizes to supraspinatus. O\u2019BRIEN TEST (active compression): arm at 90° flexion, 10° adducted, internally rotated (thumb down); resist downward force, then repeat with thumb up; pain that worsens with thumb-down position suggests SLAP or AC pathology. APPREHENSION TEST: supine, abduct and externally rotate the shoulder; apprehension (not just pain) suggests anterior instability. RELOCATION TEST: posteriorly directed pressure on the humeral head during apprehension test relieves apprehension — confirms anterior instability. SULCUS SIGN: traction on the arm produces a visible gap under the acromion; positive suggests multidirectional instability.

During full overhead abduction the scapula rotates upward about 60° via the serratus anterior, upper trapezius, and lower trapezius force couple. The scapulohumeral rhythm is approximately 2:1 — about 120° of glenohumeral motion and 60° of scapulothoracic motion for 180° total. Disruption from long thoracic nerve palsy (winged scapula from serratus anterior weakness), spinal accessory nerve injury (drooping shoulder from trapezius weakness), or postural overuse patterns produces scapular dyskinesis — visible asymmetric scapular motion during arm elevation. Scapular dyskinesis is both a cause and a consequence of cuff pathology because abnormal scapular position narrows the subacromial outlet and predisposes to impingement. Treatment usually includes scapular stabilizer strengthening alongside cuff rehabilitation.

The AC (acromioclavicular) joint sits at the top of the shoulder and is loaded in cross-body adduction. Cross-body adduction with pain at the AC joint suggests AC arthrosis. ADHESIVE CAPSULITIS (frozen shoulder) is an idiopathic restriction of glenohumeral motion, often associated with diabetes, thyroid disease, or post-immobilization; both active AND passive motion are restricted — distinguishing it from rotator cuff tear where passive motion is preserved. Three phases: freezing (painful), frozen (stiff), and thawing (improving). The LONG HEAD OF BICEPS TENDON runs in the bicipital groove and can rupture (visible Popeye deformity from distally retracted biceps belly), inflame (bicipital tendinitis), or subluxate medially in subscapularis tears. Speed\u2019s test (resisted forward flexion with elbow extended) localizes biceps tendon pain.

Upload a shoulder MRI or describe the exam findings and AnatomyIQ overlays the rotator cuff tendons, labrum, biceps anchor, and capsuloligamentous structures, and matches your exam pattern to the most likely diagnosis. The reverse exam-to-anatomy mapper takes a pattern (positive Neer + positive Jobe + Painful arc 60–120°) and returns the differential ranked by clinical likelihood. This content is for educational purposes only.