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Category	Details
Risk Factors	Young males (20-30 years) involved in contact sports or trauma (anterior); seizures, electrocution, or alcohol withdrawal (posterior); elderly patients with falls (inferior/anterior)
Etiology	Anterior: forceful abduction, external rotation, and extension of the arm. Posterior: forceful internal rotation and adduction (seizure, electric shock). Inferior (luxatio erecta): hyperabduction force
Patient Presentation	Acute shoulder pain, inability to move the affected arm, patient holding the arm in a characteristic position depending on the type of dislocation
Classic Physical Exam	Anterior: arm held in slight abduction and external rotation, loss of normal deltoid contour ("squared-off shoulder"), palpable humeral head anteriorly. Posterior: arm held in adduction and internal rotation, unable to externally rotate, posterior fullness. Inferior: arm locked in full abduction above the head
Key Diagnostic Results	Pre-reduction radiographs (AP and lateral/axillary or scapular Y view). Anterior: humeral head displaced anteriorly and inferiorly. Posterior: "lightbulb sign" on AP view. Post-reduction films to confirm congruent reduction and rule out fracture
Management	Procedural sedation followed by closed reduction. Anterior: external rotation technique, Cunningham, or traction-countertraction. Posterior: traction with external rotation. Post-reduction: sling immobilization, orthopedic follow-up, rehabilitation
Keywords	"Squared-off shoulder," "loss of deltoid contour," "arm held in external rotation," "lightbulb sign," "seizure with shoulder pain," "unable to externally rotate," "arm locked overhead," "axillary nerve injury," "decreased sensation over lateral deltoid"

1. Pathophysiology

The glenohumeral joint is the most mobile joint in the body, and this mobility comes at the cost of stability. The glenoid fossa of the scapula is shallow and covers only about one-third of the humeral head surface area. Stability depends on a combination of static restraints (the glenoid labrum, glenohumeral ligaments, and joint capsule) and dynamic restraints (the rotator cuff muscles and the long head of the biceps tendon). When an external force overwhelms these stabilizers, the humeral head is displaced out of the glenoid fossa, resulting in dislocation.

Anterior dislocation accounts for approximately 95% of all shoulder dislocations. The mechanism typically involves a combination of abduction, external rotation, and extension of the arm, such as when a football player's arm is forced backward during a tackle. In this position, the anterior capsule and the inferior glenohumeral ligament (IGHL) are stretched maximally and eventually fail. The humeral head is driven anteriorly and inferiorly, often tearing the anteroinferior labrum off the glenoid rim (Bankart lesion). This explains why the patient holds the arm in slight abduction and external rotation: the humeral head is now seated anterior to the glenoid, and any attempt to bring the arm to the side (adduction or internal rotation) increases pain because it forces the displaced head against the anterior structures. The loss of the normal rounded deltoid contour ("squared-off shoulder") occurs because the humeral head is no longer sitting beneath the acromion.

Posterior dislocation accounts for only 2-4% of dislocations but is the most commonly missed dislocation on exam and in clinical practice. The classic mechanism is a forceful involuntary internal rotation and adduction, which occurs during generalized tonic-clonic seizures or electrocution. During a seizure, the internal rotators of the shoulder (subscapularis, pectoralis major, latissimus dorsi) overpower the external rotators, driving the humeral head posteriorly. The patient holds the arm in adduction and internal rotation and is unable to externally rotate the arm, which is a critical physical exam finding. This is because the humeral head is locked behind the posterior glenoid rim, mechanically blocking external rotation.

Inferior dislocation (luxatio erecta) is the rarest type but has the most dramatic presentation. It results from a severe hyperabduction force that levers the humeral neck against the acromion, tearing the inferior capsule and driving the humeral head inferiorly. The patient presents with the arm locked in full abduction above the head, unable to bring it down. This type carries the highest rate of associated neurovascular injury, particularly to the axillary nerve and axillary artery.

The axillary nerve (C5-C6) wraps around the surgical neck of the humerus and is the most commonly injured nerve in shoulder dislocations, especially anterior dislocations. Injury results in decreased sensation over the "regimental badge" area (lateral deltoid) and weakness of deltoid contraction (impaired arm abduction). The axillary artery can also be injured, particularly in elderly patients with atherosclerotic vessels, and must be assessed by checking distal pulses.

2. Classification and Clinical Manifestation

Type	Frequency	Mechanism	Arm Position	Key Physical Exam Finding	Associated Injuries
Anterior (subcoracoid most common)	~95%	Abduction + external rotation + extension (e.g., tackle, fall on outstretched hand)	Slight abduction, external rotation	Squared-off shoulder, loss of deltoid contour, palpable humeral head anteriorly, positive apprehension test	Bankart lesion (labral tear), Hill-Sachs lesion (posterolateral humeral head compression fracture), axillary nerve injury, greater tuberosity fracture
Posterior	2-4%	Internal rotation + adduction; seizure, electrocution, lightning strike	Adduction, internal rotation	Inability to externally rotate, posterior shoulder fullness, flattened anterior shoulder, coracoid prominence	Reverse Bankart lesion, reverse Hill-Sachs lesion (McLaughlin lesion, anteromedial humeral head defect), lesser tuberosity fracture
Inferior (luxatio erecta)	<1%	Hyperabduction	Arm fully abducted, forearm resting on or behind head	Arm locked overhead, patient unable to adduct	Highest rate of neurovascular injury (axillary nerve and artery), rotator cuff tears, greater tuberosity fracture
Superior	Extremely rare	High-energy force directed superiorly	Variable	Variable	Acromion fracture, clavicle fracture, rotator cuff tear

Recurrence Classification

Patient Age at First Dislocation	Recurrence Risk	Clinical Significance
< 20 years	Up to 80-90%	Strongest indication for early surgical stabilization (Bankart repair)
20-40 years	50-70%	Moderate recurrence risk; decision based on activity level and number of episodes
> 40 years	10-15%	Lower recurrence but higher incidence of associated rotator cuff tears

3. Diagnostic Workup

Test	Role	Key Findings
Plain radiographs (AP + axillary lateral or scapular Y view)	Best initial test	Anterior: humeral head displaced anterior and inferior to glenoid. Posterior: "lightbulb sign" (internally rotated humeral head appears rounded on AP), "rim sign" (widened glenohumeral joint space > 6 mm), humeral head overlap on glenoid on AP view
Post-reduction radiographs	Mandatory after every reduction	Confirms congruent reduction, identifies associated fractures (Hill-Sachs, Bankart, tuberosity)
Neurovascular examination	Required before AND after reduction	Axillary nerve: sensation over lateral deltoid, deltoid contraction. Distal pulses for vascular integrity
CT scan	Best test for bony detail	Defines fracture pattern if plain films suggest a fracture-dislocation; quantifies glenoid bone loss (important for surgical planning)
MRI	Most accurate test for soft tissue injuries	Identifies labral tears (Bankart), rotator cuff tears, capsular injury; typically obtained after reduction in the subacute setting for surgical planning

The workup begins with a thorough neurovascular examination before any attempt at reduction. You must document axillary nerve function (light touch over the lateral deltoid and deltoid squeeze), musculocutaneous nerve function (lateral forearm sensation), and distal pulses. This documentation is tested frequently and is a required step before proceeding to imaging or reduction.

The best initial test is a plain radiograph series, which should include at minimum an AP view and a second view (true axillary lateral or scapular Y view). The AP view alone is insufficient because posterior dislocations are frequently missed on a single AP film. On a standard AP view, a posterior dislocation may appear near-normal or show only subtle findings: the humeral head appears rounded and symmetric (the "lightbulb sign") because it is fixed in internal rotation, and the joint space may appear widened (the "rim sign," with > 6 mm distance between the anterior glenoid rim and the humeral head). An axillary or scapular Y view will clearly show the posterior displacement of the humeral head relative to the glenoid.

For anterior dislocations, the AP view typically shows obvious displacement of the humeral head anteriorly and inferiorly. An associated Hill-Sachs lesion (a compression fracture of the posterosuperior humeral head caused by impaction against the anterior glenoid rim during dislocation) may be visible, particularly on an internally rotated AP view.

Post-reduction films are mandatory to confirm congruent reduction and to identify any fractures that may have been caused during the dislocation or the reduction itself. If a fracture-dislocation is identified on pre-reduction films, orthopedic consultation should be obtained before attempting closed reduction because displaced fractures through the tuberosities or surgical neck may require open reduction.

CT scan is ordered when plain films suggest a fracture-dislocation or when there is concern for significant glenoid bone loss (which changes surgical management). MRI is the most accurate test for evaluating soft tissue pathology (labral tears, rotator cuff tears, capsular damage) and is generally obtained in the subacute setting (after reduction) to guide decisions about surgical repair, particularly in young active patients with recurrent dislocations.

4. Management and Treatment

Phase	Intervention	Details
Pre-reduction	Neurovascular exam	Document axillary nerve, musculocutaneous nerve, distal pulses BEFORE any manipulation
Pre-reduction	Radiographs	AP + axillary lateral or scapular Y to confirm type and rule out fracture-dislocation
Reduction (Anterior)	Procedural sedation + closed reduction technique	External rotation method, Cunningham technique (seated, massage-based), Stimson (prone, dangling arm with weight), or traction-countertraction
Reduction (Posterior)	Procedural sedation + closed reduction	Longitudinal traction on the arm in line with deformity + gentle external rotation and lateral traction on the proximal humerus
Reduction (Inferior)	Procedural sedation + closed reduction	Traction superiorly in line with the arm overhead, then sweeping the arm into adduction
Post-reduction	Repeat neurovascular exam + radiographs	Mandatory to confirm reduction and document any new neurovascular deficit
Immobilization (Anterior)	Sling	Traditional: internal rotation sling for 1-3 weeks (shorter in older patients). Some evidence supports external rotation bracing in young patients to reduce recurrence
Immobilization (Posterior)	Sling or brace in neutral to slight external rotation	4-6 weeks; avoid internal rotation during healing
Rehabilitation	Physical therapy	Progressive range of motion followed by rotator cuff and periscapular strengthening; return to sport typically 3-6 months
Surgical repair	Arthroscopic Bankart repair or open stabilization	Indicated for recurrent dislocation (especially in young active patients), significant bone loss (bony Bankart or engaging Hill-Sachs), or irreducible dislocation

Step 1: Immediate Assessment. When a patient presents with a suspected shoulder dislocation, perform a focused neurovascular examination first. Check light touch sensation over the lateral deltoid (axillary nerve, C5-C6), lateral forearm (musculocutaneous nerve), and distal pulses (radial and ulnar arteries). Document these findings clearly because you will need to compare them after reduction.

Step 2: Imaging. Obtain pre-reduction radiographs (AP and axillary lateral or scapular Y view) to confirm the direction of dislocation and rule out associated fractures. If a fracture-dislocation is identified (e.g., displaced surgical neck fracture, large displaced tuberosity fragment), consult orthopedics before attempting reduction, as closed reduction may worsen the fracture.

Step 3: Pain Control and Sedation. For closed reduction in the emergency setting, procedural sedation (e.g., propofol, ketamine, or midazolam/fentanyl combination) is the standard of care. Intra-articular lidocaine injection (20 mL of 1% lidocaine injected into the glenohumeral joint) is an alternative when procedural sedation is not available or is contraindicated.

Step 4: Closed Reduction. For anterior dislocation, multiple techniques exist. The external rotation method is gentle and effective: with the patient supine and the elbow flexed to 90 degrees, the arm is slowly externally rotated while the elbow is kept at the patient's side. When spasm is overcome, the humeral head slips back into the glenoid. The Cunningham technique involves the patient sitting upright while the clinician massages the biceps, deltoid, and trapezius to relax spasm, without traction. Traction-countertraction uses steady in-line traction on the arm with a sheet wrapped around the thorax for countertraction.

For posterior dislocation, the arm is tractioned in line with its current position (forward-flexed and internally rotated), and then gently externally rotated while applying lateral pressure on the proximal humerus to disengage the head from the posterior glenoid rim.

For inferior dislocation, traction is applied superiorly in line with the elevated arm, and the arm is then swept into an adducted position as the humeral head reduces.

A palpable "clunk" and immediate improvement in range of motion confirms successful reduction.

Step 5: Post-reduction Assessment. Repeat the neurovascular examination and obtain post-reduction radiographs. Any new neurological deficit should be documented and referred for further evaluation.

Step 6: Immobilization. For anterior dislocations, the arm is placed in a sling. In patients older than 40, early mobilization within 1-2 weeks is recommended to prevent adhesive capsulitis (frozen shoulder), which is more of a risk in this age group than recurrent dislocation. In younger patients, immobilization for 2-3 weeks is standard, though prolonged immobilization has not been shown to reduce recurrence rates.

Step 7: Rehabilitation and Follow-up. Orthopedic follow-up within 1-2 weeks is appropriate. Physical therapy focuses on restoring range of motion first, then progressive rotator cuff and scapular stabilizer strengthening. Return to contact sports is generally delayed until full strength and range of motion are restored, typically 3-6 months.

Step 8: Surgical Consideration. Recurrent anterior instability, particularly in patients younger than 25 with documented Bankart lesions, is a strong indication for arthroscopic Bankart repair (reattachment of the torn labrum). Significant glenoid bone loss (>20-25% of the glenoid) may require a Latarjet procedure (coracoid bone block transfer) instead of soft tissue repair alone.

Contraindications and Special Considerations

A key contraindication to routine closed reduction is a fracture-dislocation with a displaced fracture of the surgical neck or a large tuberosity fragment, as reduction may further displace the fracture and risk neurovascular injury. In elderly patients, always evaluate for an associated rotator cuff tear, which occurs in more than 30% of dislocations in patients over 40 and may require surgical repair.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Rotator cuff tear	Acute shoulder pain after trauma, inability to move the arm, particularly in patients > 40 years old	Rotator cuff tear: patient may have passive range of motion but cannot actively abduct or externally rotate. No loss of deltoid contour, no bony deformity. Shoulder radiographs show the humeral head seated within the glenoid. MRI shows tendon discontinuity
Proximal humerus fracture (surgical neck)	Acute pain, swelling, inability to move the arm after a fall, particularly in elderly or osteoporotic patients	Fracture alone: humeral head remains in the glenoid on radiographs. Ecchymosis tracking down the arm and chest wall is common. No "squared-off" contour unless there is concomitant dislocation
Acromioclavicular (AC) joint separation	Pain and deformity at the shoulder after a fall or direct blow	AC separation: tenderness and deformity at the top of the shoulder (AC joint), not the glenohumeral joint. "Step-off" or prominence of the distal clavicle. Full passive glenohumeral range of motion is typically preserved
Adhesive capsulitis (frozen shoulder)	Limited shoulder range of motion, pain	Adhesive capsulitis: gradual onset (not acute trauma), restriction of both active and passive motion in all planes (especially external rotation), no deformity, no acute radiographic abnormality
Acute labral tear without dislocation (SLAP lesion)	Shoulder pain, mechanical symptoms (clicking, catching), history of forceful arm motion	SLAP tear: no acute deformity, no displacement on radiographs. Pain with overhead activities, positive O'Brien test or crank test. MRI arthrogram confirms the labral tear without dislocation
Posterior dislocation vs. anterior dislocation	Both present with shoulder pain and immobility after trauma	Posterior: arm held in internal rotation, unable to externally rotate; history of seizure or electrocution; "lightbulb sign" on AP film. Anterior: arm held in external rotation, loss of deltoid contour, humeral head palpable anteriorly

6. Traps and High-Yield Pearls

The single most tested "gotcha" with shoulder dislocation is the posterior dislocation that is missed on a single AP radiograph. Exam writers love to present a patient who had a seizure or was electrocuted and now has shoulder pain with the arm locked in internal rotation and inability to externally rotate. The AP radiograph may look "nearly normal" to the untrained eye, and the vignette will test whether you recognize the lightbulb sign or know to order a second view (axillary lateral or scapular Y). If the question stem gives you only an AP view that appears normal in a post-seizure patient with a locked shoulder, the answer is almost always posterior dislocation, and the next step is to obtain an additional radiographic view.

A second common trap involves neurovascular assessment. On exam, if you are asked "what is the next best step" after a patient presents with an obvious shoulder dislocation, and one of the answer choices is "check neurovascular status" while another is "obtain radiographs" or "attempt reduction," the correct answer is neurovascular examination first. You must document nerve and vascular function before any imaging or manipulation. The axillary nerve is tested by checking sensation over the lateral deltoid, and this is the most frequently asked nerve injury associated with anterior dislocation.

A third pearl: age determines the complication you should worry about. In patients under 30, the primary concern after a first-time anterior dislocation is recurrent instability (due to Bankart lesion). In patients over 40, the primary concern shifts to an associated rotator cuff tear. This distinction dictates the follow-up workup: young patients may need MRI to evaluate for labral pathology and surgical stabilization planning, while older patients need MRI or ultrasound to evaluate the rotator cuff.

Finally, remember that a fracture-dislocation is not the same as a simple dislocation. If the vignette describes a displaced fracture in addition to the dislocation, closed reduction in the emergency department without orthopedic involvement is not the correct answer. The next best step in that scenario is orthopedic consultation for possible operative management.

The core competency being tested across shoulder dislocation questions is your ability to recognize the type of dislocation from the clinical presentation and arm position, sequence the workup correctly (neurovascular exam before imaging, imaging before reduction, repeat examination after reduction), and identify the complications that vary by patient age and dislocation type.

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