Posterior Instability | Impression Fracture | McLaughlin Lesion
Defect Size Classification
Critical Must-Knows
- Often associated with posterior shoulder dislocation (seizures, electrocution)
- Lightbulb sign on AP X-ray due to internal rotation
- Critical defect size is greater than 25% (some sources say 20%)
- Modified McLaughlin transfers subscapularis/lesser tuberosity into defect
- Chronic locked posterior dislocations are frequently missed
Clinical Pearls
- "Demonstrate the 'Posterior Drawer' and 'Jerk Test' in viva
- "Axillary lateral view is mandatory to confirm diagnosis
- "Distinguish between 'engaging' and 'non-engaging' lesions
- "Review the 3 'E's of posterior dislocation: Epilepsy, Ethanol, Electricity
Critical Exam Points
The 'Missed' Diagnosis
Posterior dislocations are the most commonly missed major joint dislocation (greater than 50% missed initially). Always check axillary view.
Critical Size
20-25% articular surface. Defects larger than this are unstable and engage with the posterior glenoid rim.
Mechanism
Adduction + Internal Rotation. Seizures and electrocution cause massive muscle contraction (Lat/Pec/Subscap) driving head posterior.
Surgical Timing
less than 3 weeks = Acute. Over 3 weeks = Chronic. Affects reducibility and viability of the head.
Quick Decision Guide
| Scenario | Defect Size | Treatment | Key Pearl |
|---|---|---|---|
| Acute (under 3wks), Reducible | Small (under 20%) | Closed Reduction + Brace | Immobilize in ER (gunslinger) |
| Acute/Chronic, Unstable | Medium (20-45%) | Mod. McLaughlin / Allograft | Transfer L.T. into defect |
| Chronic, Head collapse | Large (over 45%) | Hemi / Total Shoulder | Head is unsalvageable |
3 EsCauses of Posterior Dislocation
| E | Epilepsy Seizures cause violent internal rotation |
| E | Electricity Electrocution shock causing contraction |
| E | Ethanol Alcohol withdrawal seizures or trauma |
| E | Epilepsy Seizures cause violent internal rotation |
| E | Electricity Electrocution shock causing contraction |
| E | Ethanol Alcohol withdrawal seizures or trauma |
Hook:The 3 Es force the head out the Back!
SMATreatment Algorithm by Size
| S | Small (under 20%) Stable - Non-op / Remplissage |
| M | Medium (20-45%) Mod. McLaughlin or Allograft |
| A | All (over 45%) Arthroplasty (Hemi/Total) |
| S | Small (under 20%) Stable - Non-op / Remplissage |
| M | Medium (20-45%) Mod. McLaughlin or Allograft |
| A | All (over 45%) Arthroplasty (Hemi/Total) |
Hook:Small, Medium, All-gone (Arthroplasty)
LIRThe 3 Signs on X-ray
| L | Lightbulb Sign Round head on AP due to internal rotation |
| I | Impression Fracture Trough line sign on AP |
| R | Rim Sign Widened joint space greater than 6mm |
| L | Lightbulb Sign Round head on AP due to internal rotation |
| I | Impression Fracture Trough line sign on AP |
| R | Rim Sign Widened joint space greater than 6mm |
Hook:Look for LIR (Lighthouse in Rain) - Lightbulb, Impression, Rim!
Overview and Epidemiology
Terminology Clarification
Reverse Hill-Sachs Lesion: An impaction fracture of the anteromedial humeral head. Reverse Bankart Lesion: Injury to the posterior glenoid labrum. Both occur in posterior shoulder instability/dislocation.
Pathology
Impaction of the anterior humeral head against the posterior glenoid rim during dislocation.
Chronicity
Often present as "Locked Posterior Dislocation". The head is impacted and cannot be reduced closed.
Pathophysiology and Mechanisms
The Engaging Lesion
A Reverse Hill-Sachs lesion is considered "engaging" if the defect falls off the posterior glenoid rim when the arm is internally rotated and flexed (functional position). This causes recurrent instability.
Mechanism of Injury
- Axial loading of the adducted and internally rotated arm.
- Violent muscle contraction: The internal rotators (Latissimus Dorsi, Pectoralis Major, Subscapularis) are stronger than external rotators.
- During a seizure, these muscles overpower the external rotators, forcing the humeral head posteriorly.
The Defect
- Location: Anteromedial humeral head.
- Comparison: A reflected image of a standard Hill-Sachs (which is posterolateral).
- Engagement: As the arm internally rotates, the defect engages the posterior glenoid.
Hill-Sachs vs Reverse Hill-Sachs
| Feature | Hill-Sachs | Reverse Hill-Sachs |
|---|---|---|
| Instability | Anterior | Posterior |
| Location on Head | Posterolateral | Anteromedial |
| Glenoid Lesion | Bankart (Anterior) | Reverse Bankart (Posterior) |
| Position of Engagement | Abduction + Ext Rotation | Adduction + Int Rotation |
Classification Systems
Classification by Articular Surface Involvement
Most clinically useful for decision making.
| Size | % of Head | Stability | Treatment |
|---|---|---|---|
| Small | under 20% | Stable | Closed Reduction / Neglect |
| Medium | 20-45% | Unstable | Reconstruction (McLaughlin) |
| Large | over 45-50% | Grossly Unstable | Arthroplasty |
Clinical Assessment
History
- Mechanism: Seizure, shock, fall on flexed/adducted arm.
- Symptoms: Pain, "locked" shoulder, inability to externally rotate.
- Missed History: Patient treats it as "frozen shoulder" for months.
Observation
- Posture: Arm held in adduction and internal rotation (".gunslinger" position).
- Contour: Flattening of anterior shoulder, prominence of coracoid (posterior fullness).
- ROM: Block to External Rotation (pathognomonic for locked posterior dislocation).
Specific Tests for Posterior Instability
- Posterior Drawer Test: Supine. Axial load + posterior force.
- Jerk Test: Seated. Flexion to 90 + Adduction + Axial Load. "Clunk" as head subluxes posteriorly.
- Kim Test: Variation of Jerk test for inferior-posterior, labral pathology.
- Load and Shift: Assess translation grading (Grade I-III).
ALWAYS compare to the contralateral side.
Investigations
Imaging Protocol
- AP: "Lightbulb sign" (head internally rotated, looks symmetrical). Loss of half-moon overlap.
- Axillary Lateral: GOLD STANDARD. Shows head posterior to glenoid.
- Scapular Y: Head posterior to intersection of Y.
- Mandatory for operative planning.
- Quantify defect size (% of articular surface).
- Assess glenoid bone loss (Reverse Bony Bankart).
- Assess posterior labrum (Reverse Bankart).
- Assess cuff integrity (subscapularis).
- Often done if diagnosis unclear or for chronic pain.
The Lightbulb Sign
On AP X-ray, the humeral head is fixed in internal rotation. The greater tuberosity rotates anteriorly, making the head look perfectly round (like a lightbulb) rather than its normal walking-stick appearance.
Management Algorithm
Acute Management
Goal: Reduce and maintain stability.
- Closed Reduction: Under conscious sedation/GA.
- Traction + gentle anterior pressure. Avoid force (fracture risk).
- Assess Stability:
- Stable: Immobilize in External Rotation (gunslinger brace) for 4-6 weeks.
- Unstable: If redislocates or engages in functional ROM to Surgery.
Early reduction minimizes cartilage damage.
Surgical Technique
The Concept
Transferring the Subscapularis tendon (original McLaughlin) or Lesser Tuberosity (modified) into the defect.
- Effect: Fills the defects and acts as a check-rein preventing internal rotation.
Steps
- Approach: Deltopectoral approach.
- Identify Defect: Locate the anteromedial impression fracture.
- Osteotomy: Lesser tuberosity osteotomy (with subscapularis attached).
- Reduction: Reduce the humeral head into the glenoid.
- Preparation: Prepare the defect bed (curette/burr) to encourage healing.
- Fixation: Secure the lesser tuberosity bone block into the defect using screws or suture anchors.
- Result: The bony plug fills the hole and the subscapularis tightens, preventing internal rotation.
Careful protection of the axillary nerve is required.
Complications
| Complication | Risk Factors | Prevention/Management |
|---|---|---|
| Recurrent Instability | Undersized graft, Missed engaging lesion | Ensure defect is filled/bypassed |
| Osteoarthritis | Damage to cartilage, hardware penetration | Countersink screws, accurate reduction |
| Subscapularis Failure | Poor fixation of osteotomy | Protect range of motion (ER) post-op |
| Hardware Prominence | Screw heads prominent | Use headless screws |
Postoperative Care
Rehab Protocol (Modified McLaughlin)
- Sling: Gun-slinger brace (neutral to external rotation).
- Restrictions: NO Internal Rotation (protects subscap transfer).
- ROM: Passive ER allowed.
- Wean sling.
- Active assist ROM.
- Begin gentle Internal Rotation stretching.
- Rotator cuff strengthening.
- Scapular stabilizers.
- Return to sport/work assessment.
Outcomes and Prognosis
Modified McLaughlin
Good to Excellent results in 75-85% of patients. Low recurrence rate if sized correctly.
Allograft
Technical procedure but shows good survival at 10 years. Risk of graft resorption/necrosis.
Neglected Cases
Associated with poor functional outcomes and rapid progression to OA if left unreduced.
Arthroscopy
Arthroscopic "Reverse Remplissage" involves filling defect with subscapularis via suture anchors. Good for small/medium defects.
Evidence Base
Locked Posterior Dislocation — the Landmark Series
- Diagnosis had been missed by the initial physician in the majority of cases; mean injury-to-diagnosis interval was 1 year
- Causes: motor-vehicle accident, seizure, alcohol-related injury, or electroshock therapy
- An axillary radiograph confirmed the diagnosis in all 41 shoulders and showed the defect size
- Lesser-tuberosity transfer succeeded in all 4 shoulders treated; subscapularis transfer succeeded in 4 of 9
Defect-Size Treatment Algorithm
- Small impression defects are usually stable after closed reduction
- Intermediate defects (roughly 25-50%) require reconstruction (McLaughlin / bone graft)
- Defects over ~50% with head collapse require prosthetic replacement
- CT quantification of the defect is central to the decision
Epidemiology & Risk of Recurrence
- Prevalence of posterior dislocation 1.1 per 100,000 population per year
- 67% caused by trauma, most of the remainder by seizures
- 17.7% of shoulders developed recurrent instability within the first year
- Age under 40, seizure mechanism, and large reverse Hill-Sachs (over 1.5 cm3) predicted recurrence
Head-Preserving Surgery for Chronic Dislocation
- For chronic posterior dislocation, the two dominant techniques were McLaughlin / modified McLaughlin and bone-graft reconstruction
- Both consistently produced good functional outcomes with few complications
- Chronic anterior dislocation techniques were far more variable with high resubluxation and early arthrosis
- Conservative neglect of chronic dislocation produced poor functional results
Osteochondral Allograft in the Shoulder
- Reverse Hill-Sachs lesions were the single most common indication for shoulder OCA (33 of 83 shoulders)
- 68 of 83 shoulders had favourable outcomes (graft incorporation, pain, function, satisfaction)
- Mean follow-up 45.7 months
- Unfavourable results clustered in concomitant surgery and pain-pump chondrolysis cases
Modified McLaughlin — Functional Outcomes
- Lesser-tuberosity transfer plus artificial bone fixed with lag screws and sutures for 30-40% defects
- Constant-Murley score improved from 46.0 to 85.8 at mean 19.8 months (p=0.001)
- No residual instability and full return to daily activity in all 5 patients
- Reinforces the modified McLaughlin as a reliable joint-preserving option for intermediate defects
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Scenario 1: The Missed Diagnosis
"A 45-year-old male presents with a 'frozen shoulder' for 3 months after a seizure. He cannot externally rotate past neutral. Show me the X-rays you would order and describe the findings."
Scenario 2: Surgical Decision Making
"You confirm a locked posterior dislocation. A CT scan shows a Reverse Hill-Sachs lesion involving 35% of the articular surface. The head is viable. How do you manage this?"
Scenario 3: The 'Engaging' Lesion
"Explain the concept of an 'Engaging' Reverse Hill-Sachs lesion and how it differs from an engaging anterior Hill-Sachs."
MCQ Practice Points
Nerve Injury
Q: What is the most common nerve injury associated with posterior shoulder dislocation? A: While the Axillary nerve is most common in anterior dislocations, checking it is still mandatory. However, posterior dislocations have a lower rate of nerve injury overall compared to anterior.
Pathognomonic Fracture
Q: Which fracture is pathognomonic for a posterior shoulder dislocation? A: A Lesser Tuberosity fracture (avulsion by the subscapularis tendon) is pathognomonic and indicates a posterior dislocation mechanism.
Radiographic Signs
Q: What is the 'Rim Sign' on an AP shoulder X-ray? A: The Rim Sign is a widening of the glenohumeral joint space greater than 6mm on the AP view, indicating posterior displacement of the head.
Cortical Lines
Q: What is the 'Trough Line Sign'? A: It appears as two parallel lines of dense cortical bone on the AP X-ray, representing the impaction fracture (Reverse Hill-Sachs) of the anteromedial humeral head.
Mechanism
Q: Which muscle group is responsible for posterior dislocation during a seizure? A: The massive internal rotators (Latissimus Dorsi, Pectoralis Major, Subscapularis) overpower the weaker external rotators, driving the head posteriorly.
Differential Diagnosis
A patient with a painful, stiff, internally-rotated shoulder after a seizure or fall is easily mislabelled. The differentials below are the classic exam traps.
Distinguishing Reverse Hill-Sachs / Locked Posterior Dislocation
| Diagnosis | Discriminating Feature | Key Investigation | Pitfall |
|---|---|---|---|
| Locked posterior dislocation (Reverse Hill-Sachs) | Fixed internal rotation, block to external rotation, lightbulb sign | Axillary lateral / CT | Missed as 'frozen shoulder' |
| Adhesive capsulitis (frozen shoulder) | Global loss of passive AND active ROM, normal joint congruity | Normal radiographs | Both block ER, but capsulitis has a congruent joint |
| Proximal humerus fracture | Discrete fracture lines, crepitus, ecchymosis | AP + axillary; CT for comminution | Fracture-dislocation may coexist |
| Anterior dislocation (Hill-Sachs) | Arm in abduction/ER, posterolateral head defect | AP + axillary | Opposite engaging position (Abd+ER) |
| Posterior labral / capsular instability (no bone loss) | Recurrent positional subluxation, no impaction defect | MR arthrogram | Soft-tissue repair alone fails if a bony defect is present |
Controversies & Areas of Uncertainty
The 'critical' defect threshold
Quoted cut-offs (20%, 25%, 40%, 50%) come from small heterogeneous series, not prospective comparisons. Most authors reconstruct intermediate defects (~25-50%) and reserve arthroplasty for over ~45-50% with head collapse, but the exact threshold for any individual head remains judgement-based.
Arthroscopic vs open reconstruction
Arthroscopic transfer / capsular fill techniques are increasingly described for small-to-intermediate engaging lesions, but comparative evidence against open modified McLaughlin is limited to case series. Open transfer remains the most validated approach for larger defects.
Disimpaction vs filling the defect
Some advocate elevating the impacted articular fragment with subchondral bone grafting to restore native cartilage, versus filling/bypassing the defect (McLaughlin, allograft). No high-level data favour one strategy.
Engagement / glenoid-track concept
The glenoid-track and "gamma angle" concepts are extrapolated from anterior instability; their predictive value for posterior engagement is biomechanically plausible but not yet validated in robust clinical cohorts.
Guidelines, Registries & Global Practice
Global Epidemiology
Posterior dislocation accounts for roughly 2-5% of all shoulder dislocations. Population-based data (Edinburgh cohort) give a prevalence of about 1.1 per 100,000 per year, with peaks in men aged 20-49 and in the elderly; roughly two-thirds are traumatic and most of the remainder seizure-related.
Consensus Across Societies
There is no single dedicated guideline; AAOS (US), BOA/BESS (UK), AO Foundation and EFORT teaching converge on the same principles — mandatory axillary/CT imaging, treat by defect size, and prefer joint preservation in young patients.
Side-by-Side Society Emphasis
| Body | Emphasis | Practical Point |
|---|---|---|
| AAOS / AO Foundation | Anatomy of impaction & fixation principles | Restore head sphericity; countersunk/headless fixation |
| BOA / BESS (UK) | Avoiding missed diagnosis; specialist referral | Three radiographic views mandatory after seizure/electrocution |
| EFORT / European consensus | Joint preservation in the young | Reconstruct rather than replace where the head is viable |
Registry Notes
Arthroplasty registries (NJR, AJRR, AOANJRR, Swedish/Norwegian) do not isolate reverse Hill-Sachs as an indication, but they inform implant choice when arthroplasty is required: reverse total shoulder is increasingly preferred over hemiarthroplasty in older patients with cuff compromise or unreconstructable bone loss.
High- vs Limited-Resource Practice
Where CT and allograft banking are available, defect quantification and osteochondral allograft expand head-preserving options. In limited-resource settings, autograft modified McLaughlin (no implant dependency) and rotational osteotomy are favoured, and late presentation of neglected locked dislocations is more common.
REVERSE HILL-SACHS RECAP
Clinical summary
Anatomy
- •Impaction of Anteromedial Head
- •Engages in Adduction + IR
- •Posterior dislocation association
- •Subscapularis tendon involvement
Classification (Size)
- •Small (under 20%) = Stable
- •Medium (20-45%) = Unstable/Recon
- •Large (over 45%) = Arthroplasty
- •Gamma angle greater than 90 deg = Unstable
Clinical Signs
- •Locked in Internal Rotation
- •Prominent Coracoid
- •Flattened anterior shoulder
- •Posterior fullness (humeral head)
Imaging
- •Lightbulb Sign (AP)
- •Trough Line Sign (Impaction)
- •Axillary Lateral = Diagnostic
- •Rim Sign: Widened joint space greater than 6mm
Treatment
- •Acute less than 3wks + Small = Reduction + Brace
- •Chronic/Med = Mod. McLaughlin
- •Large/Collapse = Hemi/Total
- •Remplissage for small engaging lesions
Complications
- •Missed diagnosis (common)
- •Recurrent instability
- •AVN (late collapse)
- •Arthritis