KNEE DISLOCATION
Orthopedic Emergency | Multiligamentous Injury | Vascular Catastrophe
ANATOMIC CLASSIFICATION
Critical Must-Knows
- VASCULAR EMERGENCY - Rule out popliteal artery injury in ALL cases with ABI and CTA
- Immediate reduction reduces vascular compromise - perform in ED under sedation
- Serial neurovascular exams - Document before and after reduction, every 2 hours
- Multiligamentous repair - Staged approach: ACL/PCL first, then collaterals at 3-6 weeks
- 20% missed initially - High suspicion if spontaneous reduction before arrival
Examiner's Pearls
- "Popliteal artery injury occurs in 30-40% - Normal pulses do NOT exclude intimal tear
- "ABI less than 0.9 = Mandatory CTA - Sensitivity 95% for arterial injury
- "Peroneal nerve injury (25%) - Check foot dorsiflexion and eversion before/after reduction
- "Schenck Classification (KD I-V) grades injury severity based on ligaments torn
Clinical Imaging
Imaging Gallery
Critical Knee Dislocation Exam Points
Vascular Assessment Protocol
MANDATORY workup. Normal pulses do NOT exclude arterial injury. Intimal tears can present with normal distal pulses initially then thrombose at 6-24 hours. ABI less than 0.9 requires immediate CTA. Vascular surgery consult if any abnormality.
Reduction Technique
Reduce immediately. Do NOT delay for imaging if neurovascular compromise present. Longitudinal traction with counter-traction. Reverse mechanism of injury. Reassess pulses post-reduction. Splint in 15-20 degrees flexion to prevent re-dislocation.
Schenck Classification
KD I-V system. KD I (one cruciate), KD II (both cruciates), KD III (M or L) (cruciate + collateral), KD IV (M or L) (both cruciates + collateral), KD V (fracture-dislocation). Higher grades = worse outcomes.
Surgical Timing
Staged repair approach. Acute: Reduce and splint, repair vascular/nerve injuries. Early (less than 3 weeks): ACL/PCL reconstruction. Delayed (3-6 weeks): Collateral ligament repair. Prevents arthrofibrosis with early total repair.
Quick Decision Guide
| Scenario | Vascular Status | Management | Key Pearl |
|---|---|---|---|
| Reduced dislocation, normal ABI (greater than 0.9) | Normal pulses, ABI 1.0 | Serial neurovascular exams, CTA if change | Intimal tears can thrombose at 6-24h |
| Reduced dislocation, ABI 0.7-0.9 | Diminished pulses | URGENT CTA + vascular surgery consult | 20% progress to thrombosis |
| Irreducible or pulseless limb | Absent pulses, cold foot | IMMEDIATE reduction + vascular surgery OR | 6-hour ischemia window - Amputation risk 86% |
PANICStructures at Risk in Knee Dislocation
Memory Hook:Knee dislocation causes PANIC - Check Popliteal artery, ACL/PCL, Nerve, Intima, Collaterals!
CLIMBSchenck Classification (KD I-V)
Memory Hook:CLIMB the knee dislocation severity ladder from KD I to KD V!
REDUCEAcute Management Priorities
Memory Hook:REDUCE is the goal - systematic approach prevents complications!
Overview and Epidemiology
Why This Topic Matters
Knee dislocation is an orthopedic emergency with potential for limb-threatening vascular injury. Despite being rare (0.02% of knee injuries), the consequences of missed diagnosis are catastrophic: amputation rates of 86% if ischemia exceeds 6 hours. The key challenge is that 20% of dislocations spontaneously reduce before arrival, making diagnosis difficult unless high clinical suspicion is maintained.
Mechanism of Injury
- High-energy trauma (60%): Motor vehicle collision, fall from height
- Low-energy in obese (40%): Simple fall, hyperextension
- Sports injuries: Dashboard injury, contact sports (football, rugby)
- Ultra-low velocity: Morbidly obese patients (BMI greater than 40)
Associated Injuries
- Popliteal artery injury: 30-40% (intimal tear most common)
- Peroneal nerve palsy: 25-30% (lateral dislocations highest risk)
- Compartment syndrome: 10-15% (especially after vascular repair)
- Meniscal tears: 50% (often peripheral detachment)
Anatomy
Ligamentous Anatomy
- ACL: Anteromedial and posterolateral bundles
- PCL: Anterolateral and posteromedial bundles
- MCL: Superficial (tibial attachment) and deep (meniscal)
- LCL: Fibular attachment, part of posterolateral corner
Posterolateral Corner (PLC)
- LCL: Primary lateral stabilizer
- Popliteus tendon: Dynamic posterolateral stabilizer
- Popliteofibular ligament: Resists external rotation
- Lateral capsule: Secondary restraint
Pathophysiology
Popliteal Artery Vulnerability
The popliteal artery is tethered proximally at the adductor hiatus and distally at the soleus arch, making it vulnerable to injury during knee dislocation. Intimal tears can occur WITHOUT complete disruption, presenting initially with normal pulses but thrombosing 6-24 hours later. This is why normal pulses do NOT exclude arterial injury.
Anatomic Constraints and Injury Patterns
| Structure | Anatomic Feature | Injury Mechanism | Clinical Significance |
|---|---|---|---|
| Popliteal artery | Tethered at adductor hiatus and soleus arch | Stretching during anterior dislocation | Intimal tear common, delayed thrombosis |
| Common peroneal nerve | Wraps around fibular neck | Traction injury in lateral/posterolateral dislocation | 25-30% injury rate, poor recovery |
| ACL and PCL | Intracapsular, minimal blood supply | Torn in 80% of dislocations | Both require reconstruction for stability |
Injury Mechanism and Cascade
Pathological sequence in knee dislocation:
- High-energy force - MVA, sports collision, fall from height
- Ligamentous failure - ACL/PCL rupture, capsular disruption
- Joint subluxation/dislocation - Tibiofemoral joint displaced
- Vascular tethering - Popliteal artery stretched over fixed points
- Intimal injury - Dissection, thrombosis, or complete transection
- Nerve traction - Peroneal nerve stretched around fibular head
- Spontaneous reduction - May occur, masking severity of injury
Classification Systems
Schenck Classification (Most Widely Used)
| Grade | Ligaments Injured | Frequency | Prognosis |
|---|---|---|---|
| KD I | Single cruciate (ACL or PCL) | 5-10% | Good with reconstruction |
| KD II | Both cruciates (ACL + PCL) | 15-20% | Moderate, requires both repairs |
| KD III-M | Cruciates + MCL | 25% | Fair, staged repair needed |
| KD III-L | Cruciates + LCL/PLC | 30% | Fair, PLC critical for rotatory stability |
| KD IV-M/L | All four major ligaments | 20% | Poor, high stiffness risk |
| KD V | Periarticular fracture + ligament | 10% | Variable, fracture healing affects timing |
Schenck Mnemonic
Think I - II - III - IV - V as increasing severity: I = one cruciate, II = two cruciates, III = three ligaments (add one collateral), IV = four ligaments (all), V = five problems (ligaments plus fracture).
Clinical Assessment
History
- Mechanism: High-energy trauma, dashboard injury, hyperextension
- Reduced in field: 20% spontaneously reduce - Ask if knee "popped out"
- Pain and swelling: Immediate hemarthrosis, inability to bear weight
- Paresthesias: Foot numbness suggests nerve or vascular injury
Examination
- Look: Gross deformity (if not reduced), swelling, ecchymosis
- Neurovascular: MANDATORY before and after reduction - ABI, pulses, peroneal nerve
- Stability testing: DO NOT stress test acutely - Risk re-dislocation
- Compartments: Palpate for tightness, especially post-vascular repair
The Spontaneously Reduced Dislocation
20% of knee dislocations reduce spontaneously before medical evaluation. High clinical suspicion is required if history suggests transient dislocation. Key clues: High-energy mechanism, severe instability on exam, inability to bear weight despite normal X-rays. Obtain MRI to assess multiligamentous injury.
Vascular Assessment Protocol (MANDATORY)
| Test | Normal Value | Abnormal Finding | Action Required |
|---|---|---|---|
| Ankle-Brachial Index (ABI) | Greater than 0.9 | Less than 0.9 | IMMEDIATE CTA + vascular surgery consult |
| Pedal pulses | 2+ bilateral | Diminished or absent | Urgent CTA (do NOT rely on pulses alone) |
| Capillary refill | Less than 2 seconds | Greater than 3 seconds | Immediate reduction, reassess post-reduction |
| Serial exams | Stable over 24 hours | Deterioration at any point | Urgent CTA - Delayed thrombosis |
Investigations
Imaging Protocol
Before reduction: Document dislocation direction, identify fractures (KD V). After reduction: Confirm concentric reduction, assess for occult fractures (tibial plateau, femoral condyle). Stress views should NOT be performed acutely.
MANDATORY in all cases. ABI less than 0.9 has 95% sensitivity for arterial injury. Perform before and after reduction, then serially every 2 hours for 24 hours. Normal ABI does NOT exclude intimal tear.
Gold standard for arterial injury. Sensitivity 95%, specificity 99% for popliteal artery injury. Detects intimal tears, pseudoaneurysms, complete disruption. URGENT vascular surgery consult if any abnormality.
Once vascular status secure. Typically performed at 5-7 days post-injury to assess ligament injury pattern and plan staged reconstruction. T2-weighted sequences show all ligament tears, meniscal injuries, and chondral damage.
Do NOT Delay Reduction for Imaging
If obvious dislocation with neurovascular compromise, reduce IMMEDIATELY in the emergency department under procedural sedation. Do NOT wait for X-rays or CT. Reduction improves vascular flow and reduces compartment pressure. Image AFTER reduction to confirm concentric alignment.
Management Algorithm
Acute Management (First 6 Hours)
Goal: Restore vascular flow, prevent limb loss, document injuries.
ED Protocol
- Identify knee dislocation (obvious or history of reduction)
- Document neurovascular status (ABI, pulses, peroneal nerve)
- Obtain AP/lateral X-rays if time permits
- Procedural sedation (propofol or ketamine)
- Longitudinal traction with counter-traction at thigh
- Reverse mechanism of injury (flex for anterior, extend for posterior)
- Reassess neurovascular status post-reduction
- Repeat ABI and pulses (document improvement or deterioration)
- Confirm concentric reduction on X-ray
- Splint knee in 15-20 degrees flexion (prevents re-dislocation)
- Vascular surgery consult if ABI less than 0.9
- Neurovascular checks every 2 hours for 24 hours
- Watch for compartment syndrome (especially post-vascular repair)
- MRI at 5-7 days to plan ligament reconstruction
Reduction Technique Pearl
For anterior dislocation (most common): Apply longitudinal traction to tibia while assistant provides counter-traction at thigh. Gently extend knee while applying posterior pressure to proximal tibia. For posterior dislocation: Flex hip to 90 degrees, apply traction, then extend knee while lifting tibia anteriorly.
Surgical Technique
Cruciate Ligament Reconstruction
Surgical Steps (ACL + PCL)
Supine on standard operating table. Lateral post at thigh. Foot of bed dropped for knee flexion. Tourniquet thigh (usually NOT inflated due to vascular concerns). Prepare for arthroscopy and open if needed.
Standard portals (anterolateral, anteromedial). Assess cruciate tears, meniscal injuries, chondral damage. Document with photos. Perform limited debridement of cruciate remnants (preserve tibial footprint).
Bone-patellar tendon-bone (BTB) autograft preferred for ACL (bone blocks aid fixation). Achilles allograft often used for PCL (larger diameter, less donor morbidity). Prepare grafts on back table with whipstitch sutures.
ACL: Femoral tunnel at 10:30 (right knee) or 1:30 (left knee), tibial tunnel at ACL footprint. PCL: Femoral tunnel at 2:00 (right) or 10:00 (left), tibial tunnel via posteromedial portal. Ensure tunnels avoid convergence.
ACL first: Pass graft, fix femur (interference screw or button), tension at 20 degrees flexion, fix tibia. PCL second: Pass via posteromedial portal, fix femur, tension at 90 degrees flexion (posterior drawer reduced), fix tibia.
Check stability: Lachman (ACL), posterior drawer (PCL). Assess ROM (should achieve 0-130 degrees). Document with fluoroscopy. Ensure no graft impingement. Close portals, apply hinged knee brace locked 0-90 degrees.
Avoid Convergence of Tunnels
When reconstructing both ACL and PCL, femoral tunnels can converge (ACL at 10:30/1:30, PCL at 2:00/10:00). Use 3D planning on CT or intraoperative fluoroscopy to ensure adequate bone bridge. If concern, stage PCL reconstruction 6 weeks later.
Complications
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Limb loss (amputation) | 5-10% overall, 86% if ischemia greater than 6h | Delayed vascular repair, compartment syndrome | Prevention: Immediate reduction and vascular surgery consult |
| Arthrofibrosis (stiffness) | 20-50% (50% if early total repair) | Early total ligament repair, inadequate ROM | Prevention: Staged repair. Treatment: Manipulation or arthroscopic lysis |
| Persistent instability | 15-30% | Missed PLC injury, graft failure | Revision reconstruction with attention to PLC |
| Permanent peroneal nerve palsy | 10-20% (25-30% have initial injury) | Traction injury, compartment syndrome | Ankle-foot orthosis, tendon transfer if no recovery at 12 months |
| Deep vein thrombosis | 10-15% | Vascular repair, immobilization | Prophylactic anticoagulation, early mobilization |
Compartment Syndrome Post-Vascular Repair
Compartment syndrome occurs in 10-15% of patients after popliteal artery repair due to reperfusion injury. Maintain HIGH clinical suspicion. Perform 4-compartment fasciotomy liberally if any concern (pain out of proportion, tense compartments). Delayed fasciotomy (greater than 6-8 hours) leads to permanent muscle and nerve damage.
Postoperative Care and Rehabilitation
ACL/PCL Reconstruction Protocol
- Hinged knee brace locked 0-90 degrees
- Weight-bearing as tolerated with crutches
- ROM exercises: Passive extension to 0 degrees, flexion to 90 degrees
- Quad sets, ankle pumps, SLR (avoid hamstring contraction)
- Unlock brace, progress ROM to 0-120 degrees
- Weight-bearing as tolerated, wean crutches by week 4
- Closed-chain exercises (wall sits, mini squats)
- Avoid open-chain hamstring exercises (protect PCL)
- Full ROM expected (0-130 degrees)
- Progress strengthening (leg press, step-ups)
- Proprioception and balance training
- Stationary bike, swimming (no breaststroke)
- Jogging at 4-6 months if quad strength greater than 70%
- Sport-specific training at 6-9 months
- Return to sport at 12 months (MINIMUM)
- Functional testing before clearance
Outcomes and Prognosis
| Treatment Approach | Stability Outcome | ROM Outcome | Notes |
|---|---|---|---|
| Staged reconstruction (cruciates early, collaterals delayed) | Good stability 70-80% | Stiffness 20%, full ROM 60% | Current gold standard approach |
| Early total repair (all ligaments at once) | Good stability 60-70% | Stiffness 50%, full ROM 30% | Historical approach, high stiffness rate |
| Delayed reconstruction (greater than 3 months) | Variable stability 50-70% | Better ROM (low stiffness) | Scar tissue makes reconstruction difficult |
Predictors of Poor Outcome
Poor outcomes are associated with: (1) Vascular injury requiring repair (higher complication rate), (2) KD IV or KD V injuries (all ligaments torn), (3) Peroneal nerve palsy (10-20% permanent), (4) Delayed reconstruction (greater than 6 months), and (5) High-energy mechanism (polytrauma, associated injuries).
Evidence Base and Key Trials
Staged Versus Early Combined Repair for Knee Dislocation
- Retrospective cohort: 65 patients with knee dislocation
- Staged repair (cruciates early, collaterals delayed): 20% stiffness rate
- Early total repair (all ligaments at once): 50% stiffness rate
- No difference in stability outcomes between groups
Popliteal Artery Injury in Knee Dislocation: Predictors and Outcomes
- Retrospective review: 38 knee dislocations with vascular injury
- 30-40% popliteal artery injury rate in all knee dislocations
- Normal pulses do NOT exclude arterial injury (intimal tears)
- Amputation rate 86% if ischemia exceeds 6 hours
Common Peroneal Nerve Palsy After Knee Dislocation
- Prospective series: 134 knee dislocations
- 25-30% peroneal nerve injury rate
- Complete palsy: 10-20% recovery rate. Partial palsy: 60-70% recovery
- Nerve exploration does NOT improve outcomes
ABI Accuracy in Detecting Popliteal Artery Injury
- Prospective study of 126 knee dislocations
- ABI under 0.9 had 95% sensitivity for significant arterial injury
- Normal ABI (over 0.9) had 95% negative predictive value
- CTA only needed if ABI abnormal or clinical concern
- Reduced unnecessary angiography by 40%
Timing of Multiligament Reconstruction
- Case series: 35 knee dislocations with multiligament injury
- Early surgery (under 3 weeks) vs delayed: similar stability outcomes
- PCL reconstruction using tibial inlay technique preferred
- PLC reconstruction critical for overall stability
- Staged approach reduces stiffness (40% vs 60%)
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Acute Knee Dislocation in ED
"A 35-year-old male presents to ED after motor vehicle collision. Paramedics report his knee was dislocated and they reduced it in the field. On arrival, knee is swollen but reduced. Pedal pulses are present but diminished. What is your assessment and management?"
Scenario 2: Surgical Planning for Multiligamentous Injury
"The patient from Scenario 1 has secure vascular status (ABI 1.0). MRI at 7 days shows complete ACL, PCL, MCL, and posterolateral corner (PLC) tears (Schenck KD IV-M/L). Walk me through your surgical plan."
Scenario 3: Delayed Vascular Thrombosis
"The patient from Scenario 1 was admitted for observation. Initial ABI was 1.0. At 18 hours post-injury, nurse reports foot is cooler and pedal pulses are diminished. What is your management?"
MCQ Practice Points
Vascular Injury Question
Q: What percentage of knee dislocations have associated popliteal artery injury? A: 30-40% - This high rate is why ABI measurement is MANDATORY in all knee dislocations. Normal pulses do NOT exclude intimal tear, which can thrombose 6-24 hours later.
Classification Question
Q: A knee dislocation with complete ACL, PCL, MCL, and PLC tears is classified as: A: Schenck KD IV - All four major ligament complexes torn. KD I (one cruciate), KD II (both cruciates), KD III (cruciate + one collateral), KD IV (all ligaments), KD V (fracture-dislocation).
Treatment Question
Q: What is the advantage of staged ligament reconstruction over early total repair? A: Reduced arthrofibrosis rate (20% vs 50%) - Staged approach reconstructs cruciates early (2-3 weeks) then collaterals delayed (3-6 weeks). Early total repair has 50% stiffness rate with similar stability outcomes.
Complication Question
Q: What is the amputation rate if warm ischemia time exceeds 6 hours in knee dislocation with popliteal artery injury? A: 86% - This is why immediate reduction and vascular repair are critical. Time is limb in this injury.
Nerve Injury Question
Q: Common peroneal nerve injury occurs in what percentage of knee dislocations, and which dislocation direction has the highest risk? A: 25-30% overall, highest in lateral and posterolateral dislocations - Nerve wraps around fibular neck and is stretched during lateral displacement. Complete palsy has only 10-20% recovery rate.
Australian Context and Medicolegal Considerations
Hospital Systems
- Trauma center activation: Major trauma call for knee dislocation (high-energy, vascular risk)
- Vascular surgery availability: Immediate consult required if ABI less than 0.9
- Tertiary referral: Transfer to center with vascular + ortho trauma if resources limited
- 24-hour observation protocol: Mandatory for all knee dislocations
Medicolegal Considerations
- Documented neurovascular exam: Before and after reduction, serially every 2 hours
- ABI measurement: MANDATORY in all cases, document specific values
- CTA if abnormal: Failure to obtain CTA with ABI less than 0.9 is indefensible
- Informed consent: 30-40% cannot return to pre-injury activity level
Litigation Risk: Missed Vascular Injury
Common medicolegal scenario: Patient with knee dislocation discharged from ED with "normal pulses" without ABI or 24-hour observation. Patient returns at 24 hours with cold, pulseless leg. Below-knee amputation required. Defense is difficult if ABI was not documented and CTA not performed for abnormal ABI. Standard of care requires vascular assessment protocol in ALL knee dislocations.
KNEE DISLOCATION
High-Yield Exam Summary
Key Anatomy
- •Popliteal artery = Tethered at adductor hiatus and soleus arch (vulnerable to injury)
- •Common peroneal nerve = Wraps around fibular neck (25-30% injury rate)
- •ACL + PCL = Both torn in 80% of dislocations
- •PLC (LCL, popliteus, popliteofibular) = Critical for rotatory stability
Classification
- •Schenck KD I = Single cruciate (rare)
- •Schenck KD II = Both cruciates
- •Schenck KD III-M/L = Cruciates + one collateral
- •Schenck KD IV-M/L = All four ligaments
- •Schenck KD V = Fracture-dislocation
Treatment Algorithm
- •ED: Immediate reduction, ABI measurement, CTA if ABI less than 0.9
- •Vascular injury: URGENT repair within 6 hours (amputation rate 86% if delayed)
- •Staged repair: Cruciates at 2-3 weeks, collaterals at 3-6 weeks
- •Serial exams: Every 2 hours for 24 hours (detect delayed thrombosis)
Surgical Pearls
- •Staged approach reduces stiffness from 50% to 20%
- •ACL/PCL tunnels: Avoid convergence (use fluoroscopy)
- •PLC reconstruction: Protect common peroneal nerve throughout
- •Collateral healing: Lock brace in extension 6 weeks
Complications
- •Amputation: 5-10% overall, 86% if ischemia greater than 6h
- •Arthrofibrosis: 50% if early total repair, 20% if staged
- •Peroneal palsy: 25-30% initial injury, 10-20% permanent
- •Persistent instability: 20-30% (often missed PLC)