What are the differential diagnoses for a knee injury following a motor vehicle collision?

Differential Diagnoses for Knee Injury Following Motor Vehicle Collision

In a motor vehicle collision, the differential diagnosis for knee injury must include fractures (femoral condyle, tibial plateau, patella), ligamentous injuries (ACL, PCL, collateral ligaments, posterolateral corner), meniscal tears, patellar dislocation, bone contusions, and critically—vascular injury to the popliteal artery which occurs in approximately 30% of knee dislocations and requires emergent intervention to prevent limb loss. 1

High-Energy Trauma Patterns

Motor vehicle collisions create specific injury patterns based on mechanism:

Dashboard Injury Pattern

• Mechanism: Direct impact of knee against dashboard
• Injuries:
  • Posterior hip dislocation or acetabular fracture (from axial loading through femur) 2
  • Anterior tibial plateau contusions 3
  • PCL tears
  • Tibial plateau fractures

Hyperextension Pattern

• Mechanism: Forceful knee hyperextension
• Injuries:
  • "Kissing contusions" (anterior proximal tibia and distal femur) 3
  • ACL tears
  • PCL avulsion fractures
  • Posterolateral corner injuries

Pivot Shift Pattern

• Mechanism: Rotational force with valgus stress
• Injuries:
  • Posterolateral tibial plateau and midlateral femoral condyle contusions 3
  • ACL tears (often complete)
  • Lateral meniscal tears
  • Anterolateral ligament injuries 1
  • Segond fractures (lateral tibial avulsion)

Critical Life-Threatening Injuries

Vascular Injuries (HIGHEST PRIORITY)

• Popliteal artery injury occurs in 30% of posterior knee dislocations 1
• Requires immediate surgical intervention to prevent limb loss
• Associated peroneal and tibial nerve injuries cause significant long-term morbidity 1
• Clinical signs: Absent or diminished pulses, expanding hematoma, limb ischemia

Knee Dislocation

• May spontaneously reduce before evaluation
• High association with multi-ligament injuries (ACL + PCL ± collateral ligaments)
• Posterolateral corner injuries present in 19.7% of ACL ruptures 1

Osseous Injuries

Fractures

• Tibial plateau fractures (Schatzker classification)
  • High-energy MVCs typically produce complex patterns (Schatzker V-VI) 4
  • Lateral plateau depression >11mm predicts lateral meniscus tears and ACL avulsion 1
• Distal femoral fractures (supracondylar/intra-articular)
• Patellar fractures (transverse, comminuted, or avulsion)
• Avulsion fractures indicating ligamentous injury:
  • Segond fracture (anterolateral ligament/capsule)
  • PCL tibial avulsion
  • Fibular head avulsion (posterolateral corner)

Bone Contusions

• Present in majority of acute knee trauma 3
• Pattern predicts associated soft-tissue injuries
• Prognostic significance: Predicts focal osteoarthritis development at 1 year 1

Soft-Tissue Injuries

Ligamentous Injuries

• ACL tears: Most common in high-energy trauma
  • Associated anterolateral ligament injury in significant percentage 1
  • Concomitant lateral meniscal lesions common 1
• PCL tears: Dashboard mechanism
• Collateral ligament sprains/tears (MCL/LCL)
• Posterolateral corner injuries: If missed, lead to considerable morbidity 1
  • Components: LCL, popliteus tendon, popliteofibular ligament

Meniscal Injuries

• Lateral meniscus tears with ACL injuries
• Medial meniscus tears with MCL injuries
• "Locked knee" presentation suggests displaced meniscal tear 1

Other Soft-Tissue Pathology

• Morel-Lavallée lesion: Closed degloving injury from high-energy shearing forces 5
  • Prepatellar location in pedestrian vs. vehicle collisions
  • Prone to infection and tissue necrosis if missed
• Joint effusion/hemarthrosis
• Lipohemarthrosis (indicates intra-articular fracture)

Associated Injuries in Polytrauma

Patients with knee injuries from MVCs have significantly higher rates of concomitant injuries 6:

• Severe chest injuries (AIS ≥3) more common
• Severe limb injuries elsewhere
• Foot and ankle fractures (from brake pedal/toepan intrusion) 2
• Hip injuries (acetabular fractures, dislocations)

Clinical Pitfalls

1. Spontaneously reduced knee dislocation: May appear benign on examination but harbor multi-ligament injury and vascular compromise
2. Delayed vascular compromise: Initial pulses present doesn't exclude intimal injury—serial neurovascular exams mandatory
3. Posterolateral corner injuries: Frequently missed, leading to failed ACL reconstruction if not addressed
4. Anterolateral ligament injuries: More visible on MRI within 6 weeks of injury; may resolve or become less apparent with chronicity 1
5. Multiple injuries masking knee pathology: Altered mental status, intoxication, or distracting injuries may prevent accurate knee examination 1

High-Risk Patient Factors

• Taller males: Increased probability of hip injury 2
• Shorter females: Increased probability of foot/ankle fractures 2
• Polytrauma patients with knee injuries: Significantly longer hospital stays (38.1 vs. 25.5 days) and higher costs 6

The key in motor vehicle collision knee injuries is maintaining high suspicion for vascular injury and multi-ligament disruption, as these determine limb viability and long-term functional outcomes. Initial radiographs are mandatory regardless of clinical decision rules given the high-energy mechanism 1, followed by advanced imaging (CT for fracture characterization, CTA/MRA for vascular assessment, MRI for soft-tissue injuries) as clinically indicated.