Knee Injuries from Motor Vehicle Accidents
For knee injuries sustained in motor vehicle collisions, immediately obtain plain radiographs (AP and lateral views) as the first imaging study, bypassing clinical decision rules like the Ottawa Knee Rule, because high-energy trauma mechanisms mandate imaging regardless of clinical findings. 1
Initial Evaluation Algorithm
Immediate Assessment
When a patient presents after a motor vehicle accident with knee injury, your priority is to identify life- and limb-threatening injuries:
- Check distal pulses and perform ankle-brachial index (ABI) - Critical for detecting vascular compromise 2
- Assess for gross deformity - Suggests possible dislocation
- Evaluate neurovascular status - Peroneal and tibial nerve injuries occur in approximately 30% of knee dislocations and cause significant long-term morbidity 1
- Look for signs of dislocation - Even if spontaneously reduced
Why Clinical Decision Rules Don't Apply Here
The Ottawa Knee Rule and Pittsburgh Decision Rule are explicitly not applicable in motor vehicle accidents 1. These rules were designed for low-energy mechanisms (falls, twisting injuries). High-energy trauma from motor vehicle collisions requires imaging regardless of:
- Ability to bear weight
- Presence or absence of effusion
- Focal tenderness
- Age of patient
Common pitfall: Don't be falsely reassured by a patient who can walk after a motor vehicle accident - obtain radiographs anyway 1.
Imaging Strategy
First-Line: Plain Radiographs
Obtain minimum two views (AP and lateral) of the affected knee 1:
- Lateral view at 25-30 degrees flexion to visualize joint effusion
- Consider cross-table lateral with horizontal beam to detect lipohemarthrosis (indicates intra-articular fracture)
- Add patellofemoral view if patellar injury suspected
When to Escalate Imaging
If knee dislocation is suspected or confirmed:
CTA of lower extremity is the preferred vascular imaging 1
MRI without contrast for soft tissue evaluation 1
- Best for ligamentous injuries (ACL, PCL, collateral ligaments, posterolateral/posteromedial corners)
- Evaluates meniscal tears
- Detects occult fractures
- Can be combined with MRA for comprehensive assessment 2
If radiographs are negative but clinical suspicion remains high:
- MRI without IV contrast is the next appropriate study for occult fractures or internal derangement 1
- CT can detect occult fractures but misses soft tissue injuries 1
Critical Management Points
Immediate Actions for Suspected Dislocation
- Closed reduction if dislocation present
- External fixation for stabilization 2, 3
- Urgent vascular surgery consultation if ABI abnormal or absent pulses
What NOT to Do
- Don't use CT as initial imaging - It misses vascular injuries without IV contrast 1
- Don't rely on clinical exam alone in altered mental status (head injury, intoxication) - these patients get imaging regardless 1
- Don't delay vascular imaging if dislocation suspected - time is limb 1
Special Populations Requiring Extra Vigilance
Motor vehicle accident patients with:
- Altered mental status (head injury, intoxication) - unreliable exam 1
- Neuropathy (paraplegia, diabetes) - may not report pain appropriately 1
- Multiple injuries - knee injury may be overlooked 1
Multi-Ligamentous Injuries
Motor vehicle accidents commonly cause multi-ligamentous knee injuries (MLKIs) involving ACL, PCL, and collateral ligaments 2. These require:
- MRI for complete soft tissue assessment
- Orthopedic surgery consultation for staged surgical planning
- Consideration of external fixation as temporizing measure
The heterogeneity of injury patterns in MLKIs means individualized surgical planning is necessary, but initial diagnostic workup follows the algorithm above 2.
Disposition
Arrange expedited orthopedic follow-up for all motor vehicle accident knee injuries, even if initial radiographs are negative, because:
- Soft tissue injuries may not be apparent initially 4
- MRI may be needed for definitive diagnosis
- Early intervention improves outcomes for ligamentous injuries