Acute Knee Trauma Management After Motor Vehicle Accident
Initial Imaging Decision
Obtain knee radiographs immediately (anteroposterior and lateral views) as the first imaging study, given this is a motor vehicle accident with significant trauma mechanism, regardless of the patient's ability to walk. 1
Rationale for Immediate Radiography
In the setting of significant acute trauma to the knee from a motor vehicle accident, radiographs should be the first imaging study and clinical decision rules (like Ottawa Knee Rule) should NOT be applied. 1 The ACR Appropriateness Criteria explicitly state that physician judgment and common sense should supersede clinical guidelines in cases of significant trauma mechanisms such as motor vehicle accidents. 1
Why Ottawa Knee Rule Does Not Apply Here
While the patient can walk (which would normally suggest low fracture risk), the Ottawa Knee Rule is specifically designed for minor trauma mechanisms (falls, twisting injuries) in patients ≥18 years. 2 The rule should be bypassed when:
- Significant trauma mechanism exists (motor vehicle accident qualifies) 1
- Gross deformity, palpable mass, or penetrating injury present 1, 2
- Unreliable examination due to altered mental status 1, 2
The ability to walk does not exclude serious injury in high-energy trauma. 1
Critical Vascular Assessment
Immediately assess for vascular injury signs given the motor vehicle accident mechanism and retropatellar/infrapatellar pain location, which could indicate posterior knee structures involvement:
- Check distal pulses (dorsalis pedis, posterior tibial) 3
- Assess for pallor, coolness, delayed capillary refill 3
- Evaluate for neurological deficits (peroneal and tibial nerve function) 3
If any vascular compromise signs are present, order CTA of the lower extremity immediately as popliteal artery injury occurs in 7.5-30% of knee dislocations and is limb-threatening. 3 CTA has similarly high accuracy to conventional angiography but is less invasive. 1, 3
Radiographic Technique
When obtaining radiographs:
- Minimum two views: anteroposterior and lateral (knee flexed 25-30 degrees) 2
- Consider adding patellofemoral (sunrise) view given the retropatellar pain location to evaluate for patellar fractures or subluxation 1
- Cross-table lateral view with horizontal beam can visualize lipohemarthrosis (indicates intra-articular fracture) 2
Next Steps Based on Initial Radiographs
If Radiographs Show Fracture
- CT without contrast may be appropriate for better fracture characterization, particularly for tibial plateau fractures 1, 3
- MRI without contrast is usually appropriate to evaluate for additional soft-tissue injuries (meniscal tears, ligamentous injuries) 1, 3
If Radiographs Are Negative
Order MRI without contrast to evaluate for:
- Occult fractures (bone marrow contusions) 1
- Meniscal tears 1
- Ligamentous injuries (ACL, PCL, collateral ligaments) 1
- Internal derangement 1
MRI has 93.5% likelihood of detecting soft-tissue injuries in acute trauma patients with negative radiographs and aids in earlier surgical decision-making. 1
Common Pitfalls to Avoid
- Do not skip radiographs based on ability to walk in motor vehicle accidents—the trauma mechanism overrides clinical decision rules 1
- Do not delay vascular imaging if any concern for arterial injury exists, as time to revascularization is critical for limb salvage 3
- Do not apply Ottawa Knee Rule in high-energy trauma mechanisms 1, 2
- Do not order MRI with IV contrast for routine evaluation—non-contrast MRI is sufficient 3
Physical Examination Priorities
While awaiting imaging, perform focused examination:
- Palpate for focal patellar tenderness and joint effusion 1
- Assess knee range of motion (ability to flex to 90 degrees) 2
- Perform Lachman test for ACL integrity (74% sensitive, 95% specific) 4, 5
- Check joint line tenderness for meniscal injury (83% sensitive, 83% specific) 6, 4
- Test collateral ligaments with valgus/varus stress 7