Lateral Right Knee Pain Following Fall with Valgus Stress Pain
Obtain knee radiographs immediately as the initial imaging study, since you have focal tenderness and weight-bearing pain following trauma, then proceed to MRI without contrast if radiographs are negative but clinical suspicion for ligamentous injury remains high. 1
Initial Evaluation Approach
Your clinical presentation—lateral knee pain after a fall with weight-bearing difficulty and pain on valgus stress testing—requires systematic evaluation to differentiate between lateral-sided injuries (lateral collateral ligament, posterolateral corner) versus medial-sided injuries (medial collateral ligament), as both can paradoxically produce pain with valgus stress.
Immediate Imaging Protocol
Start with standard knee radiographs because you meet ACR Appropriateness Criteria for acute knee trauma imaging: you have focal tenderness, weight-bearing difficulty, and trauma mechanism 1. This is the evidence-based first step for patients 5 years or older with these findings.
Critical Clinical Distinction
The valgus stress test causing pain is diagnostically important but requires careful interpretation:
If valgus stress produces medial-sided opening/pain: This suggests medial collateral ligament (MCL) injury. Greater than 3.2 mm of medial compartment gapping at 20 degrees of flexion on stress radiographs indicates grade III MCL injury 2
If valgus stress produces lateral-sided pain without medial opening: This may indicate lateral compartment pathology (lateral meniscus, lateral collateral ligament, or posterolateral corner injury)
Important caveat: A positive "dial test" (external rotation test) can occur with BOTH posterolateral corner injuries AND isolated MCL injuries 3. Therefore, you must perform valgus stress testing at both 0 and 30 degrees of flexion to differentiate medial instability from posterolateral instability 3.
Algorithmic Management Based on Radiograph Results
If Radiographs Show Fracture
- Tibial plateau fracture: Proceed to either MRI without contrast OR CT without contrast to evaluate additional bone or soft-tissue injury 1
- These are equivalent alternatives for surgical planning
If Radiographs Are Normal
Proceed to MRI knee without IV contrast as the next study for suspected internal derangement or occult fracture 1. This is the appropriate next step for adults or skeletally mature patients when radiographs don't reveal fracture but clinical suspicion remains high.
MRI will definitively identify:
- Ligamentous injuries (MCL, LCL, posterolateral corner structures)
- Meniscal tears (lateral meniscus given your lateral pain)
- Occult fractures
- Bone contusions
Red Flags Requiring Urgent Evaluation
You should be urgently referred if you develop:
- Severe instability or inability to bear weight at all
- Signs of knee dislocation (requires CTA lower extremity to evaluate vascular injury) 1
- Fever, significant swelling, erythema suggesting septic joint 4
Key Clinical Pitfall
Do not rely solely on the valgus stress test to localize pathology. The test can be misleading—MCL injuries produce increased external rotation similar to posterolateral corner injuries 3. Always correlate valgus stress findings with the specific location of tenderness (medial versus lateral) and perform comprehensive ligamentous testing including varus stress at 0 and 30 degrees.
Stress radiographs provide objective documentation of ligamentous laxity with excellent reproducibility (ICC 0.98-0.99) 2, but are typically performed under anesthesia or with standardized force application, not in the acute outpatient setting.