X-Ray Views for Knee Injury
For acute knee trauma, order a minimum of two views: anteroposterior (AP) and lateral radiographs, with the lateral obtained at 25-30 degrees of knee flexion. 1
When to Order Radiographs
Apply the Ottawa Knee Rule to determine if imaging is needed. Order X-rays if ANY of the following are present: 1, 2
- Age >55 years
- Isolated tenderness of the patella (no other bone tenderness)
- Tenderness at the head of the fibula
- Inability to flex knee to 90 degrees
- Inability to bear weight for 4 steps both immediately after injury and in the emergency department
Override clinical decision rules and obtain radiographs regardless if the patient has: 1
- Gross deformity
- Palpable mass
- Penetrating injury
- Prosthetic hardware
- Altered mental status (head injury, intoxication, dementia)
- Neuropathy (paraplegia, diabetes)
- Unreliable examination due to multiple injuries
Standard Two-View Series
Anteroposterior (AP) View
- Standard frontal projection of the knee 1
- Evaluates medial and lateral compartments, tibial plateaus, and distal femur 1
Lateral View
- Critical technical details: Obtain with knee at 25-30 degrees of flexion in lateral decubitus position 1
- Must demonstrate the patella in profile 1
- Allows evaluation for joint effusion 1
- Consider cross-table lateral with horizontal beam to visualize lipohemarthrosis (fat-fluid level), which is frequently seen with intra-articular fractures 1
Additional Views Based on Clinical Suspicion
Add supplemental views when specific pathology is suspected: 1
Patellofemoral (Sunrise/Skyline) View
- Indication: Suspected patellar fracture, patellar subluxation, or dislocation 1
- Provides optimal visualization of the patellofemoral joint 1
Internal and External Oblique Views
- Commonly performed in acute trauma settings 1
- However, recent evidence suggests limited added value: Oblique views rarely change management, with a number needed to treat of 83 for any treatment change and 643 for critical management changes 3
- Consider reserving oblique views for cases where two-view series is equivocal 3
Important Technical Considerations
The lateral view is particularly critical because several pathologies are only visible on this projection, including: 4
- Quadriceps or patellar tendon injuries
- Avulsion fractures of anterior or posterior structures
- Soft-tissue injuries
- Joint effusion and lipohemarthrosis 1, 4
Four-view series (AP, lateral, and two obliques) increases fracture detection sensitivity from 79% to 85% compared to two views alone, but this modest improvement must be weighed against increased radiation exposure and cost. 5
Next Steps if Initial Radiographs Are Negative
If Occult Fracture Suspected
- CT is superior to radiographs for detecting and characterizing fractures, with 100% sensitivity versus 83% for plain films in tibial plateau fractures 1
- Consider CT for better characterization of complex fractures 2, 6
If Internal Derangement Suspected
- MRI without contrast is the appropriate next study if: 2, 7
- Significant joint effusion persists
- Inability to fully bear weight after 5-7 days
- Mechanical symptoms suggesting meniscal injury (locking, catching)
- Joint instability suggesting ligamentous injury
Common Pitfalls to Avoid
- Don't skip radiographs in patients meeting Ottawa criteria, even if clinical suspicion for fracture is low—only 5.2% of knee radiographs show fractures, but missing these can lead to significant morbidity 1
- Don't obtain MRI as initial imaging for acute trauma—radiographs must be performed first to rule out fracture 1
- Don't forget the cross-table lateral view when intra-articular fracture is suspected, as lipohemarthrosis is a key finding 1
- Physician judgment supersedes clinical guidelines—if clinical concern is high despite negative Ottawa criteria, obtain imaging 1