X-ray Views for Knee Injury
For acute knee trauma, obtain a minimum of two radiographs: anteroposterior (AP) and lateral views, with additional patellofemoral (skyline) view when patellar injury is suspected. 1, 2
Standard Initial Views
Mandatory Two-View Series
- Anteroposterior (AP) view: Standard frontal projection to evaluate tibial plateau, femoral condyles, and overall joint alignment 1, 2
- Lateral view: Obtained with knee flexed 25-30 degrees in lateral decubitus position, with patella in profile to assess for joint effusion and lipohemarthrosis (indicating intra-articular fracture) 1
When to Add Supplemental Views
Patellofemoral (skyline/sunrise) view should be added when:
- Patellar fracture is suspected based on focal patellar tenderness 1
- Patellar dislocation or subluxation occurred 1, 3
- Osteochondral injury is suspected 3, 4
Internal and external oblique views may be obtained for:
- Better characterization of tibial plateau fractures 1
- Evaluation of subtle fractures not visible on standard views 4
Cross-table lateral view with horizontal beam is useful for:
- Visualization of lipohemarthrosis, which indicates intra-articular fracture 1
Clinical Decision Framework
When Radiographs Are Indicated
The American College of Radiology recommends obtaining radiographs when any Ottawa Knee Rule criteria are present 1, 2:
- Age >55 years
- Isolated tenderness of patella or fibular head
- Inability to flex knee to 90 degrees
- Inability to bear weight for 4 steps immediately after injury and in the emergency department
Override Clinical Decision Rules
Always obtain radiographs regardless of Ottawa criteria when the patient has 1, 2:
- Gross deformity
- Palpable mass
- Penetrating injury
- Prosthetic hardware
- Altered mental status (head injury, intoxication, dementia)
- Neuropathy (paraplegia, diabetes)
- Multiple injuries making examination unreliable
- History suggesting increased fracture risk
Evidence Quality Considerations
The ACR Appropriateness Criteria (2020) provide the strongest guideline evidence, consistently recommending the two-view minimum across multiple clinical scenarios 1. Research evidence supports that four views are more sensitive (85%) than two views (79%) for fracture detection, but the incremental benefit must be weighed against radiation exposure 5. The patellofemoral view specifically improves detection of patellar injuries that would otherwise be missed 3, 4.
Common Pitfalls to Avoid
- Don't skip the lateral view: It provides critical information about joint effusion and posterior compartment injuries that AP views miss 1, 6
- Don't forget the patellofemoral view for patellar injuries: Vertical patellar fractures and osteochondral injuries are often only visible on this view 3, 4
- Don't rely solely on clinical decision rules in high-risk patients: Physician judgment supersedes guidelines when gross deformity, altered mental status, or other red flags are present 1
- Don't assume negative radiographs rule out all significant injuries: Occult fractures and soft tissue injuries may require MRI if clinical suspicion remains high after 5-7 days 2, 7