What views should be ordered for a knee x-ray?

Standard Knee X-ray Views

For acute knee trauma or knee pain evaluation, order a minimum of two views: anteroposterior (AP) and lateral radiographs, with the lateral view obtained at 25-30 degrees of knee flexion. 1, 2

Core Two-View Protocol

The essential imaging protocol includes:

• Anteroposterior (AP) view: Standard frontal projection of the knee 1
• Lateral view: Obtained with the knee flexed at 25-30 degrees in the lateral decubitus position, demonstrating the patella in profile 1

The lateral view is critical because it allows evaluation for joint effusion and, when obtained as a cross-table lateral with horizontal beam, enables visualization of lipohemarthrosis—a key finding in intra-articular fractures 1

Additional Views Based on Clinical Suspicion

Add supplemental views when specific pathology is suspected:

• Patellofemoral (sunrise/skyline) view: Essential for suspected patellar fractures, subluxation, or dislocation 1, 3
• Internal and external oblique views: Commonly performed in acute trauma settings for improved visualization of subtle fractures 1, 2
• Cross-table lateral with horizontal beam: Specifically for detecting lipohemarthrosis in suspected intra-articular fractures 1

Weight-Bearing Views for Specific Indications

For chronic knee pain or osteoarthritis evaluation, standing (weight-bearing) radiographs provide superior diagnostic information:

• Standing AP and lateral views are the standard for routine follow-up and osteoarthritis assessment 1
• The 45-degree flexion posteroanterior (PA) view (tunnel view) is more sensitive than standard AP for detecting tibiofemoral osteoarthritis, particularly in the medial and lateral compartments 4, 5, 6
• Standing long-leg (hip-to-ankle) views may be obtained for optimal alignment assessment, though standard AP knee radiographs are valid alternatives after a baseline long-leg film 1

Clinical Decision-Making Algorithm

When to order radiographs:

Apply the Ottawa Knee Rules—order radiographs if ANY of the following are present: 1, 2

• Age >55 years
• Isolated tenderness of the patella
• Tenderness at the head of the fibula
• 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 and obtain radiographs regardless when: 1, 2

• Gross deformity present
• Palpable mass
• Penetrating injury
• Prosthetic hardware in place
• Unreliable history or examination (multiple injuries, altered mental status from head injury/drugs/alcohol/dementia)
• Neuropathy (paraplegia, diabetes)
• History suggesting increased fracture risk

Common Pitfalls to Avoid

Technical considerations that impact diagnostic accuracy:

• Minor differences in positioning can greatly alter the appearance of periprosthetic lucencies—consider fluoroscopically positioned images for improved visualization of the prosthesis-bone interface 1
• For varus/valgus alignment assessment, AP views obtained in 10 degrees of internal rotation improve interpretation compared to neutral or externally rotated views 1
• The skyline view, while recommended for patellofemoral assessment, is frequently technically unsatisfactory and may need to be excluded—the lateral view provides comparable information and is easier to acquire with high quality 7, 3
• The lateral view in full extension demonstrates 98% sensitivity for patellar dislocation, compared to only 62% for the axial view 3

Advanced Imaging Considerations

When initial radiographs are negative but clinical suspicion remains high:

• MRI without contrast is the appropriate next study for suspected meniscal or ligamentous injuries (internal derangement) 2, 8
• CT may be considered for better characterization of suspected occult fractures, with 100% sensitivity for tibial plateau fractures compared to 83% for radiographs 1