When to Order a Knee X-ray
Knee X-rays should be ordered as the initial imaging study for chronic knee pain, acute trauma meeting evidence-based criteria (Ottawa knee rules), or when fracture or degenerative changes are suspected. 1
Initial Imaging for Chronic Knee Pain
Radiography is the appropriate first-line imaging study for evaluating chronic knee pain in patients ≥5 years of age. 2 The American College of Radiology emphasizes that approximately 20% of patients inappropriately receive MRI without recent radiographs (within the prior year), representing unnecessary testing and cost. 2, 1, 3
Required Views
Knee radiographs must include at minimum: 2
- One frontal projection (anteroposterior, Rosenberg, or tunnel view)
- Tangential patellar view
- Lateral view of the affected knee
Acute Knee Pain and Trauma
For acute knee pain with trauma history, apply the Ottawa knee rules to determine if radiographs are indicated: 4
- Age >55 years
- Tenderness at the head of the fibula
- Tenderness at the patella
- Inability to bear weight for 4 steps
- Inability to flex the knee to 90 degrees
If any of these criteria are present, obtain plain radiographs. 4 The Ottawa knee rules have the strongest supporting evidence among decision rules for knee fractures, with high sensitivity for detecting fractures that require intervention.
When X-rays Are NOT Initially Needed
Do not order knee X-rays for:
- Suspected isolated soft tissue injuries (meniscal or ligamentous tears) where clinical examination is adequate 4
- Acute knee pain without trauma and without Ottawa criteria 1
The physical examination alone has reasonable accuracy for meniscal injuries (sensitivity 87%, specificity 92%) and anterior cruciate ligament tears (sensitivity 74%, specificity 95%). 4 The Lachman test is more accurate than the drawer sign for ligamentous assessment. 4
Clinical Scenarios Requiring X-rays
Suspected Osteoarthritis
Order radiographs when clinical features suggest osteoarthritis: 4
- Age >50 years
- Morning stiffness <30 minutes
- Crepitus
- Bony enlargement
These clinical criteria are 89% sensitive and 88% specific for underlying chronic arthritis. 4
Suspected Degenerative Changes or Effusion
Radiographs are indicated when examining for: 2
- Osteoarthritis or chondrocalcinosis
- Osteochondritis dissecans
- Loose bodies
- Signs of prior chronic knee injury (Segond fracture, tibial spine avulsion)
Important Considerations for Referred Pain
If knee radiographs are unremarkable but pain persists, consider referred pain sources: 2, 3
- Hip pathology: Order hip radiographs if clinical evidence suggests hip origin
- Lumbar spine pathology: Order lumbar spine radiographs if clinical concern exists
This is particularly important as referred pain from the hip or lower back commonly presents as knee pain. 2, 3
When to Advance to MRI
MRI without IV contrast is indicated only when: 2, 1, 3
- Initial radiographs are normal or show only joint effusion AND pain persists despite adequate conservative treatment
- Surgery is being considered
- Radiographs demonstrate findings requiring further characterization (OCD, loose bodies, history of cartilage/meniscal repair)
MRI is more sensitive than radiography for detecting meniscal and ligamentous damage but less specific than physical examination. 4 However, meniscal tears are often incidental findings in older patients, with the majority of people over 70 years having asymptomatic tears. 2
Common Pitfalls to Avoid
- Never order MRI without obtaining recent radiographs first (within the past year) 2, 1, 3
- Do not use radiographs to diagnose pseudogout - plain films are neither sensitive nor specific for this condition 4
- Document laterality using specific right/left codes rather than "unspecified" for proper reimbursement and clinical clarity 3, 5
- Recognize that X-ray-based diagnosis may increase patient beliefs about necessity of surgery without improving beliefs about conservative management like exercise 6