When should a knee be x-rayed, especially in patients with a history of osteoporosis, previous knee injuries, or chronic conditions like osteoarthritis?

When to X-ray a Knee

Obtain knee X-rays for acute trauma if the patient meets Ottawa Knee Rule criteria (age ≥55 years, isolated patellar tenderness, fibular head tenderness, inability to flex knee to 90°, or inability to bear weight for 4 steps), and for chronic knee pain, X-rays should be the initial imaging study before considering MRI. 1

Acute Knee Trauma

Clinical Decision Rules for X-ray Ordering

For patients ≥18 years with acute knee trauma, apply the Ottawa Knee Rule which requires X-rays if ANY of the following are present: 1

• Age ≥55 years
• Palpable tenderness at the fibular head
• Isolated patellar tenderness
• Inability to flex knee to 90 degrees
• Inability to bear weight immediately after injury
• Inability to walk 4 steps in the examination room

The Pittsburgh Decision Rule is an alternative that mandates X-rays for: 1

• Age <12 years OR >50 years
• Inability to take 4 weight-bearing steps in the emergency department

These rules reduce unnecessary X-rays by 23-35% while maintaining 100% sensitivity for detecting clinically significant fractures. 1

Override Clinical Rules and X-ray Regardless

Physician judgment supersedes clinical guidelines when patients present with: 1, 2

• Gross deformity
• Palpable mass
• Penetrating injury
• Prosthetic hardware
• Altered mental status (head injury, intoxication, dementia)
• Neuropathy (paraplegia, diabetes)
• Multiple injuries affecting reliable examination
• History suggesting increased fracture risk (osteoporosis)

Required X-ray Views

Obtain a minimum of two views: 1, 2

• Anteroposterior (AP) view
• Lateral view with knee at 25-30 degrees flexion

Additional views based on clinical suspicion: 1, 2

• Patellofemoral (sunrise) view for suspected patellar fractures
• Cross-table lateral view with horizontal beam to visualize lipohemarthrosis (indicates intra-articular fracture)
• Internal and external oblique views as supplemental projections

Chronic Knee Pain

Initial Imaging Approach

Radiography is the mandatory first imaging study for chronic knee pain, as approximately 20% of patients inappropriately receive MRI without recent radiographs (within the prior year). 1, 2

Required views include: 1, 2

• At least one frontal projection (AP, Rosenberg, or tunnel view)
• Tangential patellar view
• Lateral view of the affected knee

When to Consider Alternative Sources

If knee X-rays are unremarkable but pain persists, consider referred pain from: 1, 3

• Hip pathology - obtain hip radiographs, as hip disease commonly refers pain to the knee 1, 2
• Lumbar spine pathology - obtain lumbar spine radiographs if clinical evidence suggests spinal origin 1, 3

Progression to Advanced Imaging

MRI without IV contrast is indicated when: 1, 2

• Initial radiographs are normal or show only joint effusion AND pain persists
• Significant joint effusion is present
• Inability to fully bear weight after 5-7 days
• Mechanical symptoms suggest meniscal injury
• Joint instability suggests ligamentous injury

Special Populations and Considerations

Pediatric Patients (Ages 5-12)

Children between 5-12 years fall outside both Ottawa and Pittsburgh rule age ranges, so radiographs may be beneficial despite lack of clinical symptoms. 1

Patients with Osteoporosis or Previous Injuries

These patients have increased fracture risk and should receive X-rays regardless of clinical decision rule criteria. 1 The context of chronic conditions like osteoarthritis does not change the acute trauma algorithm, but these patients warrant lower thresholds for imaging.

Knee Dislocation

For suspected knee dislocation, obtain: 2

• Radiographs first
• MRI for soft tissue evaluation
• CTA of lower extremity (vascular injury occurs in ~30% of posterior knee dislocations)

Common Pitfalls to Avoid

Do not order MRI as the initial study - this bypasses the cost-effective screening that radiographs provide and leads to overdiagnosis of incidental findings. 1, 2

Do not assume negative X-rays rule out all significant injuries - occult fractures and soft tissue injuries may still be present, particularly if the patient cannot bear weight after 5-7 days. 2

Do not attribute all posterior knee pain to local pathology - always consider referred pain from hip or lumbar spine before ordering advanced knee imaging. 1, 3

Avoid premature reassurance - even with ability to bear weight and negative X-rays, significant meniscal or ligamentous injuries may exist requiring MRI if symptoms persist beyond one week. 2