What are the rules for ordering an X-ray (X-ray) for a patient presenting with knee pain or injury, particularly those with a history of trauma, osteoarthritis (degenerative joint disease), or rheumatoid arthritis?

X-ray Rules for Knee Injuries

Apply the Ottawa Knee Rules to determine when X-rays are indicated: order radiographs if the patient is age ≥55 years, has isolated patellar tenderness, isolated fibular head tenderness, inability to flex knee to 90 degrees, or inability to bear weight for 4 steps both immediately after injury and in the examination room. 1, 2

When to Order X-rays

Acute Trauma Setting

• Order X-rays immediately if any Ottawa Knee Rule criteria are positive 1, 2:

  • Age ≥55 years
  • Focal tenderness at the patella (no other bone tenderness)
  • Focal tenderness at the fibular head (no other bone tenderness)
  • Inability to flex knee to 90 degrees
  • Inability to bear weight for 4 steps immediately after injury AND in the examination room

• Override the Ottawa Rules and order X-rays regardless of criteria if the patient has 1, 2:

  • Gross deformity
  • Palpable mass
  • Penetrating injury
  • Prosthetic hardware
  • Altered mental status (head injury, drug/alcohol use, dementia)
  • Neuropathy (paraplegia, diabetes)
  • History suggesting increased fracture risk

Chronic Knee Pain Setting

• Radiographs should be the initial imaging study for chronic knee pain 1, 3
• Order X-rays before considering MRI—approximately 20% of patients inappropriately receive MRI without recent radiographs (within the prior year) 1, 2
• Consider hip radiographs if knee X-rays are unremarkable, as hip pathology commonly refers pain to the knee 1, 3
• Consider lumbar spine radiographs if knee X-rays are unremarkable and clinical evidence suggests spinal origin 1, 3

Required X-ray Views

Minimum Standard Views

• Obtain at minimum two views: anteroposterior (AP) and lateral (with knee at 25-30 degrees flexion) 1, 2
• For chronic knee pain, include at least one frontal projection (AP, Rosenberg, or tunnel view), a tangential patellar view, and a lateral view 1

Additional Views to Consider

• Patellofemoral (skyline) view: for suspected patellar fractures, subluxation, or dislocation 1
• Weight-bearing tunnel view (PA in flexion): significantly improves detection of joint space narrowing in both medial and lateral compartments compared to AP alone 4
• Cross-table lateral view: for specific clinical indications 2
• Internal and external oblique views: based on clinical suspicion 2

Key Evidence on View Selection

The combination of weight-bearing AP and tunnel views significantly increases detection of osteoarthritis features including joint space narrowing (p<0.001 lateral, p=0.006 medial), subchondral cysts, sclerosis, and osteophytes compared to AP alone 4. The lateral view is critical for visualizing quadriceps/patellar tendon injuries, anterior/posterior avulsion fractures, and soft-tissue pathology that may be obscured on other views 5.

When X-rays Are NOT the Initial Study

Significant Trauma with High-Risk Features

• For knee dislocation with suspected vascular injury, obtain CTA of the lower extremity (vascular injury occurs in ~30% of posterior knee dislocations) 1
• Conventional angiography is reserved for selected cases given the high accuracy of CTA 1

When to Proceed Directly to Advanced Imaging

• MRI is NOT routinely used as the initial study for acute trauma 1
• CT without IV contrast is NOT routinely used initially but is superior for fracture detection and classification 1
• MRI without contrast becomes appropriate after negative or equivocal X-rays when there is persistent pain, significant joint effusion, mechanical symptoms suggesting meniscal injury, or joint instability suggesting ligamentous injury 2, 3

Common Pitfalls to Avoid

• Do not skip X-rays and proceed directly to MRI in chronic knee pain—this occurs in 20% of cases inappropriately 1, 2
• Do not apply Ottawa Rules to patients with altered mental status, neuropathy, or gross deformity—these patients require X-rays regardless 1, 2
• Do not forget to image the patellofemoral joint—using only AP and lateral views without a patellar view misses patellofemoral pathology 1, 6
• Do not overlook referred pain from the hip or lumbar spine when knee X-rays are normal 1, 3
• Do not assume negative X-rays rule out all significant injuries—occult fractures and soft tissue injuries may still be present 2
• Physician judgment should supersede clinical guidelines when appropriate, with the goal of identifying fractures requiring immediate intervention while avoiding unnecessary radiation 2