Initial Diagnostic Approach for Knee Pain
Plain radiographs (X-rays) are the mandatory first-line imaging study for evaluating knee pain, whether acute traumatic or chronic, and should include at minimum anteroposterior and lateral views. 1, 2
When to Order Knee X-rays
For Acute Trauma
Apply the Ottawa Knee Rules to determine imaging necessity—order radiographs if ANY of the following are present: 1, 2, 3
- Age >55 years
- Focal tenderness at the patella (not diffuse)
- Focal tenderness at the fibular head
- Inability to bear weight for 4 steps immediately after injury and in the examination room
- Inability to flex the knee to 90 degrees
Override clinical decision rules and obtain radiographs immediately if the patient has: 1, 2
- Gross deformity
- Palpable mass
- Penetrating injury
- Prosthetic hardware
- Altered mental status (head injury, intoxication, dementia)
- Neuropathy (paraplegia, diabetes)
- Significant trauma mechanism (motor vehicle accident, knee dislocation)
For Chronic Knee Pain
Always obtain radiographs as the initial imaging study before considering advanced imaging, as approximately 20% of patients with chronic knee pain undergo MRI without recent radiographs within the prior year. 1
Required Radiographic Views
Standard knee series must include at minimum: 1, 2
- Anteroposterior view (or Rosenberg/tunnel view for chronic pain)
- Lateral view with knee flexed at 25-30 degrees
Add these views based on clinical suspicion: 1, 2
- Patellofemoral (tangential patellar) view for suspected patellar fractures or subluxation/dislocation
- Cross-table lateral view for specific indications
- Internal and external oblique views when indicated
What X-rays Detect
Plain radiographs effectively identify: 4, 3
- Fractures (including avulsion fractures)
- Joint effusions
- Degenerative changes and osteoarthritis
- Osteochondral defects
- Chondrocalcinosis
- Loose bodies
- Signs of prior chronic injury (Segond fracture, tibial spine avulsion)
When to Proceed to Advanced Imaging
MRI Without Contrast (Most Common Next Step)
Order MRI if radiographs are normal or show only effusion AND any of the following persist: 1, 2, 5
- Significant joint effusion
- Inability to fully bear weight after 5-7 days
- Mechanical symptoms suggesting meniscal injury (locking, catching)
- Joint instability suggesting ligamentous injury
- Persistent pain despite conservative management
MRI is superior for detecting: 1
- Meniscal tears (sensitivity 87%, specificity 92%)
- Ligamentous injuries (ACL sensitivity 74%, specificity 95%)
- Bone marrow contusions and occult fractures
- Subchondral insufficiency fractures
- Articular cartilage damage
CT Scan
CT is NOT routinely used as initial imaging but is indicated for: 1, 2
- Better characterization of fractures detected on radiographs (superior to X-ray for classification)
- Evaluation of patellofemoral anatomy in chronic subluxation or maltracking
- Assessment of tibial tubercle-trochlear groove distance
- Detection of bony avulsion fractures (80% sensitivity, 98% specificity)
Critical Pitfalls to Avoid
Never skip radiographs even when clinical findings seem obvious, as they are essential to rule out fractures requiring immediate intervention and provide baseline assessment. 2, 5
Do not order MRI as the initial study for acute trauma—it is not routinely used first-line and is premature without recent radiographs. 1
Consider referred pain sources if knee radiographs are unremarkable—obtain hip radiographs or lumbar spine films when clinical suspicion warrants. 1
Recognize that negative radiographs do not exclude significant injury—soft tissue injuries, occult fractures, and internal derangement may still be present, requiring clinical follow-up and potential MRI at 5-7 days if symptoms persist. 2, 5
Avoid unnecessary advanced imaging—bone scans, SPECT/CT, MR arthrography, and ultrasound are not routinely indicated for initial evaluation of knee pain. 1