What investigations are indicated for a patient presenting with limb pain?

Diagnostic Workup for Limb Pain

Begin with plain radiographs of the affected limb as the initial imaging study, as this is the most cost-effective and widely available test that can detect fractures, bone lesions, arthropathy, and degenerative changes. 1, 2

Initial Clinical Assessment

Before ordering any tests, obtain specific details about:

• Pain characteristics: Timing, pattern, severity, and whether pain is localized or diffuse 1
• Trauma history: Even minor falls can cause occult fractures, particularly in children 1, 3
• Red flag symptoms: Fever >101.3°F (38.5°C), night pain, unintentional weight loss, inability to bear weight, or systemic symptoms suggest serious pathology requiring urgent evaluation 1, 3, 4
• Age-specific considerations: Differential diagnosis varies significantly by age, with toddler's fractures common in young children and osteoarthritis predominant in older adults 1

Laboratory Testing

Order inflammatory markers (ESR, CRP) and complete blood count if infection is suspected based on fever, systemic symptoms, or inability to bear weight. 1, 3

• Elevated white blood cell count, ESR, or CRP suggests infection (septic arthritis or osteomyelitis) 1
• These tests help risk-stratify patients for urgent intervention 3

Imaging Algorithm

First-Line Imaging

Obtain plain radiographs (X-rays) of the symptomatic region as the initial imaging study. 1, 2

• Radiographs detect fractures, bone lesions, arthropathy, joint space narrowing, and degenerative changes 1, 2
• Include at least two orthogonal views of the affected area 1
• Critical pitfall: Normal initial radiographs do not exclude fracture—approximately 10-41% of occult fractures (especially toddler's fractures) only become visible on follow-up films obtained 7-10 days later 1, 3, 4

When to Advance Beyond Radiographs

If radiographs are normal but symptoms persist or worsen, or if red flags are present, proceed to MRI without IV contrast as the next imaging study. 1

MRI is indicated when:

• Initial radiographs are normal but pain persists beyond 7-10 days 1
• Red flag symptoms are present: fever with normal radiographs, night pain, inability to bear weight, or elevated inflammatory markers 3, 4
• Clinical concern for soft-tissue pathology, stress fracture, osteomyelitis, bone marrow edema, or early inflammatory changes 1
• Suspected nerve entrapment syndromes (e.g., Baxter's neuropathy) 1

Ultrasound Considerations

Ultrasound is appropriate for evaluating joint effusions, soft-tissue fluid collections, and specific conditions like plantar fasciitis, but has limited utility when symptoms cannot be localized due to small field of view. 1

• Hip ultrasound is specifically indicated when fever and inability to bear weight suggest septic arthritis 1, 3, 4
• Useful for detecting joint effusions and guiding aspiration 1, 3

Specialized Testing

Three-phase bone scan may localize pathology when examination is nonfocal, initial imaging is negative, and symptoms persist, though it lacks specificity. 1

CT without IV contrast is reserved for specific indications such as evaluating complex fractures or bony anatomy when MRI is contraindicated. 1

Special Populations and Considerations

Children Under Age 5

• Obtain bilateral tibia/fibula radiographs initially, as spiral tibial fractures are the most common cause of limb pain in this age group 1, 3
• Consider hip/pelvis radiographs simultaneously, as hip pathology presents as referred thigh or knee pain in up to 30% of cases 4
• Emergency evaluation required: Fever >101.3°F with refusal to bear weight mandates urgent hip ultrasound to exclude septic arthritis, which causes permanent joint damage within hours 1, 3, 4

Adults with Chronic Pain

• Consider referred pain sources: Hip pathology can present as knee pain; lumbar spine pathology can present as lower extremity pain 1
• Obtain hip or lumbar spine radiographs if knee radiographs are unremarkable and clinical examination suggests referred pain 1

Vascular Assessment

If peripheral arterial disease is suspected based on risk factors (age >50, diabetes, smoking) or examination findings (diminished pulses, skin changes), obtain ankle-brachial index (ABI) before any compression therapy. 1, 5

• The majority of patients with peripheral arterial disease are asymptomatic or have atypical leg pain rather than classic claudication 1, 5
• ABI <0.6 contraindicates compression therapy; ABI 0.6-0.9 requires adjusted compression 5

Critical Pitfalls to Avoid

• Never assume normal radiographs exclude fracture in the first 7-10 days—10-41% of occult fractures only appear on follow-up imaging 1, 3, 4
• Never delay evaluation of fever with inability to bear weight—septic arthritis requires diagnosis and treatment within hours to prevent permanent damage 1, 3, 4
• Never focus only on the reported pain site in children—young children frequently mislocalize pain, and hip pathology commonly presents as thigh or knee pain 4
• Never order MRI as the initial study without first obtaining radiographs—approximately 20% of patients receive unnecessary MRI without recent radiographs 1