What is the best initial diagnostic test for a patient presenting with right ankle swelling and pain?

Best Diagnostic Test for Right Ankle Swelling and Pain

Plain radiography with three standard views (anteroposterior, lateral, and mortise) is the best initial diagnostic test for ankle swelling and pain when clinical evaluation suggests possible fracture or bony pathology. 1

Clinical Decision Framework

Step 1: Apply Ottawa Ankle Rules to Determine Need for Imaging

Obtain ankle radiographs immediately if ANY of the following are present: 1, 2

• Inability to bear weight immediately after injury
• Inability to take 4 steps in the clinic/emergency department
• Point tenderness over the posterior edge or tip of the lateral malleolus
• Point tenderness over the posterior edge or tip of the medial malleolus
• Point tenderness over the navicular bone
• Point tenderness over the base of the 5th metatarsal

The Ottawa Ankle Rules demonstrate 92-99% sensitivity for detecting ankle fractures, with <2% false-negative rate. 1 This validated clinical decision rule applies to adults and children ≥5 years of age. 1

Step 2: If Ottawa Ankle Rules Are NEGATIVE

Do not obtain radiographs initially. 1, 2 Instead:

• Manage conservatively with RICE protocol (rest, ice, compression, elevation) 2
• Use acetaminophen as first-line analgesia for 2-7 days 2
• Encourage early mobilization with semi-rigid brace 2
• Reassess in 4-5 days 2

This approach avoids unnecessary radiation exposure while maintaining diagnostic safety. 2

Standard Radiographic Protocol When Imaging Is Indicated

Three-View Series (Standard of Care)

Order anteroposterior, lateral, and mortise views. 1 The mortise view is obtained by internally rotating the foot 15-20 degrees. 1

Weight-bearing radiographs should be obtained when possible, as they provide critical information about fracture stability, particularly for malleolar fractures where medial clear space <4mm confirms stability. 1

Evidence on Two-View vs Three-View Series

While some older research suggested two views might be sufficient 3, 4, 5, the three-view series remains the standard because it detects significantly more fractures than two-view combinations. 3 A 2022 pediatric study confirmed that eliminating views results in statistically significant decreases in both accuracy and sensitivity for fracture detection. 6

What Radiographs Can Detect

Plain radiography effectively identifies: 1

• Fractures (including stress fractures)
• Osteoarthritis and degenerative changes
• Osteochondral abnormalities
• Ankle effusions (53-74% accuracy)
• Ossific fragments indicating ligamentous injury
• Calcified or ossified intra-articular bodies
• Evidence of prior trauma
• Periostitis adjacent to tenosynovitis

When Advanced Imaging Is Needed

MRI Indications (Not First-Line)

Consider MRI as a second-line study when: 1, 2

• Initial radiographs are negative but pain persists beyond 1 week
• Suspected osteochondral lesion not visible on radiographs
• Clinical suspicion for ligamentous tears, tendon abnormalities, or soft-tissue pathology
• Evaluation needed for cartilage integrity, bone marrow edema, or synovitis

MRI is the most comprehensive imaging modality for evaluating all anatomic structures including ligaments, tendons, cartilage, and bone, but it is not routinely used as the first study. 1

CT Indications (Not First-Line)

CT is not routinely used as first-line imaging but may be helpful for: 1

• Complex fracture evaluation in polytrauma patients
• Preoperative planning
• Visualizing subchondral cysts
• Problem-solving when radiographs are equivocal

Ultrasound Role (Limited)

Ultrasound is not routinely indicated as first-line imaging for acute ankle trauma with positive Ottawa Ankle Rules. 1 It may have utility for specific soft-tissue evaluation (Achilles tendon, plantar fascia) but does not replace radiography for initial assessment. 1

Special Clinical Scenarios Requiring Modified Approach

Atraumatic Swelling (No Trauma History)

For bilateral ankle swelling without trauma, consider systemic causes (venous insufficiency, heart failure) and imaging is often not required initially. 2 Unilateral atraumatic swelling warrants Doppler ultrasound to exclude DVT, especially in obese patients. 2

Chronic Ankle Pain (>6 Weeks Duration)

Radiography remains the initial imaging study even for chronic symptoms, as it can reveal osteoarthritis, stress fractures, osteochondral abnormalities, and other pathology. 1 Advanced imaging with MRI is reserved for cases where radiographs are normal but clinical suspicion remains high. 1

Critical Pitfalls to Avoid

Do not order radiographs when Ottawa Ankle Rules are negative – this leads to unnecessary radiation exposure and cost without improving outcomes. 1, 2

Do not skip the mortise view – the three-view series is standard because two-view combinations miss a statistically significant number of fractures. 3, 6

Do not obtain foot radiographs routinely with ankle films – in the presence of ankle injury, foot radiographs (except base of 5th metatarsal) have very low yield and should not be performed. 1

Maintain low threshold for DVT evaluation in patients with unilateral atraumatic swelling, particularly those with obesity or other risk factors. 2

Do not manipulate the ankle before radiographs unless there is neurovascular compromise or critical skin injury, as this may complicate management. 1