Ankle Injury: When to order Imaging
Case: a 25-year-old male presents with left ankle pain after rolling it while walking.
What is your differential for acute ankle pain?
I like to use the mechanisms framework to guide my diagnostic approach.
*List not exhaustive. (1)
BOTTOM LINE: You can think of anatomic mechanisms, reactive, and malignant mechanisms of ankle pain to guide the diagnostic process.
History & Physical Exam Tips
Consider the following:
Onset/duration: is it acute or chronic ankle pain? This will help narrow down your differential.
Location: is it malleolar, midfoot, or elsewhere?
Traumatic or non-traumatic: if traumatic, this helps narrow down whether it is an anatomic injury (e.g. fracture, sprain, strain, pinched nerve) or not (osteoarthritis, reactive, malignant) (1).
Systemic symptoms: pain, tenderness, redness, warmth, plus associated symptoms like fever, sore throat, rash, and other joint involvement may point to a reactive cause (e.g. bacterial infection, autoimmune condition). Weight loss, drenching night sweats, fatigue/malaise may point to a more sinister cause (malignancy) (1).
Physical exam: if traumatic —> the Ottawa Ankle Rules will help determine whether imaging is necessary for suspected fracture. Rheumtutor provides a great overview of ankle evaluation.
Ottawa Ankle Rules
Ankle and/or foot x-rays are indicated if there is clinical suspicion of a fracture (1,2).
The Ottawa Ankle Rules are helpful in determining whether imaging is needed following ankle trauma (2).
Purpose of the Ottawa Ankle Rules: Identify ankle or foot injuries that require X-rays, reducing unnecessary imaging (2).
Ankle X-ray is required if there is:
Pain in the malleolar zone and any of the following:
Bone tenderness at the posterior edge or tip of the lateral malleolus
Bone tenderness at the posterior edge or tip of the medial malleolus
Inability to bear weight both immediately and in the emergency department (4 steps)
Foot X-ray (midfoot/heel) is required if there is:
Pain in the midfoot zone and any of the following:
Bone tenderness at the base of the 5th metatarsal
Bone tenderness at the navicular bone
Inability to bear weight both immediately and in the ED (4 steps)
Key Points:
X-ray has high sensitivity for clinically significant fractures
Ottawa Ankle Rules helps avoid unnecessary X-rays
Only applies to patients ≥2 years and with acute injuries
BOTTOM LINE: the Ottawa Ankle Rules helps avoid unnecessary x-rays, only applies to acute injuries in patients aged >2. X-ray is good at detecting clinically significant fractures.
Back to the case: Pain is in the malleolar zone, with bone tenderness at the tip of the lateral malleolus. He is able to weight bare. What next?
Diagnostic Imaging: Ankle
Whether you choose to image the ankle or not is dependent on your suspected pathology. Further, the imaging modality you choose also depends on the suspected pathology.
Imaging Don’ts
If a patient presents with minor ankle injury with a negative examination using the Ottawa Ankle Rules, then imaging is not warranted (3).
If a patient presents with a suspected ankle sprain - this is largely a clinical diagnosis. Acute ankle sprains are graded by severity (Grade I = mild, Grade II = moderate, Grade III = severe) to guide management and prognosis (1).
Ankle X-ray
If a patient presents with a traumatic ankle injury with positive examination using the Ottawa Ankle Rules, imaging with x-ray is warranted. X-ray for ankle fractures has high sensitivity but low specificity. A study found that it is highly sensitive in detecting fractures (around 91-100%), meaning it's good at identifying when a fracture is actually present (3). However, it also tends to produce a high number of false positives, meaning it sometimes indicates a fracture when one isn't actually there (3). Consider X-ray if:
Suspected fracture using Ottawa Ankle Rules —> initial imaging modality
Joint effusion: x-rays are sensitive for effusions >5 mL, with US and MRI being more sensitive for smaller effusions —> can use as initial imaging modality (4)
Ankle Ultrasound
Offers an accessible cost-effective method for evaluating ligaments, tendons, and joint effusions; it can assess ligament laxity in real time, but is heavily operator-dependent (1). It can also help guide injections or aspiration of fluid from the ankle joint.
Ultrasound for ankle ligament injuries demonstrates high sensitivity and specificity, particularly for the anterior tibiofibular ligament (ATFL) and the calcaneofibular ligament (CFL) (5). Consider US if:
Suspected ligamentous, tendon, or other soft tissue injury if pain persists >6 weeks despite appropriate conservative treatment (rest, ice, physiotherapy, etc.). (1)
Joint effusion: US and MR are more sensitive for smaller effusions (4)
Ankle CT
CT provides a detailed evaluation of complex fractures, suspected osteochondral injuries, or sydnesmotic injuries (1). CT scans are highly sensitive and specific for detecting ankle fractures, including non-displaced fractures, with reported sensitivity ranging from 95% to 100% and specificity from 95% to 100% (6). This means CT scans are accurate in both identifying fractures when they are present and correctly ruling them out when they are absent. Consider CT if:
Suspected occult fracture if there is an ankle effusion in the setting of normal x-ray, ongoing pain or inability to weight bear (7)
Suspected stress fracture - negative x-ray, ongoing symptoms despite conservative treatment (7)
Ankle MR
MR is the most sensitive imaging test for assessing soft tissue injuries, including ligamentous tears, osteochondral defects, tendon pathology, and syndesmotic injuries (1). Consider MR if:
Suspected occult fracture or stress fracture: Pain persists >6 weeks despite appropriate conservative treatment (rest, ice, physiotherapy, etc.) (7)
High ankle sprain suspected
Concerns for osteochondral injuries or occult fracture
Joint effusion: US and MR are more sensitive for smaller effusions (4)
If planning for surgical referral or planning for ligamentous surgical repair
BOTTOM LINE: Use imaging judiciously for ankle injuries: X-ray for suspected fractures per Ottawa Ankle Rules, ultrasound for persistent ligament/tendon issues, CT for complex or occult fractures, and MRI for detailed soft tissue injury or surgical planning.
Back to the case: using Ottawa Ankle rules, an x-ray of the ankle is ordered (Including views of the medial malleolus/tibia).
Tip: medial malleolus —> tibia; lateral malleolus —>fibula.
Results: Stable, non-displaced fracture of the distal fibula, weber A classification.
Conclusion: non-displaced fractures generally require immobilization with a cast or brace, with weight-baring restrictions and other conservative management (physiotherapy, pain control). Displaced or unstable fractures generally require surgery. When in doubt - consult orthopedics!
Want to learn more about ankle x-ray interpretation, like spotting an unstable vs. stable fracture? 👉Join NP Reasoning Masterclass to watch the latest webinar on-demand that covers this topic in depth, to increase your confidence when managing traumatic ankle injuries.
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