Sprained Ankle – Do You Need a Scan & How Do We Decide Which One?

It sounds alarming, but a crack or pop does not reliably tell us whether the ankle is fractured. One study of 464 ankle injuries found that hearing or feeling a crack did not predict fracture. Ligaments, tendons and joint movement can also produce an audible sound.

The decision regarding ankle sprain imaging should therefore be based on assessment – including the location of bone tenderness, ability to bear weight & the Ottawa Ankle Rules.

The first question is not “Should we order an X-ray, ultrasound, MRI or CT?”

It is “Will the imaging result change what we do?” Imaging may be valuable if it could change:

• if weight bearing is permitted
• +/- immobilisation is required
• whether orthopaedic or specialist referral is needed
• the rehabilitation program or expected recovery time
• return to sport decisions

If the clinical assessment indicates an uncomplicated ankle sprain and imaging would not alter treatment, an immediate scan may not be necessary.

When a fracture, syndesmosis injury, osteochondral injury, tendon injury or another important diagnosis is suspected, imaging may directly affect management. The imaging choice should be selected according to that specific concern.

The Ottawa Ankle Rules then help answer a narrower question: does this acute injury require an ankle or foot X-ray to investigate a possible fracture?

The only way to answer this is with a thorough clinical assessment. This includes examining:

• specific location of pain and tenderness are located
• ability to bear weight
• injury mechanism – how it happened
• location & degree of swelling & bruising
• ankle stability – testing the laxity of the ankle ligaments
• suspect the syndesmosis is injured
• tendons around the ankle
• ankle joint and nearby bones

A systematic review found that the Ottawa Ankle Rules were approximately 98% sensitive for excluding ankle and midfoot fractures in adults.

Their main purpose is to reduce unnecessary X-rays while maintaining a very low risk of missing a clinically important fracture.

A positive result does not necessarily mean a fracture is present—it means an X-ray should be considered.

An ankle X-ray is generally indicated when there is pain around the malleoli – the bony prominences on either side of the ankle + at least one of the following:

• tenderness along the posterior edge or tip of the lateral malleolus
• tenderness along the posterior edge or tip of the medial malleolus
• inability to take four steps both immediately after the injury and during the assessment

A foot X-ray is generally indicated when there is pain in the midfoot + at least one of the following:

• tenderness over the base of the fifth metatarsal
• tenderness over the navicular
• inability to take four steps both immediately after the injury and during the assessment

• most ankle fractures
• fracture displacement
• ankle joint alignment
• avulsion fractures (where bone is pulled away)
• syndesmosis widening or instability (this is picked up with weightbearing x-rays or MRI)

• ligament injuries (cannot see ligaments on x-ray)
• syndesmosis injury (high ankle sprain – often missed)
• tendon injuries (cannot see tensons on x-ray)
• osteochondral lesions (damage that affects both the articular cartilage (smooth surface of bone) + underlying bone
• bone bruising (oedema)
• every subtle fracture

Ultrasound can provide detailed imaging of the superficial structures. It can also assess tissues dynamically while the ankle is moved or stressed. It is accessible, relatively inexpensive and does not use ionising radiation (radiation that is dangerous to human tissue in high doses)

It may be useful for assessing:

• ankle ligaments & tendons
• superficial fluid collections
• some components of the syndesmosis

Its limitations are equally important. This examination relies heavily on the skill of the operator – training, technique and experience. Ultrasound is also less suitable for assessing:

• bone bruising (oedema)
• articular cartilage
• the complete extent of an osteochondral lesion
• deep joint structures
• complex or multiple injuries across the ankle

Therefore, an ultrasound showing a lateral ligament tear does not necessarily exclude an associated bone, cartilage, tendon or syndesmotic injury.

Ultrasound is most valuable when the clinician already has a specific superficial structure or dynamic problem they need to investigate. It is not automatically the best general-purpose scan for every swollen ankle.

MRI does not use ionising radiation and can assess bone marrow, cartilage, ligaments, tendons, muscles and the syndesmosis in one examination.

It may be considered when:

• pain remains significant despite normal X-rays
• the patient is not progressing as expected
• a syndesmosis injury is suspected
• there is deep ankle pain, catching or locking
• an osteochondral injury is suspected
• bone bruising or an occult fracture remains possible
• a tendon injury requires more complete assessment
• multiple structures may be injured
• the diagnosis will change rehabilitation, immobilisation or return to sport planning

MRI is particularly valuable when the clinical picture does not fit a straight forward lateral ankle sprain.

That does not mean every sprained ankle requires an MRI. Many uncomplicated ankle sprains can be diagnosed clinically and managed without advanced imaging. MRI is most useful when there is diagnostic uncertainty or when the result is likely to alter management.

Cost, availability, scan duration and Medicare eligibility can also affect access in Australia.

CT uses X-rays to generate detailed cross sectional & 3D images. It is particularly effective for cortical bone and fracture anatomy. It does have a significant dose of ionising radiation.

CT may be appropriate for:

• A suspected occult (broken bone that cannot be seen on a standard X-ray – “hidden”)
• A complex fracture seen on X-ray
• Defining fracture displacement or joint involvement
• Pre-operative planning
• Detailed assessment of the bony component of an osteochondral lesion
• Selected chronic syndesmosis or instability presentations

CT provides less direct information than MRI about ligaments, tendons, cartilage and bone marrow. It should therefore be selected when the main clinical question concerns bone detail or alignment – not as a substitute for MRI.

MRI and ultrasound do not use ionising radiation. X-ray and CT do.

An extremity X-ray has a very low effective dose – typically less than 0.001 mSv, although the actual dose varies. That is broadly comparable with less than several hours of normal background radiation (average Australian background radiation as approximately 1.7 mSv per year).

Ankle CT doses are also much lower than CT examinations of the head, chest, abdomen or pelvis because the scanned area is smaller and contains fewer radiation sensitive organs. However, the dose varies considerably according to:

• scanner & imaging protocol
• the length of the region scanned
• patient size
• conventional, low-dose or cone-beam CT is used
• one or both ankles are examined

Published ankle CT estimates range from approximately 0.01 mSv with some ultra-low-dose protocols to around 0.05 mSv or higher with other protocols. These figures should be treated as estimates rather than fixed doses.

The correct principle is not to avoid every investigation involving radiation. It is to select the lowest dose examination that can reliably answer the relevant clinical question.

References

Agency for Clinical Innovation. (2023). Ottawa ankle rules (adult). NSW Health.

American College of Radiology. (2023). ACR Appropriateness Criteria®: Acute trauma to the ankle.

Australian Radiation Protection and Nuclear Safety Agency. (n.d.). What is background radiation?

Bachmann, L. M., Kolb, E., Koller, M. T., Steurer, J., & ter Riet, G. (2003). Accuracy of Ottawa ankle rules to exclude fractures of the ankle and mid-foot: Systematic review. BMJ, 326(7386), 417. https://doi.org/10.1136/bmj.326.7386.417

Gomes, Y. E., Chau, M., Banwell, H. A., & Causby, R. S. (2022). Diagnostic accuracy of the Ottawa ankle rule to exclude fractures in acute ankle injuries in adults: A systematic review and meta-analysis. BMC Musculoskeletal Disorders, 23, Article 885. https://doi.org/10.1186/s12891-022-05831-7

Manske, R. C., Wolfe, C., Page, P., & Voight, M. (2024). Enhancing the diagnosis of lateral ankle sprains: The role of musculoskeletal diagnostic ultrasound in evaluating the ATFL and CFL. International Journal of Sports Physical Therapy, 19(2), 245–249. https://doi.org/10.26603/001c.92232

Polzer, H., Kanz, K.-G., Prall, W. C., Haasters, F., Ockert, B., Mutschler, W., & Grote, S. (2012). Diagnosis and treatment of acute ankle injuries: Development of an evidence-based algorithm. Orthopedic Reviews, 4(1), Article e5. https://doi.org/10.4081/or.2012.e5

Radiological Society of North America, & American College of Radiology. (2025). Radiation dose from X-ray and CT exams. RadiologyInfo.org. https://www.radiologyinfo.org/en/info/safety-xray

Reid, P. M. (1996). The relevance of hearing a crack in ankle injuries. Journal of Accident & Emergency Medicine, 13(4), 253–254. https://pubmed.ncbi.nlm.nih.gov/8832350/

Stiell, I. G., Greenberg, G. H., McKnight, R. D., Nair, R. C., McDowell, I., & Worthington, J. R. (1992). A study to develop clinical decision rules for the use of radiography in acute ankle injuries. Annals of Emergency Medicine, 21(4), 384–390. https://doi.org/10.1016/S0196-0644(05)82656-3

Stiell, I. G., McKnight, R. D., Greenberg, G. H., McDowell, I., Nair, R. C., Wells, G. A., Johns, C., & Worthington, J. R. (1994). Implementation of the Ottawa ankle rules. JAMA, 271(11), 827–832. https://doi.org/10.1001/jama.1994.03510350037034