Ankle fractures on X‑ray
Ankle injuries are one of the commonest reasons for an emergency department visit, and most are sprains. The Ottawa ankle rules decide who needs a film. When there is a fracture, the key question is not just "is it broken?" but "is the ankle stable?" Stability depends on the ring of bone and ligament around the talus, so you need to read the fibula, the medial side and the mortise together. The Weber classification gives you a quick, practical way to describe the fibular fracture and estimate the risk to the syndesmosis.
1. Who needs an X‑ray? The Ottawa ankle rules
Ottawa ankle and foot rules
Ankle X-ray is required if there is pain in the malleolar zone and any of:
- bone tenderness along the posterior edge or tip of the lateral malleolus (the distal 6 cm);
- bone tenderness along the posterior edge or tip of the medial malleolus (the distal 6 cm);
- inability to bear weight for four steps, both immediately after the injury and in the emergency department.
Foot X-ray is required if there is pain in the midfoot zone and any of:
- bone tenderness at the base of the 5th metatarsal;
- bone tenderness at the navicular;
- inability to bear weight for four steps, both immediately and in the emergency department.
Sources: Stiell IG et al., JAMA 1993 (refinement and validation); Bachmann LM et al., BMJ 2003 (systematic review: sensitivity close to 100%).
Palpating the posterior edge of the malleolus, not just the front, is what makes the rule work: the anterior talofibular ligament attaches anteriorly, so anterior tenderness is typical of a sprain. The rules are sensitive but not very specific, so most patients X-rayed under them will not have a fracture. They should be used with caution in intoxicated patients, those with reduced sensation and those with distracting injuries. See foot fractures for the midfoot.
2. Views and the mortise
A standard ankle series is an AP, a mortise view (an AP with about 15–20° of internal rotation, so the fibula no longer overlaps the talus) and a lateral. The lateral should include the base of the 5th metatarsal, because that fracture can masquerade as an ankle sprain.
The mortise is the slot formed by the tibial plafond above and the two malleoli either side. The talus should sit centrally with an even joint space all the way round.
Key measurements (approximate)
| Measure | View | Normal | If abnormal |
|---|---|---|---|
| Medial clear space (medial malleolus to medial talus) | Mortise | about ≤4 mm, and equal to the superior clear space | Suggests deltoid ligament injury and lateral talar shift |
| Tibiofibular clear space | AP and mortise | about <6 mm | Suggests syndesmotic injury |
| Tibiofibular overlap | AP / mortise | about >6 mm on AP, or >1 mm on mortise | Reduced overlap suggests syndesmotic injury |
Thresholds vary between sources and with positioning. A classic cadaver study (Ramsey and Hamilton) found that just 1 mm of lateral talar shift reduced tibiotalar contact area by about 40%, which is why small shifts matter; the exact figure is debated.
3. The Weber classification
The Weber (Danis-Weber) classification describes the level of the lateral malleolus (fibular) fracture relative to the syndesmosis, the fibrous joint between distal tibia and fibula, which lies at about the level of the tibial plafond.
| Weber | Level of fibular fracture | Syndesmosis | Usual stability |
|---|---|---|---|
| A | Below the syndesmosis (below the plafond), often transverse | Intact | Usually stable, if the medial side is intact |
| B | At the level of the syndesmosis, typically oblique or spiral, running from anteroinferior to posterosuperior | May be partially injured | Variable: depends on the medial side (deltoid ligament or medial malleolus) |
| C | Above the syndesmosis | Disrupted | Unstable; usually surgical |
The more widely used formal classification in orthopaedics is the Lauge-Hansen system (based on foot position and force direction) and AO/OTA, but Weber is easy to apply on a film and is what you will hear most in emergency practice.
Pearl: Weber B is the decision point
An isolated Weber B fracture with a normal medial clear space is often stable. The same fracture with a widened medial clear space or a medial malleolar fracture is effectively bimalleolar and unstable. When the medial side is uncertain, a weight-bearing or stress view may be requested according to local practice.
4. Stability: the ring around the talus
Think of the ankle as a ring around the talus, made of the medial malleolus and deltoid ligament, the tibial plafond, the lateral malleolus and its ligaments, and the syndesmosis. A single break in the ring is usually stable. Two breaks (bony or ligamentous) make the ankle unstable and allow the talus to shift. That is why you always assess:
- the lateral malleolus (Weber level);
- the medial malleolus, and the medial clear space as a marker of deltoid ligament injury;
- the posterior malleolus (the posterior lip of the tibia), seen on the lateral view;
- the syndesmosis (tibiofibular clear space and overlap);
- talar position in the mortise.
5. Bimalleolar, trimalleolar, Maisonneuve and pilon
- Bimalleolar fracture: fractures of both the lateral and medial malleoli, or a lateral malleolar fracture with deltoid ligament rupture (a "bimalleolar equivalent", shown by a widened medial clear space). Unstable.
- Trimalleolar fracture: lateral, medial and posterior malleolar fractures. The posterior fragment is seen on the lateral view. Large posterior fragments may be fixed; CT is often used to assess them.
- Fracture-dislocation: the talus is out of the mortise. This needs urgent reduction to protect the skin and neurovascular supply, often before formal films if the foot is compromised.
- Maisonneuve fracture: a proximal fibular fracture combined with a medial malleolar fracture or deltoid rupture and disruption of the syndesmosis and interosseous membrane. The ankle film may show only medial clear space widening or an isolated medial malleolar fracture. If the medial side is injured and there is no lateral malleolar fracture, examine and X-ray the whole fibula.
- Pilon fracture: an axial loading injury (for example a fall from height) that drives the talus into the tibial plafond, causing comminuted intra-articular fractures. High soft-tissue risk; CT is used for planning. Look for associated calcaneal and spinal fractures.
6. Pitfalls and subtle injuries
- Base of 5th metatarsal fracture presenting as an "ankle sprain": check the edge of the lateral film. See foot fractures.
- Lateral process of the talus ("snowboarder's fracture"): a small fragment seen best on the mortise view; easily dismissed as a sprain.
- Talar dome osteochondral lesions: a subtle lucency or fragment on the dome of the talus. Often missed on initial films, MRI or CT shows them.
- Anterior process of the calcaneus fractures: seen on the lateral or an oblique foot view.
- Achilles tendon rupture: loss or distortion of the triangular Kager's fat pad behind the ankle on the lateral. It is a clinical (and ultrasound) diagnosis; the film is often normal.
- Accessory ossicles (os subfibulare, os trigonum) are smooth and corticated, unlike acute avulsions.
- Children: the distal tibial physis produces specific adolescent fractures, the Tillaux and triplane fractures. See Salter-Harris fractures.
- A poor mortise view can make the clear spaces impossible to judge. Say so rather than measuring an unreliable film.
Red flags: act now
Escalate immediately
- Fracture-dislocation with skin tenting, a pale or pulseless foot: reduce urgently according to local practice, then image.
- An open fracture.
- Widened medial clear space or talar shift: unstable ankle.
- Isolated medial injury with proximal fibular tenderness: Maisonneuve until proven otherwise.
- Pilon fracture or high-energy injury with severe swelling: watch for compartment syndrome.
What to focus on, by role
| Role | Priorities |
|---|---|
| Medical student | The Ottawa ankle rules, Weber A/B/C, and the medial clear space. |
| Nurse / ENP | Apply the Ottawa rules correctly (palpate the posterior edge). Recognise the 5th metatarsal base fracture. Check the skin and foot pulses. |
| Junior doctor | Decide stable vs unstable: Weber level plus the medial side and the mortise. Examine the proximal fibula. Recognise the fracture-dislocation that cannot wait. |
| Radiographer | A true mortise view and a lateral that includes the 5th metatarsal base. Add full-length tibia and fibula views when a Maisonneuve injury is possible. |
| Advanced / FRCR | Syndesmotic measurements, posterior malleolus size, talar dome lesions and lateral process fractures. |
Practise on real lower limb X-rays →
References
- Bachmann LM, et al. Accuracy of Ottawa ankle rules to exclude fractures of the ankle and mid-foot: systematic review. BMJ 2003;326:417. bmj.com
- Stiell IG, et al. Decision rules for the use of radiography in acute ankle injuries: refinement and prospective validation. JAMA 1993;269:1127–32.
- Radiopaedia. Weber classification of ankle fractures. radiopaedia.org
- Ramsey PL, Hamilton W. Changes in tibiotalar area of contact caused by lateral talar shift. J Bone Joint Surg Am 1976;58:356–7.
- British Orthopaedic Association. BOAST: the management of ankle fractures. boa.ac.uk
For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.