3. Distinguishing Lateral vs. Medial Malleolar Fractures on AP View

Why the AP View Alone Can Mislead

On a true AP ankle radiograph, the lateral malleolus (distal fibula) projects over the lateral aspect of the talus and sits slightly posterior to the medial malleolus. The medial malleolus (distal tibia) projects medially and slightly anterior. This inherent offset means that a fracture line on one side can be partially obscured by the other malleolus, by the talar dome, or by the overlapping tibiofibular shadow. The practical consequence: you must localize the fracture to a specific bone before you describe its pattern, and you must confirm localization with a second view (mortise or lateral) whenever the AP is equivocal.

This case walks through a structured comparison of lateral versus medial malleolar fractures on the AP view, using three discriminating features: fracture-line location and orientation, displacement direction of the distal fragment, and the side and distribution of soft tissue swelling. The goal is not to memorize a list but to build a repeatable localization routine you can apply to any AP ankle film.

The Three Discriminating Features

Before looking at any single film, fix the reference conventions. On a standard AP ankle view the patient's lateral side is on the viewer's left when the image is displayed in anatomical convention (as if facing the patient). Confirm this by locating the fibula: the thinner, more distal bone is lateral. If the fibula appears on the viewer's right, the image is either a left ankle displayed in anatomical convention or a right ankle displayed non-anatomically — verify with the marker before you commit to "medial" or "lateral."

Reading the AP View: A Localization Routine

Apply the following sequence to every AP ankle film. It takes under a minute and prevents the most common error — calling a fibular fracture "medial" because the swelling looks central.

1. Confirm side and projection. Locate the fibula and the marker. State "right" or "left" and "AP" explicitly before interpreting.
2. Trace the cortical outlines. Follow the lateral cortex of the fibula from proximal to distal, then the medial cortex of the tibia from the plafond down to the medial malleolar tip. A step, break, or lucent line is your fracture candidate.
3. Localize the line to a bone. Ask: does the line lie within the fibular shadow or the tibial shadow? If it crosses the tibiofibular overlap, trace it proximally and distally to see which cortex it interrupts.
4. Describe orientation. Transverse lines suggest avulsion or tension failure; oblique/spiral lines suggest rotational injury. Note the angle relative to the tibial long axis.
5. Assess displacement. Compare the distal fragment position to the proximal shaft. Measure lateral or medial translation in millimeters at the fracture gap. Note any talar shift — a medial clear space >4 mm on AP is a red flag for instability.
6. Map the soft tissue. Follow the skin line. Swelling is a soft-tissue shadow that bulges outward. Note whether it is centered over the lateral or medial malleolus, and whether it extends into the gutter.
7. Cross-check on mortise/lateral. If the AP is equivocal, the mortise view separates the malleoli and the lateral view shows posterior displacement. Never finalize a pattern from AP alone when the line is obscured.

Worked Example: Two Films, Two Patterns

Consider two teaching films (illustrative, not from a specific patient):

• Film A: A lucent line runs obliquely across the distal fibula, beginning at the level of the tibial plafond and exiting laterally. The distal fibular fragment is translated 3 mm laterally. Soft tissue swelling bulges over the lateral malleolus and extends into the lateral gutter. The medial clear space is 3 mm. Interpretation: lateral malleolar fracture, oblique pattern, minimal displacement, no talar shift — consistent with a Weber B-type injury. The medial side is intact on this view, but the deltoid must be assessed clinically.
• Film B: A transverse lucent line crosses the base of the medial malleolus, perpendicular to the tibial axis. The medial fragment is displaced 2 mm medially. Swelling is centered over the medial malleolus. The lateral malleolus appears intact, but the lateral gutter is widened. Interpretation: medial malleolar fracture, transverse pattern, with lateral gutter widening suggesting a possible syndesmotic or lateral ligament component — the mortise view is mandatory before calling this an isolated medial injury.

Notice that in both films the swelling side matched the fracture side, and the orientation (oblique vs. transverse) matched the expected mechanism. These two features, taken together, are more reliable than either alone.

Common Pitfalls and How to Avoid Them

• Calling a fibular fracture "medial" because the swelling looks central. Swelling follows the skin, not the bone. Trace the cortical break to its bone before naming the side.
• Missing a medial malleolar fracture hidden behind the talus. The medial malleolus overlaps the talar dome on AP. If the medial clear space is widened or the medial cortex is irregular, get a mortise view.
• Ignoring the lateral gutter. A widened lateral gutter on AP (distance between the fibula and the talus) suggests syndesmotic injury even when the fibular fracture looks benign. Measure it and correlate with the mortise view.
• Describing displacement without a reference. Always state the direction relative to the proximal shaft (e.g., "distal fragment translated 3 mm laterally") and note any talar shift separately.
• Forgetting the posterior malleolus. A "double contour" on the AP view — two cortical lines where there should be one — often signals a posterior malleolar fracture. Confirm on the lateral view.

Quick Reference: Localization Checklist

Use this checklist on every AP ankle film until the sequence becomes automatic. The next step — classifying the fibular fracture by its level relative to the syndesmosis — builds directly on the localization you have just performed.