How can you determine the side of the cassette on a lateral foot x-ray (x-radiograph)?

Determining Cassette Side on Lateral Foot X-rays

On a lateral foot radiograph, the side against the cassette (plate-down side) can be determined by identifying which toes appear more inferior: when the 5th toe is down (standard positioning), the medial side is against the cassette, whereas when the great toe is down, the lateral side is against the cassette. 1

Primary Radiographic Indicators

Sesamoid Visualization

• When the great toe is positioned down (lateral-side-down), the hallux sesamoids are typically better visualized and more clearly separated on the lateral projection. 1 This occurs because the medial structures are farther from the cassette and therefore magnified with less geometric distortion.

Standard Positioning Protocol

• Standard lateral foot radiographs are routinely obtained with the medial side down (5th toe against the plate), which is the most common positioning for routine foot imaging. 1 This is the expected orientation you should encounter in clinical practice.
• The overall foot architecture and tibiotalar alignment should be properly demonstrated regardless of which side is down, but recognizing the standard positioning helps confirm proper technique. 1

Clinical Context and Practical Application

Weight-Bearing Considerations

• When weight-bearing lateral radiographs are obtained, consistent positioning becomes essential for detecting subtle malalignment between serial studies. 1 Inconsistent cassette-side positioning between examinations can make comparison difficult.

Common Pitfall to Avoid

• Do not confuse laterality markers (the "R" or "L" annotations on the image) with cassette-side determination. 2 These markers indicate which foot is being imaged (right vs left), not which side is against the cassette. AI studies have shown that laterality markers can be misleading confounding features that don't reflect actual anatomic positioning. 2

Algorithmic Approach

1. First, identify which toes appear more inferior on the image
2. If the 5th toe is down → medial side is against cassette (standard positioning)
3. If the great toe is down → lateral side is against cassette (non-standard positioning)
4. Confirm by checking sesamoid visualization: better separation suggests great-toe-down positioning 1