Determination of ankle stability is straightforward when the injury involves both the medial and lateral malleolus. However it can be challenging when the medial injury involves the deltoid ligament. Radiographic diagnosis of ankle instability highly depends on the measurement of medial clear space. As the shape of talus has been postulated akin to a trapezoid, the medial clear space may be influenced by the portion of talus occupying the mortise. Hence the medial clear space may be influenced by the position of the ankle. We sought to evaluate the impact of ankle plantarflexion and division of the deltoid ligament on the medial clear space.

For the study 10 fresh-frozen cadaveric lower limbs were used. Mortise radiographs were taken at neutral, 15 and 30 degrees of plantarflexion and neutral external rotation. These measurements were repeated after dividing the deltoid ligament. To ensure consistent ankle position, the ankle was placed in a specially constructed rig, which recreated the above positions. The medial clear space and talar tilt were measured. Differences in the means between the groups were determined with the paired ‘t’ test and ANOVA within the groups. Statistical significance was set a p-value of 0.05.

Increasing the plantarflexion from neutral to 30 degrees in both groups resulted in increase in the medial clear space and talar tilt. The mean increase in medial clear space became statistically significant at 30 degrees when compared to neutral. Between the groups there was a significant difference in medial clear space at 30 degrees plantarflexion. Dividing the deltoid ligament also had a significant effect on talar tilt.

Plantarflexion has an influence on the medial clear space in ankle mortise views therefore pre and post ankle fixation radiographs must be interpreted with caution.