Variation of the Distal Metatarsal Articular Angle with Axial Rotation and Inclination of the First Metatarsal

Background: The distal metatarsal articular angle (DMAA) is a radiographic measure of the orientation of the first metatarsal articular surface. There is debate with regards to the accuracy, reproducibility, and validity of measurement of the DMAA in the literature. This study aimed to test the validity of the measurement of the DMAA from standard radiographs, to explore the trigonometric relationship of the first metatarsal rotation and the DMAA, and to assess interobserver reliability.

Methods: Thirty-four separate dry cadaver first metatarsal bones were mounted onto a customized light-box-protractor, allowing controlled incremental changes in rotation and inclination. A series of 39 digital photographs were taken of each metatarsal in 5-degree increments of rotation between 30 degrees supination and 30 degrees pronation at 10, 20 and 30 degrees of inclination. Three reviewers performed blinded DMAA measurements from each photographic image; the data were collated for statistical analysis. The data were analysed using a mixed effects linear model comparing the DMAA with rotation of the first metatarsal.

Results: A strong statistically significant trend of increasing score with increasing pronation of the metatarsal was observed, the relationship was approximately linear. There was a strong effect of inclination, but the strength of this varied with rotation and was amplified at higher inclinations. Interobserver error was noted in line with other studies, but even allowing for this interobserver error, the linear relationship was maintained.

Conclusions: This study showed that the DMAA varies significantly in a linear pattern with axial rotation of the first metatarsal. Inclination of the first metatarsal also affects the magnitude of the angle. This study does not refute the DMAA as an entity but does confirm the inaccuracy of extrapolating the DMAA from plain anteroposterior radiographs.