Morphometric Analysis of the Foramen Magnum

The foramen magnum is morphometrically typified by two perpendicular diameters and a distinct margin. This important anatomical landmark is subject to high interindividual variation. The objective of this study was to evaluate the dimensions and morphology of the foramen magnum in a Jordanian population. In this retrospective study, 247 individuals were consecutively sampled using the institutional picture archiving and communication system. The anteroposterior and transverse diameters of the foramen magnum were measured; the foramen magnum index was calculated; and, the shape of the foramen magnum was visually assessed. The sex-pooled anteroposterior and transverse diameters were 35.1 +/- 3.2 mm and 29.3 +/- 2.5 mm, respectively. The sex-pooled foramen magnum index was 1.20 +/- 0.10. The diameters were significantly different (P<0.001) and positively correlated (r=0.52, P<0.001). Participant age was negatively correlated with the anteroposterior (r(s)=-0.15, P=0.02) and transverse (r(s)=-0.14, P=0.03) diameters. After controlling for participant age, there was a statistically significant difference in the anteroposterior diameter (P<0.001), the transverse diameter (P<0.001), and the foramen magnum index (P=0.02) between sexes. The foramen magnum was most commonly irregular in shape (36 % of all cases). In contrast to previous studies, we report, for the first time, a negative correlation between age and the diameters of the foramen magnum. In addition, a predominance of irregularly shaped foramen magnum was found. The finding makes an already heterogenous group of published data even more variable, prompting reconsideration of the role of visual assessment in morphometric studies.