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Cranial Base Structures Geometric Morphometry and Description of a Clinical Relevant Anatomical Variation

The foramen magnum is a large median oval opening in the occipital bone in the base of the skull, which leads into the posterior fossa. The foramen magnum has a forwarded shift of its anterior border in humans causing a shortening of the cranial base. The foramen is described as oval, wider posteriorly than anteriorly, and its antero‐posterior diameter exceeds the transversal one. Anteriorly, the margin of the foramen magnum is slightly covered, on each side, by an occipital condyle. These condyles, oval in shape, are located obliquely, so that their anterior end is closer to the median plane. Thirty dry skulls, from adult individuals, present in the anatomy laboratory of our institution were evaluated and a study of the dimensional anatomy of the foramen magnum and the occipital condyles was performed. With the aid of an analog caliper (precision of 0.05mm), the antero‐posterior and transverse diameters of the foramen magnum (posterior to the occipital condyles) and occipital condyles were obtained. Our results showed that the mean antero‐posterior diameter of the foramen magnum was 36.1 ± 0.5 mm and the transverse diameter was 30.0 ± 0.4 mm. The mean antero‐posterior diameter of the right occipital condyle was significantly smaller than that of the left, being 22.2 ± 0.4 mm and 23.1 ± 0.4 mm, respectively. The mean transverse diameter of the right occipital condyle was also statistically smaller than that of the left, being 12.6 ± 0.2 mm and 13.2 ± 0.2 mm, respectively. One of the skulls studied (excluded from the previous statistic) showed anatomical variation in the shape and size of the foramen magnum. This foramen was narrow, with an antero‐posterior diameter of 26.75 mm and a transverse diameter of 22.50 mm. In addition, the occipital condyles were asymmetrical, with the right condyle being practically transverse in relation to the foramen magnum, making a protrusion into its interior, contributing to the transverse narrowing of this foramen. The left condyle presented antero‐posterior (22.95 mm) and transversal (15.95 mm) diameters larger than the right (19.35 mm and 13.10 mm respectively). There were no signs of fractures or previous craniotomies in this particular skull. The present study reports the geometric morphometry of some striking structures at the base of the skull, in addition to describing an important anatomical variation of these structures. We did not find reports of significant difference between the right and left condylar diameters in the literature. The anatomy of the skull base is useful for estimating the race and sex of individuals in forensic medicine. Furthermore, geometric morphometry is an important tool in primatology and anthropology.