Morphometry of the outlet of the foramen magnum in crania with atlantooccipital fusion.

OBJECTIVE

Assimilation of the atlas to the occiput may result in symptoms that are often compressive in nature around the outlet of the foramen magnum. The aim of the present study was to elucidate the morphological features of the bone through this foramen.

METHODS

Thirteen adult skulls with atlantooccipital fusion underwent morphometrical analysis of the outlet of the foramen magnum.

RESULTS

All specimens but one were found to have a decreased area of the outlet of the foramen magnum. In those 12 specimens, a decrease of 15%-35% was seen. Fusions of the atlas that were based primarily along the anterior rim of the foramen magnum resulted in more obstruction of its outlet. In general, the horizontal diameters of the outlet of these foramina were more decreased from the normal range.

CONCLUSIONS

These findings demonstrate that in the majority of cases, assimilation of the atlas to the occiput results in a compromised outlet of the foramen magnum.