Morphometric analysis of the foramen magnum: An anatomic study

Objective: To further elucidate the importance of anatomic variations in morphology of the foramen magnum and associated clinical implications, we conducted a morphometric study. Methods: Seventy-two dry skulls were used for this study. Digital images were obtained of the foramen magnum from an inferior view. These images were studied using a computer-assisted image analysis system. Next, an image processor was used to calculate pixel differences between 2 selected points, which allowed accurate translation of pixel differences into metric measurements. Results: We found that the mean surface area of the foramen magnum was 558 mm², the mean anteroposterior diameter was 3.1 cm, and the mean horizontal diameter was 2.7 cm. For comparison, surface areas were classified into 3 types based on size. Type I foramina were identified in 20.8% of the dry skulls (15 skulls) and exhibited a surface area of less than 500 mm². Type II (66.6%, 48 skulls) was applied to foramina of an intermediate size with surface areas ranging between 500 to 600 mm². Type III (12.5%, 9 skulls) was applied to large foramina with surface areas of more than 600 mm². Conclusion: These data may be of use as a morphometric database for description of "normal" variants of foramen magnum morphology.