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 mm(2), 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 (115 skulls) and exhibited a surface area of less than 500 mm(2). Type II (66.6%, 48 skulls) was applied to foramina of an intermediate size with surface areas ranging between 500 to 600 mm(2). Type III (12.5%, 9 skulls) was applied to large foramina with surface areas of more than 600 mm(2). CONCLUSION: These data may be of use as a morphometric database for description of "normal" variants of foramen magnum morphology.