Value of MRI measurement of depths of the anterior and posterior lips of the foramen magnum in the diagnosis of basilar invagination

Objective: To investigate the depth of the anterior lip of the foramen magnum (FMAD) and the depth of the posterior lip of the foramen magnum (FMPD) by MRI, and to study their clinical significances in the diagnosis of basilar invagination. Methods: FMAD and FMPD on midsagittal SE T1WI were measured in 320 normal subjects and in 36 patients with basilar invagination. The ratio between FMAD and FMPD was calculated. The sex and age differences in the normal group and the differences between the patients and normal subjects were analyzed. Results: (1) There was no significant sex difference in FMAD in 0-9 year group, but FMAD of the male was bigger than that of the female in all age groups after 10 years (t=5.54, 6.58, 5.92, 7.13, 6.32, 5.14, and 4.48, respectively, all P<0.05); FMAD in 0-9 year group was less than that in all age groups after 10 years in the same sex (F=3.66, 4.45, 6.32, 4.08, 5.78, 7.13, and 5.99, respectively in the male, all P<0.05; F=3.88, 5.75, 6.78, 6.75, 5.96, 6.98, and 7.08, respectively in the female, all P<0.05), but there was no significant difference in FMAD among different age groups after 10 years. It ranged from 25.93 mm to 27.27 mm in patients and from 34.53 mm to 35.32 mm in normal subjects. There was no overlap between them. (2) There was no significant sex and age differences in FMPD in the control group. It ranged from 33.17 mm to 35.63 mm in patients and it ranged from 32.83 mm to 33.64 mm in normal subjects. There was overlap between them. (3) There was no significant sex and age differences in FMAD/FMPD in the control group; It ranged from 0.72 to 0.80 in patients and from 1.05 to 1.08 in normal subjects. There was no overlap between them. Conclusion: (1) The basilar invagination can be diagnosed when FMAD is less than 27 mm or FMAD/FMPD is less than 0.8. (2) There is no significant difference in FMPD between patients and normal subjects. (authors)