Aim: Patients with Chiari I malformations (CM1) have posterior fossa hypoplasia with crowding of the neural structures. We aimed to study the posterior fossa measurements to analyse the posterior fossa morphology, presence of basilar invagination (BI) and correlated the measurements with the degree of tonsillar descent.
Methods: We retrospectively reviewed data in170 patients who underwent foramen magnum decompression (FMD) for CM1. The posterior fossa measurements were compared with 30 controls. Patients were divided into those with and without BI. The variables measured included clival length, posterior fossa height, supraocciput length, posterior fossa diameter, foramen magnum width, Boogard's angle, clival angle, clival slope and the newly introduced foramen magnum (FM) angle.
Results: The average clivus length and posterior fossa height were significantly shorter with a significant increase in the Boogard's and FM angle in the patient groups. Tonsillar descent showed a negative correlation with posterior fossa height (r = -0.498, p ≤ 0.001) and clivus length (r = -0.325, p ≤ 0.001) and a positive correlation with Boogard's angle (r = 0.469, p ≤ 0.001) and FM angle (r = 0.330, p ≤ 0.001). Patients with BI had statistically significant reduced posterior fossa height (p ≤ 0.001) and increased extent of tonsillar herniation (p = 0.001) compared to patients without BI.
Conclusion: Patients with CM1 have significantly shorter clival length and posterior fossa height with smaller posterior fossa in support of published data. The presence of BI shortens the posterior fossa height and worsens the extent of tonsillar herniation. An increased Boogard's angle and FM angle result in a more horizontally placed suboccipital bone compared to a slanting bone in normal persons.
Keywords: Chiari I malformations; basilar invagination; morphometry.