Ossification patterns of the tympanic facial canal in the human fetus and neonate.

Dehiscences in the bony facial canal are comparatively common in the human adult. The highest incidence occurs in the tympanic segment of the facial nerve near the region of the oval window. Thirty-three fetal temporal bones, ranging from 16 to 40 weeks' gestation, and four from 1, 2, 4 and 12 weeks' postpartum neonates, were studied to evaluate the normal patterns of ossification of the fallopian canal of the tympanic facial nerve segment in the human. The tympanic facial nerve segment elongates three-fold during this period (from 1 mm to 3 mm). The ossification starts at 21 weeks' gestation anteriorly from apical otic ossification centers and at 26 weeks from canalicular ossification centers near the stapedius muscle. The ossification proceeds in an anterior-to-posterior direction as two periosteal shelves of bone surround the facial nerve. The superior periosteal bony ledge contributes 75% of the circumference of the fallopian canal. The anterior ossification center forms over 83% of the fallopian canal length. The two centers fuse post partum near the region of the oval window. The anatomic location of the facial nerve, nerve branching, and neural vasculature precede ossification. In 80% of the paired temporal bones, this ossification pattern appears to be symmetrical. The patterns and incidence of bony dehiscences within the tympanic fallopian canal segment can be explained by these observations. This study demonstrates that fallopian canal dehiscences are not congenital anomalies, but variations of normal developmental anatomic processes.