Position and Dimension of the Inner Ear in Midterm Fetuses

This study aimed to see the change in the area covered by the inner ear (IE) in the petrous bone with gestational age, and to determine the growth dynamics of inner ear structures such as the cochlea and semicircular canals.

Twenty temporal bones of 10 fetal cadavers (5 males and 5 females) aged 23.50 ± 2.94 weeks were included in the study.

The petrous ridge length (PRL), the inner ear length (IEL), the thickness of bone on the cochlea, the distance of the rearmost part of the superior semicircular canal to the most posterior border of the petrous ridge, and the distance of the frontmost part of the cochlea to the most anterior border of the petrous ridge were measured as 23.97 ± 4.21 mm, 13.31 ± 1.45 mm, 1.26 ± 0.20 mm, 6.86 ± 1.97 mm, and 3.85 ± 0.93 mm, respectively. The ratio of the IEL to the PRL decreased despite of proportional increase in these parameters with age. This finding shows that the growth dynamics of the IE are slower than that of petrous bone, and thus the area ratio covered by the inner ear in the petrous bone decreased with age. The size of the cochlea and the angles between the semicircular canals did not correlate with gestational age. However, the semicircular canals (their thicknesses and inner surface areas) attain adult size between 21 and 24 weeks.

Our findings may be useful for otologists to see the relation of the IE with the petrous bone. Our numeric dataset may form a basis of prenatal radiologic investigations.