Postnatal Change in Angle between the Tympanic Annulus and Surrounding Structures

Abstract

Postnatal developmental relationships in human ears were studied by a computer-aided three-dimensional reconstruction and measurement method. We measured the angle, in reference to the horizontal plane, between the tympanic annulus, the oval window, and the internal auditory canal (IAC) in 20 normal temporal bones obtained from individuals between 1 day old and 76 years old. The horizontal plane was defined as the plane sloped 30° infero-anteriorly from the plane of the horizontal canal in each specimen. The plane of the tympanic annulus changed from a nearly horizontal orientation (34.2° from the horizontal plane) in neonates to a more vertical orientation (63.3° from the horizontal plane) in adults. The tympanic annulus and oval window planes remained at the same angle to each other (11.9° ± 5.1°) throughout postnatal development, as did the plane of the tympanic annulus and the LAC (68.6° ± 5.3°). These findings have implications for ear surgeons, especially those operating on young children.