Morphology of the human tympanic membrane annulus

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Objective To study the full panoramic view with figuring of the morphology and topography of the human tympanic annulus. Study Design Postmortem material analysis. Setting University-affiliated hospital. Subjects and Methods Twenty-three single, normal human adult tympanic membranes were completely extracted from formalin-fixed temporal bones. They were faced medially and placed at the same level of a graph paper mounted on a board. High-quality images of the tissue preparations were taken, and computer-aided measurements of the annular caliber were calculated at nine reference points. The 6 o'clock direction served as a midpoint, and another four reference points were set anteriorly and posteriorly in clockwise and counterclockwise directions. Results The annulus has a horseshoe-like shape with a small part absent above the neck of the malleus. The maximal mean caliber at the manubrial axis (6 o'clock direction) was 748 ± 201 μm. The annulus gradually thins out almost symmetrically anteriorly and posteriorly, until it reaches about 15 percent of the maximal caliber at its end points (152 ± 87 and 113 ± 42 μm, respectively). Significant differences were found between adjacent reference points on both anterior and posterior sides. Conclusions The annulus has a horseshoe-like shape and gradually thins out almost symmetrically, reaching anteriorly and posteriorly about 15 percent of the maximal caliber at the manubrial axis. These new data may provide guidance in transcanal middle ear exploration and suggest the possibility of varied functions attributable to the annulus regarding middle ear sound transmission and TM vibratory properties. The data may contribute to understanding the development of marginal perforations and posterior superior retraction pockets.