Learning Objectives:

• • Variations in normal tegmen and inner ear anatomy.

• • Surgical considerations when operating near the tegmen.

Introduction: The tegmen is a thin plate of bone that separates the mastoid and middle ear cavity from the intracranial compartment. The tegmen has a very variable shape, and complications may arise when operating near the tegmen. Notably, the dura may be exposed, and if this is gone unnoticed, serious intracranial complications may result, including cerebrospinal fluid leakage and neural tissue damage. One important risk factors for dural complications is low placement of the tegmen. The purpose of this study was to determine the radiographic location of the tegmen tympani in relation to the lateral semicircular canal in adult patients with normal temporal bones.

Methods: Patients who underwent high resolution temporal bone CT scanning as part of their workup for hearing loss were examined retrospectively. We included adult patients that had normal temporal bone anatomy and no previous ear surgery. The distance between the lateral semicircular canal and the lowest point of the tegmen tympani was measured in both the sagittal and coronal planes.

Results: A total of 100 temporal bones were assessed. The mean tegmen height was 4.1 mm in the cornal plane and 2.5 mm in the sagittal plane. The measured tegmen heights demonstrated a unimodal distribution with some variance.

Conclusions: Our results demonstratethat there is generally one average tegmen height, with a range of variation around this point. Based on this finding, we propose a limited tegmen height classification scheme. Tegmens below 4.5 mm on coronal measurement and 2.5 mm on sagittal measurement are considered “low” (type A) whereas tegmens above these parameters are considered “high” (type B). This classification system might have implications in prognosticating patients undergoing middle ear surgery using preoperative temporal bone CT.