Learning Objectives:

1. 1. Variations in normal tegmen and inner ear anatomy.

2. 2. Surgical considerations when operating near the tegmen.

Introduction: The tegmen is a thin, variable plate of bone that separates the mastoid and middle ear cavity from the intracranial compartment. Because of its location, serious complications such as cerebrospinal fluid leakage and neural tissue damage may arise when operating near the tegmen. One important risk factor for dural complications is low placement of the tegmen. This study aims to determine the radiographic location of the tegmen tympani using the lateral semicircular canal as a landmark in adult patients with normal temporal bones.

Methods: 100 high resolution temporal bone CT scans from patients worked up for hearing loss were examined retrospectively. We included scans from adult patients with 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. 60 patients with cholesteatoma having undergone mastoidectomy procedures within the past 6 years where also analyzed retrospectively.

Results: The mean tegmen height was 4.1 mm in the cornal plane and 2.5 mm in the sagittal plane. The measured heights demonstrated a unimodal distribution with some variance.

Conclusions: We propose a simple and practical tegmen 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 applied to preoperative temporal bone CT might influence planning of middle ear and mastoid surgery. For instance, from our retrospective analysis of recent mastoidectomy patients, “low” tegmens were more likely to require a canal wall down mastoidectomy whereas “high” tegmens were more likely to require a canal wall up mastoidectomy.