Learning Objectives: Variations in normal tegmen and inner ear anatomy.

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. Serious complications such as cerebrospinal fluid leakage, neural tissue injury may arise when operating near the tegmen. One important risk factor for dural complications is the low placement of the tegmen. This study aims to determine the radiographic location of the tegmen tympani using the lateral semicircular canal(LSCC) as a landmark in adult patients with normal temporal bones.

Methods: 100 high resolution temporal bone CT scans from patients with 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 LSCC 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 coronal plane and 2.5 mm in the sagittal plane. The measured heights using the LSCC as our landmark demonstrated a unimodal distribution with some variance.

Conclusions: We propose a new, simple and clinically practical tegmen CT classification using the LSCC as our landmark. 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. Our retrospective analysis of recent mastoidectomy patients showed that “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.