Anatomic analysis of the mastoid tegmen: slopes and tegmen shape variances

To examine the dural slope shapes and slope depths in different axes in the mastoid. These are important for surgical guidance.

Setting-Tertiary care otologic center. Patients/materials-105 temporal bone CT scans were used from subjects who had undergone High Resolution Temporal Bone CT scanning for CSOM, using the non-diseased ear scans. In addition, 21 "diseased side" temporal bones were included.

Scans were reconstructed in 3D using Amira software to examine tegmen shapes. The highest and lowest points on the tegmen were measured along 4 axes, 2 in the coronal plane (latero-medial axes posteriorly and anteriorly [L-M ant and L-M post]), and 2 in the sagittal plane (posterio-anterior axes medially and laterally [P-A med and P-A lat]).

The highest and lowest point difference was labeled ΔH. We also measured the height from the superior external canal to the tegmen.

There was a unimodal distribution in the ΔH variable for all axes measured. Means and (SD) in mm for ΔH are as follows: L-M post 4.1 (2), L-M ant 2.9 (2.2), P-A med 6.5 (1.9), P-A lat 5.4 (2.2). Tegmen-EAC lat and med height means were 8.4 and 9.7 mm respectively. Representative 3D shapes are presented.

There appears to be a unimodal distribution of mastoid tegmen shapes and sizes, without clustering into subpopulations. The tegmen slopes have real implications for surgical exploration and disease eradication. A Tegmen classification scheme is presented.