Abstract: Objective To observe the effect of high-resolution computed tomography (HRCT) grading of low middle fossa tegmen plate on the elimination of diseased tissue in epitympanum and to investigate the grading standard of low middle fossa tegmen plate. Methods The CT images obtained from 87 chronic otitis media (COM) cases were subjected to coronal reconstruction. A horizontal tangent L1 at the highest point along the upper wall of external auditory canal, a horizontal tangent L2 at the highest point of the semicircular canal structure, and a horizontal tangent L3 at the lowest point along the middle fossa tegmen plate were created to measure the distance between L1 and L2, L1 and L3, and to calculate the r value (b/a). The r value was used to divide the low middle fossa tegmen plate into five grades: non-low, mild, moderate, severe, and absolute low. Pre-operative evaluation and postoperative results were compared. Results Surgeons had different opinions on the r value: (1) If r < 1/4, the low middle cranial fossa cover identified by the Radiation Department could not be accepted. (2) If 1/4 ≤ r < 1/3, the low middle cranial fossa cover identified by the Radiation Department could be accepted, but it was not mentioned in the operation record. (2) If 1/3 < r ≤ 1/2, wearing was found at the bottom wall of the middle fossa tegmen plate upon clearing the diseased tissue, and the operation would be affected because of bleeding. (4) If 1/2 < r ≤3/4, the surgeons were able to identify the low middle fossa tegmen plate before operation; but some cases' back wall of the external auditory canals could be preserved after the diseased tissue in the epitympanum was cleared. (5) If r > 3/4, the surgeons experienced difficulty in finding the trum tympanicum and pathway behind the ear because the low middle fossa tegmen plate wore down the back wall of external auditory canal. Conclusion The r value can be used to divide the low middle fossa tegmen plate into five grades: non, mild, moderate, severe, and absolute low. Mild grade has no effect on the removal of lesions,moderate grade makes the removal of lesions difficult, severe grade requires destruction of the back wall of external auditory canal, and absolute low grade means wearing down of the back wall of external auditory canal is inevitable.

Key words: Middle cranial fossa, Tympanic cavity lesions, Basis Cranii, X-ray computed, Tomography, Temporal Bone