Role of HRCT Temporal Bone in Predicting Surgical Difficulties Encountered in Fenestral Otosclerosis Surgery

Abstract

To study spectrum of high resolution computed tomography (HRCT) imaging findings in otosclerosis, to predict approximate length of prosthesis required from pre-operative HRCT measurements and to correlate between oval window niche (OWN) height preoperatively and difficulty in introducing foot plate perforator during surgery. A cross sectional study was conducted on 23 patients with a clinical diagnosis of otosclerosis from September 2018 to July 2020. Sensitivity of HRCT in detecting otosclerosis, correlation between pre-operative incudo-stapedial length (ISL) and intra operative prosthesis length, and correlation between OWN height and difficulty in introducing perforator were sought. The mean age of 23 patients studied was 39.9 years with a female preponderance of 56.5%. 17 out of 19 patients with foci of otosclerosis during surgery had HRCT findings of otosclerosis providing a sensitivity of 89.5%. Mean OWN height obtained was 1.29 mm preoperatively and a cut off value of 1.325 mm found using receiver operating characteristic curve method classifying OWN height as narrow or normal. Mean ISL measured pre operatively was 4.25 mm and mean length of prosthesis used was 4.56 mm with significant positive correlation using intraclass correlation coefficient method with correlation coefficient = 0.879. HRCT is an invaluable modality aiding the surgeon to detect otosclerotic foci with high sensitivity, identify thick obliterative otosclerotic foci requiring additional drilling, to predict the length of prosthesis used and to predict difficulties in approaching footplate when OWN height is below 1.325 mm.

Keywords: High resolution computed tomography; Otosclerosis; Stapedotomy.

Fig. 1

a Plain axial HRCT section of right temporal bone showing thin footplate (red line) of maximum thickness = 0.7 mm, b intra operative photograph of the same patient as in a showing blue foot plate (black arrow), c plain axial HRCT section of left temporal bone showing both fenestral (red arrow) and retro fenetsral (green arrow) lucent foci of otosclerosis, d plain axial HRCT section of left temporal bone showing a thick footplate (red line) of maximum thickness = 1.0 mm, e intra operative photograph of the patient in Fig. 2b showing white foot plate (black arrow)

Fig. 2

a Plain coronal HRCT section of left temporal bone showing normal oval window niche (OWN) height of 1.8 mm. First line was drawn along the main vestibule axis (red line); The second line perpendicular to the 1st line was drawn through the inferior part of the oval window (green line). OWN height (purple line) was measured craniocaudally as the distance between the second line and the inferior aspect of the facial nerve canal, b plain coronal HRCT section of left temporal bone showing reduced OWN height of 0.8 mm

Fig. 3

a Plain HRCT section of left temporal bone in the plane of stapes with the red line drawn between the long process of the incus and the lateral part of the stapes footplate measured as incudo stapedial length, b diagram of teflon piston used intra operatively whose functional length is shown by yellow arrow and diameter by green circle, c receiver operating characteristic (ROC) curve used to obtain cut off value of 1.325 mm for OWN height with values < 1.325 mm classified as a narrow OWN and values > 1.325 mm classified as a normal OWN. The sensitivity for the cut off value was 86.7% and specificity was 100% with area under the curve being 0.942