ANATOMICAL MEASUREMENT OF THE FACIAL RECESS AND THE ROUND WINDOW MEMBRANE USING CONE-BEAM COMPUTED TOMOGRAPHY

Takahiro Nakashima,1 Tetsuya Tono,2 Keiji Matsuda,2 Minoru Takagi,1 Tamon Hayashi,1 Yutaka Hanamure1

1Department of Otolaryngology-Head and Neck Surgery, Kagoshima City Hospital, Kagoshima, Japan; 2Department of Otolaryngology-Head and Neck Surgery, Faculty of Medicine, University of Miyazaki, Japan

Background and purpose

In Japan, high-resolution computed tomography (HRCT) is routinely used for the pre-operative evaluation of the temporal bone. Recently, cone-beam computed tomography (CBCT) has become available to daily clinical practice. Several studies have shown that CBCT can visualize fine structures of the temporal bone more precisely than conventional CT, both in vitro and in vivo.1,2 CBCT has also been used for the post-operative evaluation of cochlear implants to measure the precise insertion length of the electrode array.3 The surgical anatomy of the round window region is of increasing importance due to the round window surgical approach for hearing devices such as cochlear implants and vibrant sound bridges. The purpose of this study was to measure the width of the facial recess and the round window membrane using CBCT and to evaluate the efficacy of CBCT.

Materials and methods

Materials

Among the patients who underwent CBCT at Kagoshima City Hospital from January to February 2012, 50 ears of 37 patients (aged nine to 78 years; 16 males and 21 females) were selected for the measurements. Thirty ears were excluded because aeration of the tympanic cavities was insufficient to identify the round window membrane at the basal end of the scala tympani.

Technical data of CBCT imaging

CBCT examinations were performed using a 3D Accuitomo F17 (J. Morita Mfg. Corp., Kyoto, Japan). Imaging was performed with a tube current of 8 mA and a tube voltage of 90 kV. During the scan of each subject, 577 raw data-projection images were obtained over 360 degrees. One full rotation took 17.5 seconds. The raw data-projection images were reconstructed using the software i-View (J. Morita Mfg. Corp.) resulting in a voxel size of 0.125 mm. The diameter and the height of the cylinder-shaped reconstruction regions of interest were 60 mm.

Measurement of the facial recess and the round window membrane

The reconstructed images consisted of sagittal, coronal, and axial views, which can be set at a desired angle. The facial recess in a sagittal view is a triangular area composed of the facial canal, chorda tympani nerve and incus buttress. The length of the upper side of this triangle, which is a line parallel to the incus buttress, was defined as the width of the facial recess. The width of the round window membrane was measured at an axial view, in which a basal turn of cochlea and round window niche was visualized. Both distances were measured and the data obtained were evaluated statistically.

Results

The average width of the facial recess was 2.3 mm, ranging from 1.64 mm to 3.03 mm, and the diameter of the round window membrane was 1.6 mm, ranging from 1.23 mm to 1.96 mm. The width of the facial recess and the diameter of the round window membrane showed no statistical correlation with age or gender (Figs. 1 to 3, Spearman’s correlation).

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Fig. 1. Width of the facial recess shows no correlation with age.

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Fig. 2. Width of the round window membrane shows no correlation with age.

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Fig. 3. Width of the facial recess and round window membrane show no correlation with gender.

Discussion

A histological study4 of the facial recess and the round window membrane showed that the average width of the facial recess and the round window membrane were 4.01 mm and 1.65 mm, respectively. Although the diameter of the round window is consistent with our data, the width of the facial recess is much longer than that observed in our study. The discrepancy between the histological and CBCT studies appears to be due to different anatomical definitions of the facial recess. In the previous histological study, the facial recess was measured between the facial canal and the margin of the tympanic annulus. Thus, the thickness of the external auditory canal was included. In agreement with the previous histological study, our study showed no statistical correlation with age for both measurements. These results indicate that surgical procedures for these anatomical sites should not differ with age.

The opening of the facial recess is described as two or three mm in width in many textbooks of ear surgery or temporal bone dissection. This is supported by the results of our study. However, the range of the opening of the facial recess from 1.64 mm to 3.03 mm suggests that there may be some cases in which the facial recess cannot be opened without injuring the chorda tympani nerve or the facial nerve, even with a burr of 1.5 mm diameter.

Conclusion

In the era of implantable hearing devices, the otologist should be familiar with structures of the middle and inner ear more precisely. For this purpose, CBCT is very convenient and reliable tool for preoperative understanding of the fine structures of the temporal bone.

References

1.Gupta R, et al. Experimental Flat-panel High-Spatial-Resolution Volume CT of the Temporal Bone. Am J Neuroradiol 25:1417–1424, 2004

2.Ruivo J, Mermuys K, Bacher K, Kuhweide R, Offeciers E, Casselman JW. Cone Beam Computed Tomography, A Low-Dose Imaging Technique in the Postoperative Assessment of Cochlear Implantation. Otol Neurotol 30:299–303, 2009

3.Trieger A, Schulze A, Schneider M, Zahnert T, Mürbe D. In Vivo Measurements of the Insertion Depth of the Cochlear Implant Arrays Using Flat-Panel Volume Computed Tomography. Otol Neurotol 32:152–157, 2010

4.Su WY, Marion MS, Hinojosa R, Matz GJ. Anatomical Measurements of the Cochlear Aqueduct, Round Window Membrane, Round Window Niche, and Facial recess. Laryngoscope 92:483–486, 1982

Address for correspondence: Takahiro Nakashima, tnaka@fc.miyazaki-u.ac.jp

Cholesteatoma and Ear Surgery – An Update, pp. 363–365

Edited by Haruo Takahashi

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