CONE-BEAM CT IMAGES OF A SCUTUM DEFECT AS A TOOL FOR PRE-OPERATIVE ASSESSMENTS

Masahiro Komori

Department of Otolaryngology, Kochi University, School of Medicine, Nankoku, Kochi, Japan

Introduction

It is well known that Prussak’s orifice is the most frequent site of attic retraction pocket formation. The bony wall around Prussak’s orifice, the tympanic scute, progressively becomes eroded, resulting in a scutum defect.

Given the clinical importance of Prussak’s orifice and scutum bone defects, we developed a way to measure their sizes using a cone-beam computed tomography (CBCT). The previous report demonstrated that the width and height of the scutum defect were significantly greater in ears with an attic cholesteatoma than in normal ears or ears with chronic otitis media, regardless the stage of the cholesteatoma.1 The width of Prus-sak’s orifice was not closely related to the cholesteatoma stage. However, the height of Prussak’s orifice was significantly greater in stage-II cholesteatomas compared with stage I.

In our study, we compared the scutum defect among pre-operative eardrum findings, the volume-rendered images which are three-dimensional (3D) images and the operative findings in the same cases, and described how the image provides useful information.

Materials and methods

Subjects and operations

The study included two ears of two patients with extended middle-ear cholesteatoma who underwent staged canal-up tympanoplasty in the following manner.

In the first stage of the operation, the cholesteatoma was removed with a combined transmastoid and transcanal approach. After inserting a Silastic sheet as support, we reconstructed the tympanic scute with bone paté, and the eardrum with fascia.2 One year later, the second stage of the operation was carried out. We detached the skin of the external ear canal in order to view the reconstructed tympanic scute, and enforced the scute with the sliced cartilage.

Imagings

A cone-beam CT device (3D Accuitomo 170, manufactured by J. Morita Manufacturing Corp., Kyoto, Japan, Resolution: 0.08 mm) was used. An image of the temporal bone was made with the volume-rendering tech-nique.3 Next, the image of the lower area on the external ear canal was removed using a cutting tool. In this way, a volume-rendered image of the scutum defect was obtained. Thereafter, the volume-rendered image was rotated to an optimal view.

Results

At the assessment before the first-stage operation, the scutum defect was sometimes hidden by the skin of the external ear canal (Fig. 1). However, a large bone defect was identified on the volume-rendered image. The size and shape determined from the image were correct, as confirmed in the operative finding. At the assessment before the second-stage operation, the eardrum had clearly healed, and the volume-rendered image showed that the defect was completely reconstructed. The defect had formed completely in the operative finding in one case. In the other case, the volume-rendered image displayed that a bone defect remained although the eardrum had clearly healed. The Results suggested that the bone paté had been absorbed, but the scutum plasty of the first-stage operation had not succeeded.

image

Fig. 1. Scutum defect in staged canal-up tympanoplasty. A large bone defect was identified on the volume-rendered image, although the defect was hidden by the skin of the external ear canal (A1,2). Even though the eardrum had clearly healed (B1,C1), a bone defect was reconstructed completely or remained (B2,C2). The size and shape determined from the images were correct, as confirmed in the operative findings (2, 3). A: a case at the first-stage operation; B: a case with complete scutum plasty at the second-stage operation; C: a case with incomplete scutum plasty at the second-stage operation; 1: pre-operative eardrum findings; 2: volume rendered images; 3: operative findings.

Discussion

CT is a crucial tool for both pre-operative and post-operative assessments. The volume-rendered image is a 3D-CT image called the 3D view or virtual endoscopy in previous reports.4,5 The volume-rendering technique was introduced in 1998.3 In otology, Klingebiel et al.4 described the use of 3D images created by multi-slice CT in 2001. After that, Martin et al.5 reported that this technique was more valuable than two-dimensional (2D) CT for the diagnosis of ossicle and ossicular prosthesis dislocation.

CBCT technology gives a high-resolution view of the temporal bone from any desired direction with very little radiation hazard and no metal artifact.69 Previous studies were performed with 2D-CBCT images of the position of the ossicular prostheses or cochlear implantation.

The study demonstrated that the volume-rendered images correctly predicted operative findings of the scutum defect. This technique was particularly useful for a case in which the bone defect was hidden behind the skin of the external ear canal, and it provides valuable information for planning operations.

References

1.Kashiba K, Komori M, Yanagihara N, Hinohira Y, Sakagami M. Lateral orifice of Prussak’s space assessed with a high-resolution cone beam 3-dimensional computed tomography. Otol Neurotol 32:71–76, 2011

2.Hinohira Y, Yanagihara N, Gyo K. Surgical treatment of retraction pocket with bone pate: scutum plasty for cholesteatoma. Otolaryngol Head Neck Surg 133:625–628, 2005

3.Calhoun PS, Kuszyk BS, Heath DG, Carley JC, Fishman EK. Three-dimensional volume rendering of spiral CT data: theory and method. Radiographics 19:745–764, 1999

4.Klingebiel R, Bauknecht HC, Kaschke O, et al. Virtual endoscopy of the tympanic cavity based on high-resolution multislice computed tomographic data. Otol Neurotol 22:803–807, 2001

5.Martin C, Michel F, Pouget JF, et al. Pathology of the ossicular chain: comparison between virtual endoscopy and 2D spiral CT-data. Otol Neurotol 25:215–219, 2004

6.Dalchow CV, Weber AL, Yanagihara N, Bien S, Werner JA. Digital volume tomography: radiologic examinations of the temporal bone. AJR Am J Roentgenol 186:416–423, 2006

7.Monteiro E, Das P, Daly M, et al. Usefulness of cone-beam computed tomography in determining the position of ossicular prostheses: a cadaveric model. Otol Neurotol 32:1358–1363, 2011

8.Ruivo J, Mermuys K, Bacher K, et al. Cone beam computed tomography, a low-dose imaging technique in the postoperative assessment of cochlear implantation. Otol Neurotol 30:299–303, 2009

9.Komori M, Yanagihara N, Hinohira Y, et al. Quality of temporal bone CT images: a comparison of flat panel cone beam CT and multi-slice CT. J Int Adv Otol 8:57–62, 2012


Address for correspondence: Masahiro Komori, MD, PhD, Department of Otolaryngology, Kochi University, School of Medicine, Kohasu, Oko-cho, Nankoku, Kochi 783–8505 Japan. komori@kochi-u.ac.jp

Cholesteatoma and Ear Surgery – An Update, pp. 215–217

Edited by Haruo Takahashi

2013 © Kugler Publications, Amsterdam, The Netherlands