A new method of studying the anatomy of the posterior tympanum

Ear, Nose & Throat Journal, April, 2004 by James J. Holt

Abstract

The anatomic position of the posterior tympanum compromises direct visualization and investigation of its structures. Therefore, assessment of its anatomy is limited, and surgical removal of disease in this area remains a challenge. In this article, the author describes a new method that can be used to fully expose, visualize, and measure the structures in the posterior tympanum, thereby allowing for further definition of the anatomy and, in turn, refinement of surgical techniques for the removal of disease in this portion of the middle ear. The new technique involves removal of the anterior canal bone and its overlying tissues to eliminate the mechanical barriers to the posterior tympanum, which allows for direct visualization and measurement of the posterior tympanic structures. This method has three primary advantages over other methods used to study the posterior tympanum. First, observations of the anatomy can be made under direct visualization. Second, this method permits investigators to study the anatomy and the relationships between structures of the posterior tympanum as they occur in their normal unaltered state. Third, this method allows for wide anterior access to the posterior tympanum, thereby making scientific research into its anatomy possible. Information obtained from future studies of this method can help further refine surgical techniques for removal of disease from the posterior tympanum.

Introduction

Surgical extirpation of cholesteatoma and retracted epithelium in the posterior tympanum continues to challenge the otologic surgeon. The development of a good operative technique begins with a thorough understanding of the involved anatomy. The major structures of the posterior tympanum include lout sinuses, three eminences, two nerves, and various bony ridges. The distance between these middle ear structures has been measured, and the sinus tympani depth has been recorded. However, further anatomic knowledge is needed to refine the surgical techniques used to remove disease from the posterior tympanum.

Structures in the posterior tympanum have been studied by various methods, including computed tomography (CT), (1) tracings of microscopic sections onto paper, (2) and observation of computer-generated images from video segments. (3) However, these observations were based on reproductions of the tissue. Historically, anatomic studies are conducted by direct examination of an intact specimen. Therefore, we need a method of looking directly into the posterior tympanum in a way that preserves the integrity of the tissue.

The purpose of this article is threefold. One aim is to provide a description of a new method that can be used to fully expose and visualize the structures in the posterior tympanum. The second is to show how this new method can be employed. The third is to provide information on obtaining video and/or photographic documentation of the findings.

Materials and methods

The development of this new technique began with the harvesting of eight temporal bone plugs from human cadavers. Each specimen was stored in a separate container filled with 70% alcohol. The dissections were performed in the temporal bone laboratory at the Marshfield Clinic with the aid of a Zeiss OPMI operative microscope. An air-powered otologic drill equipped with various cutting and diamond bits was used to remove bone. Right-angle hooks were modified to range in length from 0.5 mm to 3.0 mm in 0.5-mm increments (figure 1, A). These hooks, along with thin metal rulers marked in millimeter segments (figure 1, B), were used to explore the depth of the recesses. Otologic instruments--including a Freer instrument, sickle knife, and alligator forceps--were used to perform the dissections.

[FIGURE 1 OMITTED]

A Pentax MX camera equipped with a two-power magnifier and telescopic lens was mounted on a standard tripod. The camera was equipped with a foot-controlled switch to minimize motion distortion. Kodak 320T color film was used to photograph the tissue.

An Urban UE-MOS 3000 color video camera was attached to the beam splitter on the microscope. The cable from the video camera was inserted into a Sony U-matic 3/4-inch video recorder and a Pioneer Elite DVR-7000 DVD recorder, producing both video and DVD recordings. Images were viewed on an NEC video monitor.

Results

In the standard operative orientation, the surgeon works down the ear canal from a posterior approach. However, the posterior ear canal bone compromises the line of vision into the posterior tympanum during such a procedure, and anatomic components of the posterior tympanum (e.g., the sinus tympani) cannot be easily visualized (figure 2, A). The author considered a variety of methods to improve visualization of the posterior tympanum:

* The first method involved a transcanal approach, in which the tympanic membrane was removed and a mirror was positioned in the tympanum. With this approach, the structures in the posterior tympanum were reflected in the mirror. Although this did afford some view of these structures, the exposure was not sufficient to accomplish the objectives of this study. In addition, the images in the mirror were not clear enough to precisely reproduce the finer details of the anatomy. Furthermore, although measurement instruments could be directed into the posterior tympanum, they could not be positioned in a way that allowed for the acquisition of any useful information. Hence, this method was abandoned.