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A new method of studying the anatomy of the posterior tympanum.

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.

* A second method involved removal of the posterior canal bone and resection of a segment of the facial nerve. Although this did permit better visualization into the posterior tympanum, it defeated one of the objectives of this investigation because it required the destruction of the tissue to be studied.

* Finally, an anterior approach to the posterior tympanum was investigated. The only way this could be accomplished was by removal of the anterior canal wall. Unlike the second method, removal of the anterior tissue did not affect the anatomy or the structures under investigation. Further development of this approach led to the method established, refined, and described in this article.

[FIGURE 2 OMITTED]

Exposure. In order to obtain visual and instrumental access to the posterior tympanum, certain structures had to be removed. First, the soft tissues overlying the anterior canal bone were elevated with a Freer instrument and excised by sharp dissection. Next, the anterior canal bone was removed with the otologic drill, and a sickle knife and alligator forceps were used to excise the tympanic membrane (figure 2, B). Removal of these anterior tissues eliminated the mechanical barriers to the posterior tympanum. Thus, with the anterior tissues removed and the specimen properly positioned, a direct line of vision straight into the posterior tympanum was achieved (figure 2, C).

Measurements. Various measurements of the sinuses and recesses in the posterior tympanum are possible with this means of wide anterior access. However, because sinus depths in the posterior tympanum may taper and end as deep apices, obtaining accurate measurements can be a challenge. The author used two methods to measure the depth of the sinuses in the posterior tympanum. In one method, the thin metal ruler was placed into the sinus and positioned under direct vision to the deepest part of the sinus. The millimeter mark noted at the orifice of the sinus indicated its depth. In the other method, a wire positioned in the deepest part of the sinus was grasped with a small hemostat at the orifice of the sinus. The wire was withdrawn from the temporal bone and placed on a ruler. The distance from the end of the wire to the hemostat reflected the depth of the sinus.

Recesses also can be present in the sinus itself. For instance, the facial canal, which lies in the lateral wall of the sinus tympani, may protrude into the sinus to form a recess under the nerve. In other sinuses, recesses are formed by bony vagarities in the sinus cavity. The author was able to explore these recesses with the right-angle hooks (figure 3). The depths of the recesses were measured by placing hooks of various lengths into the recesses. The measurements were most accurate when the hooks were placed under direct vision into the sinus recess.

[FIGURE 3 OMITTED]

The size and configuration of the openings into the sinuses of the posterior tympanum vary. Some are wide and offer a full view of the entire sinus, while others are narrow and offer only a minimal view. Measurement of the sinus openings in the posterior tympanum can be difficult because the small confined area limits the use of instrumentation. However, the author found that electro-cardiographic (ECG) caliper tines can be used to measure the sinus openings (figure 1, C). The long, narrow tines were positioned at the margins of the sinus orifices. The ECG caliper was then withdrawn, and the distance between the tines was measured with a ruler.

Documentation. The direct access to the posterior tympanum described here enabled the author to make video and photographic records of the structures as they are in the normal anatomic state, including those in the deepest recesses of the posterior tympanum. By removing the anterior bony canal wall, an endoscope was positioned directly toward the structures in the posterior tympanum so that photographs could be taken through the scope (figure 4). This method also allowed the line of vision from the video camera to project directly into the posterior tympanum. In this manner, photographic and video documentation was possible. Such permanent documentation of anatomic findings can be used for study, education, and/or scientific presentations.

[FIGURE 4 OMITTED]

Discussion

Anatomy. Other investigators have studied the anatomy of the posterior tympanum. In early studies, tracings made from horizontal serial sections (2) or photographs (4) were used to make measurements of various anatomic structures. More recently, investigators have used more modern methods to obtain computer-driven measurements generated from video images (3) and measurements obtained from CT sequences of the temporal bone. (1) Although these investigators provided useful information about the anatomy of the posterior tympanum, they used segmental reproductions of tissue. By using the methods that the author describes in this article, otologists can gain a new perspective of the actual structures in the posterior tympanum while preserving the integrity of the tissue.

The focus of other investigations of the posterior tympanum has been to provide a general description of its structures and anatomy; however, detailed descriptions are sparse, and many questions remain unanswered. For example, Proctor described the ponticulus, which extends from the pyramidal eminence to the promontory. (5) However, the size and thickness of the ponticulus, its proximity to neighboring structures, and the likelihood of disease becoming trapped under it have not been defined. Proctor also described the four sinuses of the posterior tympanum, (6) but specific information regarding their configuration and the accessibility of their deepest recesses is still unavailable. The method described in this article allows for more detailed investigation into the anatomy of the posterior tympanum, and thereby makes possible a better understanding of the concealed areas in the posterior tympanum where disease can be hidden from the surgeon. This knowledge can be applied toward refining or developing better surgical techniques in this obscure area.

Surgical techniques. Otologic surgeons must approach the operating room prepared to manage disease in any location in the ear. The challenge of operating in the posterior tympanum has long been recognized.

Various approaches to the structures in the posterior tympanum have been utilized. Surgical approaches include the anterior transcanal approach recommended by Farrior, (7) the antcroposterior approach suggested by Goodhill, (8) and the approach through the external auditory canal used by Lau and Tos. (9) Leonetti et al (10) described a technique to marsupialize a retraction pocket in the posterior tympanum, whereas Pulec (11) recommended transcanal exposure of the posterior tympanum by removal of the posterior canal wall bone to the facial nerve when possible, with a transmastoid approach as a second option if necessary.

Although bone of the posterior canal wall can be removed up to the limits of the facial nerve to better visualize the middle ear structures, many surgeons are reluctant to remove a significant amount of posterior canal wall bone because they fear that they might damage the facial nerve. Thus, maximal exposure of disease in the posterior tympanum is often compromised, and the surgeon must often resort to blindly scraping disease out of this area with a right-angle hook. In this manner, disease removal is guided mostly by the feel of the instrument on bone rather titan by a systematic dissection based on a thorough knowledge of the anatomy of the operative site.

The author's goal was not to invent a new anterior surgical approach to the posterior tympanum, but to augment Goodhill's method of anteroposterior intraoperative exposure of the posterior tympanum by developing a technique that can be used in the research laboratory to produce an expanded view of this middle ear structure. This expanded view allows for full exposure of the posterior tympanum, which is not possible in an operative setting. Using this refined methodology, otologists can study the temporal bone anatomy in the laboratory, gain a better perspective of this part of the middle ear, and, in turn, refine their own surgical techniques for dealing with disease in the posterior tympanum.

In conclusion, studying the posterior tympanum in this manner has several advantages over other methods. First, measurements of the anatomy are made on the tissue under direct visualization. Second, the relationships between structures are studied on the tissue as it occurs in its normal unaltered state. Third, clinicians using this anterior approach will gain better insight into the anatomy of this complex area of the tympanum. Scientific research into the anatomy of the posterior tympanum is possible with this new anterior approach. Information from future studies utilizing this method can help refine the surgical technique for the removal of cholesteatoma and retracted epithelium in the posterior tympanum.

Acknowledgments

The author thanks the Marshfield Clinic Research Foundation for providing the support of Jennifer Virnoche, who edited the article; Alice Stargardt, who prepared the manuscript; and Shirley Thompson, who rendered the illustrations.

References

(1.) Pickett BP, Cail WS, Lambert PR. Sinus tympani: Anatomic considerations, computed toarography, and a discussion of the retrofacial approach for removal of disease. Am J Otol 1995; 16:741-50.

(2.) Saito R, Igarashi M, Alford BR, Guilford FR. Anatomical measurement of the sinus tympani. A study of horizontal serial sections of the human temporal bone. Arch Otolaryngol 1971; 94:418-25.

(3.) Ozturan O, Bauer CA, Miller CC III, Jenkins HA. Dimensions of the sinus tympani and its surgical access via a retrofacial approach. Ann Otol Rhinol Laryngol 1996;105:776-83.

(4.) Donaldson JA, Anson BJ, Warpeha RL, Rensink MJ. The surgical anatomy of the sinus tympani. Arch Otolaryngol 1970;91:219-27.

(5.) Proctor B. Surgical Anatomy of the Ear and Temporal Bone. New York: Thieme Medical Publishers, 1989.

(6.) Proctor B. Surgical anatomy of the posterior tympanum. Ann Otol Rhinol Laryngol 1969;78:1026-40.

(7.) Farrior JB. Tympanoplasty: The anterior attico-tympanotomy. Surgery of the posterior tympanic recesses. Laryngoscope 1968; 78:768-79.

(8.) Goodhill V. Circumferential tympano-mastoid access. The sinus tympani area. Ann Otol Rhinol Laryngol 1973;82:547-54.

(9.) Lau T, Tos M. Treatment of sinus cholesteatoma. Long-term results and recurrence rate. Arch Ototaryngol Head Neck Surg 1988;114:1428-34.

(10.) Leonetti JP, Buckingham RA, Marzo SJ. Retraction cholesteatoma of the sinus tympani. Am J Otol 1996;17:823-6.

(11.) Pulec JL. Sinus tympani: Retrofacial approach for the removal of cholesteatomas. Ear Nose Throat J 1996;75:77, 81 3, 86-8.

From the Department of Otolaryngology-Head and Neck Surgery, Marshfield Clinic, Marshfield, Wis.

Reprint requests: James J. Holt, MD, Department of OtolaryngologyHead and Neck Surgery, Marshfield Clinic, 1000 N. Oak Ave., Marshfield, WI 54449. Phone: (715) 387-5271; fax: (715) 389 7622; e-mail: holt.james@marshfieldclinic.org
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Author:Holt, James J.
Publication:Ear, Nose and Throat Journal
Geographic Code:1USA
Date:Apr 1, 2004
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