Exoscope-assisted middle ear surgery

7.1

Historical background

In ear surgery, optical magnification and stereopsis are essential in identifying anatomical structures and performing surgical procedures safely and successfully.

The need for stereopsis and the importance of three-dimensional perception of the complex anatomy of the ear during surgery was firstly recognized 150 years ago, in 1869, by the Italian Emilio De Rossi. This concept was brought to clinical practice approximately 50 years later when Carl Olof Nylen adapted the first dissecting microscope for use in otology, recognizing the need for magnification. The operating microscope has been the optical system indissolubly linked to ear surgery since 1921. Several improvements through the years led to the actual binocular high-definition microscope, which is a standard equipment in any ENT operating room worldwide.

Although the concepts of ear surgery and the classification of tympanoplasty have been defined, Ohnsorge at the Wu¨rzburg ENT clinic (1977) was the first to describe the intraoperative use of a new tool in ear surgery: the endoscope. From the early 1990s, endoscopic ear surgery has gained increasing popularity “to look around the corner,” allowing new anatomical insight and perspectives of the middle ear structures. From that time, the endoscope was used to perform ear surgery exclusively or in adjunct to the surgical microscope.

In the last decade, the implementation of modern video technology as the 4K high-definition and three-dimensional video has allowed to gain further importance to endoscope-guided surgery. Moreover, this technological advance introduced a new surgical tool in the field of surgical technology: the exoscope. The exoscope has been largely used in the field of neurosurgery in the recent years. However, even if exoscope-assisted ear surgery was recently introduced, it will take time to see the full expression of its potential.

7.2

Principles

The operating microscope is an optical system of lenses that guarantee to the surgeon a stereoscopic sharp vision of the minute structures of the middle ear with natural colors. The magnification obtained with the microscope reduces the surgical field wideness with a deterioration of panoramic view while preserving the image quality. The microscope field of view is reduced when the magnification rise, in a cone-shaped fashion, as it is shown in Fig. 7.1A and D in endoauricular and postauricular approaches, respectively.

The exoscope is an extracorporeal video telescope, composed of an optical stereoscopic system made with a rigid rod lens with high-resolution image sensors, and an integrated illumination with optical fibers. It is suspended above the surgical field while it produces high-quality full-HD three-dimensional images. These images are visualized on a large-format HD 4K resolution flat screen by the surgeon and the other operating room staff wearing 3D glasses. It can provide good image quality, lighting, and focal and field depth. The magnification is at first optical, while it becomes digital as it increases (2–16×). This allows for higher preservation of field wideness while increasing magnification (parallelepiped shape), thanks also to the “intraoperative navigation” within the surgical field allowed by the IMAGE1 PILOT ( Fig. 7.1B and E ).

The rigid ear endoscopes are the tubular fiber-optical instrument with different angulation of view (scopes 0 degrees, 30 degrees, 45 degrees, 70 degrees, etc.) that are introduced deep in the surgical cavity to explore the middle ear with a close high-detailed view that has the shape of an inverted angled cone ( Fig. 7.1C and F ).

The microscope allows for the best high magnification rendering. However, there is a reduction of the field wideness increasing the magnification, and only those structures that can be placed directly in the line of the light cone can be visualized. Surgery can be performed through all surgical corridors with appropriate depth perception and image quality with the right microscope adjustments. This is the reason why it is the gold standard in all ear surgical procedures nowadays.

The exoscope can provide an extremely detailed high-quality image, preserving a panoramic view of the surgical field during the procedures. The images are digitally elaborated and thus less natural than an operative microscope, especially at high magnification due to the current limits of digital zoom ( Fig. 7.2 ). If high magnification is needed, the gain rises resulting in image definition, contrast, and brightness that are inferior compared to the microscope optical magnification. The need for digital magnification can be minimized by moving the exoscope 3D camera as close as possible to the surgical field.

Overall, the exoscope compared to the operating microscope has the advantages of lightness, maneuverability, and compactness. It can be easily rotated and moved in any direction using one hand with the chance of achieving even narrow view angles. On the other hand, the exoscope needs a large surgical corridor to guarantee a good performance; otherwise, the use of a microscope should be preferred. That was the reason that guided us to select only postauricular approaches in exoscope-assisted ear surgery.

An appropriate analogy can be made with smartphone technology versus professional camera. The combined use of optical and digital zoom is a technology in continuous improvement, with increased usage of the smartphone also in the professional photography and videomaking fields. The same is valid for the exoscope. Its technology can potentially have an exponential development in terms of image quality in all settings and magnification degrees, and this is the reason why it is a future-oriented tool.

The endoscope has different characteristics with respect to the other two tools. It allows to go through even narrower surgical corridors and to “look around corners,” but it needs the use of one hand and it can determine the loss of depth perception and stereopsis. For these peculiar features, even if some surgeons use this tool as a standalone in their surgery, in our experience it is complementary to the others. The highest versatility is ensured in combined exoscope–endoscope surgery, in which the use of the same video column and monitor allows a rapid and effective shift between these two tools, changing just the monitor source setting and the light source. The exoscope is a really intuitive system, and surgeons experienced in endoscopic surgery are more facilitate, given that surgeon and monitor positions are the same.