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Endoscopic Skull Base Surgery - Chap 5, Instrumentation in Endoscopic Skull Base Surger
By Hrayr K. Shahinian, M.D., FACS

Abstract:

This chapter provides an in-depth description and state-of-the-art illustrations of the instrumentation utilized in endoscopic skull base surgery. Advances in technology have complemented the advances in endoscopic skull base surgery and stimulated the creation of dedicated endoscopic equipment and microinstruments specifically designed to fulfill the unique requirements of endoscopic skull base surgery. These instruments include rigid endoscopes, lens irrigation systems, pneumatically-powered robotic holding arms, bipolars, light sources, high-definition digital cameras, digital processors, digital monitors, digital versatile disc (DVD) recorders, Polaroid digital photo printers, nerve monitoring devices (EEG, EMG, SSEP, BAEP, MEP, VEP), microdrills, micro-Cavitron ultrasonic aspirators (microCUSA), and specialized endoscopic microinstruments.

1. Introduction

Conventional neurosurgical and microsurgical instruments are too bulky for endoscopic skull base surgery. Whereas operating microscopes require wide viewing portals for adequate illumination and visualization of the operative field, endoscopes utilize minute keyholes to precisely reach the target area. Therefore, adapting and refining regular microinstruments and equipment such as bipolars and Cavitron ultrasonic aspirators (CUSA) to include longer, more slender shafts and smaller microtips has been essential for endoscopic skull base surgical approaches. In addition, major improvements such as the invention of sophisticated light sources, cameras, digital processors, lens irrigation systems, and robotic holding arms have all complemented the advances in endoscopic technology and stimulated the creation of dedicated endoscopic equipment and microinstruments specifically designed to fulfill the unique requirements of endoscopic skull base surgery (Figure 1).

2. Rigid Endoscopes

The design of the Hopkins rod lens system, developed by the British physicist Harold Hopkins in the late 1960s, has been revolutionized over the past several decades to yield endoscopes of variable lengths, diameters, and angles of view. The modern rigid endoscopes currently used for skull base surgery are either 2.7 or 4 mm in diameter and their standard working length is 18 cm. The larger the diameter of the endoscope, the better the image it displays and the more light it can transmit to the operative field. Specialized endoscopic microinstruments are inserted alongside the endoscope, and the entire surgery is performed by viewing a high-definition plasma screen or liquid crystal display (LCD).

0-degree endoscopes give a "straight-on" view, while 30-degree endoscopes permit viewing from the side, a useful tool when looking around corners at the skull base, e.g., looking into the internal auditory canal (IAC) in vestibular schwannoma surgery or the cavernous sinuses during endonasal pituitary surgery. Wider angled endoscopes, such as the 70-degree and 90-degree endoscopes, are difficult and disorienting to work with and are only occasionally used (Figures 2 (A) and (B))

3. Irrigation Sheaths and Pump

Imaging with an endoscope requires a clear medium and optimal illumination of the surgical field with the lowest amount of diffraction. Placement of the endoscope's lens into the surgical field exposes it to blood, fluid, and debris. In order to avoid the dangerous practice of frequently removing, cleaning, and repositioning the endoscope, we have fitted our endoscopes with irrigation sheaths that are attached to a pump that uses sterile saline to clear the lens while the endoscope remains in position. Irrigation sheaths are available in different diameters to fit different sized endoscopes. Upon demand, the motorized pedal-activated pump delivers a cleansing stream of saline over the tip of the endoscope. This is immediately followed by a brief period of suction whereby any remaining drops of saline that would otherwise blur the image are cleared off the lens (Figure 3).

4. Pneumatic Holding Arms

Bimanual dexterity is a necessity in endoscopic skull base surgery, and therefore holding arms are an essential component of the endoscopic equipment. The holding arms must be sturdy, stable, and capable of holding the endoscopes securely, yet they must be easily adjustable to allow the surgeon to manipulate them at will. Whereas earlier designs were mechanical, combining long metallic rods with movable joints, the latest generation of holding arms consists of ball bearing joints that are pneumatically powered. These devices are remarkably flexible and extremely reliable. Surgeons can operate them quite easily with a finger-activated mechanism that alternates between a free and a locked position (Figure 4).

5. Xenon Light Source

Illumination is generated by a powerful cold-light source and transmitted to the endoscope via a fiberoptic cable. Light travels along the length of the endoscope to fiberoptically illuminate the operative field. The different types of light sources (tungsten, halogen, metal halide, xenon) offer light of varying brightness. Currently, the most powerful and preferred light source for endoscopic skull base surgery is a xenon system (Figure 5)

6. High-Definition Digital Camera

Hopkins rod telescopes are manufactured with built-in standard eyepieces to which cameras are attached. The image is then projected onto one or several monitors where it is electronically processed and recorded. Three-chip cameras contain individual chips for each of the primary colors. These produce excellent quality images and feature automatic controls over color, exposure, white balance, and digital contrast enhancement. Devices that digitally process the endoscope's image allow for optimum enhancement and image manipulation. Recently, progressive scan, high-definition fully digital cameras have provided further improvement in image quality (Figures 6 (A) and (B)).

7. Digital Monitors

A monitor displays the camera's image during the entire endoscopic procedure; it is the "surgeon's eye," and therefore its position is critical for the surgeon. Ceiling-mounted monitors provide maneuverability that allows the monitor to be positioned in the direct line of view of the surgeon. A second monitor is useful for viewing by the remainder of the surgical team. These color monitors should have a high resolution of at least 700 lines of resolution for three-chip cameras. Recently, high-definition liquid crystal display (LCD) and plasma screen monitors have been coupled to high-definition cameras, resulting in superb image resolution (Figure 7).

8. DVD Recorder

Digital recording devices allow single-frame or continuous imaging to be produced and stored in digital format that can then be used for presentations and publications to illustrate or document the surgery. Currently, digital versatile disc (DVD) recorders provide higher resolutions and higher data storage capacity than traditional video capture recorders (VCR) or digital video camera recorders (DVCAM). They also provide direct digital formatting (Figure 8).

9. Polaroid Digital Printer

Polaroid digital printers provide color prints for documentation that are high quality, dry, durable, and fade resistant. They are particularly helpful to share with the patient's family upon completion of the surgery while the family members are in the surgical waiting room. These printers have electronic controls to manipulate brightness, contrast, orientation, image resizing, or cropping (Figure 9).

10. Cranial Nerve Monitors

Intraoperative neurophysiologic monitoring allows for real-time assessment of neurological function that helps guide the surgeon intraoperatively. In general, these devices use electrophysiological methods such as electroencephalography (EEG), electromyography (EMG), and evoked potentials to monitor the functional integrity of neural structures. Evoked potential monitoring includes somatosensory evoked potentials (SSEP), brainstem auditory evoked potentials (BAEP), motor evoked potentials (MEP), and visual evoked potentials (VEP).

EMG is extensively used in skull base surgery. The most common applications are in locating cranial nerves V, VII, X, XI, or XII in patients with cerebellopontine angle (CPA) and brainstem tumors and in patients undergoing microvascular decompressions. Furthermore, a selective electrical stimulation probe allows for intraoperative identification of individual cranial nerves. Depending on the specific location of the tumor, its size, and the vital structures at the skull base that might be involved, it is sometimes important to monitor other cranial nerves; for instance, occulomotor (III), trochlear (IV), and abducens (VI) nerves are monitored in cavernous sinus surgery. Auditory brainstem response (ABR, aka BSEP, BSER, BAEP, etc.) is particularly important in monitoring the acoustic nerve during acoustic neuroma surgery. Recordings are obtained after stimulation with auditory clicks in the ear. This is particularly useful in small- to medium-sized tumors where hearing preservation is important (Figure 10).

11. Microdrill, Handpiece, Attachments, and Burrs

Microdrills are used to create the keyhole bone flap and to drill bony structures within the skull, such as the posterior wall of the internal auditory canal (IAC) in vestibular schwannoma surgery. For endoscopic skull base surgery, pen style, compact, powerful, smooth, lightweight, high performance microdrills provide the balance and maneuverability that enables the surgeon to work in tight spaces through keyhole access. The microdrill's attachments are tapered to provide improved visibility of the cutting or diamond burrs at its tip during surgery. They include straight or angled attachments for the creation of the keyhole craniotomy and a special extended arciform attachment for deep access and delicate bone drilling within confined surgical areas at the skull base. Craniotome attachments for both pediatric and adult cases and a range of sizes and shapes for burrs (cutting/diamond) and blades are used in combination with these attachments (Figures 11 (A) - (D)).

12. Micro-Cavitron Ultrasonic Surgical Aspirator (MicroCUSA), Handpieces and Tips

MicroCUSAs are used to enable ultrasonic selective fragmentation and cavitation of the inner part of a tumor. Endoscopic microCUSA handpieces are more compact, lighter, lengthier, and more slender than the regular ones. They include ultrafine microtips and are angled with extended shafts that give the handpiece a better reach and allow optimal visibility during endoscopic skull base surgery. Generally, there are two different frequencies. The standard 35 or 36 kHz handpiece is ideal for soft tissue removal around critical structures, while the 23 or 24 Hz handpiece is used for harder masses. We have relied exclusively on the 36 kHz micro and precision tips for all of our needs (Figures 12 (A) - (E)).

13. Specialized Endoscopic Microinstruments

A wide range of specialized microinstruments have been specifically designed for use in endoscopic skull base surgery, including microscissors, suction tubes, bipolars, microdissectors, and others. These microinstruments are generally more slender with smaller tips than those used in traditional neurosurgery. Special endoscopic bipolars with insulated sheaths and small microtips for easy introduction, 360 degree rotation, and maneuverability through the keyhole opening are used. Three basic sets of instruments are used exclusively at the Skull Base Institute, as illustrated in the figures (Figures 13, 14, and 15).

Legends:

Figure 1: Skull Base Surgery EndoSuite
Figures 2: Rigid Endoscopes
  1. 4 mm Endoscopes
    Lower Case Figure 2 (A)
    1. 0-degree Endoscope
    2. 30-degree Endoscope
    3. 70-degree Endoscope
    4. 90-degree Endoscope
  2. 2.7 mm Endoscopes
    Lower Case Figure 2 (B)
    1. 0-degree Endoscope
    2. 30-degree Endoscope
    3. 70-degree Endoscope
Figure 3: Irrigation Sheath and Pump
Figure 4: Pneumatic Holding Arm
Figure 5: Xenon Light Source
Figures 6:
  1. High-Definition Digital Camera
  2. Camera Unit
Figure 7: Endoscopic Tower and Monitor
Figure 8: DVD Recorder
Figure 9: Polaroid Digital Printer
Figure 10: Cranial Nerve EMG Monitor
Figure 11: Microdrill, Handpiece, Attachments, and Burrs
  1. Foot control
  2. Adult Craniotome
  3. Pediatric Craniotome
  4. Microdrill Handpiece and Standard Attachment
  5. Curved Extended Minimal Access Attachment
Figure 12: MicroCUSA, Handpieces and Tips
  1. MicroCUSA Console
  2. Handpiece
  3. Curved Extended Standard Tip
  4. Curved Extended Micro Tip
  5. Curved Extended Precision Tip
Figure 13:
  1. Tray 1
  2. Ring Curette, Angled 90-degree left
  3. Ring Curette Tip, Angled 45-degree up
  4. Ring Curette Tip, Angled 90-degree up,
  5. Ring Curette Tip, Angled 45-degree right
  6. Ring Curette Tip, Angled 90-degree right
  7. Ring Curette Tip, Angled 45-degree down
  8. Ring Curette Tip, Angled 90-degree down
  9. Ring Curette Tip, Angled 45-degree left
  10. Ring Curette Tip, Angled 90-degree left
Figure 14:
  1. Tray 2
  2. Bipolar Forceps
  3. Bipolar Forceps 1.5mm Tip
  4. Bipolar Forceps 3.0mm Tip
  5. Micro Osteotome, Medium 3.0mm Tip
  6. Micro Osteotome Tip, 2mm
  7. Micro Osteotome Tip, 4mm
  8. Micro Cupped Forceps
  9. Micro Cupped Forceps Tip
  10. Atraumatic Suction
  11. Atraumatic Suction Tip
  12. Fisch Suction Irrigation
  13. Fisch Suction Irrigation Tip
Figure 15:
  1. Tray 3
    1. Micro Scissors
    2. Micro Scissors Tip, Straight
    3. Micro Elevator, Curved Left





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