Stereotactic Methods of Localization and Image-Guided Surgery
Frameless stereotactic neuronavigation is an excellent example of the integration of modern technology into the practice of neurosurgery. Since its initial description by Roberts and colleagues in 1986,1 the technology has advanced significantly and gained wide acceptance in the field of neurosurgery for both cranial and spinal applications. Neuronavigation assists the surgeon in maintaining spatial orientation and provides real-time information about precise anatomical locations during a procedure. This information allows a neurosurgeon to pursue a surgical objective with greater accuracy while identifying and preserving critical structures. This technology is particularly useful in surgery of the craniovertebral junction (CVJ) because this region contains a high concentration of critical neural and vascular structures.
Neuronavigation
The basic principle of neuronavigation is the correlation of a point within the stereotactic space of an operative field with a location on a digital image. Any location within the operative field can be defined as a distance from a reference point along three spatial planes, and this location can be referenced to a particular point on a three-dimensional (3D) image. There are many commercially available neuronavigational systems from companies such as BrainLAB, Medtronic, and Stryker, but we currently use the StealthStation (Medtronic Corporation, Minneapolis, MN) system ( Fig. 20.1 ). For cranial cases, this system utilizes a dynamic reference frame mounted on a rigid arm attached to the Mayfield head holder, a stereotactic pointing device, a computer workstation, and mounted infrared cameras. The reference frame and pointing device utilize light-emitting diodes (LEDs) or reflective objects that allow the cameras to localize their relative positions in space. The patients’ anatomy must be registered with the wand system by registering the points marked by fiducials placed before imaging or by mapping surface anatomy at surgery. Once the system is registered, the pointing device is placed in a particular location and the corresponding location on the magnetic resonance image (MRI) or computed tomography (CT) scan is displayed on the computer workstation. The microscope also can be mounted with an LED-containing frame that uses the focal point as the tip of the pointing device.
Neuronavigation also can be used to assist with placement of instrumentation during fusion procedures of the upper cervical spine. Because the spine is not a fixed structure like the cranial vault, the use of preoperative imaging for intraoperative navigation is precluded. We use the Iso-C (Siemens, Munich, Germany) system ( Fig. 20.2 ), which employs rotational C-arm fluoroscopy and a reference frame attached to the spine near the operative site.2 The intraoperative C-arm acquires images as it rotates around an isocentric point in space and produces CT-quality images that are immediately registered with the system for neuronavigation.2 Various spinal instruments mounted with LED frames can be used for real-time navigation as instrumentation is placed.
For the StealthStation system to operate correctly, the camera must have a direct line of sight to the reference frame, but the frame must be positioned in a manner that does not obstruct the surgeon or other equipment. Consequently, the exact configuration and location of each piece of the system must be tailored to a given procedure. For cranial cases, the position of the microscope most often determines the position of the camera and reference frame. For suboccipital approaches with a prone patient, we usually operate from directly above the patient. Doing so allows lateral movements so that various surgical angles can be achieved. The reference frame is placed at or below the patient′s shoulder level with the optical camera at the patient′s feet. For retrosigmoid or far-lateral craniotomies, the patient is usually placed in the park bench or supine position with the surgeon and microscope situated lateral to the patient. To maintain a direct line of sight, the reference frame and camera are positioned on the side opposite the surgeon. For upper cervical spine cases, the camera is often placed at the patient′s feet with the reference arc attached to the spine just below the levels of interest.
Utility in Cranial Procedures
The usefulness of neuronavigation is most evident in its application to intracranial surgery. Its use has been widely adopted for intracranial surgery, and we believe that it is particularly useful for approaches to the skull base at the CVJ. The benefit of neuronavigation in skull base surgery is realized during all stages of a craniotomy.
Incision Planning
Once a patient is secured in a Mayfield pin head holder, the Stealth system is registered and used to plan the skin incision. The trajectory to the lesion is visualized, and the incision is placed in a location that ensures that the scalp tissues do not obstruct the surgeon′s view. The guidance system also can be used to verify that the patient′s position is appropriate and that the surgeon will be able to operate comfortably while achieving the necessary working angles. Using navigation in this manner also ensures the minimum incision length and avoids an excessively large incision related to uncertainty about the exact trajectory needed. A good example of this is the hockey stick incision, which historically has been used for far-lateral approaches to easily identify midline structures and to avoid arterial injury. However, the resulting large myocutaneous flap is associated with a certain level of morbidity and often obstructs surgical views. If a smaller, linear, paramedian incision is made, neuronavigation can assist in the identification and preservation of the vertebral artery during the initial stages of the approach.
Related posts:
Posterior Neuroendoscopic Applications at the Craniovertebral Junction
Approaches to the Jugular Foramen
Transoral–Translabiomandibular Approach to the Craniovertebral Junction
Transfacial Approaches to the Craniovertebral Junction
Bypass Options for the Posterior Fossa
Suboccipital and Retrosigmoid Approaches
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