3 Technical Principles of Operating in Keyhole Craniotomies
3.1 Introduction
It is important to note that successfully switching to keyhole surgery does not require a radical alteration in operative technique, or a significant change in operative tools. The learning curve is predominantly cognitive, not manual. If done correctly, keyhole surgery is very similar to traditional brain surgery. Having said this, some minor alterations in surgical technique are necessary, and a brief description of these unique techniques is the subject of this chapter.
3.2 Overall Flow of a Keyhole Operation
We are often asked if we perform the entire operation with an endoscope, and the perceived need for advanced endoscopic skills is seen by many neurosurgeons as a barrier to entry into the field of minimally invasive brain surgery. It is important to note that although keyhole surgery is thought by many to be tightly linked to the endoscope, the majority of the operation is still performed using the microscope. Gross maneuvers, such as the craniotomy, dural opening, and even safe parts of the cytoreduction are performed first without magnification. The microscope is then brought in and used to perform the majority of the tumor resection. The endoscope is brought in, when needed, to look around corners. After obtaining hemostasis using the microscope, the dura, skull, and scalp are repaired using direct vision. In many patients the endoscope is not used at all for tumor resection, but simply to ensure the completeness of tumor resection.
3.3 Maximal Utilization of a Minimal Incision
In order to minimize the length of the skin incision, it is often necessary to retract the skin and/or muscle outward at various points during the craniotomy, to ensure that the bone flap is large enough. A common mistake early in the learning curve is to make the bone flap too small for the tumor. We have an assistant use a small skin retractor to circumferentially retract the skin during the creation of the bone flap, to ensure that the flap is maximal for the scalp incision (Fig. 3.1).
3.4 Frequent Microscope Adjustment
Returning to the idea of seeing a room through the keyhole, it is critical to emphasize that one cannot see the entire room without multiple small changes in viewing angle (to highlight this point, see Videos 3.1 and 3.2). This means that the angle of the microscope must be changed many more times during a keyhole craniotomy than with a larger craniotomy. As an example, Fig. 3.2 demonstrates all the microscope positions needed to perform a simple occipital glioma resection through a keyhole craniotomy. Patients should be secured to the bed so that the bed position can be changed as needed. Often, adjustments are very small, but in keyhole approaches these changes can make all the difference.
3.5 Anticipatory Microscope Positioning
Proper alignment of the microscope viewing angle is critical to success in keyhole surgery. The microscope viewing angle must be adjusted several times during the surgery to ensure that all necessary angles in the craniotomy are being maximally utilized. Often the ideal microscope angle for a specific maneuver is obscured by tumor remnant or brain, which will subsequently be retracted out of the way. In these situations, it is important to anticipate what one wishes to see next, and position the microscope so that the viewing angle is aimed in that direction, almost as if one is looking through the tissue (see Video 3.3). Thus, after the tissue is retracted, the microscope is directed at the ideal angle (Fig. 3.3). This is always a helpful maneuver. However, for keyhole surgery, where minor changes in viewing angle make a major difference, it is critical.
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