1.1 Introduction

This chapter introduces the keyhole concept and discusses its essential characteristics. The term “keyhole” is more than merely a gimmick used to sell surgeries or papers, but rather is a theoretical and conceptual framework that is essential to understanding the properties of smaller openings, and how to work effectively in them. More than any other chapter in this book, a thorough understanding of all of the implications of working through a keyhole has the power to alter everything about the way one performs brain surgery, and thus mastering this chapter is critical prior to serious consideration of attempting minimal openings in patients.

1.2 What Does the Term “Keyhole” Actually Mean?

The term keyhole refers to the idea of seeing an entire room through a keyhole in the door to that room. While it certainly is easier to see this entire room by taking the door off the hinges and possibly sawing open the frame to make the opening as wide as possible, this is not an elegant way to accomplish these goals. It is possible (and more elegant) to do so by viewing the room through the keyhole using multiple angles of visualization.

1.3 What Are the Properties of Keyhole?

The brilliance of the keyhole analogy lies in the depth of its descriptive properties, and their implications for minimally invasive brain surgery. Following are the properties of keyhole concept that discuss its essential characteristics:

1. 
   

   To look and work through a keyhole, the use of a single trajectory is not enough to see it all: Unlike a large opening, it is not an option to put the microscope in one position and leave it there the entire case. You must move the microscope more frequently than with a standard opening. Even minute changes in viewing angle make a significant difference. Positioning mistakes are unforgiving, so you must prepare the patient to be turned and moved frequently during the case. You simply cannot allow anything to take a critical viewing and/or working angle away from you.

   
   
2. 
   

   While a wide angle of view can be seen through a keyhole, the best view is down the middle: Thus, while changes in viewing angles can compensate for a suboptimal trajectory, especially at the depths of the opening, the best approach is usually one which places the main area of interest directly in the center of the opening. Usually this is the long axis of the tumor, however other areas of interest, such as the Sylvian fissure, can also be important over which to centralize one’s bone flap.

   
   
3. 
   

   A keyhole is good at showing one the contents of the “room” (i.e., the brain), but is not good at looking at the back of the door (i.e., underneath the edges of the bone): This feature is elegantly depicted in Fig. 1.1. The implications of this point are significant. They suggest that while keyhole surgery may be excellent at addressing pathology located at the depths, it is not very good at working around corners to address superficial disease. The vast majority of the time when we deviate from a tiny craniotomy performed through a small linear incision, it is to expose completely the surface to address superficial disease, or to safely address with a structure on the surface of the brain. While there are times one can get away without completely exposing the surface disease (techniques like cheating, which we talk about in other chapters), in general, all pathologic tissues or non-pathologic structures of interest within 1cm of the bone need to be exposed, and this means a larger craniotomy. However, with image guidance, these craniotomies can still be kept significantly smaller than traditional approaches.

   

   
   
4. 
   

   The keyhole is hard to move: Mistakes in keyhole surgery are typically cognitive, and result in part, from the lack of formal training about how to think about placing the keyhole opening in the correct location. Small openings are less forgiving to planning errors than larger openings. Those who try keyhole surgery without formal training, often struggle, not so much because of the confined space but more from placing the opening incorrectly. They then conclude that the keyhole surgery is dangerous or tedious. Our experience has shown that operating in keyhole approaches is not that different than operating through a larger opening, when done correctly. When one has a negative experience from a keyhole craniotomy, it is important to reflect on what exposure was needed but not provided with the approach performed, and to learn from that mistake. It should be kept in mind that persistence is rewarded with shorter operative times, less wound complications, and better patient outcomes.

   
   
5. 
   

   Keyhole is a concept, not a size: As elegantly stated by the great neurosurgical pioneer, Axel Perneczky, there is no size below or above which an opening is or is not a “keyhole.” Some tumors or diseases inherently require large openings. The major goal of going keyhole is to teach surgeons how to think about making their openings as small as is necessary to achieve the surgical goal, and to get away from the idea of standard approaches, which expose the same structures regardless of the pathoanatomy.