1 A Minimally Invasive Perspective: The Conversion

Abstract

A distinct difference exists in the way the mind processes an open exposure as compared to a minimally invasive exposure. In the absence of the midline structures, the basis of orientation is different. The angles converging on the spine on a lateral to medial trajectory alter the appearance of the otherwise familiar surface anatomy. The absence of the recognizable midline structures, along with an unfamiliar perspective of the surface anatomy, has the potential to disorient the mind. This chapter analyzes the difference between these exposures and explores how the mind can become oriented in minimally invasive approaches without the midline structures. Delving into the topics of recall memory and recognition memory of the spinal anatomy furthers our understanding of the basis of orientation and disorientation. Deconstructing the origin of disorientation in minimally invasive approaches becomes the foundation upon which to build a path to a more efficient learning curve for minimally invasive spine surgery. The chapter ends with covering the essential components of minimally invasive spine surgery and reviewing the premise and principles of minimally invasive spine surgery that will be the basis of the subsequent chapters in this Primer.

Keywords: exposure, facet, lamina, learning curve, orientation, recall memory, recognition memory, spinous process, three-dimensional spinal anatomy

The mind once enlightened cannot again become dark.

Thomas Paine

1.1 Introduction

The capacity to reconstruct the spinal anatomy at depth without the midline structures to orient the mind is the very essence of minimally invasive spine surgery. In traditional open approaches, we begin in the midline and work outward. In doing so, we expose the landmarks of the spinous process, lamina, facets and transverse processes in sequential order. These landmarks orient our mind and allow us to move through the procedure confidently and efficiently. Our visualization of these midline landmarks provides us certain knowledge of the anatomy. In minimally invasive approaches, we have neither the midline nor the conventional exposure of these landmarks. Instead, we must rely on visualization of the anatomy in the mind’s eye of what is not seen or even unveiled. The mind must visualize the entire facet, although only a hint of that facet may be exposed. The ability to reconstruct that anatomy with only limited exposure will prevent the mind from becoming disoriented. The capacity to maintain orientation will directly affect the proverbial learning curve in minimally invasive surgery on the spine. Therefore, the concept of what orients our minds in spine surgery is the focus of this chapter.

To borrow from nautical analogy, we should consider how, when learning to sail the ocean, we would be ill-advised to lose sight of the shore. The shoreline orients the mind of the sailor to the cardinal directions of north, south, east and west. Becoming familiar with a vessel by doing nothing more than sailing up and down the coastline provides the sailor with a foundation to venture farther and farther out to the sea. Eventually, the sailor no longer requires a view of the shoreline to remain oriented. With an understanding of the position of the sun, the stars, a sextant and even the use of computer-assisted navigation, the sailor can safely bring the ship back into a safe harbor.

In traditional midline open spine surgery, the midline structures are the shoreline that forms the basis of orientation for the surgeon. Recognizing it as such helps the surgeon to understand how the absence of the midline in minimally invasive approaches is the root cause of disorientation, the same way the loss of the shoreline is potentially disorienting for the novice sailor. The reader must always keep this key principle in mind. The limited exposure of a transverse process or the inferior aspect of a facet can become indistinguishable when viewed through a 16-mm-diameter aperture ( ▶ Fig. 1.1).

A juxtaposition of two exposures. Two minimally invasive exposures seen through a 16-mm minimal access port. At first glance, both exposures look very similar. However, each represents a completely di

Fig. 1.1 A juxtaposition of two exposures. Two minimally invasive exposures seen through a 16-mm minimal access port. At first glance, both exposures look very similar. However, each represents a completely different part of the lateral spinal anatomy, as shown by the fully dissected exposure on the right. In the absence of the midline elements, orientation with limited exposure can be quite challenging. In the end, one exposure appears ideally suited for a transforaminal approach, whereas the other exposure is too lateral to access the foramen. The length of the access port and the starting distance from the midline collectively orient the mind by incorporating the trajectory of convergence.

The mind must replace those visual reference points with its own reconstruction of the anatomy. The angle of convergence of a minimal access port onto the spine or its rostrocaudal trajectory further affects that reconstruction. Although such factors are not relevant in an open procedure, they can change the entire landscape in a minimally invasive procedure. On the one hand, the traditional midline exposures offer the visual cues of spinous processes, lamina and facets ( ▶ Fig. 1.2). These reference points allow you to keep your bearings during an operation. On the other hand, a minimally invasive approach does not unveil any of these midline bony landmarks. Instead, only limited portions of these landmarks in isolation are available to orient the mind. In the end, minimally invasive approaches require more from your mind than the open equivalent.

Traditional midline open exposure for a lumbar laminectomy. Illustration demonstrates the midline elements, which remain in the surgeon’s field of view. The midline keeps the mind oriented throughout

Fig. 1.2 Traditional midline open exposure for a lumbar laminectomy. Illustration demonstrates the midline elements, which remain in the surgeon’s field of view. The midline keeps the mind oriented throughout the procedure. The absence of these midline elements in minimally invasive exposures can make bony prominences resemble one another, as shown in ▶ Fig. 1.1.

What you accomplish with a midline exposure is a complete unveiling of the spinal anatomy at depth. As a result, there is no need for the mind to reconstruct any component of that anatomy or to speculate on whether a bony prominence is a facet or a transverse process, as demonstrated in ▶ Fig. 1.1. Whether or not you realize it, your eye is constantly scanning these visual cues in an open exposure to keep your mind oriented. In a midline open approach, you never lose sight of the shore.

A minimally invasive exposure is completely different. It forces your mind to become oriented and to stay oriented without the midline and with less visualization of the spinal anatomy. Although it is a highly efficient exposure with regard to the surgical target relative to the surgical exposure, there is no midline and very little anatomy to scan as a reference point. Your mind is left to reconstruct the anatomy around the limited exposure offered by the diameter of the minimal access port. Open approaches teach us the anatomy, but reconstruction of the anatomy is unnecessary by the nature of the exposure. We can see everything. Therefore, the capacity to accurately and efficiently reconstruct this anatomy is an acquired skill unique to minimally invasive spine procedures. To confidently and expediently move through a procedure, the minimally invasive surgeon must learn how to connect the lines of the anatomy from what is seen to what is unseen. The mind must also incorporate the trigonometry of convergence and the effect it has on the exposure at depth.▶ Fig. 1.3 illustrates this point by demonstrating how the degree of convergence can result in two different exposures despite the same incision. You will find that as the length of the minimal access port increases, the effect of convergence plays an increasingly detrimental role in envisioning the ideal position of the exposure.

The effect of convergence. (a) Illustration of a lumbar segment in the axial plane demonstrates two exposures (medial, turquoise dashed line; lateral, ruby dashed line) through the same access port (1

Fig. 1.3 The effect of convergence. (a) Illustration of a lumbar segment in the axial plane demonstrates two exposures (medial, turquoise dashed line; lateral, ruby dashed line) through the same access port (16 mm in diameter and 7 cm in length). The mind must consider several factors to reconstruct the spinal anatomy at depth. The location of the incision off the midline, the degree of convergence of the access port, and the rostrocaudal trajectory collectively affect the exposed anatomy at depth. (b) Posterior view of the spine of the previous axial image again demonstrates that both incisions can be placed 2 cm off the midline. A different exposure of the anatomy at depth may be the result, depending on the degree of convergence. The large dark ring represents the proximal aspect of the port at the level of the skin, and the smaller colored rings represent the access port at the level of the spine. The degree of convergence is the difference in these two exposures, which otherwise are identical with regard to the location of the incision and the length of the minimal access port. The medial exposure (turquoise ring) is ideal for a microdiscectomy, whereas the more lateral exposure (ruby ring) is too lateral and suboptimal for the procedure.

You must not think that the reconstruction of the spinal anatomy at depth depends solely on recognizing limited glimpses of the lamina or the facet or the transverse process. Several components other than direct visualization remain at your disposal. Use them all. Precise planning of the incision will place the exposure in the vicinity of familiar and relevant anatomy. Sounding the anatomy with the initial dilator will provide tactile feedback as to the location of the facet, the lamina and the interlaminar space. Lateral and anteroposterior fluoroscopic images reveal the degree of convergence and confirm the location. All these components combine to provide information that will begin the reconstruction process before you ever peer down the minimal access port. Incorporating every aspect of these components fills the void created by the absence of a wide exposure and the lack of orienting structures of the midline. When you finally do peer down the minimal access port, your eye will possess the absolute certainty of location from the integration of all these components. Again, it is the reconstruction of the spinal anatomy at depth with limited exposure and absence of the midline elements that is the essence of minimally invasive spine surgery. It is the skill that must be mastered. Having introduced the concept of orientation, I would like to examine the often-discussed learning curve in minimally invasive spine surgery from a different perspective.

1.2 The Proverbial Learning Curve

The learning curve in minimally invasive spine surgery is steep. I hear surgeons utter this statement time and time again in course after course and in lecture after lecture. Yet, I have never found this statement to be particularly helpful to the aspiring spine surgeon who wishes to embrace minimally invasive techniques. If anything, it serves instead as a potential deterrent. I would suggest that such a statement, which I believe does have a kernel of truth to it, requires further examination from a different vantage point. I often tell residents and fellows to forget about the slope of the learning curve and to instead consider what is actually going on in the area beneath that curve ( ▶ Fig. 1.4). What happens as the cases continue to accumulate along the x-axis and your proficiency with the procedure increases along the y-axis? What is going on that explains that change?

Fig. 1.4 The minimally invasive learning curve. (a) Learning minimally invasive surgery should require more of an emphasis on understanding what transpires in the area beneath the curve than a concern for the upward slope of the curve. The area beneath the curve represents the mind acquiring the capacity to reconstruct the spinal anatomy at depth and the surgeon being able to move through the exposure and the operation more confidently. (b) Initially, the surgeon’s eye will see only what is directly exposed. The facility of the procedure will be lowest at this juncture because the ability to reconstruct the anatomy at depth is limited. (c) As recall aptitude of the spinal anatomy builds, the mind’s eye can “see” more than what is visualized. (d) When a surgeon’s recall aptitude is highest, the mind will be able to reconstruct the anatomy at depth as shown, where the mind can “see” beyond what is exposed. Proficiency with the procedure will be highest at that point because of recall memory aptitude, not necessarily because of the number of cases completed.