2 Assessment of Surgical Exposure Neurosurgery is one of the medical specialties that has experienced a rapid technological development. The advancements made in imaging technologies, surgical equipment, and the acquired knowledge in microanatomy and pathology have dramatically improved the safety and surgical outcomes of patients worldwide. The careful evaluation of these features is essential in planning the most appropriate surgical approach, as well as in selecting the surgical tools that most afford a safe and extensive resection of the pathology. In this chapter, we explain the operability score (OS), as an application of some simple geometrical concept to preoperatively evaluate not only the surgical target, but also the trajectory used to reach it, the maneuverability space around the target, and the surgical angle of attack. Lesions in the brain can be approached from many different angles and through different approaches, with the choice of the best approach for a specific lesion being still a matter of debate in Neurosurgery. The development of different approaches has been, and is still today, one of the main topics of the research in Neurosurgery. The problem of safety and efficacy of surgeon’s maneuvers have represented the leading subject of historical neurosurgical evolution, starting from the more extensive and invasive surgical approaches since the beginning of the 20th century, to the era of “Minimally Invasive Neurosurgery,” passing through the development of the microscope, endoscope, and advanced imaging technologies. In particular, skull base surgery, with the improvement of anatomic knowledge, the introduction and development of innovative instruments and diagnostic tools, the application of different display devices, such as microscope and endoscope, has evolved from more aggressive to less invasive tailored approaches based on a careful preoperative planning. The OS represents a further advance in this area. Historically, several authors have analyzed the concept of operability under different perspectives. • Yasargil et al first described the concept of operability related both to patient-linked variables (i.e., age, general and clinical conditions, previous therapies), as well as to pathology-linked patterns. In particular the latter did consider several factors: location and number of lesions (unilateral or bilateral), composition (size, vascularization), and characteristics of the tumor (growth pattern, benign/malignant lesion, edema, presence of hydrocephalus). Considering these factors together with surgeons’ personal skills, it was possible to qualitatively assess tumor operability. Again, Yasargil, by analyzing surgery of intraventricular tumors, demonstrated that, by drilling the sphenoid wing in a pterional approach, the surgical cone was significantly implemented, making it easier the opening of the Sylvian fissure and the maneuver around the sellar region. In the last years, several authors have also comparatively analyzed different approaches and their variants in terms of operability on selected targets. • In 2001, Sindou et al described the concept of working cone by approaching central skull base lesions. Authors quantitatively analyzed surgical trajectory, depth and width of the surgical field, as well as working space, to reach a selected target, by adding different osteotomies to a standard fronto-temporo-parietal craniotomy. The different working cones did provide a multiangle visualization of the selected target, depending on its anatomical location, morphology, neurovascular relationships, and pathological features. Authors concluded that additional orbital or zygomatic osteotomies were useful in implementing the working cone for tumor removal, avoiding brain retraction. • Gonzalez et al in 2002 have further developed these concepts, defining the operability as the ability to execute surgical maneuvers on a target area. The application of the concept of defined surgical triangles in the pre-operative planning was found to be helpful in individualizing the approach, tailoring the surgical corridor according to tumor anatomic location. • Filipce et al in 2009 qualitatively and quantitatively analyzed the extent of the working area, as obtained by microscope and endoscope, by treating anterior communicating artery aneurysms. They claimed as advantage of the endoscope the direct view and illumination, and the 3D visualization as main advantage of the microscope. By combining the advantages of each technique, they stated that endoscope-assisted microscopic approaches were the best way to look around corners and to guarantee an optimal 3D view. Interestingly, angled endoscopes allowed for a better visualization, which might not necessarily implicate a direct improvement of the working corridor. • Salma et al in 2011 proposed a qualitative score system to compare the exposure obtained by pterional and supraorbital craniotomy. Even if supraorbital craniotomy did provide a less invasive way to reach the sellar region, the pterional approach presented a wider pyramidal-shaped surgical corridor as compared to the cylindrical-shaped one of the supraorbital approach, increasing the overall surgical operability. As first described by Gagliardi et al, the OS summarizes all the analyzed variables mentioned above by applying some geometrical concepts in the surgical preoperative evaluation of the lesion, in order to evaluate the main criticisms that could be encountered. These key points are easy to apply in most of the situations: • Depth of the surgical field (SF). The SF represents the length of the major axis of the surgical corridor. It is assessed, by measuring the distance between the maneuverability area and the target. The translational value of this measure is explained by the fact that dealing with deeply located lesions might represent a challenge in terms of surgical comfort and tumor control (Fig. 2.1). • Surgical angle of attack (SAA). The SAA corresponds to the angle of incidence of the surgical corridor toward an area of interest. The more the angle is wide, the more comfortable is the approach and this reflects the possibility to better control the target (Fig. 2.2). • Maneuverability arc (MAC). The MAC consists in the maximal degrees of maneuverability of surgical instruments around a target and is intrinsically influenced by the wideness of the surgical cone. As already stated for the SAA, the width of the arc directly determines the control of the target (Fig. 2.1). Assigning a numerical score to each variable by comparing different surgical approaches or different targets within the same surgical approach enables to graduate surgical complexity, optimizing the pre-surgical planning. The score system consists in assigning to each variable 0 or 1 according to Table 2.1. Other geometrical concepts could be considered in selected cases, such as • Maneuverability area (MAR). The MAR is the cross-section area, as calculated at the narrowest point in the surgical corridor. From a geometrical perspective, it corresponds to an ellipsoid (Fig. 2.3).
2.1 Introduction
2.2 Historical Perspectives
2.2.1 General Considerations
2.2.2 Assessment of Operability and Surgical Exposure
2.3 Operability Score
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