2 Assessment of Surgical Exposure

2.1 Introduction

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 aford 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.

2.2 Historical Perspectives

2.2.1 General Considerations

Lesions in the brain can be approached from many diferent angles and through diferent approaches, with the choice of the best approach for a specifc lesion being still a matter of debate in Neurosurgery. The development of diferent approaches has been, and is still today, one of the main topics of the research in Neurosurgery.

The problem of safety and efcacy 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 diferent 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.

2.2.2 Assessment of Operability and

Surgical Exposure

Historically, several authors have analyzed the concept of operability under diferent perspectives.

•Yasargil et al frst 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 signifcantly implemented, making it easier the opening of the Sylvian fssure and the maneuver around the sellar region.

In the last years, several authors have also comparatively analyzed diferent 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 feld, as well as working space, to reach a selected target, by adding diferent osteotomies to a standard fron- to-temporo-parietal craniotomy. The diferent 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, defning the operability as the ability to execute surgical maneuvers on a target area. The application of the concept of defned 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.

2.3Operability Score

As frst 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 feld (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 refects 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 infuenced 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 diferent surgical approaches or diferent 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).

Table 2.1 Score system of the variables of the OS.

Fig. 2.2 Schematic drawing depicting t ariables analyzed by the operability score: surgical angle of attack. Abbreviations: SAA = surgical angle of attack; SC = surgical corridor; T = target.

maneuverability arc.

Abbreviations: MAC = maneuverability arc; SF = depth of the sur ld; T = target.

•Conizing efect (CE). The conizing efect corresponds to a coefcient, calculated by dividing the MAR by the SF. It is directly correlated to the SF. The deeper is the feld the narrower is the surgical cone, with a consequent decrease of the MAC.

•Endoscopic index (EI). The endoscopic index is the ratio obtained by dividing the area of the surgical feld exposed by the endoscope and the whole area exposed by the approach itself. The higher is the score, the more technically demanding is the approach, being considered the endoscopic index as an indirect score of technical complexity.

2.4 Case Illustration

As case illustration, the comparative analysis on operability betweenthefrontotemporal(FT)approachandthefronto-orbito- zygomatic (FOZ) approach is reported (Fig. 2.4). In particular, as illustrative example, the operability on the anterior clinoid process (AC) as obtained by a standard FT and by adding an additional orbital osteotomy (FOZ) were evaluated and compared.

2.4.1 Material and Methods

The study was performed at the Anatomical Laboratory of the Department of Neurosurgery at the George Washington

University (Washington, DC, USA). Silicone-injected cadaveric heads were prepared using standard formaldehyde fxation techniques. Four cadaveric heads underwent a FT frst and a

FOZ thereafter on the same side. The surgical techniques are described in Chapters 15 and 17.

A Zeiss OPM 1 FC microscope was used for microsurgical techniques and morphometric measurements (Carl Zeiss, Oberkochen, Germany) and a Midas Rex drill was used for all bone drilling (Midas Rex, Fort Worth, TX, USA).

The morphometric measurements were accomplished with graded scales. The mean value of the measurements was recorded and served as the basis for the fnal tabulated data. The tip of the anterior clinoid process (AC) was selected as the anatomical target point. At this point the SF, SAA and MAC have been measured for each specimen.

2.4.2 Results

See Table 2.2 for results.

By applying the OS, even if the FT showed the best score for SF, the FOZ, providing a better SAA and MAC, did show a higher OS on the ACP. Table 2.3 summarizes the OS calculated for the two approaches.

These results must be analyzed, taking into consideration the extreme inter-individual variability of the pneumatization

Fig. 2.4 Surgical exposure of fronto-tempo- ral approach (red area) and fronto-orbito-zy- gomatic approach (blue area) of the anterior clinoid process.

Abbreviations: AC = anterior clinoid process; FOZ = fronto-orbito-zygomatic approach; FT = frontotemporal approach.

Table 2.3 Operability score for each approach to AC.

Abbreviations: FOZ = fronto-orbito-zygomatic approach; FT = frontotemporal approach; MAC = maneuverability arc; OS = operability score; SAA = surgical angle of attack; SF = depth of the sur ld.

of the sphenoid sinus and consequently of the development of the ACP.

References

1.Filipce V, Pillai P, Makiese O, Zarzour H, Pigott M, Ammirati M. Quantitative and qualitative analysis of the working area obtained by endoscope and microscope in various approaches to the anterior communicating artery complex using computed tomography-based frameless stereotaxy: a cadaver study. Neurosurgery 2009;65(6):1147–1152, discussion 1152–1153

2.Gagliardi F, Boari N, Roberti F, Caputy AJ, Mortini P. Operability score: an innovative tool for quantitative assessment of operability in comparative studies on surgical anatomy. J Craniomaxillofac Surg 2014;42(6):1000–1004

3.Gonzalez LF, Crawford NR, Horgan MA, Deshmukh P, Zabramski JM, Spetzler RF. Working area and angle of attack in three cranial base approaches: pterional, orbitozygomatic, and maxillary extension of the orbitozygomatic approach. Neurosurgery 2002;50(3):550–555, discussion 555–557

4.Salma A, Alkandari A, Sammet S, Ammirati M. Lateral supraorbital approach vs pterional approach: an anatomic qualitative and quantitative evaluation. Neurosurgery 2011;68 (2, Suppl Operative):364–372, discussion 371–372

5.Sindou M, Emery E, Acevedo G, Ben-David U. Respective indications for orbital rim, zygomatic arch and orbito-zygo- matic osteotomies in the surgical approach to central skull base lesions. Critical, retrospective review in 146 cases. Acta Neurochir (Wien) 2001;143(10):967–975

6.Yaşargil MG, Abdulrauf SI. Surgery of intraventricular tumors. Neurosurgery 2008;62(6, Suppl 3):1029–1040, discussion 1040–1041

Part II

Planning, Patient Positioning,

and Basic Techniques

3Anatomical Landmarks and Cranial

7Skin Incisions, Head and Neck