2. • 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 afford a safe and extensive
• resection of the pathology.
3. • 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.
4. • Historical Perspectives
• 2.2.1 General Considerations
• 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.
5. • 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.
6. • 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.
7. • Assessment of Operability and
• Surgical Exposure
• 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
8. • 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.
9. • In the last years, several authors have also comparatively analyzed
• different approaches and their variants in terms of operability
• on selected targets
10. • 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.
11. • 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.
12. • 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
13. • 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.
14. • Operability Score
• 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.
15. • 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).
16. Schematic drawing depicting tariables
analyzed by the operability score: depth of the surld
and
maneuverability arc.
Abbreviations: MAC = maneuverability arc; SF = depth of
the
surld; T = target.
17. • 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).
18. Schematic drawing depicting tariables
analyzed by the operability score: surgical angle of attack.
Abbreviations: SAA = surgical angle of attack; SC =
surgical
corridor; T = target
19. • 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
20. • 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
21.
22. • • 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).
Schematic drawing depicting tariables
analyzed by the operability score: maneuverability area.
Abbreviations: CE = conizin t; MAR =
maneuverability area;
SF = depth of the surld; T = target
23. • • Conizing effect (CE). The conizing effect corresponds to a coefficient,
• calculated by dividing the MAR by the SF. It is directly
• correlated to the SF. The deeper is the field 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 field 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.
24. Case Illustration
• As case illustration, the comparative analysis on operability
• between the frontotemporal (FT) approach and the fronto-orbitozygomatic
• (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.
25. Surgical exposure of fronto-temporal
approach (red area) and fronto-orbito-zygomat
approach (blue area) of the anterior
clinoid process.
Abbreviations: AC = anterior clinoid process;
FOZ = fronto-orbito-zygomatic approach;
FT = frontotemporal approach.
26. • 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 fixation
• techniques. Four cadaveric heads underwent a FT first and a
• FOZ thereafter on the same side. The surgical techniques are
• described in
27. • 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 final 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.
28. • 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
• of the sphenoid sinus and consequently of the development of
• the ACP.
