Planning of a convexity craniotomy is performed in the opposite direction to the surgery. Whereas the surgery starts with the skin incision, the craniotomy planning starts with the lesion (see ▶Fig. 3.1 and ▶Fig. 3.2).
First, the location of the lesion and the trajectory to the lesion has to be determined (which, of course, has already taken place in the mind of the surgeon during positioning of the patient).
There is a starting point for the trajectory, namely the subdural beginning of the corridor to the surgical target. This is the same whether a skull base approach or a transsulcal/transcortical approach is chosen.
The location of the corridor’s beginning (not its end) determines where the dura has to be opened. This can be marked on the patient’s skin.
When the site of the dura opening and the corridor have been determined, the craniotomy can be outlined. This applies not only to convexity craniotomies, but also to skull base craniotomies where the bony opening is determined by the location of other structures such as the transverse and sigmoid sinus (for retrosigmoid craniotomy) or the sagittal sinus (for midline craniotomies). These structures are drawn on the skin first, and then the craniotomy outline is drawn, taking the position of these structures into account.
When the contour of the craniotomy has been outlined on the patient’s skin, the length and shape of the skin incision can be determined.
Fig. 3.1 Approach planning. Inside-out planning of craniotomy and skin incision in convexity craniotomies (a). The location and size of the lesion determines the intraparenchymal or subdural corridor of the approach (b). The corridor defines the size and shape of the dural opening (c), which in turn determines the size and location of the craniotomy (d). Finally, the skin incision is planned according to the location of the craniotomy (e).Fig. 3.2 The keyhole principle. The width of the surgical corridor depends on the depth and location of the lesion. In superficial tumors, the corridor, dural opening, and craniotomy are usually the same size or larger than the lesion (a). For deeper lesions, the corridor, dural opening, and craniotomy can be much smaller because tilting of the microscope can reach the entire lesion, which is the principle of keyhole surgery (b).
3.2 Planning of Craniotomies at the Skull Convexity without the Use of Navigation
Florian Ringel and Andreas Kramer
Introduction
Convexity lesions are poorly defined by landmarks. Craniotomies in these regions are usually performed with the help of a navigation system allowing the exact placement of the craniotomy above a given pathology. Certain circumstances, e.g., emergency treatment, unavailability of sufficient imaging data, or technical failures, among others, might hamper the use of navigation systems and require alternative strategies to place an ideal convexity craniotomy over a given pathology such as the left frontoparietal epidural hematoma shown in ▶Fig. 3.3. Exact localization and projection to the skull can be difficult without the use of a navigation system (▶Fig. 3.4). When neuronavigation is not available, planning of convexity craniotomies in a standardized fashion is possible with the aid of a standard CT scan or MR images. The aim is to assess the relationship of the center and borders of a given pathology to recognizable anatomical landmarks such as the bregma or external auditory canal, among others. The examples below illustrate the necessary steps for a frontoparietal epidural hematoma using data from a CT scan for an emergency situation, and a frontoparietal meningioma using MR images for an elective case (▶Fig. 3.5, ▶Fig. 3.6, ▶Fig. 3.7, ▶Fig. 3.8, ▶Fig. 3.9, ▶Fig. 3.10, ▶Fig. 3.11, ▶Fig. 3.12, Fig. 3.13, and ▶Fig. 3.14).)
Fig. 3.3 From image to reality. Transferring a lesion visible on CT or MRI to the patient’s anatomy can be challenging.Fig. 3.4 Failure to localize a convexity lesion. Orientation and localizing of nonnavigated convexity craniotomy can lead to extension of the initial craniotomy. In this case, several bone flaps were required before the pathology was sufficiently exposed.Fig. 3.5 CT image–based freehand planning: determining the size of the lesion. CT slice thickness in this case is 5 mm. The epidural hematoma is visible on eight slices, resulting in a calculated anteroposterior size of 8 × 5 mm = 40 mm. The superoinferior diameter measured on the slice with the largest diameter is 52 mm. Thus, the whole lesion has a size of 40 mm × 52 mm.Fig. 3.6 Localizing the slice with the largest lesion diameter. The CT scout is used to mark the level of the slice with the maximal diameter of the lesion (b). A line through the center of the lesion, perpendicular to the midline, is drawn (a).Fig. 3.7 Localizing the center of the lesion. The distance from the line through the center of the lesion to the rostral border of the skull in the axial slice is measured (a, red line) and this distance is translated to the CT scout (right red line) onto the level of the slice (b, blue line). The * at the end of the line represents the position of the center of the lesion projected onto the skull. The external auditory canal is marked (b, red circle).Fig. 3.8 Determining the external auditory canal reference line. The distance from the external auditory canal to the center of the lesion (*) is measured (a, red line) and this line is extended to the limit of the bony skull (b, red line). From the crossing point of the line and the limit of the bony skull, the distance to the bregma rostrally is measured (c, red line).Fig. 3.9 Transferring the two measurements onto the patient’s head. These measurements can be transferred to the head of the patient. The bregma (1) is palpated or measured 13 cm from the nasion. From the bregma, 66 mm is measured posteriorly and marked (2). From this point, a connecting line to the eternal auditory canal (3) is drawn. The center of the lesion (4) is on this line, 89 mm superior to the external auditory canal. Finally, a craniotomy according to the diameter of the lesion around the center and ideally with a linear skin incision at least twice the length of the diameter of the craniotomy is prepared. This method is not exactly accurate for high parietal lesions because it does not take the curve of the skull into account. For high parietal lesions, a second measurement from the midline to the midpoint of the lesion on axial slices can help to better localize the target.Fig. 3.10 MRI image–based freehand planning: determining the size of the lesion. The size of the lesion is determined. Measurements are performed in the axial and coronal plane.Fig. 3.11 Determination of the tilt of the coronal plane. The bregma is identified on axial, coronal, and sagittal images. Axial as well as coronal images are brought to the level of the bregma, and sagittal images are positioned to the midline. The angle of the coronal images can be estimated already (a, b). The sagittal images are positioned to the external auditory meatus of the side of the lesion (c). The coronal plane cutting the bregma is brought from the bregma to the posterior border of the external auditory canal and the necessary slices are counted (in this case four slices → 4 × 5 mm = 20 mm) (d). Thereby, the coronal plane of the images is determined to cut the bregma and a point 20 mm anterior to the posterior rim of the external auditory canal perpendicular to the coronal plane.Fig. 3.12 Determination of the tilt of the coronal plane. The bregma is palpated or measured 13 cm from the nasion (1). The external auditory canal is identified. The coronal plane is identified from the bregma (see ▶Fig. 3.13). In our example, the bregma corresponds to a point 2 cm anterior to the external auditory canal (2) in the sagittal view along the coronal tilt. The tumor is on a line parallel to this “coronal tilt” plane 25 mm posteriorly to the bregma line (▶Fig. 3.11). The tumor borders are located on this line 52 mm lateroinferiorly from the midline (3.14) and 86 mm above the Frankfort horizontal plane (3.14). Finally, a craniotomy according to the diameter of the lesion around the center using an ideally linear skin incision at least twice the length of the diameter of the craniotomy is prepared.Fig. 3.13 Position of the center of the tumor in the coronal plane. The coronal plane is moved from the plane cutting the bregma backward to the center of the tumor and the number of slices necessary to reach the center of the tumor are counted, in this case five slices → 5 × 5 mm = 25 mm. Therefore, the tumor is on a line 25 mm behind and parallel to the line from the bregma to a point 20 mm anterior to the posterior rim of the external auditory canal.Fig. 3.14 Distance of the tumor center from the midline and the skull base on the curved skull surface. The distance of the center of the tumor from the sagittal midline and the base is measured in the coronal plane through the maximum diameter of the tumor. As most DICOM viewers do not allow measuring curved lines, several straight lines covering the convexity of the head need to be added. The borders of the tumor of this case are 86 mm above the base and 52 mm from the midline. Finally these measurements are transferred to the patient’s head.
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