Lateral Surface

The anatomy of the surface of the brain is complex, highly variable, and difficult to identify intraoperatively. Neurosurgeons often expose only a limited part of the cortex, which, in addition, is covered by the arachnoidal membrane and cortical vessels that cross gyri and small sulci. Thus, reliably recognizing the gyri and sulci of the brain surface is often not possible. Image guidance and brain mapping help with intraoperative orientation.

Preoperatively, we rely on knowledge of brain anatomy from MRI and CT scans. On the one hand, these images give more information than the classical surface anatomy view because we can scroll beneath the surface and visualize “true” deep sulci. On the other hand, we only have the three classical planes of axial, coronal, and sagittal view and miss the surface view.

In this chapter, we use a mixture of surface and neuroimaging features to provide a simplified cortical anatomy of the brain for neurosurgeons based on two landmark publications. ^{1 , 2}

Frontally, the superior and inferior frontal sulci separate the superior, middle, and inferior frontal gyrus. The inferior frontal gyrus often has a characteristic appearance. A horizontal ramus and a vertical ramus arising from the sylvian fissure divide it into three parts: the pars orbitalis anteriorly, the pars triangularis in the middle, and the pars opercularis posteriorly, abutting the precentral gyrus. Following the typical “M” configuration of the inferior frontal gyrus from anterior to posterior, the pars opercularis gyrus runs down and connects posteriorly and inferiorly to the precentral gyrus. Thus, it is used to localize the precentral gyrus and the central sulcus on lateral images. The inferior frontal sulcus connects to the precentral sulcus, also locating the precentral gyrus. The classical Brodmann area 44 (pars opercularis) and 45 (posterior half of pars triangularis) are traditionally assigned to Broca’s expressive language center, but the modern theory is much more complex.

Parietally, the intraparietal sulcus divides the superior and inferior parietal lobes. The inferior lobe consists of the supramarginal gyrus and the angular gyrus. Both are involved in higher neuropsychological or receptive language functions on the dominant side or bilaterally.

Temporally, the classical Wernicke area is localized starting anteriorly below the postcentral gyrus—the Heschl gyrus is larger and “kisses” the postcentral gyrus at the sylvian fissure—and extends to the posterior limit of the superior temporal gyrus (▶Fig. 2.1 and ▶Fig. 2.2).

Superior Surface

The superior surface is divided by the central sulcus. Frontally both the superior and the inferior frontal sulcus connect to the precentral sulcus, thus localizing the precentral gyrus. However, the precentral sulcus does not connect to the midline, creating the typical “L” sign where the superior frontal gyrus connects to the precentral gyrus (this “L” also exists for the sulcus). The central sulcus resembles an obliquely flying bird and does not entirely connect to the midline. Its mid part—the body of the bird—represents the larger and typical omega- or epsilon-like hand knob.

Parietally, the ramus supramarginalis of the cingulate sulcus cuts into the superior parietal lobe and is a consistent sign—the bracket sign—that is used to localize the central sulcus, which is one sulcus anteriorly to it. The intraparietal sulcus divides the superior and inferior parietal lobes (▶Fig. 2.3).

Inferior Surface

The inferior surface is structured frontally by the typical “H” configuration of the intraorbital sulci. At the temporal and occipital surface, there are four gyri and three fissures. From lateral to medial, the inferior temporal gyrus (T3) is separated from the lateral occipitotemporal gyrus (T4 + O4) by the occipitotemporal sulcus. In the literature, the fusiform gyrus is most often taken as being synonymous for the lateral occipitotemporal gyrus; however, some restrict the fusiform gyrus to the major central part, but not the entire occipitotemporal gyrus. The collateral sulcus separates the lateral occipitotemporal gyrus (T4 + O4) from the medial occipitotemporal gyrus (T5 + O5), which is synonymous with the parahippocampal (T5) and the lingual gyrus (O5). The lingual gyrus is an additional gyrus that runs from posteriorly into the space between the parahippocampal (T5) and the posterior part (O4) of the lateral occipitotemporal gyrus (▶Fig. 2.4).

Medial Surface

At the medial cortical surface, the sulcus cinguli runs posteriorly and superiorly, finishing as the ramus marginalis sulcus cinguli within the medial cortex, creating the typical “bracket” sign on axial images. The parieto-occipital fissure separates the parietal lobe (the precuneus) from the occipital lobe (the cuneus). The calcarine fissure and the parieto-occipital fissure form a characteristic “Y”-shaped sign which ends behind the splenium of the corpus callosum where the parahippocampal gyrus connects with the cingulate gyrus (▶Fig. 2.5).

References

2.2.1 Introduction

The human skull contains anthropological bony surface landmarks that are present in almost all human beings. These bony prominences or particular sutures are not only interesting anatomical features, but also helpful in terms of how they overlay or relate to cortical structures. Thus, a thorough knowledge of these topographical points can be crucial to neurosurgeons performing surgery in situations where no neuronavigation is available.

Some of these landmarks can easily be palpated through the scalp, whereas others can only be reliably identified after the scalp has been opened. Sutures are best visualized after removal of the periosteum with a rather sharp raspatorium, and even better by rubbing a compress soaked in hydrogen peroxide over the area of interest to enhance the contrast with the surrounding bone. Do not hesitate to use sterile pens during surgery to mark the skin and bone and to visualize and confirm these landmarks. This can facilitate the planning of surgical procedures and improve the accuracy of the approach.

Note that some of these landmarks are less reliable than others. In general, cortical key points may vary in the range of 1 to 2 cm. Although this may sound negligible, these variations may add up to considerable differences between patients (▶Fig. 2.6).

2.2.2 Craniometric Points and Lines and Their Reference to Intracranial Structures

Most Important Cranial Landmarks

Anterior Squamous Point

¹

The connection of the squamous suture to the horizontal line of the “H” of the pterion.

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  Corresponds to the base of the pars triangularis of the inferior frontal gyrus at the level of the sylvian fissure, i.e., the anterior sylvian point. It is usually a retraction in the middle of the typical “M” configuration of the inferior frontal gyrus and small widening of the fissure (see ▶Fig. 2.7).

Asterion

^{2 , 3}

Connection of the lambdoid, occipitomastoid, and parietomastoid sutures.

Important for localization of the transition from the transverse to the sigmoid sinus (sinus “knee”). Mostly on the distal transverse sinus (two-thirds); sometimes below the sinus (one-third); only rarely above (see ▶Fig. 2.8).

Around 5 to 10 mm proximal = posterior to the beginning of the sigmoid sinus, and never on the sigmoid sinus. Unlike the transverse sinus, the sigmoid sinus has a thin and fragile wall. Placement of burr holes and movement of the craniotome over the sigmoid sinus must be avoided to prevent laceration.

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  Should be visualized in retrosigmoid and lateral suboccipital approaches. The transverse sinus has a resistant wall and direct exposure helps to complete the craniotomy.

  
  
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  Cave: Can be variable, and thus tricky to identify due to additional confounding sutural components or even ossa suturarum (bony islands between sutural convolutions). Rub a compress soaked in hydrogen peroxide over the sutures concerned to allow better visualization.

Bregma

The connection between the sagittal suture and the coronal suture (see ▶Fig. 2.9). Located 12 to 13 cm posterior to the nasion. On palpation of the scalp, it can usually, but not always, be felt as a midline protuberance. It is a classical landmark, for example, for planning the placement of an external ventricular drain. For measurements lateral to the midline, the coronal suture is often used as a reference instead of the bregma.

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  The upper end of the central sulcus is located on average 5 to 5.5 cm behind the bregma at the midline; however, some sources quote distances ranging from 4 to 6 cm. ^{1 , 4 – 6}

  
  
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  The precentral sulcus is located on average 3 to 3.5 cm behind the bregma in the midline, ^{4 , 7} but also varies considerably.

Coronal Suture

For measurements lateral to the midline, instead of the bregma, the coronal suture is used as a reference. The more lateral we get, the more anterior the coronal suture runs in relation to the bregma (see ▶Fig. 2.10).

The coronal suture is located:

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  0.5 cm anterior to the bregma at a point 3 cm lateral to the midline.

  
  
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  1 cm anterior to the bregma at a point 5 cm lateral to the midline.

  
  
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  2 cm anterior to the bregma at a point 7 to 8 cm lateral to the midline (stephanion):

  
  
  
  
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    Compared to the coronal suture, the precentral and the central sulcus run even more anteriorly. Thus, the anterior border of the precentral gyrus may approach the coronal suture at the lateral part coming as close as 0.5 cm (on average 1.5–2 cm) at the level of the superior temporal line = stephanion ⁵ (see ▶Fig. 2.21).

Frontotemporale

The name “frontotemporale” is a classical term for this point, but almost not used in neurosurgery. It corresponds to the palpable bony prominence of the base of the zygomatic process of the frontal bone and forms the starting point of the superior temporal line. It is the innermost part of this line, usually immediately behind the orbital rim. In textbooks on craniometrics, it is defined as the narrowest bilateral point of the frontal bone (see ▶Fig. 2.11).

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  It is a frequently used landmark for the floor of the lateral anterior cranial fossa, but usually described rather than named with one term. When lifting the scalp flap during a pterional or frontolateral approach, this is the landmark that indicates sufficient basal exposure to reach the floor of the anterior fossa flush with the craniotomy, i.e., it is the anterolateral skull base point.

Inion

Most prominent part of the occipital bone, also referred to as the protuberantia occipitalis externa. It is formed by the midline junction of the bilateral superior nuchal lines (attachment of occipital and sternocleidomastoid muscles). Usually 7 cm inferior to the origin of the lambdoid suture (see ▶Fig. 2.12).

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  Serves as a demarcation line for supratentorial versus infratentorial (or combined supra-/infratentorial) approaches. Normally overlying torcular herophili (confluens sinuum), but varies widely (see ▶Fig. 2.13). It marks the origin of trapezius muscle.

  
  
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  Cave: Can deviate considerably to one side. May form a spoiler-like bony spur, rendering placement of burr holes hazardous. Only to be used as landmark in combination with information from CT or MRI due to high variability.

Opisthion

The midline point of the posterior rim of the foramen magnum (see ▶Fig. 2.14).

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  Useful during midline posterior fossa approaches and for approaches to the craniocervical junction, which necessitate opening of the foramen magnum, e.g., during decompression procedures for Arnold–Chiari malformations.

  
  
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  Care must be taken when palpating the opisthion to avoid forceful penetration of the atlanto-occipital membrane. The membrane can be very thick and firmly attached to this part of the occipital bone.

  
  
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  Part of many radiographic lines that help in diagnosing atlanto-occipital dislocation, basilar invagination, and platybasia, such as the McRae line, Chamberlain line, Powers ratio, etc.

Pterion

This is not a single point. It is a small region, where four bones meet, namely ala major of sphenoid bone, squama temporalis, frontal bone, and parietal bone. Thus, in this region, the coronal, sphenofrontal, sphenosquamous, squamous, and sphenoparietal sutures unite in a typical H-like configuration (see ▶Fig. 2.15).

Situated 2 to 2.5 finger’s breadth above the zygomatic arch and 1 to 1.5 finger’s breadth behind the lateral orbital rim.

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  Virtual central point for important standard craniotomies, to provide (transsylvian) access to, e.g., the anterior part of the circle of Willis and the (para- and supra-) sellar region, orbital roof and apex, insula, and temporomesial region.

  
  
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  Indicates the lower border of the inferior frontal gyrus close to the sylvian fissure at the level of the lateral orbital gyrus.

  
  
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  The base center of the “M” of the inferior frontal gyrus is just posterior to the pterion at the anterior squamous point (see ▶Fig. 2.7).

Stephanion

Intersection of coronal suture with the superior temporal line (STL).

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  Indicates the level of the inferior frontal sulcus.

  
  
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  On average, the precentral sulcus is 1.5 to 2 cm posterior to the stephanion, but it may come as close as 0.5 cm to it. ⁵

  
  
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  Can be used to localize the motor speech area by drawing a rectangle as follows:

  
  
  
  
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    Start at the stephanion.

    
    
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    Draw a line to the anterior squamous point (▶Fig. 2.16a).

    
    
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    The potential “wider motor speech area” is situated 1 cm immediately anterior to this line (includes the pars triangularis) and 3 cm posterior to this line (includes the pars opercularis and inferior precentral gyrus) (▶Fig. 2.16b).

2.2.3 Skull Base Points

These landmarks are helpful to localize the floor of the various cranial fossae. They are important during skin incision, lifting of the skin flap, and in providing the appropriate bony exposure for making the burr holes and craniotomy.

2.2.4 Other Cranial Landmarks

(see ▶Fig. 2.6a–c)

There are many other craniometric points that are for connoisseurs only: Alare, Ectoconchion, Gnation, Prosthion, Rhinion, Ophryon, Gonion, Vajkonion, Meyrion, etc., but apart from the Schallerion they have no importance for craniotomies.

When the Schallerion was first discovered, it did not mean anything. Then, after much research had been done, it continued not to mean anything. Therefore, it is regarded as the 42 of craniometry. ⁸

Most Important Craniocerebral Topography Lines

Craniocerebral topography lines	Relevance
Anterior commissure–posterior commissure (AC–PC) line (= bicommissural line)
Introduced by Talairach in 1952 as the line connecting the upper anterior commissure with the lower posterior commissure Slightly more angled (0–10°) than the canthomeatal line (= orbitomeatal line) used in CT scanning (▶Fig. 2.17)	Not a craniometric line, but the standard plane for axial MR imaging
Canthomeatal line
Also orbitomeatal line, from the center of the canthus to the center of the porion	Traditional line for tilting the CT gantry, because it is easy to identify on the CT scout A CT with this tilt is associated with a relatively high dose to the cornea Increasingly, the AC–PC line is used instead, as it is routine in MR imaging; the AC–PC line is slightly more angled (0–10°)

Craniocerebral topography lines	Relevance
Frankfurt horizontal plane and associated lines
A line connecting the lower rim of the orbit with the upper rim of the porion	Indicates the deepest point of the middle fossa (±7 mm) When planning a craniotomy without image guidance, this line may help to transfer distances and angles from the CT or MRI image to the patient By drawing a perpendicular line to the Frankfurt horizontal plane through the tip of the mastoid, the position of the apical part of the central sulcus can be estimated (▶Fig. 2.18). Note that this measurement can result in considerable error in estimation
Taylor–Haughton lines
Using these lines, the position of the central sulcus and the sylvian fissure can be estimated. There are lines, connecting: Nasion, bregma, and inion in the midsagittal line Frankfurt horizontal plane with two perpendicular planes: Condylar Mastoid	Measuring the distance from the nasion to the inion gives approximations of the following structures: Half of the distance plus 2 cm locates the most posterior point of the central sulcus in the midline (very accurate) 9 A line connecting a point three-fourths of the distance to the canthus combined with the condylar and mastoid lines locates the central sulcus and the sylvian fissure (not very accurate)

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3 Convexity Craniotomies 7 Transsphenoidal Approach 1 Basics 4 Midline Craniotomies 6 Skull Base Extensions 5 Skull Base Craniotomies

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