Anatomical Landmarks and Cranial Anthropometry

3 Anatomical Landmarks and Cranial Anthropometry

Victor Fernández Cornejo, Javier Abarca Olivas, Pablo González-López, and Iván Verdú-Martínez

3.1 Introduction

In this chapter, we describe surface landmarks of the skull and how they relate to underlying important cortical structures.

These correlations are particularly important and a great aid, when planning surgical approaches, as they assist in tailoring the approach, avoiding unnecessary exposure and minimizing risk of injuries to the brain parenchyma.

Even though there is an individual variability among skulls and underlying vascular and cortical anatomy, the knowledge of these markers is of paramount importance to “outline” the position of vital structures, which will be uncovered during the surgical approach.

3.2 Main Cranial Landmarks (Fig. 3.1)

Looking at the skull, these landmarks become readily available in the different aspects as seen in Fig. 3.1.

• Frontal view: Nasion, Bregma, and Frontozygomatic point (Fig. 3.1A).

• Lateral view: Coronal Suture, Stephanion, Pterion, Frontozygomatic Point, Squamous Suture, and Temporal Line (Fig. 3.1B).

• Superior view: Bregma, Lambda, Sagittal Suture, and Coronal Suture (Fig. 3.1C).

• Posterior view: Lambdoid suture, Opisthocranion, Inion, Asterion, and Superior Nuchal Line (Fig. 3.1D)

While some of these landmarks represent intersections of different bones of the neurocranium, some are marks left on the bone by the attachment of muscles or are simply relevant to the palpation of the skull (i.e., opisthocranion).

• Nasion (Na): Point of intersection between the two nasal bones and the frontal bone.

• Bregma (Br): Point of intersection between the coronal and sagittal suture.

• Frontozygomatic Point (FzP): Suture in the lateral wall of the orbit, between the frontal and zygomatic bones.

• Superior Temporal Line (STL): Line of attachment of the temporal fascia to the skull, crossing the middle of the parietal bone in an arched direction.

• Stephanion (St): Point of intersection between the superior temporal line and coronal suture.

• Pterion (Pt): Area where frontal, parietal, temporal, and sphenoid bones join together.

• Lambda (La): Point of intersection between the sagittal and lambdoid sutures.

• Opisthocranion (Op): Most prominent point of the occipital bone.

• Inion (In): External occipital protuberance.

• Asterion (As): Posterior end of the parieto-mastoid suture.

• Superior Nuchal Line (SNL): Line of insertion of the semispinalis capitis muscle in the occipital bone.

3.3 Cranial Anthropometry (Fig. 3.2)

Once the work of memorizing these landmarks is done, they can be put to good use in calculating distances and relationships among these landmarks and the underlying brain.

Fig. 3.1 Main cranial landmarks. (A) Frontal view. (B) Lateral view. (C) Superior view. (D) Posterior view.
Abbreviations: As = asterion; Br = bregma; Cos = coronal suture; FzP = frontozygomatic point; In = inion; La = lambda; Ls = lambdoid suture; Na = nasion; Op = opisthocranion; Pt = pterion; SNL = superior nuchal line; SqS = squamous suture; SS = sagittal suture; St = stephanion; STL = superior temporal line.

3.3.1 Main Measurements of The Skull

Approximate distance between important landmarks in adult skull:

• Nasion-Bregma: 13 cm

• Nasion-Lambda: 24-25 cm

• Bregma-Lambda: 12-13 cm

• Pterion-Frontozygomatic Suture: 3 cm

• Opisthocranion-Inion: 2 cm

• Lambda-Opisthocranion: 2-4 cm

• Inion-Lambda: 4-6 cm

3.3.2 Location of the Main Cerebral Sulci And Gyri

Some of the important cerebral landmarks can be located with good approximation with the help of artificial lines constructed on the previously described bony and surface markers, considering the variability among patients related to individual anatomy and concurrent pathologies.

• The Sylvian Fissure (SF). A line that is constructed along the sagittal convexity helps determine the distance from Nasion to Inion. It is important to locate the Sylvian Fissure as it courses parallel (a few mm above or below) a line that joins the Frontozygomatic Point to the three-quarter point of the Nasion-Inion mid-sagittal line on the lateral surface of the head (Fig. 3.2A).

• The Central Sulcus (CS). It runs parallel or along a line constructed connecting the Superior Rolandic Point (SRP) and the Inferior Rolandic Point (IRP). SRP is 5 cm behind the Bregma and IRP is 2.5 cm behind the Pterion (Fig. 3.2B).

• Precentral Gyrus (PreCG). It is located 4.5 cm posterior to the Bregma (midline) and 2.5 cm posterior to the Stephanion (lateral surface) (Fig. 3.2C).

• Postcentral Gyrus (PostCG). Situated 6.5 cm posterior to the Bregma (midline) and 4 cm posterior to the Stephanion (lateral surface) (Fig. 3.2C).

• Calcarine Sulcus. It is located 3-4 cm inferior to the Lambda and 2 cm superior to the Inion, on the mesial aspect of the occipital lobe (Fig. 3.2D).

3.4 Sulcal Key Points. Correlation with Craniometric Points

3.4.1 Anterior Sylvian Point (ASP) (Fig. 3.3)

• Description: It is an enlargement of the Sylvian Fissure, just inferior to the triangular part and anterior to the opercular part of the Inferior Frontal Gyrus (IFG) (Fig. 3.3A).

• Craniometric correlation: On the cranial surface, it relates well to the Anterior Squamous Point (ASqP). This is located on the most anterior segment of the Squamous Suture (SqSut) just posterior to the Pterion (Fig. 3.3B, Fig. 3.3C).

• Clinical implications: It represents the starting point to open the SF and localizes the insular apex. Middle cerebral artery bifurcation is deeper and 1-2 cm anterior to it.

• Correlation: The distance between the Anterior Sylvian Point (ASP) and the Inferior Rolandic Point (IRP) along the Sylvian Fissure is 2-3 cm (Fig. 3.3D).

3.4.2 Inferior Rolandic Point (IRP) (Fig. 3.4)

• Description: It represents the intersection of the lower extremity of the Central Sulcus and the Sylvian Fissure (Fig. 3.4A).

• Craniometric correlation: On the cranial surface the Superior Squamous Point (SSqP) is a good landmark. It represents the intersection between the Squamous Suture and a vertical line coming from the preauricular depression, just in front of the tragus. The average vertical height of this segment is 4 cm above the zygoma. (Fig. 3.4B). The Inferior Rolandic Point is located 2.5 cm posterior to the Pterion along the Sylvian Fissure.