Identification of Surface Landmarks of the Skull

The Base of the Skull (Figs. 10 to 13)

Rather than describing the classic features of the base of the skull, we have chosen to focus on the orientation of the head in space and the consequences of its position during the operation. How the base of the skull is positioned for a pterional approach or one via the posterior fossa is extremely important and is going to determine the outcome of the procedure.

Pterional Approach

Fig. 10. Anatomical modifications after the patient's head has been rotated to the opposite side

Fig. 10. Anatomical modifications after the patient's head has been rotated to the opposite side

Petrous Carotid Exposure

Fig. 11. Superior (a) and lateral (b) views of a drilled left dry bony labyrinth after resection of the middle ear. Note the carotid canal (CC) which is partially covered by the cochlea (C). Note the relationships between the facial nerve (FN) and the semicircular canals (SSC)

Fig. 11. Superior (a) and lateral (b) views of a drilled left dry bony labyrinth after resection of the middle ear. Note the carotid canal (CC) which is partially covered by the cochlea (C). Note the relationships between the facial nerve (FN) and the semicircular canals (SSC)

The performance of the surgeon's approach begins with proper positioning of the patient's head and proper location of the bone flap.

Important Points to Remember

The greater the inclination of the cranial axis away from the vertical, the more vertical will be the direction of the sphenoidal crest and the more horizontal will be that of the petrosal crest. It applies for the sitting position also: if the head is rotated, the petrosal crest is in a more sagital position. The positions of these bony landmarks are fixed and they can constitute a useful aid to establish spatial relationships during a procedure, as long as the ''dynamic'' characteristics of the anatomical situation are understood. The advent of surgical neuronavigation tools has not in any way dispensed with the need for a profound understanding of anatomy, and laboratory-based teaching of the spatial relationships is crucial.

Skull Bony Labyrinth
Fig. 12. (a and b) Drilling of a left bony labyrinth in a cadaver specimen to show the relationships between the facial nerve (FN) and the bony structures. Note the location of the sigmoid sinus (SS) and the temporal dura (TD)

Important Relationships at the Skull Base: The Bony Labyrinth, the Facial Nerve and the Petrous Segment of the Carotid Artery

A comprehensive review of the anatomy of all the bones of the base of the skull is beyond the scope of this chapter [4] but an awareness of certain anatomical relationships is essential if the principle of the trans-petrous approaches is to be grasped.

Middle Cranial

Fig. 13. Superior view of a drilled right middle cranial fossa in a cadaver specimen to show the relationships between the trigeminal nerve (TN), the carotid artery (CA), the facial nerve (FN), the cochlea (C), the semicircular canals (SSC) and the tympanic cavity (TC)

Fig. 13. Superior view of a drilled right middle cranial fossa in a cadaver specimen to show the relationships between the trigeminal nerve (TN), the carotid artery (CA), the facial nerve (FN), the cochlea (C), the semicircular canals (SSC) and the tympanic cavity (TC)

A dry specimen of a left sided drilled bony labyrinth can be used to show the ventral segment (cochlea) and the dorsal segment (vestibule and semicircular canals). Its length is about 2 cm located in the long axis of the petrous pyramid. The relationships between the lateral tract of the lateral semicircular canal and the second portion of the facial nerve, and between the ampullae of the superior and lateral semicircular canals and the genu of the facial nerve are to be emphasized. These landmarks are routinely used as a guideline when drilling the bone to avoid damage to the facial nerve during posterolateral approaches.

The cochlea lies near the cortical substance of the petrous apex. The carotid canal is usually partially covered by the cochlea so it is impossible to displace the carotid artery downwards from above without entering the cochlea. The petrous segment of the carotid artery may be or may not be covered by bone at the level of the middle cranial fossa. If not, it is separated from the trigeminal enlargement by a dural layer. If the floor of the middle cranial fossa is drilled, the facial nerve within the internal auditory meatus, the geniculate ganglion, and the middle ear will be exposed. Drilling the petrous apex medial to the carotid artery gives access to the posterior fossa along the inferior petrosal sinus.

The Key Surface Structures (Figs. 14 to 16)

The ''keyhole'' concept makes appropriate placement of the first burr hole of importance. Two of these are particularly important, the one in the pter-ional region and that around the sinuso-jugular axis.

The Pterional Approach

The hole should be made behind the lateral crest of the external orbital rim of the frontal bone. This will afford access to the inferior surface of the frontal lobe just above the roof of the orbit.

Keyhole Burr

Fig. 14. (a and b) The pterional keyhole burr hole is show for the pterional approach b

Fig. 14. (a and b) The pterional keyhole burr hole is show for the pterional approach

Bony Landmarks Skull
Fig. 15. Corrosion cast showing the relationships between the sigmoid sinus (SS) and the bony structures at the skull base

Venous Sinus Relationships to Surface Landmarks of the Skull

The sinuso-jugular axis is one of the keys to access to the base of the skull. It is dangerous (being surrounded by many other structures) but accessible (superficial, posterior, continuous). All corrosion casts show that the transverse sinus, the transverse-sigmoid junction and the sigmoid sinus are deeply located within a large bony groove which makes the first burr hole quite difficult and risky if placed just on the venous structure.

Placement of the burr holes for craniotomy must be compatible with the position of the transverse and sigmoid sinuses. Posterior surface landmarks have been proposed in the literature [2]. The asterion, the mastoid groove and the superior nuchal line were found to be the most valuable landmarks. Because the asterion could not be identified in almost 60% of the cadaver specimens, various methods have been proposed. We believe that the first burr hole can be placed in the angle between the asterion, the parietomastoid and the occipitomastoid sutures. The second burr hole can be placed in the angle between the asterion, the lambdoid and the oc-cipitomastoid sutures. Doing so on both sides of the occipitomastoid suture, you stay below the venous structures. Making the holes on either side of the occipitomastoid suture ensures avoidance of the venous structures above. For combined exposure with via a supratentorial approach, third and fourth burr holes can be placed on both sides of the squamosal suture.

Burrhole Keyhole

Fig. 16. (a and b) The keyhole burr holes are shown along the right sinuso-jugular axis; (A asterion; LS lambdoid suture; OS occipitomastoid suture; PS parietomastoid suture)

Fig. 16. (a and b) The keyhole burr holes are shown along the right sinuso-jugular axis; (A asterion; LS lambdoid suture; OS occipitomastoid suture; PS parietomastoid suture)

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