Anatomy of the Craniofacial Region

1.1  Anterior Skull Base

The anterior cranial fossa is of vital importance in traumatology of the craniofacial region (Manson 1986). In close proximity there are connections to the neighboring cerebral regions, to the olfactory bulb and tract, to the frontal cerebral lobe, to the anterior temporal lobe, to the pituitary gland, and to the brainstem (Fahlbusch and Buchfelder 2000).

From anterior to posterior, the skull base is divided into the anterior, middle, and posterior cranial fossa. The inner surface of the anterior cranial fossa is composed of the ethmoidal, frontal, and sphenoidal bones (Lang 1985, 1987, 1988, 1998; Lang and Haas 1979; Schiebler and Schmidt 1991). Topographically,­ the anterior cranial fossa is subdivided into a medial section (lamina cribrosa and crista galli), a lateral section (orbital roof/ethmoid/posterior wall of the sinus), and a posterior section (sphenoid/sella) (Fig. 1.1).

The most important transverse interfaces are the sutura fronto-ethmoidalis and the sutura spheno-­ ethmoidalis. The sutura spheno-ethmoidalis forms the anatomical border between the ethmoidal bone and the lesser wing of the spenoid bone, which, in turn, marks the border to the middle cranial fossa.

This line borders on the lesser wing of the sphenoid bone and the connecting line between the optical foramina. The canals for both optical nerves are situated here, anterior to the hypophyseal fossa. The sphenoidal plane belongs to the anterior cranial fossa.

1.1.1  Cribriform Plate/Crista Galli

The ethmoidal bone lies prominently appendant to the paired cribriform plate and crista galli in the medial section of the floor of the anterior cranial fossa.

Fig. 1.1  Topographical subdivision of the anterior cranial fossa into a medial section (1 lamina cribrosa/crista galli), a lateral section (2 orbital roof/ethmoid/posterior wall of the sinus) and a posterior section (3 sphenoid/sella)

The cribriform plate itself is thicker than the roof of the ethmoid bone. The falx cerebri, which separates the two cerebral hemispheres, is almost anchored onto the crista galli, a usually pneumatized bony ridge rising sagitally in the middle of the cribriform plate. The foramen caecum lies in front of the crista galli and is surrounded by the ethmoidal bone posteriorly and laterally and anteriorly by the frontal bone (Rohen and Yokochi 1982; Vesper et al. 1998).

The cribriform plate is a perforated strip of bone. Between 26 and 71 foramina, on average 44, can be

found on both sides of the midline, through which the olfactory fibers pass, enclosed in the dural sheath and the subdural space, as they lead to the olfactory bulb (Schmidt 1974; Lang 1998).

Lateral to the cribriform plate, the orbital parts of the frontal bone constitute the greater part of the orbital roofs and the floor of the anterior cranial fossa. The sphenoidal plane forms the posterior border of the cribriform plate (Lang 1998). Normally, the sphenoidal plane slightly overlaps the posterior margin of the cribriform plate.

As a rule, the posterior ethmoidal artery and its fine collateral nerve enter the region in the lateral section of this overlap (Krmptocic-Nemancic et al. 1995).

In the anterior sector of the cribriform plate there is a large, elongated aperture (foramen cribro-ethmoidale), through which the thickest branches of the anterior ethmoidal nerve and artery pass on their way to the nasal cavity (Jackson et al. 1998; Donald 1998) (Fig. 1.2).

1.1.2  Fossa Olfactoria

The olfactory fossa meets the medial and upper paranasal walls laterally. As a rule, the major part of the upper wall is formed by the orbital section of the frontal bone. Seldomly, the ethmoidal bone is also involved in forming the upper wall of the ethmoidal cells (Lang 1988).

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Fig. 1.2  Anterior cranial base with cribriform plate and penetrating medial and lateral olfactory foramina (after Lang 1983a, b, 1998). 1: Foramen caecum 3 Crista galli, 5 Cribriform plate,

6 Roof of the anterior superior ethmoidal cells,7 Ethmoidal sulcus,

9 Fronto-ethmoidal sutura,

10 Lateral lamina of the lamina cribosa, 11 Os frontale 12: Spheno- ­ethmoidal sutura / Planum sphenoidale 13: Sphenoidal limbus 15: Anterior clinoid process 16: Optic channel

Concurrently, the exceptionally thin ascending osseous lamellae of the cribriform plate form the medial part of the ethmoidal labyrinth. The deeper the lamina cribrosa lies, the higher are the ascending lamellae (Fig. 1.3).

On average, the olfactory fossa is approximately 15.9-mm long and 3.8-mm wide (Lang 1988). Keros (1962) determined the depth of the olfactory fossa to be 5.8 mm anteriorly and 4.8 mm in the posterior third. The distance between the lamina cribrosa and the highest point in the ethmoidal labyrinth measures 6.9 mm in the anterior third and 5.8 mm in the posterior third. The lowest inner cranial point of the lamina cribrosa lies approximately 7.9 mm below the nasion on both sides (Lang 1987, 1998; Krmptocic-Nemancic et al. 1995).

Keros (1962) accounted for a shallow fossa (1–3 mm deep) in 12%, a fossa with average depth (2–7 mm) in 70% and a deep fossa (8–16 mm) in 18%. These differences in niveau can also be described as “encaissement des ethmoids” (Probst 1971).

•The deeper the cribriform plate lies in relation to the ethmoidal roof, the wider is the very fine ascending osseous lamella (os frontale) between the cribriform plate and the roof of the ethmoid. In viscero-cranial injuries, the difference in height between the lamina cribrosa and the residual base predisposes fracturing in the thin lamina with an imminent danger of dural injury (Sakas et al. 1998) (Fig. 1.4).

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