1.1.3  Roof of the Orbit

The orbital roof of the frontal bone exhibits a filmy osseous structure displaying impressions and ridges. The digital impressions and crest-like jugae form an irregular relief with hills and valleys in the region of the orbital and ethmoidal roofs. The prominent bone is substantially thicker than that in the depressed zones (Probst 1986). Here there is a major site of predilection for base ­fractures and dural injuries and, consequently, also a primary site for liquor fistulae (Probst and Tomaschett 1990).

foramina cribrosa on their way to the medial and lateral walls of the nasal cavity (Lang 1998).

Branches of the arteria carotis interna and arteria cerebri anterior may be involved in supplying the farmost medial floor regions of the anterior cranial fossa.

The lateral floor region of the anterior cranial fossa gets its supply from the frontal ramus of the middle meningeal artery, whose meningo-orbital branch penetrates the floor of the anterior cranial fossa and ­anastomoses with the rami of the ophthalmic artery (Lang 1998).

Aberrant to other regions of the cranial skeleton and skull base, the frontobasal region displays anatomical anomalies in the configuration of an osseous cranial vault with depressions, ridges, and septa. The association to the dura mater padding is closer in the frontobasal region than the remaining skull interior. The dura itself is comparatively thin and particularly tightly anchored to the bone along the sutures and foramina. There is an exceptionally strong fixation of the dura to the cribriform plate, roof of the labyrinth and crista galli. The epidural translational displacement layer, as found in the middle and posterior cranial fossa, is missing here (Vajda et al. 1987). Furthermore, in the region of the foramina the dura is attached to the sheath of the first cranial nerve. It is histologicaly proven that the subarachnoidal space occasionally extends caudaly along the olfactory fila, through the cavities of the cribriform plate (Probst 1971).

•In the region of the olfactory foramen, the virtual dural cover is lacking and there is a mere arachnoidal covering; so, in the case of fracture, liquor fistulas may easily occur (Samii et al. 1989; Okada et al. 1991; Sakas et al. 1998).

From an evolutionary point of view, the paranasal sinuses are convexities of the nasal cavity into the neighbouring­ bone. Their mucosa are a continuation of the nasal mucosa; thus, a close relation exists between the varying paranasal sinuses. They are very variable with regards to dimension and shape (Lang 1985, 1998) (Fig. 1.5).

1.2.1  Frontal Sinus

Dimension and form of the frontal sinuses vary greatly. They may be totally absent (aplasia) or extend asymmetrically into the orbital roof. In the latter case, they may even reach the anterior margin of the lesser wing of the sphenoid bone. Laterally, the frontal sinus can extend as far as the zygomatic process of the frontal bone and occasionally comprise the lateral orbital wall. The roof of the frontal sinus partially constitutes the floor of the anterior cranial fossa.

The extent of the frontal sinus in the orbital roof section of the frontal bone is particularly important during surgery, when approaching the orbit from the anterior cranial fossa (Lang 1998).

1.1.5  Arterial Supply: Skull Base/Dura

The floor of the anterior cranial fossa and the dura mater are supplied by the anterior ethmoidal artery, whose branches ascend into the falx cerebri, forming the arteria falcea anterior, and pass through the

1.2.2  Ethmoid

The ethmoidal labyrinth – in the center of the facial skeleton, with proximate anatomical connections to

the orbit, nose, and residual paranasal sinuses, and also situated in the anterior cranial fossa – has exceedingly great significance as:

•A link between the visceroand neurocranium

•A central midfacial component

•The ontogenetic origin of the paranasal sinus system

•The site of olfactory cognition

The ethmoid measures 3–4 cm in length, 2–2.5 cm in height and 0, 0.5–1.5 cm in width (Lang 1987).The ethmoidal cells border medially on the nasal cavity, caudally on the maxillary sinus, and cranially on the anterior cranial fossa, respectively the frontal sinus. The orbital boundary is formed anteriorly by the lacrimal bone, posteriorly by the papyraceous lamina of the ethmoid and caudally by the maxillary complex. The sphenoid is attached posteriorly.

As a rule, the adjacent medial and anterior regions of the orbital roof are pneumatized by the frontal sinus extensions (Kastenbauer and Tardy 1995) (Fig. 1.6).

The ethmoid labyrinth is composed of a system of partially disjoined chambers, which one can divide

into an anterior and posterior ethmoidal cell system according to their position (Anon et al. 1996).

The horizontal lamella of the middle nasal concha forms the border. Genesis and anatomy of the anterior ethoidal cells are constitutionally (fetal period) more complex than that of the posterior cell group.

The anterior ethmoidal cells drain into the hiatus semilunaris in the middle nasal meatus, the posterior ethmoidal cell system into the superior nasal meatus.

The posterior ethmoidal cells are located dorsal to the basal appendage of the middle nasal concha and ventral to the sphenoidal sinus. They are usually composed of three to four larger cellular cavities without having any complex anatomical connection to other paranasal sinuses. The posterior ethmoidal cells may extend as far as the ventral wall of the sphenoid sinus and laterally as far as the cavernous sinus. Occasionally they may even extend as far as the optical canal and the middle cranial fossa. A frontal bulla may protrude into the dorsal wall of the frontal sinus, where it is separated from the orbital cavity by a thin osseous lamella (Krmptocic-Nemancic et al. 1995).