M01.02.002 The Anterior Cranial Fossa

The skull’s base isn’t just a flat platform for the brain; it’s divided into three distinct compartments called cranial fossae. Each fossa houses a specific brain region.

The anterior cranial fossa is the shallowest and uppermost compartment. It sits above the nasal and orbital cavities, cradling the frontal lobes’ front and lower portions. Let’s delve into its borders, contents, and clinical significance.

Borders:

Imagine a box made of three bony plates:

• Front and sides: The inner surface of the frontal bone forms these walls.
• Back and middle: The sphenoid bone’s limbus – a bony ridge marking the prechiasmatic sulcus (a groove between the optic nerve canals) – defines this border.
• Back and sides: The lesser wings of the sphenoid bone (triangular bony projections) contribute here.
• Floor: A combination of the frontal bone, ethmoid bone, and the sphenoid bone’s anterior aspects and lesser wings form the floor.

Contents:

Several key landmarks reside within the anterior cranial fossa:

• Frontal crest: A midline ridge on the frontal bone, providing an attachment point for the falx cerebri (the membrane separating the brain hemispheres).
• Crista Galli: A midline projection on the ethmoid bone, resembling a rooster’s comb. It also anchors the falx cerebri.
• Cribriform plate: Flanking the crista galli, this perforated bone plate supports the olfactory bulb and transmits nerves and vessels through numerous openings.
• Lesser wings of the sphenoid bone: These project from the sphenoid’s body within the fossa. Their rounded ends, called the anterior clinoid processes, attach to the tentorium cerebelli (another dural membrane separating the cerebrum and cerebellum). They also separate the anterior and middle cranial fossae.

Foramina (Openings):

The ethmoid bone is the main foramen hotspot:

• Cribriform plate: This sieve-like structure houses numerous tiny foramina transmitting olfactory nerve fibers (smell) to the nasal cavity. It also has two larger openings:
  • Anterior ethmoidal foramen: Carries the anterior ethmoidal artery, nerve, and vein.
  • Posterior ethmoidal foramen: Provides passage for the posterior ethmoidal artery, nerve, and vein.

Clinical Relevance: Cribriform Plate Fracture

Being the thinnest part of the anterior cranial fossa, the cribriform plate is susceptible to fractures. These fractures can lead to two main problems:

• Anosmia (loss of smell): Shearing of the olfactory nerve fibers passing through the plate can cause smell loss.
• CSF rhinorrhea: Bone fragments can tear the brain’s protective membranes, allowing cerebrospinal fluid (CSF) to leak into the nasal cavity, presenting as a clear runny nose.