Skull Fracture - Neurocranium

Neurocranium

The human skull is anatomically divided into two parts: the neurocranium formed by eight cranial bones which houses and protects the brain, and the facial skeleton (viscerocranium) composed of fourteen bones not including the three ossicles of the inner ear. The term "skull fracture" is typically used to describe fractures to the neurocranium while fractures of the facial portion of the skull are classified as "facial fractures, or if the jaw is fractured, a "mandibular fracture".

The eight cranial bones are separated by sutures : one frontal bone, two parietal bones, two temporal bones, one occipital bone, one sphenoid bone, and one ethmoid bone.

There are three layers in the bones of the skull: the hard compact layer of the external table (lamina externa), the diploë which is a spongy layer in the middle containing red bone marrow and the compact layer of the inner table (Lamina interna).

Skull thickness is variable depending upon location thus the traumatic impact required to cause a fracture depends on the site of the impact. The skull is thick at the glabella, the external occipital protuberance, the mastoid processes, and the external angular process. Where the skull is covered with muscle, there is no underlying diploë formation between the internal and external lamina which results in a thin area of bone more susceptible to fractures.

Skull fractures occur more easily at the thin squamous temporal and parietal bones, the sphenoid sinus, the foramen magnum (the opening at the base of the skull through which the spinal cord passes), the petrous temporal ridge, and the inner portions of the sphenoid wings at the base of the skull. The middle cranial fossa, a depression at the base of the cranial cavity forms the thinnest part of the skull and is thus the weakest part. This area of the cranial floor is weakened further by the presence of multiple foramina as a result this section is at higher risk for basilar skull fractures to occur. Other areas more susceptible to fractures are the cribriform plate, the roof of orbits in the anterior cranial fossa, and the areas between the mastoid and dural sinuses in the posterior cranial fossa.