The bones of the face and skull contain and protect critically important structures, including the brain; organs of vision, hearing, balance; and related neurovascular structures. The core concepts of this section include recognition of areas of relative strength and vulnerability, the neurovascular and sensory contents that may be threatened with fracture or other pathology, and some anatomic patterns or signs.

A key purpose of these bones is protective, for example: a thick skull surrounding intracranial contents, a dense otic capsule around inner ear structures, and a series of thickened vertical struts and horizontal buttresses forming a protective lattice for the face and orbit. Other roles are cosmetic and functional (mastication).

It is important to recognize areas of relative weakness, for instance the thin lamina papyracea of the medial orbital wall and the inferior orbital wall (relative to the thicker orbital rim), suture lines of the zygomatic bone (common areas of fracture lines with a zygomaticomaxillary complex fracture pattern), or exposed nasal bones.

Any conduit (fissure, canal, foramen) in the skull base, through the orbit or other facial regions will typically house important arteries, veins/venous sinuses, and nerves. This is important since fracture can threaten the associated vessel, nerve/sensory organs. Additionally, congenital variants of vessels may have associated osseous findings-for example an absent foramen spinosum due to a persistent stapedial artery with aberrant origin of the middle meningeal artery.

While described anatomic signs are less numerous than those describing the cerebral and cerebellar surface anatomy, some useful patterns can be recognized.

Notes-sutures, age-dependence,

The calvaria (skull) is comprised of portions of the paired frontal bones, parietal bones, sphenoid bones (greater wing), temporal bones (squamosal), and the occipital bone. Throughout, the skull is comprised of an outer table and inner table with intervening diploic space. The diploic space contains veins and bone marrow. Thus, it may contain normal lucencies representing venous lakes, may expand with anemias or be a site of metastatic disease. The pattern of involvement of these skull layers with a lesion can give clues to the etiology. For example, langerhan's cell histiocytosis may asymmetrically involve the tables, leading to a beveled edge lesion appearance.

Lucencies involving the inner table with variable size and extent of involvement of the other layers near dural venous sinuses, the posterior fossa, or the middle cranial fossa sphenoid bone may represent arachnoid granulations. Such arachnoid granulations are typically considered normal variants, but may be associated with herniated brain tissue, seizures, or CSF leak in certain cases (and potentially a history of elevated intracranial pressure). Both venous lakes and arachnoid granulations have potential to be confused with lytic lesions such as metastases/myeloma if not properly recognized.

The bones of the calvaria are separated by sutures and fontanelles that typically close early in childhood. Recognition of the cranial sutures is important for assessment of craniosynostosis, wormian bones, and for fracture assessment. After closure of the fontanelles, the pterion is the convergence of the frontal, parietal, temporal, and sphenoid bones at the anterior extent of the squamosal suture; similarly the asterion is the convergence of the lamdoid suture, parietomastoid suture, and occipitomastoid suture at the posterior extent of the squamosal suture (at the site of the closed mastoid or posterolateral fontanelle).

Overall, assessment of the calvaria and skull base can begin with a midline sagittal T1 image on MRI (again the midline sagittal T1 is a key image). Further assessment on the off-midline sagittal T1 images is also useful as well as as fat-suppressed and diffusion-weighted images.

Finally, the inner table of the skull has important relationships with intracranial contents. The outer layer of dura matter is associated with the periostium of the inner table. This dura is tightly bound at suture lines, explaining why epidural hematomas do not cross them unless the suture is fractured. Middle meningeal artery branches course along grooves of the inner table. The dural venous sinuses, found within the layers of the dura.

3D surface rendered image of a pediatric patient, posterior oblique view. An occipital bone fracture in a pediatric patient extends inferolaterally into the occipitomastoid suture. The occipitomastoid suture is an extension of the lambdoid suture. The parietomastoid suture bridges the asterion/lambdoid suture with the squamosal suture.