The skull, located in the most superior aspect of the axial skeleton, plays a vital role in protecting the brain while providing structural support for tendinous muscle attachments. It gives the human face form and allows for a wide variety of visual differences among human individuals. The skull is a complex of 22 bones that are subdivided into eight cranial bones and 14 facial bones. Cranium is the Latin term for the skull. The eight cranial bones include the frontal bone, two temporal bones, two partial bones, a sphenoid bone, an ethmoid bone, and an occipital bone. The skull bones are separated by coronal, lambdoid, sagittal, and squamosal sutures. The 14 facial bones include two nasal conchae, two nasal bones, two maxilla bones, two palatine bones, two lacrimal bones, two zygomatic bones, the mandible, and the vomer. Each of these bones plays a vital role in the structural formation of the skull and a protective role within the human body.
The function of the skull is both structurally supportive and protective. It protects its inner contents: the cerebrum, cerebellum, brainstem, and orbits. Structurally, it supports the muscles of the face and scalp, providing an anchor for muscular and tendinous attachments. It also protects complexes of nerves and vessels that feed and innervate the brain, facial muscles, and skin.
The embryological origin of the skull originates mostly from the neural crest cells that provide formation of connective tissue and skeletal tissue to this region. The remainder of the skull is formed from cranial mesoderm. The two sets of tissue are separated by the coronal suture. This suture is located between the partial and the frontal bones of the skull. Through migration, the neural crest cells form five embryonic arches. Neural crest cells from the first two arches form portions of the skull. The skull base undergoes endochondral ossification, while the remainder of the skull vault and face undergo intramembranous ossification. However, cartilage and mesenchymal formation do not occur until after nerves and vessels have formed and migrated, allowing for the formation of the foramen, especially in the skull base. Several genes play an important role in the formation of the skull and skull base, including the Dickkopf family, matrix metallopeptidase 9, Indian hedgehog, and Sonic hedgehog (Shh). Issues during the development of the skull are thought to play a role in anencephaly as well as many other syndromes and disorders.
Most of the blood supply to the skull and its contents comes from the common carotid, with the remainder coming from the vertebral artery. The common carotid splits into the internal and external carotid. The external carotid travels up the side of the neck and branches multiple times, feeding the superficial structures of the skull and face. One of the major branches of the external carotid artery is the maxillary artery. The middle meningeal artery, a branch of the maxillary artery, becomes clinically significant during the trauma of the skull. This artery and its branches are also the main blood supply to the skull bones. The internal carotid has no branches within the neck and enters the base of the skull, supplying structures on the inside of the cranial cavity. The internal carotid and the vertebral artery combine in a large anastomosis to form the Circle of Willis. The circle is formed by the anterior communicating artery, the two anterior cerebral arteries, two middle cerebral arteries, two posterior communicating arteries, two posterior cerebral arteries, and a basilar artery that is the most superior combination of the two vertebral arteries. The blood vessels drain through the dural venous sinuses, a complex channel of venous drainage leading to the internal jugulars and ultimately the right side of the heart.
The brain and central nervous system are thought not to contain any lymphatics. However, some believe that the cerebral spinal fluid does have some connection with the lymphatic system and drains through the cervical lymph nodes.
The skull base has multiple foramina that allow passage into and out of the skull for vessels and nerves, including the cranial nerves. The optic nerve canal contains the optic nerve and the ophthalmic artery. The superior orbital fissure contains oculomotor nerve, trochlear nerve, the ophthalmic branch of the trigeminal nerve, and abducens nerve. The foramen rotundum contains the maxillary branch of the trigeminal nerve, and the foramen ovale contains the mandibular branch of the trigeminal nerve. The internal carotid artery enters the carotid canal. The foramen spinosum contains the middle meningeal artery. The largest of the foramen is the foramen magnum which allows the passage of the spinal cord into the spinal canal of the vertebral column. The ethmoid bone of the skull contains a specialized bone formation called the cribriform plate that allows the passage of the olfactory nerve from the nasal chamber into the skull.
The sutures of the skull allow for movement of the bones as a newborn infant, and this persists into adulthood. Eventually, these sutures fuse and are no longer moveable. There is a great deal of variation among the timing of closure with each of the sutures. The sagittal suture closes first around age 22, then the coronal suture, followed by the lambdoid at 26 years and the squamous sutures around 60 years old. The metopic suture splits the frontal bones and typically closes at 3 months of age but can take up to 9 months. The premature fusion of these sutures is termed craniosynostosis and creates different shapes to the skull. Through development, the embryonic arches can miss fold or be absent and leave portions of the facial bones deformed. Treacher Collins syndrome is a disorder of craniofacial abnormalities due to interruption of normal embryonic growth. It stems from a defect in the development of the first and second embryonic arches. Thus mandibular hypoplasia and facial defects predominate. It also can lead to craniosynostosis of the sutures within the skull.
The middle meningeal artery has a large significance in the diagnosis of intracranial hematomas. An epidural hematoma is believed to be derived from the severing of the middle meningeal artery due to traumatic injury of the skull, leading to an epidural hematoma. The epidural space arises from a potential space between the dura mater and the skull.
Skull Base Fractures
Fractures of the skull base should be considered, especially in high impact motor vehicle accidents. The type of fracture and the impact that it has on the contents of the skull are determined by the location of the fracture and the mechanism of injury. However, the treatment management of the fracture is determined by the injury to the inner contents of the skull and their complications. Thin sliced CT has become an integral part of the diagnosis of subtle skull base fractures.
When interpreting imaging of the skull bones, a strong understanding of the sutures and their normal variants is needed because of the easy misinterpretation of a suture for a fracture, especially with increased usage of enhanced CT scanning and its ability to pick up the fine nature of the sutures. The anatomy of a child’s skull is constantly changing, and these variations are important to note.