The skull is an entity in itself. It is a jigsaw puzzle of irregular bones and very specific puzzle edges that combine to form a masterpiece. It is a helmet for the brain and a scaffold for the face. Even though the bones are separated by very narrow and shallow jagged sutures, they skull is not divided by every single point of articulation, but by the areas that are created via these bony connections. The main areas of the skull loosely termed are the face and the head. Anatomically and developmentally, they are known as the Viscerocranium and the Neurocranium respectively. This article will highlight the key points of the neurocranium and elaborate on the bones and adjacent anatomical structures which surround it.
- Neurocranium fractures
The primary functions of the neurocranium are to form the shape of the head and to protect the brain and the organs that control the five senses; the eyes, the ears and the areas of the brain that control touch, taste and smell. The bones that make up the neurocranium include the sphenoid, temporal, ethmoid, parietal, occipital and frontal bones. Remember these bones quickly with the mnemonic "STEP OF" which is combined out of the first letters of the every word.
The frontal bone creates the smooth curvature of the forehead and protects the frontal lobe of the brain, especially the ethmoid bone's horizontal plate known as the cribriform plate which allows the olfactory nerve bundles to pass through its perforated surface and bring the ceiling of the nasal cavity its sense of smell. As the frontal bone is the most anterior bone in the neurocranium, it is bordered by both the surrounding neurocranial bones and the most superior viscerocranial bones.
Laterally, it articulates with the zygoma through the frontozygomatic suture, the maxilla through the frontomaxillary suture and the nasal bones via the frontonasal suture. It also sometimes has remnants of the metopic suture from childhood, which run down the midline of the forehead. Posteriorly, in the neurocranium, the frontal bone is surrounded by the two parietal bones and is linked to them by the coronal suture and the bregma, where all three bones meet. Posterolaterally, is the sphenoid bone and it articulates with the frontal bone via the frontosphenoidal suture.
The parietal bones are situated at the crown of the skull and provide a roof over part of the anterior, the entire middle and some of the posterior cranial fossae, which contain the various lobes of the brain, along with the sinuses, the meninges, and their corresponding vessels. While they are anchored to the frontal bone anteriorly, they also anchor to each other in the midline through the sagittal suture. Anterolaterally, they connect with the sphenoid bone through the sphenoparietal suture and posterolaterally to the temporal bone via the squamosal suture. Posteriorly, the occipital bone borders both parietal bones using the lambdoid suture and the lambda.
The occipital bone is the most posterior superior bone of the neurocranium. It creates the rounded bulb of the head at the nape of the neck and covers the cerebellum and the brainstem in a bony capsule. Its anterior superior connections have previously been discussed, however, its anterior inferior border articulates with the lesser wings of the sphenoid bones anteriorly and the temporal bone laterally. The sutures are named the spheno-occipital suture and the petro-occipital suture respectively.
The ethmoid bone consists of a horizontal and a vertical plate. From the bony orbit only the vertical plate can be seen on the medial wall. It cascades down from the roof of the nasal cavity and separates it into two nasal passages. Its appendages include the superior and middle nasal conchae. Within the bony orbit it is surrounded anteriorly by the lacrimal bone, superiorly by the frontal bone, inferiorly by the maxilla and the orbital process of the palatine bone and posteriorly by the sphenoid bone.
The sphenoid bone is in a unique position, because it is surrounded anteriorly by the frontal bone, superiorly by the parietal bone and posteriorly, as well as laterally, by the temporal bone. It borders the anterior and middle cranial fossae, with its most prominent feature being the sella turcica which holds the hypophysis. Internally, the bone articulate anteriorly with the cribriform plate via the sphenoethmoidal suture and the frontal bone anterolaterally via the sphenofrontal suture. Externally, the pterion can be seen on the lateral side of the skull and it is the connection between the parietal bone and sphenoid bone. Posteriorly the squamous suture connects the temporal bone and the sphenoid bone.
These are some of the most complex bones of the skull due to their various portions. The superior sutures have all been covered above, however their inferior connections are yet to be mentioned. The following sutures are unique because the are very small and separate the portions within the temporal bone! The petrosquamous suture unites the petrosal and squamous parts of the bone. The petrotympanic suture links the petrous and the tympanic areas of the bone and lastly, the squamotympanic suture articulates the squamous and the tympanic areas of the temporal bone. The temporal bone forms part of the middle and posterior cranial fossae and its most important feature is that it encapsulates the auditory and vestibular organs.
The most frequent fracture that can be seen to affect the neurocranium is the frontobasilar fracture. This is usually due to a blunt force trauma to the head and it fractures the frontal bone which in turn creates a trauma in the base of the skull which can be either a fracture or internal bleeding. An MRI will show the extent of the bleeding and internal damage.
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