The frontal bone is one of the skull bones enclosing the brain (neurocranium) and it consists of three parts:
- the squamous part
- the orbital part
- the nasal part
The largest area of the frontal bone is the squamous part which encompasses the area of the forehead. It comprises the frontal sinuses which are situated superior to the orbit on the medial side and are separated by a septum. The superior border of the orbit (supraorbital margin) contains the supraorbital notch where the supraorbital vessels and nerve pass through. The arches above them are known as the superciliary arches (Latin “cilia” = eyelashes). The smooth and slightly elevated surface above the nasal root is referred to as glabella. The zygomatic processes arise caudolaterally from the squamous part and articulate with the zygomatic bone.
The orbital part forms the roof of the orbit and the ethmoidal sinuses. The ethmoid air cells lie within its ethmoidal notch. The trochlear spine serves as a site of insertion for the superior oblique muscle at the medial eye angle.
The orbital part comprises two openings: the anterior ethmoidal foramen ( for the anterior ethmoidal vessels and nerve) and posterior ethmoidal foramen (for the posterior ethmoidal vessels and nerve).
The bone is surrounded by no less than seven articulating bones. The adjacent bones and the corresponding sutures are:
- the lacrimal bone (→frontolacrimal suture)
- the nasal bone (→frontonasal suture)
- the ethmoid bone (→frontoethmoidal suture)
- the maxilla (→ frontomaxillary suture)
- the zygomatic bone (→ zygomaticofrontal suture)
- the sphenoid bone (→sphenofrontal suture)
- the parietal bones (→ coronal suture)
The ossification of all three parts of the frontal bone is intramembranous. Sometimes a persistent metopic suture can be seen running down the midline of the frontal bone. This is a remnant of the original frontal suture that normally ossifies at two years of age. Another relic that can be present on the squamous part are the arachnoid foveae. These are thinner areas that have undergone bone resorption due to the pressure placed by the arachnoid granulations pushing on the dura mater.
In newborns, the frontal sinuses are merely small cavities without any openings. These start to develop from two years of age due to the upward migration of ethmoid air cells (secondary pneumatization) until early adolescence. The left and right frontal sinuses develop independently which is why one may have one dominant and one hypoplastic side.