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Frontal bone

The frontal bone is a shell-shaped, unpaired, flat bone of the skull located in the forehead region.

The frontal bone consists of six main parts: the squamous part, nasal part, two orbital plates and two zygomatic plates.

The main function of the frontal bone is to create the smooth curvature of the forehead and to provide protection for the frontal lobe of the brain. In addition, it participates in the formation of several cavities of the skull such as the orbit and the anterior cranial fossa.

This article will discuss the anatomy and function of the frontal bone.

Key facts about the frontal bone
Definition Shell-shaped cranial bone located in the forehead region
Main parts Squamous part, nasal part, two orbital parts (plates) and two zygomatic processes
Functions Creates the smooth curvature of the forehead; protects the frontal lobe of the brain
Articulations Lacrimal bone (frontolacrimal suture)
Nasal bone (frontonasal suture)
Ethmoid bone (frontoethmoidal suture)
Maxilla (frontomaxillary suture)
Zygomatic bone (zygomaticofrontal suture)
Sphenoid bone (sphenofrontal suture)
Parietal bones (frontoparietal or coronal suture)
Contents
  1. Anatomy
    1. The squamous part
    2. Nasal part
    3. Orbital part
    4. Zygomatic process
  2. Articulations
  3. Osseous development
  4. Sources
+ Show all

Anatomy

The frontal bone is one of the skull bones enclosing the brain (neurocranium) and it consists of the following parts:

  • The squamous part;
  • The nasal part;
  • Two orbital plates;
  • Two zygomatic processes.

The squamous part

The squamous part forms the largest portion of the frontal bone. It is composed of two surfaces: An external and an internal surface.

The external surface is convex and corresponds to the forehead. The external surface above the supraorbital margin hosts a curved elevation known as the supraciliary arch. Between the two arches, there is a prominence known as the glabella. Above the supraciliary arch, the external surface displays an elevation known as the frontal tuber (frontal eminence). In addition, located along the frontal part of the orbital margin, at the superior border of the orbit (supraorbital margin) is the supraorbital notch where the supraorbital vessels and nerve pass through. The zygomatic processes arise caudolaterally from the squamous part and articulate with the zygomatic bone.

The internal surface is deeply concave and presents a ridge located in the midline known as the frontal crest. This crest is continuous with the sagittal sulcus. The foramen cecum are two small foramina situated on each side of the posterior end of the frontal crest.

Nasal part

The nasal part is a small portion of the frontal bone that projects inferiorly between the right and left supraorbital margins. The main feature of this part is the nasal notch that serves for the articulation with the nasal bones and frontal processes of the maxilla. Projecting from the central portion of the nasal notch is a median process known as the nasal spine. The nasal spine supports the bridge of the nose.

Orbital part

The orbital parts contain orbital plates. Each plate is a triangular-shaped flat sheet of bone that extends posteriorly from the supraorbital margin. It is named 'orbital' since its inferior surface comprises most of the roof of the orbit. In contrast, its superior surface forms the greater part of the floor of the anterior cranial fossa.

When viewed from the inferior view, the anterolateral portion of the orbital part of the frontal bone presents with the lacrimal fossa (for the lacrimal gland) while the anteromedial portion has a trochlear fossa that forms a point of attachment for the corresponding superior oblique muscle.

Zygomatic process

The zygomatic processes project inferolaterally on each side of the frontal bone. They begin at the lateral aspect of the supraorbital margin and end at the zygomaticofrontal suture that connects the frontal bone and the frontal processes of the two zygomatic bones. In addition, the posterior border of the zygomatic process forms the beginning of the temporal line.

Articulations

Posteriorly, the frontal bone articulates with the two parietal bones (frontoparietal or coronal suture) and the greater wing of the sphenoid bone (sphenofrontal suture). Laterally, it articulates with the frontal processes of the zygomatic bones via its zygomatic processes forming the zygomaticofrontal suture. The nasal plate articulates with the two nasal bones (frontonasal suture) and two frontal processes of each maxilla (frontomaxillary suture). The orbital part articulates with the sphenoid bone, ethmoid bone (frontoethmoidal suture), and the lacrimal bones (frontolacrimal suture).

Osseous development

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 is 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.

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