Frontal bone

The frontal bone is a skull bone that contributes to the cranial vault. It contributes to form part of the anterior cranial fossa.

Gross anatomy

The frontal bone has two portions:

• vertical portion (squama): has external/internal surfaces
• horizontal portion (orbital): has superior/inferior surfaces

External surface of the vertical portion features:

• frontal/metopic suture
• frontal eminence (tuber frontale)
• superciliary arches which join to form the glabella
• supraorbital margin
• supraorbital notch/foramen containing the supraorbital nerve and vessels
• supratrochlear notch/foramen containing the supratrochlear nerve and vessels
• zygomatic process laterally
• upper and lower temporal lines run backwards and are the sites of attachment for the temporalis fascia and temporalis muscle, respectively
• nasal part features the nasal notch, nasion (middle of frontonasal suture), and nasal process (sharp spine, forms part of nasal septum)
• coronal suture
• bregma
• pterion

Internal surface of the vertical portion features:

• sagittal sulcus: vertical groove for the superior sagittal sinus
• frontal crest: ridge, formed from edges of sulcus that gives attachment to the falx cerebri
• foramen cecum: small notch, converted into foramen, emissary vein from nose
• groove for anterior meningeal artery (branch of anterior ethmoidal artery)

The horizontal portion is composed of two thin, orbital plates separated by the ethmoidal notch.

Inferior surface of the horizontal portion is smooth and concave. It features:

• lacrimal fossa: lateral shallow depression for lacrimal gland
• fovea trochlearis / trochlear spine near nasal part, for attachment of cartilaginous pulley for superior oblique muscle

Superior surface of the horizontal portion is convex and contains depressions for cerebral convolutions. It features:

• openings for the frontal sinuses on either side of the nasal process; each frontal sinus communicates with the ipsilateral middle nasal meatus via a frontonasal duct
• two grooves converted into anterior and posterior ethmoidal canals when articulating with the ethmoid bone

Articulations

The frontal bone articulates with twelve bones.

Unpaired bones include:

• ethmoid: via ethmoidal notch
• sphenoid:
  • greater wings via border of squama
  • lesser wings via posterior borders of orbital plates

Paired bones include:

• nasal: via either side of midline of nasal notch
• frontal process of maxilla: via lateral portion of nasal notch
• lacrimal: via lateral portion of nasal notch
• parietal: via border of squama
• zygomatic: via zygomatic processes

Variant anatomy

• persistent metopic suture
• hypoplasia/aplasia of the frontal sinus(es)

Development

The frontal bone undergoes intramembranous ossification. The metopic suture fuses in 2^nd year but may persist.