Frontal Bone

The frontal bone is a single, flat cranial bone located in the anterior part of the skull. It forms the forehead, the roofs of the orbits (eye sockets), and contributes to the anterior cranial fossa. As a key part of the neurocranium, the frontal bone protects the frontal lobes of the brain and plays an important role in facial structure and sinus drainage. It is one of the most visible and anatomically significant bones of the skull.

Structure

The frontal bone is composed of dense cortical bone with a spongy diploë layer in between. It is broadly divided into two main parts:

• Squamous part: The vertical portion that forms the forehead.
• Orbital part: The horizontal portion that forms the roof of each orbit and part of the floor of the anterior cranial fossa.

Internally, the frontal bone contains two air-filled cavities called the frontal sinuses, which communicate with the nasal cavity and contribute to voice resonance and mucus drainage.

Location

The frontal bone is located at the front of the skull. It lies superior to the nasal bones and maxilla, and anterior to the parietal bones. It articulates with multiple bones, forming joints at the front of the face, sides of the cranium, and the skull base. It also forms part of the orbital roof and contributes to the anterior cranial base that supports the frontal lobes of the brain.

Articulations

The frontal bone articulates with 12 bones:

• Parietal bones (2) – via the coronal suture
• Nasal bones (2)
• Zygomatic bones (2)
• Maxillae (2)
• Lacrimal bones (2)
• Ethmoid bone
• Sphenoid bone

Key Anatomical Features

1. Squamous Part

• Frontal eminences: Rounded elevations on the forehead marking centers of ossification.
• Glabella: Smooth, median area between the eyebrows.
• Supraorbital margins: The superior boundary of each orbit.
• Supraorbital foramen/notch: Transmits the supraorbital nerve and vessels.

2. Orbital Part

• Orbital plates: Flat surfaces forming the roofs of the orbits and separating them from the anterior cranial fossa.
• Lacrimal fossa: Depression in the lateral part of the orbital plate for the lacrimal gland.

3. Nasal Part

• Nasal spine: Projects downward between the nasal bones to form part of the nasal septum.

4. Frontal Sinuses

• Located posterior to the glabella within the squamous part of the bone.
• Vary in size and shape; typically asymmetrical.
• Drain into the middle nasal meatus via the frontonasal duct.

Development and Ossification

The frontal bone develops from two primary centers of ossification — one in each half of the developing bone. These centers fuse at the midline during early childhood.

• Ossification type: Intramembranous ossification
• Frontal (metopic) suture: Initially separates the two halves of the frontal bone; usually fuses between 6 and 8 years of age. If it persists into adulthood, it’s called a persistent metopic suture.

Functions

• Protection: Forms a shield for the frontal lobes of the brain and the superior parts of the orbits.
• Structural: Provides shape to the forehead and upper face.
• Attachment: Serves as a site for muscles of facial expression and the scalp.
• Sinus function: Houses the frontal sinuses, which help lighten the skull and aid in resonance and mucus drainage.

Muscle Attachments

• Frontalis muscle: Part of the occipitofrontalis; attaches to the skin of the forehead and plays a role in facial expression (raising eyebrows).
• Temporalis fascia: Attaches along the superior temporal line, which partially lies on the frontal bone.

Clinical Relevance

1. Frontal Sinusitis

Inflammation of the frontal sinus can result from infections and may cause forehead pain and pressure. In severe cases, the infection can spread to adjacent structures, including the brain (e.g., frontal lobe abscess).

2. Trauma and Fractures

Frontal bone fractures commonly occur in high-impact injuries. They may involve the anterior table (outer surface) or posterior table (near the brain). Posterior table fractures are more dangerous due to proximity to the frontal lobe and meninges, potentially leading to cerebrospinal fluid (CSF) leakage.

3. Metopic Suture

In some individuals, the metopic suture persists into adulthood. While it is a normal variant, it may be mistaken for a fracture in radiographic imaging if not recognized properly.

4. Surgical Access

Neurosurgical procedures may involve craniotomies that access the frontal lobe by removing a section of the frontal bone. Surgeons must consider the position of the frontal sinuses and avoid damaging the orbital roof or dura mater.

5. Forensic Identification

Frontal sinus patterns are unique to individuals and are sometimes used in forensic cases for identification purposes via radiographs or CT imaging.

Radiological Appearance

On X-ray and CT scans, the frontal bone appears as a dense, curved structure forming the anterior skull. The frontal sinuses are visible as radiolucent (dark) areas within the squamous part. Persistent metopic sutures or fractures must be carefully differentiated in imaging studies.

Associated Foramina

• Supraorbital foramen (or notch): Transmits the supraorbital nerve and vessels.
• Frontal notch: Sometimes found medial to the supraorbital notch; may transmit the supratrochlear nerve.