Anatomy, Head and Neck: Frontal Bone

Structure and Function

The frontal bone forms the anterosuperior part of the cranium and borders several other bones on its external surface. This bone divides into several anatomical regions. The squamous part comprises the superior 2/3 and is commonly referred to as the forehead. A midline prominence called the "glabella" appears in its lower portion.

Just inferior and anterior to the squamous part lies the nasal segment, which joins the nasal bones at the midline to form the nasion. On each side of the nasion, the frontal bone articulates with the frontal processes of the maxilla. The orbital part forms the roof of the orbit and consists of 2 triangular plates extending posteriorly from the supraorbital margins. Lateral to the maxilla, the frontal bone connects with bones forming the posterior orbit, namely, the lacrimal, sphenoid, and zygomatic bones (see Image. Orbit, Anterior View).[4] Posterior and lateral to the zygomatic bones, the frontal bone articulates with the greater wing of the sphenoid and, more posteriorly, with the parietal bones.

On the internal cranial surface, the frontal bone articulates with the ethmoid bone medially and inferiorly and with the sphenoid posteriorly and inferiorly (see Image. Frontal Bone, Inner Surface). Ascending posteriorly and laterally, this bone connects with the temporal bone before meeting the parietal bones at the cranial vertex.

Dense fibrous connective tissue syndesmoses (sutures) between formed bones in the adult calvarium maintain structural integrity.[5] The syndesmosis connecting the frontal bone to the paired parietal bones is called the "coronal suture." Running transversely across the skull’s crown, this suture forms the posterior boundary of the frontal bone. The point where the coronal suture intersects the sagittal suture, uniting the 2 parietal bones, is known as the bregma. In infancy, this region remains open as a nonossified fibrous membrane called the "anterior fontanelle."[6] Laterally, the coronal suture joins the sphenoid, parietal, and temporal bones at the pterion, an anatomically significant H-shaped junction.[7]

The frontal bone contains 3 foramina: a paired set of supraorbital foramina and a singular foramen caecum. The bilateral supraorbital notches lie at the superior orbital rim, just medial to the midpupillary line, transmitting the supraorbital artery, veins, and nerves.[8] The foramen caecum appears on the internal surface near the posterior border, where the frontal bone meets the ethmoid bone, and carries an emissary vein to the superior sagittal sinus. This sinus follows a longitudinal groove running anteroposteriorly along the frontal bone.[9]

The anteroinferior portion of the frontal bone houses 2 frontal sinuses located superior to the orbital roofs. These sinuses vary widely in size and volume between individuals.[10]

Embryology

The frontal bone originates from neural crest cells, along with the anterior cranial bones.[11] At birth, the bone exists as 2 halves separated by the frontal (metopic) suture, which typically fuses during early childhood. In approximately 5% of individuals, the suture remains partially visible beyond infancy, usually just superior to the nasion—a condition known as metopism.[12] The bregma in adults corresponds to the location of the anterior fontanelle in infants. This junction of the coronal, sagittal, and frontal sutures serves as a critical landmark in neonatal assessments. Premature fusion of these sutures may result in craniosynostosis, a developmental abnormality affecting cranial shape (see Image. Frontal Bone at Birth).[13]

Blood Supply and Lymphatics

Blood supply to the head primarily originates from the external and internal carotid arteries (see Image. Arteries of the Scalp and Face). The vertebral arteries contribute vascular supply to the brainstem and form the posterior portion of the circle of Willis. The internal carotid arteries supply the anterior segment of the circle of Willis.[14] The external carotid artery supplies the superficial structures of the head, including the muscles of facial expression and mastication.

Intracranial venous drainage occurs through dural venous sinuses, which converge and ultimately drain into the internal jugular vein. Superficial structures of the head drain primarily via the external jugular vein. Both jugular veins empty into the brachiocephalic vein in the thorax, which then drains into the superior vena cava.

Lymphatic drainage of the head and neck occurs through a network of lymphatic vessels. The right hemicranium drains into the right lymphatic duct, while the left hemicranium drains into the thoracic duct. These ducts then empty into the corresponding right and left subclavian veins.[15]

Nerves

The dermatomal region overlying the frontal bone receives sensory innervation from the ophthalmic branch of the trigeminal nerve (cranial nerve V), which exits the cranium through the paired supraorbital foramina to supply the skin anterior to the frontal bone. Motor innervation to the muscles overlying the frontal bone is provided by the facial nerve (cranial nerve VII). The distribution area includes the frontalis, procerus, and orbicularis oculi muscles, which contribute to facial expression and are clinically relevant in both neurologic and aesthetic evaluations.

Muscles

On the lateral aspect of the frontal bone, just anterior to the parietal bone and superior to the sphenoid bone, the frontal bone contributes to the anterior portion of the temporal fossa. The temporalis muscle occupies this space and is innervated by the trigeminal nerve, playing a key role in jaw closure.[16]

Three muscles associated with the frontal bone are essential for facial expression: the frontalis, procerus, and orbicularis oculi. The frontalis muscles are broad, paired structures that elevate the forehead and eyebrows. The procerus muscles lie superior to the orbit and lateral to the glabella, contributing to frowning. The orbicularis oculi consists of concentric fibers surrounding the eye, enabling blinking and eye closure.[17]

Surgical Considerations

Frontal bone injuries in adults most often result from trauma. A 2014 retrospective study of nearly 4,000 frontal bone fractures identified motorcycle accidents as the leading cause, accounting for approximately 1/3 of cases.[18] Surgical management depends on the location, displacement, and complexity of the fracture, as well as any associated maxillofacial or intracranial injuries. Surgeons may take advantage of preexisting lacerations for access or, when necessary, develop a bicoronal flap to expose the fracture. Fixation typically involves titanium plates with screws, titanium mesh, or a combination of both to achieve stable open reduction and internal fixation.[19]

Potential postoperative complications include surgical site infection, paresthesias of the supraorbital and supratrochlear nerves, cerebrospinal fluid leak, and pneumocephalus. Frontal bone osteomyelitis, although rare, may occur when implanted materials used for fixation become secondarily infected.[20]

Clinical Significance

The anterior fontanelle, which later becomes the bregma, serves as an important landmark during the physical examination of infants. A bulging fontanelle may indicate elevated intracranial pressure and prompt further diagnostic evaluation.[21] Conversely, a sunken fontanelle can signal dehydration. Fontanelles, sutures, and overall cranial shape should be monitored for signs of premature closure or positional abnormalities such as plagiocephaly—a unilateral flattening of the anterior or posterior quadrant of the skull.[22] If left unaddressed, this condition may impair normal brain growth and development.

The pterion, located on the lateral aspect of the frontal bone, is a junction where 4 cranial bones converge: the frontal, temporal, sphenoid, and parietal bones. This region holds clinical importance due to its proximity to the middle meningeal artery, a major vessel supplying the dura mater. Trauma to the pterion warrants urgent evaluation to exclude an epidural hematoma, as arterial injury in this area may result in rapid neurological decline, coma, or death if not managed promptly.[23]

The frontalis muscle is also relevant in aesthetic and reconstructive medicine. Frequent activation of this muscle contributes to horizontal forehead wrinkles, making it a common target for botulinum toxin injection to reduce dynamic lines and improve facial aesthetics.[24]

Frontal bone fractures may arise from facial trauma. Presenting symptoms may include pain, edema, ecchymosis, facial asymmetry, paresthesia, and diplopia.[25]