Sinus of frontal bone

At a glance

English

Sinus of frontal bone

Latin (TA2)

Sinus ossis frontalis

System

Bone

Region

Head

Side

Single, on the midline

About the sinus of frontal bone

The frontal sinuses are two air spaces hollowed out of the lower frontal bone, one on each side of the midline behind the brow ridges. They belong to the paranasal sinuses, the mucosa-lined cavities in the bones around the nose, with the maxillary, ethmoidal and sphenoidal sinuses. The two are seldom the same size, because the thin bony septum between them usually leans to one side. On average each is a little under 3 cm high, about 2.5 cm wide and 2 cm deep, holding some 6 to 7 ml of air, though some are far larger and in about one person in twenty they are missing.

Each sinus has three walls. The front wall, or anterior table, is thick bone under the forehead skin; the back wall, or posterior table, is thinner and faces the dura and frontal lobe; and the floor is the roof of the orbit. An opening in the floor near the midline leads into a narrow channel that runs down through the front of the ethmoidal labyrinth and empties into the ethmoidal infundibulum and semilunar hiatus of the middle meatus.

The lining makes mucus that traps dust and microbes and is rich in immune cells and antibodies. Cilia on its surface beat the mucus out through the opening, into the nose and back towards the choanae, from where it is swallowed. The sinuses are absent at birth. The frontal bone is still largely membrane around then, and the sinus begins as a recess that cannot be seen on radiographs until roughly the second year; it is usually well formed by six to eight years and keeps growing slowly into the late teens.

Because the sinus sits next to the orbit and the cranial cavity, frontal sinusitis can spread to cause orbital cellulitis, an abscess outside or beneath the dura, or meningitis. A blow to the forehead in a fall or road crash can break the sinus walls; a dent in the front wall is the typical sign, while clear fluid from the nose suggests the back wall has been broken and the dura torn. A slightly displaced front-wall break is simply watched, a displaced one is fixed with plates, and the sinus is sealed off if its drainage path is blocked.

Function and supply

Function

Its lining produces mucus that is swept into the nose, adding to the filtering and defence of inhaled air.

Nerve supply

The supraorbital nerve, a branch of the ophthalmic division of the trigeminal nerve, gives sensation to the lining; secretory parasympathetic fibres come from the facial nerve through the pterygopalatine ganglion.

Blood supply

The supraorbital and anterior ethmoidal arteries, both from the ophthalmic artery; veins drain to the superior ophthalmic vein and, through the back wall, to the dural sinuses.