The orbital bone, commonly known as the orbit, is the bony socket within the skull that encases and supports the eye. It is a paired cavity, with one orbit located on each side of the face, separated by the nasal cavity and sinuses. The orbit provides a defined space for the eyeball and the associated structures needed for vision and movement.
Anatomy and Composition of the Orbit
The orbit is not a single bone but a complex, four-sided, pyramid-shaped cavity with its base opening toward the face and its tip pointing toward the back of the head. This bony housing is assembled from parts of seven distinct cranial and facial bones:
- Frontal
- Zygomatic
- Maxillary
- Ethmoid
- Sphenoid
- Lacrimal
- Palatine bones
The roof, or superior wall, is primarily formed by the frontal bone, while the floor, or inferior wall, is composed mainly of the maxilla, along with portions of the palatine and zygomatic bones. The lateral wall is the thickest and is made by the zygomatic bone and the greater wing of the sphenoid bone.
The medial wall, located closest to the nose, is the thinnest, featuring a delicate plate of the ethmoid bone called the lamina papyracea. The orbital rim, which forms the opening of the pyramid, provides a robust, circular frame around the eye. The adult orbit houses the eyeball, extraocular muscles, fat, nerves, and vessels within its approximately 30 milliliter volume.
The Protective Role of the Orbital Bones
The primary function of the bony orbit is to shield the eyeball and its related structures from external force and trauma. The thick, rounded orbital rim is designed to absorb and deflect impacts, protecting the globe that sits slightly recessed behind it. The bony walls also serve as stable anchors for the extraocular muscles, which control the precise movements of the eye.
Beyond protection, the orbit contains specialized openings that act as passageways for essential neurovascular structures. The optic canal, located at the apex of the pyramid, transmits the optic nerve, which carries visual information to the brain, and the ophthalmic artery. Other openings, like the superior and inferior orbital fissures, allow for the passage of various cranial nerves and blood vessels that supply the eye and surrounding tissues.
Understanding Orbital Fractures
Trauma to the face can result in an orbital fracture, which is a break in one or more of these surrounding bones. The most common type is a “blow-out” fracture, which involves the floor or the medial wall of the orbit. This fracture typically occurs when a blunt object, such as a fist or a ball, strikes the eye, causing a sudden increase in pressure within the socket.
The force is then transmitted to the weakest walls, causing them to fracture and the orbital contents to occasionally herniate into the adjacent air-filled sinuses, usually the maxillary or ethmoid sinuses. Immediate signs that require medical attention include double vision, or diplopia, which may be vertical or oblique, especially when looking up. This double vision can be caused by the entrapment of eye muscles, such as the inferior rectus, within the fracture site.
Patients may also experience numbness or tingling in the cheek, upper lip, and teeth, due to damage to the infraorbital nerve that runs along the orbital floor. As swelling subsides, the eye may appear sunken, a condition called enophthalmos, because the fracture has increased the total volume of the bony socket. Other symptoms include limited eye movement, pain with eye movement, and bruising around the affected area.