Orbital Bones

Abstract

The paired orbital cavities are formed by the facial bones and serve as sockets for the eyes. The orbital bones and the structures contained within the orbit (connective tissue, fat, nerves, vessels) act to support, protect, and maximize the function of the eye. In form, the orbit is roughly a quadrilateral pyramid with rounded angles and resembles a pear. Its volume in the average individual is 30 cc, of which the eyeball contributes about 7.5 cc (range: 6.9–9.0 cc). There are four surfaces: the roof, floor, lateral wall, and medial wall. The base of the pyramid is the opening onto the face (orbital entrance) and is circumscribed by the orbital margin (or orbital rim). The orbit narrows inward to its termination, the apex. The widest portion of the orbital cavity lies 5 to 10 mm behind the orbital rim. The orbit is made up of seven bones: frontal, sphenoid, zygomatic, malar, palatine, lacrimal, and ethmoid. Superiorly, the orbit is bordered by the anterior cranial fossa and the frontal sinus. Nasally, the ethmoid sinus is separated from the medial orbital wall by the thin lamina papyracea of the ethmoid bone. Inferiorly, the maxillary sinus lies beneath the orbital floor. The lateral orbit is bordered anteriorly by the temporalis fossa, and posteriorly it borders the middle cranial fossa. The lateral and medial walls of each orbit form an angle of approximately 45 ° with each other. The two medial walls diverge somewhat posteriorly but are almost parallel to each other (being about 3 mm farther apart posteriorly than at the orbital margin). The lateral orbital walls of the two orbits form a 90 ° angle with each other. The four walls of each orbit converge posteriorly toward the apex, where the optic canal and superior orbital fissure pass into the middle cranial fossa. The overall dimensions of the orbit, especially its depth, are quite variable. An orbital surgeon cannot rely on precise measurements as a guide to the exact location of the optic canal or superior orbital fissure.