21st February 2024

The Orbit

The orbit is a complex structure that protects the eye. It is pyramidal in shape, with the base sitting anteriorly and the apex of the pyramid situated posteriorly. A detailed overview of the structure of the orbit is detailed below.

Image Source: Wikimedia Commons (2013). 704_Skull-01, Licensed under a creative commons licence. Available for free at:  https://commons.wikimedia.org/wiki/File:704_Skull-01.jpg (Accessed: 28 September 2023).

Bone Anatomy

The orbit is primarily composed of seven bones, which include the frontal bone, zygomatic bone, maxilla, palatine bone, ethmoid bone, lacrimal bone, and sphenoid bone. These seven bones come together to create a protective socket for the eye. Their intricate arrangement provides structural support, safeguards the eye from external forces, and offers pathways for various nerves and blood vessels crucial for vision and eye function. 

Frontal Bone

The frontal bone forms the upper part of the orbit (the roof) along with the lesser wing of the sphenoid bone. It is a ‘shell-shaped’ bone which consists of squamous, orbital and nasal parts. It extends from the forehead down to the eyebrow region and contributes to the roof of the eye socket. The frontal sinus, an air-filled cavity within this bone, lies just above the orbit. The frontal bone separates the orbit from the anterior cranial fossa.

Clinical relevance: frontal sinus fractures can occur following vehicle accidents or falls, with the characteristic clinical sign being the frontal depression in the anterior table of the skull.

Zygomatic Bone

The zygomatic bone forms the lateral and inferior (‘the floor’) border of the orbit. It plays a vital role in maintaining the overall shape of the face and protecting the eye from lateral impacts.

Clinical relevance: Zygomatic arch fracture is associated with trauma to the side of the face. This type of fracture can displace the nearby infraorbital nerve, leading to ipsilateral paralysis of the cheek, nose and lip. 

Maxilla

The maxilla contributes to the orbital floor, providing structural support beneath the eye. The maxilla also houses the maxillary sinus, which is located below the orbit.

Palatine Bone

The palatine bone is a small, L-shaped bone located near the back of the orbit. It forms a portion of the orbital floor and is situated posteriorly to the maxilla.

Ethmoid Bone

The ethmoid bone forms the medial wall of the orbit. It is a complex bone with numerous thin, bony plates that help create the ethmoidal air cells. These air cells are important for reducing the weight of the skull and enhancing voice resonance. The ethmoid bone contains the cribriform plate, through which olfactory nerve fibres pass.

Clinical relevance: the ethmoid bone can be fractured in facial trauma – most commonly following a dashboard injury or a fall from a height. Fracture to the cribriform plate can cause an anastomosis between the nasal cavity and the central nervous system, which can present as watery discharge from a nostril (rhinorrhoea). Spontaneous CSF rhinorrhoea often requires surgical input.

Lacrimal Bone

The lacrimal bone is a small, fragile bone that contributes to the medial wall of the orbit. It contains the lacrimal sac, which is involved in tear drainage, and has a small groove for the nasolacrimal duct.

Sphenoid Bone

The sphenoid bone is a critical component of the orbit. Structurally, it is butterfly-shaped and consists of a body, a greater wing, a lesser wing and pterygoid processes. It forms the posterior part of the orbit and contributes to the main skull foramina, including the superior orbital fissure, the foramen rotundum and the foramen ovale. The important bony landmarks of the sphenoid bone include the sella turcica and the chiasmatic groove, a sulcus formed by the optic chiasm. The sphenoid bone also houses the sphenoidal sinus.

Clinical relevance: the sphenoid bone is closely associated with the pituitary gland; the pituitary can be accessed endoscopically through the sphenoid bone and sinus. Transsphenoidal surgery is usually the treatment choice for pituitary adenoma rather than an extensive craniotomy.

Borders of the Orbit

• Roof: Frontal and Sphenoid (lesser wing)
• Floor: Maxilla, Palatine, Zygomatic
• Medial: Ethmoid, Lacrimal, Maxilla, Sphenoid
• Lateral: Zygomatic and Sphenoid (greater wing)

Image Source: Wikimedia Commons (2013). 713 Bones Forming Orbit, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:713_Bones_Forming_Orbit.jpg (Accessed: 28 September 2023).

Orbital Fissures and Foramina

Optic Canal

The optic canal, within the lesser wing of the sphenoid bone, serves as a crucial passage for the optic nerve (cranial nerve II) and the ophthalmic artery. Any compression or damage to the optic canal can lead to vision impairment. 

Superior Orbital Fissure

Positioned between the greater and lesser wings of the sphenoid bone, the superior orbital fissure accommodates several critical structures, including cranial nerves III (oculomotor), IV (trochlear), and VI (abducens), along with the ophthalmic division of the trigeminal nerve (cranial nerve V1). These nerves are responsible for eye movement, pupil constriction, and sensory innervation to various parts of the face and eye. A lesion or compression within the superior orbital fissure can result in debilitating eye movement disorders or sensory deficits.

Inferior Orbital Fissure

The inferior orbital fissure is located at the intersection of the sphenoid, maxilla, and zygomatic bones. It provides a pathway for branches of the maxillary nerve (cranial nerve V2) and blood vessels. Disorders affecting this fissure can lead to sensory disturbances in the face and orbit, particularly the lower eyelid and cheek.

Ethmoidal Foramina

The ethmoidal foramina, found in the ethmoid bone, permit the passage of anterior and posterior ethmoidal vessels and nerves. These structures play a role in the sensory innervation and vascular supply to the nasal cavity and ethmoidal air cells. Blockages or damage to these foramina can disrupt normal nasal physiology and potentially affect the eye due to their proximity.

Supraorbital and Infraorbital Foramen

The supraorbital foramen is the small opening at the superior orbital margin in the frontal bone just below the superciliary arches that transmits the supra-orbital nerve, artery and vein. The infraorbital foramen is within the maxillary bone. It is the anterior opening of the infraorbital canal, which is the anterior continuation of the infraorbital groove, which runs through the floor of the orbit.

References

• The Bony Orbit – Borders – Contents – Fractures – TeachMeAnatomy [Internet]. Available from: https://teachmeanatomy.info/head/organs/eye/bony-orbit/
• Kaplan HJ. Anatomy and function of the eye. Immune Response and the Eye. 2007; 92:4-10.
• Gaillard F. Orbit | Radiology Reference Article | Radiopaedia.org [Internet]. Radiopaedia. Available from: https://radiopaedia.org/articles/orbit
• Snell RS, Lemp MA. Clinical anatomy of the eye. Malden, Mass. Blackwell Science; 1998.