21st February 2024

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.