Bones of the Orbit
The bony orbit is the skeletal cavity which is made up of several cranial structures and surrounds the soft tissue that make up the eye.
Its function is to provide a stable and enclosed environment for the eyeball and its adjacent structures.
Although it may appear to be a spherical cavern, the bony orbit is actually more like a canal. It has a large opening anteriorly in the upper face, where the part of the eye that is used directly to see emerges from and two smaller fissures and a canal posteriorly, that connect the inner cranium with the exterior part of the skull, allow several anatomical structures to pass through them.
Firstly, the individual bones that make up the bony orbit will be discussed, along with how they connect to one another. Secondly, the anatomical landmarks and fissures of the bones, including those made due to their articulations (between bones) will also be described in detail, especially those which have structures passing through them. Lastly, in the final paragraph, the most important pathological disorder will be mentioned.
Bones & Walls of the Orbit
In a clockwise direction (using the right orbit, which is on the reader’s left side), the bones that contribute to the anterior margin and anterior walls of the bony orbit include the frontal bone, which provides the anterior superior rim of the orbit and the majority of the ceiling of the orbital cavern.
Next is the maxilla, which contributes via the frontal and nasal portions of its entire mass to the anterior medial margin and anterior inferior medial margin of the rim of the orbit, as well as the majority of the floor of the bony orbit.
The last bone in rotation that contributes to the anterior lateral internal and external walls of the bony orbit is the zygoma or the facial portion of the zygomatic bone.
Internal Medial Wall
The internal medial wall continues posteriorly and superiorly from the maxilla up to the frontal bone and consists of first the lacrimal bone and just behind it, the vertical plate of the ethmoid bone.
The posterior margin of the ethmoid plate articulates with the greater and lesser wings of the sphenoid bone, as does the maxilla inferiorly (but only with the greater wing).
The sphenoid bone forms the entire posterior wall of the bony orbit. A tiny slither of the vertical plate of the palatine bone, known as the orbital process can be seen between the maxilla, the ethmoid bone and the sphenoid bone.
Articulations of the Orbit
Inside the orbit the articulations are not openly named and are not expected as common knowledge from medical students in an exam. For now, only the main sutures that are mentioned in the anatomy books will be listed here. As previously mentioned, the order will be clockwise using the right orbit for reference:
- The frontomaxillary suture articulates the frontal bone and the maxilla in the superior anterior margin of the orbital rim
- The zygomaticomaxillary suture can be seen just lateral of the infraorbital foramen and has a lateral diagonal stance. it links the maxilla to the facial portion of the zygomatic bone
- The frontozygomatic suture joins the most superior aspect of the zygoma to the frontal bone on the superior lateral margin of the anterior orbital opening.
Anatomical Landmarks & Fissures
The anatomical landmarks of the orbit consist of the three external and six internal openings, with the actual eye socket opening not being counted due to it’s size.
The three external landmarks are foramina:
- the supraorbital foramen on the superior margin of the orbital rim in the frontal bone
- the infraorbital foramen on the inferior margin of the orbital rim in the maxilla
- the zygomaticofacial foramen slightly more lateral to the latter, in the facial portion of the zygomatic bone
These foramina contain the superior and inferior orbital nerves and vessels and the zygomaticofacial nerve and vessels respectively.
The six internal openings are situated on the posterior and medial walls of the orbit. The posterior openings are bordered by the greater and lesser wings of the sphenoid bone and the ethmoid bone. The inferior orbital fissure is an exception and has only the greater wing of the sphenoid bone and the maxilla as an opposing border, rather than the ethmoid bone. The superior and inferior orbital fissures and the optic canal are the landmarks currently under review.
The optic canal is much smaller than the other two landmarks and is considered the most superior of the three. It contains:
- the optic nerve
- the ophthalmic artery
The next below the latter is the superior orbital fissure, which is the second largest of the fissures, however it holds the most content, including:
- the ophthalmic branches of the trigeminal nerve (nasociliary, frontal and lacrimal)
- the oculomotor nerve
- the trochlear nerve
- the abducent nerve
- the superior and inferior ophthalmic veins
Lastly, the inferior orbital fissure which is the largest and most inferior structure in the posterior wall of the bony orbit, contains:
- the maxillary branch of the trigeminal nerve
- the zygomatic nerve
- the infraorbital vessels
The medial wall landmarks include the nasolacrimal canal and fossa, which can be seen on the most anterior aspect of the lacrimal bone at its border with the maxilla. It holds the lacrimal sac which continues on as the nasolacrimal duct.
The final structures are the anterior and posterior ethmoidal foramina, which sit in the suture between the ethmoid bone and the frontal bone and contain the anterior and posterior ethmoidal nerves and vessels respectively.
One of the most severe fractures that include the bony orbit is the blow out fracture. This is when the floor of the bony orbit, namely the maxilla caves in as a result of a combination of facial fractures such as the Le Fort II or III and zygomatic fractures. Clinically, the condition presents with:
- decreased visual acuity in the affected eye
- periorbital ecchymosis and edema
- pupillary dysfunction
- ocular misalignment
- hypo- or hypertropia
- a step in the inferior margin upon palpation
Treatment includes reconstructing the orbital floor and stabilizing the bones that surround and contribute to its formation.