Video: Bones of the orbit

Most of us have watched a pirate movie at some point in our lifetime. You know the type. Big old pirate ships with enormous sails, interesting characters with unusual names and, of course, the classic pirate flag, featuring a white skull with empty orbits, sometimes covered by a patch. By now, you’ve probably figured out that this talk isn’t about pirates, but rather about those spooky orbits of the eye, and despite the fact that they’re presented so simplistically in pirate movies, these bony formations have, in fact, a really intricate architecture.

For example, did you know that the orbit is actually formed from as many as seven different bones? These form a somewhat a delicate cavern which must accommodate the eyeball along with its accompanying muscles, nerves, and blood vessels. You may be thinking, “Holy smokes! This looks complicated.” But worry not, this tutorial will prove to be an anatomical treasure if you stay with me as we explore the bones of the orbit.

So as you may have gathered, today’s tutorial is all about the bony orbits – a set of incomplete cavities which surround the soft tissues that make up the eye. Like many other bony cavities, the function of the bony orbit is to provide a stable and enclosed environment for the eyeball and its adjacent structures.

To give you an overview of what we will be discussing today, we’re going to start by quickly identifying the individual bones which make up the bony orbit along with some points of connection between them. I’m then going to take you through each individual bone in detail, discussing their specific parts, landmarks, and surfaces that contribute to the walls of the orbit. We’ll also be looking at openings or passages in the bony orbit known as foramina and fissures, which allow for passage of nerves and blood vessels. Finally, I’m going to finish up by taking our knowledge of the bony orbit to look at its anatomy from a more clinical perspective.

So let’s begin our tutorial today by first identifying the cranial bones which contribute to the orbit of the eye socket. Beginning at the top or the superior aspect, the orbit is formed by the frontal bone, appropriately named after the Latin word frons which means ‘forehead’. Moving clockwise, we’ll find the maxilla, which contributes to much of the anterior facial skeleton as well as the inferomedial part of the orbit. Next, we’ll highlight the zygomatic bone, also known as the cheekbone, which forms much of the inferolateral part of the orbit. These three bones collectively contribute to much of the anterior orbital cavity, forming what is known as the orbital margin or the rim of the eye socket.

If we move further into the orbit, we’ll find several other bones that also contribute to its walls. The sphenoid bone, seen here along the posterior wall of the orbit. We also have a number of smaller bones which contribute to the inner surfaces of the orbit. These are the palatine bone, the lacrimal bone, and the ethmoid bone. As I mentioned, we’ll be looking at each of the bones in a lot more detail especially the parts that contribute to the socket. So, let’s waste no time and start with our first bone of interest – the frontal bone.

We’ve already established that the frontal bone is the large dome-shaped bone of the forehead, but, of course, that’s not all it contributes to. Since we’re focusing on the orbit, we’ll concentrate our attention on the inferior most portion of the frontal bone, which is appropriately known as its orbital or horizontal part.

If we take a sagittal section of the frontal bone, we can get a better image of this part of the bone. The majority of the orbital part of the frontal bone contributes to what’s known as the orbital plate. It’s a horizontal shelf of bone which separates the frontal lobe of the cerebrum from the orbit. Its superior surface contributes to the floor of the anterior cranial fossa. It’s called the orbital surface and forms the superior surface or roof of the orbital cavity. So when we’re looking for anterior perspective once again, we now know that the specific part of the frontal bone which forms the roof of the orbit is its orbital surface.

Staying with the frontal bone, let me point out a few more features of it before we move on.

The first is the zygomatic process, which is this inferolateral extension of the frontal bone seen here in green. And true to its name, this part of the frontal bone is that which articulates with the zygomatic bone, which we’re going to take a closer look at in just a few moments. Moving medially, we have what’s known as the supraorbital margin, which marks the superior boundary of the orbit. The supraorbital margin is pierced by a small hole called the supraorbital foramen, which transmits the supraorbital vessels and nerve as they pass superiorly onto the forehead. I should note that the supraorbital foramen is sometimes incomplete, meaning it appears as more of a notch in the supraorbital margin as opposed to a defined hole or foramen.

So that’s all we need to know about the frontal bone for now. So, we’re going to continue clockwise around the orbit to look at our next bone of interest, the maxilla.

The maxilla is a bilateral or paired bone which makes up much of the anterior facial skeleton and serves many functions, such as holding the roots of the maxillary teeth forming the hard palate and nasal aperture, in addition to contributing to the floor and the inferomedial part of the orbit, as you can see in the illustration. The superior part of the maxilla presents two processes. The first of these is the frontal process, which is a prominent bony projection stretching superiorly and posteriorly from the maxilla by the side of the nose. It forms the lateral boundary of the bridge of the nose where its medial border articulates with the nasal bone. Its superior border articulates with the frontal bone, hence its name, and this articulation is known specifically as the frontomaxillary suture. Posteriorly, the frontal process of the maxilla articulates with the lacrimal bone, which we’ll look at later on.

As you can also see in the illustration, the lateral border of the frontal process forms the inferomedial border of the orbital margin. The anterolateral aspect of the frontal process is marked by a well-defined ridge, which is known as the anterior lacrimal crest. This forms the lateral border of the lacrimal fossa, which holds the lacrimal sac. If you look at the frontal process from the lateral aspect, you’ll see this small indentation called the lacrimal notch where the lacrimal bone fits into the frontal process.

If we continue clockwise around the infraorbital margin, you will see a small opening onto the anterior surface of the maxilla known as the infraorbital foramen. It gives passage to the infraorbital nerve – a branch of the maxillary nerve – in addition to the infraorbital blood vessels.

Turning our attention now to the inside of the orbital cavity, we can see the orbital surface of the maxilla here, which contributes significantly to the orbital floor, in addition to acting as the roof of the maxillary sinus located within the maxilla.

The posterior portion of the orbital floor is defined by a small depression which is known as the infraorbital groove or the infraorbital sulcus which gives passage to the infraorbital vessels and nerve. The infraorbital groove continues into the orbital floor as the infraorbital canal before terminating on the anterior or facial surface of the maxilla as the infraorbital foramen, which we’ve already discussed.

Let’s continue now to our next bone of interest – the zygomatic bone.

Also known as the zygoma, the zygomatic bone is situated at the upper and lateral part of the face lateral to the maxilla and forms the prominence of the cheek parts of the lateral wall and floor of the orbit and parts of the temporal and infratemporal fossa. As you can see, it completes the orbital margin between the frontal and the maxillary bones. It articulates with the maxilla via the zygomaticomaxillary suture and with the frontal bone via the frontozygomatic suture.

If you take your finger and follow along the lateral orbital margin, you should be able to feel a small elevation which is known as the orbital tubercle, which serves as an attachment point for several soft tissues of the orbit. Inside the orbital cavity, we can see the zygomatic bone also contributing to the floor and the lateral walls of the orbit, and similar to the two previous bones we’ve looked at, the surface of the zygomatic bone is known as its orbital surface.

And with that, you can now see that we’ve completed exploring the bones which form the anterior portion of the orbit. Let’s turn our attention now to the posterior wall of the orbit and take a look at the many bones which come together to form it.

First up is the sphenoid bone. The sphenoid bone is a large irregular bone which contributes to both the orbital and cranial cavities. Its shape somewhat resembles that of a butterfly or bat with its wings extended, and as you can see in the illustration, it has several processes and landmarks which give it its unusual shape. Of course, we’re only going to concern ourselves with the parts of the bone which specifically contribute to the orbital wall, and if we look at the sphenoid bone in situ, we will see exactly which parts concern the bony orbit.

Our first area of interest is the greater wing of the sphenoid bone, which you can now see highlighted in green on both of our illustrations, or more specifically, the orbital surfaces of the sphenoid bone. If we bring our attention back to the sphenoid bone in situ, we can see the greater wing forming a large portion of the posterior wall of the orbit.

The other part of the sphenoid bone which we need to take a note of is located slightly superior to the greater wings. We’re talking about the lesser wings of the sphenoid bone. These also contribute to the posterior wall of the orbit, albeit less than their greater counterparts. One landmark of importance close to the lesser wing is the optic canal, which you can now see here anteroinferior to the lesser wing. The optic canal gives passage to the optic nerve, in addition to the ophthalmic artery, as they make their way to the ocular bulb or eyeball. Some of the extraocular muscles also have their origins close to this landmark.

A little lateral to the optic canal, you can see a rather large opening, or cleft, between the orbital surface of the lesser and greater wings of the sphenoid bone. This is known as the superior orbital fissure. It gives passage to a number of significant anatomical structures such as the superior and inferior divisions of the oculomotor nerve, the trochlear nerve, some branches of the ophthalmic nerve, the abducens nerve, as well as the superior and inferior ophthalmic veins.

Now if we have a superior orbital fissure, it must mean that we also have an inferior orbital fissure, which is this cleft or opening now highlighted in green. It’s formed largely by the opening found between the greater wing of the sphenoid bone and orbital surface of the maxilla. It, too, transmits several structures such as the zygomatic branch of the maxillary nerve, the infraorbital nerve, and the infraorbital artery and vein, which is a branch of the inferior ophthalmic vein.

So we’ve spoken about the major contributing bones to the walls of the orbital cavity, however, let’s quickly check out some of the smaller bones which fill in the gaps of the orbit.

The first of them is this bone here, the ethmoid bone, and we’re specifically looking at what’s known as the orbital plate of the ethmoid bone. The ethmoid bone is located centrally within the cranium, and like the sphenoid bone, contributes to the medial wall of both the right and left orbits. It is bordered superiorly by the frontal bone, and this is where you’ll find another two small foramina piercing the orbital wall. These are the anterior and posterior ethmoidal foramina, and like the other foramina and fissures we’ve seen so far, these also transmit neurovascular structures from the cranial cavity. In this case, we’re talking about the anterior and posterior ethmoidal nerves.

Slightly inferior to the orbital plate of the ethmoid bone, you’ll see another small piece of bone found in the posterior orbit. And you’re looking at the orbital process of the palatine bone. This is the most superior part of the vertical part of the palatine bone, and as we’ve already seen, it is bordered superiorly by the orbital plate of the ethmoid bone, inferiorly by the orbital surface of the maxilla, and posteriorly by the inferior orbital fissure.

And that brings us to our final bone of this tutorial which is the lacrimal bone. The lacrimal bone is a paired or bilateral bone which contributes to the medial wall of the orbit. As you can see from the illustration, its orbital surface is relatively small and roughly rectangular in shape. It presents one major landmark of interest to us, which is this one here, the posterior lacrimal crest. The posterior lacrimal crest forms the posterior boundary of the lacrimal fossa, which we mentioned earlier when discussing the maxilla.

And that’s it! We have now explored and discussed each of the cranial bones which collectively form the orbit.

I hope you’re still with me because we’re going to apply our knowledge of the bony orbit to a clinical setting.

Since we’ve been speaking about the bones of the orbit, you’re probably not be surprised that our clinical correlation today is related to a bony orbit fracture, specifically known as an orbital blowout fracture. This is the most common type of orbital fracture and is most often incurred as a result of trauma to the facial skeleton – a clue as to why it is most prevalent amongst young men. Most often, an orbital blowout fracture is seen in the inferior or medial walls of the orbit. In the case of fracture to the inferior orbital wall, orbital fat will prolapse into the underlying maxillary sinus and may be joined by the prolapse of the inferior rectus muscle. Medial wall fractures display a similar pathology with orbital fat and the medial rectus muscle prolapsing into the ethmoidal air cells.

Clinically, the condition presents with decreased visual acuity in the affected eye, periorbital ecchymosis and edema, pupillary dysfunction, pain, ocular misalignment and a step in the inferior margin upon palpation. Orbital blowout fractures can be diagnosed using x-ray; however, a CT scan is the modality of choice if possible as it allows not just for evaluation of bone-related injury, but also intraorbital hemorrhage, globe injury or rupture, condition of the extraocular muscles, and indications of prolapsed orbital fat.

In general, treatment of orbital fractures tends to be conservative with surgery or reconstruction reserved for more serious cases where a risk of impairment of ocular structures is present. When surgery is required, it is often delayed to allow for better evaluation of damage once the initial swelling has subsided.

And that’s it! We’ve come to the end of our tutorial on the bones of the orbit. Let’s wrap this up with the summary of what we’ve talked about today.

So we began with the superior wall or roof of the orbital cavity where we discussed the frontal bone – the large bone of the forehead. We identified a number of relevant parts and landmarks of the frontal bone pertaining to the orbit, which included the orbital surface of the frontal bone, the superior orbital margin, the zygomatic process which articulates with the zygomatic bone at the frontozygomatic suture, and finally, the supraorbital foramen or notch which gives passage to supraorbital vessels and nerves.

We then moved on to the maxilla, which contributes to the medial and inferior orbital margin in addition to the medial part of the floor of the orbit. Once again, we looked at some important landmarks of this bone which contribute to the orbit such as the frontal process of the maxilla which articulates with the frontal bone at the frontomaxillary suture; the anterior lacrimal crest, a landmark of the frontal process; the inferomedial orbital margin, and the orbital surface of the maxilla with its most prominent landmark here known as the infraorbital groove. This gives way to the infraorbital canal which courses through the maxilla before opening here at the infraorbital foramen.

Our next bone of interest was the zygomatic bone or the cheekbone, another bilateral bone of the facial skeleton. It contributes largely to the inferolateral part of the orbit by the inferolateral orbital margin and orbital surface.

After looking at the three anterior bones of the orbit, we turned our attention towards the bones which make up the posterior and medial walls of the orbit, beginning first with the sphenoid bone. Here we identified the greater wing of the sphenoid bone; specifically, its orbital surface, which contributes to the posterior wall of the orbit. Just superior to the greater wing was the lesser wing, which can also be partially seen on the posterior orbital wall, and when discussing the greater wing of the sphenoid bone, we identified two clefts or fissures known as the superior and inferior orbital fissures found along the superior and inferior margins of the greater wing.

Next up was the ethmoid bone, another centrally located bone which contributes to both the left and right orbits via its orbital plates which are found on the lateral surfaces of the bone. We finished up with two smaller bones, which were the palatine bone, a paired bilateral bone whose orbital processes contribute to a small portion of the posterior orbital wall; and finally, the lacrimal bone, another bilateral bone, which is largely defined by a large crest on its anterior surface – the posterior lacrimal crest.

Finally, we looked at orbital blowout fractures in our clinical note section.

And that’s it. Done and dusted. I hope you enjoyed feasting your eyes on the bony orbit. Please make sure to spread the word about Kenhub’s awesome video tutorials, and until next time, happy studying!