Anatomy of Orbit
A careful study of anatomy of orbit is very important to an ENT surgeon because of its proximity to the para nasal sinuses. A comprehensive knowlege of orbital and peri orbital anatomy is necessary to understand the various disorders of this region and in its surgical mangement.
The shape of the orbit resembles a four sided pyramid to begin with but as one goes posterior it becomes three sided towards the apex. The volume of the orbital cavity in an adult is roughly about 30cc. The rim of orbit in an adult measures about 40mm horizontally and 35 mm vertically. The medial walls of orbit are roughly parallel and are about 25 mm apart in an adult. The lateral walls of orbit angles about 90 degrees from each other.
Osteology of orbit: The orbital rim is more or less spiral with its two ends overlapping medially on either side of lacrimal fossa. The inferior orbital rim is formed by the maxillary bone medially and zygomatic bone laterally.
Image showing the bones taking part in the formation of orbit
The zygomatic bone forms the lateral orbital rim, while the frontal bone forms the superior orbital rim. The superior rim is commonly indented by a small notch known as the supra orbital notch. This notch is invariably present at the junction of medial and lateral 1/3. The supra ortbital nerve and artery pass through this notch to reach the forehead.
The medial portion of the orbital rim is formed by the frontal process of maxilla and the maxillary portion of the frontal bones. A depression known as the lacrimal fossa is formed in the infero medial orbital rim. This fossa is formed by the maxillary and lacrimal bones. This lacrimal fossa is bounded by two projections of bones i.e. the anterior lacrimal crest of maxillary bone and the posterior lacrimal crest of lacrimal bone. This fossa houses the nasolacrimal sac. This fossa opens in to the naso lacrimal canal through which the naso lacrimal duct traverses.
The naso lacrimal duct is 3 - 4 mm in diameter, courses in an infero lateral and slightly posterior direction towards the inferior turbinate under which it opens into the inferior meatus. This duct is roughly 12mm long. All the walls of the lacrimal duct except its medial wall is formed by the maxillary bone. The medial wall is formed by the lateal nasal wall inferiorly and the descending process of lacrimal bone superiorly.
In the frontal process of maxilla just anterior to the lacrimal fossa a fine groove known as the sutura longitudinalis imperfecta of Weber. This suture runs parallel to the anterior lacrimal crest. Small branches of infraorbital artery pass through this groove to supply the nasal mucosa. The presence of these vessels should be anticipated in any lacrimal sac surgery to avoid unneccessary troublesome bleeding.
Embryology of orbit: The walls of the orbit formed by 7 bones, are embryologically derived from neural crest cells. Ossification of the orbit is complete at birth except at its apex. Except the lesser wing of sphenoid which is cartilagenous the other bones develop by intramembranous ossification.
The roof of the orbit is mostly formed by the frontal bone, only the posterior 1.5 cms of the roof is formed by the lesser wing of the sphenoid bone. The optic foramen through which the optic nerve traverses is located in the lesser wing of the sphenoid bone. The optic nerve enters the orbit at an angulation of 45 degrees.
The lacrimal gland fossa is located in the lateral portion of the orbital roof, while the trochlear fossa is located in the anterio medial portion of the orbital roof.
The medial wall of the orbit is formed from anterior to posterior by :
1. frontal process of maxilla
2. lacrimal bone
3. ethmoid bone
4. lesser wing of sphenoid bone
The thinnest portion of the medial wall is the lamina papyracea which separates the ethmoidal sinuses from the orbit. It is one of the components of ethmoid bone. Infections from ethmoidal sinus can easily breach this paper thin bone and affect the orbital contents. The medial wall of the orbit is thicker posterior where the sphenoid bone is present and anteriorly where the posterior lacrimal crest is present.
The fronto ethmoidal suture line marks the approximate level of ethmoidal sinus roof, hence any dissestion above this line may expose the cranial cavity. The anterior and posterior ethmoidal foramina through which branches of ophthalmic artery (anterior and posterior ethmoidal arteries) and branches of naso ciliary nerve passes are present in this suture. The anterior ethmoidal foramen is located at a distance of 24 mm from the anterior lacrimal crest, while the posterior ethmoidal foramen is located at a distance of 36mm from the anterior lacrimal crest.
Image of skull lateral view showing posterior relations of orbit
A vertical suture that runs between the anterior and posterior lacrimal crests is the anastomotic area between the maxillary and the lacrimal bone. If this suture is located more anteriorly it indicates a predominance of lacrimal bone, while a more posteriorly placed suture line indicates a predominance of maxillary bone in the anastomotic relationship. The lacrimal bone at the level of lacrimal fossa is pretty thin (106 micrometer). This bone can be easily penetrated during dacryocystorhinostomy surgery. If the maxillary component is predominant it becomes difficult to perform the osteotomy in this area to access the sac because the maxillary bone is pretty thick. Hence lacrimal bone predominance makes it easy to expose the sac during dacryocystorhinostomy.
The floor of the orbit is the shortest of all its walls and is bordered laterally by infra orbital fissure. Medially the floor is bounded by the maxillo ethmoidal strut. The floor of the orbit is almost entirely formed by the orbital plate of maxilla, palatine contributes to a small portion of the floor posteriorly. Zygoma also makes a small contribution to it anterolaterally. The infra orbital groove becomes a canal anteriorly, through this groove passes the infra orbital nerve and artery. The floor of the orbit medial to the infra orbital groove is thin because of the expansion of the maxillary sinus. With the growth of facial bones the infra orbital foramen migrates to about 6-10mm below the infra orbital rim.
The lateral wall of the orbit is formed mainly by the greater wing of sphenoid bone with contributions from zygoma and zygomatic process of frontal bone anteriorly. The recurrent meningeal branch of middle meningeal artery may be seen coursing through a foramen in the suture line between the frontal and sphenoid bones. This artery forms a anastomosis between the external and internal carotid arterial systems. Roughly 4 - 5 mm behind the lateral orbital rim and 1 cm inferior to the frontozygomatic suture is the lateral tubercle of Whitnall. The following structures gets attached to this tubercle:
1. Lateral canthal tendone
2. Lateral rectus check ligament
3. Suspensory ligament of lower eyelid (Lockwoods ligament).
4. Orbital septum
5. Lacrimal gland fascia.
The frontal process of zygomatic bone and the zygomatic process of frontal bone are thick and they protect the globe from lateral trauma. Just behind this facial buttress area the posterior zygomatic bone and the orbital plate of greater wing of sphenoid are thinner thus making the zygomatico sphenoid suture a convenient land mark for lateral orbitotomy. The zygomatico facial and zygomatico temporal nerves and vessels pass through the lateral wall of the orbit to reach the cheek and temporal regions. Posteriorly the lateral wall thickens and meets the temporal bone which forms the lateral wall of the cranial cavity. When lateral orbitotomy is being done only 12 - 13 mm separate the posterior aspect of lateral orbitotomy to that of the middle cranial fossa. This distance could still be shorter in females.
Superior orbital fissure: is a linear notch between the greater and lesser wings of sphenoid. The superior portion of the fissure is narrower and here the lacrimal, frontal and trochlear nerves passes through outside the annulus of zinn. The annulus of zinn is a ring of fibrous tissue surrounding the optic nerve at its entrance into the apex of orbit. This ring gives origin to the extra ocular muscles. The following structures pass through the superior orbital fissure within the annulus of zinn:
1. Superior and inferior divisions of oculomotor nerve
2. The abducent nerve
3. Naso ciliary branch of ophthalmic branch of trigeminal nerve
4. Major venous orbital drainage exit via the superior orbital fissure to drain into the cavernous sinus.
Medial to the superior orbital fissure is the optic foramen through which the optic nerve passes. This formen which is present in the lesser wing of sphenoid also conveys the ophthalmic artery. The optic foramen and optic canal are separated from the superior orbital fissure by a bony optic strut. In adults the optic canal is 8 - 10 mm long and 5 - 7 mm wide. The optic foramen is about 6.5 mm in diameter. The optic canal is known to attain its full adult size by the age of 3. The optic foramen on both sides are universally symetrical. Any variation in size even to the extent of 1mm should be considered as pathological.
Superior orbital fissure
The inferior orbital fissure lies between the lateral orbital wall and the floor of the orbit. It is about 20 mm long. The following structures pass through this fissure:
1. Maxillary division of trigeminal nerve
2. Zygomatic nerve
3. Branches from the sphenopalatine ganglion
4. Branches of inferior ophthalmic vein leading on to pterygoid plexus.
The maxillary division of trigeminal nerve and the terminal branch of internal maxillary artery enter the infra orbital groove and canal to become the infra orbital nerve and artery. These structures exit through the infra orbital foramen to supply the lower eye lid, cheek, upper lip and upper anterior gingiva.
The roof of the orbit slopes down medially. In fact this slope continues up to fronto ethmoidal suture to form the roof of the ethmoid sinus. This is otherwise known as fovea ethmoidalis.
The anatomical relationship between the anterior ethmoidal air cells and the lacrimal fossa should be borne in mind to avoid confusion between the ethmoid and nasal cavities during dacryocystorhinostomy surgery.
Soft tissues of orbit:
Orbital septum is the anterior soft tissue boundary of the orbit. It acts as a physical barrier against pathogens. This is a thin multilayered fibrous tissue derived from the mesodermal layer of eyelid. This septum is covered anteriorly by the preseptal orbicularis oculi muscle.
Periorbita: is the periosteal lining of orbital walls. The periorbita is attached to the suture lines, fissures and foramina of the orbit. Posteriorly the periorbita is continuous with the optic nerve sheath.
Orbital fat: Adipose tissue present in the orbit has a cushioning effect on the contents of orbit.
The extra ocular muscles of orbit arise from the annulus of zinn and are responsible for the movement of the globe. These muscles are:
lateral and medial rectus
Superior and inferior rectus
Superior and inferior oblique
The lacrimal system:
The main lacrimal gland is located in the supero temporal portion of orbit. It lies in the shallow lacrimal fossa of the frontal bone. The gland is composed of numerous secretory units known as acini which progressively drain in to small and larger ducts. The gland measures 20 mm by 12 mm. A fibrous band incompletely devides the lacrimal gland into two lobes i.e. posterior larger orbital lobe and a smaller anterior palpebral lobe. 2 - 6 ducts from the orbital lobe pass through the palpebral lobe joining with the ducts from the palpebral lobe to form 6 - 12 tubules to empty into the superio lateral conjunctiva. Hence damage to the palpebral lobe may block drainage from the entire gland. About 20 - 40 accessory lacrimal glands of Krause are located in the superior conjuctival fornix, about half this number is located over the lower fornix.
The lacrimal gland is innervated by branches from 5th and 7th cranial nerves, sympathetic supply to lacrimal gland is via the nerves from the superior cervical ganglion. The parasympathetic fibers are supplied via the 6th nerve. Sensory supply is via the branches of trigeminal nerve.
Figure showing lacrimal gland and its relationships
The lacrimal excretory system begins at a 0.3 mm at the medial end of each eyelids known as the punctum. These puncta are directed posteriorly. The punctal opening widens into ampulla, which is perpendicular to the eye lid margin. The ampulla makes a sharp turn to drain into the canaliculi. The canaliculi measures 0.5 - 1mm in diameter and courses parallel to the lid margins. The superior canaliculus is 8 mm long and the inferior canaliculus is 10 mm long. In majority of individuals the superior and inferior canaliculi merge into a common canaliculi before draining into naso lacrimal sac. The opening of common canaliculi into the naso lacrimal sac is known as the common internal punctum. There is a valve at the junction of common canaliculus and lacrimal sac at the common internal punctum level. This is known as the Rosenmuller valve. Another valve known as the valve of Hasner is found at the lower end of the naso lacrimal duct at the level of inferior meatus of nose.
If this Hasner's valve is imperforate in new born infants it causes congenital naso lacrimal obstruction.
The lacrimal sac resides in the lacrimal fossa. It measures about 12 - 15 mm vertically, and 4 - 8 mm antero posteriorly.