Anatomy – Orbit

Anatomy - Orbit

Our eyes are probably the most important vital structures we have in our body. They discovered on the surface by a thin layer of skin and soft tissue called the eyelids. The eyelids serve multiple purposes including protecting the eyeball from injury, controlling the amount of light that enters the eye and also constantly lubricating the eyeball with tears secreted by the lacrimal gland during blinking. All these functions together help maintain the structural integrity of the eyeball and protect them from external influences.

From an anatomical point of view, the eyelid consists primarily of skin, underlying soft tissue also called a subcutaneous tissue and a thin layer of muscle called the orbicularis oculi. Under this muscle are other issues that divide the area into different planes. These are called septum and include the fibrous orbital septum and tarsi. In addition to this, in order for the eyelids to open are lid retractors that help assist blinking. Finally, there also exists a small amount of fat tissue as well. The eyeball is covered by a thin layer of tissue called the conjunctiva.

Anatomy of the eyelid

The description above only offers a superficial overview of the anatomy of the eyelid. If one were to look at the eyelid in a more detailed manner, a sagittal section taken across the eyelid will offer a clear view of the various structures that form it. Of course, it must be borne in mind that the structures that are visualised depend on the plane at which the sections are taken.

As mentioned above, the tissues can be divided into planes by structures called the septum. The orbital septum differentiates the orbital tissue from the lid. Behind the septum are a number of different other structures, a knowledge of which is essential if surgery is to be performed. In particular, it is essential to identify the anterior and posterior lamellae. In essence, the anterior lamella consists of the skin and the orbicularis oculi muscle while the posterior lamella consists of the conjunctiva and the tarsus.

Let’s take a look at the structures of the eyelid in a bit more detail.

Muscle Innervation Primary function Secondary function Tertiary function
Levator palpebrae superioris Oculomotor nerve Elevation of the superior (upper) eyelid
Superior rectus Oculomotor nerve Elevation Intorsion Adduction
Inferior rectus Oculomotor nerve Depression Extorsion Adduction
Lateral rectus Abducens nerve Abduction
Medial rectus Oculomotor nerve Adduction
Superior oblique Trochlear nerve Intorsion Depression Adduction
Inferior oblique Oculomotor nerve Extorsion Elevation Adduction

Orbital Anatomy

  • Orbital volume and dimensions: = 30 cc, 35(Height) x 45(Width) x 45 mm(medial wall depth), globe 25 x 25 mm
  • Bones: (F)rontal, (M)axillary,(Z)ygomatic, (L)acrimal, (E)thmoid, (P)alatine, (S)phenoid

Orbital Walls and Openings

 

  • Superior orbital fissure (SOF):
    • 22 mm long
    • separates greater wing of sphenoid from lesser wing of sphenoid
    • transmits third, fourth, sixth and V1 AND SYMPATHETIC FIBERS
    • lateral rectus origin separates into superior and inferior divisions
      • Superior division transmits lacrimal, frontal and trochlear nerves
      • Inferior division transmits superior and inferior divisions of CN III, nasociliary branch of CN V, CN IV, superior ophthalmic vein, and sympathetic nerve plexus
      • Venous system: superior ophthalmic vein
  • Inferior orbital fissure (IOF)
    • Located between lateral orbital wall and the orbital floor
    • Transmits V2 ( maxillary) , pterygoid nerves nerve arising from pterygopalatine ganglion
    • infraorbital nerve (a branch of V2) enters the infraobital groove and infraorbital canal for sensation to lower eyelid, cheek, upper lid, upper teeth
    • Venous system inferior ophthalmic vein
  • Holes/Notches/Canals

Nasolacrimal canal Nasolacrimal canal

  • lacrimal sac fossa to the inferior meatus
  • separates greater wing of sphenoid from lesser wing of sphenoid
  • transmits third, fourth, sixth and V1 AND SYMPATHETIC FIBERS
  • lateral rectus origin separates into superior and inferior divisions
    • Superior division transmits lacrimal, frontal and trochlear nerves
    • Inferior division transmits superior and inferior divisions of CN III, nasociliary branch of CN V, CN IV, superior ophthalmic vein, and sympathetic nerve plexus
    • Venous system: superior ophthalmic vein
  • Supraorbital foramen/notch
    • transmits blood vessels
    • supraorbital nerve
    • Anterior/posterior ethmoidal foramen: transmits ethmoidal blood vessels and nerve
    • Zygomatic foramen: transmits zygomaticofrontal and zygomaticotemporal nerves, zygomatic artery
    • Nasolacrimal duct (NLD): exits into inferior meatus
    • Infraorbital canal: transmits infraorbital nerve (V2)
  • Ethmoidal foramina
    • anterior ethmoidal artery
    • posterior ethmoidal artery
    • allows infections and neoplasms to enter to orbit from the sinuses
  • Optic Canal
    • 8-10 mm long
    • located within the less wing of sphenoid
    • separated from SOF by OPTIC STRUT
    • Transmits. Ophthalmic nerve, ophthalmic artery, sympathetic nerves
    • Optic foramen is 6.5 mm wide: it could be enlarged in the presence of optic nerve glioma; 1 mm of asymmetry between right and left is abnormal