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Ocular Anophthalmia Anatomy

 

Key Anatomical Facts

The human eye is an organ which reacts to light for several purposes.

The eye is not properly a sphere, rather it is a fused two-piece unit. The smaller, less curved unit called {the} cornea, is linked to {the} larger unit called {the} sclera.

The cornea and sclera are connected by a ring called {the} limbus.The iris and its black center, {the} pupil, are seen instead of {the} cornea due to {the} cornea's transparency.

 
 

Dimensions differ among adults by only one or two millimeters. The vertical measure, generally less than {the} horizontal distance, is about 24 mm among adults, at birth about 16-17 mm. The eyeball grows rapidly, increasing to 22.5-23 mm (approx. 0.89 in) by {the} age of three years. From then to age 13, {the} eye attains its full size. The volume is 6.5 ml (0.4 cu. in.) and {the} weight is 7.5 g. (0.25 oz.)

The eye is made up of three coats, enclosing three transparent structures. The outermost layer is composed of {the} cornea and sclera. The middle layer consists of {the} choroid, ciliary body, and iris. The innermost is {the} retina, which gets its circulation from {the} vessels of {the} choroid as well as {the} retinal vessels, which can be seen in an opthalmoscope.

Within these coats are {the} aqueous humor, {the} vitreous body, and {the} flexible lens.

The aqueous humor is a clear fluid that is contained in two areas: {the} anterior chamber between {the} cornea and {the} iris and exposed area of {the} lens; and {the} posterior chamber, behind {the} iris and {the} rest. The lens is suspended to {the} ciliary body by {the} suspensory ligament (Zonule of Zinn), made up of fine transparent fibers. The vitreous body is a clear jelly that is much larger than {the} aqueous humor, and is bordered by {the} sclera, zonule, and lens.

 

 

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.

The eyelids

The upper eyelid starts at {the} eye and extends up words joined {the} skin of {the} forehead. It is distinguished from {the} forehead skin by {the} presence of eyebrows. Similarly, {the} lower eyelid starts at {the} eye and extends to join {the} skin of {the} cheek. Upon close inspection, it is evident that {the} Lord eyelid is a lot looser than {the} upper eyelid, particularly because {the} tissue within {the} cheek that blends with {the} lower eyelid is a lot denser.

At {the} top of {the} upper eyelid is a fold in {the} skin called a skin crease or {the} superior palpebral sulcus. It lies around 8 to 11 mm above {the} margin of {the} upper eyelid and consists of fibres of {the} levator aponeurosis. Similarly, there also exists another skin fold in {the} lower eyelid called {the} inferior palpebral sulcus. However, this skin fold is often more prominent in children and can become less prominent as one gets older. Anatomically, {the} inferior skin crease is seen around 3 to 5 mm below {the} outer aspect of {the} lid margin.

The inner aspect of {the} eyelid is called {the} inner canthal region. At this region runs a fold of skin called {the} nasojugal fold. From an anatomical point of view, this fold lies between {the} orbicularis oculi and {the} levator labii superioris. To put it in simpler terms, {the} nasojugal fold is that area of {the} inner aspect of {the} eye where tears roll down and can accumulate. It is also called {the} tear trough. Similar to {the} nasojugal fold is another fold of skin that is evident in {the} outer aspect of {the} eyeball. This is called {the} malar fold and runs from {the} outer aspect towards {the} nasojugal fold.

When {the} eyes are open, {the} space between {the} upper and lower eyelids is typically described as ‘fusiform’. This space is also called {the} palpebral fissure. Typically {the} palpebral fissure measures between 28 to 30 mm wide and around 9 to 10 mm in height. If one were to examine {the} palpebral fissure in a bit more detail, it would be evident that {the} highest point of {the} fissure lies at that point on {the} upper eyelid that corresponds to a point at {the} inner aspect of {the} pupil. Younger individuals have {the} upper eyelid slightly higher than older individuals in whom it lies at about 1.5 mm below {the} margin of {the} eyeball called {the} limbus. Similarly, {the} lower eyelid lies at {the} border of {the} lower limbus.

There are two points at which {the} upper and lower eyelids meet. The one on {the} inner aspect is called {the} medial canthus while that at {the} outer aspect is called {the} lateral canthus. Both of these have a unique angle at which {the} upper and lower eyelids meet. When examined along a horizontal plane, {the} medial canthal angle is located around 2 mm lower than {the} lateral canthal angle in Caucasians; it is 3 mm lower in Asians. The nose lies around 15 mm on {the} inside of {the} medial canthus.

In a nutshell, {the} palpebral fissure consists of {the} medial and lateral canthus, {the} lacrimal papillae (part of {the} tear glands, also called lacrimal glands) and a small opening of {the} lacrimal glands through {the} lower eyelid at {the} medial canthus called {the} punctum lacrimale.

Skin and subcutaneous tissue

The eyelid is primarily made of skin. It is {the} thinnest skin in {the} body and is less than 1 mm thick. Within {the} skin are a number of glands called sebaceous glands that secrete an oily substance called sebum. These glands are in larger numbers at {the} nasal aspect of {the} eyelid. If one were to trace back {the} skin of {the} upper and lower eyelid, it would be clearly evident that once it joins {the} forehead or {the} cheek, {the} texture of {the} skin changes and becomes a lot thicker. Furthermore, {the} texture of skin is also different at {the} various folds described above. Below {the} skin is a layer of thin connective tissue called a subcutaneous tissue (sub = under, cutaneous = skin).

Underneath {the} skin, along with {the} subcutaneous tissue is a thin layer of fat. However, {the} amount of fat is negligible when compared to other parts of {the} body. Typically, subcutaneous tissue is absent at points where {the} skin is attached directly to underlying ligaments such as {the} medial and lateral palpable ligaments. The skin and subcutaneous tissue can be subject to certain clinical conditions such as dermatochalasis and blepharochalasis.

Orbicularis oculi muscle

The orbicularis oculi muscle plays an important part in {the} function of {the} eyelids and also in facial expressions. When it contracts and relaxes, {the} skin over {the} muscle tends to move as well. The orbicularis oculi muscle is attached to {the} skin through fibres tissues that form what is called {the} superficial musculoaponeurotic system.

Broadly divided, {the} orbicularis oculi muscle consists of two main parts. The orbital part plays a role when {the} eyelids need to be tightly shut. It is further divided into pretarsal and preseptal segments. The other part is called {the} palpebral portion that plays a role in winking and blinking. The muscle is supplied by {the} facial nerve then divides into different branches to supply these different muscles. Anatomically, {the} facial nerve travels under {the} muscle groups and supplies it from under it surface.

The orbital part of {the} orbicularis oculi muscle has a close relationship with other muscles responsible for facial expression. It originates from {the} inner margin of {the} orbit, further attaching to {the} upper and inner aspect of {the} orbital bone, {the} maxillary process that arises from {the} frontal bone, {the} lower and inner aspect of {the} orbital bone and {the} frontal aspect of {the} maxillary bone. The path taken by {the} muscle is typically described as 'horseshoe shaped'. The muscle fibres mingle with {the} surrounding facial muscles such as {the} corrugator supercilii and {the} frontalis muscle. As they travel around, they also interdigitate with {the} anterior temporalis fascia.

The preseptal portion of {the} orbicularis oculi muscle consists of a superficial and deep muscle head. The fibres of this portion that lie within {the} upper and lower eyelid join and {the} outer aspect to form a structure called {the} lateral palpebral raphe. The pretarsal portion also has similar origins and its fibres run under {the} lateral palpebral raphe, inserting into a bony structure at {the} outer aspect of {the} orbital bone called {the} lateral orbital tubercle through {the} lateral canthal tendon.

Submuscular areolar tissue

This is a loose connective tissue that lies beneath {the} orbicularis oculi muscle. It can form an anatomical plane that divide {the} eyelid into a front (anterior) and back (posterior) portion. The fibres of {the} levator aponeurosis then passed through this plane in {the} upper eyelid. A small portion of these fibres contribute towards {the} development of {the} upper eyelid crease. Similarly, in {the} lower eyelid, {the} fibres of {the} orbitomalar ligament passed through this plane.

If this anatomical plane were to be tracked towards {the} eyebrow area, {the} retro-orbicularis oculi fat will be traversed. If {the} plane were to be tracked towards {the} cheek, {the} sub-orbicularis oculi fat would be traversed.

Tarsi and Orbital septum

Tarsal plates

In order for {the} eyelids to maintain their shape and integrity, within them is dense fibrous tissue called tarsal plates. Each of these tarsi is around 1mm thick and 29 mm in length. There are 2 main types of tarsi – {the} superior tarsus and inferior tarsus. The superior tarsus is crescentic in shape and measures around 10mm vertically in its central aspect. It narrows outs as it traverses towards {the} nose and outer aspect of {the} eyelid. Its lower area is what forms {the} back of {the} eyelid that lies next to {the} conjunctiva of {the} eyeball. Similarly, {the} inferior tarsus lies in {the} lower eyelid, measures 3.5 – 5mm in height at its centre, and also lies in contact with {the} conjunctiva. Each of {the} tarsi are attached to {the} margin of {the} orbits through {the} medial and lateral palpebral ligament.

Within {the} tarsal plates are 25 tiny glands called mebomian glands. These glands are as tall as {the} tarsus, and they open at a point just in front of {the} lid margin where {the} conjunctiva meets {the} skin (mucocutaneous junction). If one were to look closely, they lie behind a grey line on {the} margin of {the} eyelid.

Medial palpebral ligament

Also called {the} medial canthal tendon (MCT), {the} medial palpebral ligament is a band of fibrous tissue that holds {the} inner aspect of {the} tarsal plates in place. It is closely related to {the} orbicularis oculi muscle and {the} tear ducts. The MCT is composed of an anterior limb which is formed by a small part of {the} superficial aspect of {the} orbicularis muscle that lies behind {the} tarsus. It traverses along a horizontal plane but is also attached to {the} frontal bone through a superior extension. The deeper part of {the} orbicularis muscle inserts into {the} back aspect of {the} lacrimal crest and {the} lacrimal sac fascia. The fascia of {the} lacrimal sac is therefore closely related to {the} various aspects of {the} MCT.

Lateral palpebral ligament

Also called {the} lateral canthal tendon (LCT), {the} lateral palpebral ligament is also a band of fibrous tissue that originates from {the} tarsus, traverses outwards under {the} orbital septum and eventually inserts into {the} lateral orbital tubercle (this lies around 1.5mm behind {the} lateral orbital rim). The LCT is around 10.5mm long and 6.5 mm wide, and at its midpoint attaches around 10mm below {the} frontozygomatic suture. The orbital septum and {the} LCT are separated by a pocket of fat called {the} Eisler pocket. In addition, {the} LCT is attached to {the} outer part of {the} orbital rim through a superficial plane of fascia. This has been also called {the} superficial lateral canthal tendon and helps to keep {the} lateral canthus stable.

When traced above and below, {the} LCT attaches to {the} lateral horn of {the} levator aponeurosis above while {the} lower aspect forms an arc where it attaches. During outer movement of {the} eye (abduction), {the} lateral canthal angle moves around 2mm as well, and this occurs due to {the} fibres that attach from {the} back of {the} lateral check ligament of {the} lateral rectus muscle.

A structure called {the} lateral tarsal strap has been described by Flowers. This is believed to be different to {the} LCT, and connects {the} tarsal plate of {the} lower lid to {the} lower, outer aspect of {the} orbital rim. Anatomically, it is a broad and rather sturdy structure. It lies just 3mm below and 1 mm behind {the} LCT, and 4-5 mm behind {the} anterior orbital rim. If excised, it allows for elevation of {the} lateral canthus.

Fat Pads

There are a number of different fat pads that are present within and around {the} eyelid. One layer of fat called {the} pre-aponeurotic fat is found right behind {the} orbital septum and in front of {the} levator aponeurosis. Also within {the} upper eyelid are two more areas that contain fat pads that are centrally and medially (towards {the} nose) located. The medial fat pad is pale yellow in colour and lies in front of {the} levator aponeurosis.

On {the} other hand, {the} central pad of fat is broader and yellow in colour. As it travels outwards, it wraps around {the} inner aspect of {the} lacrimal gland. The lacrimal gland can be clearly seen and differentiated from this fat by its pink colour and lobulated structure. The lacrimal gland is positioned just posterior to {the} orbital margin but might prolapse slightly making it more prominent when {the} eye is examined.

While {the} above described {the} fat pads within {the} upper eyelid, {the} lower eyelid fat pads are slightly different in structure. The inferior oblique muscle separates {the} central fat pad from {the} dinner medial fat pad. There is a small amount of fat that lies in front of {the} inferior oblique muscle as well. The inferior oblique muscle originates from a small indentation in {the} lower border of {the} orbital floor, moving behind {the} orbital margin and at {the} upper aspect of {the} nasal lacrimal canal. It passes underneath {the} inferior rectus muscle and through {the} Tenon capsule, ultimately inserting at {the} point close to {the} macula of {the} eye. This rather winding course of {the} inferior oblique muscle makes it vulnerable to injury during this section of {the} fat pads around {the} eyelid and eye.

Blood supply

The eyelids are supplied by branches of {the} internal and external carotid arteries. The ophthalmic artery branches off {the} internal carotid artery and supplies different parts of {the} eyelid. At {the} inner part of {the} upper eyelid, {the} ophthalmic artery splits into two and traverses outwards {the} supply both {the} upper and {the} lower eyelid. The branch that supplies {the} lower eyelid is in fact a branch that arises from {the} superior marginal vessel (that supplies {the} upper eyelid). The superior and inferior marginal vessels that arise from {the} ophthalmic artery together form {the} marginal arcade.

The marginal arcade arteries are located at {the} front of {the} tarsus, 4 mm from {the} upper eyelid and 2 mm from {the} lower eyelid margin each. The superior marginal arcade gives rise to a peripheral arcade that runs in front of {the} Muller muscle, giving it a superficial plane and making it prone to injury during eyelid surgery. The peripheral arcade in {the} lower eyelid is often rudimentary.

Another branch of {the} internal carotid artery is {the} lacrimal artery that passes through {the} orbital septum along each eyelid and ultimately joins {the} marginal arcade.

While {the} above described {the} branches of {the} internal carotid artery, {the} external carotid artery supplies {the} eyelids as branches of {the} facial artery, infraorbital artery and {the} superficial temporal artery. Each of these pride branches that anastomosis with other arteries on {the} face. For example, {the} branch of {the} superficial temporal artery that supplies {the} eyelids joins with {the} zygomatic branch and transverse facial branch.

Lymphatic drainage

The lymphatic drainage of {the} eyelid is rather extensive. The majority of {the} upper eyelid and {the} outer half of {the} lower eyelid drain into {the} pre-auricular lymph nodes, while a small part of {the} middle of {the} upper eyelid and {the} inner half of {the} lower eyelid drains into {the} submandibular lymph nodes.

Muscles

There are numerous muscles around {the} eyeball that control of different movements. These muscles are called extraocular muscles. They include {the} medial rectus, lateral rectus, inferior and superior oblique and inferior and superior rectus muscles. These are responsible for {the} various directions of movement of {the} eyeball, including rotation of {the} eyeball. Within {the} eyelid, {the} levator palpebrae superioris is responsible for elevation of {the} upper eyelid.

The extraocular muscles are supplied by a variety of different cranial nerves. These include {the} oculomotor nerve, {the} trochlear nerve and abducens nerve.

Bones

The socket within which {the} eyeball is located is called {the} orbit. It is a pyramidal shaped fossa that is created by {the} fusion of different orbital bones. These bones originate from {the} different aspects of {the} skull such as {the} frontal bone, sphenoid bone, zygomatic bone and {the} Palatine bones. In addition, {the} maxillary bone and {the} lacrimal bone also form a part of it. The arrangement of these bones is such that {the} walls are parallel to each other. The orbit measures 4 cm in height, 3.5 cm in width and is around 5 cm in depth.

Within {the} orbit are a number of blood vessels and nerves. These passed through {the} bone through various openings called fissures. There are three main openings - {the} superior orbital fissure, {the} inferior orbital fissure and {the} optic canal. Through these openings, various cranial nerves passed through and supply {the} muscles and blood vessels in {the} orbit. The superior orbital fissure allows for {the} passage of {the} frontal nerve, lacrimal nerve, nasociliary nerve and {the} recurrent branch of {the} lacrimal artery along with {the} superior orbital and ophthalmic veins. The inferior orbital fissure allows for {the} passage of {the} infraorbital nerve, zygomatic nerve, infraorbital artery and vein and parasympathetic nerve supplying {the} lacrimal gland. Through {the} optic canal passes {the} optic nerve, central retinal vein and ophthalmic artery.

Lacrimal gland

The lacrimal gland is responsible for tear production. It is divided by {the} levator aponeurosis into an orbital lobe and a palpebral lobe. It has a characteristic appearance as has been previously described. It is supplied by {the} lacrimal nerve which is a branch of {the} ophthalmic division of {the} trigeminal nerve.

The lacrimal gland secretes tears that are drained through a series of ducts. The lacrimal system consists of lacrimal papillae, canaliculi, lacrimal sac and naso-lacrimal duct. The canaliculus joins {the} lacrimal sac at an angle which is protected by {the} valve of Rosenmuller.

Connective tissue

We have already taken a brief look at {the} different types of connective tissue that are seen in {the} eyelid and {the} structures around it. The fascia that is present around {the} eyeball divides {the} orbit into a number of different connective tissue planes. Within each of these planes lie different structures. Having a knowledge of {the} structures helps {the} surgeon in locating them.