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Eyelid and Orbit Anatomy

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.

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.

Orbital septum

A septum often refers to a band of tissue that separates a structure. The orbital septum is a connective tissue band that attaches to {the} border of {the} orbital bone at {the} periosteum (outer aspect of a bone). Within its central structure, {the} septum joins {the} lid retractors at {the} lid margins. If one were to look at {the} septum a lot more closely, it contains a number of layers (lamina) that are in close relationship with {the} anterior connective tissue framework. From a functional point of view, {the} septum has mobility similar to {the} eyelids.

We have already discussed how {the} septum attaches to {the} lid margin. If one were to trace {the} septum in an outward direction (laterally), it is evident that it is attached to {the} margin of {the} orbit, around 1.5mm in front of {the} attachment of {the} LCT. The orbital septum is separated from {the} LCT by {the} previously discussed Eisler’s pocket of fat. While traversing laterally, {the} septum runs along {the} rim of {the} orbit at {the} arcus marginalis. When traversing above and towards {the} nose, {the} septum runs across {the} supraorbital groove, in front of {the} trochlea and along {the} posterior aspect of {the} lacrimal crest. From an anatomical point of view, this position results in {the} septum lying in front of {the} medial check ligament and behind {the} lacrimal sac and Horner muscle.

When tracing {the} attachment of {the} septum, it passes {the} lacrimal sac fascia, eventually reaching {the} anterior lacrimal crest at a point corresponding to {the} lacrimal tubercle. It then passes below this crest and along {the} lower rim of {the} orbit, ultimately leaving {the} rim at a point beyond {the} zygomaticomaxillary suture. This results in creation of a small space (recess) due to its separation from {the} zygomatic bone - this is called {the} premarginal recess of Eisler, and is filled with fat. Eventually, {the} septum reaches {the} lateral orbital margin at a point that lies just below {the} Whitnall ligament.

Another extension of {the} septum exists from {the} point where {the} orbital septum joins {the} levator aponeurosis. Described by Reid et al, this extension travels over {the} tarsal plate and ultimately reaches {the} ciliary margin. The function of this septum is to aid {the} levator aponeurosis, and should be borne in mind when operating on {the} eye.

Orbicularis retaining ligament

Also called {the} orbital retaining septum or orbitomalar ligament, this ligament attaches {the} orbicularis oculi muscle to {the} lower rim of {the} orbit. It is weak in its central aspect, and a lot stronger in {the} lower-outer aspect. When traced laterally, it is contiguous with tissue that is formed by fusion of {the} outer part of {the} orbicularis oculi and {the} deeper periosteum and temporalis fascia. This fusion is called {the} orbital thickening. This orbital thickening covers {the} frontal process of {the} zygomatic bone.

As one gets older, {the} orbicularis retaining ligament tends to get thinned out and stretched, with these changes more prominent in {the} central aspect. When excised along with {the} orbital thickening, it results in full release of {the} superficial fascia that lines {the} orbital rim.

Upper lid retractors

The upper lid retractors are a group of muscles whose main function is {the} keep {the} upper eyelid elevated. The muscle that forms a part of this is called {the} levator palpebrae superioris (LPS). This muscle originates from {the} bottom aspect of {the} lesser wing of {the} sphenoid bone located within {the} skull. It consists of 2 heads - {the} levator muscle and {the} superior rectus muscle. They are joined together by fibrous tissue. From its origin, {the} LPS traverses horizontally forward for about 40mm, ending in an aponeurosis that is around 10mm posterior to {the} orbital septum. It then takes a more vertical course toward {the} Whitnall ligament (superior transverse ligament).

The Whitnall ligament is similar to {the} previously described orbital fascia and lies in close proximity to {the} aponeurotic and muscular junction of {the} LPS. It extends around {the} upper margin of {the} orbit in a plane that lies between {the} lacrimal gland fascia and {the} trochlea. The LPS varies in thickness, and is relatively thin in areas between {the} upper orbital rim and {the} Whitnall ligament.

When traced inwards and outwards, {the} LPS aponeurosis forms ‘horns’ called medial and lateral horns. The lateral horn runs through {the} lacrimal gland, dividing it into 2 lobes - {the} palpebral lobe and {the} orbital lobe. Having done this, it goes on to attach to {the} lateral retinaculum located at {the} lateral orbital tubercle. On {the} other hand, {the} medial horn has a more direct course and is fixed to {the} posterior lacrimal crest.

The aponeurosis eventually reaches {the} border of {the} superior tarsal plate having fused initially with {the} orbital septum. At {the} bottom end of this fusion, a small part of {the} aponeurosis attaches to {the} lower aspect of {the} anterior part of {the} tarsal plate. One part of this fusion extends forwards to insert into {the} pretarsal orbicularis oculi muscle and skin, resulting in {the} formation of {the} skin crease in {the} upper eyelid. 

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.