Limbal Landmarks: Bridging the External and Internal Anatomy of the Eye

In this article we discuss in detail about the anatomy, physiology of a bustling hub of ocular activity; limbus. In our previous posts we have discussed regarding the anatomy and physiology of cornea in detail! if you missed that, you can click here and have a quick read . If you are an audio visual learner you can visit my video on the anatomy of the limbus here. Also you may refer to the text below as a read along notes for the same.

What is Limbus?

The term, limbus, means ‘a border‘ between two different types of tissues.
In ocular context, it refers to the border zone or the junction between the cornea and sclera.
Similar to geographic border zones, there is no clear consensus on its exact boundaries.

Why is Limbus important?

The limbus is a big deal as it has a lot of things going on.
It demarcates the optically clear cornea from the conjunctiva and opaque sclera.
It harbors the pleuripotent limbal stem cells present in the palisades of vogt.
These stem cells have a very high proliferative capacity and have a role in corneal regeneration.
The limbus is highly vascular . Therefore, it maintains nourishment of the peripheral cornea.
Corneal cells derive nourishment by diffusion from the aqueous, the capillaries at the limbus and oxygen dissolved in the tear film
It contains the pathways of aqueous humour outflow.
It is the site of surgical incisions into the anterior chamber for cataract and glaucoma.

Gross Anatomy Of Limbus

Externally the limbus is formed by the junction between the corneal epithelium and the conjunctival epithelium.
There is a gradual transition at the limbus from the stratified, non-keratinized
columnar epithelium of the conjunctiva to the stratified, non-keratinized squamous epithelium of the cornea.
There are seven to ten layers of cells in the limbal epithelium
The basal limbal epithelial cells have dividing capacities and they harbor the stem cells in palisades of Vogt. (For more refer to anatomy of Cornea)
The substantia propria of the conjunctiva is thin at the limbus and fuses with the episclera and tenons capsule at the limbus.
This provides a surgeon a firm grip of globe with forceps during any ocular surgery.
The radius of curvature changes abruptly at the junction of cornea and sclera creating a shallow furrow or sulcus externally. This is known as the external scleral sulcus or sulcus sclerae
Corresponding to the external scleral sulcus; an internal inward groove is formed.
This is known as the internal scleral sulcus.
The posterior edge of this sulcus forms the scleral spur.
This contains the trabecular meshwork and the aqueous humor outflow apparatus (more in anatomy of the angle of eye)

Understanding the Limbal Zone from Inside

The prominent posterior end of Descemet’s membrane of the cornea is known as the Schwalbe’s line.
When seen from inside of the eye, the limbal zone begins at the junction of the peripheral extent of the Descemet’s membrane (schwalbes line)- endothelium and the anterior most end of the trabecular meshwork.
Its important to note that the corneal endothelium does not continue over trabecular meshwork.

DID YOU KNOW?

During embryologic development, the trabecular meshwork is covered by an endothelial layer confluent with corneal endothelium.
This membrane ‘retracts‘ to the cornea only in the later stages of the third trimester.
What prevents the corneal endothelium respect this border is not understood, but occasionally it crosses over, overgrowing trabecular mesh work and iris and leading to secondary glaucoma.
Proliferation of similar endothelial tissue has been described in the Irido-corneal-endothelial (ICE) syndromes and after trabecular trauma, especially laser trabeculoplasty.

Anatomical Limbus

The limbus is defined differently by pathologists, by histologists and by ophthalmic surgeons.
ANATOMICAL LIMBUS : PATHOLOGIST DEFINITION OF A LIMBUS :–
Pathologists define a limbus as a block of tissue bordered anteriorly by a line joining the peripheral extremes of Bowman’s and Descement’s membrane and posteriorly by a line from scleral spur perpendicular to the tangent of the external surface of the globe.
This makes the limbus an oblique transition zone which is narrower above(i.e between the corneal and conjunctival epithelium) and wider below between the corneal stroma and sclera.
In superior and inferior quadrant the limbal epithelium extends more into the cornea leading to early termination of the bowmans layer.
Therefore, the limbal zone is broader in the vertical meridian(12o clock-6o clock) than the horizontal meridian.

Histology Of Limbus

The epithelial characteristics of the limbus have already been defined before.
A histologist identifies corneal-scleral junction or limbus based on histologic cross sections. Note that both cornea and sclera are made up of collagen.
This limbal junction is difficult to define precisely with conventional histology although corneal collagen is somewhat less eosinophillic than scleral collagen.
In cornea the collagen fibre size is about 700 Angstrom and these fibers are regularly arranged making cornea transparent or clear.
However in sclera the fibre size is greater (700-1000 angstroms) and the fibres are irregularly arranged; making the sclera opaque.
The scleral fibres extend further anteriorly on the external than on the internal surface of the corneo-scleral junction.
Thus in cross-section the histologic limbal junction from external to internal forms a curvilinear diagonal.
This diagonal configuration is important for the appearance of the surgical limbus( read below)

Cellular Makeup Of the Limbus

The limbus is not a very cellular place. Majority of it is formed by collagenous connective tissue.
The only cellular areas of the limbus are the border of the cornea epithelium and conjunctival epithelium and the trabecular meshwork.
To know more about the stem cells present in the limbus refer to the article on anatomy of cornea.

DID YOU KNOW: Importnace Of Cells in the anterior trabecular meshwork near limbus

The majority (60%) of the newly dividing cells are concentrated in the most anterior region of the trabecular meshwork.
The trabecular cells have vast phagocytic capability and infact they can detach themselves from the trabecular beams in a very macrophage-like manner when exposed to foreign material.
This denuding of the trabecular beams has been suggested as a contributor to outflow obstruction in some forms of glaucoma.
Also, it has been seen that during the weeks following laser trabeculoplasty the newly divided cells are seen to migrate into the region of the trabecular burns.

External Landmarks Of Limbus

The limbus is characterized by the following external landmarks

ANTERIOR LIMBAL BORDER
MID LIMBAL LINE
POSTERIOR LIMBAL BORDER.
The borders divide the limbus into two zones; an anterior blue grey zone and a posterior white zone.

Kasner has popularised the correlation between these external landmarks of the limbus and the internal structures of the angle. Understanding this immensely helps a surgeon in planning the incision during intraocular surgeries.

Anterior Limbal Border (limbo-corneal border)

The anterior Border is formed due to firm attachment of the tenons and conjunctiva on the sclera.
Internally the anterior limbal border corresponds to the termination of the bowman’s membrane.
The Blue Zone Lies posterior to the anterior Limbal border in between it and the mid limbal line.

Mid Limbal line or Border

Situated in the middle of anterior and posterior limbal border. .
It corresponds the termination of the Descemet’s membrane ( Schwalbe’s Line)
It acts as a junction between the blue and white zones of the limbus.
In literature, this is also known as the posterior border of the Surgical limbus.

Posterior Limbal border

Forms the posterior Border of the limbus.
It can be difficult to visualise, hence is seen better with the use of a sclerotic scatter illumination.
Therefore it has also been defined in literature as the posterior limit of the sclerotic scatter.
The posterior limbal border roughly corresponds internally with the scleral spur
To know more about the anatomy of the scleral spur and angle structures, click here

Blue Limbal Zone V/s White Limbal Zone

BLUE LIMBAL ZONE (Aka Surgical limbus)

It is a bluish translucent zone between the anterior limbal border and the mid limbal line.
This corresponds to part of cornea between termination of bowmans and descemets membrane.
It can be seen after the conjunctiva and tenons have been reflected; like after peritomy.
The extent of the blue zone is 1mm in superior quadrant, 0.8mm in inferior quadrant and 0.4mm in nasal and temporal quadrant.
It is also known as the surgical limbus.

WHITE LIMBAL ZONE

It is 1mm zone situated between the mid limbal line and posterior limbal border.
It corresponds to the trabecular meshwork.
Its width is constant throughout.

Surgical Limbus

Majority of operations for cataract and glaucoma involve incision through the limbus.
Therefore limbus holds great importance for the surgeons.
The limbus is populary described by the surgeons as a blue-grey transition zone seen around the cornea after reflection of the conjunctiva.
This blue-grey ring spans a width of about 1.2 mm between the white sclera posteriorly and the transparent cornea anteriorly.
The blue grey zone is narrower in the horizontal than in the vertical meridian.

DO YOU KNOW: Why Limbus appears Bluish grey in color?

The blue grey appearance derives from a scattering of light through the oblique interface between cornea and sclera.
The interface between the cornea and sclera is not straight as discussed above.
Instead it is curvilinear diagonal. At this transition zone differential scattering of light giving a blue grey color to the limbus.

Relationship between limbal incisions and the structures of the anterior chamber angle

It is essential for the ophthalmic surgeon who is incising the limbus to know exactly where
his knife will penetrate the interior of the eye.

Anterior Limbal Incision

When a incision is made at the anterior limbal border ,whether the incision is made with the conjunctiva in place or after reflecting a conjunctival flap, it was always be found to enter the anterior chamber well anterior to the termination of Descemet’s membrane. (since the Descemet’s ends at the mid limbal line)
This point of entry into the anterior chamber will be separated from the termination of Descemet’s membrane by the following average distances: 1.2 mm. superiorly, o.8 mm inferiorly, and 0.4 mm. nasally and temporally.
These distances naturally depend on the degree of conjunctival overlap seen.
Since the incision passes through the Descemet’s membrane; there is risk of Descemet’s stripping and detachment. this applies to incisions taken at the anterior border or in the blue zone alike.

Mid Limbal Incision / Posterior border of the Blue zone /Posterior Border of the Surgical limbus

The incision here is given at the junction of the blue zone and the white zone.
This can be easily seen after reflecting the conjunctiva.
As this corresponds to the Schwalbe’s line.
Using this incision one can enter the anterior chamber at or just anterior to the termination of Descemet’s membrane. This is about 0.5 mm anterior to the centre of the lumen of Schlemm’s canal.
Such incisions do not cut into the trabecular meshwork and all the structures of the angle are posterior to the site of entry.
Hence , it can be considered as a pretty safe site for incisions and forms an important landmark.

Posterior Limbal Border Incision /Incisions at the posterior limit of the scleral scatter

Incisions placed perpendicular to the scleral surface at the posterior limit of the scleral scatter have a constant relation to angle structures.
Irrespective of the meridian, such incisions cut internally at a point in the sclera behind the scleral spur,
This means that here they do not enter the anterior chamber but the supraciliary space about o.5 mm behind the scleral spur.
However, when the incision is given in the white limbal zone i.e posterior to the mid limbal border, but anterior to the posterior limit of sclerotic scatter, it cuts through the trabecular meshwork located anterior to the scleral spur.
This may occasionally be done on purpose, but other times it may cause harm to the trabecular meshwork.

Scleral Incisions

This is when the incision is taken posterior to the posterior limbal border.
The main disadvantage of this incision is that it can cause bleeding as they can pass through the pars plicata causing bleeding.
Examples: sulcus surgeries (like Capsule tension ring placement, SFIOL surgery)
In an ab interno ciliary sulcus suture fixation of IOL using straight needle, the neddle must emerge around 2-2.5mm from the midlimbal line and 3.0 mm in pars plana fixation of IOL

ALSO KNOW :-

CLEAR CORNEAL INCISION: This incision is given anterior the anterior limbal border. It has greater risk of Descemet’s detachment.
Also it can induce high astigmatism.

Conclusion

So that was the detailed anatomy of the limbus. I hope now we understand well the various landmarks of the limbus and its importance.

If you are an audio visual learner you can check out our video on the anatomy of limbus here.