Detailed Anatomy of Sclera and Limbus

1. Limbus and Sclera
Shraddha
M. Optom

2. Anatomical Limbus
• Anatomically the limbus refers to a circumcorneal transition zone of the
conjunctivocorneal and corneoscleral junction .

3. Conjunctivocorneal Junction
• At this point the bulbar conjunctiva is firmly adherent to the underlying
structures.
• The substantia propria of the conjunctiva stops here but its epithelium
continues with that of the cornea.
• At the transition zone, the epithelium becomes several layers thick (10-12) and
arranged irregularly at the limbus
• The cells contain melanin and differ from those of the cornea in having no
prickles between them

4. Conjunctivocorneal Junction

5. sclerocorneal Junction
• At this point the transparent corneal lamellae becomes continuous with the
oblique ,circular and opaque fibres of the sclera.

6. Surgical Limbus
• The surgical limbus is a 2 mm wide circumcorneal transitional zone between
the clear cornea on one side and opaque sclera on other side.
• The surgical limbus is characterized by these external landmarks: anterior
limbal border, mid limbal line and posterior limbal border.

7. Anterior limbal border
• It is the anterior boundary of the surgical limbus seen from the exterior .
• It is marked by a prominent ridge created by insertion of the conjunctiva and
Tenon’s capsule into the cornea.
• The anterior limbal border overlies the termination of the bowman’s
membrane

8. Blue limbal zone
• It is bluish translucent zone seen posterior to the anterior limbal border after
dissecting the limbus free of the conjunctiva and Tenon’s capsule.
• Adjacent to the bluish zone posteriorly is the white sclera .
• Since the position of insertion of the conjunctiva and tenon’s capsule into the
cornea varies in different quadrants so the extent of blue limbal zone also
varies accordingly.
• The extent of blue limbal zone is about 1mm in superior quadrant, 0.8mm in
inferior quadrant and about 0.4 mm in nasal and temporal quadrant.

9. Mid limbal line
• It is the junction of the blue zone with white area.
• It overlies the termination of the Descemet’s membrane .

10. Posterior limbal border
• The posterior limbal border lies about 1mm posterior to the mid limbal line .
• It overlies the scleral spur and can only be seen with the use of sclerotic
scatter illumination .
• It forms the posterior border of the surgical limbus.

11. White limbal zone
• It is a 1 mm wide whitish area which lies between the mid limbal line and the
posterior limbal border.
• It overlies the trabecular meshwork
• Since the width of the white limbal zone is constant the total width of the
surgical limbus is greatest in the superior quadrant where width of the blue
zone (1mm) is maximum.

12. Sclera

13. Anatomy
• The sclera forms the opaque 5/6th part of the external fibrous tunic of the
eyeball.
• Its whole outer surface is covered with tenon’s capsule and bulbar conjunctiva
in the anterior part.
• Its inner surface lies in contact with the choroid with a potential
suprachoroidal space in between.

14. Thickness of sclera
• Thickness of sclera varies considerably in different individuals with different
age.
• Sclera is generally thinner in children than the adults and in females than in
males.
• Sclera is thickest posteriorly (1mm) and gradually becomes thin when traced
anteriorly.
• It is thinnest at the insertion of extraocular muscles (0.3mm) .
• Along with the muscle thickness tendon thickness becomes 0.6mm
• Thickness of sclera at equator: 0.4 to 0.6mm and adjacent to limbus is about
0.8mm

15. Special regions of the sclera
• Scleral sulcus :
• It is an indentation (furrow) on the inner surface of the anterior most point of
the sclera near the limbus.
• It houses the schlemm’s canal.

16. Special regions of the sclera
• Scleral spur :
• It is a circular flang of the anterior most part of the sclera which lies deep in
the schlemm's canal.
• It appears wedge shaped in section and is known as scleral spur.
• The corneo scleral part of the trabecular meshwork extends from the scleral
spur to Schwalbe’s line.
• The meridional fibres of the ciliary muscles are attached to the scleral spur

17. Scleral spur

18. Special regions of the sclera
• Lamina cribrosa :
• It is a sieve like sclera from which the fibres of the optic nerve pass.
• When the IOP is increased for a prolonged period of time, the lamina cribrosa
gradually increases in posterior curvature.

19. Scleral apertures (emissaria)
• Sclera has three sets of apertures:
1. Posterior apertures are situated around the optic nerve and transmit long
and short ciliary nerves and vessels.

20. Scleral apertures (emissaria)
• Sclera has three sets of apertures:
2. Middle apertures are situated 4-7mm posterior to the equator. Through this
pass four of more vortex veins(vena verticosae). The superior vortex veins are
more posterior (7mm) than the inferior veins (5-6mm).

21. Scleral apertures (emissaria)
• Sclera has three sets of apertures:
3. Anterior apertures are situated 3-4mm away from the limbus and transmit
anterior ciliary vessels, perivascular lymphatics and the ciliary nerves.

22. Microscopic structures
• Sclera consists of three layers :
1. Episcleral tissue
2. Sclera proper
3. Lamina fusca

23. Episcleral tissue
• It is thin, dense vascularized layer of the connective tissue which covers the
sclera proper.
• Anteriorly it becomes continuous with the tenon’s capsule.
• The capillary network present in the anterior part of the episclera becomes
prominent during inflammation giving rise to ciliary flush.

24. Sclera proper
• It is an avascular structure which consists of dense bundles of collagen fibres
crossing each other in all directions.
• This arrangement makes the sclera opaque in contrast to cornea where the
collagen bundles are arranged orderly.
• Mucopolysaccharides are present in interfibrillar space of the collagen fibres
• Few fibroblasts are also present in this layer.
• With advancing age the scleral collagen fibres tend to become sclerosed and
also there occurs deposition of lipids: consequently sclera becomes yellow

25. Lamina Fusca
• It is the innermost part of sclera which blends with suprachoroidal and
supraciliary lamina of the uveal tract.
• It is brownish in colour owing to presence of pigment cells.

26. Nerve supply of sclera
• The sclera is supplied by branches from the long ciliary nerves anteriorly and
short ciliary nerves behind the equator.

27. Physiological consideration
 Biochemical composition
• The water content of the sclera is about 70% compared to the 78% of the
corneal stroma; of the solids about 75% is collagen, 10% other proteins, 10%
glycosaminoglycans and the rest salts .

28. Physiological consideration
 Corneal lamellae vs scleral lamellae
• The uniformly arranged corneal stroma on crossing through the limbus
increasingly interweave.
• The individual collagen fibrils of each lamella also change from their uniform
30 nm diameter and 60 nm center to center spacing in the cornea to wide
variation in diameter (30 to 300) and irregular spacing in the sclera
• This particular arrangement of the lamellae accounts for the cornea being
transparent and sclera opaque

29. Physiological consideration
 Swelling pressure, diffusion and bulk flow
• The hydration of sclera is about 2 gram water / gram dry material near the
limbus and corresponding swelling pressure is 10 to 17 mm Hg while the
adjacent corneal stroma the hydration is 3.5 grams water /gram dry material
and swelling pressure is about 60 mm Hg.
• This difference is attributed to the lower GAG content of the sclera.

30. Physiological consideration
 Bulk fluid transport and uveal effusion syndrome
• Although most of the bulk transport of fluid out of the eye takes place through
the anterior chamber drainage and /or uveoscleral meshwork, there is
appreciable transretinal transport of fluid towards the choroid .
• Some of this is drained via the normal choroidal vessels but a proportion is
drained directly transclerally.
• Fluid flowing across the sclera is absorbed by the matrix proteoglycans
• Thus the sclera is maintained in its normal state by having proteoglycans with
a low water binding capacity

31. Physiological consideration
 Tissue mechanism of sclera
• The sclera is constantly under stress by IOP
• The pressure pushing outward is contained by the sclera thereby providing a
stable viscoelastic structure for the globe as a whole