1 Anatomy(Ant.segment-1) .pptx

CONATCT LENS

1. Contact Lens Anterior Segment of the Eye -i

2. Anatomy of Anterior Segment of the Eye To identify anatomical structures of the anterior eye which are important in contact lens wear. Understanding of the physiological processes of the anterior eye and especially those that may be affected by contact lens wear .

3. Anterior Segment of the Eye The anterior segment is composed of the eyelids (and margins), conjunctiva, sclera, cornea , limbus and iris.

4. Anterior Segment of the Eye

5. Anterior Segment of the Eye Corneal shape is elliptical because of the encroachment of the opaque limbus into the cornea’s superior and inferior borders . Horizontal visible iris diameter (HVID) is 11.7 mm and vertical visible iris diameter (VVID ) is 10.6 mm on average. These may be about 0.1 mm less in females Corneal area is 1.3 cm or 1/14 of the total area of the globe .

6. Anterior Segment of the Eye The globe is bi-spherical with the cornea having the smaller posterior spherical radius of curvature of 6.2 to 6.8 mm (average 6.5 mm). According to Maurice (1969), the average corneal thickness is 0.52 mm in the centre and 0.67 mm at the limbus . Sagittal depth of the cornea is 2.6 mm with variations largely dependent on the radius of corneal curvature .

7. Corneal Shape The cornea is not symmetrical and is not a solid of rotation about the optic, geometric or any other axis

8. Corneal Shape Corneal curvature flattens towards the periphery and the degree of increase in the radii of curvature varies from one meridian to the next. This variation in flattening indicates that the curvature is not a solid of rotation about any particular axis. The eye is never still. Constant movement of the eye (saccadic movement) does not allow for a fixed centre of rotation.

9. Corneal Shape The cornea is a meniscus lens The average front apical radius is 7.8 mm = 43.27 The average back apical radius of 6.5 mm gives the posterior cornea a power of -6.15 D The actual refractive index of the cornea is 1.376 The cornea is not optically homogeneous. However the actual refractive indices of the individual layers are not known accurately n ground substance = 1.354 : n collagen = 1.47

10. Corneal Profile Cornea is Transparent The principal optical surface of the eye, accounting for approximately two-thirds of the eye’s refractive power.

11. Corneal Composition The cornea is composed of: 78% water. 15% collagen. 5% other proteins. 1% glycosaminoglycans (GAGs). 1% salts.

12. Transverse Section of the Cornea (Warwick, 1976)

13. Transverse Section of the Cornea (Warwick, 1976) Epithelium. Bowman's layer. Stroma . Descemet's membrane. Endothelium. Examination and assessment of the cornea is best done using the slit-lamp biomicroscope . Observation of a transverse section of the cornea requires a narrow slit-beam, together with high magnification

14. Epithelium The outer surface is smooth and regular to provide a tear layer substrate. It is of uniform thickness Any slight irregularity is compensated for by the overlying tear film.

15. Layers of Epithelium (Hogan et al., 1971) The epithelium is made up of three types of cells: squamous cells (surface) wing cells columnar (or basal) cells (deepest).

16. Microvilli are seen on the surface. Microvilli (singular: microvillus ) are microscopic cellular membrane protrusions that increase the surface area of cells, and are involved in a wide variety of functions, including absorption , secretion , cellular adhesion Layers of Epithelium (Hogan et al., 1971) May be partially responsible for tear film retention?

17. Epithelium The epithelium is 50 microns thick. It is a five-layered structure: outer surface cells: two layers of flattened squamous cells middle wing cells: two to three layers of polygonal cells inner basal cells: one layer of columnar cells. Cellular turnover, i.e. the time required for basal cells to migrate anteriorly to become surface cells, is approximately seven days .

18. BOWMAN’S LAYER • Acellular • Differentiated anterior stroma • Mainly collagen, some ground substance

19. Stroma 0.50 mm thick 90% of corneal thickness It contains 2 - 3% keratocytes (also known as corneal fibroblasts) and about 1% ground substance ( glycosaminoglycans or GAGs).

20. Stroma

21. Ground Substance (GAGs) ( glycosaminoglycans ) Very hydrophilic Responsible for exact spacing of fibrils H2O imbibitions pressure of the cornea. This is due largely to their hydrophilicity .

22. Descemet's Membrane Thickness: 10-12 μ m (microns) It is structureless , slightly elastic and secreted by the endothelium. It is a very regularly arranged basement layer of the endothelium .

23. Hassall- Henle Warts (Hogan et al., 1971) Periodic thickenings of Descemet’s membrane Can protrude into anterior chamber The disturbance to the cornea/anterior chamber interface alters its specular reflection properties. Light is lost by scatter and reflection from the irregular surface. If sufficient light is lost, the affected area appears black.

24. Endothelium Single layer 500,000 mainly hexagonal cells 18-20 μ m diameter 5 μ m thick

25. Endothelium The endothelium can be viewed by specular reflection using a SLE at high magnification (25X - 40X).

26. AGE-RELATED CELLULAR CHANGES Cell degeneration and nonreplacement decreased uniformity decreased thickness with age Polymegethism

27. Corneal Vasculature Peripheral cornea supplied by circumcorneal vessels Minor role in corneal nutrition Remainder of cornea is avascular

28. Corneal Innervation One of the richest sensory nerve supplies Ophthalmic division of trigeminal nerve (N 5)

29. Conjunctiva

30. Conjunctiva The conjunctiva is a mucous membrane consisting of loose, vascular connective tissue. It is transparent. Its transparency is less than that of the cornea when observed with a slit-lamp. It Is loose, to allow free and independent movement of the globe. It Is thinnest over the underlying Tenon's capsule

31. Conjunctiva Continuous with: lining of globe beyond cornea upper and lower fornices innermost layer of upper and lower lids skin at lid margin corneal epithelium at limbus nasal mucosa at lacrimal puncta

32. Conjunctiva Palpebral Fornices Bulbar Plica semilunaris Caruncle

33. Conjunctiva is composed of 2 layers: Epithelium Stroma

34. Conjunctival Glands Goblet Cells - unicellular and sero -mucous secreting. Glands of Wolfring - contributors to the aqueous layer of the tears (along with the lacrimal gland) Glands of Krause - contributors to the aqueous layer of the tears (along with the lacrimal gland) Crypts of Henle - mucous crypts or invaginations found in the superior peripheral palpebral conjunctiva.

35. Conjunctival Arteries Palpebral branches of nasal and lacrimal arteries of lids - larger branches form peripheral and marginal arterial arcades - lower lid peripheral arcade not always present Anterior ciliary arteries

36. Conjunctival Arteries