Corneal curvature and thickness

Corneal curvature and thickness

1. CORNEAL CURVATURE AND THICKNESS Presented by- KAUSTAV GOGOI B.OPTOM 2 nd year RIDLEY COLLEGE OF OPTOMETRY

2. WHAT IS CORNEA ? Cornea is a avascular,transparent,watch glass like structure It forms anterior one-sixth of the outer fibrous coat of the eyeball

3. HUMAN EYE

4. DIMENSIONS The anterior surface of cornea is elliptical with an average horizontal diameter of 11.75 mm and vertical diameter of 11 mm The posterior surface of cornea is circular with an average diameter of 11.5 mm

5. Radius of curvature: The central 5mm area forms the powerfull refracting surface of the eye.The anterior and posterior radii of curvature of the central part are 7.8 mm and 6.5 mm , respectively

6. Refractive power : Refractive power of the anterior surface of cornea is about +48D and that of its posterior surface is about -5D Thus,the net refractive power is +43D which is three-fourth of the total refractive power of the eye Refractive Index of cornea is 1.37

7. Measurement of Corneal Curvature KERATOMETER IOL MASTER CORNEAL TOPOGRAPHY ORBSCAN PENTACAM

8. PROBLEMS RELATED TO CORNEAL CURVATURE- “KERATOCONUS’’ Keratoconus is a progressive eye disease that causes a thinning of the clear front surface of the cornea and distorts the cornea into a cone like shape. In the mildest form of keratoconus , eyeglasses or soft contact lenses may help. But as the disease progresses and cornea thins and becomes increasingly more irregular in shape,eyeglasses and regular soft contact lenses no longer provide adequate vision correction.

9. Treatment for progressive KERATOCONUS - Corneal collagen crosslinking with riboflavin(C3-R) Gas permiable contact lenses Scleral and semi- scleral lenses Corneal transplant

10. VISION OF A KERATOCONUS PATIENT-

11. ASTIGMATISM Astigmatism is a refractive condition that is the result of two principle meridians of the eye having different refractive power It occurs when the cornea is shape more like an oblong football than a spherical baseball which is the normal shape Astigmatig cornea(oblong or oval shape) causes light rays to focus on two points in the back of the eye rather than just one

12. CORNEAL THICKNESS

13. Thickness of cornea in the centre is about 0.52mm while at the periphery it is about 0.67 mm

14. Thick Cornea vs Thin Cornea An average cornea is between 540 µm and 560 µm Greater than 565 µm or more is called thick cornea Lower than 540 µm or less is called thin cornea Thicker corneas are flatter and thinner corneas are steeper

15. ANATOMY OF THE CORNEA

16. EPITHELIUM Made up of stratified squamous epithelium Thickness - 50-90µm 5-6 layers of cells Once destroyed,it can regenerate. Replaced every 6-8 days Made up of three types of cells- basal,wing,flattened Cells are attached to each other by means of desmosomes

17. BOWMAN’S MEMBRANE Consists of acellular mass of condensed collagen fibri Once destroyed,it can’t be regenerate It is not a true membrane,but it is a condensed superficial part of stroma Shows considerable resistance to infection and injury Thickness – 8-14 µ m

18. STROMA This layer is about 0.5 mm in thickness and constitutes most of the cornea (90 % of the total thickness) It consists of collagen fibrils and cells embedded in hydrated matrix of proteoglycans Lamellae are arranged in 200-250 layers Also consits of keratocytes,histocytes,leucocytes

19. DUA’S LAYER (PRE DESCEMET’S MEMBRANE) This layer is present between stroma & descemet’s membrane It is tough,well -defined and acellular Thickness- 10-15 µ m

20. DESCEMETS MEMBRANE Also known as posterior elastic lamellae Once destroyed,it cannot be regenerate It is strong,elastic,homogenous basement membrane of endothelium and it bounds stroma posteriorly Highly resistant to chemical agents,trauma & pathological process Thickness- 3µm(birth) , 10-12µm (young adults) Maintains integrity of eyeball

21. ENDOTHELIUM It consists of single layer of hexagonal cells It is involved in active secretion & energy production Consists of large number of mitochondria Cell density – 3000 cells/mm sq in young adults Cell density decreases with increasing age The cells have active pump mechanism

22. THICKNESS OF CORNEA IN NEWBORN AND CHILDREN : Corneal configuration in newborn is similar to that of adult cornea It has been found that cornea on day one is significantly thicker and decreases in thickness as the child grows older The average corneal thickness in infants is 585+52 microns The superior peripheral cornea is thinnest in newborn

23. Techniques of Pachymetric Measurements

24. CORNEAL ECTASIA These are a group of uncommon, noninflammatory,eye disorders characterised by bilateral thining of the central,paracentral or peripheral cornea Most common- 1) Keratoglobous 2) Pellucid marginal degeneration

25. KERATOGLOBOUS It is a condition in which the entire cornea is abnormally thin. Onset: at birth Signs: acute hydrops occur less commonly,cornea is more prone to rupture on trauma Management:1) Contact lens- Scleral contact lens

26. PELLUCID MARGINAL DEGENERATION: It is a condition of inferior(sometimes superior) thinning of cornea Bilateral Onset: 20 years to 40 years Signs: crescent shapeband of inferior cornea,intact epithelium,high astigmatism(against the rule) Treatment: Spectacle : Not useful Contact lens: RGP contact lens Surgery:Keratoplasty