L20 - Fibrous Layer (cornea)

Question 1

What defines the circular boundary of the cornea?

Answer 1

The limbus of the sclera.

Question 2

What fraction of the eyeball does the cornea form?

Answer 2

The anterior 1/6.

Question 3

What is the main optical function of the cornea?

Answer 3

It is the most important optical element of the eye and accounts for 75% of the ocular dioptric power.

Question 4

What is the approximate dioptric power of the cornea?

Answer 4

+43 diopters (Dpt).

Question 5

What are the mechanical and protective functions of the cornea?

Answer 5

It acts as a barrier preventing entry of foreign substances and protects intraocular components.

Question 6

What is the typical horizontal visible iris diameter (HVID) in adults?

Answer 6

11.7 mm.

Question 7

What is the typical vertical visible iris diameter (VVID) in adults?

Answer 7

10.6 mm.

Question 8

At what age does corneal diameter stabilize?

Answer 8

From about 2 years of age.

Question 9

What is the corneal diameter at birth?

Answer 9

9.5 mm.

Question 10

What is considered microcornea and macrocornea in terms of HVID?

Answer 10

HVID < 10mm = microcornea; HVID > 13mm = macrocornea.

Question 11

How does corneal radius change towards the periphery?

Answer 11

It becomes flatter (eccentricity 0.4–0.6).

Question 12

What does a larger radius of curvature indicate about corneal shape and power?

Answer 12

A larger radius means a flatter cornea with less dioptric power.

Question 13

What does a smaller radius of curvature indicate?

Answer 13

A steeper cornea with more dioptric power.

Question 14

What is pachymetry?

Answer 14

The measurement of corneal thickness.

Question 15

What is the normal range for central corneal thickness (CCT)?

Answer 15

490 to 560 µm.

Question 16

What is the normal range for limbal corneal thickness?

Answer 16

700 to 900 µm.

Question 17

What does a CCT > 700 µm indicate?

Answer 17

Endothelial decompensation.

Question 18

What does a CCT < 400 µm indicate?

Answer 18

Keratoconus.

Question 19

How does corneal thickness change with age?

Answer 19

It decreases with age.

Question 20

How does gender affect corneal thickness?

Answer 20

Women have slightly thinner corneas than men.

Question 21

How does ethnicity affect corneal thickness?

Answer 21

Mean CCT of Black children is thinner than that of White children.

Question 22

How does high myopia affect corneal thickness?

Answer 22

High myopias have decreased corneal thickness.

Question 23

How does circadian rhythm affect corneal thickness?

Answer 23

CT increases during the night by about 5.5%; daytime fluctuation is about 7.4%.

Question 24

What clinical conditions are associated with corneal thinning?

Answer 24

Fuch’s endothelial dystrophy, keratoconus, persistent epithelial defects, normal tension glaucoma, pseudoexfoliation, primary open-angle glaucoma.

Question 25

What clinical conditions are associated with corneal thickening?

Answer 25

Pseudophakic bullous keratopathy, diabetic eyes, ocular hypertension.

Question 26

Why is measuring corneal thickness clinically significant?

Answer 26

It helps diagnose and monitor ocular pathologies, affects IOP measurement, and is important for contact lens fitting and ophthalmic surgery.

Question 27

What is the relationship between intraocular pressure (IOP) and corneal thickness?

Answer 27

There is a positive correlation between IOP and CT.

Question 28

What is the refractive index of corneal collagen?

Answer 28

1.470.

Question 29

What is the refractive power at the air/tear film interface?

Answer 29

+43.1D.

Question 30

What is the refractive power at the tear film/cornea interface?

Answer 30

+5.1D.

Question 31

What is the refractive power at the cornea/aqueous humour interface?

Answer 31

-6.2D.

Question 32

What are the optical functions of the cornea?

Answer 32

Refraction and transmission of light.

Question 33

What factors are essential for corneal transparency?

Answer 33

Minimal scattering, smooth optical surface, absence of blood vessels, maintenance of stromal microstructure, minimal distortion.

Question 34

What is the composition of the cornea by percentage?

Answer 34

78% water, 15% collagen, 5% other components (elastin), 1% GAG.

Question 35

What are the five layers of the cornea (from anterior to posterior)?

Answer 35

Epithelium, Bowman’s membrane, stroma, Descemet's membrane, endothelium.

Question 36

How thick is the corneal epithelium?

Answer 36

50 µm.

Question 37

How many cell layers does the corneal epithelium have?

Answer 37

5–7 cell layers.

Question 38

What are the three layers of the corneal epithelium?

Answer 38

Apical cells (non-keratinised squamous), wing cells (polygonal), basal columnar cells (germinative).

Question 39

What type of junctions are found between corneal epithelial cells?

Answer 39

Tight junctions and desmosomes.

Question 40

What is the function of tight junctions in the corneal epithelium?

Answer 40

Maintain homeostasis, act as a mechanical barrier, ensure transparency, and anchor cells to the basal lamina.

Question 41

What is the thickness of Bowman’s membrane?

Answer 41

8–14 µm.

Question 42

What are the functions of Bowman’s membrane?

Answer 42

Anchoring site for epithelial cells, provides mechanical support, prevents stromal keratocytes from exposure to epithelial growth factors.

Question 43

What is the thickness of the corneal stroma?

Answer 43

About 450–500 µm (about 90% of corneal thickness).

Question 44

What is the main collagen type in the corneal stroma?

Answer 44

Type I (with types III, V, and VI also present).

Question 45

What percentage of the stroma is occupied by keratocytes?

Answer 45

5%.

Question 46

What is the function of keratocytes?

Answer 46

Synthesize and maintain collagen and proteoglycans, help in corneal regeneration, part of antioxidant defense.

Question 47

How many lamellae are in the corneal stroma?

Answer 47

About 200 layers.

Question 48

How are stromal lamellae arranged?

Answer 48

Regularly, almost at right angles to each other.

Question 49

What is the width and thickness of stromal lamellae?

Answer 49

Width: 9–260 µm; thickness: 1.15–2 µm.

Question 50

What is the diameter of corneal collagen fibrils?

Answer 50

22 (±1) nm.

Question 51

What is the center-to-center distance between collagen fibrils?

Answer 51

55–60 nm.

Question 52

What are the functions of stromal GAGs?

Answer 52

Confer hydrophilic properties, maintain transparency by controlled hydration, help in regular spacing of collagen fibers.

Question 53

What are the two main theories of stromal transparency?

Answer 53

Maurice’s lattice theory and Goldman et al.'s theory.

Question 54

What is Maurice’s lattice theory?

Answer 54

Transparency is due to uniform collagen fibrils arranged in a regular lattice; scattered light is destroyed by mutual interference.

Question 55

What is Goldman et al.'s theory?

Answer 55

Perfect lattice is not necessary; as long as fibril separation and diameter are less than a third of the wavelength of light, transparency is maintained.

Question 56

What other factors contribute to corneal transparency?

Answer 56

Epithelial non-keratinization, regular arrangement of epithelium, cell junctions, controlled hydration, and tear film.