Corneal Shape

Question 1

usual visible iris diameter range is

Answer 1

10.5 to 13.5 mm

Question 2

HVID is about

Answer 2

11.7 mm

Question 3

VVID is about

Answer 3

10.6 mm

Question 4

HVID is about ___ less for females

Answer 4

0.1 mm

Question 5

VID can be measured via:

Answer 5

• limbus-to-limbus topography (Medmont)

- OCT (optical coherence tomography)

Question 6

when can VID (corneal diameter) have clinical indications?

Answer 6

• soft contact lens base curve (sagittal height)
• corneal GP overall diamter
• infants
• microcornea/megalocornea
• scleral GP contact lenses

Question 7

sagittal height formula is

Answer 7

r- (square root r^2-(d/2)^2)

Question 8

are corneal curvature and sagittal depth related?

Answer 8

they are not the same thing!

Question 9

list 4 contact lens parameters that can vary with sagittal height (thereby changing fit)

Answer 9

• base curve (BC)
• peripheral curve (PC)
• overall diameter (OAD)
• optic zone diameter (OZD)

Question 10

do changes to the CL front surface (F1) alter sagittal height?

Answer 10

no, they do not alter inside sagittal height or lens-to-cornea fitting relationship

Question 11

what does “mechanical fit” mean?

Answer 11

matching the sagittal depth of the CL to the sagittal depth of the corneal surface

Question 12

how is corneal thickness measured? (what instruments can be used)

Answer 12

• pachymetry (ulrasound or optical)
• orbscan topography
• inferometry
• OCT (optical coherence tomography)

Question 13

where is corneal thickness the thinnest and how many microns is it?

Answer 13

thinnest: centrally, 536 microns

Question 14

where is corneal thickness the thickest and how many microns is it?

Answer 14

thickest: peripherally, 650 to 1,200 microns

Question 15

in the morning right as you wake up, how does corneal thickness change?

Answer 15

in a.m., 4% decrease upon waking

Question 16

when calculating if a patient has a thick enough cornea for lasik, how much is the flap thickness in microns?

Answer 16

about 100 microns

Question 17

when calculating the thickness of a cornea after LASIK, how much is vaporized per diopter of correction?

Answer 17

16 microns for every D of correction

Question 18

95% of the population has a corneal thickness between ___ and ____

Answer 18

473 - 595 microns

Question 19

average corneal thickness is approximately

Answer 19

534 microns

Question 20

after lasik, the post-surgical corneal bend thickness has to be between ___ and ___ microns

Answer 20

250 to 300 microns

Question 21

what could you consider if a patient is borderline for thickness of LASIK?

Answer 21

PRK (photorefractive keratectomy), a surface ablation procedure

Question 22

how do thin corneas affect IOP readings?

Answer 22

thin corneas cause under-reading of true IOP

Question 23

keratoconus patient IOP readings

Answer 23

many measurements are single digits, ocular rigidity??

Question 24

Goldmann is calibrated for ___ microns corneal thickness

Answer 24

550 microns

Question 25

the ocular hypertension treatment study (OHTS) showed what concerning data about thin corneas and IOP?

Answer 25

thin corneas cause under-reading of true IOP- could lead to delay in diagnosis and establishing the wrong target reading for glaucoma tx

Question 26

average corneal curvature data is about

Answer 26

43.00 D (7.85 mm)

Question 27

corneal curvature changes from infancy until 5 y/o

Answer 27

flattens from infancy 'til about age 5

Question 28

corneal curvature is pretty stable from about ___ to ___ age

Answer 28

5 to 95 unless corneal injury, corneal surgery, CL wear

Question 29

why does the corneal not act like a steel ball?

Answer 29

- patient fixation, especially if decreased vision - tear film - change over time - measurement variability

Question 30

if cornea is steeper than 52D and you use a +1.25 D trial lens, how much do you have to +/- to get the reading?

Answer 30

add 8 to 9 D

Question 31

if cornea is steeper than 52D and you use a +2.25 D trial lens, how much do you have to +/- to get the reading?

Answer 31

~ 16 D

Question 32

if cornea is flatter than 38D and you use a -1.00 D trial lens, how much do you have to +/- to get the reading?

Answer 32

subtract 6 D from drum reading

Question 33

how repeatable is your keratometer data for central cornea?

Answer 33

K = 0.50 D (or better) | Sim K= 0.50 D (or better

Question 34

how repeatable is your keratometer data for peripheral cornea?

Answer 34

less accurate than central

Question 35

tomography may not be how the cornea is in real life, but what data does it produce?

Answer 35

anterior surface flow, posterior surface flow, thickness map, change map, etc.

Question 36

clinically, how could you test for possible keratoconus on the keratometer ?

Answer 36

measure central cornea and then inferior. if inferior steeper, then maybe keratoconus

Question 37

the old school K Shape theory describes the spherical corneal cap as:

Answer 37

central 4mm of maximum (steep) and essentially constant (sphere) curvature

Question 38

what is the K Eccentricity (e) value?

Answer 38

rate of flattening

Question 39

K eccentricity (e) has an inverse relationship to:

Answer 39

shape factor

Question 40

average K eccentricity value (e) and range

Answer 40

~ 0.5 eccentricity (0.48 +/- 0.11)

Question 41

___ (lower/higher) e = more spherical

Answer 41

lower

Question 42

a new study shows liable area and first part of sclera, which have always been assumed to be curved, may actually be

Answer 42

in some cases the sclera often continues in a straight line (tangential) from the peripheral cornea onward

Question 43

with new data from OCT on the peripheral cornea, limbus, and sclera: the best way to now describe the geometry of these regions is

Answer 43

a tangent angle rather than a radius curve

Question 44

typically in the average eye, the ___ portion is flatter compared with the rest

Answer 44

nasal

Question 45

many eyes are non rotationally symmetrical in nature beyond the corneal borders. this may call for:

Answer 45

non rotationally symmetrical lenses such as toric and quadrant specific lenses

Question 46

prolate=

Answer 46

normal, steeper centrally and flattens in periphery

Question 47

oblate =

Answer 47

flatter in center, steeper in edge, post-refractive (LASIK, RK), or post orthoK

Question 48

lens design needed for prolate corneas

Answer 48

spherical lens design

Question 49

lens design needed for oblate cornea

Answer 49

reverse geometry lens design