Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Cornea and Contact Lenses I > Corneal Shape > Flashcards

Corneal Shape Flashcards

1
Q

usual visible iris diameter range is

A

10.5 to 13.5 mm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

HVID is about

A

11.7 mm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

VVID is about

A

10.6 mm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

HVID is about ___ less for females

A

0.1 mm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

VID can be measured via:

A
  • limbus-to-limbus topography (Medmont)

- OCT (optical coherence tomography)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

when can VID (corneal diameter) have clinical indications?

A
  • soft contact lens base curve (sagittal height)
  • corneal GP overall diamter
  • infants
  • microcornea/megalocornea
  • scleral GP contact lenses
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

sagittal height formula is

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

are corneal curvature and sagittal depth related?

A

they are not the same thing!

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

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

A
  • base curve (BC)
  • peripheral curve (PC)
  • overall diameter (OAD)
  • optic zone diameter (OZD)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

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

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what does “mechanical fit” mean?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

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

A
  • pachymetry (ulrasound or optical)
  • orbscan topography
  • inferometry
  • OCT (optical coherence tomography)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

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

A

thinnest: centrally, 536 microns

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

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

A

thickest: peripherally, 650 to 1,200 microns

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

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

A

in a.m., 4% decrease upon waking

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

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

A

about 100 microns

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

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

A

16 microns for every D of correction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

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

A

473 - 595 microns

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

average corneal thickness is approximately

A

534 microns

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

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

A

250 to 300 microns

21
Q

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

A

PRK (photorefractive keratectomy), a surface ablation procedure

22
Q

how do thin corneas affect IOP readings?

A

thin corneas cause under-reading of true IOP

23
Q

keratoconus patient IOP readings

A

many measurements are single digits, ocular rigidity??

24
Q

Goldmann is calibrated for ___ microns corneal thickness

A

550 microns

25
Q

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

A

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

26
Q

average corneal curvature data is about

A

43.00 D (7.85 mm)

27
Q

corneal curvature changes from infancy until 5 y/o

A

flattens from infancy ‘til about age 5

28
Q

corneal curvature is pretty stable from about ___ to ___ age

A

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

29
Q

why does the corneal not act like a steel ball?

A
  • patient fixation, especially if decreased vision
  • tear film
  • change over time
  • measurement variability
30
Q

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?

A

add 8 to 9 D

31
Q

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?

A

~ 16 D

32
Q

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?

A

subtract 6 D from drum reading

33
Q

how repeatable is your keratometer data for central cornea?

A

K = 0.50 D (or better)

Sim K= 0.50 D (or better

34
Q

how repeatable is your keratometer data for peripheral cornea?

A

less accurate than central

35
Q

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

A

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

36
Q

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

A

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

37
Q

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

A

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

38
Q

what is the K Eccentricity (e) value?

A

rate of flattening

39
Q

K eccentricity (e) has an inverse relationship to:

A

shape factor

40
Q

average K eccentricity value (e) and range

A

~ 0.5 eccentricity (0.48 +/- 0.11)

41
Q

___ (lower/higher) e = more spherical

A

lower

42
Q

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

A

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

43
Q

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

A

a tangent angle rather than a radius curve

44
Q

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

A

nasal

45
Q

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

A

non rotationally symmetrical lenses such as toric and quadrant specific lenses

46
Q

prolate=

A

normal, steeper centrally and flattens in periphery

47
Q

oblate =

A

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

48
Q

lens design needed for prolate corneas

A

spherical lens design

49
Q

lens design needed for oblate cornea

A

reverse geometry lens design

Cornea and Contact Lenses I (5 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions