Brainscape's Knowledge GenomeTM

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


See full index

Optics > 12. Contact Lenses > Flashcards

12. Contact Lenses Flashcards

1
Q

why are CLs better in some cases for patients

A
  • reduce or elminiate the aberrations assocaited with glasses used in the correction of high refractive errors
  • reduce aniseikonia associated with anisometropia and high degrees of atigmatism
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

examples of CLs used in ophthal

A
  • bandage contact lenses - precroneal membranes
  • used in electroretinography
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what determines the power of the optical (central) zone of the contact lens

A
  • anterior curvature
  • posterior curvature
  • thickness
  • refractuve index
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

when is a CL described as spherical

A

same radius of surgace curvature in each meridian

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

when can cyclindrical refractive erros be corrected by contact lenses

A

front surfac,e, back surface or both are toric

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

how is torsion of a toric CL prevention

A

either
incorporating an up to 2.00 D base down prism to weight the lower pole or lens
OR
removing the lower 0.5-1mm of the lens (truncation) to allow it sit on the edge of the lower eyelide

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what is the base curve of a CL

A

posterior surface of the optical zone is defined by the posterior central curvature

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

how is a correct fit ensured for CL

A

base curve should conform closly to the aspheric surface of the cornea

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

how is an aspheric shape created

A

encircling the optical zone with one or two concentrix zones of increasing radius of curvate to produce a bircurve or tricurve C;

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

how are the junctions between teh zones made smooth

A

blneding
or computer controlled production

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

corneal contact lenses have a smaller diameter than the cornea on which they are supported - T/F

A

true

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

sclearal contact lenses

A

rarely used
have a peripheral rim which is supported by the sclera

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

how is the corneal surface oxygenated

A

by the tear film

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

CL are made of gas-imperable materials

A

thf incorporate fenestrations slot or grooves to facilitate the circulation of tears behind the lens

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

why do high power concave (minus) lenses ride high

A

tendency of the upper lid to grip the thick upper edge of a high power lens
countered by a periheral bevel

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

why do high power convex (plus) lenses drop lower

A

due to weight
counted by a minus peripheral carrier postion which tends to be lifted by the upper lid

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

refractive index of the precorneal tear film

A

1.333

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

refractive index of the cornea

A

1.3375

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

refractive index of precorneal tear film = cornea

A

YES

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

tear film helps to reduce what

A

corneal surface irregulalrity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

deinfe the tear lens

A

tear film between the posteruor surface of the CL and anterior surface of the eye

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

if the tear lens has a uniform thickness it has

A

plano power

23
Q

a steeper base curve of the CL (i.e more vaulted CL) does what

A

increases the axial ehigh of the tear lens making it more strongly positive

24
Q

what does the tear lens help

A

allows a speherical CL to neutralise corneal astigmatism

25
Q

the base curve whould be what in regards ot the corneal surface

A

be the same as the corneal surgface curvature at the FLATTEST meridian

so that were the cornea is steepr the tear lens is thicker thf neutrlisies atigmatism

26
Q

why does only the sphercial component need to be prescribed

A

tear lens neutrlaises astigmatism

27
Q

how much astigmatism can soft CLs corrected

A

1.00D

28
Q

rigid CLs can correct more stigmatism - T/F

A

truse

29
Q

astigmatism arising from crystalline lens or implanted IOL can only be corrected by

A

the front surface of a toric lens

30
Q

why do CL give good vision in all positione of gaze

A

the cl moves with the eye

31
Q

when may a ridig contact produce a halo effect

A

when the pupil is dilate
because of refraction through the peripheral zone of the lens or adjacent tear film

32
Q

why do myoptic patients have a reduced field of veiew

A

the lens peripheray has a primatic effect with the base towards the visual axis

33
Q

why do hypermetropic patients have an increased field of veiew

A

primatic effect with the base away from the visual axis

34
Q

aniseikonia is

A

reduced with CLs when compared to glasses

35
Q

CL give image mag/minification for myopic

A

magnification

36
Q

CL give image mag/minification for hypermetropia

A

minification

37
Q

CL and oblique aberration

A

is minimised as you can’t look through the non-axial portion of a well fitting CL

38
Q

why does a change to CL for myopes cause eye straing

A

a change to CL increased the deman for greater convergnece and accomation which may cause eye strian

39
Q

how much prism power can be incoportaed in corneal CL

A

up to 3 dioptres

40
Q

what orientation is the prisms

A

base down thf can’t do horizontal primatic correction

41
Q

a scleral lens can incorporate up to how much dioptre corection

A

6 prims in vertical or horizontal split between two lenses

42
Q

times when more than one object distance needs to be corrected

A

presbyoic, pseudophakic and aphakic

because their accomodation is reduced or absent

43
Q

options for CL w/o accomodation

A

1/ glasses and cl
2/ biofocal CL
3/ monofocal vision

44
Q

described monofocal vision

A

one eye for distance 9usually better eye)
one eye for near

reduced binoculalirty and steropsis

45
Q

options for bifocal or multifocal lensses

A

annular aspheric segmental and diffrative

46
Q

annular biofocal CL

A
  • central zone corrects for distance
  • annular one for near
  • in down gaze the CL rises relative to the cornea placing the nar portion in front of the visual acis
    Peripheral portion of the lens is not as effective when the pupil is msllaer
47
Q

Aspheric multifocal CL

A
  • central part corrects for distance
  • gradual trasnition in power to the peripheral zone which corrects for near
  • onle a small ampint of light focuses on the retina
48
Q

Segmental biofocal CL

A
  • incorportae the near additiona over the lower portion of the lens
  • inin down gaze the CL rises relative to the cornea placing the nar portion in front of the visual acis
  • must be preventied by rotation –> truncation or ballasting with a base down prims
49
Q

Diffractive bifocal LC

A
  • concentric diffraction rings on their posterior surface which are desined t focus equal amoutns of lght from distand and near objects
50
Q

keratonconus

A

increased corneal surface curvature causes increasing myopia and irregular astimaticm

51
Q

short term problems with CL

A
  • if posterior surface is too flat, it will move
  • movement during blinking cayses changes to vision
52
Q
  • small changes occur when lens is in use -
A

evaopration of water increases the refractive index and increse in temperature increase the curvatre

incresa the negative power of the lens

53
Q

long term problems with CL

A

warpage is the change of corneal curvature induced by CL
regresses after lens is removd over hours or day
more prondouc in long term wear

54
Q
A

CL

Optics (18 decks)

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