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OV2006 contact lenses > optics of CLs > Flashcards

optics of CLs Flashcards

1
Q

what is subjective refraction?

A

. spectacle prescription in front of the ocular surface ( spec RX)
. BVD=12mm

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2
Q

what is spectacle RX?

A

. spectacle prescription in front of the ocular surface ( spec RX )

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3
Q

why do we need to convert spectacle RX into ocular RX?

A

. when we give px cls, we will get rid of vertex distance and we will push the prescription all the way onto the cornea
. therefore we need to convert spectacle RX into ocular RX, which takes into account the disappearance of the 12 mm vertex distance

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4
Q

how do we convert spec RX to ocular RX?

A

by using the back vertex distance table

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5
Q

why is there a difference between spec RX and ocular Rx?

A

the difference is due to the back vertex distance

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6
Q

why does the back vertex distance table start at 4?

A

. anything below a prescription of 4 will have the same spec Rx and ocular Rx

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7
Q

what to do when px is astigmatic ?

A

need to correct spec Rx to ocular Rx in both meridians

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8
Q

what rules to follow when prescribing cls?

A

. usually prescribe the least negative ( round down )

. round sph down
and round the cyl up

. you can round both sph and cyl down and make sure you do an over refraction on top of the cls

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9
Q

where is refractive astigmatism located ?

A

. located in the cornea or the crystalline lens

. refractive astigmatism=corneal + lenticular

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10
Q

how do we get refractive astigmatism ?

A

. from spectacle Rx

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11
Q

where do we get corneal astigmatism from ?

A

. from keratometry or topography

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12
Q

how do we measure the curvature of the cornea ?

A

. keratomerty - measures the curvature along the two principle meridians of the cornea over three to four mm

. topography

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13
Q

what is topography ?

A

. topography is used to identify the full shape of the cornea by measuring thousands of points along a nine mm diameter
. gives more information than keratomerty
. shows the power of the cornea along the principal meridians

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14
Q

why is a topographer more useful than a keratometer ?

A

. in keratometry we only get information about the principal meridian centrally, where as in a topographer we get information about the principle meridian both centrally and in the periphery

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15
Q

how is the radius of the cornea calculated?

A

. the radius of the cornea is calculated in mm
. if you put a circle along the curvature that follows the curvature of the cornea you can measure the radius by going from the centre of the circle all the way to the side
. the distance between the centre of the circle to the side is measured- vertical measurement
. there is a difference between the 2 principle meridians
. when the radius is bigger that means that the circle is bigger and that means that the curvature is flatter

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16
Q

how do we calculate corneal astigmatism ?

A

. rule of thumb: 0.05 difference = 0.25D corneal astigmatism

. 0.05= smallest available unit radius
. 025D= smallest unit power

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17
Q

how do we calculate lenticular astigmatism ?

A

lenticular astigmatism = refractive astigmatism - corneal astigmatism

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18
Q

what is WTR ( with the rule astigmatism )?

A

. the vertical meridian is steeper, so the radius is smaller

19
Q

what is ATR ( against the rule astigmatism )?

A

. the vertical meridian is flatter, so the radius is bigger

20
Q

what does it mean that if all the astigmatism the patient needs is located in the cornea?

A

. it means that the patient is an excellent candidate for rigid corneal lens
. this is because a rigid corneal lens doesn’t change shape when we put it on the patient’s eye

21
Q

what is the problem with a soft toric lens ?

A

. lenses don’t stabilise very much
. every time you blink the cl will move
. rotation in contact lens when you blink- due to the axis changing
-you get poorer vision than in specs

22
Q

how does a rigid lens correct astigmatism ?

A

. the tears will fill the gap between the cornea and contact lens
. RGP CL neutralises almost 90% of corneal astigmatism - so dont have to give cyl- so can just give them the sphere -
. doesn’t neutralise the lenticular astigmatism
-if patient wants toric lens- then you would have to prescribe the whole prescription for e.g -2.00/-2.00x180 whereas just RGP needs -2.00 .

23
Q

what happens to corneal power when you spherical rigid corneal lens ?

A

. instead of calculating the air/tear interface, we now calculate the astigmatism in the tear/cornea interface

. when spherical RGP lens is placed on the cornea, tears replace air ( and cornea replaces tears )

. (n) for air is replaced by (n) for cornea

. corneal astigmatism is reduced when a spherical RGP is placed on the cornea

24
Q

what does the choice for cl depend on ?

A

. the choice for cls depends on the amount of astigmatism there is

25
Q

when can you order a spherical RGP?

A

if corneal astigmatism is anything between 0 and 2.50D
you can order spherical RGP lens
-IF TOO Much difference in both principal meridians then the lens will start rocking on the eye- not good- fit good in one merdian but not in the other- lots of tears which will need to fit gap- distance too big- uncomfortable - px will blink lens out

26
Q

what to order if corneal astigmatism is more than 2.50DC?

A

. a toric lens must be ordered

.back surface toric RGP will be ordered if corneal astigmatism is more than 2.50D

27
Q

what is a back surface toric RGP?

A

. you will order the curvature of the lens to the same curvature as you found in keratomerty ot topography
. back surface of lens is ordered to the two principle meridians that you have measured
-corrects the back of CL - cornea and tears side

28
Q

what happens when you have remaining lenticular astigmatism ?

A

. you order a front surface toric lens for any remaining lenticular astigmatism
. lenticular astigmatism is corrected on the outside of lens- side of the air and tears

29
Q

what is induced astigmatism ?

A

. happens when you are having a back surface toric because there is a difference in the refractive index of the lens and tears which can cause induced astigmatism

30
Q

what is a bi toric lens?

A

. when you order back and front surface toric

31
Q

what to order with SCL?

A

for any astigmatism you need to order a toric CL

32
Q

when are SCL not manufactured ?

A

. SCL are not manufactured unless they have a CYL of 0.75D or more

33
Q

what happens if CYL is less than 0.75D?

A

. you order a mean spherical equivalent , so you add half of the cyl to the sphere

34
Q

what is problems with SCL?

A

. need to be stabilised to avoid rotation , they move

35
Q

what to do if corneal and lenticular astigmatism cancel each other out ?

A

. spherical SCL is best solution
. if you are putting a RGP lens , the lenticular astigmatism will be uncorrected
- so if you read the keratometry readings of a px with no astigmatism (no cyl) and they have keratometry reading with a difference therefore if you prescribe them RGP lenses the LA will remain uncorrected therefore better to give Spherical SCL

36
Q

what do large oblique cyl do when changing to CLs?

A

. the patient might experience distortion if they have spectacle rx with astigmatism
. large oblique cylinders may produce some spatial distortion when changing to CLs
. when you wear a spectacle Rx with oblique cyl and you look at an object, the retinal image is slightly tilted , however when you wear a spectacle for a while your perception is that the object is straight
. when you fit them with cls, the retinal image will be straight and the perception is of object is tilted

37
Q

what is corrected retinal image ?

A

when a spectacle or cls are worn this is referred to as corrected retinal image

38
Q

how do cls affect visual acuity ?

A

. myopes get more magnification with cls compared to spectacles , improved VA

. hyperopes get less magnification with cls compared to spectacle, reduced VA

. this is because in cls we decrease the BVD, myopes receive more magnification

39
Q

what is the prismatic effect when wearing cls?

A

. myopes use more convergence when wearing cls compared to wearing glasses
. when myopes wear spectacles they receive base in at near
. when myopes wear cls, cls move with the eye, so when eyes turns in, the base in is lost and no decentration
. myopic eye needs to accommodate and coverage more with cls

. the opposite for hyperopes, they might like reading with glasses

40
Q

what is the effect of accommodation when wearing cls?

A

. myopic eye needs to use more accommodative efforts when wearing cls

. in hyperopes : delay presbyopia when wearing CLS

41
Q

what is anisometropia ?

A

. there is a significant difference in the spherical refractive errors between2 eyes

42
Q

what is the effect of relative spectacle magnification when wearing cls?

A
  1. refractive anisometropia: good for cl correction
    - this is because you get similar retinal images
  2. axial anisometropia: this means one eye is longer than other
    - larger retinal image, cls not good choice
43
Q

what is the effect of field of view when wearing cls?

A 
. larger FoV with CLs
. loss of spectacle frame
. less blind spot
. especially appreciated by high positive Rx
. more natural view with CLs
44
Q

why is there a big advantage of spectacle wear over contact lenses when it includes prismatic prescription ?

A
  1. in spectacles you have option to divide prism between eyes, you can go vertical or horizontal
    . you can use fresnel prisms for high values
  2. in contact lenses
    - normally only base down prism fitting
    - maximum is base down
    - vertical prism cannot be divided
    - horizontal and base up prisms only in scleral lenses

OV2006 contact lenses (11 decks)

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