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Contact Lenses > Toric RGP lenses > Flashcards

Toric RGP lenses Flashcards

1
Q

what are the two criteria for fitting topic RGP lenses

A

to:
improve the physical fit
- to avoid lens rocking on cornea, so improving the radii of the lens

improve the visual acuity
- when the spec rx and k readings don’t match, indicating lenticular astigmatism

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2
Q

when is residual astigmatism expected

A

when the spectacle astigmatism doesn’t equal the corneal astigmatism

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3
Q

when is residual astigmatism found

A

when placing a spherical lens on the cornea

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4
Q

what is residual astigmatism

A

it is the uncorrected astigmatism from the crystalline lens

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5
Q

which type of astigmatism is residual astigmatism generally

A

against the rule astigmatism (Horizontal meridian is steeper)

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6
Q

when can you maybe get away with not correcting residual astigmatism with contact lenses

A

if it is in the non-dominant eye, it may not be noticed by the px

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7
Q

when is induced astigmatism expected

A

when using toric back surface - otherwise the lens is rocking about

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8
Q

why is induced astigmatism found

A

because of the difference in refractive indices (RI) between the lens material and the tears
hence the px does not see what you expect them to see/as well as they’re supposed to

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9
Q

how can you sort out the flourescein patten for a induced astigmatism for a back surface toric lens

A
  • can compromise by changing the cylindrical element of the back surface
    or
  • put some cyl on the front, so won’t have cyl on the back
    = bitoric lens
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10
Q

list 5 different reasons/patients you will want to fit a toric RGP on, for a improved physical fit

A
  • difference between principle meridians is greater than 0.6mm (3.00D of corneal astigmatism) - to avoid the spherical lens rocking around
  • when spherical lens is unstable
  • when spherical lens decentres
  • when the spherical lens gives unacceptable bearing areas
  • when the cornea is more toric towards the periphery - by looking at the FP
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11
Q

list 2 different reasons when you will want to fit a toric RGP for improved visual acuity

A
  • residual astigmatism >1.00D, put power on the front

- induced astigmatism >1.00D, it is induced by the back but put the power on the front

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12
Q

list 2 non-toric RGP lens designs

A
  • spherical
  • aspheric
    simply and less costly for the patient
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13
Q

list 4 types of toric RGP lenses

A
  • front surface toric
  • toric periphery
  • back surface toric - mostly used
  • bitoric - mostly used
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14
Q

how can a spherical lens be fitted on a toric cornea and why

A

fitted with flat fitting lenses to minimise the lens flexure

use diagnostic of a specific material - so make sure you don’t use too soft of a material

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15
Q

how do you observe the flexure of a spherical lens on a toric cornea

A

observe flexure with the keratometer mires

  • when the patient blinks, the mires should always look clear
  • if the lens is flexing, then the mires will distort and then go clear
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16
Q

when will the flourescein pattern be acceptable, when there is a spherical lens on a toric cornea

A

if there is adequate edge clearance along the flattest meridian and no excessive stand off in the steeper meridian
can be useful to use a small TD design

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17
Q

which lens parameter is best kept small when fitting a spherical RGP on a toric cornea

A

small TD - as gets more toric as you go away from the centre

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18
Q

why may an aspheric lens provide better fitting on a toric cornea than a spherical lens

A

better fitting characteristics because of the close alignment to the cornea - fits the shape of the cornea more closely

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19
Q

what is the ideal fit of an aspheric lens on a toric cornea

A

alignment or slightly flatter than flattest k

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20
Q

what are the two advantages of using an aspheric lens on a toric cornea

A
  • minimises lens flexure

- maintains good acuity

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21
Q

what sort of flourescein pattern will an aspheric lens have on a toric cornea

A

the mainly aspheric designs produce the least peripheral stand off in the steeper meridian

they produce less edge clearance than spherical lenses
and don’t need so much tear flow under the lens as the material helps with the oxygen going through

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22
Q

which power cyls of corneal astigmatism are aspheric most useful for

A

CA between 3.00 and 4.00D, so try this as a first option before toric lenses

23
Q

when may a aspheric lens not work on a toric cornea, hence needing to use a toric lens as a solution

A

if the lens decentres on the cornea

24
Q

what are the three methods of stabilisation used on toric lenses

A
  • prism ballast
  • truncation
  • back surface design
25
Q

which type of toric lenses do you need to use one or more of the stabilisation methods

A

for front surface toric or toric periphery

26
Q

which method of stabilisation is used if a lens doesn’t click into place on a front surface toric lens

A

prism ballast

27
Q

list how a prism ballast is used as a method of stabilisation

A
  • 1.5 to 3.0 prism base down
  • usually marked to help observation
  • prism is at periphery of lens as no aspect for it is for vision
  • it is only used for weighing down the lens as its thicker base on the bottom and it is kept away from the optic zone
  • has a tendency for 5-10 degrees nasal rotation on blinking due to lid tension and eyelid position, but is only irritating on dominant eye or on one eye, otherwise not
28
Q

name a disadvantage to the prism ballast method of stabilisation

A

has a tendency for 5-10 degrees nasal rotation on blinking due to lid tension and eyelid position, but is only irritating on dominant eye or on one eye, otherwise not

29
Q

list how truncation is used as a method of stabilisation

A
  • used with or without a prism ballast
  • single or double - can cut off just the bottom or top and bottom
  • chord of 0.5-1mm removed
  • truncation should sit on lower eyelid
  • ineffective with small TD or if lid is below the limbus
  • TD is increased by 0.5mm to assist the stability
30
Q

when will a truncation method of stabilisation be ineffective, and how is the problem generally solved

A

with a small TD or if the lid is below the limbus

TD is increased by 0.5mm to assist the stability

31
Q

which type if eyelid will you want to have a truncation but off at the top and bottom

A

on a tight eyelid

32
Q

what else can a truncation be used with or without

A

a prism ballast, it varies with the rx and lid tension

33
Q

as well as used in toric lenses, when else is a truncation method of stabilisation used

A

in bifocal CL’s to weight the lens

34
Q

list how a back surface design is used as a method of stabilisation

A
  • the lens will click into place if the curve is chosen appropriately
  • will help when fitting a bitoric
35
Q

what is a front surface toric lens

A
  • a lens with a spherical or aspheric back surface

- the cylinder is worked on the front

36
Q

why does a front surface toric lens need stabilising

A

to keep the cyl orientation in the right place/axis

37
Q

what type of lens may be more appropriate over a front surface toric lens

A

soft lens

38
Q

when may a soft lens be more appropriate over a front surface toric RGP lens

A
  • if k readings show a toric cornea
  • but the spec rx is near spherical
  • then lenticular cyl becomes the reason for a toric design

if you use a thin SCL, it will take up the shape of the cornea and it will come onto the front of the soft lens, which will be continued to be neutralised by lenticular astigmatism, so SCL is better than rigid lens optically

39
Q

why are toric periphery lenses not generally used

A

due to aspheric lenses being used instead

40
Q

what design is a toric periphery lens

A
  • spherical BOZR
  • toric BPORs - peripheral curves are toric
  • oval optic (BOZR oval)
41
Q

what can you use with a toric periphery lens is there is manufacturing problems

A

a spherical last curve

42
Q

what is available with toric periphery lenses

A

fitting sets, to try on, but used more in hospital practice

43
Q

what design is a back surface toric lens

A
  • full back toric surface
  • front spherical surface
  • no extra stabilisation needed, as back surface toric will usually click into place
44
Q

what three ways can a back surface toric lens be fitted by

A
  • toric fitting set
  • spherical fitting set
  • fitting my calculation
45
Q

how is a toric fitting set used to fit a back surface toric lens

A
  • the trial lens chosen is based on keratometry
  • flattest curve is chosen along Kf (flattest K)
  • steepest curve is 0.10mm flatter than Ks (steepest K)
  • FP = spherical fitting, if the k readings are correct, as have matched the shape of the eye
  • over refraction gives the BVP along the flatter meridian
46
Q

how is a spherical fitting set used to fit a back surface toric lens

A
  • flattest meridian is worked out the normal way
  • steepest meridian is decided from the steepest k = 0.10 flatter than Ks
  • the power is worked out using the steepest k, using the normal power calculation: BVP: F = n-1/r
47
Q

if deciding to fit a back surface toric lens by fitting by calculation, which 4 details must be sent to the lab to do so

A
  • spectacle rx
  • BVD
  • keratometry readings needed
  • refractive index of material (as it will have implications as it can alter how much astigmatism is induced)
48
Q

why does the induced astigmatism need to be considered, in the lab when fitting a back surface toric lens, via fitting by calculation method

A

because if the induced astigmatism corrects the residual astigmatism (i.e. the difference between spec rx and k’s) then the front surface remains spherical

a thin air gap separates all surfaces = between the tears-lens and lens-cornea, must all be considered with calculations

49
Q

what is the formula for induced astigmatism

A

I = n-n’/rs - n-n’/rf

n = RI of tears 
n' = RI of CL material 
r = k readings
50
Q

how is a bitoric lens designed

A

it has a toric: BOZR (central), BPOR (peripheral) and FOZR

the cylinder is worked on the front as there is enough induced astigmatism which causes the problem

51
Q

when is a bitoric lens needed

A

when induced astigmatism is a visual problem

  • when corneal astigmatism = ocular astigmatism - hence no residual astigmatism
  • when corneal astigmatism is less than the ocular astigmatism:
    • so lenticular astigmatism is revealed so a front surface toric lens is needed to compensate as the resulting astigmatism is increased when a rigid lens is fitted
52
Q

what 3 details need to be sent to the lab when making a bitoric lens

A
  • refraction
  • keratometry
  • HVID
  • use a computer programme
  • use a topographer CL programme
53
Q

what will a good fitting toric lens look like

A

an alignment spherical fit

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