Week 1.10 Toric Lenses

Question 1

What is the definition of astigmatism

Answer 1

Refractive condition, is the difference in refractive power between the two principal meridians of the eye

Question 2

What are the types of regular astigmatism

Answer 2

With the rule - flatter meridian close to 180, steeper closer to 90
+/- 30 horizontal
50% of the younger pop has WTR

Against the rule - flatter meridian closer to 90, steeper close to 180
+/-30 vertical
Approx 50% of older pop have ATR

Oblique astigmatism
Approx 20%
Where u don’t have it within +/- 180 or 90

Question 3

Corneal astigmatism increases….

Answer 3

….towards the periphery

Question 4

What is the important rule for corneal astigmatism and k readings

Answer 4

Each 0.05mm difference in k readings = 0.25DC

Or 0.10mm difference in k readings = 0.50DC

Question 5

Corneal and lenticular astigmatism

Answer 5

• Each 0.05mm difference = 0.25DC
• E.g. K readings of 7.85 along 180, 7.75 along 180
• = 0.50DC corneal astigmatism
• Spec rx -2.00/-1.50 x180
• Mismatch between spec cyl and corneal astigmatism
• 1.00D lenticular or residual astigmatism
• Residual astigmatism ocular astigmatism – corneal astigmatism

Question 6

What are some signs of irregular astigmatism

Answer 6

Due to corneal irregularity signs are:
- k mires irregular (distorted)
- k readings may not be at 90 degrees to each other
- best viewed using topographer
- associated with corneal injury, dystrophies (including keratoconus) warpage etc

Question 7

Spherical soft lenses vs spherical RGP lenses

Answer 7

Soft lenses flex to take up the shape of the cornea
Corneal shape (incl corneal astigmatism is transmitted through lens)
Thin tear lens is focal

Spherical RGP
RGP lenses don’t flex to take up the shape of the cornea
Corneal shape is not transmitted through the lens
Corneal astigmatism is corrected by the tear lens

Question 8

Why correct astigmatism?

Answer 8

Uncorrected astigmatism causes:
Decreased VA
Decreased contrast sensitivity
Glare/streaks at night
More variable accomodation hence headaches/asthenopia (eye strain)
Improved quality of vision leads to greater px satisfaction
Better contact lens fitting skills
Greater job satisfaction

Question 9

Why leave astigmatism uncorrected?

Answer 9

Suitable toric lens may not be available in material/modality required
- oblique axis
- high rx
- cost - px pays twice as much as spherical lens, chair time

Question 10

What degree of astigmatism do we usually not correct

Answer 10

Under 0.75DC

Question 11

A px with 0.25-0.75DC may be happy with astigmatism uncorrected if:

Answer 11

• low visual demands
• not driving at night
• CLs for occasional daytime social only
• more likely if axis ~90/180 deg rather than oblique
• more likely if astigmatism is <1/4 sphere power
• occasionally higher astigmatism can be tolerated, particularly in non-dominant eye

Question 12

Front surface aspheric soft lenses may…

Answer 12

..reduce spherical aberration hence some individuals with up to 1.50DC find them beneficial

Vision still not as good as toric correction

Question 13

By how many letters improvement in eyes with 1D of astigmatism when fitted with a toric soft CL

Answer 13

3-5letters

Question 14

What percentage of pop have some degree of astigmatism

Answer 14

90%
15% of those gave between 0.75-1.00DC

Question 15

Lens choices for astigmatism depends on…

Answer 15

• level/axis of astigmatism
• source of astigmatism (corneal/lenticular)
• px requirements
• lens availability

Question 16

What are some contact lens options for astigmatics

Answer 16

• soft toric (corneal and/or lenticular)
• RGP - spherical for corneal astigmatism only,
  toric back surface - corneal astigmatism,
  Toric front surface - some lenticular astigmatism
  Bitoric - corneal and corneal/lenticular/induced

Question 17

Induced astigmatism

Answer 17

Difference in refractive indices between cl and tears

Tears has less refractive index

Therefore RGP with toric back surface u might put too much bending power and too much refractive index material in the gap between the tear lens will work so will induce astigmatism.

So do 2/3rds correction on back surface and the 1/3 will be the tears film

Question 18

What’s the suitable lens for someone who
Rx -2.00/-1.25x90
7.75 along 180, 7.75 along 90

Answer 18

No corneal astigmatism all lenticular
Suitable lenses: soft toric or front surface RGP toric

If the px was fitted with a spherical RGP lens there would be uncorrected residual astigmatism

Question 19

What is the suitable lens for this px:
-2.00/-1.50x180
7.95 along 180, 7.65 along 90

Answer 19

6x0.25DC = 1.50 corneal astigmatism only
Astigmatism is all corneal
Suitable lenses: soft toric or spherical RGP

The tear lens between the back of the spherical
RGP lens and the front surface of the cornea corrects the corneal astigmatism

Question 20

What is the suitable lens for this px
Rx -3.00/-1.50x180
7.50 along 180, 8.00 along 90

Answer 20

10x0.25 =2.50DC corneal astigmatism
Corneal astigmatism partially compensated for by lenticular astigmatism
Suitable lenses: soft toric (or bitoric RGP)
A spherical RGP lens would only correct the corneal astigmatism

Question 21

How do you take into account back vertex distance in this px
Rx -6.00/1.75x170, BVD 12mm

Answer 21

Consider each meridian separately so we have
-6.00 along 170 &
-7.75 along 80

1/-6.00=-0.167-0.012=-0.179
1/0.179=5.587

-6.00 along 170 becomes -5.50
D

1/-7.75=-0.129-0.012=-0.141
1/0.141=7.0922

-7.75 along 80 becomes -7.00

Ocular rx = -5.50/-1.50x170

Remember reciprocal of power, subtract old BVD, add new BVD (in this case 0) then reciprocal

Question 22

How do you take into account the back vertex distance in this patient
+8.00/-1.75x10, BVD 12mm

Answer 22

+8.00 along 10
+6.25 along 100

1/8.00= 0.125-0.012=0.113
1/0.113=8.85

1/6.25= 0.16-0.012=0.148
1/0.148=6.757

+8.00 along 10 becomes +8.75
+6.25 along 100 becomes +6.75

Ocular rx +8.75/-2.00x10

Question 23

Soft toric contact lenses stability of vision

Answer 23

Historically poor with soft toric contact lenses
Historically poorly reproducible
Toric back surface alone rarely sufficient to maintain orientation
Lids interact with thicker portion of lens
Gravity plays a small role
Error now however is much smaller

Question 24

All toric lens design rely on what for stabilisation

Answer 24

Rely on the lids interacting in a typical way with the lens
Individuals with atypical lids are less likely to be successful - slanting eye lids, incomplete blink, lax lids common with age

Question 25

What are 2 other methods of stabilising a toric lens

Answer 25

Prism ballast Dynamic stabilisation

Question 26

Prism ballast

Answer 26

- 1-1.5 prism base down - upper lid squeezes thicker lens portion down - also affected by gravity - Reduced comfort and Dk/t - interaction with low lid If put prism in both eyes shouldn’t make a difference however if only fitting one eye with a toric then is a problem optically

Question 27

Dynamic stabilisation

Answer 27

- Both lids take an active role in stabilisation of the lens - thin zones superiority and inferiority lids squeeze against thickness differential - maintains minimal thickness of less so good for O2 supply, most effective for high minus lenses - greater thickness difference can be created Stabilisation can be a problem in those with oblique astigmatism as additional thick zones at oblique locations 1 zone of stabilisation As px blinks lids move inwards the lens spins inwards with each blink so vision wont be as stable with normal dynamic stabilisation - so accelerated stabilisation design comes in

Question 28

Accelerated stabilisation design

Answer 28

Developed by j&j 4 zones of thickness located within palpebral aperture Minimum interaction with lids when aligned correctly

Question 29

Fitting stock SCL topics

Answer 29

- discuss with px to fit toric SCL - choose lens modality/material required - calculate effect of BVS - each meridian separately - choose a prescription as near to that required as possible - select mid-BOZR trial lens or flatter of two - consider diff design for each eye - under correct cyls rather than over correct - allow to settle for a few minutes

Question 30

Assessing orientation of SCL toric

Answer 30

- ensure lens has settled first - find the orientation markings (different for different lens designs) - might be lines at 3 and 9. Or 3 lines at 6 o’clock - examine orientation with blink - markings do no indicate axis of astigmatism’s they indicate the orientation of the lens

Question 31

What cyl would we give to a -2.00/-1.00x40

Answer 31

-2.00/-0.75x40 Under correct and manufacturers tend to only do 0.75 and next one up is 1.25

Question 32

How can you check the degree of rotation of the toric markings

Answer 32

To check the degree of rotation u can change the axis of the beam on an slit lamp to see where it is

Question 33

What is the compensating for rotation rule

Answer 33

Rule: clockwise add, anticlockwise subtract (CAAS) For vertical markings LARS - left add right subtract Don’t get confused it means subtract from the axis don’t start working out what the new axis is and all that. Don’t over complicate just subtract or add from the rx axis NOT the new axis that its as now. That’s only used to indicate the direction of the rotation

Question 34

Compensating for rotation example: If a px -2.00/-0.75x40 and toric marker usually sits at 6oclock, degree off rotation is 20degrees anticlockwise so what is the axis we need to order for the px

Answer 34

Is an anticlockwise rotation of the toric markers so we add to the original axis which was 40 So now the new axis its sat at is 60 So we order -2.00/-0.74x20 because we know when we put that in and the lens rotates 20degrees anticlockwise it will lie at 40. So we did 40 SUBTRACT 20 (because remember CAAS) Forget about the 60 that was just there to indicate the direction of rotation

Question 35

Examples of SCL torics: Bausch and Lomb - pure vision toric (balafilcon A)

Answer 35

1st gen Sihi Aspheric front surface Prism ballast, toric back surface BOZR 8.7mm TD 14mm Sphere powers - +6.00 to -9.00D Cyl in 10dgree steps - -0.75, -1.25,-1.75,-2.25… Monthly Dk68

Question 36

Examples of SCL torics: Johnson and Johnson acuvue oasis for astigmatism

Answer 36

3rd gen SiHi Toric back surface Prism ballast BOZR 8.6mm TD 14.50mm Sphere powers available: +6.00 to -9.00 Cyl powers also in 10degree steps - -0.75, -1.25, -1.75, -2.25 Monthly or 2 a month (weekly is wear overnight) Dk77 higher o2 perm than B&l pure vision

Question 37

Example SCL torics: Alcon - focus DAILIES toric

Answer 37

High water content conventional hydrogel Back surface toric BOZR 8.6mm, TD 14.20 Sphere powers available: +4.00 to -8.00 Cycl powers -0.75 and -1.50 at 20,70,90,110,160 or 180 Daily disposable Dk 20 Poor transmissibility - only for for occasional wear

Question 38

Do we always need to take into account the BVD

Answer 38

Not if all the astigmatism is corneal RE 8.10mm along 180, 7.60mm along 90 Rx -1.00/-2.50x180 BVD=12mm 0.5mm change so that’s 2.50DC No BVD adjustment needed Soft cl or spherical RGP

Question 39

Aftercare issues with soft torics

Answer 39

- discomfort due to thicker lens - handling issues due to larger diameter - hypoxia problems under thicker portions of the lens - less of a problem with new stabilisation techniques - less of problem now that more SiH are available

Question 40

Advantages of spherical RGP

Answer 40

- cheapest option for corneal astigmatism - sharpest vision - no stabilisation issues - no hypoxia problems - assuming high Dk material - astigmatic pooling of tears corrects most of corneal astigmatism - acceptable fit usually possible up to approx 3.00DC of corneal astigmatism

Question 41

Disadvantages of RGP lenses

Answer 41

- comfort - difficulties in dusty environments - occasional wear - corneal distortion with higher cyls