Week 1.10 Toric Lenses Flashcards
What is the definition of astigmatism
Refractive condition, is the difference in refractive power between the two principal meridians of the eye
What are the types of regular astigmatism
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
Corneal astigmatism increases….
….towards the periphery
What is the important rule for corneal astigmatism and k readings
Each 0.05mm difference in k readings = 0.25DC
Or 0.10mm difference in k readings = 0.50DC
Corneal and lenticular astigmatism
- 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
What are some signs of irregular astigmatism
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
Spherical soft lenses vs spherical RGP lenses
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
Why correct astigmatism?
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
Why leave astigmatism uncorrected?
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
What degree of astigmatism do we usually not correct
Under 0.75DC
A px with 0.25-0.75DC may be happy with astigmatism uncorrected if:
- 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
Front surface aspheric soft lenses may…
..reduce spherical aberration hence some individuals with up to 1.50DC find them beneficial
Vision still not as good as toric correction
By how many letters improvement in eyes with 1D of astigmatism when fitted with a toric soft CL
3-5letters
What percentage of pop have some degree of astigmatism
90%
15% of those gave between 0.75-1.00DC
Lens choices for astigmatism depends on…
- level/axis of astigmatism
- source of astigmatism (corneal/lenticular)
- px requirements
- lens availability
What are some contact lens options for astigmatics
- 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
Induced astigmatism
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
What’s the suitable lens for someone who
Rx -2.00/-1.25x90
7.75 along 180, 7.75 along 90
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
What is the suitable lens for this px:
-2.00/-1.50x180
7.95 along 180, 7.65 along 90
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
What is the suitable lens for this px
Rx -3.00/-1.50x180
7.50 along 180, 8.00 along 90
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
How do you take into account back vertex distance in this px
Rx -6.00/1.75x170, BVD 12mm
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
How do you take into account the back vertex distance in this patient
+8.00/-1.75x10, BVD 12mm
+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
Soft toric contact lenses stability of vision
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
All toric lens design rely on what for stabilisation
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
What are 2 other methods of stabilising a toric lens
Prism ballast
Dynamic stabilisation
Prism ballast
- 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
Dynamic stabilisation
- 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
Accelerated stabilisation design
Developed by j&j
4 zones of thickness located within palpebral aperture
Minimum interaction with lids when aligned correctly
Fitting stock SCL topics
- 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
Assessing orientation of SCL toric
- 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
What cyl would we give to a -2.00/-1.00x40
-2.00/-0.75x40
Under correct and manufacturers tend to only do 0.75 and next one up is 1.25
How can you check the degree of rotation of the toric markings
To check the degree of rotation u can change the axis of the beam on an slit lamp to see where it is
What is the compensating for rotation rule
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
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
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
Examples of SCL torics:
Bausch and Lomb - pure vision toric (balafilcon A)
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
Examples of SCL torics:
Johnson and Johnson acuvue oasis for astigmatism
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
Example SCL torics:
Alcon - focus DAILIES toric
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
Do we always need to take into account the BVD
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
Aftercare issues with soft torics
- 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
Advantages of spherical RGP
- 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
Disadvantages of RGP lenses
- comfort
- difficulties in dusty environments
- occasional wear
- corneal distortion with higher cyls
