Title: FITTING THE TORIC CORNEA WITH RGP LENSES
1FITTING THE TORIC CORNEA WITH RGP LENSES
- Lens designs available
- Spherical Base and Peripheral Curves
- Aspheric Lenses
- Spherical Base Curve and Toric Peripheral Curves
- Toric Base Curves
- Prism Ballasted Front Surface Cylinders
- Hydrogel Torics
2CHOOSING THE TYPE OF LENS TO USE
Corneal cylinder approximately equals the
spectacle cylinder (with 2.00 D. or less of
corneal toricity) Rigid spherical or aspheric
lens -tear layer corrects cylinder equal to the
corneal toricity, therefore no cylinder power
required in CL Example -2.50 -1.50 x 180
spectacle Rx 43.50 _at_ 180 45.00 _at_ 090
K-reading Toric hydrogel lens -need toric lens
since there is no power in the tear layer
3CHOOSING THE TYPE OF LENS TO USE
Corneal cylinder with little or no spectacle
cylinder -use spherical hydrogel lens (rigid
lens would induce residual astigmatism due to a
toric tear lens) Example -3.00 -0.25 x 180
spectacle Rx 43.00 _at_ 180 44.25 _at_ 090
4CHOOSING THE TYPE OF LENS TO USE
Corneal cylinder gt2.00 D. and corneal cylinder
does not equal spectacle cylinder -rigid
spherical lens would create residual
astigmatism -use a toric base curve rigid or
toric hydrogel. -with high cylinders (over 3.00
D.) RGP bitoric lenses usually give better
vision (rotation of the RGP usually has
insignificant effect on vision).
5CHOOSING THE TYPE OF LENS TO USE
Corneal cylinder does not equal spectacle
cylinder (with less than 2.00 D. corneal
toricity). A spherical rigid lens would create
residual astigmatism Use 1. toric hydrogel
2. prism ballast front cylinder
6CHOOSING THE TYPE OF LENS TO USE
With-the-rule corneal toricity and
against-the-rule residual astigmatism with a
non-flexing lens use 1. thin RGP which will
flex and correct the residual astigmatism. 2.
a toric hydrogel to correct the
cylinder. Example K-reading 43.00 _at_ 180 45.00
_at_ 090 Spectacle Rx -3.00 -1.25 x 180 If RGP
does not flex there is -0.75 x 090
over-refraction If RGP lens is made thin and
flexes 0.75 D. the residual astigmatism
would be corrected
7Using Spherical Base Curve RGP Lenses on Toric
Corneas
A. Advantages 1. simple 2. inexpensive B.
Disadvantages 1. Lenses may not center well 2.
May be excessive movement 3. May cause corneal
distortion 4. Fluctuations in vision-because of
lens movement and decentration 5. Lens
flexure
8Using Spherical Base Curve RGP Lenses on Toric
Corneas
- If corneal toricity less than 2.00 D. then
spherical lens - is lens of choice
- 2. On 2 to 3 D. corneal toricity may be able to
use sphere - 3. On K fit- results in excessive movement and
decentration - 4. Best compromise fit (common rules of thumb)
- -base curve 1/3 of toricity added to flat K
- -mean K minus 0.50 D.
- 5. Diameter same as for near spherical
cornea-depends on - lid position
- 6. Peripheral curves- same as standard spherical
lenses.
9FITTING A TORIC CORNEA WITH A SPHERICAL BCR
On K fit (lens BCR matches flat meridian of
cornea) -horizontal band of touch on WTR
corneas -edge stand-off at 12 and 6
oclock -excessive movement and rocking on
blinking -lens tends to ride high if held up by
lid or drops and rides low -lens may be easily
lost from eye due to edge catching lens edge
due to stand-off
10FITTING A TORIC CORNEA WITH A SPHERICAL BCR
Spherical base curve on a 3.00 D. toric cornea.
Horizontal band of touch.
Lenses fitted this flat typically ride ride high
if pulled up by the upper lid or drop and ride
low.
11FITTING A TORIC CORNEA WITH A SPHERICAL BCR
Lens fitted steeper than K -lens rests on cornea
in mid-periphery -dumbell or H fluorescein
pattern -Best compromise fit Lens fitted
steeper than K by approx. 1/3 of corneal
toricity Example 43.00/44.50 cornea 43.50
base curve lens -Can cause corneal distortion
with spectacle blur
12FITTING A TORIC CORNEA WITH A SPHERICAL BCR
Best compromise fit is one where lens base curve
is steeper than K by about 1/3 of corneal
toricity. For example if Ks are
42.00/45.00, then lens base curve should be
43.00 D. This gives a dumbell or H pattern.
This lens is fitted too steep with lens resting
on cornea at the peripheral curve of the lens.
13Toric Base Curve RGP Lenses
Use when there are high amounts of corneal
toricity (2.00 D. or more corneal toricity)
Physical fit of lens to cornea is much better
than with a spherical lens. Saddle on the
horse concept.
14FITTING TORIC BASE CURVE RGP LENSES
Overall and optical zone diameter
determination -use same overall diameter
(OAD) and optical zone diameter (OZD) as you
would for a spherical lens. -OAD determination
based on lid positions -OZD based on lens
diameter and pupil size
15FITTING TORIC BASE CURVE RGP LENSES
Base curve radii selection a. select the flat
meridian of lens to match flat corneal
meridian -with a large diameter-large optical
zone lens you may need to go 0.25 D flatter than
flat K and with a small lens you may have to go
0.25 or 0.50 D. steeper than flat K. b. Select
the steeper meridian of the base curve to give
the CL about ¾ of the toricity of the
cornea. Example K-readings 43.00 _at_ 180 47.00
_at_090 CL base curve 43.00/46.00 D
(7.85/7.34 mm)
16FITTING TORIC BASE CURVE RGP LENSES
Peripheral (secondary) curve selection
Use the same criterion used for spherical base
curve lenses -for example if for a given
diameter/optical zone of a spherical lens the
SCR is 1.5 mm flatter than the BCR do the same
for the toric BCR lens Keep difference between
meridians of the SCR the same as BCR -For
example if BCR 7.70/7.30 mm then SCR maybe
9.20/8.80 mm. This keeps the OZD round and
secondary clearance even.
17FITTING TORIC BASE CURVE RGP LENSES
Fluorescein pattern of a toric BCR lens on a
toric cornea should look like a spherical lens
on a spherical cornea.
7.6/8.0 BCR on toric cornea Even fluorescein
pattern
Toric base curve lens with both meridians too
steep. Try next lens 0.50 D. flatter in each
meridian.
18FITTING TORIC BASE CURVE RGP LENSES
Toric BCR RGP lens with too much toricity in the
lens. Note the central pooling and the touch at
12 6 oclock
19FITTING TORIC BASE CURVE RGP LENSES
A toric BCR lens with too little toricity in the
lens. Looks like a spherical lens on a low
toricity cornea with a central band of touch.
20FITTING TORIC BASE CURVE RGP LENSES
- Using diagnostic lenses to fit toric corneas
- a. can use spherical BCR diagnostic lenses to
determine - fit on flat meridian and use 3/4 rule for steep
meridian. -
- b. best to use toric base curve lenses
- most used set has 2.00 D. toricity
- 3.00 D. and 4.00 D. sets useful
- SPE designed sets are best
21FITTING TORIC BASE CURVE RGP LENSES
Spherical Power Equivalent (SPE) Toric Base Curve
Lenses Design -toric base curve lens
with a front toric to give spherical power
effect when on the eye. -in air the
cylinder power will be the same as the
difference in the base curve toricity specified
in diopters. -example base curve
toricity 42.00/46.00 power plano/-4.00
in air.
22FITTING TORIC BASE CURVE RGP LENSES
SPE diagnostic sets
23FITTING TORIC BASE CURVE RGP LENSES
Spherical Power Equivalent (SPE) Toric Base Curve
Lenses Advantage of SPE lenses -corrects
patient's cylinder when the spectacle cylinder
equals corneal toricity -example
K-readings 43.00/46.00 spectacle cylinder
-3.00 D. -if lens rotates on eye it has no
detrimental effect on patient's vision-tear
lens compensates. -ideal for diagnostic lenses
since it allows for easy, accurate
over-refractions.
24FITTING TORIC BASE CURVE RGP LENSES
With SPE lens (or any diagnostic lens) on the
eye, do a sphere-cylinder over-refraction. Using
the diagnostic lens base curve and power with
the over-refraction to determine lens to
order. Compensate for any base curve change from
the diagnostic lens. Keep it simple, calculate
power needed in each meridian and order lens. Do
not worry about what the front surface will be,
let lab calculate that. Compare lens to order
using Ks spectacle Rx to that using the
diagnostic lens and over-refraction. Both should
give same lens power to order.
25Determining the lens power to order in a toric
BCR lens Using the spectacle Rx and K-readings
26Mandell-Moore form for calculating bitoric lens
power. Available on the RGPLI web site
http//www.rgpli.org
27Determining lens power using a diagnostic lens
and over-refraction.
28Excel spreadsheet for calculating bitoric lens
power using the K-readings and spectacle
Rx. Available on the web at http//www.
29Excel spreadsheet to calculate bitoric lens power
using a diagnostic lens and over-refraction.
This spreadsheet program can be downloaded from
http//www.opt.indiana.edu/lowther/index.htm
30LENS ORDER
31SUMMARY OF FITTING TORIC BASE CURVE RGP LENSES
- Using K-readings and Spectacle Rx
- Do exam and obtain K-readings and spectacle Rx
- Determine lens diameter and optical zone
- Select base curve flat K and ¾ corneal toricity
in lens. - Calculate lens power
- calculate using power crosses
- Use spreadsheet program
- Mandell-Moore form
- Order lens
- Can use spherical diagnostic lens and do
over-refraction. - From this calculate power needed in bitoric lens
as a check - on the power required.
32SUMMARY OF FITTING TORIC BASE CURVE RGP LENSES
- Using bitoric diagnostic lens
- Do exam and obtain K-readings and spectacle Rx
- Determine lens diameter and optical zone
- Select base curve flat K and ¾ corneal toricity
in lens - Evaluate fluorescein pattern-change diagnostic
lens if necessary to get best fit. - Do an over-refraction
- Calculate lens power
- Order lens