Toric rgp fit

FITTING THE TORIC CORNEA WITH RGP LENSES
Lens designs available:
A. Spherical Base and Peripheral Curves
B. Aspheric Lenses
C. Spherical Base Curve and Toric Peripheral Curves
D. Toric Base Curves
E. Prism Ballasted Front Surface Cylinders
F. Hydrogel Torics

CHOOSING 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 @ 180; 45.00 @ 090 K-reading
Toric hydrogel lens
-need toric lens since there is no power in the tear layer

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 @ 180; 44.25 @ 090

Corneal cylinder =>2.00 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).

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

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 @ 180; 45.00 @ 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

Using 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

1. 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.
Using Spherical Base Curve RGP Lenses on Toric Corneas

FITTING 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 o’clock
-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

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.

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

Best compromise fit is one where
lens base curve is steeper than K
by about 1/3 of corneal toricity.
For example if K’s 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.

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.
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
FITTING 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 @ 180; 47.00 @090
CL base curve: 43.00/46.00 D (7.85/7.34 mm)

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.
Peripheral (secondary) curve selection:

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.

Toric BCR RGP lens with too much toricity in the lens.
Note the central pooling and the touch at 12 & 6 o’clock

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.

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

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.

SPE diagnostic sets:
Possible 2.00 D. Toric Spherical Power Equivalent Diagnostic Set
(diameter 9.5 mm; OZD 8.0 mm; PCW 0.3 mm)
Base Curves (D) Base Curves (mm) Powers Secondary curves Peripheral curves
39.00/41.00 8.65/8.23 -3.00/-5.00 10.2/9.8 12.5/12.1
39.50/41.50 8.54/8.13 -3.00/-5.00 10.1/9.7 12.3/11.9
40.00/42.00 8.44/8.04 -3.00/-5.00 10.0/9.6 12.0/11.6
40.50/42.50 8.33/7.94 -3.00/-5.00 9.9/9.5 11.7/11.3
41.00/43.00 8.23/7.85 -3.00/-5.00 9.8/9.4 11.3/10.9
41.50/43.50 8.13/7.76 -3.00/-5.00 9.7/9.3 10.9/10.5
42.00/44.00 8.04/7.67 -3.00/-5.00 9.6/9.2 10.5/10.1
42.50/44.50 7.94/7.58 -3.00/-5.00 9.4/9.0 10.4/10.0
43.00/45.00 7.85/7.50 -3.00/-5.00 9.2/8.8 10.3/9.9
43.50/45.50 7.76/7.42 -3.00/-5.00 9.0/8.7 10.1/9.8
44.00/46.00 7.67/7.34 -3.00/-5.00 8.8/8.5 10.0/9.7
44.50/46.50 7.58/7.26 -3.00/-5.00 8.6/8.3 9.9/9.6
45.00/47.00 7.50/7.18 -3.00/-5.00 8.4/8.1 9.8/9.3
45.50/47.50 7.42/7.11 -3.00/-5.00 8.2/7.9 9.7/9.4
Lenses designed to have an axial edge lift of 0.12 mm in the flat meridian

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.

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 K’s & spectacle Rx to that
using the diagnostic lens and over-refraction. Both should
give same lens power to order.

Determining the lens power to order in a toric BCR lens
Using the spectacle Rx and K-readings
43. 00
K-READING
46.50
43.50
BASE CURVE
-0.50
+0.50
LACRIMAL LENS
-0.50
+0.50
LACRIMAL LENS
-6.50
-2.00
SPECT. RX
-6.00
-2.50
CL RX
47.00
43.00

Mandell-Moore form for calculating bitoric lens power.
Available on the RGPLI web site: http://www.rgpli.org

45.00
45.00
DIAGNOSTIC CL BC
46.50
43.50
BC TO BE ORDERED
+1.50
-1.50
CHANGE IN LL P0WER
-3.00
-3.00
DIAG. LENS POWER
-1.50
-1.00
OVER-REFRACT. CHANGE IN LL P0WER
+1.50
-1.50
-6.00
-2.50
CL POWER
Determining lens power using a diagnostic lens and
over-refraction.

Excel spreadsheet for calculating bitoric lens power using
the K-readings and spectacle Rx.
Available on the web at http://www.****
CALCULATION OF POWER FOR BITORIC RGP LENSES
USING K-READINGS AND SPECTACLE RX
Enter the requested values in the blank (white) cells.
Flat K Steep K
K-readings 45.00 49.00
Sphere Cylinder Vertex distance (mm)
Spectacle Rx -5.00 -3.50 12
(minus cyl form)
Flat K Sphere Power Steep K Sph + Cyl
(corrected for VD) (corrected for VD)
45.00 -4.72 49.00 -7.71
Fit factor* 0.25 -0.75
CL Rx Flat BCR (D.) Power Steep BCR (D.) Power
45.25 -4.97 48.25 -6.96
Flat BCR (mm) steep BCR (mm)
7.46 6.99
*if lens is fitted steeper than flat meridian put in dioptric value steeper as a plus value;
if fitted flatter than flat K put diopter value flatter as a minus value.
*for the steep meridian fit factor enter the diopter value flatter than the steep meridian
as a minus value.

Excel spreadsheet to calculate bitoric lens power using
a diagnostic lens and over-refraction.
CALCULATION OF POWER FOR BITORIC RGP LENSES
USING DIAGNOSTIC LENSES AND OVER-REFRACTION
Enter the requested values in the blank (white) cells.
Flat meridian Steep meridian
Diagnostic Lens BC (D.) 44.00 44.00
Power of diagnostic lens* -3.00 -3.00 *enter total power in each meridian
Over-refraction** -1.00 -3.00 vertex distance (mm) 12
Over-refraction vertexed -0.99 -2.90
Base curve ordering (D.) 43.00 46.00
**total power of over-refraction in each meridian
CL Rx Flat Meridian power Flat meridian BC Steep meridian power Steep meridian BC
-2.99 43.00 -7.90 46.00
This spreadsheet program can be downloaded from:
http://www.opt.indiana.edu/lowther/index.htm

SUMMARY OF FITTING TORIC BASE CURVE RGP LENSES
Using K-readings and Spectacle Rx
1. Do exam and obtain K-readings and spectacle Rx
2. Determine lens diameter and optical zone
3. Select base curve: flat K and ¾ corneal toricity in lens.
4. Calculate lens power
a) calculate using power crosses
b) Use spreadsheet program
c) Mandell-Moore form
5. 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.

SUMMARY OF FITTING TORIC BASE CURVE RGP LENSES
Using bitoric diagnostic lens:
1. Do exam and obtain K-readings and spectacle Rx
2. Determine lens diameter and optical zone
3. Select base curve: flat K and ¾ corneal toricity in
lens
4. Evaluate fluorescein pattern-change diagnostic lens if
necessary to get best fit.
5. Do an over-refraction
6. Calculate lens power
7. Order lens

Toric rgp fit

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Toric rgp fit