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 Rx43.50 @ 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 usuallyhas 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 @ 090Spectacle Rx -3.00 -1.25 x 180If RGP does not flex there is -0.75 x 090 over-refractionIf 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. simple2. inexpensive

B. Disadvantages:1. Lenses may not center well2. May be excessive movement3. May cause corneal distortion4. Fluctuations in vision-because of lens

movement and decentration5. 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 catchinglens edge due to stand-off


Spherical base curve on a 3.00 D.toric cornea. Horizontal band oftouch.

Lenses fitted this flat typicallyride 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

toricityExample: 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 witha 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 BCRdo 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 corneaEven fluorescein pattern

Toric base curve lens withboth 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 12.5/12.139.50/41.50 8.54/8.13 -3.00/-5.00 10.1/9.7 12.3/11.940.00/42.00 8.44/8.04 -3.00/-5.00 10.0/9.6 12.0/11.640.50/42.50 8.33/7.94 -3.00/-5.00 9.9/9.5 11.7/11.341.00/43.00 8.23/7.85 -3.00/-5.00 9.8/9.4 11.3/10.941.50/43.50 8.13/7.76 -3.00/-5.00 9.7/9.3 10.9/10.542.00/44.00 8.04/7.67 -3.00/-5.00 9.6/9.2 10.5/10.142.50/44.50 7.94/7.58 -3.00/-5.00 9.4/9.0 10.4/10.043.00/45.00 7.85/7.50 -3.00/-5.00 9.2/8.8 10.3/9.943.50/45.50 7.76/7.42 -3.00/-5.00 9.0/8.7 10.1/9.844.00/46.00 7.67/7.34 -3.00/-5.00 8.8/8.5 10.0/9.744.50/46.50 7.58/7.26 -3.00/-5.00 8.6/8.3 9.9/9.645.00/47.00 7.50/7.18 -3.00/-5.00 8.4/8.1 9.8/9.345.50/47.50 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 asphere-cylinder over-refraction.

Using the diagnostic lens base curve and power with theover-refraction to determine lens to order.

Compensate for any base curve change from the diagnostic lens.

Keep it simple, calculate power needed in each meridianand order lens. Do not worry about what the front surfacewill 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. 00K-READING46.5043.50BASE CURVE-0.50+0.50LACRIMAL LENS-0.50+0.50LACRIMAL LENS-6.50-2.00SPECT. RX-6.00-2.50CL RX47.00

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 usingthe K-readings and spectacle Rx.

Available on the web at http://www.****CALCULATION OF POWER FOR BITORIC RGP LENSES USING K-READINGS AND SPECTACLE RXEnter the requested values in the blank (white) cells.Flat K Steep K

K-readings 45.00 49.00Sphere 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 -7.71Fit factor* 0.25 -0.75

CL Rx Flat BCR (D.) Power Steep BCR (D.) Power45.25 -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 meridianas a minus value.

Excel spreadsheet to calculate bitoric lens power usinga diagnostic lens and over-refraction.

CALCULATION OF POWER FOR BITORIC RGP LENSESUSING DIAGNOSTIC LENSES AND OVER-REFRACTIONEnter the requested values in the blank (white) cells.Flat meridian Steep meridian

Diagnostic Lens BC (D.) 44.00 44.00Power of diagnostic lens* -3.00 -3.00 *enter total power in each meridian

Over-refraction** -1.00 -3.00 vertex distance (mm) 12Over-refraction vertexed -0.99 -2.90Base curve ordering (D.) 43.00 46.00 **total power of over-refraction in each meridianCL 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

LENS ORDER

SUMMARY OF FITTING TORIC BASE CURVE RGP LENSES

Using K-readings and Spectacle Rx

1. Do exam and obtain K-readings and spectacle Rx2. Determine lens diameter and optical zone3. Select base curve: flat K and ¾ corneal toricity in lens.4. Calculate lens power

a) calculate using power crossesb) Use spreadsheet programc) 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 checkon the power required.

SUMMARY OF FITTING TORIC BASE CURVE RGP LENSES

Using bitoric diagnostic lens:

1. Do exam and obtain K-readings and spectacle Rx2. Determine lens diameter and optical zone3. Select base curve: flat K and ¾ corneal toricity in

lens4. Evaluate fluorescein pattern-change diagnostic lens if

necessary to get best fit.5. Do an over-refraction6. Calculate lens power7. Order lens

toric rgp fit

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