FITTING THE TORIC CORNEA WITH RGP LENSESLens designs available:A. Spherical Base and Peripheral CurvesB. Aspheric LensesC. Spherical Base Curve and Toric Peripheral CurvesD. Toric Base CurvesE. Prism Ballasted Front Surface CylindersF. Hydrogel Torics
CHOOSING THE TYPE OF LENS TO USECorneal 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 cornealtoricity, therefore no cylinder power required in CLExample: -2.50 -1.50 x 180 spectacle Rx43.50 @ 180; 45.00 @ 090 K-readingToric hydrogel lens-need toric lens since there is no power in the tear layer
CHOOSING THE TYPE OF LENS TO USECorneal cylinder with little or no spectacle cylinder-use spherical hydrogel lens(rigid lens would induce residual astigmatismdue to a toric tear lens)Example: -3.00 -0.25 x 180 spectacle Rx43.00 @ 180; 44.25 @ 090
CHOOSING THE TYPE OF LENS TO USECorneal cylinder =>2.00 D. and corneal cylinder does not equalspectacle 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 lensesusually give better vision (rotation of the RGP usuallyhas insignificant effect on vision).
CHOOSING THE TYPE OF LENS TO USECorneal 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 hydrogel2. prism ballast front cylinder
CHOOSING THE TYPE OF LENS TO USEWith-the-rule corneal toricity and against-the-rule residualastigmatism with a non-flexing lens use:1. thin RGP which will flex and correct the residualastigmatism.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. theresidual astigmatism would be corrected
Using Spherical Base Curve RGP Lenses on Toric CorneasA. Advantages:1. simple2. inexpensiveB. Disadvantages:1. Lenses may not center well2. May be excessive movement3. May cause corneal distortion4. Fluctuations in vision-because of lensmovement and decentration5. Lens flexure
1. If corneal toricity less than 2.00 D. then spherical lensis lens of choice2. On 2 to 3 D. corneal toricity may be able to use sphere3. On K fit- results in excessive movement and decentration4. 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 onlid position6. Peripheral curves- same as standard spherical lenses.Using Spherical Base Curve RGP Lenses on Toric Corneas
FITTING A TORIC CORNEA WITH A SPHERICAL BCROn 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 andrides low-lens may be easily lost from eyedue to edge catchinglens edge due to stand-off
FITTING A TORIC CORNEA WITH A SPHERICAL BCRSpherical base curve on a 3.00 D.toric cornea. Horizontal band oftouch.Lenses fitted this flat typicallyride ride high if pulled up by theupper lid or drop and ride low.
FITTING A TORIC CORNEA WITH A SPHERICAL BCRLens 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 cornealtoricityExample: 43.00/44.50 cornea 43.50 base curve lens-Can cause corneal distortion with spectacle blur
FITTING A TORIC CORNEA WITH A SPHERICAL BCRBest compromise fit is one wherelens base curve is steeper than Kby about 1/3 of corneal toricity.For example if K’s are 42.00/45.00,then lens base curve should be43.00 D. This gives a “dumbell” or“H” pattern.This lens is fitted too steep withlens resting on cornea at theperipheral 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 zonediameter (OZD) as you would for a spherical lens.-OAD determination based on lid positions-OZD based on lens diameter and pupil sizeFITTING TORIC BASE CURVE RGP LENSES
FITTING TORIC BASE CURVE RGP LENSESBase curve radii selection:a. select the flat meridian of lens to match flat corneal meridian-with a large diameter-large optical zone lens you mayneed to go 0.25 D flatter than flat K and with asmall lens you may have to go 0.25 or 0.50 D. steeperthan flat K.b. Select the steeper meridian of the base curve to givethe CL about ¾ of the toricity of the cornea.Example: K-readings: 43.00 @ 180; 47.00 @090CL 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 aspherical lens the SCR is 1.5 mm flatter than the BCRdo the same for the toric BCR lensKeep difference between meridians of the SCR the same as BCR-For example if BCR 7.70/7.30 mm then SCR maybe9.20/8.80 mm.This keeps the OZD round and secondary clearance even.FITTING TORIC BASE CURVE RGP LENSESPeripheral (secondary) curve selection:
FITTING TORIC BASE CURVE RGP LENSESFluorescein pattern of a toric BCR lens on a toric corneashould look like a spherical lens on a spherical cornea.7.6/8.0 BCR on toric corneaEven fluorescein patternToric base curve lens withboth meridians too steep.Try next lens 0.50 D. flatterin each meridian.
FITTING TORIC BASE CURVE RGP LENSESToric 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. Lookslike a spherical lens on a low toricity cornea with a centralband of touch.FITTING TORIC BASE CURVE RGP LENSES
Using diagnostic lenses to fit toric corneas:a. can use spherical BCR diagnostic lenses to determinefit on flat meridian and use 3/4 rule for steepmeridian.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 bestFITTING TORIC BASE CURVE RGP LENSES
Spherical Power Equivalent (SPE) Toric Base Curve LensesDesign:-toric base curve lens with a front toric to give sphericalpower effect when on the eye.-in air the cylinder power will be the same as thedifference in the base curve toricity specifiedin diopters.-example: base curve toricity 42.00/46.00;power plano/-4.00 in air.FITTING TORIC BASE CURVE RGP LENSES
FITTING TORIC BASE CURVE RGP LENSESSPE 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 curves39.00/41.00 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 9.7/9.4Lenses designed to have an axial edge lift of 0.12 mm in the flat meridian
Spherical Power Equivalent (SPE) Toric Base Curve LensesAdvantage of SPE lenses:-corrects patients cylinder when the spectacle cylinderequals corneal toricity-example: K-readings: 43.00/46.00;spectacle cylinder -3.00 D.-if lens rotates on eye it has no detrimental effect onpatients vision-tear lens compensates.-ideal for diagnostic lenses since it allows for easy,accurate over-refractions.FITTING TORIC BASE CURVE RGP LENSES
FITTING TORIC BASE CURVE RGP LENSESWith 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 thatusing the diagnostic lens and over-refraction. Both shouldgive same lens power to order.
Determining the lens power to order in a toric BCR lensUsing the spectacle Rx and K-readings43. 00K-READING46.5043.50BASE CURVE-0.50+0.50LACRIMAL LENS-0.50+0.50LACRIMAL LENS-6.50-2.00SPECT. RX-6.00-2.50CL RX47.0043.00
Mandell-Moore form for calculating bitoric lens power.Available on the RGPLI web site: http://www.rgpli.org
45.0045.00DIAGNOSTIC CL BC46.5043.50BC TO BE ORDERED+1.50-1.50CHANGE IN LL P0WER-3.00-3.00DIAG. LENS POWER-1.50-1.00OVER-REFRACT. CHANGE IN LL P0WER+1.50-1.50-6.00-2.50CL POWERDetermining lens power using a diagnostic lens andover-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 LENSESUSING K-READINGS AND SPECTACLE RXEnter the requested values in the blank (white) cells.Flat K Steep KK-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 -0.75CL Rx Flat BCR (D.) Power Steep BCR (D.) Power45.25 -4.97 48.25 -6.96Flat 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 meridianDiagnostic Lens BC (D.) 44.00 44.00Power of diagnostic lens* -3.00 -3.00 *enter total power in each meridianOver-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 46.00This spreadsheet program can be downloaded from:http://www.opt.indiana.edu/lowther/index.htm
SUMMARY OF FITTING TORIC BASE CURVE RGP LENSESUsing K-readings and Spectacle Rx1. 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 powera) calculate using power crossesb) Use spreadsheet programc) Mandell-Moore form5. Order lensCan 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 LENSESUsing 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 inlens4. Evaluate fluorescein pattern-change diagnostic lens ifnecessary to get best fit.5. Do an over-refraction6. Calculate lens power7. Order lens