Correcting Astigmatism with Contact Lenses John M. Laurent, OD, PhD William J. Benjamin, OD, PhD

Transcription

1 Correcting Astigmatism with Contact Lenses John M. Laurent, OD, PhD William J. Benjamin, OD, PhD Photo: Annie Chang, OD, and Dawn Lam, OD.

2 Before we start... Accurate refraction critical in most cases Vertex adjustment Spectacle power +/ D In any meridian Less minus / More plus in CLs Change in required focal length Plus power focal length Minus power focal length

3 Tyler s Quarterly

4 Vertex change - cylinder Plus Lenses Refraction in any meridian D Cylinder will increase at corneal plane Minus Lenses Refraction in any meridian D Cylinder will decrease at corneal plane SPR = x 180 SPR = 180, 090 CPR = 180, 090 CPR = x 180

5 Toric Soft CLs Power = Corneal plane refraction Standard cylinder powers -0.75D, -1.25D, -1.75D, -2.25D (-2.75) Up to -5.75D w/ Extended Range Custom most to D cylinder Rotational stability Critical with high cylinder Variable rotation may want to under-correct Worse with dry eye Often unreliable if rotation > 20 degrees Photo: Lindsay A. Sicks, OD, FAAO

6 GP Correction of Astigmatism Spherical GP lens on toric cornea Creation of lacrimal lens (LL) Front surface = CL BC Back surface = corneal shape (K) Neutralizes corneal surface New spherical optical surface Corrects all corneal astigmatism Assuming no lens flexure Rotation has no effect on optical correction

7 Residual Astigmatism Residual astigmatism = amount of astigmatism remaining through an optical correction. When CA = RA and IA = 0, there is no residual astigmatism w/ an inflexible GP CL, where RA = Refractive (total) Astigmatism CA = Corneal Astigmatism IA = Internal (lenticular) Astigmatism Residual astigmatism (over an inflexible CL) = internal (lenticular) astigmatism

8 GP Front surface toric lenses Spherical or near-spherical cornea Spherical base curve on contact lens Significant residual (lenticular) astigmatism Front surface toricity = residual astigmatism Orientation by prism ballast Possible comfort issue Market taken over by toric SCL Andrew Franklin and Ngaire Franklin - Optician

9 GP Correction of Astigmatism Spherical GP lenses on toric corneas may have limitations due to the amount of corneal toricity Excessive edge lift in steep meridian Lens may rock with blink, disturbing vision Lens may flex, creating residual astigmatism Spherical lens usually ok with < 2.00D corneal astigmatism Photo: GPLI

10 GP Back surface toric lenses Used when CA > 2.00 D Poor fit with spherical lens Lens flexure, residual astigmatism BC toricity to fit cornea Flat meridian close to alignment Steep meridian fit slightly flat calculated misfit Front surface toricity if needed (bitoric) Corrects residual astigmatism Orientation - toric BC aligns w/ toric cornea Photo: Laguna Eye Care

11 GP Back surface toric lenses Back surface toricity May be limited to BC / OZ May need to extend to PC s Dependent on topography Photos: Randy Kojima

12 Spherical Power Effect (SPE) Toric BC / Spherical front surface Cylinder power in GP lens (lensometer) = BC toricity (radiuscope) Toric SPE lens acts optically like a spherical GP lens masks corneal astigmatism but not internal astigmatism SPE lens can rotate without any change in cylinder power SPE lenses work only when RA = CA

13 SPE Lens - Example Spec Rx: x 180 (-1.00 / -4.00) K s: / 090 Choose CL BC: / LL: Plano / Required power in CL: / Cylinder power in CL = BC cylinder Rotate lens 90 degrees: CL Rx: / // / LL: / CL Rx + LL = / -4.00

14 Cylindrical Power Effect (CPE) Bitoric BC and front surface toricity Cylinder power equal BC toricity Lensometer Radiuscope Bitoric CPE lens masks corneal cylinder like SPE lens but has additional front surface cylinder to compensate for IA IA will rotate with CPE lens IA = difference between CPE and SPE IA usually small and rotation minimal

15 GP CL Optical Calculations GP Contact Lens Rx Power Calculation Radius of curvature in diopters (BCR typically in mm for CL order) Keratometry curvature based on n = Conversion of mm to diopters / mm = D (337.5 / 7.50 mm = 45 D) / D = mm (337.5 / 44 D = 7.67mm)

16 CL Optical Calculations Tyler s Quarterly

17 GP Rx Power Calculation Two methods: (1) Empirical calculation Refraction and K s Choose BC of GP CL (front surface LLP) Final CL Power = CPR LLP (2) Diagnostic lens and over-refraction Ideally, FCLP = DCLP + OR Often, FCLP = DCLP + OR - LLP SAM - FAP

18 GP Rx Power Calculation GP Contact Lens Example: CPR = x 180 K s = / 090 DCLBC = 7.85 mm (43.00 D) DCLP = DS OR = x 090 FCLBC = 7.80 mm (43.25D) What FCLP should be ordered?

19 GP Rx Power Calculation Estimate 1: From K s and Refraction FCLP = CPR LLP Horizontal LLP = FCLBC - K = = D Vertical LLP = = D FCLP(H) = (+0.25) = D FCLP(V) = (-0.75) = D FCLP = DS

20 Empirical Power Calculation REF K s FCLBC LL FCLP

21 GP Rx Power Calculation Estimate 2: Refraction Over Diagnostic CL FCLP = DCLP + OR ΔLLP = ( x 090) - ΔLLP ΔLLP = FCLBC DCLBC = = D FCLP(H) = (-0.75) (+0.25) = D FCLP(V) = (-0.50) (+0.25) = D FCLP = Either DS or DS

22 Diagnostic Lens and Over-refraction DCLBC DCLP OVR-REF FCLBC ΔLL FCLP

23 GP Rx Power Calculation GP Contact Lens Example: CPR = x 020 K s = / 110 DCLBC = 7.50 mm (45.00 D) DCLP = DS OR = x 020 FCLBC = 7.58 mm (44.50 D) What FCLP should be ordered?

24 Empirical Power Calculation REF K s FCLBC LL FCLP

25 Diagnostic Lens and Over-refraction DCLBC DCLP OVR-REF FCLBC ΔLL FCLP

26 Fitting GP Toric Lenses 1. Use spherical diagnostic lenses Helps determine flat BC and OA Flat meridian on-k or -0.25D FTK 2. Use SPE toric fitting set (corneal astigmatism) Select BC to fit flat K and 2/3 3/4 of CA Over-refraction (should be spherical) Compare to calculated power 3. Order initial lenses from refraction & K s Mandell-Moore Bitoric Fitting Guide Calculator on GPLI website

27 Using Diagnostic Lenses Same basic principles as spherical GP lenses Applied to two primary meridians Estimate degree of lid attachment May order OAD different from DCL Determine BC of first diagnostic lens BC = 0.25D-0.50D FTK for lid attachment BC = On-K for interpalpebral fit SPE BC = Flat K + 2/3 to 3/4 of corneal toricity (SPE DCLs usually Pl -2.00, or Pl -3.00)

28 Using Diagnostic Lenses Assess lens position and fluorescein pattern Change BC as needed to achieve acceptable central fit (central clearance) Estimate changes for optimal central fit Assess peripheral clearance Over-refraction, spherical and sph/cyl Design lens, including power, to order Use lab consultant as needed

29 Mandell-Moore Bitoric Lens Guide

30 Mandell-Moore Bitoric Lens Guide / Form or Calculator

31 Fitting GP Bitoric Lenses Case 1 Spec Rx: x 180 K s: / 090 Case 2 Spec Rx: x 180 K s: / 090

32 Case 1

33 Empirical Power Calculation M-M REF K s Choose FCLBC Flat = 0.25 flat Steep = 0.50 flat LL FCLP

34 Empirical Power Calculation - GPLI REF K s Choose FCLBC Flat = on-k Steep = 0.75 flat PL LL FCLP

35 GP Contact Lens Order M-M

36 Fitting GP Bitoric Lenses Case 1 Spec Rx: x 180 K s: / 090 Case 2 Spec Rx: x 180 K s: / 090

37 Case 2

38 Empirical Power Calculation M-M REF K s Choose FCLBC Flat = 0.25 flat Steep = 0.75 flat LL FCLP

39 Empirical Power Calculation - GPLI REF K s Choose FCLBC Flat = on-k Steep = 0.75 flat PL LL FCLP

40 GP Contact Lens Order M-M

41 Fitting GP Bitoric Lenses Case 3 Spec Rx: x 180 K s: / 090

42 Fitting GP Bitoric Lenses

43 Empirical Power Calculation M-M REF K s Choose FCLBC Flat = 0.25 flat Steep = 0.75 flat LL FCLP

44 Empirical Power Calculation - GPLI REF K s Choose FCLBC Flat = on-k Steep = 0.75 flat PL LL FCLP

45 Against-the-Rule, Oblique, and Interpalpebral Fits Against-the-Rule Corneas Vertical rocking not expected Start fitting On-K in both meridians Oblique Corneas Lens may not center with 2/3 to ¾ of CA Start fitting On-K in both meridians Interpalpebral Fits Lid attachment not feasible Start On-K or slightly steep in both meridians

46 (Final, OS: 2.50)