Toric RGP base curve stability

Clinical Article

Toric RGP Base Curve Stability Bruce A. Bridgewater,

The use of toric RGP fitting sets should provide pmctitioners with valuable information for initial kns designs. This study evaluated the hydration/dehydration stability of various RGP toric fitting sets. A prism ballasted and a back su&ce toric set was designed from FluoroPenn 30 material. Bitoric sets were made from both Fluorol’em 30 and FluroPerm 60 materials. Although statisticaUy signijicant, no clinically significant base curve changes were noted with any of the sets. Practitioners could, therefore, count on stable toric diagnostic lens parameters during trial kns fittings. Keywords: lenses

RGP contact lenses; base curve stability; toric contact

Introduction Toric rigid gas permeable (RGP) fitting can be one of the most challenging, yet rewarding, opportunities contact lens practitioners can face. Various theoretical fitting models have been presented in the literature. Some advocate treating each principal toric lens meridian as an independent lens. lp2 Other theories utilize the differences in indexes of refraction between tear film and lens.3 As with spherical lens fitting, however, the use of a trial lens should provide the most accurate means of initial lens design. Practitioners must, however, feel certain that the diagnostic lens parameters are consistent and accurate. Also, diagnostic lenses should be stored dry to ensure sterility and avoid warpage should the storage solution dehydrate or escape from the lens case.’ Various reports concerning hydration effects on RGP

Address reprint requests to Dr. Bridgewater at Paragon Optical, 947 East Impala Avenue, Mesa, AZ 85204, USA. Accepted

for publication

February 1991.

0 1991 Butterworth-Heinemann

OD

lens parameters have been presented in the literature. Drs. Barr and Hettle? performed hydration studies on a series of Boston II lenses. They found that high minus lenses flattened greater than low minus and plus lenses. Also, all lenses returned to their pre-hydration state within 1 hour after lens removal. Another study comparing a variety of materials (PMMA, silicone acrylates, and fluorosilicone acrylates) found that all plus lenses except PMMA steepened. All of the minus lenses flattened, with the high minus exhibiting greater flattening. The author suggested that most labs cut lenses steeper to compensate for hydration included flattening.6 This study was designed to test the base curve stability upon hydration of prism ballasted, back surface toric and bitoric fitting sets made from fluorosilicone acrylate RGP materials. To ensure the accuracy of a toric RGP trial fitting, practitioners need to know the stability of their fitting sets.

Methods A prism ballasted fitting set was produced to the specifications as listed in Table 1. The set was fabricated using standard lathing procedures and made from FluoroPerm 30 material. The base curve and lens power tolerances were 2 0.05 mm and Ifr 0.50 D, respectively, for all trial sets. The toric fitting sets were manufactured using standard lathing and crimping techniques for toric lens designs. The back surface toric set was designed to the specifications as listed in Table 2 and was made from FluoroPerm 30 material. Two bitoric fitting sets were generated as listed in Table 3. One set was produced from FluoroPerm 30 and another, material.

for comparative

After manufacturing,

reasons,

from

FluoroPerm

60

all lenses were allowed to stabilize

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Clinical Artick Table 1. Prism Ballasted Set

BC (mm)

DlA/OZD (mm)

7.11 7.18 7.26 7.34 7.42 7.50 7.58 7.67 7.76 7.85 7.94 8.04 8.13 8.23 8.33

8.817.6 8.817.6 8.817.6 9.017.6 9.017.6 9.217.8 9.217.8 9.217.8 9.217.8 9.217.8 9.217.8 9.417.8 9.4i7.8 9.618.0 9.618.0

POWER (D) -

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

PRISM (D) 1.75 1.75 1.75 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.25 1.25 1.25 1.25

BC from 40.50 D to 47.50 D in 0.50 D steps. Apex edge thickness .I2 mm. FluoroPerm 30.

Table 2. Back Surface Toric Set

BC (mm)

DIA/OZD (mm)

POWER (D)

CT (mm)

7.4216.96 7.5017.03 7.5817.11 7.6717.18 7.7617.26 7.8517.34 7.9417.42 8.1317.58 8.2317.67 8.4417.85 8.5417.94

8.817.6

-2.50

9.017.6 9.017.6 9.217.8 9.217.8 9.217.8 9.217.8 9.417.8 9.618.0 9.618.0 9.618.0

-2.50 -2.50 -2.50 -2.50 -2.50 -2.50 -2.50 -2.50 -2.50 -2.50

.15 .16 .21 .21 .17 .17 .17 .17 .18 .I8 .18

BC from 39.50D to 48.50D in 0.50 D steps.

Bc difference 3.00 D. Power in flat meridian. FluoroPenn 30.

American Optical (Reichert Ophthalmic Instruments, Buffalo, NY) binocular radiuscope. All lenses were soaked in Barnes-Hind Gas Permeable Wetting and Soaking Solution. The lenses were then stored dry for at least 48 hours for a second dry state recording. Statistical analysis, utilizing the matched pairs t-test, was performed to determine the statistical significance of any base curve changes from baseline.

Results Figure 1 illustrates the mean base curve radius change through hydration of the prism ballasted lenses. At a P < 0.05 (95%) level, there was a statistically significant increase in base curve radius for all time periods including dehydration. A maximum mean increase of 0.0107 + 0.016 mm was noted after 48 hours of hydration. Figure 2 illustrates the mean base curve radius change of the toric base curve lenses in the steep and flat meridians. In the steep meridian, at the p < 0.05 level, there is a significant difference at all time periods except the dehydration reading. A maximum mean increase of 0.0291 2 0.0281 mm was noted at 48 hours. In the flat meridian at the P < 0.05 level, there was a significant difference at all time periods, except the 4-hour and dehydration readings. of note, although not statistically significant, was the radius steepening, upon dehydration, of both meridians. Figure 3 illustrates the mean base curve radius change of the FluoroPerm 30 bitoric lenses. At a p < 0.05 level, there is a significant change at all time intervals except dehydration in the steep meridian. A maximum mean radius increase of 0.0367 + 0.025 mm was noted at the 24-hour interval. In the flat meridian at the p < 0.05 level, there was a significant change only at the 12 and 48-hour intervals. Figure 4 illustrates the mean base curve radius change of the FluoroPerm 60 bitoric lenses. In the steep meridian at the P < 0.05 level, there was a significant difference between baseline and all time intervals except dehydration. The maximum radius increase of 0.0317 + 0.023 mm was

Table 3. Bitoric Set

BC (mm)

DIA/OZD (mm)

POWER (D)

7.4216.96

8.817.6

- 2.50/- 5.50

.15

-2.501-5.50 -2.501-5.50 -2.501-5.50 -2.50/-5.50 -2.501-5.50

.21 .17 .17 .18 .18

7.5817.11 7.7617.26 7.9417.42 8.2317.67 8.5417.94

9.017.6 9.217.8 9.217.8 9.6J8.0 9.618.0

CT (mm)

BCR

CHANGE

(mm)

0.05

0.04

-

0.03

-

0.02

-

BC from 39.50 D to 48.50 D in 1.00 D steps. Bc difference 3.00 D. FluoroPerrn 30 and FluoroPerm 60. for at least 48 hours prior to evaluation. Base curve measurements were made on all lenses in their dry state at 4, 12, 24, and 48-hour soaking intervals. These base curve measurements were done by one individual utilizing an

64

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4 HI75

12 HRS

HOURS

24 HRS

48 HRS

DEHYDRATION

OF HYDRATION

Figure 1. Changes in BCR with hydration Fluoroperm 30 prism ballast lenses.

Toric RGP Base CuTwe Srubi!Ay: Bridgewater BCR CHANGE (mm) ::I

7-1

-0.01

-0.02

4 HRS

12 HRS

24 HRS

48 HRS

DEHYDRATION

HOURS OF HYDRATION Steep meridian

0

Flat meridian

Figure 2. Changes in BCR with hydration Fluoropenn 30 toric BCR lenses. BCR CHANGE (mm)

1::

77

0.03

0.01 0.00 4 HRS

12 HRS

24 HRS

48 HRS

DEHYDRATION

HOURS OF HYDRATION Steep meridian

0

Flat meridian

Figure 3. Changes in BCR with hydration Fluoroperm 30 bitoric E?CR lenses. SCR CHANGE (mm)

1::

r-_-I

0.03 0.02 0.0 1 0.00 -0.01

I

’ 4 HRS

12 HAS

24 HRS

48 HRS

DEHYDRATION

agnostic fitting set to more accurately design toric RGP lenses, they must be assured constant parameters during testing. Concerns of instability could arise considering possible parameter changes with either dry or hydrated lens storage and hydration/dehydration effects with on-eye testing. This study illustrates that all of the theoretical fitting sets underwent statistically significant base curve radius changes upon hydration/dehydration. Of note, the largest changes were found in the steeper meridians. These meridians also had the higher minus powers in toric lens designs. These findings would then correlate with the findings reported by others. 5,6 that show a greater flattening effect with minus lens designs. However, no statistical analysis was performed to determine if the base curve changes in this study were significantly different in the steep versus flat meridians. Although statistically significant, the base curve changes noted in this study would have little, if any, clinical effect. In the most recent ANSI publication, the range of tolerance for base curve variation is kO.05 mm for RGP lenses. 7 Further, base curve changes less than 0.05 mm have been described as clinically acceptable8 and nominal.’ During this study, no mean base curve radius change was found to be greater than + 0.05 mm, and the vast majority of changes were significantly less. This study illustrates that diagnostic fitting sets could provide consistent parameters for clinical evaluations. Subtle parameter changes could, therefore, be made based upon the diagnostic lenses. Also, over-refractions should be stable and could accurately be incorporated into the final lens prescription. With these assurances, the use of toric RGP fitting sets should allow practitioners to “fine tune” the original lens prescription and minimize the margin of error found with theoretical lens designing. It must be stated that this study was designed to evaluate the stability of diagnostic lenses with parameters and materials chosen as clinically relevant by the author. It should not be concluded that parameters and materials, other than those studied, would be as clinically stable. Future studies would be necessary to generally accept all toric RGP diagnostic lens as stable, independent of design or material.

HOURS OF HYDRATION Steep meridian

0

Flat meridian

Figure 4. Changes in BCR with hydration Fluoroperm 60 bitoric BCR lenses.

noted at 12 hours of hydration. noted in the flat meridian.

No significant

changes were

Discussion Various reports have been published concerning RGP contact lens instability upon hydration.536 These reports have illustrated various parameter changes based upon material and lens power. If practitioners were to utilize a di-

Summary This study illustrated the stability of prism ballasted, back surface toric and bitoric FIuoroPerm 30 and FluoroPerm 60 RGP fitting sets under both hydration and dehydration conditions. Although statistically significant, no clinically significant base curve changes were found. Therefore, practitioners should feel confident that no clinically significant parameter changes would occur during trial fitting evaluations due to hydration/dehydration effects.

References 1. Henry VA, Bennett ES: Contact lenses for the difficult-to-fit patient. Contact Lens Forum, 1989;14(10):49-67.

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Clinical

Ad&

2. Kastl PR: Fitting bitoric and front toric rigid contact lenses. Contact Lens update 1989;8(5):77-80. 3. Maltzman BA, Koeniger E, Dabezies OH: Correction of astigmatism: hard lenses. Contact Lenses, The CLAO Guide to Basic Science and Clinical Practice, Second Edition. Boston,

Little Brown and Company, 50.1-50.29, 1991. 4. Snyder C, Campbell JB: Considerations in the maintenance of large RGP fitting sets. Contact Lens Spectrum 1990; 5(7\:37-39. 5. Barr JT, Hettler DH: Boston II base curve changes with hydration. Contact Lens Forum, 1984;9(8):65-67.

6. Walker F: Radical flattening-a laboratory enigma. Opiciun 1988;195:21-23. 7. American National Standard for Ophthalmic-Rigid Contact Lenses-Requirements. New York, American National Standards Institute, September 8, 1988. 8. Grohe RM: RGP problem solving. Identifying and diagnosing common RGP complications. Contact Lens Spectrum 1990;5(9):82-99. 9. Schwartz C: Radical flattenine and RGP Lenses. Contact Lens Forum 1986;11(8):49-52. -

Bruce A. Bridgewater, OD, received his degree from Ferris State University, College of Optometry, in 1983. He is the director of the Contact Lens Clinic at the Gary Hall Eye Surgery Institute and the director of clinical research at Paragon Optical. Dr. Bridgewater has lectured nationally and is a member of the Contact Lens Section of the American Optometric Association and the Contact Lens Association of Ophthalmologists.

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