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Astigmatism revisited: A case report
Clinical Article
Astigmatism Revisited: A Case Report Langis Michaud,
OD, FAAO,
This report presents a specific case of anisometropia: the right eye showing a myopic condition with a slight amount of ugainstthe-n& astigmatism, with the I.efteye with hyperopia and a large amount of with-the-& astigmatism. The main interest in such a case is how to fit both eyes properly using the materials and techniques availabk. Four different types of lenses are considered. In this particular case, a bitoric lens with toroidal periphery represents the most adequate altemdtive to meet all the clinical standards of a good fit. Keywords: Comeal astigmatism; back-toric RGP lens; sphericalpower-effect bitoric; residual astigmatism
Every practitioner wants to fit the most difficult cases. A large amount of cornea1 astigmatism in one eye is such a case.
Methods We have selected one particular case of over-threediopters monocular cornea1 astigmatism. It represents a type of case we see in our office several times each year. Our purpose in this paper is to evaluate four types of lenses and to compare their clinical performance. To make the comparison, we have selected the same material for each lens, respecting the same diameter and similar peripheral curves, lenticular design, and standard thickness between 0.20 and
Address reprint requests to Dr. Langis Michaud at the Carrefour La Pocatiere CP 577, 601, lere rue, bureau 400, LaPocatiere GOR 120, Canada. for publication
0.22 mm. The base curve and the power vary according to the appropriate philosophy of fitting, and the necessary adjustments are made to the other parameters. All the lenses have been made by the same laboratory according to our specifications. The fitting was made in the same afternoon by the same practitioners. The evaluation was made by biomicroscopy, overkeratometry for flexure, and overrefraction for visual acuity. The clinical value of each lens was rated according to the following factors: centration of the lens, lag of the lens, visual acuity, comfort, and fluorescein pattern.
Background
Introduction
Accepted
and Jacques Sevigny, OD, FAA0
June 15, 1992.
0 1992 Butterworth-Heinemann
C.S., a 38-year-old Caucasian female, is a professional. She was seen for an examination of her contact lenses in May 1991. She had been wearing R.G.P. lenses for more than 10 years, and for 3 months, she has been having some discomfort from her left lens. She was still wearing her lenses, but for not more than 8 hours a day. Her eyes became infected and irritated after several hours of wear. At the time of her visit, she was under no medication. Her family health and ocular history were both negative.
Clinical Findings The refractive results were stable in comparison with previous findings: OD - 1.75 - 0.50 x 110 (20/20); OS + 1.00 - 3.25 X 25 (20140 no improvement with a pinhole). Binocularity was unremarkable except for the Worth Dot test where she demonstrated a reduced stereoacuity (4/9). Ocular health was within normal limits. The most important findings were those related to the contact lenses. The parameters of the lenses were the following:
ICLC, Vol. 19, September/October
1992
215
Clinical Articles
Material Base curve (mm) Diameter (mm) Power (diopters) Visual acuity
OD
OS
Boston II 7.88 8.6 - 2.25 20/20 - 1
Boston II 8.1017.62 8.6 + 1.50 20/20 - 1
Biomicroscopic evaluation indicated that the left lens had very little movement on the cornea with poor lag and a steep fluorescein pattern. The right lens fluorescein pattern indicated an on-K fit with nearly 2 mm movement with a blink. A large circular area of staining was present under the peripheral curves of the left lens and a diffuse SPK was present. Some dessication was present in the 3-9 o’clock area of the right cornea. There was inferior positioning of the left lens (Figure I) and an interpalpebral positioning of the right lens. No other negative signs were identified; the rest of the structures being evaluated were within normal limits, except for a moderate amount of deposit, principally due to mascara, on both lenses. After the withdrawal of the lenses, we found these results:
OD
OS
42.62 (7.92) at 90 43.75 (7.72) at 180 11.5
42.25 (7.99) at 110 45.25 (7.46) at 20 11.5
(mm) Pupillary diameter
5
5
(mm) Palpebral aperture
11.5
11.5
Over 15
Over 15
Keratometry Cornea1 diameter
(mm) B.U.T. (second) Lens surface
lipids + + organic deposits + mascara + + +
ICLC, Vol. 19, September/October
sign, (3) a back-surface toric, spherical front surface, and (4) a spherical-power-effect bitoric lens. Discussion For the right eye, the clinical data showed a slight against-the-rule cornea1 astigmatism with myopia. Two options are possible to reach a good correction of the ametros pia: First, spherical on-K fit can be tried, but such a fit will tend to eliminate the with-the-rule toricity and generate a higher amount of against-the-rule residual astigmatism.’ This type of fitting needs a prism-ballasted lens with an anterior toric design.6 In this case, the prism might disrupt the patient’s binocularity. ’ For these reasons, this design was dismissed. Second, a steeper-than-K fit can be tried. That kind of fit performs well with an interpalpebral design and with some flexure of the lens to minimize the residual astigmatism. 7 A secondegeneration material gives better flexure control rather than does the third-generation material, which is too flexible and can reduce the visual stability. 5s,9 We chose this latter technique to fit the right eye with good results (Figure 2). For the left eye, four different techniques were tried. Each of them is valuable, but obviously, their application varies with each case.
Same
From these evaluations, we diagnosed a poor fit of the left lens, being too tight on the cornea and probably too thick to permit a good oxygenation of the cornea in regard to the material’s Dk. The tear pump was insufficient to be an important factor and, thus, a metabolic diffuse SPK was present that we felt created the discomfort. Recent reports and new materials led the authors to consider a number of theories to fit such astigmatic corneas. iJ( To refit our patient, some of the following designs could be considered: (1) a spherical lens, (2) a standard bitoric de-
216
Figure 1. Original decentered back-toric left lens.
1992
Figure 2. Right lens. Against-the-rule fitting with steeper than K base curve.
Astigmatism revisited: Michaud and Sevigny
(I) SphericalLens A spherical base-curve lens has been reported as successful to correct low-to-moderate amounts of astigmatism,“4 even with as much as 5 diopters of astigmatism. i” These reports are often associated with PMMA lenses or secondgeneration RGP lenses such as Boston II. The technique is simple and easy to understand and to perform. In fact, anyone can fit 2-to-3 diopters of astigmatism in almost the same way as for a spherical myope. Unfortunately, these advantages are outweighed by several problems: The lensto-cornea relationship does not respect the comeal topography. This is visible by an excessive bearing in the flat meridian, when fluorescein is applied.” Sometimes, that bearing can create mechanical trauma to the epithelium of the comea.4 In our case, such a lens showed this bearing, leading to the characteristic dumbbell-shape pattern (Figure 3). Other disadvantages include lens decentration and, sometimes, failure to achieve good comfort. The use of a third-generation polymer with a spherical lens on a 3-diopter-astigmatic cornea brings the problem of increased flexibility. 6 In general, the higher the Dk, the greater the flexure. 7 A high amount of flexure can result in optical problems where vision fluctuates and residual astigmatism prevails.’ Some authors”4’5 suggested increasing the thickness of the lens to eliminate the excessive flexure, but this compromise brings major concerns about the health of the cornea, especially edema and cornea1 curvature changes. Finally, third-generation materials are found more sensitive to warpage, which tends to alter the lens properties on a long-term basis. (2) Buck-Surface Toric The next alternative to fitting an astigmatic patient is a back-surface toric lens. The design is relatively easy to make: Two curvatures on the posterior surface of the lens are designed to correspond to the curvature of the cornea, in order to respect its contour and shape. This design represents the first choice of lenses for many practitioners in
Figure 3. Spherical lens on toroidal cornea with fluorescein pattern.
Quebec, even for a large amount of astigmatism. There are many different approaches to determining the base-curve values. i2-15 The most popular one here is the Brazeau techusing a rule-of-thumb nique. 3,16 Brazeau recommends called the two-third rule to compensate for the power loss at the lens-tear interface. In fact, it is easy to demonstrate that, for a lens with an index of 1.49, its power, at the surface, is 1.45 greater. l6 The flatter curvature is usually 0.50-0.75 flatter than the steeper meridian. This allows good tear exchange, especially in the vertical meridian. The other curvature is determined by the astigmatism in the spectacles: 2/3 of the amount found in the astigmatic power of the spectacles equals the difference allowed between the two curvatures (in diopters). This method appears to work best with a small-toaverage amount of astigmatism and its success has been further enhanced by the recent oval design described by Brazeau3 That contribution to the design of this type of lens permits a regular and equivalent thickness around the lens, which tends to create a better tear exchange and, thus, better health for the cornea. It also results in better centration of the lens according to the clinical trials of Brazeau.3 Few problems were pointed out with such lenses. For a higher amount of astigmatism, lens bearing and flexure can become significant and generate the same problems as a spherical base curve design.” A misalignment of the lens will also create a cylindrical power effect because of its off-axis position. Some recent concerns were also noticed with respect to the rotation of these lenses, which seems to be more important with low astigmatism and can create asthenopia and discomfort.” Finally, this method is not very useful with high Dk materials where the flexure is difficult to compensate. l6 (3) Spherical Power Effect Bitoric When the amount of the cornea1 and refractive astigmatism are almost the same, a lens can be designed to act like a sphere on the eye. Sarver et al. is described such a lens in RGP materials in 1985. The main advantage of this lens is that it can rotate on the cornea without affecting acuity. This technique, while optically and technically complex, offers the same simplicity and ease of fitting as do the previous ones. The practitioner is concerned with the fitting itself, the position of the lens, its movement, and the analysis of the fluorescein pattern, rather than with the mechanica1 procedures and the calculations required by other methods. Ideally, it is a good idea to proceed with a diagnostic set, as designed by Sarver et al. ‘s in the 2-4 diopters range with 0.50 D b ase-curve increments. In this case, we ordered two lenses with a slight oval design of 0.2 mm to balance the thickness of the lens in the steep and flat meridians The bitoric design negates a part of the thickness induced by the minus axis. The difference between our two lenses was in the design of the peripheral curves: The first lens was designed with spherical peripheral curves, and the second one, with toroidal ones. The spherical curves were
ICE,
Vol. 19, September/October
1992
217
Clinical Articles made with regular tools after the lens was released from crimping. The toroidal peripheries were made on the generator with the lens crimped in order to achieve an evenwidth periphery. The lenses were also lenticularized to achieve a light minus carrier effect. Clinically, the bitoric design with spherical peripheries represents the least desirable design since the width of the periphery varies with the axis. Accordingly, bearing can occur in the narrow part of the periphery, whereas the wider part may offer a somewhat greater axial edge stand-off and subsequently more peripheral pooling (Figure 4). With the toroidal peripheral curves, the width and the standoff are the same around the lens. The fluorescein pattern looks like a spherical lens on a spherical cornea. One cannot tell the axis of the cornea1 astigmatism (Figure 5). This also results in good centration and a good comfort.
Results (A) The Spherical Design First we tried the spherical Base curve (mm) Peripheral curves Optic zone (mm) Diameter (mm) Material Power (diopters) Thickness (mm)
lens. Its parameters
were
7.98 0.2/11.50; 0.2/9.50; 0.2/ 9.00 (total: 0.6 mm) 8.3 9.5 S.G.P. II (SA/Dk: 39.5 at 35°C) + 1.25 0.20
This lens resulted in considerable decentration, positioning under the upper lid and more than desired with the Korb technique.6 The lag was higher than normal: more than 2 mm. The fluorescein pattern indicated an astigmatic pattern and the visual acuity was 20/30. Because of the position of the lens, which was displaced, and because of the poor acuity, we rejected it. Another factor to consider is the flexure of the lens. S.G.P. II is somewhat more flexible than the Boston IV lens but gives better oxygen-
Figure 5. S.P.E. bitoric with oval optical design and toroidal periphery. ation of the cornea, which is important.4 It is also less flexible than FSA materials and more resistant to deposits, another important factor in this case. With a spherical lens, a lot of flexure can occur during a blink. This is enough to reduce the visual acuity significantly and we feel that this is unacceptable. The comfort was good. (B) The Back-Surface Toric Lens Following
are the specifications
Base curve (mm) Peripheral curves Optic zone (mm) Diameter (mm) Material Power (diopters) Thickness (mm)
for this lens:
7.9817.62 0.3/11.50 0.2/9.5 0.2/ 9.50 (total: 0.7 mm) 8.1 9.5 S.G.P. II + 1.25 0.20
This lens positioned high under the upper lid with some rotation during blinking. The rotation gives fluctuating vision when the lens is off-axis and then rotates back to its previous position.‘7 Visual acuity was 20/30with the lens. The fluorescein pattern showed a partial astigmatic pattern, which indicates that the toricity was slightly undercorrected (Figure 6). That contributes to reducing the visual acuity. The lag was normal (between 1.5 and 2 mm). The lens is thinner and, thus, the oxygenation is better. The comfort was better than with the bitoric lenses. We feel that this is a direct effect of the oval design, which equalizes the thickness of the lens. Because of a rather poor vision given by the lens and the superior centration of the lens, this design was not chosen. (C) Spherical Power Effect Bit&c Lens The specifications Base curve (mm) Peripheral curves
Figure 4. S.P.E. bitoric with spherical P.C.‘s and oval design.
218
ICLC, Vol. 19, September/October
1992
of this lens are as follows: 8.0817.50 0.2/11.50 0.2/9.5 0.21 9.00 (total: 0.6 mm)
Astigmatism revisited: Michaud and Sevigny lens represents the best choice to refit the patient particularly since it gave good comfort.
with,
Conclusion This patient was fitted with four different lens designs. Each was designed to maintain the best visual acuity, comfort, and ocular health. According to our clinical evaluation, the Sarver SPE lens, as modified, was the most accurate lens for a patient with a large amount of cornea1 toricity. This is true for the SA material S.G.P. and should be true for most RGP materials. However, it may be different with other materials. A number of parameters could change from one patient to the next and require changes in design. These may include
Figure 6. Back-toric lens and oval design.
Optic zone (mm) Diameter (mm) Material Power (diopters) Thickness (mm)
a.3 9.5 S.G.P. t-1.75 0.22
??amount
of cornea1 astigmatism
??refraction
II -3.25
??interpalpebral
x 25
This is an excellent lens to prescribe, as many authors have indicated. 1~4~9~‘2~13~15~18 Clinically, we have observed good centration of the lens, in line with Korb’s fitting technique, 6 i.e., just under the upper lid, with a normal lag of 1.5 mm. This position is easier to achieve with a plus prescription when a minus carrier is ordered. This lens gave good visual acuity of 20/30, improving to 20125 + with an overrefraction of - 0.75. The only problem with this lens was with the periphery clearance as observed with the fluorescein pattern. A bitoric lens with a spherical periphery tends to create an uneven edge lift around the lens (Figure 4), which is true for small amounts of cornea1 astigmatism or with small diameter lenses. In this case, the diameter of 9.5 mm, which was selected in order to have a stable lens, and the 3 diopter cornea1 astigmatism caused a peripheral touch in the flat meridian and good standoff in the steep meridian. The central fluorescein pattern appeared to be almost spherical. Finally, the lens was uncomfortable. For these reasons, we did not use this lens.
??thickness
aperture of the lens
??materials ??lid
tension of material ??diameter of the lens.
??flexure
To have some success with the Sarver technique, we must have a patient with almost the same amount of corneal toricity as of cylindrical refraction and an appropriate material with medium-to-high Dk, an acceptable flexure during the blink, and chosen according to the requirements of the patient (oxygenation, resistance to deposits, etc.). We believe that other improvements to the lens, such as the oval design, the toroidal peripheries, and minus carriers, have helped to bring the successful conclusion of this case.
Acknowledgments We thank Les Laboratories Blanchard for supplying lenses used during this study and especially Mr. Gilles Castonguay for his technical support.
References (D) The Spherical Power Effect Bitoric Lens (Toroidul P. c. ‘s) This lens is the same design as the previous one except the laboratory produced a toric periphery as explained previously. This improves the pumping of tears under the lens.16 With such a modified design, there is a better fluorescein pattern with even clearance around the lens periphery (Figure 5). Both the central and peripheral part of that pattern showed good alignment with good tear exchange. The lens was well centered as Korb recommends, the lag was normal ( 1.5 mm), and the visual acuity was 20/25 + with a power of + 1.00 - 3.25 X 25. All other specifications were the same as for the previous lens. This
1. Mandell RB: Contact Lens Practice, 3rd ed. Springfield, 2. 3. 4. 5. 6. 7.
IL, Charles C. Thomas, 1981. 918 DD. Lowther GE: RGP manufac&ing and fitting. ZCLC 1990;17(5):117. Brazeau D: An oval-shaped back surface toric RGP lens design. Contact Lens Forum 1991;16(3):2629. Bennett ES, Weissman BA: Clinical Contact Lens Practice. Philadelphia, J.B. Lippincott, 1991, Chaps. 20-27, 40-52. Andrasko GS: Residual astigmatism with various RGP materials. Contact Len5 Spectrum 1991;5:49-50. Bennett ES, Grohe RM: Rigid gas-permeable contact lenses. New York; Professional Press Books, 1986, 537 pp. Herman JP: Flexure of rigid contact lenses as function of base curve fitting relationship. ] Am Optom Assoc 1983;54(3): 209-2 13.
/UC,
Vol. 19, September/October
1992
219
Clinical Articles 8. Stevenson RW: Young’s modulus measurements of gas permeable contact lens materials. Optom Vis Sci 1991;68(2): 142-145. 9. Phillips A, Stone J: Contact Lenses, 3rd ed. London, Butterworths, 1989, 1016 pp. 10. Ghormley NR: Update on Polycon: It’s hard to beat. Contact Lens Forum 1980;5( 10):33-41. 11. Goldberg JB: Clinical application of toric B.C. contact lenses. Optom Weekly 1962;53(39):1911-1915. 12. Goldberg JB: Designing RGP toric and bitoric cornea1 lenses. Contact km Forum 1986;7:27-30. 13. Remba MJ: The A.B.C.‘s of toric C.L. fitting. Rev Optom 1962;99(24):25-30.
Langis
Michaud
received
his OD degree
14. Barr J: Toric G.P. hard C.L. design. ICLC 1984;11(11):652653. 15. Edwards KH: The calculation and fitting of toric lenses. Ophtkzl Opt 1982;16:106-114. 16. Brazeau D: Adjustment des lentilles toriques intemes et extemes en lentilles P.A.G. Universite de Montreal, 1985 (conference). 17. Lapierre M, Giasson C, Steben M: Evaluation de la rotation des lentilles comeennes rigides a courbure inteme. Can J @torn 1991;53(2):50-54. 18. Sarver M, Kame R, Williams CE: A biotoric gas permeable contact lens with spherical power effect. J Am @tom Assoc 1985;56(3):184-189.
from the University
of Mon-
treal in 1986. He is a Fellow of the American Academy of Optometry and is in private practice in Quebec, Canada. He has published articles in optometric journals, particularly in primary care and c.1. fields.
Jacques Sevigny is a graduate of the University of Montreal with a BSc and an OD degree. He is a member of the American Academy of Optometry, the Canadian and Quebec Associations of Optometrists, and the Canadian Contact Lens Society. He is a diplomate of the Cornea and Contact Lens Section of the American Academy of Optometry. He has published numerous articles in optometric journals and presented papers at many meetings.
220
ICLC, Vol. 19, September/October
1992
Astigmatism Revisited: A Case Report Langis Michaud,
OD, FAAO,
This report presents a specific case of anisometropia: the right eye showing a myopic condition with a slight amount of ugainstthe-n& astigmatism, with the I.efteye with hyperopia and a large amount of with-the-& astigmatism. The main interest in such a case is how to fit both eyes properly using the materials and techniques availabk. Four different types of lenses are considered. In this particular case, a bitoric lens with toroidal periphery represents the most adequate altemdtive to meet all the clinical standards of a good fit. Keywords: Comeal astigmatism; back-toric RGP lens; sphericalpower-effect bitoric; residual astigmatism
Every practitioner wants to fit the most difficult cases. A large amount of cornea1 astigmatism in one eye is such a case.
Methods We have selected one particular case of over-threediopters monocular cornea1 astigmatism. It represents a type of case we see in our office several times each year. Our purpose in this paper is to evaluate four types of lenses and to compare their clinical performance. To make the comparison, we have selected the same material for each lens, respecting the same diameter and similar peripheral curves, lenticular design, and standard thickness between 0.20 and
Address reprint requests to Dr. Langis Michaud at the Carrefour La Pocatiere CP 577, 601, lere rue, bureau 400, LaPocatiere GOR 120, Canada. for publication
0.22 mm. The base curve and the power vary according to the appropriate philosophy of fitting, and the necessary adjustments are made to the other parameters. All the lenses have been made by the same laboratory according to our specifications. The fitting was made in the same afternoon by the same practitioners. The evaluation was made by biomicroscopy, overkeratometry for flexure, and overrefraction for visual acuity. The clinical value of each lens was rated according to the following factors: centration of the lens, lag of the lens, visual acuity, comfort, and fluorescein pattern.
Background
Introduction
Accepted
and Jacques Sevigny, OD, FAA0
June 15, 1992.
0 1992 Butterworth-Heinemann
C.S., a 38-year-old Caucasian female, is a professional. She was seen for an examination of her contact lenses in May 1991. She had been wearing R.G.P. lenses for more than 10 years, and for 3 months, she has been having some discomfort from her left lens. She was still wearing her lenses, but for not more than 8 hours a day. Her eyes became infected and irritated after several hours of wear. At the time of her visit, she was under no medication. Her family health and ocular history were both negative.
Clinical Findings The refractive results were stable in comparison with previous findings: OD - 1.75 - 0.50 x 110 (20/20); OS + 1.00 - 3.25 X 25 (20140 no improvement with a pinhole). Binocularity was unremarkable except for the Worth Dot test where she demonstrated a reduced stereoacuity (4/9). Ocular health was within normal limits. The most important findings were those related to the contact lenses. The parameters of the lenses were the following:
ICLC, Vol. 19, September/October
1992
215
Clinical Articles
Material Base curve (mm) Diameter (mm) Power (diopters) Visual acuity
OD
OS
Boston II 7.88 8.6 - 2.25 20/20 - 1
Boston II 8.1017.62 8.6 + 1.50 20/20 - 1
Biomicroscopic evaluation indicated that the left lens had very little movement on the cornea with poor lag and a steep fluorescein pattern. The right lens fluorescein pattern indicated an on-K fit with nearly 2 mm movement with a blink. A large circular area of staining was present under the peripheral curves of the left lens and a diffuse SPK was present. Some dessication was present in the 3-9 o’clock area of the right cornea. There was inferior positioning of the left lens (Figure I) and an interpalpebral positioning of the right lens. No other negative signs were identified; the rest of the structures being evaluated were within normal limits, except for a moderate amount of deposit, principally due to mascara, on both lenses. After the withdrawal of the lenses, we found these results:
OD
OS
42.62 (7.92) at 90 43.75 (7.72) at 180 11.5
42.25 (7.99) at 110 45.25 (7.46) at 20 11.5
(mm) Pupillary diameter
5
5
(mm) Palpebral aperture
11.5
11.5
Over 15
Over 15
Keratometry Cornea1 diameter
(mm) B.U.T. (second) Lens surface
lipids + + organic deposits + mascara + + +
ICLC, Vol. 19, September/October
sign, (3) a back-surface toric, spherical front surface, and (4) a spherical-power-effect bitoric lens. Discussion For the right eye, the clinical data showed a slight against-the-rule cornea1 astigmatism with myopia. Two options are possible to reach a good correction of the ametros pia: First, spherical on-K fit can be tried, but such a fit will tend to eliminate the with-the-rule toricity and generate a higher amount of against-the-rule residual astigmatism.’ This type of fitting needs a prism-ballasted lens with an anterior toric design.6 In this case, the prism might disrupt the patient’s binocularity. ’ For these reasons, this design was dismissed. Second, a steeper-than-K fit can be tried. That kind of fit performs well with an interpalpebral design and with some flexure of the lens to minimize the residual astigmatism. 7 A secondegeneration material gives better flexure control rather than does the third-generation material, which is too flexible and can reduce the visual stability. 5s,9 We chose this latter technique to fit the right eye with good results (Figure 2). For the left eye, four different techniques were tried. Each of them is valuable, but obviously, their application varies with each case.
Same
From these evaluations, we diagnosed a poor fit of the left lens, being too tight on the cornea and probably too thick to permit a good oxygenation of the cornea in regard to the material’s Dk. The tear pump was insufficient to be an important factor and, thus, a metabolic diffuse SPK was present that we felt created the discomfort. Recent reports and new materials led the authors to consider a number of theories to fit such astigmatic corneas. iJ( To refit our patient, some of the following designs could be considered: (1) a spherical lens, (2) a standard bitoric de-
216
Figure 1. Original decentered back-toric left lens.
1992
Figure 2. Right lens. Against-the-rule fitting with steeper than K base curve.
Astigmatism revisited: Michaud and Sevigny
(I) SphericalLens A spherical base-curve lens has been reported as successful to correct low-to-moderate amounts of astigmatism,“4 even with as much as 5 diopters of astigmatism. i” These reports are often associated with PMMA lenses or secondgeneration RGP lenses such as Boston II. The technique is simple and easy to understand and to perform. In fact, anyone can fit 2-to-3 diopters of astigmatism in almost the same way as for a spherical myope. Unfortunately, these advantages are outweighed by several problems: The lensto-cornea relationship does not respect the comeal topography. This is visible by an excessive bearing in the flat meridian, when fluorescein is applied.” Sometimes, that bearing can create mechanical trauma to the epithelium of the comea.4 In our case, such a lens showed this bearing, leading to the characteristic dumbbell-shape pattern (Figure 3). Other disadvantages include lens decentration and, sometimes, failure to achieve good comfort. The use of a third-generation polymer with a spherical lens on a 3-diopter-astigmatic cornea brings the problem of increased flexibility. 6 In general, the higher the Dk, the greater the flexure. 7 A high amount of flexure can result in optical problems where vision fluctuates and residual astigmatism prevails.’ Some authors”4’5 suggested increasing the thickness of the lens to eliminate the excessive flexure, but this compromise brings major concerns about the health of the cornea, especially edema and cornea1 curvature changes. Finally, third-generation materials are found more sensitive to warpage, which tends to alter the lens properties on a long-term basis. (2) Buck-Surface Toric The next alternative to fitting an astigmatic patient is a back-surface toric lens. The design is relatively easy to make: Two curvatures on the posterior surface of the lens are designed to correspond to the curvature of the cornea, in order to respect its contour and shape. This design represents the first choice of lenses for many practitioners in
Figure 3. Spherical lens on toroidal cornea with fluorescein pattern.
Quebec, even for a large amount of astigmatism. There are many different approaches to determining the base-curve values. i2-15 The most popular one here is the Brazeau techusing a rule-of-thumb nique. 3,16 Brazeau recommends called the two-third rule to compensate for the power loss at the lens-tear interface. In fact, it is easy to demonstrate that, for a lens with an index of 1.49, its power, at the surface, is 1.45 greater. l6 The flatter curvature is usually 0.50-0.75 flatter than the steeper meridian. This allows good tear exchange, especially in the vertical meridian. The other curvature is determined by the astigmatism in the spectacles: 2/3 of the amount found in the astigmatic power of the spectacles equals the difference allowed between the two curvatures (in diopters). This method appears to work best with a small-toaverage amount of astigmatism and its success has been further enhanced by the recent oval design described by Brazeau3 That contribution to the design of this type of lens permits a regular and equivalent thickness around the lens, which tends to create a better tear exchange and, thus, better health for the cornea. It also results in better centration of the lens according to the clinical trials of Brazeau.3 Few problems were pointed out with such lenses. For a higher amount of astigmatism, lens bearing and flexure can become significant and generate the same problems as a spherical base curve design.” A misalignment of the lens will also create a cylindrical power effect because of its off-axis position. Some recent concerns were also noticed with respect to the rotation of these lenses, which seems to be more important with low astigmatism and can create asthenopia and discomfort.” Finally, this method is not very useful with high Dk materials where the flexure is difficult to compensate. l6 (3) Spherical Power Effect Bitoric When the amount of the cornea1 and refractive astigmatism are almost the same, a lens can be designed to act like a sphere on the eye. Sarver et al. is described such a lens in RGP materials in 1985. The main advantage of this lens is that it can rotate on the cornea without affecting acuity. This technique, while optically and technically complex, offers the same simplicity and ease of fitting as do the previous ones. The practitioner is concerned with the fitting itself, the position of the lens, its movement, and the analysis of the fluorescein pattern, rather than with the mechanica1 procedures and the calculations required by other methods. Ideally, it is a good idea to proceed with a diagnostic set, as designed by Sarver et al. ‘s in the 2-4 diopters range with 0.50 D b ase-curve increments. In this case, we ordered two lenses with a slight oval design of 0.2 mm to balance the thickness of the lens in the steep and flat meridians The bitoric design negates a part of the thickness induced by the minus axis. The difference between our two lenses was in the design of the peripheral curves: The first lens was designed with spherical peripheral curves, and the second one, with toroidal ones. The spherical curves were
ICE,
Vol. 19, September/October
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Clinical Articles made with regular tools after the lens was released from crimping. The toroidal peripheries were made on the generator with the lens crimped in order to achieve an evenwidth periphery. The lenses were also lenticularized to achieve a light minus carrier effect. Clinically, the bitoric design with spherical peripheries represents the least desirable design since the width of the periphery varies with the axis. Accordingly, bearing can occur in the narrow part of the periphery, whereas the wider part may offer a somewhat greater axial edge stand-off and subsequently more peripheral pooling (Figure 4). With the toroidal peripheral curves, the width and the standoff are the same around the lens. The fluorescein pattern looks like a spherical lens on a spherical cornea. One cannot tell the axis of the cornea1 astigmatism (Figure 5). This also results in good centration and a good comfort.
Results (A) The Spherical Design First we tried the spherical Base curve (mm) Peripheral curves Optic zone (mm) Diameter (mm) Material Power (diopters) Thickness (mm)
lens. Its parameters
were
7.98 0.2/11.50; 0.2/9.50; 0.2/ 9.00 (total: 0.6 mm) 8.3 9.5 S.G.P. II (SA/Dk: 39.5 at 35°C) + 1.25 0.20
This lens resulted in considerable decentration, positioning under the upper lid and more than desired with the Korb technique.6 The lag was higher than normal: more than 2 mm. The fluorescein pattern indicated an astigmatic pattern and the visual acuity was 20/30. Because of the position of the lens, which was displaced, and because of the poor acuity, we rejected it. Another factor to consider is the flexure of the lens. S.G.P. II is somewhat more flexible than the Boston IV lens but gives better oxygen-
Figure 5. S.P.E. bitoric with oval optical design and toroidal periphery. ation of the cornea, which is important.4 It is also less flexible than FSA materials and more resistant to deposits, another important factor in this case. With a spherical lens, a lot of flexure can occur during a blink. This is enough to reduce the visual acuity significantly and we feel that this is unacceptable. The comfort was good. (B) The Back-Surface Toric Lens Following
are the specifications
Base curve (mm) Peripheral curves Optic zone (mm) Diameter (mm) Material Power (diopters) Thickness (mm)
for this lens:
7.9817.62 0.3/11.50 0.2/9.5 0.2/ 9.50 (total: 0.7 mm) 8.1 9.5 S.G.P. II + 1.25 0.20
This lens positioned high under the upper lid with some rotation during blinking. The rotation gives fluctuating vision when the lens is off-axis and then rotates back to its previous position.‘7 Visual acuity was 20/30with the lens. The fluorescein pattern showed a partial astigmatic pattern, which indicates that the toricity was slightly undercorrected (Figure 6). That contributes to reducing the visual acuity. The lag was normal (between 1.5 and 2 mm). The lens is thinner and, thus, the oxygenation is better. The comfort was better than with the bitoric lenses. We feel that this is a direct effect of the oval design, which equalizes the thickness of the lens. Because of a rather poor vision given by the lens and the superior centration of the lens, this design was not chosen. (C) Spherical Power Effect Bit&c Lens The specifications Base curve (mm) Peripheral curves
Figure 4. S.P.E. bitoric with spherical P.C.‘s and oval design.
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of this lens are as follows: 8.0817.50 0.2/11.50 0.2/9.5 0.21 9.00 (total: 0.6 mm)
Astigmatism revisited: Michaud and Sevigny lens represents the best choice to refit the patient particularly since it gave good comfort.
with,
Conclusion This patient was fitted with four different lens designs. Each was designed to maintain the best visual acuity, comfort, and ocular health. According to our clinical evaluation, the Sarver SPE lens, as modified, was the most accurate lens for a patient with a large amount of cornea1 toricity. This is true for the SA material S.G.P. and should be true for most RGP materials. However, it may be different with other materials. A number of parameters could change from one patient to the next and require changes in design. These may include
Figure 6. Back-toric lens and oval design.
Optic zone (mm) Diameter (mm) Material Power (diopters) Thickness (mm)
a.3 9.5 S.G.P. t-1.75 0.22
??amount
of cornea1 astigmatism
??refraction
II -3.25
??interpalpebral
x 25
This is an excellent lens to prescribe, as many authors have indicated. 1~4~9~‘2~13~15~18 Clinically, we have observed good centration of the lens, in line with Korb’s fitting technique, 6 i.e., just under the upper lid, with a normal lag of 1.5 mm. This position is easier to achieve with a plus prescription when a minus carrier is ordered. This lens gave good visual acuity of 20/30, improving to 20125 + with an overrefraction of - 0.75. The only problem with this lens was with the periphery clearance as observed with the fluorescein pattern. A bitoric lens with a spherical periphery tends to create an uneven edge lift around the lens (Figure 4), which is true for small amounts of cornea1 astigmatism or with small diameter lenses. In this case, the diameter of 9.5 mm, which was selected in order to have a stable lens, and the 3 diopter cornea1 astigmatism caused a peripheral touch in the flat meridian and good standoff in the steep meridian. The central fluorescein pattern appeared to be almost spherical. Finally, the lens was uncomfortable. For these reasons, we did not use this lens.
??thickness
aperture of the lens
??materials ??lid
tension of material ??diameter of the lens.
??flexure
To have some success with the Sarver technique, we must have a patient with almost the same amount of corneal toricity as of cylindrical refraction and an appropriate material with medium-to-high Dk, an acceptable flexure during the blink, and chosen according to the requirements of the patient (oxygenation, resistance to deposits, etc.). We believe that other improvements to the lens, such as the oval design, the toroidal peripheries, and minus carriers, have helped to bring the successful conclusion of this case.
Acknowledgments We thank Les Laboratories Blanchard for supplying lenses used during this study and especially Mr. Gilles Castonguay for his technical support.
References (D) The Spherical Power Effect Bitoric Lens (Toroidul P. c. ‘s) This lens is the same design as the previous one except the laboratory produced a toric periphery as explained previously. This improves the pumping of tears under the lens.16 With such a modified design, there is a better fluorescein pattern with even clearance around the lens periphery (Figure 5). Both the central and peripheral part of that pattern showed good alignment with good tear exchange. The lens was well centered as Korb recommends, the lag was normal ( 1.5 mm), and the visual acuity was 20/25 + with a power of + 1.00 - 3.25 X 25. All other specifications were the same as for the previous lens. This
1. Mandell RB: Contact Lens Practice, 3rd ed. Springfield, 2. 3. 4. 5. 6. 7.
IL, Charles C. Thomas, 1981. 918 DD. Lowther GE: RGP manufac&ing and fitting. ZCLC 1990;17(5):117. Brazeau D: An oval-shaped back surface toric RGP lens design. Contact Lens Forum 1991;16(3):2629. Bennett ES, Weissman BA: Clinical Contact Lens Practice. Philadelphia, J.B. Lippincott, 1991, Chaps. 20-27, 40-52. Andrasko GS: Residual astigmatism with various RGP materials. Contact Len5 Spectrum 1991;5:49-50. Bennett ES, Grohe RM: Rigid gas-permeable contact lenses. New York; Professional Press Books, 1986, 537 pp. Herman JP: Flexure of rigid contact lenses as function of base curve fitting relationship. ] Am Optom Assoc 1983;54(3): 209-2 13.
/UC,
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Clinical Articles 8. Stevenson RW: Young’s modulus measurements of gas permeable contact lens materials. Optom Vis Sci 1991;68(2): 142-145. 9. Phillips A, Stone J: Contact Lenses, 3rd ed. London, Butterworths, 1989, 1016 pp. 10. Ghormley NR: Update on Polycon: It’s hard to beat. Contact Lens Forum 1980;5( 10):33-41. 11. Goldberg JB: Clinical application of toric B.C. contact lenses. Optom Weekly 1962;53(39):1911-1915. 12. Goldberg JB: Designing RGP toric and bitoric cornea1 lenses. Contact km Forum 1986;7:27-30. 13. Remba MJ: The A.B.C.‘s of toric C.L. fitting. Rev Optom 1962;99(24):25-30.
Langis
Michaud
received
his OD degree
14. Barr J: Toric G.P. hard C.L. design. ICLC 1984;11(11):652653. 15. Edwards KH: The calculation and fitting of toric lenses. Ophtkzl Opt 1982;16:106-114. 16. Brazeau D: Adjustment des lentilles toriques intemes et extemes en lentilles P.A.G. Universite de Montreal, 1985 (conference). 17. Lapierre M, Giasson C, Steben M: Evaluation de la rotation des lentilles comeennes rigides a courbure inteme. Can J @torn 1991;53(2):50-54. 18. Sarver M, Kame R, Williams CE: A biotoric gas permeable contact lens with spherical power effect. J Am @tom Assoc 1985;56(3):184-189.
from the University
of Mon-
treal in 1986. He is a Fellow of the American Academy of Optometry and is in private practice in Quebec, Canada. He has published articles in optometric journals, particularly in primary care and c.1. fields.
Jacques Sevigny is a graduate of the University of Montreal with a BSc and an OD degree. He is a member of the American Academy of Optometry, the Canadian and Quebec Associations of Optometrists, and the Canadian Contact Lens Society. He is a diplomate of the Cornea and Contact Lens Section of the American Academy of Optometry. He has published numerous articles in optometric journals and presented papers at many meetings.
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