GP Lens Case Grand Rounds Troubleshooting Guide

Diagnostic Fitting for Spherical Power Effect Bitoric Lenses: Joel A. Silbert, OD, FAAO

Background

M.A. is a 28-year-old female, presenting to be fitted with contact lenses for the first time. She has worn glasses since childhood. She has thought about refractive surgery, but has been advised that she is not a good candidate. As such, the cosmetic benefits of contact lenses are desirable to help meet her needs. She is in good health, and takes no medications. She is a business person who spends a good deal of time on the computer and doing desk work.

Test Procedures, Fitting/Refitting, Design and Ordering (Part One)

Manifest Refraction:

OD: +2.00 -4.00 X 180 20/20-
OS: +2.50 -3.75 x 178 20/20-

Keratometry:

OD: 42.00 @ 180 /46.25 @ 090 (good mire quality)
OS: 41.75 @ 180 /45.75 @ 090 (good mire quality)

Slit Lamp Examination:

The cornea and adnexae are clear; no vascularization, staining, scarring, or any signs of ectasia or keratoconus.

Patient Consultation and Education

After the history and pre-fitting testing has been completed, it is important to discuss the options available with the patient. Although both custom soft toric and gas permeable (GP) toric contact lenses could be utilized in this case, M.A. was educated as to the likelihood that even a small mislocation off axis with a high-powered soft toric would lead to visual fluctuation and/or outright blurring, despite the advantages of better initial comfort compared to GP lenses.

GP toric lenses, conversely, would be able to provide a high quality of vision that would be stable, given the close approximation of corneal and refractive astigmatism. Additionally, GP lenses would be more economical in the long term compared to custom soft toric lenses, due to increased lens life and ease of cleaning.

M.A. was expected to be successful with GP toric lenses, and was educated as to the initial adaptive period of adapting to lens wear.

Test Procedures, Fitting/Refitting, Design and Ordering (Part Two)

Although toric GP lenses can be ordered empirically (for example, by the use of optical crosses or more easily with the Mandell-Moore Bitoric Fitting Guide available through the GPLI web site), the patient was instead fitted using a diagnostic SPE (Spherical Power Effect) Bitoric GP fitting set.

Having such a set makes for very rapid and accurate assessments, as one can observe the degree of lens alignment on the toric cornea using sodium fluorescein, as well as the lens position, movement, and peripheral clearance. Additionally, over-refraction can be performed quickly and accurately, once the fitting characteristics have been determined.

Looking at M.A.’s clinical data, it should be apparent that almost all the refractive cylinder is corneal. Whenever there is close agreement between refractive and corneal cylinder, and when their axes are similar, then optically a spherical lens is indicated. With an SPE bitoric fitting set, we have just the right tool with which to fit the toric cornea, as well as a lens that has been already rendered spherical by its very design.

[This is done by application of a front surface cylinder placed at the flat axis, which is equal but opposite to the amount of unwanted cylinder that is “induced” by the lens/tear-lens interface. The manufacturing laboratory calculates this value as approximately 40% of the amount of rear surface cylinder on the toric lens, and applies it as a front surface cylinder. The net result is a lens that fits the highly toric cornea well, but is also a lens that is optically spherical.]

Determining the Base Curve Radii (Guidelines):

Flat Meridian: Fit the lens on-K or slightly flatter than K
Steep Meridian: Undercorrect this meridian by about 1D to ensure adequate tear exchange and prevent a physiologically tight fit.

Since M.A. had 4D of corneal toricity, we only need to use 3D of cylinder on the back surface of our lens. Thus, the following toric base curve radii were chosen from the 3D SPE bitoric fitting set (Note: The right lens flatter base curve radius was fit “On K” the left lens flatter base curve radius was fit 0.25D flatter than “K”):

Note that the powers of the SPE diagnostic lens have 3D of “air” cylinder which is identical to the amount of toricity on the back surface of the lens. By definition, this constitutes a spherically optical lens.

It is best to have both a 2D and a 3D SPE fitting set. The 2D set is useful for patients with 2 to 3.25D of corneal toricity; The 3D set is useful for patients with corneal toricity of 3.50 to 5 diopters. With both sets, the practitioner can handle over 95% of any high astigmatic cases presenting in his or her office!

Fluorescein Pattern Analysis:

Fluorescein is applied and the diagnostic lens is evaluated (Use a Wratten yellow filter for the best pattern). Look for an alignment pattern, or one in which a low degree of toricity is present, as we have deliberately undercorrected the full amount of corneal cylinder. Should the pattern appear steep or with apical clearance, the lens should be removed and replaced with the next flatter lens in the fitting set.

If the lens appears flat or apical bearing, the next steeper lens in the fitting set should be used. If the natural flat cylinder axis appears to have reversed, then too much cylinder is in the diagnostic lens, and a lens with less cylinder should be applied. (SEE APPENDIX for Fluorescein images).

Determining the Final Power of the SPE Bitoric Lens:

Once the “best fit” has been determined, you can simply over-refract with loose spherical lenses from your diagnostic lens set. The visual acuity should be noted, which should be very good. If this is so, then simply add the over-refraction power to both meridians of the labeled diagnostic lens powers, and the final lens powers of the SPE lens have been determined.

In M.A.s case, the over-refraction was:
OD: +2.00 (20/20)
OS: +2.75 (20/20)

Thus, the final lens powers are:
OD: +2.00 / -1.00
FM SM
OS: +2.75 / -0.25
FM SM

Other Parameters:

Size dimensions are typically the same as for conventional GP lenses. A good diameter for many patients, and one that is also utilized in the fitting sets is 9.2/7.8mm but this can be modified based on lid aperture size, pupil considerations, etc. Peripheral curves are spherical when the back surface toricity is 3D or less and toric peripheral curves are recommended for cylinders above 3D for added rotational stability.

Final Lens Specifications for Patient M.A:

BCR = Base Curve Radius (mm)
SCR/W = Secondary Curve Radius/width (mm)
PCR/W = Peripheral Curve Radius/width (mm)
BVP = Back Vertex Power (D)
OAD/OZD = Overall Diameter/Optical Zone Diameter (mm)

Follow-Up Care/Final Outcome

The use of a 3D SPE bitoric GP lens provided M.A. with excellent visual acuity. The use of a topical anesthetic during the diagnostic evaluation allowed for good patient acceptance and high motivation, especially when visual acuity as good as she obtains with glasses was demonstrated, and reduced the tearing time lenses were applied to a non-adapted wearer. M.A. had an uneventful adaptation to her new lenses. Evaluated two weeks later, she was delighted with both her vision as well as comfort of her lenses. Slit-lamp examination showed excellent physiological tolerance OU, with full day wearing time achieved.

Discussion/Alternative Management Options

The use of SPE bitoric diagnostic fitting sets is very helpful to clinicians who desire to help patients with high corneal and refractive astigmatism. Visual acuity is typically sharper and more stable than when attempting to fit a high-powered soft astigmatic lens (even if an SPE toric lens were to rotate off axis, which is unlikely, there would be no decline in vision, as the compensating front surface cylinder also rotated with the lens, thus negating effects of the “induced cylinder” in any position that the lens assumes).

Secondly, compared to empirical fitting (which is perfectly fine if one does not have diagnostic fitting sets) the use of these sets gives a truer representation of the fitting characteristics of the lens, as well as a more accurate means of determining the final powers and predicting the visual acuity.

Diagnostic sets (SEE APPENDIX) make the fitting of these patients easy and with chair time that is similar to that used in fitting conventional GP lenses. Fitting sets are reusable, not expensive, and once purchased do not need replacement over the typical professional lifetime of the practitioner.

Supplemental Readings

1. Sarver MD: A toric base corneal contact lens with spherical power effect. J Am Optom Assoc 34(14): 1136-1137, 1963.
2. Sarver MD, Kame RT, Williams CE: A bitoric gas permeable hard contact lens with spherical power effect. J Am Optom Assoc 56(3): 184-189, 1985.
3. Silbert JA: Take the bother out of bitorics. Rev Optom 123(4): 75-82, 1986.
4. Mandell RB, Moore CF: A bitoric lens guide that really is simple. Contact Lens Spectrum 3(11):83-85, 1988.
5. Silbert JA: RGP Correction of High Astigmatism. Contact Lens Spectrum 5(10): 25-31, 1990.
6. Silbert JA: Rigid Contact Lenses and Astigmatism. Chapter 25, in Bennett ES and Weissman BA (eds): Clinical Contact Lens Practice. Lippincott Williams & Williams, Philadelphia, 2005, pp.487-514.

Appendix

Template Design for SPE Bitoric Diagnostic Sets:

Diameter/OZD: 9.0/7.6 mm or 9.2/7.8 mm or 9.4/8/0 mm
Flat meridian air power: Plano
Steep meridian air power: If buying a 2.00 D bitoric set: -2.00 D.
If buying a 3.00 D bitoric set: -3.00 D

Toric Base Curves:

2.00 D Bitoric Set3.00 D Bitoric Set

Fluorescein Images Using SPE Diagnostic Fitting Sets

Example of diagnostic SPE lens, which has adequate rear surface toricity for the corneal astigmatism, but is too steep, causing apical clearance. The next flattest lens in the set should be tried, and the lens re-evaluated. (Courtesy Gerald Lowther, OD, PhD)

Toric SPE diagnostic lens which is slightly too steep for this with-the-rule cornea. Flattening the base curve of the flat meridian, while keeping the same toricity, will relieve the compression in the flat meridian, and reduce the pooling in the steep meridian. (Courtesy, Gerald Lowther, OD, PhD)

Example of diagnostic SPE lens with an excessive degree of rear surface toricity for the corneal astigmatism. Excessive compression of the steeper corneal meridian can be seen, necessitating a change to a set with lower toricity. (Courtesy of Gerald E. Lowther, OD, PhD)

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