By Dr. Joel Silbert
Case 1: Diagnostic Fitting for S.P.E. Bitorics
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 LASIK, but has been advised that she is not a good candidate. As such, the cosmetic benefits of contact lenses are not 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.
Latest Manifest Refraction:
OD +2.00 = -4.00 X 180 (20/20-)
OS +2.50 = -3.75 x 178 (20/20-)
OD 42.00 @ 180 / 46.25 @ 090 (good mire quality)
OS 41.75 @ 180 / 45.75 @ 090 (good mire quality)
Slit Lamp Examination:
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 rigid 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.
Rigid toric lenses, on the other hand, would be able to provide a high quality of vision that would be stable, given the close approximation of corneal and refractive astigmatism.
Additionally, rigid lenses would be more economical in the long term compared to custom soft toric lenses, due to increased lens life and ease of cleaning.
Fitting Procedures, Design & Ordering
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 RGPLI), 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 done quickly and accurately, once the fitting characteristics have been determined.
Looking at M.A.’s clinical data, it should be apparent that almost all of her 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 lab 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 (Guidelines)
- Flat Meridian: Fit the lens on-K or slightly flatter than K
- Steep Meridian: Undercorrect this meridian by about 1 D. to ensure adequate tear exchange and prevent a physiologically tight fit.
Since M.A. had 4 D. of corneal toricity, we only need to use 3D of cylinder on the back surface of our lens. Thus, the following toric base curves were chosen from the 3D SPE bitoric fitting set (Note: The OD was fit on the flat K; the OS was fit 0.25D flatter than the flat K):
Flat Meridian (FM) = 8.04 (42.00D)
Steep Meridian (SM) = 7.50 (45.00D)
Diagnostic lens power: PL/-3.00
Flat Meridian (FM) = 8.13 (41.50D)
Steep Meridian (SM) = 7.58 (44.50D)
Diagnostic lens power: PL/-3.00
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 and is the hallmark of SPE bitorics!
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, remove the lens and replace with the next flatter lens in the fitting set.
If the lens appears flat, use the next steeper lens in the fitting set.
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.
Determining the Final Rx Power of the SPE Bitoric Lens
Once the “best fit” has been determined, simply over-refract with loose spherical lenses from your trial lens set. Note the acuity, 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:
FM / SM
FM / ;SM
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 9.2/7.8 but this can be modified based on lid aperture size, pupil considerations, etc.
Peripheral curves are spherical when the rear surface toricity is 3D or less. Use toric peripheral curves for cylinders above 3D for added rotational stability.
Final Lens Specifications for Patient M.A.
|OD:||8.04/7.50||9.2(.4)||10.5(.3)||+2.00/-1.00||9.2||7.8||Boston EO blue|
|OS:||8.13/7.58||9.3(.4)||10.5(.3)||+2.75/-0.25||9.2||7.8||Boston EO blue|
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 when 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. Seen 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.
Take Home Discussion
- The use of SPE bitoric diagnostic fitting sets is very helpful to clinicians who wish 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).
- 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 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.
This case will be presented without a case history. I will just give the pertinent K and refractive data for one eye (the fellow eye is similar). Suffice it to say that the decision to use a GP toric lens has already been made.
K OD 44.00 @ 180 / 46.50 @ 90
Rx OD -3.00 = – 3.50 x 177 (20/20)
Can this patient be fit with a spherical GP lens?
There is 2.50 D. of toricity, so while a spherical base curve lens could be used, there will be a lot of “rocking” around the flat corneal meridian. This will probably lead to excess lens movement, more discomfort and instability, and may lead to low riding lenses that could become stagnant in terms of tear exchange.
Can the patient be fit with an aspheric GP lens?
This would be a better possibility, as an aspheric lens would provide more uniform bearing on the toric cornea with better weight and contact distribution than the more harsh spherical base lens. This would be done with an aspheric with an ellipsoidal e-value (e approximately .65) and not with an aspheric that is used for multifocal purposes (which have very high e values).
What kind of vision would be produced with either of the above two options?
There is a high possibility that acuity would be compromised, as in both cases, there is residual astigmatism of about 1.00D. This is a with-the-rule residual astigmatism!
What about a front toric to correct for this residual astigmatism?
Well, that is a possibility, but this would be making a difficult situation even more complicated.
Is there any other way that we could correct the residual astigmatism without using a front toric prism ballasted design?
Yes! Why not design a simple “Back Toric” lens. This is a rear surface toric that has a spherical curve on the front (not a bitoric). We know that the use of a rear surface toric will fit this 2.5D toric cornea much better than a spherical or aspheric lens. The lens should center well, and conform better to the actual corneal shape. We will design it to provide adequate tear exchange, so that it won’t become too snug.
More importantly… we know from our optic studies that when a back toric lens is applied to a toric cornea, there will be a “cylindrical error induced” (the “induced cylinder”) that forms along the flat axis and which is approximately 50% of the toricity of the rear surface of the contact lens! Well, in this case, if we use a lens with 2D of rear surface toricity, it will induce somewhere between 0.75D and 1.00D of cylinder along the flat axis (180). And in our patient’s case, the Residual Astigmatism is just about the same amount!
In other words, we are using the “induced cylindrical error” as a “correcting lens” for the patient’s residual astigmatism! Even though it is a tear lens, it behaves like any other lens, and will correct the residual nicely, leading to excellent visual acuity for our patient!