A Guide to Prescribing Toric Lenses
(Part 2)

BY PETER D. BERGENSKE, O.D.
JULY 1996

Last month we discussed the various vision correction options for your astigmatic patients, with an emphasis on rigid toric contact lenses. This month we'll concentrate on the soft toric designs and discuss a definitive method of categorizing them.

There was a time when astigmatism limited the contact lens options for many patients, but new, sophisticated, repeatable soft lens designs for astigmatism have greatly broadened our choices.

In choosing a toric lens, you must take into account not only what is optically and physically required, but also the patient's history, needs and demands. The patient interview will help guide you in making your first decision -- whether to proceed with a rigid lens or a soft lens.

The amount of astigmatism alone does not dictate the type of lens as much as does the type of astigmatism and its relationship to corneal curvature. With-the-rule astigmatism, which tends to be mostly corneal nearly always lends itself to correction with either spherical or bitoric rigid lenses. And, while most practitioners would think first of a toric soft lens when correcting low degrees of against-the-rule astimatism, there are some cases where a rigid lens is the lens of choice.

As a general rule, the very flat, large diameter cornea presents a challenge when applying rigid lens designs. These types of corneas are often relatively easy to fit with the very large, flat designs available from many soft toric lens manufacturers.

Other physical factors that can affect the decision of which lens type to use include the quality of the lids, tears and blinking. Persistent 3 and 9 o'clock staining, for example, may indicate the need for a hydrogel lens despite possible optical benefits of a rigid lens. On the other hand, poor tear quality that affects lens hydration may have significant impact on soft lens stability and visual performance, but little impact on rigid lens visual performance.

FITTING TECHNIQUES

The key to fitting a toric soft contact lens is to control rotation, which depends on the lid/lens interaction, the cylinder axis of the lens, the lens design, and the base curve-to-cornea relationship. Because most of the blink action is in the upper lid, it's generally safe to consider only the action of the upper lid when analyzing lens rotation, keeping in mind that the upper lid generates not only a vertical motion but also a temporal to nasal vector. The lid is most likely to rotate the lens when it encounters the thickest meridian of the lens. Understanding this dynamic will help you solve rotation problems by choosing lens designs that will either minimize or enhance lid effects.

The cylinder axis of the lens, at least in theory, indicates that the thickest part of the lens should be at 90 degrees from the axis. This will be true in evaluating the overall thickness profile of the lens, but modern lens designs incorporate broad lenticulation of at least the upper edge, thus minimizing lid interaction differences dictated by the axis of the correcting cylinder. What may be more significant is the lens design and how it will interact with the lid action, and how it interfaces with the corneal curvature.

SOFT TORIC CHOICES

There are really four major groups of toric soft lenses: prism ballast spherical base; prism ballast toric base; thin zone spherical base and; thin zone toric base (Fig. 1).

It's helpful to be aware of these differences in lens design, particularly when the first lens you choose is not performing optimally. On some eyes, stability may be poor with a front cylinder design and much better with a back toric, and on other eyes the opposite may be true.

Knowing the differences among these lens designs will help you make logical, rather than random choices when trying alternate lenses. A significant factor in many cases is how the back curve of the lens relates to the corneal curve. Soft lenses drape or conform to the corneal curve, which is why spherical soft lenses do not correct corneal astigmatism. You can expect a spherical base, toric front surface lens to conform to the corneal curve (assuming appropriate base curve and diameter) regardless of corneal cylinder. However, a soft toric lens with a toric base should orient itself somewhat to the corneal curvatures. This can be particularly advantageous with significant corneal astigmatism. It makes a back surface cylinder design a logical choice for patients with the corneal astigmatism axis in agreement with the refractive astigmatism axis. On the other hand, if there is significant disparity between the corneal astigmatism axis and the refractive astigmatism axis, back surface cylinder designs can be unpredictable. For greatest stability, choose a front surface cylinder design in cases of nearly spherical corneal curvature and against-the-rule refractive cylinder. There are numerous toric base and spherical base lens designs. Most are prism ballast incorporating thin zone features.

CUSTOM DESIGNS

You can achieve very good results with toric soft contact lenses, even for patients with high degrees of astigmatism. Front and back toric designs are available in custom parameters from several sources. It's always of value to place a trial lens on the eye, but in some cases the trial lens may need to be the first lens ordered. If you're using a spherical base design, it can be helpful to use a trial lens even if it has far less cylinder correction than anticipated. An unfortunate limitation of many 'custom' designs is that only the power is custom; the curve and diameter are predetermined. Some manufacturers, however produce both spherical and toric base lenses in a wide variety of diameters and base curves.

TAKE THE TRIAL OUT OF TRIAL FITTING

When faced with a patient whose degree of astigmatism is outside the parameters available as a stock lens, you can still approach the fit as you would a 'stock' fit.

In assessing a patient for a toric soft lens fit, consider his goals, refractive power, degree and axis of refractive astigmatism, degree and axis of corneal cylinder, lid position and lid tension. Based on this information, you may decide to order lenses empirically, or to place trial lenses on the eye to gain further information.

Trial lenses that are close to the sphere power needed will enable you to assess rotation, refraction and general fit quite reliably. For spherical base lenses, a trial lens with a low degree of cylinder, regardless of the axis, should reliably predict rotation if the sphere power is close to what is needed. Toric base trial lenses should have both cylinder axis and power close to that needed to predict fit and rotation accurately.

For the patient with significant corneal astigmatism, order a toric base trial lens that is close to the right axis and cylinder power. A trial lens of the planned base curve and diameter should ensure that the saggital depth relationship is acceptable.

Once you've achieved a fit that is stable, use the overrefraction to finalize the lens prescription.

Overrefraction information is useless if the trial lens is not stable. Crossed cylinder calculations are accurate assuming the lens is stable on the eye and the power of the trial lens is known. You can input the formulae for crossed cylinder calculation into a spreadsheet on your computer, or you can program a calculator to perform this function (Table 1). Some lens manufacturers have calculators that are already programmed, and most lens consultants can perform the calculation for you when you phone in your order.

The other approach, and perhaps the one that is more commonly used, is to allow a trial lens to stabilize on the eye, note its rotational orientation, and then follow the 'left add, right subtract' rule to order a lens with the correct axis.

VERIFICATION

Toric soft trial lenses present two concerns with regard to in-office management of lenses: verification and storage (see sidebar).

Verifying soft lenses has always been a challenge, and toric soft lenses increase the degree of difficulty. At the same time, it's imperative that you know that lenses used for trial fitting are exactly as indicated, particularly if you are relying on overrefraction information.

PATIENT EXPECTATIONS

With all contact lens fittings it's important to prepare patients for what to expect, but this is particularly important for toric soft lens patients. Inform them that they may have to return to the office several times and that lens exchanges are common during the fitting process. Patients accept that a higher fee is required if they understand that a high degree of expertise, as well as increased time and material costs go into fitting toric soft contact lenses. Make sure patients understand that you see them as 'special' rather than as 'trouble.' Remember, patients who know you've had to work harder to obtain the best contact lens fit are much less likely to go elsewhere for their vision care in the future. CLS

Dr. Bergenske is in private practice in Madison, Wisc. He is a graduate of the University of California, Berkeley, School of Optometry, and a diplomate of the Section on Cornea and Contact Lenses of the American Academy of Optometry.

Dr. Bergenske welcomes your feedback via e-mail at: pbergenske@mailbag.com

References are available upon written request to the editors at Contact Lens Spectrum. To receive references via fax, call 1-800-239-4684 and request Document #15. (Be sure to have a fax number ready.)