A Guide to Prescribing Toric Lenses
(Part 1 of 2)

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

Are you providing your astigmatic patients with the best possible vision? Be sure to consider all your options for contact lens correction.

Although it's difficult to define "significant" astigmatism, it's safe to say there are many individuals for whom correction of astigmatic refractive error would be preferable over simple spherical correction. The success of spherical soft contact lenses certainly speaks for the adaptability of many patients to live without full correction. On the other hand, it's possible that some contact lens wearers stop wearing their lenses due to poorly corrected vision. Estimates of the prevalence of high degrees of astigmatism in the contact lens-seeking population also indicate the need for eyecare professionals to be well-versed in this area.


There are many approaches to correcting astigmatism for the contact lens wearer. One of the most common approaches has been to ignore astigmatic error, applying spherical equivalent correction. This is a legitimate approach for low amounts of astigmatism or situations where simplicity and cost are of higher priority than acuity. Performance in sports such as soccer or hockey, for example, while requiring good vision, is not likely to suffer due to a visual acuity that is not a perfect 20/20.

One option that is rarely discussed is using an astigmatic spectacle correction over contact lenses, especially for patients who have relatively extreme spherical components to the refractive error. Many high myopes may be happy to have their correction "debulked" and then wear thin spectacle lenses for optimum vision. Presbyopic high myopes or hyperopes and monocular patients who need to wear spectacles for protection may also be well served by astigmatic spectacles over a contact lens correction.

The ability of spherical rigid contact lenses to neutralize corneal astigmatism is well-known and provides a fundamental part of contact lens practice. When corneal astigmatism and refractive astigmatism are more or less equal, and corneal toricity is not extreme, a spherical rigid lens should provide excellent optical results.

Toric rigid lenses are important tools for dealing with large amounts of corneal toricity as well as residual astigmatism. Although rarely used today, front toric and back toric designs offer advantages for some patients and should not be dismissed as outdated. Bitoric rigid lenses are especially useful for their excellent visual as well as optical benefits.

THE RIGID LENS ADVANTAGE

With their superior physiology, durability and ease of care, you should consider rigid lenses at every contact lens consultation. For most astigmatic patients, there is good agreement between corneal and refractive cylinder, and a spherical rigid lens will provide excellent optical results.

The major optical advantage of the rigid lens is its ability to neutralize, rather than correct, astigmatism. With a soft lens, all the astigmatism is residual, whereas a rigid lens neutralizes most of the astigmatism, and the residual is usually only a small factor.

A spherical rigid lens almost always neutralizes the majority of astigmatic error. In fact, the main use by far, for rigid toric design is not for correcting astigmatism, but for stabilizing the lens on a toric cornea. When we encounter residual astigmatism in rigid lens fitting, it is typically minimal and relatively easy to correct.

To understand the optical characteristics of rigid lenses, we must review the fundamental concept of surface power, especially as it relates to the difference in how a lens performs in air as opposed to on the eye. It's important to understand that the convenience of the units of diopters K, as applied to the keratometer and contact lens curves, is useful but this convention will lead to error if you assume that lens curves expressed in diopters K are the same as the lens surface power. This makes little difference when dealing with spherical lenses, but creates large errors when applied to a back surface toric rigid lens. Fortunately, we really only need to concern ourselves with back surface cylinder power in the rare instance of using a back toric rigid lens that is not designed to have spherical power effect. Front toric rigid lenses are optically no more complex than spectacle lenses or soft toric lenses, and the Sarver spherical power effect bitoric design so simplifies the calculation that you can usually do it in your head.

If a spherical lens does not provide adequate cylinder correction, use a lens that employs one or more toric curves. Back surface cylinder is often appropriate to stabilize fit and improve centration on a toric cornea, but it may create more cylinder power effect than you want.

CHOOSE YOUR TORIC

When choosing a toric lens, consider your patient's history, needs and demands in addition to optical and physical requirements. Previous contact lens wear experiences will also provide valuable clues to help you develop a successful contact lens program. Demands of perfect vision or perfect comfort, as well as occupational or athletic needs will guide you in your first decision: whether to proceed with a soft lens or a rigid lens.

The amount of astigmatism alone does not dictate the type of lens as much as 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 one of the two simplest types of rigid lenses, spherical and bitoric.

Low degrees of against-the-rule astigmatism, a condition in which there is often a nearly spherical central corneal curve, might make you think first of a toric soft lens. But what if the patient with two diopters of with-the-rule corneal astigmatism also plays second base, or the patient who has happily worn rigid lenses for 15 years is now noting a reduction in acuity which improves with plano -1.00 x 90? Factors other than the type of astigmatism can often "overrule" the traditional rules.

Another optical consideration is the situation in which the cylinder is partly corneal. If the choice comes down to either a toric rigid lens or a soft toric, keep in mind that you'll deal with less residual astigmatism with the rigid lens than with the soft lens. In cases of high degrees of against-the-rule astigmatism, often half or more of the cylinder is corneal and a rigid lens only needs to account for half or less of the total astigmatism. The soft toric lens must correct the entire cylinder. Then it becomes a matter of which lens will be better tolerated from both a visual and physical standpoint. Patients are much more likely to be sensitive to the soft lens three-diopter cylinder that is slightly off axis than they are to the RGP one-diopter cylinder that is slightly unstable.

EVALUATE PHYSICAL FACTORS

Overall corneal shape plays a role in selecting a lens type, particularly when it's outside the typical dimensions. Certainly, extreme conditions, either very flat or very steep, can be fit with either a soft or a rigid lens. As a general rule, however, the very flat, large diameter cornea presents difficulty 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. On the other hand, it's often quite simple to design a rigid lens for the small, steep cornea, for which most soft toric designs will be quite unstable and unacceptably large in diameter.

Other physical factors that can influence the decision of which lens type to use include the quality of the lids, tears and blinking. Persistent three o'clock and nine o'clock staining, for example, may indicate the need for a hydrogel lens despite the 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, and little impact on rigid lens visual performance.

FRONT TORIC RIGID LENSES

A front toric rigid lens is especially useful when correction of oblique cylinder by a spherical lens results in a modest degree of against-the-rule residual astigmatism. In this situation, rigid lens fitting may be very simple and soft lens fitting particularly frustrating. This type of lens is also useful when a long-term rigid lens wearer develops lenticular astigmatism.

Front toric rigid lenses should be designed with a relatively small diameter (8.6-8.9), but with a proportionately larger optical zone (7.7 with 8.9 diameter). The aim is to achieve a lens that tends to center, but that has enough stability to resist excess rotation. A small degree of prism is added to maintain lens orientation, so the smaller the diameter, the less average thickness will be required.

Due to the temporal-nasal vector of the upper lid motion, you can expect some counter-clockwise rotation, and should take this into consideration when you order the lenses. If a spherical trial lens overrefracts cylinder axis 90 for each, order lenses with axis 80 for the right eye and axis 100 for the left. (This follows the "left add, right subtract" rule we use with soft lenses, so it's easy to remember.) You can usually control excessive rotation by thinning the superior edge, especially at the temporal aspect.

BACK TORIC RIGID LENSES

The back toric, spherical front lens is probably one of the most misunderstood contact lenses. In cases when a spherical lens neutralizes astigmatism, the back toric often creates unacceptable residual astigmatism. However, the back toric lens is an ideal lens for correcting against-the-rule astigmatism associated with against-the-rule corneal toricity and greater refractive astigmatism than corneal cylinder. Due to the effect of surface power, the back surface cylinder that fits a toric cornea will create a cylinder power on the eye that is one and a half times the back surface toricity expressed in diopters K. This is useful in cases of against-the-rule corneal cylinder, where the refractive cylinder is usually greater than the corneal cylinder, so the excess correction is desirable. For example, if the cylinder component of the refractive error is -3.00 x 90, a lens with a base curve toricity of 2.00 diopters K will yield approximately 3.00 diopters cylinder correction on the eye. Such a lens must be stable on the eye. The back surface cylinder will inhibit rotation, however, you may need to add a small degree of prism.

BITORIC RIGID LENSES

The most common toric rigid lens is the bitoric lens because most often the excess cylinder correction induced by the back surface cylinder must be neutralized by a front surface cylinder. When the front cylinder exactly neutralizes the excess cylinder that is created with the lens on the eye, the bitoric lens behaves optically as a spherical lens on the eye and rotation is not a concern.

When a spherical lens will correct the astigmatism, but physically will not fit well, use a spherical power effect (SPE) lens. Even if you suspect that you may need to correct a small residual cylinder, it's often wise to make an initial attempt with an SPE lens. Overrefraction of this lens will be the best test to see if cylinder power should be added.

Design of an SPE lens is elegantly simple: the cylinder power of the lens in air (the difference in absolute powers in the principle meridians) is equal to the back surface cylinder expressed in diopters K. For example, a lens requiring a base curve of 43.00 x 46.00 (3.0 diopters K cylinder), must have a 3.00 d.c. such as -1.00 @ 43.00 and -4.00 @ 46.00 for the lens to have spherical power effect on the eye. The principal advantage of this design is that lens rotation will have no effect on the vision.

Another significant advantage of this lens design is that you can calculate the power without knowing K readings or refractive error. The only necessary information is that obtained from trial lens fitting. The overrefraction, along with the specifications of the trial lens, give all that you need to calculate lens power. This is particularly useful when fitting distorted corneas where keratometer readings are of little value.

If the SPE bitoric results in significant cylinder overrefraction, and the orientation of the principal meridians of the contact lens are close to or the same as the principal meridians of the residual cylinder, a cylinder power effect bitoric lens is useful. The overrefraction can simply be added to the SPE design powers.

The simplest, and most reliable way to determine the power for a cylinder power effect bitoric is to trial fit with an SPE lens to determine the residual cylinder and orientation when the lens is on the eye. Simply add the overrefraction to the SPE lens powers, keeping the base curves the same. For example if an SPE lens that measures 43.00 x 46.00, -1.00/-4.00 (-1.00 c. -3.00 cyl.) results in overrefraction of -1.00 cyl., you would order the cylinder power effect lens as -1.00/-5.00 (-1.00 c. -4.00 cyl.).

FIG. 2: A SPHERICAL LENS ON A TORIC CORNEA DEMONSTRATING POOR CENTRATION.

FIG. 3: A TORIC GAS PERMEABLE LENS ON A TORIC CORNEA WITH GOOD CENTRATION

LENS MATERIALS

It's often best to leave lens material selection to the laboratory. They know which material will adapt well to your lens design and be most stable under their manufacturing techniques. A non-flexing, easy-wetting material of reasonable permeability is all you need.

TRIAL LENSES

Rigid toric trial lens sets are a luxury, and often the first lens you order must be your trial lens. Most laboratories will lend trial lenses, but this causes delays that you may want to avoid.

You can gather a great deal of information by trial fitting with a spherical lens using a topical anesthetic for patient comfort. If this lens centers and doesn't flex, the overrefraction provides much of the information you need to determine toric lens power and design. Assess lens flexure by taking a keratometer reading of the front surface of the lens on the eye. Observe the fluorescein pattern with a spherical lens on the eye to assess corneal topography.

It's imperative that you verify toric lenses in the office. Only when you know the exact specifications of trial lenses and ordered lenses can you make sound decisions for changes in lens design. In addition, when lenses are working out well, it's extremely important to record the complete specifications of the lenses so they can be duplicated if they're lost or damaged.

Often, patients who need toric rigid lenses know that they pose a challenge. It's likely that they've had difficulty obtaining comfortable, clear vision with contact lenses, and they usually understand the additional time and cost involved. It's important to explain the complexity of the lens fitting to patients so they're willing to work with you in achieving success. CLS

E-mail Dr. Bergenske at: pbergenske@mailbag.com

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.

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 #14. (Be sure to have a fax number ready.)

Link to A Guide to Prescribing Toric Lenses Part 2