A Primer on Toric Lens Fitting

This instructional review may help you overcome the challenges of fitting toric contact lenses.

BY J. BART CAMPBELL, OD
FEBRUARY 1999

Providing satisfactory vision for patients with astigmatism has always been a challenge for contact lens practitioners. In the days before soft contact lenses, astigmatism resulting primarily from the cornea was approached rather simply with RGPs. However, internal astigmatism resulted in the problems of residual astigmatism, which practitioners occasionally addressed by fitting prism ballast rigid torics. The advent of spherical hydrogel contact lenses meant that even moderate amounts of astigmatism could be problematic, because these lenses are not very effective in masking this refractive error. One strategy was to fit the spherical equivalent power, and although some patients found this acceptable, others were not satisfied with the compromise. Thus, hydrogel toric lenses were developed to provide optimum visual acuity to patients with astigmatism.

Optically, soft toric lenses function very much like spectacle lenses, except the astigmatic correction is achieved by cylindrical optics in the contact lens. While this essentially eliminates the problem of internally generated residual astigmatism, it introduces the need for the lens to orient properly in order for the patient to obtain good visual acuity.

Patient Factors Affecting Lens Selection

Studies have determined that soft contact lenses do not predictably mask corneal astigmatism the same way that rigid contact lenses do (Harris, et al., 1979; Wechsler et al., 1986; and Snyder & Talley, 1989). Therefore, it's crucial when fitting soft torics contact lenses that you give careful consideration to the patient's refractive status.

Low amounts of astigmatism (1.00D or less) -- Patients with 1.00D of astigmatism or less may be successfully fit using the spherical equivalent technique. Above 1.00D, patients may not obtain satisfactory visual acuity, but this can vary considerably from patient to patient. Perform both a sphere-only and a spherocylindrical overrefraction to determine the difference in acuity between the best possible spherical power and the best acuity with the astigmatism fully corrected.

More astigmatic error than spherical error -- Patients with more astigmatic error than spherical error (e.g., -0.25 -1.75 x 090) are often especially challenging to fit successfully. Patients with this type of refractive error tend to be sensitive to even small amounts of off-axis rotation. The same sensitivity may be an issue in higher cylinder powers as well. This sensitivity phenomenon gave rise to the "4:1 rule," which states that the best candidates for soft toric contact lenses are patients with a sphere to cylinder power ratio of 4:1 (e.g., -4.00 -1.00 x 090). This rule should not preclude an attempt to fit patients not meeting its criteria, but it should serve as a guideline so you're aware of when it may be challenging to obtain satisfactory visual acuity.

High spherical or astigmatic refractive errors and oblique axes -- These visual conditions should prompt you to evaluate potential lens designs more closely. While the availability of lenses in these powers and axes has increased markedly over what was available in the past, most brands still have some limitations. Many brands also increase in price when ordering lenses outside common parameters, a factor that some patients find significant.

Presbyopia -- Presbyopia raises issues that you should consider carefully. There are few hydrogel multifocal toric lenses currently available. Consequently, monovision using toric soft lenses is the method of choice for correcting the astigmatic presbyope who doesn't want to wear spectacles. You may encounter problems, however, if the lenses rotate unexpectedly when a patient's eyes converge for near tasks. The amount and direction of this rotation can be difficult to assess. Also, monovision patients' vision is always blurred in one eye or the other, so there would be no backup from the other eye if the toric lens in the eye being used should rotate off-axis reason. Choosing lenses with consistent rotation is therefore very important for monovision patients.

Demonstrating Astigmatism

Effective patient counseling is essential for fitting toric soft lenses successfully. Patients generally respond favorably if you give them an explanation of their condition and what you will attempt to do to address it. One technique that I have found to be particularly effective is as follows. Have the patient remove his spectacle prescription but hold it in front of his face to look through the lenses at your acuity chart. Have him then close one eye (the eye with the least astigmatism if there is a difference), then slowly rotate the spectacles while looking through the correct lens for the open eye. The patient will notice that the image becomes blurred as the lens is rotated off axis. Have the patient then slowly rotate the spectacles back to the correct axis and note the improvement in the image.

Follow this demonstration with an explanation that even with the proper power in the prescription, the lens must be oriented correctly for it to provide good vision. Finally, contrast the spectacle lenses, which have a frame to hold them, to contact lenses, which are free to rotate on the eye. This simple explanation is effective in giving patients an understanding of why astigmatism is more complex. Aim to establish reasonable expectations emphasizing that correcting astigmatism with contact lenses requires more time and expense than correcting spherical refractive errors and that they may experience transient blurring of vision if the lenses rotate off-axis momentarily.

Lens Selection

Toric hydrogel contact lenses may be classified according to the location of the toric surface, front or back. Most current designs are back surface torics. Traditionally, front surface toric lenses have been recommended for patients with primarily internal astigmatism (i.e., not on the cornea). Back surface toric lenses have been recommended for patients with primarily corneal astigmatism. This rule of thumb provides a starting point only.

You should be familiar with the design used to stabilize the rotation of the lens. Thin zones at the top and bottom of the lens, prism ballast and eccentric lenticulation are commonly used methods of stabilization. All of these depend on interaction between the eyelids and the contact lens. Lid forces, not gravity, hold the thicker edge of the lens toward the bottom of the eye. If a particular design doesn't provide an acceptable fit, a lens that uses a different method may be successful.

Fitting

Toric lenses are traditionally fitted using trial lenses. Select a lens that's as close to the anticipated lens powers and axis as possible, based on the patient's refractive error. Using a trial lens with an axis or power that's significantly different from the patient's refraction may produce misleading rotational characteristics. Keratometry readings provide a starting point for base curve selection. Palpebral fissure width and corneal diameter should be considered in the selection of overall diameter. Apply the lens to the patient's eye and allow it to equilibrate. The point is to avoid assessing the lens fit, especially rotation, too quickly after insertion.

The ins and outs of assessing rotation -- Rotation of the lens refers to the position of the axis of the cylinder power when the lens has equilibrated on the eye. It is not related to the amount of vertical lens movement that occurs after a blink. In most situations, rotation is evaluated with the patient looking straight ahead. If you allow the lens to settle for five to 10 minutes and the rotation has not varied after five additional minutes, the lens is likely to be positioning as it will when the patient wears it. If a lens exhibits no rotation, the axis of the cylinder power that the patient is looking through should be the same as the axis marked on the lens vial. Rotation will result in an axis other than that indicated for the lens.

If you visualize the contact lens as the face of a clock, assessing rotation is straightforward. There are 30 degrees between each clock position. If the lens has an orientation mark at the six o'clock position, but on the eye it rests halfway between the five and six o'clock position, then the lens is exhibiting 15 degrees of counterclockwise rotation. Some slit lamps have built-in protractor scales which measure the amount the slit beam is rotated. You can determine the amount of rotation by narrowing the beam and aligning it with the orientation mark on the lens, then reading the amount of rotation off the scale. The slit lamp beam should be straight ahead.

Assessing rotation is critical when fitting toric hydrogels, all of which have orientation marks placed on them during the manufacturing process. The more complex orientation mark designs are intended to assist you in making a more precise determination of the amount of rotation.

Consistent rotation is essential to success in fitting toric lenses. If the rotation varies constantly after allowing the trial lens sufficient time to equilibrate, another lens must be evaluated. Rotation itself is not a problem as long as it is consistent (assuming that the compensatory axis is available). To compensate for rotation, first determine the amount and direction of rotation exhibited by the trial lens. Using our clockwise-counterclockwise description, add the amount of rotation to the spectacle axis if the rotation is clockwise. Subtract the amount of rotation from the spectacle axis if the rotation is counterclockwise. This determines the axis of the toric lens to order for that eye. Various other methods, such as LARS (Left Add, Right Subtract), accomplish the same thing. According to the LARS acronym, if the lens rotates to your left, add the amount of rotation to the spectacle axis. If rotation is to your right, subtract it. The importance of documenting the rotational characteristics of the lens cannot be overemphasized. Probably the most common reason for decreased visual acuity with a soft toric lens is rotational inconsistency.

Overrefractions -- There are differing opinions on the usefulness of performing an overrefraction when fitting a toric contact lens (Koers & Quinn, 1997; and Englehart et al., 1996). If the lens is fitted empirically, obviously an initial overrefraction isn't possible. Even when using a trial lens, an overrefraction with a cylindrical component often requires the need for relatively complex crossed-cylinder equations to determine the appropriate power. Some manufacturers have developed calculators preprogrammed with the crossed cylinder equations for clinical use (Koers & Quinn, 1997). These devices simplify the process of calculating the resultant spherocylindrical power. They often do not offer a preprogrammed means to compensate for rotation. Therefore, if the lens exhibits any significant rotation, you will need to account for this before ordering the final lens (Koers & Quinn, 1997). Performing an overrefraction is probably most useful if the trial lens is close to the anticipated power and axis of the patient's final lens.

Before ordering the contact lenses, verify that the lenses to be ordered are close as possible in power to the power of the spectacle correction when adjusted for vertex distance. In fact, it is feasible to use the trial contact lenses simply to gauge rotation, then apply the appropriate vertex distance correction to each meridian of the spectacle prescription and change the axis as necessary. This method offers a usable middle ground between empirical fitting and calculating the resultant contact lens power using overrefraction.

In addition to lens power calculations, overrefraction data can serve as a general indicator of the quality of the lens fit. If acceptable visual acuity cannot be obtained with a spherocylindrical overrefraction, or if the overrefraction does not make sense optically, change the base curve or use a different lens design (Contact Lens Spectrum, Staff, 1997). Trial fitting also allows the clinician to evaluate the lens for acceptable centration, movement and limbal coverage. It further allows the patient to provide subjective input regarding the comfort of the lens.

Fitting Empirically -- Some manufacturers encourage practitioners to obtain refractive data and forward it directly to them. The manufacturer then selects the appropriate lenses and sends them to the practitioner. This method, known as empirical fitting, is often successful, especially with the improvements in reproducibility of toric hydrogel lenses that have occurred over the years. It tends to be most successful with lenses that rotate very little when placed on the eye. Advantages of empirical fitting include not having to stock trial lenses and reduced chair time at the initial visit. If the initial pair of lenses don't work, use them as trial lenses and assess as you would during a traditional fitting. Englehart, et al., found that empirical fitting was a viable alternative to trial fitting.

Dispensing

Before dispensing contact lenses, verify the parameters printed on the vial or package. Verification of the lens itself is difficult in most clinical settings, and probably not productive as a routine procedure. Certainly, lenses that are being dispensed to a patient for the first time should be applied to the patient's eyes and evaluated after they have had time to equilibrate.

Follow-up visits are handled the same as with any soft lens. Note rotational characteristics. Obtain and consider the patient's subjective impressions. Preparing the patient to achieve reasonable expectations will reduce the time spent explaining the difference between expected sensations and significant problems. Complaints of reduced visual acuity are the most common subjective issue encountered.

Toric RGP Lenses

Toric RGP lenses have an undeserved reputation for being difficult to fit. The optics of these lenses are straightforward, the key point being to analyze each meridian separately using an optical cross. As with soft torics, they can be fitted empirically, keeping in mind the need to compensate for the power created by the tear lens. Most toric RGP lenses are of bitoric design and achieve stability through the lens base curve-to-cornea interaction. RGP torics provide excellent visual acuity, durability and a wide range of parameters. Possible disadvantages compared to soft lenses include increased chair time and initial reduced comfort.

Today's toric lenses are vastly improved over the first generation designs. While they certainly require more chair time and skill than spherical lenses, they are a potential source of very loyal patients. Practitioners would do well to maintain their skills in fitting this highly viable modality. CLS

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #45. (Be sure to have a fax number ready.)

Dr. Campbell is an associate professor at Southern College of Optometry in Memphis, Tenn. He is also chief of the College's Contact Lens Service and director of Residency Programs.