Toric Soft Contact Lenses: They're Not Hard To Fit

BY TIMOTHY B. EDRINGTON, OD, MS
June 1999

Prescribing toric soft contact lenses isn't what it used to be. Why these underappreciated lenses shouldn't be overlooked.

If you avoid prescribing soft toric contact lenses due to inconsistent product reproducibility, less than optimal vision, limited parameter availability or lack of disposability, it's time to reconsider. The reproducibility of toric soft contact lenses has improved within the past few years, and the increasing availability of quality frequent replacement options have made them more attractive to eyecare practitioners and their patients.

More Tools for the Toolbox

Several manufacturers now include opaque, enhancing and handling tints in their toric soft contact lens armamentarium. For example, Wesley Jessen and Alden Optical both market cosmetic tinted toric soft contact lenses, and Wesley Jessen offers UV protection with it's Durasoft 3 Optifit Toric lens.

The availability of toric soft contact lenses in multipacks for disposable or frequent replacement schedules has dramatically increased in recent years. Disposable (2-week replacement) toric soft lenses are currently marketed by Wesley Jessen (Fresh Look Toric) and Bausch & Lomb (Softlens 66 Toric). Monthly and quarterly replacement toric soft contact lenses are also available.

Manufacturers of toric soft lenses provide a wealth of fitting information via their fitting guides, which offer step-by-step instructions for maximizing success. They have in-house fitting and trouble-shooting consultants available to talk you through the learning curve or to commiserate with you (and offer advice) when contradictory data prevail.

Where To Start?

Toric soft lenses can be prescribed either by diagnostic trial lens fitting or by empirical fitting (without trial lens fitting). Both methods offer different advantages. Proponents of trial lens fitting feel that time is ultimately saved if a diagnostic lens is placed on the eye, allowed to equilibrate and then evaluated for base curve-to-cornea fitting relationship, lens rotation and rotational stability prior to ordering. Overrefractions with trial lens fitting also allow powers to be fine-tuned.

Supporters of empirical fitting feel that less chair time is required and that the patient has a more positive first impression when they are fit with a lens that they can see well through. Also, most toric soft lens manufacturers offer generous exchange policies with empirically ordered lenses, which benefits the practitioner because it is less of a financial risk. I prescribe certain toric soft contact lens brands empirically, but I still diagnostically fit other brands.

Your personal philosophy regarding trial contact lens use will dictate, to some degree, the number of lenses in your diagnostic inventory. I recommend that you keep at least two toric lens brands for quarterly-to-yearly replacement schedules and two brands for biweekly-to-monthly replacement.

Prescription Guidelines

I prefer to prescribe toric soft contact lenses for patients with spherical refractive errors ranging from approximately +4.00D to -6.00D. My concern for patients whose errors fall outside of these parameters is that their corneas might not receive adequate amounts of oxygen. I might prescribe toric soft lenses for a patient with a +6.00D sphere finding and recommend part-time wear or even full-time wear if no signs of corneal stress, such as striae, are detected at follow-up visits.

I generally prescribe toric soft contact lenses for patients with 1.00D to 3.00D of refractive cylinder. Again, this is only a guideline since every patient's demand for precise vision differs. If a patient with more than 3.00D of refractive cylinder is less critical about their vision, I might consider prescribing a toric soft lens. I have found that the greater the amount of sphere component, the more likely the patient is to accept vision through larger amounts of cylinder correction.

In the past, many practitioners preferred to prescribe toric soft contact lenses for patients with against-the-rule astigmatism because the thick and thin zones created by this lens profile enhanced rotational stability. Many of these are constructed with the stabilizing prism contained in the lenticular or carrier portion of the lens, making successful results possible with all axis.

Before selecting a certain brand of lenses for your patient, consider these factors: daily wear versus flexible wear, the availability of lens parameters, the patient's desire for cosmetic tint and the lens replacement schedule. I seldom prescribe flexible wear contact lenses for my toric soft lens patients since the increased overall thickness of the design reduces the amount of oxygen available to the cornea. Make sure that the brand you select is available in the sphere power, cylinder power and axis you need. Also determine if the lens is outside standard or stock parameters. If so, it's a custom design and will be more expensive to prescribe. Finally, verify whether a warranty or "guaranteed fit" program is available with the lens and what, if any, the specific guidelines are.

Many optometric authors recommend prescribing a back surface toric contact lens design if the patient's astigmatism is primarily corneal and a front surface design if it's primarily lenticular, but I don't feel that this is necessary. Instead, I tend to prescribe patients requiring high amounts of cylinder with contact lenses that have more prism for better rotational stabilization.

The base curve of the initial diagnostic lens may be determined using the manufacturer's guidelines, which are usually dictated by keratometry readings. Vertex the sphere and cylinder powers of the manifest refraction (if greater than �4.00D) to determine powers. Some manufacturers recommend adding plus power to the sphere or cylinder to adjust for lens flexure effects. Generally, thicker profile toric soft contact lenses require more plus power, but consult the fitting guide for the contact lens you're using to make sure this is indicated. If a diagnostic lens isn't available in the powers your patient needs, select one with an axis close to the desired axis or order the lens empirically.

Fitting Soft Torics

When fitting soft toric lenses, allow them to settle or equilibrate for at least 20 minutes (if practical) prior to evaluating the fit, rotation, rotational stability and vision. Corneal coverage and lens centration is generally good with toric soft designs because of their large overall diameters, and they should move adequately (0.5mm to 1.0mm) on a blink.

Lens rotation, as determined by the position of the base down prism marking (or 3 and 9 o'clock scribe marks), isn't as important as the lens' rotational stability. Rotation that's stable and within 30 degrees of base down, (base down marking located between the 5 and 7 o'clock positions), is acceptable. If the rotation is more than 30 degrees, suspect a steep fitting relationship and refit with a different base curve radius, lens diameter or lens brand. Correct a steep fit by prescribing a flatter base curve or smaller overall diameter. Rotational stability (i.e., the amount of rotation during or after a blink), is critical for successful vision. If the lens rotates more than 10 degrees on a blink, or if it doesn't return to relatively the same position after each blink, consider refitting. Suspect a flat fitting relationship if the lens swings excessively during or after a blink. To steepen the fit, order a steeper base curve or larger overall diameter. The larger the cylinder correction, the more critical the rotational stability.

I recommend performing a spherocylinder overrefraction. If the correcting cylinder in the toric contact lens is aligned with the eye's refractive cylinder, then the overrefraction cylinder axis will be similar to this axis or 90 degrees away. If so, the overrefraction findings can be arithmetically added to the diagnostic contact lens power to determine the prescription. If the correcting cylinder in the toric contact lens isn't aligned with the eye's refractive cylinder, then the overrefraction cylinder axis will be oblique to the eye's cylinder. The amount of the overrefraction cylinder will be related to the amount of axis misalignment. If the correcting lens cylinder is 30 degrees away from the refractive cylinder axis, the overrefraction cylinder will be approximately the same as the correcting cylinder amount. If the correcting cylinder is 15 degrees away, the overrefraction cylinder will be approximately one-half of the correcting cylinder amount.

The spherocylinder overrefraction data should be entered into a calculator that uses cross cylinder calculations to determine prescriptions. Keep in mind that the resultant sphere is often more plus (by 0.25D to 1.00D) than the vertexed manifest refraction would indicate due to the fact that the back surface of the contact lens bends or flexes more on the eye than does the front surface.

Reviewing the LARS Method

To prescribe the cylinder axis, most practitioners use the Left Add, Right Subtract (LARS) method. The amount of lens rotation found using the LARS method should be added to or subtracted from the axis of the manifest refraction, not from the axis of the diagnostic lens. Remember, when you apply the prescribed lens to the eye, the base down marking should be positioned at the same place as the diagnostic lens marking. A well-fitted toric soft contact lens will exhibit a consistent amount of lens rotation and excellent rotational stability.

However, the prism base down marking doesn't have to position exactly at 6 o'clock for optimal vision. If the base down marking (as observed with a slit lamp), is positioned to the left of the 6 o'clock position, add the amount of rotation in degrees to the manifest refraction cylinder axis. If it's located at the 6:30 position, 15 degrees needs to be added to the manifest refraction cylinder axis (See example in Table 1). Note: Each clock hour corresponds to 30 degrees of rotation.

If the base down marking is located to the right of the 6 o'clock position, subtract the amount of rotation in degrees from the manifest refraction cylinder axis (See example in Table 2).

Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry. He serves on the Executive Committee of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study.