TORIC LENSES
Contact Lenses for Patients With Astigmatism
This mini toric lens fitting guide helps you manage the visual needs of your astigmatic contact lens patients.
By Jennifer Y. Jung, OD, and Timothy B. Edrington, OD, MS

Fitting toric contact lenses has traditionally been a challenging skill to master. However, over the years practitioners have become more proficient at prescribing these specialty lenses. This article provides an overview of toric options available to manage the vision needs of astigmatic patients.

SOFT TORIC CONTACT LENSES

Improved lens reproducibility and expanded parameter options have contributed to increased success in prescribing soft toric contact lenses. Toric soft contact lenses are also now available in daily disposable and two-week replacement modalities.

Toric soft contact lenses are generally prescribed for patients with 1.00D to 3.00D of refractive astigmatism. However, many patients with 0.50D to 1.00D of astigmatism can benefit from wearing toric or aspheric soft contact lenses to optimize their vision. Also, patients with more than 3.00D of astigmatism successfully wear soft torics.

Keep in mind that as the amount of the correcting cylinder increases, rotational stability becomes increasingly important. For example, a 10-degree misalignment of the correcting cylinder will result in only 0.50D of induced over-refraction cylinder through a 1.50D toric cylinder correction; whereas 1.00D of residual cylinder would be induced if a 3.00D cylinder correction was misaligned by 10 degrees.

Figure 1. Scribe mark near 6 o'clock position on soft toric lens.

Soft Toric Design

With so many soft toric lens options available, contact lens practitioners need to understand the differences among designs to determine the optimal lens for each patient. The key to success is to maximize on-eye rotational stability without compromising corneal physiology and patient comfort. Various methods have been employed to stabilize lens rotation, including prism ballast, eccentric lenticulation, double slab-off, toric back surfaces, truncation or a combination of these designs.

Prism ballast is the most common stabilization method used in manufacturing today's toric soft lenses. Many toric lens designs achieve this effect by eccentric lenticulation. In eccentric lenticulation, the prism is located in the peripheral carrier portion of the lens (Figure 1). Increased prism generally results in improved on-eye rotational stability. However, increased prism implies a thicker inferior lens profile which may result in decreased oxygen transmissibility. Patients prescribed prism "ballasted" soft contact lenses should be regularly monitored for physiological complications of the cornea, such as neovascularization.

In the double slab-off, or dual thin zone design, the thinnest areas of the lens periphery are located superiorly and inferiorly (Figure 2). Rotational stability is achieved by the thin zones of the lens positioning beneath the eyelids.

	Figure 2. Horizontal scribe mark on double slab-off design.

Empirical vs. Diagnostic Prescribing

The availability of toric trial lenses in your office often dictates whether you order empirically or utilize diagnostic lenses. Ordering empirically using the keratometry readings and the manifest refraction will save you chair time and generally results in the patient experiencing excellent initial vision, but empirical prescribing may result in increased patient office visits to fine-tune lens parameters. Remember to vertex any refraction findings exceeding 4.00D and to vertex the sphere and cylinder powers separately when determining the final contact lens power. Diagnostic fitting with trial lenses provides you with valuable information on the fit, rotation, rotational stability and vision. This allows you to fine-tune lens parameters prior to ordering the patient's lenses.

Soft Toric Troubleshooting

When evaluating the fit of a toric soft lens, observe centration and movement in primary and up-gaze, as well as the stability and location of the scribe marks. Scribe marks typically indicate the base of the prism. In toric soft lenses using the thin zone method of stabilization, the scribe marks are located at the 3 and 9 o'clock position on the lens.

Suppose a toric soft lens rotates from the base down position, but appears stable between and during blinks at that position. Apply the LARS principle (Left Add, Right Subtract) to the manifest refraction cylinder axis and order a new lens. For example, if the scribe mark(s) is positioned counter-clockwise (Right), measure the amount of rotation and Subtract that amount from the manifest refraction axis. When the new lens is applied to the patient's eye, the scribe mark should rotate to the same counter-clockwise position observed with the diagnostic lens.

You can also perform a sphero-cylinder over-refraction (OR) to obtain additional information on the amount of lens rotation. The OR axis will be the same as the spectacle axis or 90 degrees away only if the axis of the toric lens and axis of the refractive cylinder are aligned. Otherwise, an oblique OR axis will result. If the OR cylinder is the same as the refractive cylinder (or 90 degrees away), add the contact lens and OR powers together on an optical cross to determine the new lens powers. When the OR axis is oblique to the refraction axis, the amount of OR cylinder compared to correcting cylinder will assist you in determining axis misalignment. For example, if a toric lens axis is 30 degrees misaligned relative to the refractive cylinder axis, the amount of the OR cylinder and toric lens cylinder should be the same but at different axes. If the toric lens axis is misaligned 15 degrees, the OR cylinder should be approximately one-half the amount of the toric lens cylinder.

An efficient and accurate method to determine the next toric lens prescription using the OR data is to enter the sphero-cylinder over-refraction along with the current toric lens parameters into a cross-cylinder program. CooperVision's Toric Track program is available online at coopervision.com.

Fluctuating vision through a toric soft contact lens is usually attributed to poor rotational stability. If the lens rotates excessively, consider steepening the fit by ordering a steeper base curve or increasing the overall diameter of the lens. Another option: select a different lens design and/or manufacturer.

GAS PERMEABLE TORIC CONTACT LENSES

Gas permeable (GP) sphere and toric contact lenses may be successfully prescribed for patients with almost any refractive error. Patients with high refractive errors (>6.00D) requiring toric lens designs are excellent candidates for gas permeable lenses because of the increased oxygen transmissibility obtained with GP lenses when compared to currently available soft toric lenses. Astigmatic patients with precise vision needs should also consider GP toric lenses.

GP Toric Design

Prescribe a base curve toric, spherical front surface lens design for patients with at least 1.50D of corneal toricity and a manifest refraction cylinder that is approximately 3/2 the corneal toricity at the same axis. Another way to identify patients who would benefit from a base curve toric lens design: perform a sphero-cylinder OR through a spherical diagnostic GP lens. If the resulting OR cylinder is at the same axis and approximately half the amount of corneal toricity, then a base curve toric is the design of choice. For example, a patient with keratometry values of 41.00@180/44.00@90 and a residual OR cylinder of ­1.50 x 180 through a spherical GP contact lens is a good base curve toric lens candidate. The advantage of prescribing a base curve toric design, as opposed to a bitoric, is the ability to make in-office modifications, such as polishing the front surface of the lens and adding minus or plus power.

Prism ballast, front surface torics are generally prescribed with a spherical base curve. This lens design is indicated when there is minimal corneal toricity (<1.00D) and an unacceptable amount of residual astigmatism in the over-refraction. However, be aware that with this design, lens rotation may adversely affect the patient's vision. Front surface toric lenses tend to position inferiorly on the cornea due to the profile of the prism.

Bitoric lenses are useful for cases in which a sphere, base curve toric or front surface toric are not appropriate. This lens design can be ordered empirically using the Mandell-Moore guide or it can be diagnostically designed using a bitoric trial lens. The Mandell-Moore guide is available on the RGP Lens Institute website at www.rpgli.org.

The remaining parameters of a bitoric GP lens can be ordered using your standard design or consulting with your laboratory. Prescribing a peripheral curve system toric by the same amount as the base curve toricity will ensure a round optic zone. Your choice of lens material depends on the lens profile, the patient's corneal health and their daily activities.

Troubleshooting

If, at the dispensing visit, your bitoric GP lens patient is unable to achieve 20/20 vision, perform a sphero-cylinder OR. If the lens is rotationally stable on the eye, the axis of the OR cylinder should match one of the two major corneal toricity meridians. Instructing the laboratory to dot the flattest meridian will allow you to determine if the lens is rotationally stable on the cornea. If the lens is rotationally stable, place the OR and GP lens raw powers on an optical cross and add them together to determine the new GP lens powers. (See P. 42 of the December 2001 issue, or check the Contact Lens Spectrum website [www.clspectrum.com] archives for the Primer column in the December 2001 issue.) If the lens is not rotationally stable, consider increasing the amount of base curve toricity. The base curve toricity should be approximately 0.50D less toric than the cornea. A flat fitting relationship may also contribute to excessive lens rotatation. Remember that when changing the base curves of the lens, adjust the lens powers accordingly. Change the power in both meridians of the lens by the same amount as the base curve (in diopters) is changed (SAM-FAP).

Conclusion

Often in the past, contact lenses were not presented or recommended for astigmatic patients. Providing lenses that offer crisp, clear vision to these patients can be extremely rewarding. We hope this overview of soft and gas permeable toric lenses has provided you with additional tools to tackle those challenging astigmatic patients desiring contact lenses.

Dr. Jung is the current cornea and contact lens resident at the Southern California College of Optometry.

Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at tedrington@scco.edu.