Figure 1: The prism-ballasted, back surface O2 Optix Toric for Astigmatism with fluorescein.

Over a year ago, I authored an article for OM on reasons to fit silicone hydrogels.¹ At the time, I said, "silicone hydrogels are the most innovative advance in soft contact lenses in recent years." Things have changed since that article appeared. In 2006, silicone hydrogel torics are the most innovative advance in contact lenses. Just over a year ago, there was, perhaps, one silicone hydrogel toric to choose from. Now there are three, with even more yet to come. Here are ten reasons to prescribe them.

REASON 1: EXCELLENT VISION

Vision is the primary reason to fit torics. Spherical lenses do not quite cut it when it comes to vision for the astigmatic patient. Aspheric lenses bring us closer to great vision, but nothing works better for highly astigmatic patients than a toric lens.

Gas permeable (GP) lenses afford tremendous vision. But I remember in my early years of practice, refitting a GP wearer into soft lenses sometimes proved to be difficult. After the cornea changed shape and I adjusted the lens fit and power, many patients would still have visual complaints. On the chart, they would read 20/20 with their new soft lenses, but they were still uncomfortable.

Pitted against spherical soft lenses, the GP usually wins out in visual performance. The stiffness of the GP material allows for better optics versus the more flexible soft lenses, whether they're silicone hydrogel or not.

When we compare GP lenses with soft torics, however, the story changes. One study used multiple, objective tests to demonstrate that visual acuity with soft toric lenses is equivalent to GP lenses. Results showed the majority of patients preferred the soft toric due to better comfort and handling.² The invention of soft toric lenses had definitely raised the bar for soft lens vision.

REASON 2: EASY TO PRESCRIBE

Toric lenses have improved to the point where fitting is much simpler than in the past. This is largely due to the advances in stabilization design. Most of the silicone hydrogel torics today are very stable. Occasionally, I see lenses that rotate more than 15Þ, but not with the same frequency as in previous years.

Stabilization methods for silicone hydrogel torics take two basic forms: prism ballast and dynamic stabilization. Both designs are based on past forms of hydrogel toric lenses. Additional design elements have made today's lenses more stable, with a smooth front surface and seamless junctions. This means the lids are less likely to catch the edges of the junctions and cause rotation. (See "Empirical Fitting Pearls," on page 55.)

Sometimes, empirical methods do fail. Research shows that empirical fitting success can vary according to the type of astigmatism a patient has. In the past, our definition of astigmatism has been largely limited to the central 3mm of the cornea as measured by keratometry. But corneal topography has identified five different types of astigmatism and demonstrates that the astigmatism in the peripheral cornea (greater than 3.5mm centrally) can differ from that of the central cornea. In some corneas, this difference has a significant effect on toric soft contact lens fitting. In one study, fitting success dropped from about 75% to 50%, depending on the type of astigmatism the patient experienced.⁴

When peripheral astigmatism is greater than central astigmatism, empirical success rates are around 50%. Why does the fitting success drop with these types of corneas? Peripheral astigmatism changes the lens' fit on the eye. The thicker toric lenses drape and flex differently than predicted by keratometry on corneas with peripheral astigmatism and the empirical fitting success rate drops.

A corneal topographer can identify the patient's type of astigmatism and help you troubleshoot your contact lens fittings.³

Empirical methods are not perfect, but they do simplify fitting immensely and make torics easier than ever to prescribe.

REASON 3: YOUR PATIENTS NEED THEM

Estimates suggest some 45% of patients wear astigmatic spectacle corrections. Almost half of your patient population is walking around with 0.75D of correction or more. Most astigmatic contact lens patients stand to benefit from toric lenses, so consider all patients with 0.75D of refractive astigmatism candidates. This translates to quite a few toric patients.

From a clinical point of view, some practitioners consider the "rule of four" when prescribing soft toric lenses. This rule mandates that a toric lens is indicated if the patient's cylinder power is greater than 25% of the sphere power. If the cylinder power is less than 25%, use spherical lenses. Commonly, practitioners consider 0.75D the lower limit and would prescribe a soft toric for all but the highest spherical prescriptions.⁵

Personally, I think it is rather unfortunate that there are patients wearing spherical lenses when they really should be wearing toric lenses. In my mind, the visual benefits are well worth any extra time or expense fitting may entail. Some-times I think back to when we did not have viable toric lenses available. We were creative in trouble-shooting these fits; glasses over the contact lenses, over-minusing, etc. Toric lenses have made life much simpler for both the contact lens fitter and patient.

REASON 4: BETTER OXYGEN TRANSMISSION

Oxygen transmission is one of the reasons silicone hydrogels were invented. The beauty behind silicone hydrogel torics is the combination of the ability to correct for astigmatism and a high-Dk material. Higher oxygen means healthier corneas.

Under hydrogel technology, higher water content results in higher-Dk values. The problem is higher water content tends to produce more fragile lenses. Generally, the overall hydrogel Dk values are much lower than silicone hydrogels. With silicone hydrogels, the opposite occurs: the lower the water content, the higher the Dk value. The overall values for oxygen transmission are much higher than hydrogels can reasonably offer. (See chart "Selected Dk Values and Parameters of Soft Toric Lenses," on page 54.)

REASON 5: FEWER HYPOXIC COMPLICATIONS

Better oxygen transmission means that silicone hydrogel torics have fewer complications than their hydrogel counterparts. Hypoxic changes to the cornea are few and far between with silicone hydrogels. We study edema, striae, thickening, folds, etc. more for historical reasons than as probable clinical findings.

REASON 6: VASCULARIZATION

Hydrogel soft toric lenses reportedly have a higher incidence of vessel encroachment or vascularization. This is largely attributed to the thickness of some lens designs.⁶ Toric lenses, by design, have thick and thin zones for stabilization. Prism ballasting is the most commonly used method to add a thick zone to a lens. Dynamic stabilization, or thin zones, is another method to add the dual thickness profile to a toric lens design. CIBA Vision's O2 Optix Toric for Astigmatism and Bausch & Lomb's PureVision Toric are prism ballasted. Vistakon's Acuvue Advance for Astigmatism uses a form of dynamic stabilization.

Studies from the early 1990s cite the increased stability of prism ballast design, especially if combined with a back surface toric.^7-10 The back surface toric will contour to the corneal surface much like a saddle on a horse. This becomes more important as the cylinder power increases, when a majority of the power is in the cylinder or when stability becomes an issue.

Silicone hydrogel torics have greatly minimized the chance for vascularization. The high-Dk material allows greater oxygen transmission and reduces hypoxia (see figure 1). Figure 2 shows a map of oxygen transmission with a silicone hydrogel toric and a hydrogel toric. There is greater oxygen transmission throughout the entire lens, even within the prism-ballasted area. Silicone hydrogel prism ballasted torics have the advantages of better stability without the pitfalls of thicker zones.

Figure 2: A map of oxygen transmission is shown for a silicone hydrogel toric (top) and a hydrogel toric (bottom).

REASON 7: BETTER FOR DRY EYES

Decreased dryness has been reported with silicone hydrogels. Drs. Schafer and Barr conducted a study in which 318 patients interested in continuous wear were fit with silicone hydrogels. At the baseline exam, patients responded to a questionnaire regarding symptoms and satisfaction with their current contact lenses or spectacles. At six-months, patients responded to an identical questionnaire regarding symptoms, including dryness, and satisfaction. There was a significant decrease in dryness for frequency (p<0.0001) and severity (p<0.0001) during the day and frequency (p<0.0001) and severity (p<0.0001) of dryness at the end of the day. Dryness symptoms were significantly related to symptoms of redness (p<0.0001) and itching (p<0.0001). The authors concluded that dryness symptoms decreased for patients refitted into silicone hydrogel contact lenses after six months of wear.¹¹

REASON 8: OVERNIGHT WEAR

Silicone hydrogels enable the patients to sleep in their lenses. Patients sleep in lenses overnight even when we tell them not to — an estimated 40% of patients occasionally or frequently sleep in their lenses.¹² Patients desire a lens that they can be wear overnight, whether their practitioner sanctions it or not. Overnight wear is advantageous because it offers the ultimate in convenience of full-time visual correction similar to that achieved with refractive surgery, with the added benefit of reversibility. The O2 Optix Toric for Astigmatism and B&L PureVision Toric are approved for overnight wear and PureVision is approved for continuous wear. Acuvue Advance for Astigmatism is approved for daily wear.

REASON 9: LENS SPOILAGE

Silicone is intensely hydrophobic and lipophilic. Plasma surface treatments are applied or wetting agents are added to modify the surface to make them more wettable. An often-overlooked attribute is the impact of the surface on the occurrence of deposits with the various silicone hydrogel lenses.

The surfaces of CIBA Vision's O2Optix Toric lenses are modified in a gas plasma reactive chamber to create an ultra-thin, smooth, continuous and permanent hydrophilic surface. Bausch & Lomb's PureVision Toric lenses are surface treated in a gas plasma reactive chamber, which transforms the surface silicone components into hydrophilic silicate compounds. Vistakon's Acuvue Advance for Astigmatism uses an internal wetting agent (Hydraclear) based upon PVP to modify the surface and wettability of the lenses. In a study I conducted, I found lotrafilcon B lenses to have lower deposits than galyfilcon A materials. Differences in the way the surface is modified may impact each material's likelihood of causing lens deposits.¹³

REASON 10: WIDENING PARAMETER RANGE AND DESIGNS

Hydrogel soft toric lens availability can range from +20.00D to -20.00D of sphere power and up to 10.00D of cylinder power in one-degree axis steps. There is virtually no limit to the potential number of astigmatic patients who can be fit with hydrogel toric lenses. One present disadvantage of silicone hydrogel torics is their limited parameter ranges. But, as time goes on, the parameter range will catch up to hydrogel torics. Because manufacturers are concentrating their resources on silicone hydrogels, we have probably seen the peak of technological innovation with hydrogels. From here on out, silicone hydrogels torics will surpass hydrogel torics in parameters, designs, modalities and capabilities.

Even today, we are seeing a preview of what is to come: multifocals, daily disposables, etc. How does a silicone hydrogel high minus, high-cylinder, toric, daily disposable, in a multifocal design sound? I just can't wait.

1. Hom MM. Ten reasons to fit silicone hydrogels. OM. May 2005.

2. Silbert JA. Conquering residual astigmatism and RGP flexure. Contact Lens Forum. 1990;15(11):15-28.

3. Hom MM. Tips for soft toric success. Optometric Management. July 2001.

4. Reddy T., Szczotka L.B., Roberts C. Peripheral corneal contour measured by topography influences soft toric contact lens fitting success. CLAO 2000;26(4): 180-185.

5. Anastasio M. Considering a soft toric lens. CL Spectrum 2002: 17(3):(Suppl 0):1s-15s.

6. Mannis, M. Contact Lens in Ophthalmic Practice. New York: Springer, 2003; (11): 90-108.

7. Remba MJ, Blaze P. Toric Hydrogel Correction. In ES Bennett, BA Weissman (eds), Clinical Contact Lens Practice. Philadelphia: Lippincott, 1991(41)1-12.

8. Goldsmith WA, Steel S. ICLC 1991;18:227-229.

9. Blaze P. Refining Toric Soft Lens Correction. C L Forum 1988; 11:53-58.

10. Barron C, Dishman A, Akerman D, Bekritsky G, Garofalo R. Correcting astigmatism with the Wesley-Jessen DuraSoft 3 Optifit back surface toric lens. CL Spectrum 1991; 3:51-59.

11. Schafer J, Barr J, Mack C. A characterization of dryness symptoms with silicone hydrogel contact lenses. Optometry & Vision Science; 2003. Annual Meeting Abstract and Program Planner accessed at www.aaopt.org. American Academy of Optometry.

12. Jones L, Dumbleton K, et al.: Comfort and compliance with frequent replacement soft contact lenses. Optom Vis Sci 2002; 79;12s: 259.

13. Hom, MM. Comparison of visible lens deposits between lotrafilcon B and galyfilcon A when used with different lens care solutions. American Academy of Optometry 2006.

This Strategic Skill Builders Continuing Education article is made possible by a grant from CIBA Vision. The content is independently produced by Optometric Management. Please submit your answer card by May 1, 2007.

1.   How many different silicone hydrogel torics are available in the United States for 2006?

a.    3. b. 8. c. 10. d. 15.

2.   Companies that manufacture silicone hydrogel torics include:

a.   CIBA Vision, Vistakon and CooperVision.
b.   CIBA Vision, B&L and CooperVision.
c.   CIBA Vision, Vistakon and B&L.
d.   B&L, Vistakon and CooperVision.

3.   Gas permeable (GP) lenses usually provide better vision than spherical soft lenses because:

a.   GPs are more flexible than spherical soft lenses.
b.   GPs are more difficult to handle than spherical soft lenses.
c.   GPs are stiffer and have better optics than spherical soft lenses.
d.   GPs have greater initial comfort than spherical soft lenses.

4.   Using multiple objective tests, one study showed vision with soft toric lenses:

a.   Is superior than GPs.
b.   Is about the same as GPs.
c.   Is much worse than GPs.
d.   Is worse than spherical soft lenses for astigmatic patients.

 5.   Over the years, toric lens stabilization has:

a.   Improved.   
b. Worsened.
c.   Remained the same.
d.   Radically changed in design for silicone hydrogels. 

6.   Empirical fitting methods:

a.   Use tonometry and retinoscopy.
b.   Use tonometry and vertexed spectacle refraction.
c.   Use K readings and vertexed spectacle refraction.
d.   Use K readings and retinoscopy readings. 

7.   Empirical fitting methods:

a.   Can use tables.
b.   Can account for nasal rotation.
c.   Can add plus for thicker lenses.
d.   All of the above.

8.   To account for nasal rotation for the patient's right lens:

a.   Add 20Þ to 30Þ to the ordered axis.
b.   Add 5Þ to 10Þ to the ordered axis.
c.   Add 5Þ to 10Þ to the ordered axis.
d.   Add 20Þ to 30Þ to the ordered axis.

9.   Corneal topography has identified how many different types of astigmatism?

a.    3. b. 5. c. 8. d. 10.

10. When peripheral astigmatism is greater than central astigmatism, empirical success rates are around:

a.   10%. b. 20%. c. 50%. d. 80%.

11. What percentage of patients wear astigmatic spectacle corrections of 0.75D or more?

a.   45%. b. 40%. c. 35%. d. 30%.

12. Under the "Rule of Four," a toric lens is indicated if cylinder power is:

a.   More than 50% of sphere power.
b.   Less than 50% of sphere power.
c.   More than 25% of sphere power.
d.   Less than 25% of sphere power.

13. With respect to water content and Dk values with silicone hydrogels:

a.   The lower the water content, the higher the Dk value.
b.   The lower the water content, the lower the Dk value.
c.   The lower the water content, the lower the Dk value.
d.   The lower the water content, the higher the Dk value. 

14. Hypoxic complications include:

a.   Edema, striae, thinning.
b.   Edema, striae, folds.
c.   Edema, thinning, folds.
d.   Thinning, striae, folds. 

15. For silicone hydrogels:

a.   Overall oxygen transmission is lower; higher oxygen means healthier corneas.
b.   Overall oxygen transmission is higher; higher oxygen means healthier corneas.
c.   Overall oxygen transmission is higher; lower oxygen means healthier corneas.
d.   Overall oxygen transmission is lower; lower oxygen means healthier corneas.

16. Toric lens stabilization is achieved with:

a.   Prism ballast, back surface toricity, dynamic stabilization.
b.   Prism ballast, small diameter, dynamic stabilization.
c.   Small diameter, back surface toricity, dynamic stabilization.
d.   Prism ballast, back surface toricity, small diameter. 

17. Which lenses are prism-ballasted?

a.   CIBA Vision's O2 Optix toric and Vistakon's Acuvue Advance for Astigmatism.
b.   CIBA Vision's O2 Optix toric and B&L's PureVision toric.
c.   Vistakon's Acuvue Advance for Astigmatism and B&L's PureVision toric.
d.   Vistakon's Acuvue Advance for Astigmatism. 

18. According to the author, approximately what percentage of patients sleep in their contact lenses?

a.   50%. b. 70%.   c. 40%. d.   25%.

19. According to the author, all patients with 0.75D of astigmatism are toric lens candidates

a.   True. b. False.      

20. Drs. Schafer and Barr found:

a.   Dryness symptoms were significantly related to symptoms of redness and itching.
b.   Dryness symptoms were not significantly related to symptoms of redness and itching.
c.   Dryness symptoms were significantly related to wear time.
d.   Dryness symptoms were significantly related to the patient's environment.