TORIC FITTING

Intermediate Toric Lens Prescribing

By Rhonda S. Robinson, O.D.
February 2000

Today's modern contact lens designs, advanced manufacturing processes and affordable prices make most toric soft contact lenses relatively simple to fit and easy for patients to wear. But what happens when the "fit" doesn't "read the script," or when the patient becomes extraordinarily frustrated with the adjustment?

The key to making all toric lens fits less frustrating is to reach an understanding of which lens designs optimize the lens' stability on the eye and which water content materials optimize comfort and oxygen permeability. First, ask yourself the following questions:

• Can you predict stability?
• How do you decide the initial lens of choice?
• Is your "go to" lens always your first lens of choice, regardless of
  cornea type?
• Do you keep topography results in mind when deciding which lens design to try first?

I believe that successful toric lens fitting depends heavily on lens stability. However, when stability is not achieved through your favorite diagnostic trial, failure is not inevitable. Try looking back at the keratometry, corneal topography, refractive error and patient history to help you decide which lens will increase your chances of achieving success.

Sharp vision is obviously one of the goals of successful toric contact lens fitting, but don't rule out comfort, since it's usually the reason why patients discontinue lens wear. Understanding the difference between different lens materials and designs will help you choose a lens that will increase your overall success rate and achieve patient satisfaction.

Choosing a Material

A typical example of material dependency for comfort and wearability occurred just the other day in my practice. A patient who, after having unsuccessfully tried soft contact lens wear, heard about disposable lenses, and thought they might solve her comfort problems presented to my office. Her previous conventional lenses provided clear vision, but they became uncomfortable toward the end of the day. Her previous eye care practitioner told her that her comfort problems were due to dry eye, and subsequently attempted to fit her with extended wear contact lenses on a daily wear basis. Unfortunately, she felt that the newly prescribed extended wear lenses didn't improve her level of comfort very much. So, after a month of trying to adapt to wearing the lenses all day, she gave up and went back to wearing spectacles.

We all experience the frustrations of inadequate records regarding patient history of previously worn contact lenses. Without previous records, it's hard to know if a patient was fit with a high oxygen permeable lens, a higher or lower water content lens or a thinner lens. Practitioners' classifications of "extended wear" can mean a variety of things.

My first consideration in cases like the one mentioned above is to concentrate on the lens material (water content). This is especially true for soft toric lenses because of the nature of their design. I use the FDA's classification of categorizing soft lens materials into four groups. As a review, the four groups are shown below.

Group I: low water content, nonionic

Group II: high water content, nonionic

Group III: low water content, ionic

Group IV: high water content, ionic

(High water content >55% water)

My experience has allowed me to rely on the following guidelines:

• High water content lenses may dry the cornea due to excessive evaporation.
• Lower water content lenses, when manufactured in very thin designs, may also have a drying effect. Therefore, thickness and movement are important parameters to consider.
• Nonionic materials resist protein buildup better than ionic materials, so patients with buildup problems would benefit from a Group I or Group II material.
• Ionic materials may coat a bit better with mucin than nonionic materials upon blinking and ultimately, provide better overall comfort.

The Bottom Line

Unfortunately, there's no one material group that fully corrects dryness or comfort. The bottom line is that each lens material is different and helps to accomplish different things. Problem-solving becomes easier when you learn which lenses belong to which groups and thus, are not duplicating one problematic lens with another from the same category. When all else fails, try a different lens.

Our dry eye patient may benefit from a Group I lens initially, but if it proves unsuccessful, the next lens of choice obviously shouldn't be another lens from Group I.

Choosing a Design

Whether you have a corneal topography system or not, the key to selecting the right lens design is accurately identifying the type of astigmatism you are correcting. Is this residual astigmatism? Is there a relatively spherical cornea with a slight regular astigmatism? Or is this a highly toric cornea requiring different lens design considerations?

My experience has proven that for overall stability and success, back surface toric lens designs give better stability to toric corneas and front surface toric designs work better for spherical corneas. It sounds easy, but remember to take material and design into consideration together. This way, your choices become more clear. I've developed a handy chart that gives examples of some common toric contact lenses that I use (Table 1). It's not meant to be all-inclusive, but it's designed to be a practical tool to aid you in distinguishing between contact lenses within a specific FDA assigned group and lenses within the groups themselves. I use this practical tool to validate my first lens choice and my favorite lens design, and also to determine any subsequent follow-up problem-solving attempts.

If your initial favorite diagnostic lens produces symptoms of excess protein buildup, dry eye and residual astigmatism, try a low-water, nonionic, front surface toric.

Using the chart, I found that Bausch & Lomb's Optima toric lens and CIBA Vision's Torisoft lens are also good alternatives.

If the patient is interested in planned replacement or disposable lenses, the chart indicates that the Gold Medalist lens by Bausch & Lomb would be a good first choice to try. If you are unable to achieve stability with a front surface toric lens design, try a back surface toric design.

Finally, remember to use lens rotation evaluations and overrefraction calculations to fine tune your
patient's final lens prescription. There's more to eliminating frustration beyond rotation than mathematical gymnastics and algebraic equations. Try the methods highlighted in this article to help you decide which toric lens design is right for your patient. You'll experience more success with less overall chair time, not to mention less frustration when things go wrong. 

See the Contact Lenses and Solutions Summary (CLASS) supplement to the July 1999 issue for a more complete listing of toric contact lenses.

Dr. Robinson is in private practice in Indianapolis, IN. She is also an advisory board member and academic consultant for Bausch & Lomb and holds contact lens problem- solving workshops in optometry schools across North America.

 Table 1:  Common Toric Soft Lenses