Article Date: 9/1/2004

toric tactics
Make the Most of Toric Contact Lenses
Astigmatic patients may need your help in seeing the value of this lens option.
BY DEEPAK GUPTA, O.D., F.A.A.O., Stamford, Conn.

 

ILLUSTRATION BY PATRICK GNAN

Research shows that more than one-third of spectacle-corrected patients have cylindrical corrections of ­0.75D or greater. However, many of these patients who have low amounts of astigmatism wear spherical lenses to mask their astigmatism instead of fully correcting for it.

Unfortunately, with the dominance of planned replacement lenses, these patients experience less-than-optimal visual acuity. In addition, many patients who have higher degrees of astigmatism are wearing eyeglasses instead of contact lenses -- many aren't even aware that they're candidates for contact lenses. Or they've been told by their eye doctor that they have to wear GP or conventional toric lenses.

The worst part about this data is that many of these patients are operating on information that's five to 10 years old. So this is where you, as the primary care optometrist, must make a point to ask every eligible patient about their interest in contact lens correction.

Get realistic

With the diversity and abundance of contact lenses on the market, such notions of limited lens wear options are ridiculous -- whether your patient has 0.75D of astigmatism or 5.75D. Furthermore, with the quality of lenses on the market, there's no reason why we shouldn't aim for 20/15 vision, as we often do with our spherical patients. Even presbyopic patients will find happiness with distance-only toric contact lenses or even monovision (until good quality toric bifocal contacts become available). Following are some of the finer points of soft toric lens fitting for your astigmatic patients.

Nuances of lens design

Regardless of modality, soft toric lenses require some means of keeping their cylinder axis properly aligned with that of the patient's refractive error. Most employ one of three methods, or a combination of these three:

1. Prism ballasting. This method of stabilization entails adding more lens material mass to the inferior portion of the lens, making that part of the lens thicker. Doing this helps prevent the lens from rotating and is one of the more common methods of lens stabilization, as it offers a good compromise between patient comfort and stability.

2. Lens truncation. This design involves cutting off the bottom portion of the lens, leaving it flat or truncated. The flat edge usually aligns with the lower lid, and the bottom "corners" help hold the lens in place. Lens truncation is an effective stabilization system, but it tends to make the lens less comfortable.

3. Thin zones. Also called "dynamic stabilization" and "double slab-off," this design involves removing lens material at both the apex (superior portion) and base (inferior portion) of the lens, creating thin zones at the top and bottom. These zones allow the eyelids to exert pressure on the lens and hinder its rotation. This design offers the greatest comfort of the three stabili zation methods, but the the least resistance to rotation.

 

The Becherer Twist Test

Rotation  Success Rate
+ 20� 90% with one lens
+ 15 90% with two lenses
+ 10 70% with three lenses
+ 5 Patient must accept variable vision

Assessing rotation

All toric lenses have their own unique marking to allow us to assess lens rotation. Many manufacturers put their markings at 6 o'clock, while others put them at 3 o'clock and 9 o'clock. When we get the markings aligned ideally, the cylinder axis is located as marked on the trial lens. If the markings are rotated, so is the cylinder axis. We can take advantage of this by visualizing the rotation as clock hours where each clock hour equals 30 degrees.

When a particular lens doesn't align properly, compensate for lens rotation via the LARS rule: Left Add Right Subtract. Clockwise rotation means the bottom portion of the lens has rotated to the left. Therefore, add 10 degrees to the lens axis when ordering the diagnostic contact lens. Counterclockwise rotation means that the bottom portion of the lens is rotated to the right; subtract this value when ordering the diagnostic contact lens.

For example, for a refraction of ­3.25 ­1.25 x 90 and a diagnostic lens rotation 10 degrees clockwise, order a contact lens with power ­3.00 ­1.25 x 100. This assumes that when we place the new lens on the eye, it will again rotate that same 10 degrees clockwise and will make the effective axis at 90.

Fitting guidelines

With the emergence of planned replacement lenses, most clinicians should have at least one toric fitting set in their office. Many practitioners, including myself, advocate that you have several different fitting sets so you can customize your fits according to your patients' needs. By doing this, you'll save yourself and your patients much time and aggravation. As an added bonus, this is the easiest way to build your contact lens practice.

While a patient may return for a contact lens evaluation for a toric lens that you have to order, nothing compares to immediate gratification. A good way to get started is to pick one lens to use as your lens of choice for all appropriate new fits and ask for a fitting set from your sales rep. Once your practice grows, you may consider adding alternative fitting sets for a back up or for those rare occasions where your first lens didn't work.

Borrow my pearls

Following are some important points I've learned in dealing with toric contact lenses:

► While it's okay to be loyal to one brand for a starting point, the key to having a successful and healthy contact lens practice is to be fickle. If one brand doesn't work or isn't appropriate for a patient, don't force it; instead move on to another brand.

► Charge fees appropriate for a more difficult fitting process. This means that we should stop telling patients that correcting for astigmatism is no big deal; it is a huge deal. Although many of your toric fits will go well, occasionally a patient will require multiple visits and multiple trial lenses. Not only should you prepare your patient to financially deal with this up front, he should also be willing to return for multiple visits if necessary.

► Gauge your patient's motivation to wear contact lenses and check his sensitivity to astigmatic correction. One of the easiest ways to do this is to apply The Becherer Twist Test (see table). With the refraction in the phoropter, rotate the cylinder axis until the patient first notices blurring of the acuity chart. The more the patient allows you to rotate the axis, the less sensitive he is and the more likely that you'll have a successful fitting.

Personally, I don't rely on the Becherer Twist Test method quantitatively in terms of lenses used, but rather qualitatively. The more sensitive a patient is to axis rotation, the less likely you are to have a successful fitting experience.

Know your options

Here's a review of the major brands on the market:

Acuvue Toric (Vistakon). A two-week disposable lens featuring double slab-off prism. This lens offers a Dk/t that helps patients who want to wear their lenses long hours, but it sometimes provides less-than-optimal rotational stability resulting in poor visual acuity.

Vertex Toric (Cooper Vision). Originally called Encore Toric, this two-week disposable lens features a back surface prism ballast. This lens provides exceptional rotational stability and good patient comfort. The material offers adequate comfort for patients with mild dry eyes.

Frequency 55 Toric (CooperVision). This monthly disposable lens features back surface prism ballast and offers good acuity and rotational stability. However, with the emergence of the two-week lenses, practitioners don't use it as frequently any more. The lens is available in cylinder powers up to 5.75D, so virtually any astigmatic patient can wear it as a planned replacement soft contact lens.

Soft Lens 66 Toric (B&L). This two-week disposable lens recently expanded its cylindrical power up to 2.75D and was approved for one-week extended wear. It offers a nice rotational stability and excellent vision.

Biomedics Toric (Ocular Sciences). This two-week disposable lens is my favorite because it offers excellent rotational stability and visual acuity.

Earn the title

Regardless of which brand of soft toric lens you choose as your go-to lens, every practitioner should see a high rate of fitting success. Our patients will accept nothing less than optimal visual acuity and comfort. Regardless of what we're legally allowed to call ourselves, as optometrists we're all contact lens specialists. By using the latest in contact lens technology, our patients will accept us as such unconditionally.

References available on request

Dr. Gupta has no financial interests in any of the companies or products mentioned in the article. Reach him at deegup4919@hotmail.com.

 



Optometric Management, Issue: September 2004