Article Date: 3/1/2008

Not All Bitoric GPs Are Created Equal
GP insights

Not All Bitoric GPs Are Created Equal


Recently, I compared the results of two eerily similar patients who'd previously worn GP lenses. Both had last gone for eye care somewhere other than where their original GPs had been fit, for one reason or another. Both had significant refractive cylinder (>3.00D in each eye) and were refit into soft toric contact lenses by the practitioner who last examined them. Both were unhappy with their vision with the soft toric lenses and felt they should at least consider going back to GPs. Both patients' refractions measured about 1.50D more astigmatism than that measured by keratometry. So what to do next?

Reviewing Bitoric Designs

Different designs of bitoric GPs can be utilized depending on the nature of how the spectacle cylinder compares to the corneal toricity. To compare the outcome of two such designs, I ordered both patients two different pairs of bitoric GP lenses.

The first pairs ordered were designed to precisely correct the measured refractive error and, of course, to fit the astigmatic cornea well. Because the amount of astigmatism measured with keratometry was unequal to the refractive measurement in this design, the bitoric lenses are referred to as having cylindrical power effect (CPE). CPE lenses act much like soft torics in that rotational effects are evident. If lid interaction creates rotation with the blink, transient blur will occur. When little rotation occurs, vision should be excellent because of the precise design and because of the GP lens optics.

I designed the second pairs of bitoric lenses to cut the full refractive cylinder back to nearly equate to the measured keratometric astigmatism. This design is referred to as spherical power effect, or SPE. To refresh your memory on this design, SPE lenses act just like spherical lenses in that rotational effects are eliminated. Because the change in back vertex power approximately equals the change in base curve (±0.50D) between any two meridians, the power change in any meridian induced by lens rotation is negated by the concurrent, equal change in tear lens power. In other words, these lenses can rotate without any noticeable change in vision.

Evaluating the Lenses

I asked both patients to wear each of their respective pairs of GPs for at least a week and then to return to the office to report on their lens wearing experience. Interestingly, both subjects preferred the SPE design when asked to choose. One patient noted that the CPE lenses were slightly clearer when he found the 'perfect' spot, but this benefit was outweighed by the intermittent blur he experienced upon blinking, and he therefore preferred the SPE design.

The other patient actually reported clearer and more consistent vision with the SPE lens design, and clinical evaluation of the lenses confirmed a strong lid interaction that explained the patient's dislike for the CPE design.

Not What You'd Expect

An additional method I didn't try would have been to call a second lab and simply give them the patients' parameters (Ks and refraction) and have them design the lenses, which is something that many practitioners do. It's likely they would have made the lenses in the CPE design, given the parameters for both patients.

As my lens comparison showed, these patients might not have succeeded had I ordered the lenses this way. As a result, they may even have decided to resume their less-than-perfect soft toric lens wear. The implications to both chair time and patient retention are obvious. CLS

Dr. Giedd entered private practice with the Eye Associates of Winter Park in 2000, where he specializes in fitting specialty contact lenses and also performs clinical research for Vistakon.

Contact Lens Spectrum, Issue: March 2008