Article Date: 2/1/2007

Layout 1

Benefits of Diagnostic Fitting for Bitoric GP Lenses
This non-LASIK candidate rid herself of spectacles via a bitoric GP lens fit with a diagnostic S.P.E. bitoric set.



A 28-year-old Caucasian woman wearing spectacles presented for a contact lens consultation. She said she recently sought LASIK to rid herself of glasses, but was now seeking contact lenses, as she was not deemed a good candidate for the refractive surgery due to the high degree and type of her astigmatism. Her visual demands as an insurance auditor included a good deal of computer time and deskwork. She took no medications other than birth control. Familial ocular and medical history was unremarkable. She said she had tried to wear soft contact lenses several years earlier, but hadn't been able to achieve good vision despite several attempts with custom astigmatic lenses.

Exam findings
This patient's latest manifest refraction was +2.00D – 4.00D X 180 (20/20-) O.D. and +2.50D – 3.75D X 178 (20/20-) O.S. Keratometry revealed 42.00 @180/46.25 @ 090 O.D. and 41.75 @ 180/45.75 @ 090 O.S., with good mire quality O.U. Slit lamp exam showed clear corneas and adnexae, no vascularization, staining, scarring nor any signs of ectasia or keratoco-nus. The key finding: Almost all this patient's refractive cylinder was corneal.

Management
Because this patient displayed a close agreement between refractive and corneal cylinder and similar refractive and corneal cyl-inder axes, I suggested that rath-er than pursue soft astigmatic lenses, she would more likely find success with a rigid lens. So, I informed her of my recommendation: a bitoric gas permeable (GP) lens, which would be able to provide her with clear and stable vision, unlike her experience with soft astigmatic lenses. (Her difficulties with these highly powered, soft toric lenses were likely due to misalignment of the toric lens cylinder axis, as well as some instability due to lens rotational movement.)

I further informed the patient that the bitoric GP lens would be more economical long-term than the custom-made, high-powered, soft toric lens, due to the widely known advantages of rigid lenses having longer wearing life and greater ease of maintenance than soft lenses. The patient agreed with my recommendation.

To optimally fit this patient, you can either use the Mandell-Moore Bitoric Fitting Guide (GPLI www.gpli.info) to empirically design a bitoric lens, or use a diagnostic "spherical power effect" (SPE) bitoric GP fitting set. The Mandell-Moore Guide allows you to design a lens using just refractive and keratometer readings. It takes into consideration both vertex power adjustments (for high prescriptions over 4.00D) and a built-in fitting allowance (or "fit factor") to make sure that the selected lens meridians don't align too snugly on the patient's cornea. If you don't own a diagnostic toric GP fitting set, realize that the guide is very helpful in determining the initial parameters. It also reduces the chances of mathematical mistakes that may occur when you attempt to empirically design rigid toric lenses using optical crosses, by its simple "add up each column" template. Its downsides: The Guide is based on keratometry, which often may be inaccurate and only provides data over a very small part of the central cornea (about 3mm). It also doesn't allow you to see the lens position, movement and effects of the lids, or provide a fluorescein assessment to evaluate tear exchange or accuracy of the desired toric alignment.

Bitoric SPE diagnostic fitting sets enable me to help patients who have high corneal and refractive astigmatism. For those like this patient who have good agreement between the amounts of corneal and refractive astigmatism, we know that spherical lens optics provide sharp and stable visual acuity, which won't be affected by potential lens rotation. This is due to the special nature of SPE bitoric lenses in which the laboratory applies a compensating cylinder on the front surface of the lens, to offset the acuity-degrading effects caused by the rear surface of the lens when it encounters the tear film. Thus, the lens always provides stable vision, without regard for lens rotation (although most toric GP lenses are rotationally stable anyway). Also, when compared with empirical fitting, bitoric GP diagnostic fitting sets provide a truer representation of the fitting character- istics of the lens and a quick and more accurate means of determining the final lens powers and potential visual acuity. Another bonus: SPE bitoric fitting sets are reusable, relatively inexpensive and once purchased, don't need to be replaced over the typical practitioner's lifetime. Finally, these sets make fitting patients easy, with chair time comparable with fitting conventional GP lenses.

Fitting

Four steps went into properly fitting this patient: determining base and peripheral curves, overall diameter and powers.

Base Curves, peripheral curves, lens diameter. I began by fitting the lens on-K or slightly flatter than K on the flat meridian. Then, I under-corrected the steep meridian by about 1.00D to ensure adequate tear exchange and prevent a physiologically tight fit.

Since this patient has about 4.00D of corneal toricity (actually 4.25D), I only want 3.00D of cylinder on the back surface of the lens, following the undercorrection guideline. I therefore chose to use my 3.00D SPE bi-toric fitting set for my evaluation (a 2.00D and a 3.00D SPE bi-toric set work optimally for most O.D.s, as these can be used for patients who have up to 5.00D of corneal astigmatism).

I fit the right eye on the flat K and the left eye 0.25D flatter than the flat K. So, if we look at the K reading for the right eye, then the base curve (BC) of the flat meridian (FM) is 8.04mm (or 42.00D). Since I am using a

3.00D SPE set, then I know that the steep meridian (SM) must be 3.00D steeper than the FM, and so the BC of the steep meridian will be 7.50mm, (or 45.00D). All lenses in my diagnostic sets have Plano (PL) powers in the flat me-ridian, and the steep meridian is either -2.00D or -3.00D depending on which fitting set I use.

So, the lens for the right eye of my patient has:
BC: 8.04mm (FM)/7.50mm (SM)
Power: PL (FM)/-3.00D (SM)

And, the left eye of my patient has:
BC: 8.13mm (FM)/7.58mm (SM)
Power: PL (FM)/-3.00D (SM).

I recommend the use of spherical peripheral curves when lens toricity is 3.00D or less. For ad-ded rotational stability when needed, or when lens toricity exceeds 3.00D, I use toric peripheral curves. Lens diameter for bitorics generally is similar to conventional GP lenses and is influenced by pupil diameter and palpebral aperture size. My diagnostic sets typically are 9.2mm diameter, with 7.8mm optic zones. This lens size worked well with my patient.

Analyze fluorescein pattern. I employed a topical anesthetic, instilled fluorescein and used a yellow filter to evaluate the selected diagnostic lenses. Then, I looked for an alignment pattern (see figure 1), or one in which a low degree of toricity was present (see figure 2), as I deliberately under-corrected the full amount of corneal cylinder. Should the pattern appear steep or with apical clearance (see figure 3), remove the lens and replace with the next flattest lens in the fitting set. If the lens appears flat, use the next steepest lens in the fitting set (see figure 4). If the natural flat cylinder axis appears to have reversed, then there is too much cylinder in the diagnostic lens (see figure 5). Try applying a lens that has less cylinder. In this case, both diagnostic lenses looked nicely aligned, with slightly greater clearance in the vertical meridian, consistent with the under-correction in this steeper merid- ian. The lenses centered and moved well and showed good peripheral clearance.

Determine the final prescription powers. Once I determined the "best fit" for the patient (in this case it was the initially selected base curves), I simply over-refracted with loose spherical trial lenses. Visual acuity should be very good if there is good agreement between corneal and refractive astigmatism, as was the case with my patient (who achieved 20/20 in each eye after over-refraction). If this is so, then simply add the spherical over-refraction power to both meridians of the labeled diagnostic lens powers. Then, you've determined the final lens powers of the SPE lens for the initial lens order.

In cases in which spherical over-refraction doesn't provide good acuity (e.g. less than 20/25), it's likely due to the presence of residual astigmatism. In these cases, all is not lost. Repeat your refraction spherocylindrically. Then carefully add each respective meridian's power to its counterpart in the diagnostic lens. The result will be a lens known as a cylindrical power effect (CPE) bitoric. As noted earlier, bitoric lenses are rotationally stable. However, in the case of a CPE bitoric, should the lens dem- onstrate axis mislocation or instability, then acuity is reduced, unlike the SPE bitoric.

In this patient's case, the spherical over-refraction was +2.00D (20/20) O.D. and +2.75D (20/20) O.S. Thus, her final lens powers were: +2.00 (FM)/-1.00 (SM) O.D. and +2.75 (FM)/-0.25 (SM) O.S. (See "Lens Order Information," page 56.)

Follow-up

I saw this patient two weeks after lens dispensing. She report-ed uneventful adaptation to her new lenses. She also said she was extremely satisfied with both her vision and comfort. Slit-lamp exam revealed excellent physiological tolerance O.U.  Another follow-up at six months confirmed these findings. She was wearing her lenses at least 12 hours each day.



Optometric Management, Issue: February 2007