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