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Special Fits for Special Eyes
Soft toric tailored to patient's corneal
architecture.
ROBERT L. DAVIS, O.D.&
P. DOUGLAS BECHERER, O.D., F.A.A.O
Imagine
no longer being at the mercy of the features of soft toric contact lenses when attempting
to treat your difficult-to-fit astigmatic patient. In essence, neither you nor your
staff would have to search among the numerous prefabricated soft toric lenses for
the optimal lens design for this type of patient. Sound too good to be true? Then
you haven't heard of the SpecialEyes soft toric contact lens. This lens is available
in either a yearly replacement called SpecialEyes 49% or a quarterly replacement
called SpecialEyes 59%. Both lenses are tailored to the patient's individual corneal
architecture, allowing you to provide him with the best lens fit possible.
Here are two cases
that illustrate the success of the SpecialEyes soft toric contact lens.
1. Unavailable parameters for
policeman
A white, 48-year-old
policeman, presented for his annual comprehensive eye exam and expressed an interest
in soft lenses. He said his occupation made spectacle wear difficult, and he was
concerned about frame displacement. He reported previous, unsuccessful gas permeable
(GP) lens wear due to lens discomfort from lid irritation. He further said that
his previous
eyecare practitioner told him that soft lenses were not an option, as he has a high
astigmatic prescription. This patient's brother had referred him to our practice.
The policeman's
subjective refraction was 3.50D — 10.00D X 10 with an Add of +1.50D O.D. and
3.50D — 9.50D X 170 with an Add of +1.50D O.S. Topography readings revealed
O.D.: 39.62 X 48.37 and O.S.: 39.62 X 48.25.
Truth
be told, patients who have high amounts of astigmatism, especially patients who
have predominant astigmatism compared with the spherical prescription, and patients
who have irregular astigmatism, tend to be less successful in soft toric lenses.
This is due to lens rotation. It's influenced by lid dynamics and the amount of
rotation creating blur. This defines the patient's blur tolerance. But, with the
advent of robotically integrated optical blocking and the numerically controlled
lathes of the SpecialEyes lens, you can order a custom soft toric lens manufactured
to precisely fit the cornea without lens rotation.
Lid
dynamics and blinking characteristics influence lens rotation and the success of
lens fit. Tolerance to cylinder axis rotation varies from patient to patient. You
can evaluate this using the Becherer Twist Test: Use the phoropter
and rotate the cylinder axis during the spectacle refraction
until the
patient
first notices blur. The more rotation a patient can tolerate, the more likely he
is to achieve success in a soft toric lens.1 Our patient was able to
tolerate 6° of axis rotation before blur. So, if the lens is fit with less
than 6Þ of rotation, the fit will be successful.
I
ordered a SpecialEyes 59% quarterly frequent replacement hioxifilcon lens that had
a 14.9mm diameter and an 8.0mm base curve. The prescription was 3.50D —
8.00D X 10 O.D. and 4.50D —
7.50D X 175 O.S. Both lenses exhibited excellent centration with less than 2°
of lens rotation with blink. The lenses moved 0.5mm vertically
with blink and showed adequate lag with superior gaze.
The patient reported
being happy with his lens vision and comfort. At a follow-up appointment one-week
later, he reported no complaints and said he was wearing the lenses at all waking
hours.
2. Megalocornea mess
A 39-year-old Hispanic
male presented complaining of discomfort from his GP lens. So, he requested a different
approach to correct his ammetropia. History revealed that his past eyecare practitioners
had difficulties fitting him in soft toric lenses because of his extremely large
corneas. This patient was referred by a successful contact lens-wearing friend.
Slit
lamp exam revealed megalocornea — a nonprogressive enlargement of the cornea
to 13mm or greater.2 (For some unknown reason, this patient's prior doctors
did not identify him as having megalocornea.) In this condition, the cornea and
limbus are enlarged, but the cornea itself is histologically normal. Megalocornea
patients are difficult to fit in contact lenses because the sagittal height of the
eye creates a steeper than normal relationship to the central base curve. And, fitting
paradigms don't work because they are developed for corneal diameters between 11.4mm
to 12.2mm. Further, a soft lens gaps on the cornea in the periphery, and a GP lens
tends to fit flat with excessive edge lift unless you implement corrective measures
to modify the contact lens/cornea-fitting relationship. One corrective measure is
to make the lens is larger and increase its sagittal depth.
Subjective refraction was 7.50D Sphere,
with a best visual acuity (BVA) of 20/20 O.D. and -8.00D Sphere with a BVA of 20/20
O.S. Automated keratometry readings were 47.87/48.87@95 O.D. and 47.87/48.37@85
O.S. Topography revealed normal surface regularity O.U. and a corneal diameter of
13.3mm O.U. His pachymetery was 502 O.D. and 506 O.S.
I ordered a Special
Eyes 49% hioxifilcon lens with a 15.4mm diameter and a 7.6mm base curve with a prescription
of -6.50D O.D. and -7.50D O.S. The patient reported good comfort, and his visual
acuity was 20/20 monocularly. In addition, both lens designs centered perfectly,
and lens movement in primary gaze was adequate and exhibiting sufficient lag with
superior gaze. Also, no edge lift of the lens or gapping was evident in the periphery.
The 15.4mm diameter demonstrated adequate corneal coverage. The desired amount of
lens coverage on the cornea was 2mm to 3mm larger than the horizontal visible iris
diameter. This is important because when designing a lens, the optimal coverage
is 2mm to 3mm larger than the corneal diameter. The reason for this: Lenses equal
to or smaller than the corneal diameter can cause adverse corneal outcomes, such
as neovascularization or corneal insult.
The
SpecialEyes lens has virtually no limitation on parameters, and the company can
alter the lens
diameter — the most significant variable in determining sagittal height, quality
of fit, stability, comfort and resultant acuity — in one tenth of a millimeter
increments, giving you total control of lens parameters. In
providing this option, you can design an exact fit for the patient. Also, you can
order both spherical and cylinder powers in one tenth of a diopter increments (more
critical than the human eye). This is significant because patients can identify
blur when you don't prescribe the exact prescription. SpecialEyes gives
you the option to provide an exact prescription for every patient. Finally, you
can specify the Axis — one of the most significant variables in toric performance
— in 1° increments around the clock. This means your patient can attain
comfort as well as maximum and stable vision.
1.
eMedicine Article Search. Megalocornea. Emedicine.
www.emedicine.com/cgi-bin/foxweb.exe/searchengine@/em/searchengine?boolean=and&book=all&maxhits=40&HiddenURL=&query=megalocornea
(Accessed January 16, 2007).
2.
Clinical Management of Contact Lenses. ES Bennett, VA Henry. Philadelphia: Lippincott,
Williams & Wilkins, 2000. Page 43.
Optometric Management, Issue: February 2007