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Formcon
Contact Lens Design for Keratoconus
BY
CHARLES R. EDMONDS, OD, FAAO
In 1974, Leonard Bronstein invented the original Formcon aspheric rigid contact lens consisting of a spherical base curve, an aspheric
intermediate area and a spherical peripheral curve with a 12.25mm
radius and a width of 0.4mm. The key to the design was the aspheric intermediate
curve, accomplished initially by crimping the lens and more recently by computerized
lathing creating anywhere from 0.25mm to as much as 2.00mm of radius change.
The lens was specifically designed for use in keratoconus,
corneal rehabilitation and reorganizing corneal symmetry. By varying back surface
eccentricity values, the bearing area of the contact lens could be spread out over
larger areas of the peripheral cornea to reduce pressure points and allow lens movement,
good tear flow and centration.
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Figure 1. The superior portion of the keratoconic
cornea was considered the more normal portion on which the Formcon lens fit was
based
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The original design advocated fitting relatively large
diameter
lenses flatter than K. At the time, the 9-ring corneoscope was the diagnostic topography
tool for evaluating keratoconus. The superior portion of the keratoconic cornea
was considered the more normal portion on which the Formcon lens fit was based (Figure
1). The result was often a very flat fitting contact lens design that put excessive
bearing on the cone, causing unacceptable inferior lens edge standoff.
Modern topography evaluation and research gives us a better understanding
of the keratoconus corneal surface changes from center to periphery and of cone
size, shape and location. The new information allowed the unique Formcon design
to be modernized and reformatted.
A
new Formcon trial kit is now available for fitting keratoconus patients with a systematic
approach. The lenses consist of varying eccentricity values of realistic and logical
increments, lens powers characteristic of keratoconus, small optic zones with appropriate
diameters to properly align the cone and peripheral curves to reduce excess edge
standoff.
We've used the Formcon design in the dry desert climate of Arizona
to successfully fit hundreds of keratoconus patients over the last 25 years. I'll
describe the resulting fitting philosophy, following a retrospective evaluation
of 500 keratoconus contact lens wearers using the reformatted Formcon GP lens design.
Lens Design
Histological and biomolecular research shows that the thin apex
of the keratoconus cone is fragile. Mechanical stress on the cone causes a cascade
of enzymatic pathway events that lead to modification of corneal cellular behavior,
corneal thinning, apoptosis and ultimately loss of corneal transparency. The lens
design should avoid excessive bearing on the fragile steep apex and instead bear
on the thicker, flatter intermediate peripheral area without causing excessive lens
edge standoff.
A small enough base curve optic zone is paramount to align the
cone apex without tear pooling around the base of the cone, but too small may compromise
vision and promote unwanted glare. An aspheric intermediate peripheral curve that
flattens from the edge of the optic zone and blends into the peripheral curve provides
a large bearing area over the irregular, flatter part of the cornea without seal
off. The spherical peripheral curve is adjusted to reduce excessive edge standoff
by steepening as the stage of keratoconus progresses.
Trial Lens Design
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Figure
2. Formcon fluorescein pattern. |
Research shows that keratometry is inadequate for
describing the
cornea's shape in keratoconus. Corneal topography evaluation will best show the
cone's steepest curve of the apex, size, shape and location. The axial topography
map provides the best indication of the steepest cone area. Today, topographical
maps are preferred for keratoconus patients.
Corneal topography studies of keratoconus show that as the central
cone steepens, the peripheral cornea flattens. This valuable information is incorporated
into the Formcon trial fitting kit. Table 1 describes the Formcon trial lens parameters.
Logical incremental changes allow for flexibility in fitting a
variety of cone types. As the base curve steepens, the optic zone decreases, the
diameter decreases, the intermediate aspheric rate of flattening increases and the
peripheral curve steepens. Retrospective evaluation of past keratoconus patients
shows the need for higher minus power lenses as the base curve steepens. The trial
lens optic zone size compensates to avoid pooling around the cone, the aspheric
intermediate curve compensates for increased peripheral flattening as the cone steepens,
the decreasing diameter retains proper sag depth as the cone increases and the steepening
peripheral curve prevents excessive inferior edge standoff.
Base
Curve and Optic Zone Size
To determine a starting base curve, simply find the steepest curve
of the cone apex on the axial corneal topography map and use that as your starting
base curve. You can make a less accurate determination using the steepest keratometry
or simulated keratometry reading. Place the closest Formcon trial lens with the
calculated base curve on the eye. We recommend using one drop of a topical anesthetic
on the eye before applying the trial lens to reduce patient apprehension, excess
tearing and adaptation time. We use a small amount of fluorescein to evaluate the
amount of central bearing on the cone apex. Generally for a globus or sagging cone
you need a less steep base curve with a larger optic zone, and for a nipple cone
you need a steeper base curve with a smaller optic zone.
The desired fluorescein pattern is a slight apical touch of 2mm
or less to slight apical clearance (Figure 2). Too much apical clearance and pooling
under the lens results in lens flexure, reduced acuity and peripheral lens seal
off. The optic zone size will decrease as the base curve steepens. The
relationship of BC and OZD is already calculated in the Formcon trial lenses as are the appropriate
aspheric eccentricity values and the peripheral curves.
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TABLE 1 |
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Formcon Trial Lenses in Boston XO MaterialDr. Edmonds practices in
Tucson, AZ, and is a Diplomate in the Cornea and Contact Lens Section of the American
Academy of Optometry. |
BASE
CURVE (MM) |
POWER
(D) |
DIA
(MM) |
OZ
(MM) |
ASPH
|
PCR
(MM) |
7.90
|
–3.50
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9.0
|
7.70
|
0.60
|
12.25
|
7.80
|
–3.50
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9.0
|
7.60
|
0.60
|
12.25
|
7.70
|
–3.50
|
8.9
|
7.50
|
0.70
|
12.25
|
7.60
|
–4.00
|
8.8
|
7.40
|
0.80
|
12.25
|
7.50
|
–4.50
|
8.8
|
7.30
|
0.80
|
12.25
|
7.40
|
–5.00
|
8.6
|
7.20
|
0.90
|
12.25
|
7.30
|
–5.50
|
8.6
|
7.10
|
0.90
|
12.25
|
7.20
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–5.50
|
8.4
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7.00
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0.90
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12.25
|
7.10
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–6.00
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8.4
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6.90
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1.00
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11.25
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7.00
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–6.50
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8.4
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6.80
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1.00
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11.25
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6.90
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–7.00
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8.4
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6.70
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1.00
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10.25
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6.80
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–7.50
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8.4
|
6.60
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1.00
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10.25
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6.70
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–8.00
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8.3
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6.50
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1.00
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10.25
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6.60
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–8.50
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8.2
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6.40
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1.00
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10.25
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6.40
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–9.00
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8.2
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6.20
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1.20
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10.00
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6.30
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–10.00
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8.2
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6.10
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1.20
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10.00
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6.20
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–10.50
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8.2
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6.00
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1.20
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10.00
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6.10
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–11.00
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8.2
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5.90
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1.20
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10.00
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6.00
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–12.00
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8.2
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5.80
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1.20
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10.00
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Overall Lens Diameter
For most keratoconus patients fit with Formcon lenses, a
diameter
of 9.0mm and smaller is indicated. As the cone steepens and inferior edge standoff
problems increase, smaller lens diameters maintain a better cornea/lens relationship.
The trial lenses incorporate the needed size changes as the base curve steepens
to maintain appropriate sag depth, proper positioning, bearing area relationship
and proper edge lift.
Intermediate Asphericity
The amount of intermediate aspheric flattening is already calculated
in the Formcon trial lens. The steeper the base curve needed, the more peripheral
flattening is indicated. You may custom order the asphericity for advanced nipple
cones with excessive peripheral flattening and for large inferiorly positioned globus
cones. Remember, the higher the aspheric number, the faster the flattening rate.
Usually with advanced nipple cones, you can't avoid some central touch. Patients,
however, usually well-tolerate such fits.
Reducing Edge Lift
Keratoconus topography typically shows inferior steepening around
the cone. Inferior lens edge standoff often occurs because of this irregular topography.
The Formcon trial lens parameters usually compensate for this problem. If you observe
excess inferior edge lift with trial lens fitting, you can make four changes:
1. Steepen base curve by at least 0.75D.
2. Reduce optic zone diameter by at least 0.2mm.
3. Reduce overall lens diameter by at least 2.0mm.
4. Steepen peripheral curve radius.
Power Calculation
Usually you need a higher minus power as the cone progresses and
you need steeper base curves. The trial lenses take this into account. When measuring
power through the trial lenses, you'll generally find a spherical over-refraction.
Due to the irregular nature of the astigmatism, you rarely need spherocylindrical
power. Remember not to over-minus the refraction. A mispositioning lens with the
blink puts the pupil in front of the flat aspheric zone incorrectly, indicating
the need for more minus power.
Lens Material
A balance of lens wettability, durability and oxygen permeability
is always advantageous. The Formcon can be manufactured in any material. In the
past a mid-Dk material such as Fluoroperm 30 or Boston EO worked well for our patients.
However, with the clinical availability of specular microscopy, allowing for evaluation
of low endothelial cell counts, you may need higher-Dk materials. Today we exclusively
use GP lens materials more than 100 Dk. Your clinical judgement will dictate which
material you use.
Piggyback Combination
Sometimes regardless of lens design, a keratoconus patient can't
comfortably tolerate a GP contact lens. In such cases, you can place a soft lens
on the eye as a base upon which to fit a Formcon GP. To date, the recently developed
ultra high-Dk silicone hydrogel soft lenses are our first choice for the soft lens
base. The stiff mechanical modulus of the materials, plus their high oxygen transmissibility
and disposability, make them ideal for combination with the Formcon lens.
Because keratoconic corneas present with a highly prolate
corneal profile, use a low minus power silicone hydrogel lens (–0.50D to –1.00D)
as a base because it presents a more oblate foundation upon which to place the Formcon
lens. To a limited degree, you can compensate for more or less minus power by changing
the power of the soft lens base. Power changes above 1.50D are less successful with
the soft lens base, and you must change the Formcon GP lens power.
Conclusion
Even though rigid contact lens management of keratoconus has been
the mainstay of treatment for the last 50 years, there's no consensus on which lens
design works best. Advanced topography instrumentation, new wettable, high-Dk materials
and computer-driven lathes allow for production of sophisticated aspheric lens designs
for keratoconus treatment. I believe the Formcon lens is a successful contact
lens alternative to offer your keratoconus patients.
Dr. Edmonds practices in Tucson, AZ, and is a
Diplomate in the Cornea and Contact Lens Section of the American Academy of
Optometry.
Contact Lens Spectrum, Issue: November 2006