HYBRID SPECIALTY FITS
Fitting Irregular
Corneas with a Hybrid Lens
Learn
step by step how to fit keratoconic, post-surgical and other irregular corneas with
hybrid lens designs.
By Michael DePaolis, OD, FAAO, Jane Beeman,
COT, FCLSA, & Robert Ryan, OD, FAAO
The
emergence of a newer-generation soft skirt hybrid lens modality has resulted in
as much of an evolution in fitting methodology as in material science. The SynergEyes
family of lenses (SynergEyes, Inc.) offers multiple FDA indications ranging from
the full continuum of naturally occurring ametropia and presbyopia to irregular
astigmatism resulting from keratoconus, refractive surgery and corneal trauma. The
broad indications have opened the door for clinicians to harness the potential of
these new designs.
New Materials, New Fitting Techniques
During the SynergEyes FDA clinical trials, clinicians
used fitting methods similar to those with previous-generation hybrid lenses. The
learning that took place was manifold. Apical clearance quickly replaced alignment
fitting in an effort to increase post lens tear film thickness given the surface
properties of the hyper-Dk rigid material. We now generally recommend central clearance
of at least 30μm on most eyes.
Controlling apical clearance goes beyond modulating
the base curve radius. SynergEyes offers multiple skirt curve radii for a given
base curve radius. The Total Clearance Technique recommended for the SynergEyes
KC (keratoconus) and PS (post surgical) lenses determines the optimum parameters
from the center of the lens outward.
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Figure
1. SynergEyes KC on a post-LASIK patient.
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Total Clearance Technique for Keratoconus
The SynergEyes KC design is a prolate ellipse
with a fixed eccentricity for all base curve radii. The base curve continues to
a diameter of 9mm where it transitions to a spherical skirt curve radius that terminates
at a peripheral bevel. The lens is designed to clear the keratoconic cornea until
a feather touch at 9mm. The skirt should align with the peripheral cor-nea and suspend
the lens above the cornea from the 9mm chord inward.
For the initial diagnostic lens, we recommend
that you use keratometry or topography (simulated keratometry) values to determine
a lens closest to the steep meridian as a starting point. Consider cone morphology,
assessing the keratoconus as either a globus, oval or nipple type. A base curve
approximately 2.00D flatter than the steep meridian is more appropriate for a nipple
cone, while a base curve closer to the steep meridian is required for a globus cone.
Use the flattest skirt curve radius in the initial diagnostic lens.
The
goal is to observe total apical clearance without bubbles larger than 1mm. Occasionally,
bubbles indicate air trapped in the retrolens space upon application and are transitory
in nature. Simply removing the lens, placing a drop of high molecular weight fluorescein
in the bowl and reapplying the lens will resolve this issue. However, persistent
bubbles warrant a base curve change. When a base curve provides total clearance
free of bubbles, the next step is to optimize the skirt curve radius.
Optimizing Skirt Curve
Radius The base curve radius that meets the above conditions in conjunction
with the flattest skirt may be ideal. The determining factor is the presence or
absence of a dark ring of touch at about a 9mm chord. If a dark ring is present,
the touch at the base curve/skirt curve is too heavy, which may lead to late onset
tightening. Reduce the dark ring of touch by using a steeper skirt curve radius.
The proper step in this case is to
keep the same base curve radius and go to the next steeper skirt curve radius. Continue
to steepen the skirt curve radius until you observe a feather touch at the 9mm chord,
which assures adequate contact lens vaulting and continual blink-induced movement.
Optimizing Base Curve Radius
Bubbles may sometimes reappear under the optic zone when the proper skirt curve
is in place. When this occurs, flatten the base curve by 0.1mm to eliminate the
bubble. Once the bubble is gone and you observe a feather touch at 9mm, the diagnostic
fitting is complete and you can over-refract to determine lens power.
Case Study A 47-year-old
female presented having undergone well-healed LASIK OU four year ago with two enhancements
OU. She had a post-surgical ectasia and complained of blurred vision and dryness.
Biomicroscopy revealed inferior thinning and epithelial pigment OU. Her K readings
were OD 48.00/56.00 and OS 42.50/49.75, and her refraction was OD +2.00 –7.50
x92 = 20/100 and OS +2.00 –6.50 x88 = 20/60. We fit her with SynergEyes KC
with parameters of 6.90mm base curve, 8.5mm skirt curve, 14.5mm diameter, –8.50D
20/30+ OD and 6.70mm base curve, 8.5mm skirt curve, 14.5mm diameter, –6.50D
20/25 OS. Figure 1 shows the fluorescein pattern OS. The patient wears the lenses
for approximately 14 hours each day.
Total Clearance Technique for Irregular Oblate Corneas
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Figure 2. SynergEyes PS features three sagittal
depths. |
Refractive surgery for myopia including RK, PRK,
LASIK and Intacs (Addition Technology) renders corneas oblate in geometry. In addition,
some corneas that undergo penetrating keratoplasty, experience mechanical trauma
or develop pellucid marginal degeneration may be best suited for an oblate lens
design. The SynergEyes PS lens features a spherical base curve that transitions
into an oblate ellipsoidal zone that continues to a diameter of 9mm, where it transitions
to a spherical skirt curve radius that terminates at a peripheral bevel.
The
lens is designed to clear the irregular central cornea until a feather touch at
9mm. The skirt should align the peripheral cornea and suspend the lens above the
cornea from the 9mm chord inward.
The PS lens offers fewer constraints
on the base curve radius of the initial diagnostic lens because you can use the
oblate ellipsoid portion of the base curve to create clearance. For post refractive
surgery, the optimal base curve is frequently what results in a near plano lens
power. The goal of the optimum base curve is to represent the closest reference
sphere to the cornea to reduce the potential for bubbles. A base curve approximately
equal to the average of the flat and steep meridians is a good starting point. As
with the KC design, we recommend the flattest skirt curve in the initial diagnostic
lens.
The goal is to observe total apical
clearance without bubbles larger than 1mm. When a base curve provides total clearance
free of bubbles, the next step is to optimize the skirt curve radius.
Optimizing Skirt Curve Radius
It's possible to observe total alignment with little highs and lows in the base
curve but not complete clearance. If you observe alignment with inadequate clearance
or if clearance with a dark ring at the 9mm chord is present, the lens is not adequately
suspended over the cornea and late onset lens tightening may occur. You can reduce
the central alignment or the dark ring of touch by going to a steeper skirt curve
radius.
The proper step in this case is to
keep the same base curve radius and go to the next steeper skirt curve radius. Continue
to steepen the skirt curve radius until you observe clearance over the cornea with
a feather touch at the 9mm chord.
Optimizing Base Curve Radius
Once again, bubbles may sometimes reappear under the optic zone when the proper
skirt curve (that lifts the optic zone and the junction to a feather touch) is in
place. When this occurs, flatten the base curve by 0.1mm to eliminate the bubble.
Once you eliminate the bubble with the flatter base curve and proper skirt curve,
diagnostic fitting is complete. The final step is an over-refraction to determine
lens power.
Sagittal Depth Factor Options
Irregular corneas are the sequelae of disease, trauma and surgery and demonstrate
extreme geometric diversity. A single oblate sagittal depth isn't adequate to properly
fit the continuum of post-surgical irregular eyes. SynergEyes PS is available in
three sagittal depths or Sagittal Depth Factors (shallow, medium and deep, Figure
2) to accommodate the geometric diversity. Standard diagnostic lenses are in the
medium depth.
When the otherwise optimum base curve
that parallels the cornea exhibits too much depth with the flattest skirt curve,
the shallow depth is indicated. You may also observe a lens in the proper base curve
that demonstrates heavy touch or alignment on the central cornea that you can't
eliminate by suspending the lens with the steepest skirt curve radius. These cases
require the deep Sagittal Depth Factor. This option provides a strategy to modulate
sagittal depth in the presence of an optimal base curve/skirt radius relationship.
Without this option, you could only steepen the base curve in an effort to achieve
clearance, and central bubbles would frequently result due to the mismatch of the
base curve and the central corneal radius.
Case
Study A 41-year-old male presented having undergone well-healed LASIK OU.
Biomicroscopy revealed corneal thinning OU. His topography maps showed oblate post-LASIK
corneas OU. Keratometry readings were OS 37.25/39.00. We fit him with a SynergEyes
PS lens OS with parameters of 8.76mm base curve, 8.2mm skirt curve, 14.5mm diameter,
plano power, 20/25 (Figure 3), giving the patient a wear time of 14 hours each day.
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Figure
3. SynergEyes PS lens on a post-LASIK, oblate cornea.
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Diagnostic Lens Considerations
The SynergEyes KC and SynergEyes PS each come
in a 24-lens diagnostic set configured with three skirt curve radii for each of
eight base curve radii. The SynergEyes KC base curves are in 0.2mm steps while SynergEyes
PS are in 0.3mm steps and the skirt curve radii for each are steep, median and flat.
We recommend large-molecule fluorescein, and evaluating the pattern is essential.
You can observe lenses upon application and in some, but not all, cases we recommend
another observation after 15 minutes.
Achieving a pattern that is free of central
touch or alignment and free of a heavy 9mm touch is the best prevention of late
onset tightening. In our experience, patients will report a mild awareness of the
lens when 9mm touch is too heavy.
Hybrid lenses are known for the counter
intuitive characteristic of exhibiting more movement when fit steeper. A proper
sagittal depth clearance of about 30μm under the rigid portion is the key to
ensuring all day comfort, consistent vision and an optimal physiologic response
for compromised corneas.
Dr. DePaolis is in group practice in Rochester,
NY. He also serves as a clinical associate faculty member at the University of Rochester
Medical Center and is on the Board of Directors at TLC Vision.
Ms. Beeman holds an advanced level certification in contact
lens fitting from the National Contact Lens Examiners and is also certified as an
ophthalmic assistant. She is in clinical practice in Rochester, NY, and is the Academic
Education Coordinator for the GP Lens Institute.
Dr. Ryan is in group practice in Rochester, NY. He has also been
appointed Clinical Associate in Ophthalmology at the University of Rochester, School
of Medicine.
Contact Lens Spectrum, Issue: October 2006