contact lens
materials
How
Modulus Affects Soft Contact Lenses
BY
N. REX GHORMLEY, OD, FAAO
When selecting
a soft contact lens for a patient, most practitioners usually evaluate lens oxygen
permeability, design, polymer deposit characteristics and available parameters.
But what effect does also does
modulus have on the fitting and performance of a soft lens?
Demystifying Modulus
The modulus of a lens material is the measurement of the material's
resistance to deformation under tension. In practical terms, the stiffer the lens
material, the higher the lens modulus.
In the past, clinicians rarely discussed modulus because there
was little difference in the modulus of mid-water low-Dk soft lenses. But, with
today's silicone hydrogel lenses, we see significant differences in lens modulus.
For example, Figure 1 shows that there's almost no difference
in modulus when comparing the Acuvue (Vistakon) and Proclear (CooperVision) lenses.
But, you'll see a large difference in modulus among the five currently available
silicone hydrogel lenses. The Acuvue Advance lens has the lowest modulus and the
Night & Day (CIBA Vision) lens has the highest modulus. But what does this mean
to us in clinical practice?
Comfort Questions
Are higher-modulus lenses more uncomfortable? Do they cause more
ocular complications? The original lotrafilcon A (Night & Day) lens caused more
cases of SEALs and GPC. But, the second generation lotrafilcon A lens has an aspheric
design in two base curves, which has significantly reduced these complications.
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Figure
1. Modulus of some soft lens materials. Graph
courtesy of Lyndon Jones, PhD, University of Waterloo. |
Some
patients don't obtain maximum comfort with a high-modulus silicone hydrogel lens.
In these cases I usually switch the patient to a lower-modulus silicone hydrogel
lens.
Stiffer, high-modulus lenses may need a larger diameter or several
base curves to achieve a good physical fit. Improper peripheral alignment may result
in edge fluting. The lotrafilcon B (O2Optix, CIBA) lens has a lower modulus
and a larger diameter. Edge fluting is rare with this lens.
Better Lens Handling
Patients find that high-modulus lenses are easy to apply and remove
and that it's simple to tell if they're "everted." My patients like the way a high-modulus
lens "stands up" on their finger and doesn't "roll up."
The Bottom Line
If I prescribe high-modulus silicone hydrogels for a patient,
I tell him that they're "stiffer," easy to handle and that lens awareness is normal
for a few days. Most of my patients easily adjust in a short time. If after several
weeks comfort is an issue, I then change the lens design and modulus.
We need to understand the different qualities in all silicone
hydrogels. Modulus is another tool that we can use to increase the performance of
this new lens technology.
Dr. Ghormley is in private
practice in St. Louis, MO. He is a past president of the American Academy of Optometry
and a Diplomate of its Cornea & Contact Lens Section. He's also a consultant
for CIBA Vision.
Contact Lens Spectrum, Issue: November 2005