Tech Connection
The Challenge of HOA-Correcting Contact
Lenses
In pursuit of "perfect" vision.
By
Louis J. Catania, O.D., F.A.A.O.
IF SPECTACLES
THAT CORRECT higher order aberrations
(HOAs) turn you on, think about HOA-correcting contact lenses! They will change
the face of vision correction and create a "feeding frenzy" among contact lens marketers.
Thus, it's critical for clinicians to know the challenges and limitations of HOA-correcting
contact lenses as well as their potential.
Off-the-rack vs. Tailor-made
HOA-correcting contact lenses will fall into two
main categories:
1. Generic aspheric lenses. These
lenses, which are already being marketed, will benefit patients who have a lot of
positive spherical aberrations.
2. Individually designed lenses. These lenses
will have programmed central optics, requiring pupil registration for point-to-point
Zernike correction.
An aspheric correction is like a suit off the
rack. It may not be a bad fit and usually looks pretty good. But a point-to-point
correction is a custom-made suit that looks and fits "perfectly."
This tailored-fit analogy raises an
interesting business consideration for eyecare practitioners in this evolving field
of HOA-correcting contact (and even spectacle) lenses. When you measure and fit
a true, point-to-point correcting lens, you hold a unique, personalized prescription
for that patient. Once a person adjusts to and enjoys HOA correction, "off-the-rack"
lenses will no longer suffice. This is no small consideration in this era of drug-store
and mail-order contact lenses and eyeglasses.
Achieving this personalized correction
has some formidable challenges well beyond those of the generic, aspheric form of
correction. However, meeting these challenges has the potential of producing the
optimal vision-correcting modality.
Seeking Stability
The main challenges associated with point-to-point
pupil correction with a contact lens relate to stabilizing fit. The movement (like
lag), decentration, flexure and the like must be controlled. Otherwise, these factors
will negate precise alignment or registration between correcting points and pupil
powers.
The trick, of course, is to fit the
lens tight enough to eliminate these factors, while maintaining corneal physiological
and morphological integrity. One way to accomplish the former (tightness) is obvious:
Make the lens larger and steeper. We've been doing that since the good old days
of scleral haptic shells.
But tightness breeds hypoxia in con-
tact
lens wear. Current and future high-Dk materials will need to address the challenge
of balancing perfect lens stability with uncompromised corneal health. Other creative
ways of stabilizing lenses — bioadhesive materials, implantable lenses (onlays)
anchored by the epithelium and collagen lenses — are under investigation.
Another Variable: Tear Film
HOA-correcting contact lenses have the potential
to provide vision that's superior to that achieved from any other form of correction,
either nonsurgical or surgical. The key to achieving this potential lies in the
lens surface. Point-to-point HOA correction combined with control of the variable
tear film — the true refracting surface of the eye — is the real likelihood
of producing "piston" in human vision. And, if you think about it, a contact lens
is the only device that can accomplish both.
So there's our future!
Dr. Catania is with Nicolitz Eye Consultants
in Jacksonville, Fla. He does clinical research; consults for ophthalmic companies
and professional journals; and writes and lectures worldwide. You can reach him
at lcatania@bellsouth.net.
Optometric Management, Issue: April 2006