Contact Lens Design & Materials
Contact Lens Design & Materials
Getting at the “Center” of GP Lens Flexure
By Neil Pence, OD, FAAO
In conversations with relatively recent graduates, the topic of GP lens flexure has come up. With GP lenses dropping in market share compared to soft lenses over the last 30 years, less time is spent on teaching the intricacies of rigid lens design now compared to, say, 20 years ago. Some of the more seminal studies of GP flexure are also from a relatively distant time (Hermann, 1983; Pole, 1983), and not many have appeared in the recent literature.
I’ll provide a brief review of GP flexure and how lens design affects it.
Sources of GP Flexure
Common recommendations to reduce lens flexure are to flatten the base curve, reduce the optic zone size, increase the center thickness, or change materials. Both Hermann and Pole showed that base curve radius is clearly related to GP flexure. Flatter fitting relationships exhibit less flexure compared to steeper ones.
Generally, there have not been dramatic differences in flexure among different GP materials, although stiffer materials may resist flexing a small bit more. Center thickness can affect flexure, but the increase often needs to be much greater than what most think to alter flexure, and the added mass may alter the lens fit and comfort.
Thus, the fitting relationship seems to be the main variable for reducing flexure. There is also an effect from the upper lid, with lid-attached fits likely to exhibit less with-the-rule flexure. Lid-attached fits and looser, flatter fitting relationships both have in common a tendency to be less well-centered on the cornea. Steeper fits are generally more well-centered, and this encourages with-the-rule flexure. Thus, centration is a key indicator of the likelihood of GP flexure.
A Lens Flexure Visualization Aid
Imagine placing a thin yardstick and a rolling pin on a flat countertop. Now picture placing the 18-inch mark (midpoint) of the yardstick over the rolling pin. This represents the horizontal meridian of our GP lens. For the lens to flex, both ends of the yardstick need to move toward the countertop. If only one side moved down, the lens would simply tilt. Picture pushing down at both ends until they touch, and imagine how much force this might take.
Now imagine placing the 24-inch mark over the rolling pin. The amount of force it will now take to touch both ends to the table is much greater. It is easiest to flex the stick when it is exactly centered over the rolling pin.
So it is with a GP contact lens. The forces needed to achieve flexure are lowest when the lens is perfectly centered, and the less well-centered a lens is, the less conducive it is to flexure. These have often been referred to as adhesive forces, but cohesion, surface tension, gap or separation distances, lid forces, and even tear volume are likely involved in how and why a lens actually flexes.
Whatever the actual physics and surface chemistry behind the flexure, a good rule of thumb is to just think “centration.” The next time you have a situation in which you do not want flexure, fit the lens so that it will be less well-centered. This means looser fits, flatter base curves, small optic zones, or lid-controlled fits. If you want a lens to flex, do whatever is needed to better center the lens, which often means tighter, steeper, non-lid-attached fits. This rule gets right to the “center” of what you need to do in terms of lens design and fit. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #207.
Dr. Pence is the associate dean for Clinical and Patient Care Services, Indiana University School of Optometry in Bloomington, Indiana. He has received travel expenses, stipend, or reimbursement from Alcon and B+L. You can reach him at firstname.lastname@example.org.
Contact Lens Spectrum, Volume: 28 , Issue: February 2013, page(s): 21