ask the labs
Processing & Prescribing Possibilities
by Karlen McLean, ABOC, NCLC
POLY PRESSURE
Q We have problems with patients returning to us with stress fractures in their lenses after wearing their semi-rimless frames with polycarbonate lenses. Is there any way to prevent future stress fractures from happening?
A Stress on the lens or contact with a solvent causes poly fractures. In the case of rimless string-mount frames, there is a chance both causes are at play.
The grooving process can easily put stress on a lens. Sharpness of the grooving wheel/blade or swarf buildup needs to be checked during the edging process.
The width of the groove must be sized to the nylon or metal cord inside it. If it's too narrow, it puts pressure on the thin outside edges of the lens. If it's too wide, then the cord needs to be tightened to secure the lens.
If the edge is polished, care must be taken so there's as little heat buildup on the edge of the lens as possible, and the condition of the polishing wheel is critical in reducing stress on the lens.
If additional hand-polishing is done to achieve a high-luster edge (generally done on a felt wheel with dry polish), care must be taken to avoid heat buildup on lens edges.
Contact with solvents can weaken lenses, and many organic solvents can damage poly. These include acetone, nail polish remover, paint thinner, some hair sprays, some body oils, and even some lens cleaners. Isopropyl alcohol is a viable option if soapy water is not adequate for cleaning.
Care when processing and educating consumers will minimize the likelihood of this exposure. The choice of a different lens material (CR39, 1.60, or 1.67) greatly reduces the effect of these solvents.
If poly was chosen for its high impact-resistant qualities, in most cases the same lens will be available in Trivex material, which is chemical- and impact-resistant.
Remarkably, this fracturing occurs in a very small percentage of dispensed lenses. If care is taken by the lab (removing these solvents from their process and replacing cutters and wheels often), opticians, and consumers, poly can be a successful choice as a material for string-mount frames.
—Warren Meyer, vice president of lab operations, VSP labs
| Underwater Wear |
|---|
| Q I have a patient whose spectacle Rx is: OD) -5.75 -2.25 × 177; OS) -6.00 -3.75 × 180. What power should I prescribe for her customized swimming goggles? A I would do this prescription as written; it should provide more than adequate vision. The only compensation I would consider would be for vertex distance change, in which case the Rx would be -5.62 -2.12 × 177 OD and -5.87 -3.50 × 180 OS. Water has an index of refraction of 1.33, so images will naturally be magnified. But I don't see the need for any additional adjustment to the Rx. This patient with prescription goggles will actually have better underwater vision than an emmetrope without goggles. —Jason Mansuy, vice president–operations, Vision Craft, Inc., Walled Lake, Mich. |
If you have a question you'd like to have answered in Ask the Labs, send it to Karlen McLean. Email: karlen.mclean@wolterskluwer.com. An archive of past Ask the Labs columns can be found on the Eyecare Business website at www.eyecarebusiness.com.
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