contact lens primer
Bend Me, Shape Me: Lens Flexure
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
The majority of rigid contact lenses are prescribed as spherical designs. Over time, heat or normal patient wear and tear on the lens can cause a number of these lenses to warp. If the spherical R
RGP is designed with a thin overall profile, it may also flex or bend while on the cornea. Flexure and warpage lead to the same clinical outcomes: They alter the over-refraction cylinder and the fitting characteristics of the lens.
Analyze the difference in base curves found with the radiuscope to verify the amount of lens warp. If you measure 7.85mm (43.00 diopters) by 7.76mm (43.50 diopters), the lens is warped by 0.50D. The lens power as measured by lensometry will generally continue to show a sphere. If you measure more than 1.00D of difference between base curve meridians, suspect a toric design.
The amount of flexure can be determined by performing over-keratometry on the lens front surface while the lens is on the eye. The difference between the over-keratometry toricity and any measured lens warpage is the amount of on-eye flexure.
If a lens warps or flexes on the eye, the over-refraction cylinder will be altered by the amount of warp plus flexure. The flat and steep meridians of the warped lens will tend to align with the flat and steep meridians of the cornea, resulting in an increase in with-the-rule over-refraction cylinder on a with-the-rule cornea and an increase in against-the-rule cylinder on an against-the-rule cornea.
Why Does It Happen?
On-eye flexure depends on a number of variables: center thickness, overall lens thickness, corneal toricity, lens material, fit and lens position on the cornea. A thinner center or overall lens profile will tend to flex more; therefore low minus power rigid lenses will flex more than high minus or plus power lenses. If a cornea is near spherical in toricity, a lens will tend to flex less than if it was placed on a toric cornea. This is due to the lack of a fulcrum for the lens to flex over. Fluoropolymer lenses tend to flex more than silicone acrylate rigid lenses, and higher Dk lenses tend to flex more than lower Dk lenses. Fluoro-silicone RGP contact lenses can flex clinically significant amounts with center thicknesses of less than 0.15mm to 0.18mm. Flexure is also lens-position dependent. Superior positioning and flatter lenses flex less than lenses fit intrapalpebrally because of the enhanced fulcrum effect obtained with a centered lens.
How Can I Avoid It?
Sometimes flexure is good. For example, if the patient's cornea is with-the-rule and you obtain an against-the-rule over-refraction cylinder with a non-flexing rigid lens in place, flexure will reduce the over-refraction cylinder by the amount of flexure.
If the patient would not benefit from lens flexure, you can alter your standard spherical lens design by increasing the center and overall thickness of the lens, prescribing a lower Dk material or fitting a superior positioning or lid-attached lens design. Also, if the corneal toricity is sufficient (>1.50D), consider a bitoric design to negate the bending created by the fulcrum effect.
Small amounts of lens warpage or flexure will probably not affect the success of your RGP prescription, and it may even improve the patient's vision or the fitting characteristics of the lens. Understanding the effect of lens warp-age and flexure on over-refraction cylinder, will enhance your problem-solving acumen.
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at email@example.com.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean of Clinical Affairs at The Ohio State University College of Optometry.