Prescribing for Astigmatism
Flexure or Just Plain Warped?
By Timothy B. Edrington, OD, MS, FAAO
Frequent lens replacement has become the standard of care for soft lens patients, with lenses typically replaced in one month or less. Of course, this is not the case for GP-wearing patients. Granted, many eyecare practitioners justifiably recommend six-month or annual replacement schedules for their GP wearers. But GP lens patients tend to replace their lenses only when they experience decreased comfort or vision, or when they lose or damage a lens.
Check for Warpage
I feel that it is important to inspect GP lenses with a slit lamp, hand-held magnifier, or “shadowscope” at annual examinations. You can check for excessive lens surface scratches or tiny chips in the lens edge.
I also recommend verifying the base curve radius to determine whether there has been any lens warpage. If the lens is warped, you will measure two different radii of curvature. For example, if the originally dispensed lens had a base curve radius of 7.85mm (43.00D) and the lens warped by 1.00D, you would anticipate measuring base curve radii of 7.94mm (42.50D) X 7.76mm (43.50D).
Warpage probably results from heat generated by routine lens handling. Generally, I would recommend a new GP lens for patients whose lenses had warped by 0.50D or more.
Continue reading to see whether this—or any—amount of warpage would be detrimental to your patients' vision.
While lens warpage refers to off-eye changes to the lens base curve, lens flexure refers to on-eye changes in base curve toricity. Lens flexure and lens warpage became more of a clinical issue when GP contact lenses entered the market. It was common to prescribe GP lenses for patients wearing PMMA lenses to upgrade the oxygen permeability and enhance corneal physiology. Even though the GP parameters were ordered the same as, or similar to, the PMMA parameters, patients often complained about their vision through their new GP lenses. Over-refractions often revealed residual cylinder that was not present through their old PMMA lenses. That meant that the lens had warped or was flexing during wear.
Refresher Course—Flexure 101
If a GP lens flexes on the eye, the over-refraction cylinder will change by the amount of the flexure. Flexure on a with-the-rule (WTR) cornea will increase WTR cylinder in the over-refraction; lens flexure on an against-the-rule (ATR) cornea will increase ATR over-refraction cylinder.
Why? The presence or absence of warpage and on-eye flexure are dependent on center and overall lens thickness, corneal toricity, lens material, and the fitting relationship (Table 1).
That's So Yesterday
Fast forward to today, and it is common for scleral GP lenses to flex on the eye if the center thickness is not adequate. If the over-refraction cylinder is excessive, perform over-keratometry or topography to measure for front-surface toricity. If excessive, increase the center thickness of the contact lens.
If you obtain an unanticipated and unwanted over-refraction cylinder, suspect and investigate lens warpage and/or lens flexure as the culprit. CLS
Dr. Edrington is a professor at the Southern California College of Optometry. He has also worked as an advisor to B+L. You can reach him at firstname.lastname@example.org.