Contact Lens Design & Materials
Material and Design Influences in an Interesting Case
BY NEIL PENCE, OD, FAAO
While it is never a good idea to draw firm conclusions from a study with an “n” of 1, single cases can provide interesting learning examples. A recent patient who had posterior subcapsular cataracts needed to have lens replacement surgery. She had a history of more than 50 years of rigid lens wear. The first 20 years were in PMMA material, followed by 30+ years of wearing GP lenses of increasingly higher Dk. She has been wearing GP multifocal lenses for the past 15 years.
Effects of Long-Term PMMA and GP Lens Wear
Experienced practitioners may recall that it was not uncommon for patients who had worn PMMA lenses for 8 to 10 years or more to experience what was referred to as Corneal Fatigue Syndrome. Corneal edema occurred under the lens during the day, presumably resolved prior to lens application the next day, and then proceeded to swell again.
It seemed as if the cornea became less stable and more pliable or moldable. Patients’ refractions were less myopic in the morning than they had been the previous evening (sometimes markedly different), often with less astigmatism. Switching to a GP lens helped stabilize their corneas, and as contact lenses became available in more permeable materials, Corneal Fatigue Syndrome became a historical footnote.
Our patient had a history of Corneal Fatigue Syndrome; she even experienced a slight recurrence after a number of years of low-Dk GP wear, but had no problems with higher-Dk materials. Yet, she cannot stop GP wear today and have a stable refraction tomorrow. There are many suggestions as to how long she should wait before surgery (two weeks; one month; three months; one month for every decade of rigid lens wear). As she is roughly a –13.50D myope, she was not eager to wear glasses for possibly several months.
Because she had little astigmatism, we fit –12.00D silicone hydrogel (SiHy) lenses with reading glasses. That way, any corneal changes induced by the GPs could hopefully resolve while she continued to enjoy the benefits of contact lens wear. At the one-week visit, the right eye showed a nearly –1.50D increase in correction, while the left eye changed minimally. Because –12.00D was the highest power in standard SiHy, she wore it in both eyes and enjoyed a monovision effect.
There was nearly no change in either eye when comparing the one-week findings to two- or four-week visits. Both eyes’ topography and refraction seemed to stabilize relatively rapidly despite five decades of rigid lens wear and continuing to wear a SiHy lens. This might be attributed to the benefit of having relatively high-Dk GP lenses; we are thus thankfully far removed from the old corneal fatigue days.
Design and Material Impact
Interestingly, when five different –12.00D SiHy lenses were trial fit, several required a 0.25D lens in front for best acuity, and others did not. She also had a difference of 1 to 1.5 lines in acuity among lenses, even with the best power in place. Optical design differences are exaggerated at such high powers, but this case serves as a reminder that not every lens with the same labeled power is going to have the exact same effect.
Finally, as an interesting last note, this patient saw more than 1 line better in a –12.00D daily disposable SiHy compared to a frequent replacement SiHy lens in identical material (over-refraction with both was plano). Design changes between the two lenses likely account for this difference. CLS
Dr. Pence serves as associate dean, Clinical and Patient Services, Indiana University School of Optometry in Bloomington, Ind. He is a consultant or advisor to Alcon, has received educational funding from Alcon, Johnson & Johnson Vision Care, and B+L, and has received travel funding from CooperVision. You can reach him at email@example.com.