Overcoming Skepticism About RGP Scleral Lenses

Featuring calculations and commentary from the late Irving Fatt Ph.D., this article reinforces the clinical benefits of using rigid gas permeable scleral contact lenses.

BY DONALD F. EZEKIEL, AM, Dip Opt, DCLP, FACLP, FCLSA, FAAO &
DAMON J. EZEKIEL, B Optom, FAAO
DECEMBER 1998

Scleral contact lenses have evolved from glass lenses to lenses made from high Dk gas permeable materials. PMMA material replaced glass as the material of choice for scleral lenses in 1948. The advantage of PMMA over glass is that it's a much easier material from which to make a lens, and PMMA lenses are much easier to modify. Dallos and Bier in 1948 described how fenestration allows for the interchange of tears, resulting in an increase of lens wearing time to all day lens wear. To further assist with the exchange of tears, manufacturers made various contact lens modifications in the form of vents and channels. As gas permeable materials became available, it was logical to use them for scleral lens designs.

Results Questioned

At the Annual Meeting of the British Contact Lens Association at Harrogate in 1983, Donald Ezekiel first described the results of fitting 43 patients with gas permeable scleral lenses, which included traumatic, injured corneas, keratoconics, pediatrics, sclerals with a ptosis bar and protective lenses. A report in the U.K. publication The Optician on the meeting and on the paper commented that the consensus from practitioners at the meeting was that with the necessary thickness for a gas permeable scleral lens, the results as reported were questionable. A gas permeable scleral lens would not be of any appreciable benefit over a PMMA scleral lens in terms of oxygen transmission.

Since that time, the gas permeable scleral lens has become accepted as a lens that the serious contact lens practitioner must have available for his patients. The scleral contact lens has been described as "the medical contact lens par excellence" by Dr. Roger Buckley, consultant ophthalmologist and director of the Contact Lens and Prosthetic Department at Moorfields Eye Hospital in London.

"There is no eye shape that cannot be fitted with this type of lens and degree of long sight, short sight or astigmatism that cannot be corrected," he wrote in a personal letter.

Although the gas permeable scleral contact lenses we fit our patients with are all fenestrated, there are scleral lens practitioners, such as Ken Pullen in London and Perry Rosenthal in Boston, who fit gas permeable scleral lenses without fenestrations and achieve excellent results -- even with a scleral lens of some thickness!

Thoughts from Irving Fatt

I (Donald Ezekiel) had the opportunity to discuss my work and the results I was achieving in gas permeable scleral lenses with Irving Fatt, Ph.D., after a presentation I gave on gas permeable scleral lenses at the International Society of Contact Lens Specialists meeting in France in 1995. He subsequently wrote to me, and I believe his theory explains why the gas permeable scleral lens (at the necessary thickness required for scleral lens design) allows more oxygen than the cornea requires, even when the lens is not fenestrated. Irving Fatt wrote, and I quote:

"I think I know why your RGP sclerals work. My explanation requires good mixing of the tears that move around under the scleral section, the haptic section and the corneal section. I based my calculations on the cross-section of a scleral lens shown in Mandell's 4th Edition, page 869. I calculated the area under the whole lens. That came out to be about 7cm². Then I took an average thickness of 0.1cm (1.0mm) and assumed that the oxygen tension could go down as low as 20mmHg under the lens. This would still allow a reasonably healthy cornea. I assume that Quantum II had a Dk of 100*. I haven't measured it because this material is not available in the United States, but for a rough calculation, a Dk of 100 is a good starting point. When I put all of the numbers into Fick's law of diffusion, I get a diffusion rate through the lens of 36 microliters per hour. The cornea needs (according to the 1963 measurements of Hill and Fatt) about 7 microliters. So we see that if the numbers for the lens are reasonably close and there is a good tear mixing under the entire lens, then we actually get about five times more oxygen through the lens than the cornea needs. This is good news because it means that we can relax the requirement of total mixing under the lens. Even if the mixing is only 20 percent efficient we are getting enough oxygen for the cornea. The trick is that a scleral RGP lens collects oxygen over a large area and funnels it to the cornea."

*The gas permeable scleral contact lenses that we use are manufactured from Equalens II, having a Dk value of 127.

The Proof is in the Fitting

In further correspondence, Irving described the work he wanted us to do to prove his theory and to "put this together to show the skeptics..." Regrettably, we have not yet completed this work. Although for now we cannot scientifically prove why wearing a gas permeable scleral lens of optimal thickness allows normal corneal metabolism to occur without ocular stress, it is gratifying that the excellent clinical results have turned early skeptics into advocates.

Fitting scleral lenses for keratoconic, post-graft and traumatic, injured corneas has gained popularity. Fitting sets have been developed, enabling the keratoconic patients and patients with distorted corneas to be readily fitted with a gas permeable scleral lens, requiring minimal modification and fitting time and providing comfortable all day wear with optimal acuity. A standard lens for the pediatric aphakic patient has been developed and proven over the past 15 years. Figures 1 through 4 are examples of cases where RGP scleral lenses are worn successfully and with significant improvements in vision.

FIG. 1: Post-graft patient with distorted corneas.	FIG. 2: Traumatic injury -- corneal scar with iris loss.
FIG. 3: Down's syndrome congenital cataract aphakic patient wearing gas permeable aphakic scleral lenses.	FIG. 4: Scleral RGP protects cornea in ectropism patient.

The clinical results obtained with patients clearly demonstrate the advantages of gas permeable over PMMA scleral lenses. The gas permeable scleral lens is kind and forgiving to the eye. It is a much easier lens for practitioners to fit, and it is a comfortable lens for our patients to wear. The gas permeable scleral lens has enabled us to achieve excellent results with debilitated eyes -- results that are just not possible with a PMMA scleral lens.

With the development of further advanced materials, fitting sets and techniques, this "par excellent" lens will become more readily available for patients who may have no other option.

Acknowledgements: Leon Ozapantis and Quang Le of Gelflex Laboratories, Perth, Western Australia, for assistance with the design and manufacture of the advanced fitting lenses for keratoconic and post-graft corneas.

To receive references via fax, call (800) 239-4684 and request document #43.

Dr. Donald Ezekiel is the founder and CEO of Gelflex Laboratories. He is a fellow of the American Academy of Optometry and a founding fellow of the contact lens society of Australia. He lectures throughout Australia, Europe, Asia and the United States.
Dr. Damon Ezekiel is a member of the design and evaluation team at Gelflex Laboratories. He lectures in Australia, Asia and the United States. Both doctors are in a practice together in Perth, Western Australia, which specializes in contact lenses.