Optimizing Oxygen Without Compromising Performance

BY CHRIS AMOS, Bsc, Msc, & TIM GRANT, BOptom, MBA
June 1999

Does lotrafilcon A measure up to other current contact lens materials? Clinical studies put this new high Dk material to the test.

The ultimate goal of delivering more oxygen to the cornea during contact lens wear has been elusive. The critical levels of oxygen transmission (Dk/t) needed to support normal corneal function had remained unmet until recently.

In 1984, Holden and Mertz established the critical oxygen transmissibility required to avoid edema for daily and extended wear contact lens materials. The level needed for zero daytime swelling during daily wear of contact lenses is 24.1 x 10^-9 (cm x mlO₂) / (ml x mm Hg). The level for extended wear is 87.0 x 10^-9 (cm x mlO₂) / (ml x mm Hg) to limit overnight edema to 4 percent. In addition, Harvitt and Bonanno state that the Dk/t required to avoid anoxia of the stroma in the closed eye is 125 x 10^-9 (cm x mlO₂) / (ml x mm Hg). Many of today's soft contact lens materials barely meet these daily wear oxygen requirements.

Past attempts to increase Dk/t in current forms of hydrogel contact lenses have failed in terms of overall lens performance. A popular method of increasing Dk/t is to decrease center thickness. However, the use of ultrathin lenses is complicated by desiccation staining of the cornea and poor handling. Increasing water content also increases Dk/t, but leads to increased lens deposition, giant papillary conjunctivitis and decreased lens life. With current hydrogel chemistry, oxygen transmissibility depends on water content. However, even if a contact lens could be made out of 100 percent water, its Dk/t would only be about 90.

Today, only silicone and some RGP lens materials meet the critical level of oxygen transmission for extended wear. The silicone lens was the first extended wear lens recommended for up to 30 days by the FDA in 1983. But despite high oxygen transmissibility, silicone contact lenses have demonstrated reduced comfort, poor surface wettability, lens adherence and increased lens deposition, which has led to their decline in clinical usage and availability.

Some RGP contact lenses also have a high Dk/t, but have demonstrated problems with corneal binding and poor initial comfort, in addition to a greater potential for foreign bodies, such as dust, to get lodged behind the lens. RGP contact lenses are also prone to peripheral corneal desiccation, which has caused as many as one-third of RGP extended wear discontinuations.

Given the oxygen limitations of current hydrogel contact lens materials, a key technology breakthrough is required to deliver a high Dk soft contact lens material that is durable, deposit resistant, demonstrates good surface wettability, offers excellent comfort and doesn't bind to the cornea. This clinical trial was conducted as part of the development of a new surface-treated, high Dk, fluorosiloxane soft contact lens material (lotrafilcon A) by CIBA Vision. It evaluated the new material's durability and on-eye performance, while subjecting it to the rigors of daily cleaning and disinfecting.

Study Design

This study was a randomized, controlled clinical trial involving 58 subjects for a 3-month time period. The trial was designed to compare CIBA Vision's new high Dk lens with a currently marketed contact lens (CIBASoft). The 58 subjects (38 test, 20 control), were divided evenly among five independent investigators and dispensed 3 months worth of either the high Dk daily contact lens wear or the conventional daily contact lens wear, in addition to monthly replacements. The care regimen used for both the test and the control groups was AOSept (CIBA Vision). The identities of the lenses were concealed from both the subjects and the practitioners. After signing the informed consent form, subjects were scheduled for an initial baseline and lens dispensing visit, followed by 7-day, 14-day, 1-month, 2-month and 3-month visits. Lens parameters are listed in Table 1.

Clinical evaluations during follow-up visits were used to rate the following performance criteria: front surface wetting characteristics, front surface deposits, lens movement, lens mobility when pushed with the lower lid (push-up test), overall fit assessment and biomicroscopy. In addition, patients were asked to subjectively evaluate factors such as comfort, handling and vision.

Fifty-three of the initial 58 subjects completed the study. The five discontinuations (3 test, 2 control) were due to initial discomfort from poor contact lens fit (4 subjects) and to geographical relocation (1 subject). The ages and male-to-female ratios in both groups were similar. The mean age of the test group was 27, with a male-to-female ratio of 1:3, while the mean age of the control group was 29, with a 1:2 male-to-female ratio.

Lens Surface Performance

Ninety-seven percent of the high Dk test lenses and 99 percent of the control contact lenses had either perfect wettability or only a slight disturbance (Table 2). Front surface deposits were also comparable, with 92 percent of the high Dk contact lenses and 96 percent of the control contact lenses having either no deposition or a slight deposition that was easily removed with lid pressure (Table 3) and statistically, there was no significant difference between front surface wettability measures. In addition, lens replacements were also similar, with 12 percent of the high Dk test lenses and 11 percent of the control lenses being replaced for reasons other than the predetermined replacement schedule.

Fitting Characteristics

The lenses used in the study were only available in one base curve, and of the 53 subjects who completed the trial, 97 percent of the high Dk lens participants and 95 percent of the control lens participants had an acceptable overall fit. In addition to a similar overall fit, the high Dk lens and the control lens demonstrated similar lens movement and push-up, and no reports of lens adherence or binding were noted with either lens. There were no restrictions for keratometry values in the inclusion criteria and corneal curvatures ranged from 40.37D to 47.50D in the high Dk lens group and 41.62D to 47.25D in the control lens group.

Subjective Patient Evaluations

Subjective patient evaluations of contact lens comfort at insertion, comfort during the day, comfort at the end of the day, handling ease at insertion, handling ease on removal and quality of vision, using a scale of 1 to 100 (1 being the poorest and 100 being excellent or perfect), are shown in Table 5.

There were no statistically significant differences between patient evaluations for the high Dk test lenses and the control lenses. Both lenses were rated as having very good comfort at all of the above intervals. The high comfort ratings for the Dk test lens were further supported by the low number of symptoms reported by test subjects. Both lenses had similar ratings pertaining to dryness, lens awareness, blurred vision and burning/stinging (Table 6). Quality of vision was also rated high, with a mean rating greater than 90 for both lenses and a low report rate of blurred vision. But interestingly, the high Dk lenses were reported to be easier to handle at both insertion and removal.

Biomicroscopy

Biomicroscopy findings with an incidence rate greater than 5 percent are shown in Table 7. In addition to the low incidence of biomicroscopy findings, there were no severe reactions or grade 4 biomicroscopy findings reported for either contact lens during the trial.

Corneal epithelial staining, palpebral conjunctival redness and bulbar conjunctival redness were similar in both the test lens and the control lens. However, there was a greater incidence of tarsal abnormalities with the control lens than with the high Dk lens.

Optimizing Oxygen Along With Performance

Throughout the study, CIBA Vision's high Dk lens performed as well as, if not better than, the conventional control lens. There were no signs of surface performance changes associated with the daily cleaning of the surface-treated test lens. The high Dk material (lotrafilcon A) performed well when exposed to the rigors of daily wear. The test contact lens' surface treatment withstood daily handling, cleaning and disinfecting, and had a smooth, wettable, deposit-free surface. The high Dk material didn't demonstrate lens adherence, durability or comfort problems, such as those found in previous attempts to create a high Dk soft contact lens material. In addition, the lens exhibited excellent fit and movement characteristics similar to a currently marketed hydrogel contact lens.

Due to a Dk of 140, and a Dk/t of 175 (-3.00 D), the test material (lotrafilcon A) exceeded the oxygen requirements necessary for both daily and extended wear. Because its clinical performance was equivalent to a successfully established daily wear lens (CIBASoft), this surface treated high Dk material truly offers a breakthrough soft contact lens material that meets the demands for oxygen while also maintaining overall performance expectations. CLS

The investigators participating in this trial were: Peter Bergenske, O.D. (Middleton, WI), Carmen Castellano, O.D. (St. Louis), Ronald Cedrone, O.D. (Portland, ME), Walter Choate, O.D. (Madison, TN) and Evan Thomas, O.D. (Newport Beach, CA). Statistical support was provided by Mike Lynn, M.S., Department of Biostatistics, The Rollins School of Public Health, Emory University, Atlanta, GA. We thank Joe Schwallie and Sally Dillehay for assistance with the manuscript.

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #49. (Be sure to have a fax number ready).

Tim Grant received his Optometry training in Australia and his MBA in Atlanta. He is currently Head of Marketing at CIBA Vision Australia.

Chris Amos was trained in the UK and fitted lenses at Emory University medical school. Since 1994 he has been working on clinical trials with CIBA Vision, Duluth, GA and currently specializes in data management.