LENS STUDY

A New Material on the Block of Frequent Replacement Lenses

By Urs Businger, OD, FAAO
December 2000

A new frequent replacement lens is put to the test against the current industry leader.

Most practitioners today prescribe frequent replacement lenses as a routine, based on fewer complications and a desire for simple care. A practitioner may choose a specific lens for its material properties, fitting parameters for specific patients, ocular health and patient preference. A point often not mentioned by practitioners is lens availability, meaning which lens the practitioner has in stock and how the practice benefits through fitting that specific lens brand. Any new lens entering the market has to address these concerns to demonstrate its usefulness for any specific practice.

Material Properties

The Extreme H₂O lens is made of hioxifilcon A, a non-ionic material with approximately 60 percent water content. Its components GMA (glycerol methacrylate) and HEMA (2-hydroxyethyl methacylate) are both well known in the contact lens industry but were thought to be too difficult to bring together. Because this combination holds very tightly to its bound water via strong hydrogen bonds, the material's hydration level remains almost unchanged during the entire wearing period. The material stays at a constant near saturation level not only due to its strong capability to hold water, minimizing dehydration, but also its characteristic fast rehydration. The ratio between the dehydration and the rehydration is called water balance. Taking p-HEMA as a reference (p-HEMA = 1), the Extreme H₂O lens has a water balance value five times greater.

Hioxifilcon A has a very low protein uptake, is dimensionally stable and not susceptible to pH or temperature changes. In contrast, ionic materials are known for their fast protein uptake, pH sensitivity and a greater effect of temperature on lens parameter changes. Hioxifilcon A also showed the lowest amoebal absorbence even when the lenses were coated with a biofilm.

Fitting Characteristics

Since the lens does not lose water, its fitting characteristics are pretty much the same when inserting the lens as well as after several hours of lens wear. The lens is available in only one base curve (8.6mm) and diameter (14.2mm), so it seemed initially that only a relatively small percentage of patients could be fitted with the lens. However, practice has shown that a very large range of cornea radii from 48 diopters to 39.50 diopters can be fitted with the lens. With this broad fitting range, despite having only one base curve, only a small stock is needed to dispense the lens out of an inventory. The lens is available from +6.00D to ­6.00D in 0.25D steps and from ­6.50D to ­8.00D in 0.50D steps. Since the hioxifilcon A lenses remain significantly more hydrated on the eye than most other lenses, prescribe the lens power of an Extreme H₂O lens based on the spectacle Rx and not on the power of previously worn contact lenses.

Ocular Health Study

Does the Extreme H₂O lens meet the excellent ocular health criteria set by other frequent replacement lenses? To address that question a study was initiated to compare the clinical performance of Extreme H₂O versus Acuvue 2, a lens known for its excellent performance. The study was designed as a single center, simultaneous, randomized, single-masked clinical trial.

Some 30 patients, all previously adapted soft contact lens wearers with a wearing time of at least eight hours per day, took part in the study. The control lens was the new Acuvue 2. Prior to the study, all patients were fitted with an Acuvue 2 lens to make sure that an optimum fit could be achieved with either the 8.7mm or the 8.3mm base curve. The patients wore an Extreme H₂O lens on one eye and an Acuvue 2 lens as a control on the other eye. The eye wearing the Extreme H₂O lens was chosen randomly. Patients were instructed to continue their previously used care regime.

All patients wore the lenses for 14 days, at which time they returned for the first progress check. New lenses were dispensed, but the lens type for each eye was switched. The new lenses were again worn for two weeks followed by a final check. The study subjects were seen prior to the study for lens fit evaluation at the time of dispensing to record water content of the new lens before being worn and high and low contrast acuity, as measured by counting the number of correctly identified letters on the Bailey-Lovie chart. Those measurements were repeated after the lenses had been worn for two weeks. At the same visit, biomicroscopy was performed, fluorescein was used to judge the staining of the cornea and lissamine green for the conjunctiva. At the four-week visit, in addition to the above mentioned tests, refraction and K readings were repeated. At the end of the four-week period, each patient was asked to choose which lens system they would like to continue after the study.

For statistical analysis Wilcoxon Signed Rank Test was used by non normal distribution and paired t-test by normal distribution.

Results

The study confirmed that the Extreme H₂O lens showed essentially no dehydration while being worn. The mean of the measured water content of the Acuvue 2 lenses after two weeks of daily wear was 53.2 percent, originally starting from 58 percent. The Extreme H₂O lenses showed a mean water content after the two-week period of 61.6 percent from an original water content of 62 percent (Figure 1). Both lenses showed no significant corneal response as observed by biomicroscopy and fluorescein staining. There was significantly stronger conjunctival staining with the Acuvue 2 lens than with the Extreme H₂O lens as observed by lissamine green staining (Figures 2 and 3). Over the short term this difference is probably of no clinical significance. However, over the long term, the constant conjunctival irritation could lead to a cell change of the conjunctiva with the possibility of promoting dry eye symptoms.

An interesting difference was observed in the visual performance. The Extreme H₂O lens showed statistically better acuity on high as well as low contrast. It was surprising that the acuity was better than with the Acuvue 2, a lens type that is known to have extremely good optics and usually ranks best in acuity when compared to other lenses. This difference was apparent after the lenses had been dispensed as well as after a two-week wearing period (Figure 4). There was also a significant difference between initial VA measurement and VA at the end of the two weeks. Over this time period, measurement with Acuvue 2 lenses showed a slight loss in VA, with Extreme H₂O lenses showing a slight gain.

The number of test lenses replaced due to tearing was the same as for control lenses. At the end of the study, 73 percent of the patients decided to continue wearing the Extreme H₂O lenses after the test period and 26 percent decided to continue wearing the Acuvue 2 lens (Figure 5).

The Extreme H₂O lens is a good alternative to current frequent replacement lenses based on material properties, lens parameters, clinical performance and ocular health. These factors, along with the lens's availability to patients, may be convincing enough for some practitioners to consider a change to a new product.

References are available upon request to the editors of Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #66. (Have a fax number ready.)

Dr. Businger is in private practice in Luzern, Switzerland, and provides technical evaluation and analysis of contact lenses and related products for numerous corporations.