Clinical Performance Of A Monthly Lens And Patient Comfort
By Brenda Hall,
BSc, PhD, and Stephen Jones, BSc, PhD
The unusual material properties of biomimetic lenses deliver significant benefits in clinical performance, particularly patient comfort.
A biocompatible contact lens causes no significant change to ocular physiology and shows low levels of spoilation. In 1995, medical device company Biocompatibles introduced a new class of hydrogels, designed to be biomimetic, to mimic the chemistry of natural cell membranes. The first of these materials, omafilcon A, incorporated phosphorylcholine (PC), a substance present in the cell membrane of red blood cells and natural material responsible for biocompatibility. Lenses manufactured from PC-containing polymers were expected to show better biochemical characteristics than available soft contact lens materials.
Proclear, a lathe-cut 59 percent water content daily wear contact lens, and Proclear Compatibles, a cast-molded 62 percent water content disposable lens for daily wear and monthly replacement, were the first contact lenses manufactured from omafilcon A. Omafilcon A is classified by the FDA as a Group 2 material, and in 1998 Proclear and Proclear Compatibles were the first lenses to be cleared by the FDA for patients with mild to moderate dry eye. This article reviews the clinical performance of these lenses.
The two key properties of omafilcon A are its capacity for sustained hydration and its resistance to lens deposits. The affinity of PC for water helps the material dehydrate less on the eye than other materials of similar water content, and its water content is less affected by changes in temperature and pH. Resistance to dehydration enables the lenses to maintain high oxygen transmissibility. An important factor in the material's deposit resistance is that, at physiological pH, the PC molecule is more electrically neutral. Resistance to deposition minimizes clinical problems associated with spoilation and makes the lenses easier to keep clean.
A recent study to evaluate surface wettability of Proclear Compatibles found that the omafilcon A lens supported a more stable pre-lens tear film and a better tear film structure than other frequent replacement lenses.
Comfort and preference. Comfort is the patient's most important requirement of contact lens wear. Discomfort ranks high among the principal reasons given for abandoning contact lenses. Several studies have compared comfort with omafilcon A lenses to the comfort of conventional soft lenses.
Figure 1. Mean comfort score comparison for Proclear Compatibles and Acuvue at different times throughout the day (error bars indicate 1SD). There was a significant difference throughout the day for both lenses (P<0.0001, repeated measures ANOVA) (Hall 2000).
Lebow and Bridgewater investigated the clinical performance of Proclear and Acuvue (etafilcon A, Vistakon) lenses among 100 successful Acuvue wearers. Proclear was significantly more comfortable at each follow-up visit over the three months of the study. More subjects expressed a preference for Proclear than for Acuvue at all visits, and the reason given tended to be comfort.
The same authors compared comfort with Proclear to that of CSI (crofilcon A, Wesley Jessen) lenses in 54 subjects. Proclear lenses tended to become more comfortable as the three-month study progressed, while the CSI lenses became less comfortable. Twelve of the 24 subjects who had previously worn CSI lenses preferred the Proclear lens, while eight preferred CSI. Four had no preference.
When the clinical performance of Proclear was compared with that of another Group 2 lens, Permaflex (surfilcon A, Pilkington Barnes-Hind), comfort was graded significantly better with Proclear after one month of daily wear. All subjects who expressed a preference opted for the Proclear lens, all but one citing comfort as the reason for their choice.
Comfort throughout the day. Many soft lens studies evaluate lens comfort, but few published reports exist on lens comfort at different times during the day. One recent study assessed comfort throughout the day in subjects wearing Proclear Compatibles in one eye and Acuvue in the other. The Acuvue lenses were replaced on days 14 and 28, whereas the Proclear Compatibles lenses were worn for the duration of the one-month study. Subjects recorded subjective comfort with the lenses at stages throughout the day.
Figure 2. Lipid and protein spoilation at one month for omafilcon A, polymacon, etafilcon A, Weicon CE and atlafilcon A. (Negative result for atlafilcon A was due to the handling tint) (after Young et al).
Comfort was graded significantly better with Proclear Compatibles than with Acuvue at each time point (Figure 1) and on all days, including day 14 after Acuvue lens replacement. Overall, only five percent of subjects preferred Acuvue, compared to 38 percent opting for Proclear Compatibles. Where a preference was expressed, subjects preferred Proclear Compatibles for comfort and lens handling.
Lens deposits. Despite the trend toward frequent replacement, spoilation remains an important factor in successful lens wear and in determining replacement schedules. Although Group 2 lenses are normally susceptible to lipid spoilation, and high water lenses tend to deposit high levels of protein or lipids, omafilcon A shows low levels of binding to both key tear components. In contrast, Group 2 Permaflex lenses show significant levels of lipid spoilation. Protein and lipid spoilation on Proclear appears to be independent of length of wear and is consistent for most wearers.
Deposits on worn Proclear lenses have also been compared with a range of typical lens materials. Proclear lenses showed significantly less lipid spoilation than other materials tested and significantly less protein than the Group 2 Excelens (atlafilcon A, CIBA Vision) and Group 4 Acuvue lenses (Figure 2). High protein levels were found on Group 4 lenses, as well as high lipid levels on other Group 2 lenses.These findings confirm the results of laboratory tests and highlight the fact that the spoilation characteristics of omafilcon A cannot be assumed from its FDA classification, water content or ionic nature.
Dehydration. On-eye dehydration remains a problem with many high water soft lenses. Factors affecting water content include temperature, humidity, pH and protein deposits. In turn, dehydration has been linked to several aspects of clinical performance.
Omafilcon A has been shown to resist dehydration, and studies have confirmed that Proclear lenses dehydrate less on the eye than ionic and non-ionic materials of similar water content. Although high water materials generally show greater on-eye dehydration than low water materials, Proclear's clinical performance demonstrates that a high water, non-ionic material can possess similar dehydration-resistant characteristics to low water, non-ionic materials.
Dehydration with Proclear Compatibles has also been compared with a range of frequent replacement lenses during over 12 hours of lens wear. Proclear Compatibles dehydrated less than Acuvue and SofLens 66 (alphafilcon A, Bausch & Lomb), both of which showed an increase in dehydration with increasing wearing time (Figure 3). Comfort was graded higher with Proclear Compatibles than with Acuvue. A significant negative correlation was found between dehydration and comfort but only with the Acuvue lens, which suggested that other factors such as pre-lens tear film stability and environmental conditions also influenced comfort.
Morgan and Efron investigated the dehydration and comfort of soft lenses over wearing periods of up to four weeks. Six subjects wore each of four lenses (Acuvue, CooperVision's Frequency 38, Wesley Jessen's Gentle Touch and Proclear). The pattern for daily dehydration remained consistent for Acuvue and Gentle Touch but decreased with Proclear and Frequency 38. Only Proclear showed no significant decrease in comfort by day's end. For one lens type (Frequency 38), greater dehydration was associated with a greater loss in comfort during the day.
A recent Canadian study found no correlation between lens dehydration and subjective dryness and comfort, in either symptomatic or asymptomatic wearers. Here, 20 soft lens wearers with dryness-related symptoms and 20 asymptomatic wearers wore an Acuvue lens in one eye and a Proclear Compatibles lens in the other. There was also no significant difference in lens dehydration between the symptomatic and asymptomatic groups during seven hours of lens wear. These authors concluded that dehydration was not a major factor associated with symptoms of dryness and discomfort.
Dry eye. Approximately half of all lens wearers report symptoms of dryness. Dryness-related discomfort is one of the main causes of patients discontinuing lens wear. Proclear and Proclear Compatibles lenses were the first to be cleared for use in dry eyes. The specific label claim allowed by the FDA is: "The lenses may provide improved comfort for contact lens wearers who experience mild discomfort or symptoms related to dryness during lens wear associated with evaporative tear deficiency or from aqueous tear deficiency (non-Sjögrens only)."
FDA clearance resulted from a study by Lemp et al of the clinical performance of omafilcon A lenses in a dry-eye population. In this three-month, cross-over study, 76 mild to moderate dry-eye sufferers wore either Proclear lenses or their own daily wear soft lenses. When the subjects wore Proclear, they experienced a significant improvement in comfort, dryness (Figure 4), frequency of eye irritation and frequency of burning. There was also less on-eye dehydration and corneal staining with Proclear than with the subjects' own lenses.
Dehydration and Oxygen Performance
The oxygen permeability of omafilcon A, coupled with the thin design of Proclear and Proclear Compatibles, results in high oxygen transmissibility. Early reports suggested that resistance to on-eye dehydration would allow the material to maintain its high oxygen transmissibility during wear.
Figure 3. Relative percentage dehydration after 4, 8 and 12 hours of lens wear (error bars indicate 1SD). Proclear Compatibles data have been combined (n=30) at each time point; control lens data points are based on 10 measurements. (P=0.004, P<0.0001 respectively) (after Hall et al)
Efron and Morgan measured dehydration after four hours of wear using 17 different lens types and assessed the clinical relevance of any consequent changes in transmissibility. They found that the ionic nature of a soft lens material was more closely associated with its on-eye dehydration than its water content. Group 2 lenses, including Proclear, showed the least reduction in water content and Group 4 lenses, such as Acuvue, the greatest. Their conclusion was that changes in transmissibility could be clinically significant for some lenses, and dehydration was a factor clinicians should consider when choosing which soft lens to prescribe.
Figure 4. Subjective symptoms of dryness/moistness for Proclear and control lenses. At the cross-over between Parts 1 and 2 of this study, the Proclear wearers in Part 1 wore the control lenses in Part 2 and vice versa. The scale of 0 to 10 indicated lens feeling from very dry to perfectly moist (after Lemp et al).
Many studies have been published comparing the clinical performance of Proclear and Proclear Compatibles contact lenses with that of other soft lenses in normal and dry-eye populations. The omafilcon A material has delivered significant patient benefits, particularly in patient comfort. Studies have confirmed the material's resistance to dehydration and spoilation. These characteristics contribute to improving patient comfort and meeting patient expectations of soft contact lens wear.
Acknowledgement: Our thanks to Visioncare Research for help in preparing this paper, which is based on an article previously published in the UK journal Optician. Figures 2-4 are reproduced with permission of The CLAO Journal.
Referenced are available upon request. To receive references via fax, call (800) 239-4684 and request document #66. (Have a fax number ready.)
Dr. Hall is Director of Clinical Affairs, North America, and Dr. Jones is Director of Research and Development, for Biocompatibles Hydron.