Daily wear performance of Silicone Hydrogel Lenses

Background

It is more than five years now since the first silicone hydrogel lenses were introduced onto the worldwide contact lens market. While these lenses were originally developed for a continuous wear (CW) modality, it soon became apparent that lenses made from these materials were also able to offer many physiological advantages on a daily wear (DW) basis, equating with superior corneal health and very often greater comfort and decreased dryness. As a result, silicone hydrogel lenses are being increasingly prescribed for DW. Although estimates for this modality vary significantly by country and region (8 to 83% of silicone hydrogel wearers)¹ , it is widely accepted that silicone hydrogels worn on a DW basis represent a rapidly growing segment of the contact lens market . Moreover the most recent silicone hydrogels to enter the contact lens arena have been developed specifically for DW or occasional overnight lens wear.

This is the first of a series of three editorials which will investigate how silicone hydrogel lenses behave when they are prescribed and worn on a DW basis. The first editorial will describe the overall performance of silicone hydrogels and the subsequent two editorials will discuss the adverse responses which can sometimes be associated with silicone hydrogels and suggest strategies for silicone hydrogel contact lens fitting based on modulus of the lens materials.

Lens Comfort and Adaptation

Initial comfort is known to greatly influence the patient’s perception of contact lenses and their ultimate success (Efron, McMonnies). There have been anecdotal reports of initial discomfort when refitting some patients from conventional lens materials to silicone hydrogels. In a recent study conducted at the Centre for Contact Lens Research (CCLR)² a group of successful long-term soft contact lens wearers were refitted with silicone hydrogel lenses (CIBA Vision Night & Day ™ ) to be worn on a daily wear basis and replaced monthly. Evaluations were made on study entry while wearing their habitual lenses, and then with the silicone hydrogel lenses at dispensing and after 1 week, 1 month and 2 months of lens wear. Ninety three percent of the participants successfully completed the 2-month study and no differences were found in comfort on insertion or overall daytime comfort between the silicone hydrogel lenses and their own lenses. This finding indicates that it should be possible to refit the majority of conventional lens wearers without comfort and adaptation concerns.

End of Day Comfort and Dryness

The most commonly reported problem with soft contact lens wear continues to be discomfort, often presenting with symptoms of dryness^3;4 as the day progresses. In the CCLR study described above, study participants were asked to record their end-of-day comfort and dryness after at least 12 hours of lens wear, on several occasions over the two-month study period. Reliability and compliance were improved by requiring the participants to complete analogue grading scales and then telephone their responses to an answering system which time-stamped their calls. The results showed significantly better end-of-day comfort and less end of day dryness when compared with their habitual lenses. While end-of-day comfort remained the same throughout the study, there was actually less end of day dryness as the study progressed. The intensity of dryness symptoms at the end of the first month and throughout the second month was significantly lower than after the first day and first week wearing the silicone hydrogel lenses. It is important to emphasize that this was in a group of successful long term wearers who did not consider that they had a problem wearing their lenses for long periods of time. Silicone hydrogel lens wear may lengthen the period of comfortable lens wear for even our happy patients.

Durability of Lens Surfaces

Concern has been expressed by practitioners that repeated daily cleaning of silicone hydrogel lenses may affect the surface treatments which can be employed to improve wettability. CCLR data⁵ have shown lens surface characteristics to be unaffected by daily wear and handling of PureVision™ and Night & Day lenses™. No changes in lens wettability, as measured by non-invasive tear break-up time, were found and break-up times were consistent with conventional hydrogel materials. A slight decrease in graded wettability over time and a slight increase in deposits over the wearing period were found, but these findings are also comparable with conventional lens materials when worn on a daily wear basis. Even the regular use of alcohol-based surfactant cleaners has been shown to have no effect on lens surface appearance⁶.

Limbal Hyperemia

Even successful contact lens wearers report undesirable redness with their lenses, particularly as the day progresses. Significantly less limbal hyperemia has been reported to occur with silicone hydrogels when compared with conventional hydrogel lenses by a number of authors^7;8 but it is only more recently that studies specifically investigating the effect of daily wear of silicone hydrogel lenses have been conducted. One such study showed a significant decrease in limbal hyperemia within one month of refitting conventional hydrogel lens wearers with silicone hydrogels on a daily wear basis² . As would be expected, the reduction was most marked for those participants who presented with higher baseline levels of limbal hyperemia (Figures 1a and 1b). Participants also reported a corresponding reduction in end-of-day redness and these findings were also in agreement with gradings made at a later date by a masked investigator from photographs taken at each of the study visits (Figure 2).

Figure 1a Figure 1b

Resolution of Chronic Hypoxic Signs

One of the most commonly observed signs of chronic corneal hypoxia is neovascularization⁹. The typical presentation is a moderate circumlimbal or regional encroachment into the cornea from the limbus of 1mm or less (Figure 3a). While many patients are unaware that they have this complication, it is unquestionably in their best interests for this condition to be resolved. We have observed remarkable resolution of corneal neovascularization when refitting study participants with silicone hydrogel lenses (CCLR data on file). One such example is shown in Figure 3b which demonstrates the apparent reduction in both the extension of the vessels into the cornea and also their “fullness”. This photograph was taken one month after refitting the eye shown in Figure 3a with Night & Day™ lenses worn on a daily wear basis. With time, the vessels empty and “ghost vessels” remain¹⁰. It is important however to remember that a number of stimuli can result in the refilling of these vessels including mechanical trauma and inflammatory reactions.

Figure 3a Figure 3b

All our patients deserve nothing but the best!

In view of the fact that silicone hydrogels are able to offer both superior ocular health and the potential to provide improved comfort characteristics, it will only be a matter of time before eye care practitioners begin promoting these lenses for the majority of their patients, not just those who want to sleep in their lenses. As demand for silicone hydrogel lenses grows, more options will rapidly become available, including toric lenses, multifocal designs and possibly even daily disposable lenses. Worldwide, the majority of patients choose to wear their lenses on a daily wear basis. It is imperative that we refit these patients with silicone hydrogel materials as soon as they become available in the required parameters and designs. All our patients deserve nothing but the best!

Reference List

1. Morgan P, Efron N, Jones D, Grein H, Tranoudis IG, Montani G et al. International contact lens prescribing in 2004. Contact Lens Spectrum 2005;20:34-7.
2. Dumbleton KA, Keir N, Moezzi A, Jones L, Fonn D. Redness, dryness and comfort following refitting long term low Dk hydrogel wearers with silicone hydrogels. Optom.Vis.Sci. 2004;81:31.
3. Doughty MJ, Fonn D, Richter D, Simpson T, Caffery B, Gordon K. A patient questionnaire approach to estimating the prevalence of dry eye symptoms in patients presenting to optometric practices across Canada. Optom Vis Sci 1997;74:624-31.
4. Orsborn G, Robboy M. Hydrogel lenses and dry-eye symptoms. Transactions of the British Contact Lens Association Annual Clinical Conference Birmingham 1989;66:37-8.
5. CCLR. Data on File 2005.
6. CCLR 2. Data on File 2003.
7. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom.Vis.Sci. 2001;78:147-51.
8. 8. Papas EB, Vajdic CM, Austen R, Holden BA. High-oxygen-transmissibility soft contact lenses do not induce limbal hyperaemia. Current Eye Research 1997;16:942-8.
9. McMonnies CW, Chapman-Davies A, Holden BA. The vascular response to contact lens wear. American Journal of Optometry and Physiological Optics. 1982;59:795-9.
10. McMonnies CW. Contact lens-Induced corneal vascularization. Int Cont Lens Clin 1983;10:12-21.