In most practitioners' minds, the crucial factor for successful extended wear is sufficient oxygen supply to the cornea. Although early silicone elastomer lenses were able to achieve this, problems such as surface deposition, poor wettability, and lens binding resulted in clinically unacceptable lenses. With new generation silicone hydrogel lenses, the incorporation of silicone into the hydrogel matrix provides sufficient oxygen to the cornea, while a plasma surface has produced a lens that provides enhanced biocompatibility compared with silicone elastomer lenses. Have these lenses been able to overcome the problems of their predecessors with respect to wettability and deposition? Since the release of silicone hydrogel materials, much research has been done attempting to answer this question.

Deposits on a Silicone Hydrogel Lens

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Long et al.¹, and Montero et al.² have found consistently low levels of front surface deposits and back surface debris with silicone hydrogel lenses. However, Brennan et al.³ found that, although low, these levels of deposition were still higher than with extended wear of traditional hydrogel materials. It is of significance though, that in the Brennan ³ study, the silicone hydrogel lenses resulted in better subjective responses, and similar vision, wettability, and tarsal anomalies to the hydrogel materials. Lens age is also an issue with deposits and extended wear, as 'on-eye' time is significantly greater than with daily wear lenses. Studies have shown deposits and film to increase with length of wear with high water content disposable lenses⁴. Interestingly, this has not been the case with silicone hydrogels, where the greatest deposition was found to occur within the first week of wear, and remain at a constantly low level for a month¹.

Wettability is thought to be an index of biocompatibility⁵. Deposition has a great influence on wettability, as does material type, cleaning regime, wearing time, replacement frequency, and an individual's tear film characteristics. In objective measurements of in-vitro wettability, silicone hydrogel materials have higher wetting angles (lower wettability) than conventional hydrogel materials, particularly when used with non-surfactant regimes, such as saline⁶. However, these results are not mirrored in clinical trials of silicone hydrogel lenses, where they have similar wettability values to low Dk lenses^7-3-5. As for the levels of deposition, wettability changes slightly in the first week and then remains fairly stable for at least one month¹.

Literature to date indicates that the surface treatments of silicone hydrogel materials have been successful in keeping deposits to a minimum, even over extended periods of wear. Even with cases of slightly higher deposition, there has been no effect on subjective comfort, wettability, or other findings normally adversely effected by deposition.

This year, at ARVO, Thai and Colleagues⁸ added to the volume of research investigating the question of silicone hydrogels and wettability, with their poster, "Effect of Different Soft Contact Lens Materials on the Tear Film". Their study compared five different contact lens materials, and their effects on pre-lens tear film (PLTF). The PLTF was evaluated by the measurement of evaporation rate, thinning characteristics, and lipid layer changes before lens insertion, as well as after 30 minutes of wear. Lenses under consideration were polymacon (Optima 38), omafilcon A (Proclear Compatibles), phemfilcon A (Durasoft 2), etafilcon A (Acuvue), and a silicone hydrogel lens, balafilcon A. Twenty habituated lens wearers wore the 5 lenses in random order on the left eye.

Tear film evaporation rate was measured by a modified ServoMed evaporimeter. This instrument measures tear evaporation rate by measuring water vapour pressure at two points above the ocular surface. Tear thinning time was measured by HirCal grid, a non-invasive method of assessing tear stability. Both of these tests are useful diagnostic and research tools, since they provide an important indicator of PLTF lipid layer integrity (a poor lipid layer will result in an unstable tear film, generally causing poor wetting of lenses and ocular discomfort). In this study, there were no significant differences between evaporation rates and tear thinning time for any of the five lens types. In addition, a Doane tear film video interferometer was used to record tear film structure and thinning. This instrument non-invasively evaluates dynamic changes in tear film thickness, thickness distribution, and wetting properties of contact lens surfaces. Although there were differences between lens types with this instrument, there were no differences between balafilcon A and any other lens type. Essentially, this study confirmed that contact lenses all affect the tear film by increasing the evaporation rate, and decreasing tear thinning time, and that the effect of a silicone hydrogel material on the tear film was similar to that of all other lens types.

Changes in tear film structure and characteristics affect the wettability and deposition of contact lenses. As such, careful evaluation of the tear film is essential to the investigation of contact lens wettability. Since previous research has highlighted the difficulties in accurately assessing the tear film, the results of this study are rendered more valid by the use of multiple techniques. Even though measurements were taken after only 30 minutes of lens wear, they are in agreement with long term clinical studies, where silicone hydrogels have been shown to have similar wettability to low Dk lenses^7-3-5.

This recent study by Thai et al.⁸ confirms previous reports on the wettability of silicone hydrogels. It appears that, unlike early silicone elastomer lenses, silicone hydrogel materials with a surface treatment overcome the problems of poor wettability and deposition and in most cases perform just as well as, or better than conventional lenses. The result is a safe and comfortable environment for extended wear.


References:

1. Long B, Robirds S et al. Six months of in-practice experience with a high Dk lotrafilcon A soft contact lens. Contact Lens and Anterior Eye 2000;23:112-118

2. Montero I, Nebot R et al. Practical experience with a high Dk lotrafilcon A fluorosilicone hydrogel extended wear contact lens in Spain. CLAO J 2001;27(1):41-6

3. Brennan NA, Coles M-L C. A 1-year prospective clinical trial of balafilcon A (purevision) silicone hydrogel contact lenses used on a 30-day continuous wear schedule. Ophthalmology 2002;109(6):1172-1177

4. Maissa C, Franklin V. Influence of contact lens material surface characteristics and replacement frequency on protein and lipid deposition. Optometry and Vision Science 1998;75(9):697-705

5. Fonn D. Factors affecting the success of silicone hydrogels. Optician 1999;218:27-29

6. Jones L, Long J et al. The impact of contact lens care regimens on the in vitro wettability of conventional and silicone hydrogel contact lens materials. Poster presented at ARVO 2002, abstract available at www.arvo.org.

7. Sweeney D, Keay L et al. Clinical performance of silicone hydrogel lenses. In: Sweeney DF, ed. Silicone hydrogels: The rebirth of continuous wear contact lenses. Oxford: Butterworth-Heinemann; 2000:90-149

8. Thai L, Doane M et al. Effect of different soft contact lens materials on the tear film. Poster presented at ARVO 2002, abstract available at www.arvo.org