The association between wettability and contact lens comfort – have SiHs improved our knowledge?

One of the major breakthroughs in the development of silicone hydrogel (SH) contact lenses has related to the ability of manufacturers to overcome the surface hydrophobicity that occurred with silicone elastomer lenses.  This has been accomplished by various techniques, including the incorporation of plasma into the surface processing and internal wetting agents. 
In the eye, the term “wettability” refers to the ability of the tear film to cover and maintain itself over the contact lens surface1 and it is often determined by a clinician observing the lens surface using a slit lamp.  Wettability is considered an essential factor in determining the physiological compatibility of biomaterials [2] and a wettable contact lens surface is important for many reasons, including reduced front surface deposition [3, 4] and improved optical quality. [5] 

Laboratory studies

A laboratory-based test of wettability is typically performed by determining the ability of a fluid to spread over a contact lens surface, and is measured in terms of the “contact angle” (CA).  The smaller the CA, the more wettable the surface is.  Laboratory-based (in vitro) techniques to assess CAs include the sessile drop, captive bubble and Wilhelmy plate techniques, which are reviewed elsewhere. [1, 6] Either the advancing or receding CA is typically reported and there are conflicting opinions as to whether the receding [7] or advancing [8] CA is more clinically relevant.  The advancing CA is measured as a fluid is moved over a previously unwetted surface, whereas the receding CA is measured as the material is withdrawn from the fluid. [8] CA hysteresis is the difference between the advancing and receding CAs and provides a measurement of molecular mobility at the lens interface. [8] 

Laboratory-based studies investigating CAs have provided a great deal of potentially useful clinical information, establishing that the blister pack solution, care systems containing surface-active agents (surfactants), [8] and various tear film components such as lipids [9], lysozyme and/or mucin [10] may all significantly lower the CAs measured (or improve the wettability) of various contact lens materials.  In vitro studies have also shown that CAs associated with conventional hydrogel materials are quite similar, [7] while CAs of different SH materials tend to vary. [1]   

Despite studies showing that the CA can be manipulated in various ways, the relationship between CA and in-eye comfort, remains elusive. Theoretically, lower CA wettability should provide enhanced comfort; however, the clinical relevance of in vitro CAs is yet to be established.  Thai et al. [11] investigated the influence of a multipurpose contact lens solution containing an ocular lubricant (hydroxypropyl methylcellulose (HPMC)) and reported improved in vitro and in vivo wettability; however, no subjective preference was reported when the lenses were worn by 35 subjects for one week.   Other researchers have also confirmed the lack of a direct relationship between CA and subjective performance.  A  study performed by Dumbleton and colleagues showed relatively little difference in reported in-eye comfort between SH materials, [12] compared with the large differences in reported CAs. [1]  This may be due to interaction between the tear film and the lens surface, lowering CAs, and masking material differences while the lenses are on the eye. [10] 

Despite the fact that this line of research has been explored for many years,13 it remains unknown whether there is a link between subjective outcomes and CAs.  Maldonado-Codina et al. [7] suggest that no single CA measurement can predict the complex interaction between the tears and a lens.  Additionally, Jones et al. [14]  suggested that CA analysis following lens removal may be more predictive of comfort than CAs determined pre-wear.  Consequently, interpretation of laboratory-based CA analysis should be done with these points in mind.

Clinical studies   

There are various ways to measure pre-lens tear stability, which is an indirect measure of the hydrophilic nature (or wettability) of soft lenses in vivo, including pre-lens non-invasive break-up time (NITBUT) (using the image of a placido ring from a corneal topographer [15] or the Tearscope [16]) and clinician grading using a slit lamp. [17] Soft contact lens wear disrupts normal tear physiology, causing NITBUT to decrease [17, 18] and a shorter NITBUT has been reported in symptomatic contact lens wearers. [19-21] 

Differences exist in the methodologies used for the in vivo evaluation of soft contact lens wettability.  Certain SH lens materials exhibit a shorter NITBUT compared to conventional hydrogels, [22] while other studies have shown similar [23] or improved [24] in vivo wettability with SH lens materials.  In vivo wettability can also vary between SH lens materials. [25]  Regardless, while it has been reported that improved in vivo wettability may be related to better comfort with certain SH lenses; [24] a direct correlation between in vivo wettability and comfort is yet to be established. 

NITBUT is only one snapshot of in vivo wettability, which is dynamic in nature, and it may be that the pre-lens tear film structure (i.e. thickness or quality) has a greater impact on comfort, compared to NITBUT alone. [26, 27]  There is also compelling information relating to the association between poor front surface lens wettability and reduced optical quality. [5, 28]  Fluctuating vision (e.g. having to “blink to clear vision”) due to tear film disruption is a symptom that has gained more attention in recent years, highlighted by the modified definition of dry eye in the 2007 DEWS report. [29]  The measurement of optical quality may prove to be a viable, indirect method to monitor pre-lens tear stability, [30] and as suggested by Papas et al., [31] the question remains as to whether poor optical quality has a psychological impact on perceived ocular comfort.


Summary

Despite improved overall comfort with SH lenses over conventional hydrogel lenses in many subjects, [32, 33] reduced comfort at the end of the day continues to be a problem. [12, 34] Ongoing research will continue to improve our understanding of in vivo wettability with SH lenses and provide further insight into the impact of lens wettability on comfort.  Additional work on the clinical relevance of CA assessment and its relationship to in-eye lens performance is clearly necessary and more accurate methods to link in eye wettability, ex vivo CA, optical quality and comfort remains an area of intense interest.
       

References:

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