Despite the possibility of using disposable soft contact lenses to avoid complications associated with long time wear, improving hydrogel contact lens materials is one of the main focuses of research in this field.19 The surface properties of contact lenses and interface interactions between the lens and eye surface may induce biological deposits, corneal damage, and infection.33,34 To improve tolerance and reduce the adsorption of deposits, more research needs to be targeted at modifying surfaces and developing new polymer materials.
A different surface roughness in a new lens can be the result of the manufacturing method and the material's properties. The spin casting method generates contact lenses with the smoothest surfaces, followed by cast-molding and then lathe-cut lenses.13,35 All the lenses tested in our study were cast-molded, and their roughness parameters were similar to the ranges reported for other non-surface-treated cast-molded lenses.13 Thus, the roughness differences between lenses observed here cannot be attributed to the manufacturing procedure. Besides the mode of elaboration, other authors have linked the presence of MA10 or a reduced water content4,13 to a greater lens surface roughness.
In this study, highest roughness values were recorded for the daily replacement hydrophilic contact lenses (nelfilcon A and ocufilcon B), which showed similar roughness scores for both surface areas analyzed. In contrast, comfilcon A showed the smoothest, or flattest surface (Ra = 1.56 nm), followed closely by omafilcon A (Ra = 1.90 nm). Similar roughness values were observed for the hioxifilcon-based material and senofilcon A, yet their surface appearance was different (Figs. 3 and 4). Although the hioxifilcon-based contact lens contains MA, which should determine a greater surface roughness, its similar Ra to senofilcon A could be attributed to its high water content. As may be observed in Fig. 3, senofilcon A shows a granulated surface structure, which is similar to that previously reported for the AFM observation of senofilcon A16, of galyfilcon A12 and for the cryogenic SEM visualization of the latter.36 Galyfilcon A is a non-surface-treated silicone hydrogel contact lens that contains polyvinylpirrolidone (PVP) as an internal wetting agent.
Silicone-hydrogel contact lenses exhibit different surface characteristics depending on their chemical composition and surface treatments.37 Surface treatments are targeted at obtaining wettable surfaces38 although the surfaces of the silicone-hydrogel contact lenses examined here were untreated. Thus, senofilcon A incorporates an internal wetting agent (polyvinyl pyrrolidone) that apparently leaches to the lens surface, and the Aquaform™ technology used in comfilcon A minimizes lens dehydration by forming hydrogen bonds with water molecules, creating a naturally hydrophilic contact lens that retains water inside the lens.39,40
The roughness parameters obtained for these lenses were similar to those observed previously in silicone-hydrogel contact lenses lacking surface treatment, such as galyfilcon A and comfilcon A,12,15 but lower than those reported for surface-treated lenses of this type.13,14 Although not significant, the differences observed in the present study between senofilcon A and comfilcon A could be related to the effect of water content on surface roughness.13 Despite the similar surface appearance of our silicone hydrogels and those examined by others,15,16 Teichroeb et al. observed higher roughness parameters for senofilcon A than comfilcon A when measuring a 25 μm2 area. These differences could be related to the fact that the lenses were analyzed after drying in ambient conditions for 15 min.
Occasionally, a contact lens wearer will suffer an adverse response to a lens. These problems are frequently caused by bacterial contamination of the contact lens surface, and keratitis is one of the most feared complications.41,42 Contact lenses absorb tear film proteins and lipids, and this induces lens contamination and deterioration. Moreover, the build-up of tear film components on contact lenses can cause discomfort and inflammatory complications such as giant papillary conjunctivitis,43,44 and this may occur with any type of daily or extended wear lenses.45 This adsorption depends mainly on the contact lens material and varies according to the tear secretion rate and certain pathological conditions. Research on conventional poly-HEMA-based lens materials has shown that the deposition of lysozyme and albumin depends on the polymer's composition,46 charge,47,48 and water content.49 Silicone-hydrogel materials give rise to different deposition profiles to those associated with the use of conventional poly-HEMA hydrogel lenses in that they induce less protein deposition and more lipid deposition.50–52 Surface roughness also needs to be considered, because deposits are more likely to form on imperfections of the lens surface.2 Our findings would seem to suggest that daily replacement contact lenses are more prone to deposit formation during wear because these lenses showed the highest roughness scores. Accordingly, a strict replacement regime would be recommended for the use of nelfilcon A and ocufilcon B contact lenses.
As mentioned above, when measuring larger areas, a higher variability of roughness parameters values is observed on a non-homogeneous surface. Because the present study is the first to apply the parameters Rku and Rsk to contact lenses, and to reduce the possible effect of variations in their values, we only determined these shape parameters across a single surface area (25 μm2). Measuring Rku and Rsk over larger areas could nevertheless be the aim of future studies. The Rku and Rsk values obtained for the nelfilcon A and ocufilcon B lenses reveal a low peak distribution profile such that despite a predominance of peaks, the surfaces of these lenses have few high peaks and deep troughs. For the lenses displaying the lowest roughness parameters (comfilcon B and omafilcon A), Rku values indicated the highest peaks and deepest troughs with more peaks than troughs because Rsk was positive. The clinical implications of high peaks and/or deep troughs over a contact lens surface could be enhanced deposit formation, even for a surface with a low Ra. Thus, the present observations indicate that including statistical parameters such as Rku and Rsk when characterizing a contact lens surface will complete the information on the finished quality of the surfaces. Further work is needed to establish whether a surface with low Ra and Rms is really less prone to deposits if its Rku is high and Rsk is negative. An improved understanding of the surface roughness of different types of contact lenses will better prepare practitioners to prescribe the most suitable lens for a given patient, and interpret the clinical performance of the lenses they prescribe in terms of patient symptoms and ocular surface signs.
We thank Prof. Cesar Sanchez Sellero (Department of Statistics and Operations Research, University of Santiago de Compostela, Spain) for his help with the statistical analysis.
M. Jesús Giraldez
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