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Optometry & Vision Science:
doi: 10.1097/OPX.0b013e3181f3e2fc
Original Article

Interactions of Lens Care with Silicone Hydrogel Lenses and Effect on Comfort

Willcox, Mark D. P.*; Phillips, Bronwen†; Ozkan, Jerome†; Jalbert, Isabelle‡; Meagher, Laurence*; Gengenbach, Thomas*; Holden, Brien‡; Papas, Eric*

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Author Information

*PhD

BOptom

PhD, FAAO

Brien Holden Vision Institute, The University of New South Wales, Sydney, New South Wales, Australia (MDPW, BP, JO, IJ, BH, EP), Vision Cooperative Research Centre, Sydney, New South Wales, Australia (MDPW, BP, JO, IJ, BH, EP), School of Optometry and Vision Science, The University of New South Wales, Sydney, New South Wales, Australia (MDPW, IJ, BH, EP), and CSIRO, Molecular and Health Technologies, Clayton, Victoria, Australia (LM, TG).

Received February 25, 2010; accepted June 22, 2010.

Mark D. P. Willcox; Brien Holden Vision Institute; Level 4, Rupert Myers Building; Gate 14, Barker Street; Kensington Sydney; New South Wales 2052, Australia; e-mail: m.willcox@ier.org.au

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Abstract

Purpose. The purpose of this study was to investigate the effect of lens care products on short-term subjective and physiological performance silicone hydrogel lenses.

Methods. Ten subjects wore either lotrafilcon B or galyfilcon A silicone hydrogel contact lenses soaked in a lens care product containing either Polyquad/Aldox or PHMB or control lenses inserted directly from the pack. Subjects wore the lenses for 6 h. Ocular comfort (graded on a 1 to 10 scale) and ocular physiology were assessed. Unworn but soaked lenses were analyzed for metrological changes, release of excipients into phosphate buffered saline, and changes to their surface chemical composition.

Results. None of the lens metrology measures or clinically observed conjunctival or limbal redness changed. Corneal staining was significantly (p < 0.008) raised, albeit to low levels, after 6 h wear for either lens type when soaked in the PHMB solution compared with the control lens (lotrafilcon B 0.4 to 0.9 ± 0.7 to 0.4 vs. 0.1 to 0.4 ± 0.3 to 0.5; galyfilcon A 0.2 to 0.3 ± 0.2 to 0.4 vs. 0.0 ± 0.0). For lotrafilcon B lenses, there were decreases in comfort (p = 0.002), increases in burning/stinging (p = 0.002) after 1 h of wear, and increases in lens awareness on lens insertion (p = 0.0001) when soaked in PHMB. However, lotrafilcon B lenses soaked in Polyquad/Aldox showed increases in burning/stinging after 1 and 6 h (p < 0.008) of lens wear. For galyfilcon A lenses, most significant (p ≤ 0.002) changes to symptomatology occurred after soaking in Polyquad/Aldox solution. More PHMB was released from lotrafilcon B lenses, and more MPDS material was released from galyfilcon A lenses. The surface of galyfilcon A lenses changed but irrespective of lens solution type, whereas the changes to the lens surface was dependent on solution type for lotrafilcon B lenses.

Conclusions. Lens care products can change corneal staining and comfort responses during wear. These changes may be associated with release of material soaked into lenses or changes to the lens surface composition.

Since their market release in the late 1990s, the prescription and use of silicone hydrogel contact lenses on continuous wear (CW), extended wear (EW), and daily wear (DW) schedules has been consistently increasing.1,2 The popularity of these lens types can be attributed to benefits such as the increased oxygen permeability of the material, resulting in decreased limbal hyperemia and other hypoxic-related problems, compared with the use of the more traditional hydrogel materials.3–5

Despite initially being marketed as CW or EW lenses, silicone hydrogels are most commonly prescribed and used on a DW basis, probably because of the historical reluctance of practitioners to endorse EW regimens.6,7 DW schedules introduce the need for an effective and efficient lens care product for cleaning and disinfection. The majority of lens care products prescribed for use with silicone hydrogel lenses are multipurpose solutions.6

Silicone hydrogel materials are, by nature, hydrophobic. Thus surface treatment is often used to create a hydrophilic lens surface with improved wettability and comfort.8 However, when used in conjunction with multipurpose lens care systems, it is possible that some polymeric solution additives may adsorb onto the surface of the lens. These adsorbed components may affect physiological and/or subjective responses and, perhaps, cause the discontinuation of use of the contact lenses.

Multipurpose disinfecting solutions (MPDSs) require an antimicrobial active component or components, and commonly used examples are polyquarterium-1 (Polyquad), myristamindopropyl dimethylamine (Aldox; Alcon Laboratories, Fort Worth, TX), and polyaminopropyl biguanide (PHMB; CIBA Vision, Duluth, GA). PHMB has alternatively been referred to as polyhexanide or DYMED by different manufacturers.9 In addition to the antimicrobial agents, the lens care products contain buffer systems, surfactants, and preservatives to yield an economic but effective lens care solution.

One study investigating the physiological and subjective responses of subjects wearing a balafilcon A silicone hydrogel lens compared the ocular response to use of a lens care product containing Polyquad/Aldox with one containing PHMB.10 The PHMB lens care product was associated with more corneal and conjunctival staining and more stinging or burning on lens insertion compared with the product containing Polyquad, which conversely showed less limbal hyperemia.10 Further research in this area has shown that certain lens/ solution combinations are associated with the production of corneal staining, with a combination of balafilcon A lenses and a PHMB-containing solution usually having the highest levels.11,12 This solution-induced corneal staining has been associated with but not proven to cause corneal inflammation.13

Lens care product components also have the potential to affect the comfort of silicone hydrogel contact lenses.3,11,12 Indeed, it has been reported that the higher the levels of micropunctate corneal staining, the lower the level of comfort.11 As discomfort is the most common reason attributed to discontinuation of contact lens wear,14 the aim of this study was to investigate the physiological and subjective responses of the short-term use of various lens care products with silicone hydrogel contact lenses of lotrafilcon B or galyfilcon A materials and to examine whether changes to the surface of lenses was correlated with the responses.

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METHODS

Study Design

This study was a prospective analysis of the performance of lotrafilcon B (CIBA Vision) and galyfilcon A (Johnson and Johnson Vision Care, FL) silicone hydrogel contact lenses. The lenses were soaked before wear for 7 days on a 12:12-h cycle, with the lenses stored in a lens care product overnight and placed in unit-dose unpreserved saline during the day. The soaking occurred in sterile contact lens cases (CIBA Vision.). The control condition (control) used lenses inserted on eye directly from the pack. For the 12-h before wear, and if lenses were soaked, they were soaked in one of the multipurpose solutions.

The material characteristics of the lens used in the study are described in Table 1. The lens care products used contained polyquarternium-1 (Polyquad: 0.001% Polyquad) and Aldox (myristamindopropyl dimethylamine 0.0005%), or polyhexanide (PHMB: 0.0001%; CIBA Vision). Table 2 details the constituents of the solutions used in this study.

Table 1
Table 1
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Table 2
Table 2
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All subjects (age 31 ± 8 years; male:female (%) 30:70) were free from ocular disease or any condition contraindicated to contact lens wear. The protocol was approved by the Committee for Experimental Research Involving Human Subjects at the University of New South Wales, Sydney, Australia, and complied with the Declaration of Helsinki 1975 as revised in 1983. All subjects signed a record of informed consent. A sample size of 10 subjects per lens type was required for the study to determine a difference of 1 ± 1 in subjective ratings and 0.5 ± 0.5 in clinical grades (such as corneal staining) with 80% power and 5% level of significance. Subjects wore either of the lenses, in a power close to their spectacle prescription, bilaterally for 6 h on three different days under three different conditions. The order of the two test conditions and one control condition were randomly assigned to each subject in a between-subjects study design. Lenses were soaked in the laboratory and then masked as to whether they were soaked in an MPDS or from the blister packs. Thus, both clinicians and lens wearers were masked to the solution being used. There was at least 12 h of no lens wear for each subject between each lens/soaking combination.

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Lens Parameter Measurements

Lens parameter measurements were taken for unworn lenses in each of the three conditions, i.e., lenses that had been cycled in a multipurpose solution and lenses taken directly from the pack. Three unworn lenses of each polymer type were used for each of the measurements. Parameter measurements included back curvature of the lens or back optic zone radius measured with the Zeiss Keratometer (Carl Zeiss Vision, Jena, Germany); lens diameter measured with the Nikon V-12 profile projector (Nikon Corporation, Tokyo, Japan); back vertex power measured using the Nikon PL-2 focimeter; center thickness, thickness at 3 mm, and mean edge thickness measured using the Heidenhain MT-10 soft lens thickness gauge (Heidenhain Corporation, Schaumburg, IL); and back surface sag calculated by deducting center thickness from the front surface sag measured using the Nikon 6C-2 profile projector.

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Responses of the Outer Ocular Surface During Lens Wear (Objective Measures)

Biomicroscopy procedures were performed to assess the ocular surface at lens insertion and on removal. Observers were masked to the lens solution that had been used. Limbal and bulbar conjunctival redness and conjunctival staining were graded on the Cornea and Contact Lens Research Unit scale of 0 to 4 in 0.1 increments in each of the four quadrants, nasal, temporal, inferior and superior.15 Corneal staining was graded according to the Cornea and Contact Lens Research Unit scale of 0 to 4 for extent, depth, and type of staining in each of five corneal areas, namely, central, nasal, temporal, superior and inferior, and in 0.5 increments.15 Visual acuity was measured using wall mounted Bailey-Lovie Log-MAR charts.

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Subjective Responses

Subjective responses to lens wear were assessed by directly asking the subjects to rate various aspects (on a 1 to 10 scale) during lens wear at each visit (insertion, 1 h and 6 h) or particular visits. Aspects rated were ocular burning and stinging; comfort on lens insertion (asked at insertion visit); comfort during wear (asked at 1 h visit); comfort at the end of the day (asked at 6 h visit); overall vision quality; redness of eyes; and overall lens awareness.

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Analysis of Release of MPDS Material From Lenses after Soaking

After soaking in MPDS/saline as described above, lenses were removed from the final soak in MPDS, gently blotted dry on filter paper, and added to a new lens case containing 1 ml saline solution (0.95% wt/vol NaCl). After incubating at ambient temperature (21°C) for 1 and 6 h, the saline solution was removed and absorbance of the solution read at 236 nm to measure the amount of PHMB release16 and scanned from 200 to 300 nm for release of material from Polyquad/Aldox-soaked lenses. Rosenthal et al.17 have reported the use of UV spectroscopy to measure the uptake of Polyquad into lenses. Negative controls of saline only and lenses soaked in saline for 12 h were used. Three lenses for each lens/solution combination were tested.

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Analysis of Lens Surface Composition by X-Ray Photoelectron Spectroscopy

Lenses were cycled as described above or, in the case of the control condition, removed directly from the pack and rinsed eight times in ultrapure water to remove any salts before drying overnight in a laminar flow cabinet and mounted for analysis. X-ray photoelectron spectroscopy (XPS; AXIS HSi Spectrometer; Kratos Analytical, Manchester, United Kingdom) was used to obtain quantitative analysis of elements by integrating the area under photoelectron peaks, corresponding to the various elements present in low-resolution survey spectra. This analysis allowed elemental composition (atomic percentages) and elemental ratios to be calculated. High resolution spectra were obtained to assess the chemical environment (or chemical functionality) of the elements present. Since the high-resolution spectra obtained for some elements are not well resolved, even with monochromated X-ray sources, deconvolution of the spectra via curve fitting allowed us to obtain information regarding the chemical functionality, oxidation state, and chemical environment of C, O, and N species contained within the peaks. In the high-resolution carbon spectra (C1s) C1 and C2 components correspond to CHx or hydrocarbon, C3 corresponds to carbon singly bonded to O or N (C―O/C―N), C4 corresponds to carbon doubly bonded to O (C[double bond]O), C5 corresponds to carbon with three bonds to oxygen atoms (O―C[double bond]O). In the case of the high-resolution nitrogen spectra (N1s), the N1 component corresponds to C―N, N2 to nitrogen bonded to carbon that is in turn doubly bonded to oxygen, e.g., amides (N―C[double bond]O), or N doubly bonded to C (N[double bond]C), N3 corresponds to protonated N bonded to carbon (+HN―C), and N4 to quaternarized nitrogen (+N―C) groups. For each lens polymer type and each soaking solution, two to three lenses were analyzed with each lens being analyzed at three points on its surface.

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Data Analysis

Eye specific data were collected for all clinical variables and subject specific for subjective ratings. A linear mixed model was used determine whether there were significant differences between solutions and lens types, and the interaction of solution and lens. The correlation between right and left eyes and repeat observations were accounted for in the model using random intercepts. If the interaction of lens and solution was significant, then the differences between solutions were determined for each lens type. If the effect of solution was significant, post hoc multiple comparisons were performed. Statistical significance for a variable was set at 5%. Post hoc comparisons were considered significant if p < 0.017 for three-group comparisons and p < 0.008 for six-group comparisons. Subjective symptoms were summarized as percentages at each visit and compared between the solution groups using the χ2 test.

For the surface analysis by XPS, data were recorded from four points each on the front and back of lenses and an average taken for the whole lens. Two lenses were analyzed for each test condition. Differences in the lens surface compositions, elemental ratios, and C1s and N1s spectral components for each condition were assessed using analysis of variance. The significance of comparisons between the control and test conditions was assessed using a post hoc (Bonferroni) analysis. Statistical significance was set at p values < 0.008.

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RESULTS

Lens Parameter Measurements

Lens parameters assessed were back curvature of the lens, lens diameter, center thickness, thickness at 3 mm, edge thickness, back surface sag, and back vertex power. Lens parameter measurements were performed on unworn soaked lenses in the two test solutions or unworn control lenses measured directly from the pack. No significant differences resulted when lenses were soaked in either solution compared with those measured directly from the packaging and equilibrated in saline.

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Responses of the Outer Ocular Surface During Lens Wear (Staining, Redness, and Visual Acuity)

Before lens insertion, there were no significant differences noted between the subjects' eyes. There were no differences in redness (conjunctival or limbal), conjunctival staining, or visual acuity between control lenses or lenses soaked in Polyquad/Aldox or in PHMB. After 6 h of lens wear, differences were noted for corneal staining (Fig. 1). The significant differences for lotrafilcon B lenses were for extent of corneal staining in area 2 (PHMB > control, p = 0.006), 3 (PHMB > control, p = 0.001), and 5 (PHMB > control, p = 0.004); for depth of corneal staining in area 3 (PHMB > control, p = 0.001); and for type of corneal staining in area 3 (PHMB > control, p = 0.001). The significant differences for galyfilcon A were for extent of corneal staining in area 2 (PHMB > control, p = 0.006) and 3 (PHMB > control, p = 0.001); for depth of corneal staining in area 3 (PHMB > control, p = 0.001; PHMB > Polyquad/Aldox, p = 0.006); and for type of corneal staining in area 3 (PHMB > control, p = 0.001). No other significant differences in corneal staining were found.

Figure 1
Figure 1
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Subjective Responses

For lotrafilcon B lenses, the only significant differences in responses were found for comfort after 1-h lens wear (Polyquad/Aldox-soaked lenses worse than control, p = 0.002), burning/stinging after 1 h (Polyquad/Aldox worse than control, p = 0.002; PHMB worse than control, p = 0.007) and 6 h of wear (PHMB worse than control p = 0.001), and lens awareness at insertion (Polyquad/Aldox worse than control, p = 0.0001; Table 3). For galyfilcon A lenses (Table 3), differences were noted for comfort at insertion (Polyquad/Aldox worse than control p = 0.0001), ocular comfort after 1 h of wear (Polyquad/Aldox worse than control p = 0.002), burning/stinging at insertion (Polyquad/Aldox worse than control, p = 0.0001), after 1 h of wear (Polyquad/Aldox worse than control p = 0.0001; PHMB worse than control, p = 0.007), and after 6 h of wear (Polyquad/Aldox worse than control p = 0.002), redness of eyes at insertion (Polyquad/Aldox worse than control, p = 0.0001), and lens awareness at insertion (Polyquad/Aldox worse than control, p = 0.0001). For either lens type and for either soaking solution, there were no differences for comfort at the end of the day (asked at 6 h visit) or overall quality of vision at the 1-h or 6-h visits.

Table 3
Table 3
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Release of Solution Components from Soaked Lenses

Release of PHMB was higher from lotrafilcon B lenses compared with galyfilcon A at either 1 or 3 h. Also, release of components from the Polyquad/Aldox solution was highest from galyfilcon A lenses compared with lotrafilcon B at either 1 or 3 h. Release of components from Polyquad/Aldox that absorbed at 200 to 300 nm was faster than release of PHMB (i.e., absorbance peaked at 1 h for Polyquad/Aldox but at 3 h for PHMB) regardless of the lens material.

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Chemical Analysis of Contact Lens Surfaces

Unworn lenses from each of the three conditions were analyzed using XPS. The results from quantitation of the peaks corresponding to the elements present in the survey spectra revealed differences in lens surface compositions and elemental ratios depending on the lens condition (Table 4). Although there are statistically significant differences between the surface composition data (atomic percentages) obtained for the different conditions, it is more reliable to compare elemental ratios. In this case, the atomic concentration of each element was normalized by the carbon atomic concentration, as carbon is the most abundant element. Therefore, interpretation of the results is based on the normalized data (Table 4). The contributions to the high-resolution C1s and N1s spectra, obtained via curve-fitting procedures, for each lens condition are shown in Table 5.

Table 4
Table 4
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Table 5
Table 5
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For lotrafilcon B lenses, soaking in PHMB increased the N/C on the lens surface (p = 0.0001) and soaking in Polyquad/Aldox decreased the N/C (p = 0.0001) on the lens surface compared with control. For galyfilcon A lenses, soaking in PHMB increased the O/C (p = 0.002) and Si/C (p = 0.003) on the lens surface and soaking in Polyquad/Aldox increased in O/C (p = 0.003) on the lens surface compared with control.

For the high-resolution C1s spectra, for lotrafilcon B lenses soaked in PHMB, there were decreases in C3/C (p = 0.0001) but increases in C4/C (p = 0.003) and C5/C (p = 0.001) compared with control. For lotrafilcon B soaked in Polyquad/Aldox, there was a significant decrease in C4/C (p = 0.0001). For galyfilcon A lenses soaked in PHMB, there were significant increases in C1+C2/C, C4/C, and C5/C (p = 0.0001) and a significant decrease in C3/C (p = 0.0001). For galyfilcon A lenses soaked in Polyquad/Aldox, there were increases in C1+C2/C, C4/C, and C5/C (p = 0.0001) and a significant decrease in C3/C (p = 0.0001).

For high-resolution N1s spectra for lotrafilcon B lenses soaked in PHMB, there were significant increases in N1/C (p = 0.002) and N2/C (p = 0.0001). For lotrafilcon B lenses soaked in Polyquad/Aldox, there were significant decreases in N1/C and N2/C (p = 0.0001) and a significant increase in N4/C (p = 0.0001). For galyfilcon A lenses soaked in PHMB or Polyquad/Aldox, there were no significant differences.

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DISCUSSION

This study has demonstrated that both lens types caused corneal staining (solution-induced corneal staining) if soaked in the PHMB containing MPDS but not when soaked in the Polyquad/Aldox containing MPDS. Andrasko and Ryen,11 found a similar result, although lenses in their study were worn for only 2 h and the PHMB-containing solutions demonstrating the highest level of staining were ReNu Multiplus and ReNu Multipurpose (Bausch and Lomb, Rochester, NY) rather than Aquify (Cibavision, Duluth, Georgia) as used here. In contrast, Carnt et al.12 demonstrated that corneal staining responses differ when lenses were worn on a normal, DW cycle for 2 to 4 weeks. For example, in combination with the same Polyquad/Aldox solution used in this study, lotrafilcon B lenses showed a higher rate of staining than with the corresponding PHMB product. These differences are likely to reflect the different length of wear of lenses (6 h in this study and 2 to 4 weeks in Carnt et al.12) and perhaps, therefore, the contribution of tear deposits. Green-Church and Nichols18 and Zhao et al.19,20 have shown that different lens materials and lens/solution combinations can attract different types and amount of proteins and lipids during wear. Santodomingo-Rubido21 has shown that, with either lotrafilcon A and galyfilcon A lenses, the Polyquad/Aldox solution used in this study was more likely to produce corneal staining than a new formulation containing PHMB. The fact that silicone hydrogel materials can behave differently with various PHMB formulations suggests a possible role for other solution excipients in the interactions between the lens materials and the active ingredients.

The formulation of lens care systems has been implicated as a cause of the corneal staining and symptoms of discomfort observed in soft lens wearers.9 The contribution of MPDS to symptoms associated with lens wear was also different between the two lenses and solutions. For lotrafilcon B lenses, use of the PHMB or Polyquad/Aldox solution was associated with worsening of certain comfort responses. On the other hand, Polyquad/Aldox MPDS tended to be more highly associated with degradation of comfort responses when used with galyfilcon A lenses. This is broadly in line with the findings of Andrasko and Ryen.11 In this study, there were no significant differences in the comfort response for galyfilcon A lenses when soaked in PHMB apart from burning/stinging after 1 h of wear (Table 3). Interestingly, in this study, there appeared to be no direction relation between corneal staining and comfort, because corneal staining was seen to be increased only when the PHMB solution was used whereas comfort was reduced when either MPDS type was used.

This study sought to determine whether release of ingredients from lenses after soaking or adsorption of ingredients onto the lens surface was associated with changes in comfort and ocular responses during wear. For lotrafilcon B lenses, Polyquad/Aldox material was released quicker than PHMB, which may be the reason that staining was only seen when the PHMB-soaked lenses were assessed after 6 h of wear. Also, lotrafilcon B released more PHMB than galyfilcon A, which correlates with the degree of staining seen for the two materials soaked in PHMB, where there was between 0.3 and 0.5 difference in staining compared with only 0.2 and 0.3 for galyfilcon A lenses. Previous studies have used XPS to analyze the surface of contact lenses, either after deliberate chemical modification of the lens surface or after lens wear.21–23 Chemical analysis of the lotrafilcon B lens surface exposed to MPDS showed that lotrafilcon B lenses had significant increases N/C, C5/C, N1/C, and N2/C and a significant decrease in C3/C compared with lenses from the packaging and differences to C4/C and N1/C depending on the MPDS type. This raises the possibility that these differences may be associated with the decrease in comfort responses during lens wear that were worse with the PHMB-soaked lenses. For example, lotrafilcon B lenses soaked in Polyquad/Aldox showed changes to the chemical composition of the lens surface that were of a different character to those seen after soaking in PHMB. Perhaps the increases in N/C, C4/C, N1/C, and N2/C seen with PHMB are factors in driving this discomfort. Lotrafilcon B soaked in Polyquad/Aldox did have decreases in comfort and increases in lens awareness, and perhaps, the increase in N4/C are associated with this. Given the chemical structure of PHMB, the changes in the high-resolution C1s and N1s spectral components (decreased C3/C, increased C4/C and C5/C and increases in both N1/C and N2/C) were consistent with the presence of a thin layer of adsorbed PHMB on the surface of the lenses. Because the antibacterial active PHMB is polymeric, it is unlikely to be able to penetrate the surface of the lotrafilcon B lenses as they have a highly cross-linked organic coating.8 Although this coating does not affect the water and ion transport properties of the lenses, it is most likely not permeable to polymers with a molecular weight of more than a few thousand daltons.8

The contribution of either release of MPDS ingredients from lenses or adsorption of components to the lens surface seems to be very different for the galyfilcon A lenses compared with the lotrafilcon B lenses. Galyfilcon A lenses release more components of Polyquad/Aldox. Changes to the lens surface are fundamentally different between galyfilcon A and lotrafilcon B lenses. There are almost exactly the same changes to the surface of galyfilcon A lenses whether they were soaked in PHMB or Polyquad/Aldox solutions: increases in O/C, C1+C2/C, C4/C, and C5/C and decreases in C3/C. This suggests that it was the soaking alone and not the different solutions in which lenses were soaked that changed the surface of galyfilcon A lenses. Galyfilcon A lenses are not surface coated, at least in the same way that lotrafilcon B lenses are. Galyfilcon A lenses contain a hydrophilic wetting agent, polyvinyl pyrrolidone within their lens matrix, which presumably associates with the lens surface making in wettable. Perhaps soaking removed this layer from the surface of galyfilcon A. Furthermore, the lack of difference in the lens surface between galyfilcon A lenses soaked in either MPDS suggests that it was not changes to the surface of these lenses that contributed to the worsening of the comfort response with Polyquad/Aldox-soaked galyfilcon A. Perhaps for this lens type, the larger amount of release of material from Polyquad/Aldox that adsorbed between 200 to 300 nm was the driving factor for changes to the comfort response.

This study has shown that MPDS can affect corneal physiology (increased staining) and symptomatology associated with lens wear. The study has also highlighted that lenses interact differently with MPDS according to their polymeric make up. Hence, the mechanisms contributing to corneal staining and/or symptomatology during lens wear may vary accordingly.

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ACKNOWLEDGMENTS

The authors thank Ms. Indrani Pereira for lens parameter measurements and Dr. Thomas Nadivulath for statistical analysis.

Mark D. P. Willcox

Brien Holden Vision Institute

Level 4, Rupert Myers Building

Gate 14, Barker Street

Kensington Sydney

New South Wales 2052, Australia

e-mail: m.willcox@ier.org.au

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CrossRef
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Keywords:

silicone hydrogel contact lenses; comfort; adsorption; Polyquad; PHMB; hydrogen peroxide; surface analysis

© 2010 American Academy of Optometry

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