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Optometry & Vision Science:
Articles: Original Article

Effect of Contact Lens Materials on Tear Physiology

THAI, LEE CHOON BSc, MCOptom; TOMLINSON, ALAN DSc, FCOptom, FAAO; DOANE, MARSHALL G. PhD

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Author Information

Department of Vision Sciences, Glasgow Caledonian University, Scotland (LCT, AT), Schepens Institute, Boston, Massachusetts (MGD)

Submitted October 25, 2002; accepted December 4, 2003.

Alan Tomlinson

Department of Vision Sciences

Glasgow Caledonian University

Cowcaddens Road

Glasgow G4 0BA

United Kingdom

e-mail: A.Tomlinson@gcal.ac.uk

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Abstract

Purpose. This study measured evaporation rate, thinning characteristics, and lipid layer changes in the prelens tear film (PLTF) associated with wearing of different soft contact lens materials, in an attempt to determine the biocompatibility of the material with the PLTF.

Methods. Twenty habituated contact lens wearers wore five different soft materials in a random order on the left eye at visits separated by at least 24 h. The soft contact lens materials were polymacon (Optima 38), omafilcon A (Proclear Compatibles), phemfilcon A (DuraSoft 2), balafilcon A (PureVision), and etafilcon A (Acuvue). Tear film evaporation rate was measured by a modified Servo Med Evaporimeter and tear thinning time by HirCal grid. Tear film structure, elimination rate, and lens wetting ability were recorded dynamically with a Doane tear film video interferometer and graded according to a new system developed for the study. Baseline measurements were taken of the precorneal tear film before lens insertion, and PLTF was determined 30 min after commencing lens wear.

Results. No statistically significant differences were found for any of the baseline (precorneal tear film) data. There was also no significant difference in evaporation rate change (analysis of variance) and in tear thinning time (Friedmann) between the five contact lenses. In the PLTF structure grading, omafilcon A had significantly more stable grades than phemfilcon A (Friedmann, p = 0.0033) and polymacon (p = 0.004). In PLTF observation of tear thinning and elimination rate, there was a significantly slower rate of elimination observed for omafilcon A than phemfilcon A (Friedmann, p = 0.0023) and polymacon (p = 0.0023). There was no significant difference in the overall PLTF wetting ability grading between any of the lenses worn.

Conclusion. Generally, all soft contact lens materials significantly and adversely affected tear physiology by increasing the evaporation rate and decreasing tear thinning time. The surface wetting ability of all contact lens materials exhibited no significantly difference irrespective of the special surface treatments. Only in PLTF structure and in PLTF elimination rate were differences found from the conventional low water content materials; omafilcon A was better in PLTF structure and in PLTF elimination.

The presence of a contact lens in the eye can produce the condition of dry eye in an otherwise normal individual. This is the condition of contact lens-induced dry eye (CLDE), which is said to occur in 20% to 30% of soft lens wearers 1 and >80% of hard lens patients. 2,3 The contact lens causes CLDE by disrupting normal tear physiology through thinning and break up of the tear film, 4–6 interrupting tear film reformation, 4,7 rupturing the lipid layer 8,9 with consequent increases in tear film evaporation, 10 and (in the case of soft lenses) by per-evaporation of fluid from the corneal tissue. 11

In a survey of problems with hydrogel contact lens wear carried out in 310 contact lens practices randomly selected from across the United States, 12 it was found that CLDE was ranked the third major problem, after lens deposits and lack of patient compliance. The methods of managing CLDE were also surveyed and ranked. Refitting the CLDE patient with a contact lens of a different hydrogel material is recommended as one of the first management options available to practitioners. 13,14 If a contact lens wearer is complaining of CLDE, the recommended management is to fit the patient with a hydrogel lens from Group I of the four U.S. Food and Drug Administration lens categories. 13,14 This group consists of hydrogel materials that have <50% water content and are nonionic. Previous studies have established that a higher water content hydrogel lens dehydrates more than a lower water content lens on the eye. 15–21

There are >30 hydrogel contact lens materials approved by the U.S. Food and Drug Administration, and they are categorized into four groups based on their chemical and material properties. 22,23 In recent years, new additions to this list are some hydrogel materials that are biomimetic and others that are composed of silicone hydrogel polymers.

The biomimetic hydrogel material omafilcon A is a synthetic analogue of the natural phospholipid phosphatidycholine incorporated into the copolymer backbone. The essential element is phosphorylcholine, a substance present in the outer surface of red blood cell membranes, which is the primary, natural material responsible for cell membrane biocompatibility. 24 The structure of the biomimetic material is able to create or mimic the biological surface and convince the host to accept it. 25 Initial findings suggest that the material’s resistance to hydration and lens deposits help to reduce CLDE and increase contact lens wearers comfort and decrease spoilation. 15,17–19,21,26,27

Silicone hydrogel lenses were first patented by Tanaka et al. 28 in the United States. Silicone chains are used as the backbone of the hydrogel polymer, and by increasing the silicone content, the oxygen permeability is improved. However, potential problems with these new hydrogels are decreased wetting ability and increased lipid interaction due to the migration of hydrophobic moieties to the lens surface. To enhance the compatibility of these silicone hydrogel lenses with the anterior ocular structures, the surfaces are treated with gas plasma techniques. The end products are hydrogel lenses that have low water content and high Dk value, which overcomes the major barrier of corneal hypoxia in contact lens wear. They are the first generation of new range of extended-wear products with a real potential for continuous wear. 29

The purpose of this study was to compare the effect of different soft contact lens materials on aspects of tear physiology that feature in the etiology of CLDE. In this comparative study, five different soft contact lens materials were chosen from each of the four U.S. Food and Drug Administration classification groups. They included two commonly prescribed daily wear lenses, one disposable lens, one silicone hydrogel lens, and one “biocompatible” lens. The physiological effects on the preocular tear film of these lenses was evaluated by measuring the preocular tear film evaporation rate, tear thinning time (TTT), and the interferometric observation of tear film structure, wetting capability, and rate of elimination. The physiological effect was assessed by comparing the measurements before and after lens insertion. The aim of this study was to determine whether any of these five lens materials could, through improve biocompatibility, preserve the integrity of the tear film.

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METHODS

Twenty habituated hydrogel contact lens wearers were recruited for the study. All the subjects were in good health with no anterior ocular surface abnormality and were taking no medication that was likely to adversely affect their tear film. The group comprised 17 females and three males with a mean age of 22.5 ± 5.3 years (range, 18 to 41).

The five lens materials in the study are described in Table 1, each representing one of the U.S. Food and Drug Administration classification groups. All the contact lenses were −3.00 D to standardize the central thickness between lenses.

Table 1
Table 1
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Protocol

Subjects were instructed not to wear any contact lens for at least 24 h before each measurement visit. This “wash-out” period was necessary to avoid carryover effects from the subject’s own contact lenses. At each visit, the baseline precorneal tear film data were recorded; this comprised tear structure, wetting ability, elimination rate, TTT, and evaporation rate for each subject. A contact lens was chosen at random, inserted onto the subject’s left eye by the investigator, and allowed to settle for 30 min. The five different lenses were masked from both the investigator and subjects. Measurements were taken in the same manner and order as the baseline data. These sets of pre- and post-insertion measurements constituted the data for each lens type. The same procedures were repeated at separate visits for each of the hydrogel materials.

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Measurements
Tear Thinning Time.

The noninvasive precorneal TTT was observed with the HirCal-Grid system fitted to a standard Bausch & Lomb keratometer. 30,31 The grid was focused on the precorneal tear film, and the time before the first distortion of the grid image was recorded. In each set of observations, three measurements were taken, and the mean was calculated.

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Evaporation Rate.

Tear evaporation was measured as described previously. 32,33 Briefly, a Servo-Med EP-3 Evaporimeter was attached to a modified swimming goggle mounted on a slit lamp. This determined the vapor pressure gradient between two sensors placed above the ocular surface. 34–36 Evaporation rate was derived from the measurement of relative temperature and humidity at the two points. 32,37 Data were stored and analyzed by a computer. 33 Measurements were made with the eye open and closed, and the open eye area was calculated from a digital image. To determine evaporation of the ocular surface from measurement of the total evaporation within the goggle, which included moisture from the facial skin, it was necessary to know the area of facial skin in the goggle and the rate of skin evaporation. This was calculated from the knowledge of the open- and closed-eye evaporation rates and the area of facial skin within the goggle. 33,38 To offset the effects of differences in blinking rate between subjects, all subjects were trained to blink every 3 s in response to an auditory signal. 38

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Tear Structure, Wetting Ability, and Elimination Rate.

The tear structure was recorded by a video interferometer. Doane 39,40 designed and constructed an instrument that allows measurement of the in vivo tear film through an application of the principle of thin film interferometry. This was connected to a miniature video camera system to provide a dynamic record of the tear film. The use of this thin film interferometry allowed examination of changes in the tear film (with or without contact lens wear) during sequential interblink periods. From the recordings, an objective evaluation of the quality of the tear film on the corneal or the contact lens surfaces, its wetting properties, tear break-up characteristics, evaporation pattern, and the thickness distribution could be made. The effects of different hydrogel contact lens materials on the tear film could be compared.

The thin film interferometry recording procedure took about 2 min. The subject was required to look straight ahead into the instrument, where a dim light was seen. The instrument was focused using white light illumination on the interference pattern produced by the nonsoluble layer (mucin and lipid) over the aqueous layer of tear film covering the cornea or the contact lens. The image was relayed to a high-resolution color video monitor with Y/C input mode to facilitate the view by the observer. The image was recorded with a VCR. The recording format was NTSC 3.58 with the Super VHS system. This was to ensure the best image quality for reproduction and display.

In the second segment of each recording, the broadband green filter (λ 546 nm) was slotted in to produce a monochromatic green light source. This illumination enhanced the contrast of the interference patterns generated by the aqueous layer of tear film, particularly of the observed lens area (an illuminated disk of light approximately 5.5-mm diameter) located centrally on the contact lens surface.

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Tear Film Grading by Tear Film Interferometry.

One of the advantages of the interferometer linked to a recording system is the ability to repeatedly and retrospectively analyze the recording of the tear film interference fringe patterns. To describe the characteristics of the tear film in a consistent and comprehensive manner, the following grading system was developed. This system is based on the original grading system by Doane, 39,40 which included observations of the tear film appearance using white and green (monochromatic) light. Doane’s system is elaborated in the present work to include a description of the structure of the precorneal and prelens tear films and a more comprehensive system for grading wetting ability and fringe movement on a contact lens.

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Precorneal Tear Film Structure.

In the observation of the naked cornea, only interference patterns from the superficial lipid layer are visible. An overall observation of the tear film with white light allows classification into five distinct patterns with the following appearances (Fig. 1 A to E).

Figure 1
Figure 1
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Grade 1: This pattern has the appearance of “islands” on a light background. This is due to contamination by mucin strands and poor mixing properties of the lipid layer. The background is formed of a thinner lipid layer.

Grade 2: With this pattern, there is still some localized nonmixing lipid that appears as clusters on a light wavy background. This pattern is generally stable.

Grade 3: A stable wavy pattern with the appearance of micelles. These are globules of lipid molecules enveloped in aqueous, which burst and release lipid as the tear film spreads.

Grade 4: This pattern has an even appearance, with vague fringes. The lipid layer seems to be mixing well and the lipid is evenly spread.

Grade 5: There is hardly any noticeable fringe seen in this pattern. A thin lipid layer spreads evenly over the aqueous layer.

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Pre-Contact Lens Tear Film.

In the presence of a contact lens, the interference fringes from the different layers of the prelens tear film are visible during different illumination conditions. The anterior lipid layer is clearly visible in white light and can be classified into the five grades of prelens tear film structure similar to those for the precorneal tear film. In addition, the underlying aqueous layer can be viewed and assessed with the green filter. This filter allows improved contrast and visibility of the interference fringe pattern of the aqueous layer. For the prelens tear film, three grading systems are proposed describing different properties of the film: these are the tear film structure, contact lens surface wetting ability and the tear film elimination rate.

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Structure.

This is derived from the general observation of the pattern of prelens tear film (including aqueous layer) with white light. The description of each of the grades is similar to that for the precorneal tear film (Fig. 2 A to E).

Figure 2
Figure 2
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Grade 1: Excess mucin strands or lipid globules contaminate this pattern and appear as islands on a light background on the anterior lens surface.

Grade 2: In this pattern, clusters of localized nonmixing lipid lie over a light wavy background.

Grade 3: The pattern may be wavy, with the presence of micelles.

Grade 4: Vague fringes that are even in appearance, i.e., well-spread lipid layer.

Grade 5: This pattern has nearly no observable fringes, indicating the thin lipid spreading evenly over the aqueous layer.

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Wetting Ability.

This grading system records the tear wetting properties for contact lens materials. The appearance and the percentage of wetted area are noted immediately after the first blink (after full drying of the surface of the lens). This is observed with white light (Fig. 3 A to D).

Figure 3
Figure 3
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Grade 1: Generally, there is no wetting of the lens surface. The lens looks dry, with strands and lumps of desiccated mucin on the anterior surface of the lens. The percentage of wetted area is zero.

Grade 2: Wetting of the lens surface is poor, with approximately 25% of the lens surface wetted after the blink.

Grade 3: Moderate wetting of the lens occurs with about half of the lens surface covered with tear film.

Grade 4: The wetting is extensive, with only localized dry areas after the blink. The total wetting is up to 75% of the lens surface.

Grade 5: The tear film covers 100% of the anterior lens surface after every blink, showing excellent wetting properties of the material.

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Elimination Rate.

This grading system categorizes the rate at which the tear film is eliminated by observation of the movement of the fringe pattern. The rate of the fringe movement is observed with the green filter. This elimination rate is analogous, but not exactly comparable to, the evaporation rate.

Grade 1: There is no interference fringe observed on the lens. The lens surface is dry, but mucin deposits may be present in some cases.

Grade 2: This can be classified by the rapid and variable interference fringes movement.

Grade 3: The rate of the fringes movement is moderate and even.

Grade 4: In this grade, the fringes move slowly and at a stable rate.

Grade 5: The interference fringe pattern is very stable and near stationary.

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Establishment of the New Grading System

The validation of any new qualitative grading system can be carried out through a comparative study with a “gold standard” and/or a normative study. At present, there is no universally accepted system or gold standard for the grading of tear film interference patterns. The comparison of this grading system with other interferometric methods 8,41,42 is not possible because of the differences in the wavelength of the illuminating light sources. Therefore, a separate study was carried out to describe the distribution of patterns in a large, normal population.

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Normative study

Forty-four female and 20 male subjects aged 28 ± 8 (SD) years were recruited. These subjects were screened and found to be asymptomatic of dry eyes, using the modified McMonies’ questionnaire. 43 Precorneal tear structure recordings were made before contact lens (etafilcon A) insertion, and the prelens tear structure, wetting ability, and elimination rate were recorded after 0.5 to 6 h of lens wear.

The precorneal tear film structure grades were distributed normally (Fig. 4), with about half of the subjects in grade 3. The peak of the pre-contact lens tear film distribution was shifted to the right, with about 50% of the subjects classified as grade 4 (Fig. 5). This shift was analyzed by Wilcoxon matched-pairs signed rank test and found to be significant (p < 0.001), indicating that there was a significant change in the appearance of the tear film structure with contact lens wear.

Figure 4
Figure 4
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Figure 5
Figure 5
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The wetting of the corneal surface of normal (non-dry eye) subjects was graded as 100% in all subjects. Similarly, the precorneal tear film elimination rate was graded as 5 in all cases. It is for these reasons that these two grading systems are generally useful only for pre-contact lens tear films. Most of the soft contact lens surfaces were adequately wetted by tears, with about two thirds and one third of the subjects at grade of 4 or 5, respectively (Fig. 6). Pre-contact lens tear films were eliminated more rapidly than precorneal tear films. The elimination data were distributed normally, with half of the films being graded 3 (Fig. 7).

Figure 6
Figure 6
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Figure 7
Figure 7
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The intraobserver repeatability of this new system was evaluated. First, the interferometric recordings of precorneal and prelens tear structure, wetting ability, and elimination rate were graded. Then the recordings were regraded by the same observer. To test the agreement between the two evaluations, weighted Kappa descriptive statistical analysis was applied separately to the intradata for all 4 grades (Table 2). A value of one indicates perfect agreement, and a value of 0 indicates that agreement is no better than chance. All the grades had values of about 0.6, which indicates good agreement between the observations by the same observer. This revealed that the new grading system is repeatable.

Table 2
Table 2
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As part of the ongoing development of this new grading system, more studies are being carried out. These include a study of repeatability of the system between observers and the distribution of grades for different populations of mild to moderate and severe dry eyes.

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RESULTS

An initial analysis of the data from the precorneal tear films in this study was carried out to determine whether there was a difference in the baseline data on the five visits for each subject. All the data sets were tested for normality with the Anderson-Darling test. There are large variations for individual subjects in evaporation rate, TTT, and the structure of precorneal and prelens tear films. For measurements of precorneal evaporation rate and TTT, no significant variability between visits was found (parametric analysis with analysis of variance resulted in p = 0.084 for evaporation rate, and a nonparametric Freidman test gave p = 0.853 for TTT). Because the precorneal tear film structure data were ordinal, analysis with the nonparametric Freidman test indicated no significant (p = 0.809) variability between visits for this parameter.

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Evaporation Rates

The mean evaporation rate for the precorneal tear film was 39.05 ± 19.03 g/m2/h, and for the prelens tear film, the average evaporation rate increased to 52.33 ± 18.03 g/m2/h. The latter was an increase of >34% from the precorneal value. Analysis of variance of the prelens evaporation rates revealed no significant difference between any of the five lenses (F = 1.61; p = 0.181) (Table 3). But a significant (F = 3.08; p < 0.00) difference was found between the subjects evaporation rate, which is expected due to the wide individual variation.

Table 3
Table 3
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Tear Thinning Times

For the preocular TTT, there was a reduction of >65% in the presence of a hydrogel contact lens; an average precorneal TTT of 21 ± 19.54 s was reduced to 5.62 ± 4.69 s for the prelens TTT. The prelens TTT data were not distributed normally, so a nonparametric Friedman test was used to show that the prelens TTT was not significantly different for any of the five hydrogel lenses (s = 8.90; p = 0.064) (Table 4).

Table 4
Table 4
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In this study, the data for prelens TTT’s were considered in two different ways. First, the prelens TTT changes were calculated as a percentage of the initial, baseline precorneal TTT. This technique looked at the relative change in TTT from the precorneal tear film to lens in vivo. The Friedman test revealed no significant (s = 6.12; p = 0.19) difference between the nonparametric data for the five different hydrogel lenses.

In an alternative analysis of tear thinning data, the number of subjects who had a prelens TTT <5 s was compared for each lens type. A 5-s break-up time is the level that is clinically indicative of CLDE. 30,44–47 It can be seen in Fig. 8 that only 30% of subjects had prelens TTT <5 s when wearing omafilcon A lenses. This was the lowest level for all five lens materials. However, a χ2 test revealed no statistical significant difference for the five hydrogel materials in the frequency of subjects with prelens TTT’s <5 s.

Figure 8
Figure 8
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Tear Film Interferometry

The interferometry recordings showed a statistically significant difference in the gradings of pre-contact lens structure between the five hydrogel materials (Friedman test, s = 14.27, p = 0.0065). The omafilcon A was found to have lower grade than phemfilcon A and polymacon (p = 0.0033 and p = 0.004, respectively, Wilcoxon post hoc test) (Table 5).

Table 5
Table 5
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Also, the prelens tear elimination rate between the five hydrogel materials was found to be statistically different on a Friedman test (s = 13.33; p = 0.0098). A post hoc Wilcoxon test showed that for omafilcon A, the prelens tear film had a significantly slower elimination rate (p = 0.0023) than for the phemfilcon A and (p = 0.0023) polymacon lenses (Table 6).

Table 6
Table 6
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However, the Friedman test indicated no significant difference for lens wetting between the five hydrogel materials (s = 3.87; p = 0.4239).

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DISCUSSION

This study showed that there was little difference in the effect of the contact lens materials on the prelens tear film; all materials had significant negative effects on normal tear physiology, with increases in evaporation rate and decreases in TTT. Only the thin film interferometry showed any statistically significant effects between materials for the pre-contact lens tear film.

The evaporation rate of the prelens tear film was about 35% higher than that of the precorneal tear film. All contact lens materials had similar effects in increasing the rate. This is consistent with the various studies of the effect of contact lenses on tear physiology, 4,34–36,48,49 and this effect is independent of the initial water content or material of the lenses. The differences in evaporation rate between the five hydrogel lens types were small compared with the large increase in rate produced by the presence of any contact lens. This may mask any significant level of difference between the lenses. Another factor that may have contributed to the lack of statistical difference found between materials was the wide range of variation in tear evaporation rates for individual subjects with the same lens material. 14,36,48,50

The average baseline precorneal TTT found in this study was 21 ± 19.54 s, slightly lower than previous studies utilizing noninvasive techniques. 30,51,52 Some previous studies measured actual tear film break up, in which the criterion was the first full break rather than the first distortion of the projected mire image. As the tear film thins, the reflected mires of the HirCal grid distort, and, eventually, the tear film breaks up. Thus, our TTT had a lower value than previous break-up measurements. On the other hand, TTT had slightly higher values than the fluorescein break-up times because fluorescein, in itself, disturbs the integrity of the tear film. 4, 48–55

Others have reported that hydrogel lenses create localized tear thinning at the lens edge, 49 which interferes with the continuity of the lipid layer of the prelens tear film 8,56 and the spread of mucin over the cornea. 46 This disrupts the lipid layer 36,44,46 and causes a decrease in the tear stability 4,46 and break-up time. 6,51,57 The average prelens TTT in this study was reduced to 5.62 ± 0.5 s compared with the previous findings of prelens break-up time of 6.1 ± 0.7 s with Igel (67%) lenses, 6 6.1 ± 1.1 s with Optima and Igel 67% hydrophilic contact lenses, 51 and 7.3 ± 0.7 s with eight different hydrophilic lenses. 58 A value for tear break up of 6.3 ± 0.08 s is reported for rigid gas-permeable lenses. 59

Tear thinning time, as with many other tear physiological measurements, shows a large intersubject variability. To minimize the subject effect, the relative change of the TTT with contact lens wear was calculated. This also accounted for the subject who had a low precorneal TTT in which even a small reduction of the TTT would have a potentially important clinical effect. The TTT data are computed in terms of percentile change in the TTT compared with the baseline precorneal values. An average reduction of 65% on the prelens TTT occurred after insertion of a hydrogel contact lens. Against this large drop in TTT, any difference in change between the various hydrogel lens materials was comparatively small. The large decrement in TTT may cause CLDE in subjects who would otherwise have a normal tear function in the absence of a contact lens. In clinical practice, a criterion level of TTT of 5 s may be used to describe the dry-eye condition. 60 There was no significant difference between the frequency data of subjects with prelens TTT <5 s with most of the hydrogel lenses (Fig. 8), and >50% of the subjects would fall into the dry-eye category. In contrast, only 30% of the subjects who wore omafilcon A had prelens TTT <5 s. This may be important clinically for success of the marginal dry-eye patient who wants to wear contact lenses.

Interferometry is a noninvasive technique of assessing the preocular tear film by observing the surface interference fringes of the lipid layer of the tear film. 40–42,44,57,61–63 This method is based on the principle of Fizeau fringe patterns, in which a monochromatic light source is specularly reflected from a thin (partially) transparent layer. Interference of the reflected, coherent waves from the different incident surfaces occurs, forming a series of light and dark fringes that correspond to the contours of constant optical thickness in the layer. These interference fringe patterns from the tear film are analogous to contours on ordnance survey topographic maps (Fig. 9). As with all isolines, when the fringes lie close together, they represent a steep slope of tear film thickness, and separated lines represent a gradual slope.

Figure 9
Figure 9
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With white light, colored fringe patterns can be observed on the precorneal tear film. Fringes of the same color represent the same thickness of the precorneal tear film. 63 The nature of the lipid layer is assessed by observing the appearance of these fringe patterns. 40,64 With a contact lens in vivo, the interference fringe patterns of the prelens tear film lipid layer are formed due to the reflection at the interface between the aqueous layer and the contact lens. They have a similar appearance to that of the precorneal interference pattern. The interference patterns from the aqueous layer can also be viewed with a broadband green filter centered at about 546 nm. 40 The patterns display a series of dynamic green and dark fringes, and movement of these fringes demonstrates the elimination of the aqueous layer from the anterior surface of the lens.

The proposed grading systems used for the tear film interferometry in this study adds to the understanding of the complex interaction between the hydrogel contact lens structure and the tear film. The first aspect of the new grading system, tear film structure, applies to the precorneal and prelens tear films. The air-lipid interface and the lipid-aqueous interface serve as the two reflecting surfaces at which interference occurs during these conditions. With white light or monochromatic green light, this grading provides an overview of the superficial lipid layer in terms of its structure and its mixing properties. These vary from the poorly mixed lipid and contaminated tear film of grade 1 to the well-mixed, continuous lipid of grade 5. Our data indicate that the most existing soft contact lens wearers have a grade of 3 for the precorneal tear structure.

With the contact lens in vivo, interferometry was used to grade tear film structure, lens wetting ability, and aqueous tear elimination rate. These three and separate grading systems have been proposed to more fully describe the visual information obtained from interferometry. They were based on the original Doane grading system, which was described in his previous works. 39,40 The tear film on a contact lens was graded in Doane’s original system by observation with white and green light, and each grade was a combined assessment of the degree of surface wetting and the rate of movement of the fringe pattern after a blink.

The categorization of the prelens tear film structure in this study was performed using the same criteria as that for precorneal tear film. From our data, we observed that the lipid layer spreads evenly, but is thinner across the anterior surface of the prelens tear film. Similar results were found in other studies. 62,65 This thinner prelens tear film is prone to contamination by microparticles on the contact lens surface, 66,67 which, in turn, affects the integrity of the lipid layer.

The present study found that subjects wearing omafilcon A lenses had significantly lower grades for tear structure than when wearing phemfilcon A or polymacon lenses. This suggests that the lipid layer of the prelens tear film was thicker with the omafilcon A lens than for a conventional, low water content hydrogel lenses. 68 The biomimetic nature of omafilcon A, incorporating phosphorylcholine, appears able to sustain a thicker lipid layer. Guillon et al. 69 found a similar result in their study. With a stable lipid layer, the in vivo dehydration of the lens may be reduced, as suggested in other studies. 15,17–19,26,27

The wetting of all the contact lens materials in this study was essentially the same irrespective of any surface treatment. In previous studies, the material and the water content of the hydrogel lens was not found to influence the surface wetting ability of the contact lens. 68–72 The wetting ability of hydrogel contact lenses increases after 15 min in vivo and reaches a maximum level after 30 min, 73 probably as a result of interaction with the tears. However, with longer wear, greater mucous coating of the lens occurs, 71 and the hydrophilic properties of the contact lens are reduced. 73 Therefore, the advancing and receding contact angles, provided by manufacturers, may not be the best way to define in vivo wettability. 68 Also, the manufacturing techniques for the hydrogel contact lenses are important because the chemical structure of the surface is directly influenced by the method of preparation, causing possible differences in wettability. 68,74

Tear film elimination from the contact lens surface (an analogue for tear film evaporation) was graded according to the rate of movement of the interference fringe patterns as the tear film thinned and dried out. This observation was facilitated with the monochromatic green filter of the interferometer. In this study, omafilcon A lenses showed the slowest elimination rate, which was significantly slower than for phemfilcon A and polymacon lenses. These results suggest that the biomimetic lens material dehydrates less than conventional lower water content materials and is able to sustain a thicker prelens tear film for a longer time than other lenses.

Contact lens practitioners are aware of the wide range of individual patient variations in the biocompatibility of contact lens materials. A plausible explanation of these variations might be found in the complexity of mucin adhesion in different wearers of the various hydrogel materials. 75 This is highlighted in interferometry recordings in this study for subjects C and S (Fig. 10). Both were wearing new omafilcon A lens, but presented diametrically opposite outcomes. Subject C with the omafilcon A lens had a stable, evenly spread lipid layer (grade 3 tear structure) over the prelens tear film, with good wetting ability of the anterior contact lens surface (grade 5 wetting ability). In contrast, subject S exhibited a dry anterior surface on the omafilcon A lens, with an absence of prelens tear film. The anterior lens surface was coated with mucin lumps and strands (grade 1 tear structure), and no wetting of the lens surface occurred, even after blinking (grade 1 wetting ability).

Figure 10
Figure 10
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CONCLUSIONS

All soft contact lens materials significantly and adversely affect tear physiology by increasing the evaporation rate and decreasing TTT. These changes are large, and the differences between various soft lens materials in this study were statistically insignificant. Thin film interferometric observations of the prelens tear films indicate that the surface wetting ability of all contact lens materials are not significantly different irrespective of the new biomimetic materials or the special plasma-treated surface of silicone hydrogel lens. The structure of the prelens tear film and the rate of elimination show some differences between lenses. The biomimetic hydrogel contact lens has a better prelens tear film structure than the conventional low water contact hydrogel lenses. The new biomimetic lens also has a lower prelens tear film elimination rate than conventional low water content lenses. The new biomimetic lens material shows some promising results that warrant further investigation. The prelens tear film characteristics on the silicone hydrogel lenses are similar to those of the tear films over conventional low water content hydrogel materials.

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ACKNOWLEDGMENTS

Supported, in part, by a grant from the College of Optometrists (UK).

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REFERENCES

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Keywords:

contact lens; tear film; interferometry

© 2004 American Academy of Optometry

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