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.
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.
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.
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.
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).
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.
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).
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).
However, the Friedman test indicated no significant difference for lens wetting between the five hydrogel materials (s = 3.87; p = 0.4239).
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.
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).
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.
Supported, in part, by a grant from the College of Optometrists (UK).
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Keywords:© 2004 American Academy of Optometry
contact lens; tear film; interferometry