Analysis of tear film breakup on Etafilcon A hydrogel lenses
Introduction
The tear film stability on the surface of hydrogel contact lenses is often reduced compared to the stability of the normal pre-ocular tear film [1], [2], [3]. The wearing of hydrogel lenses is also associated with an increased incidence of symptoms of dryness [4], [5], [6]. In order to improve vision and comfort during lens wear, there is a need to understand better the biomaterial characteristics responsible for tear film stability during hydrogel lens wear.
A variety of methods have been applied in the assessment of tear film stability. The pre-lens tear film stability is defined as the time for breaks or dry spots to appear in the surface tear film layer, in the absence of blinking. Clinical instruments such as the keratometer have been applied to detect tear film breakup, without the instillation of fluorescein; however, only a limited area of the tear film is monitored [7]. The custom [8] and clinical [9] instruments that assess non-invasive breakup time over the entire cornea or lens, provide the most complete information as to pre-lens tear film stability.
The underlying cause of tear film instability on hydrogel biomaterials may be indicated by the distribution of sites of breakup. One theory of tear film breakup in the normal eye is that of McDonald and Brubaker, which contends that the tear film menisci induce thinning and breakup of the tear film adjacent to the menisci [10], [11], [12]. The more commonly cited theory of tear film breakup is that of Holly, which attributes tear film breakup to contamination of the mucus layer by lipid, causing hydrophobic areas to develop [13]. A number of other theories of tear film instability are also present [14]. In the latter theories, the tear film breakup sites would be expected to be more randomly distributed.
In this study, we used a non-invasive assessment technique to examine the distribution of pre-corneal tear film rupture sites in normal subjects, and compared them to tear film breakup location patterns during wear of Etafilcon A hydrogel lenses.
Section snippets
Subjects
The research procedures adopted in this study followed the tenets of the Declaration of Helsinki. Ten subjects were recruited for the study, five males and five females. Subjects were drawn from the staff and student population at the Queensland University of Technology (QUT) School of Optometry, and ranged in age from 20 to 47 yr. All ten subjects were in good general health, were free of significant anterior ocular pathology, and were asymptomatic for dry eye, achieving a score of less than 14
Pre-corneal tear film
Pre-corneal tear film breakup locations for each of the ten subjects are illustrated in Fig. 2. Table 1 gives parameniscal and central corneal zone proportions and breakup counts for each subject. The breakup densities in these zones are also given for each subject. Mean results for the ten subjects are shown. It may be seen from Table 1 that for each subject, the total number of breakup sites recorded exceeded the number of NIBUT measurements made (40), and this total varied between subjects.
Discussion
The Etafilcon A biomaterial appears to modify the surface properties of the system, in that the distribution of tear film rupture sites differed to that of the normal eye. The tendency for pre-corneal tear film breakup to occur more commonly in parameniscal zones than in areas overlying the central cornea is consistent with predictions based on the meniscus model of tear film stability [10]. Results for tear film breakup in the Etafilcon A lens wear do not support a predilection for any of the
Acknowledgments
This paper is dedicated to the memory of Dr. Tim Golding. The authors thank all subjects who participated.
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