Determinants of postlens tear film morphology in hydrogel contact lens wear

The postlens tear film (PTF) in hydrogel lens wear has received relatively little attention in the literature. Early studies of the effects of hydrogel contact lens wear on corneal physiology indicated that there would be only a minimal increase in oxygen tension at the cornea with blink-induced postlens tear exchange. More recently, the potential importance of the PTF in lens movement and in flushing debris and necrotic cellular matter from beneath the lens has been contemplated. However, there have been few attempts to establish the roles or determinants of the PTF in hydrogel lens wear.

In early investigations with fluorexon, the PTF was considered to be constant after an initial period of lens settling. Studies of lens dehydration indicated that the PTF did not have an aqueous component since hydration gradients were not produced between the surfaces of the lens during wear. However, more recent studies have reported debris and particle movement in the PTF with some hydrogel lenses, indicating that a substantial aqueous component can be present. Furthermore, preliminary studies using a non-invasive biomicroscopic technique have suggested that the PTF can vary during lens wear.

In the biomicroscopic method of PTF examination, the specular reflection at the back surface of the lens is examined. Amorphous, coloured, mottled and striated PTF appearances have been observed. Non-amorphous PTF morphologies are thought to be produced by optical interference between the back surface of the lens and the ocular surface, ie within the full thickness of the PTF. However, the exact location of the reflecting interface at the ocular surface is uncertain. In addition, it is possible that the different patterns are produced by a postlens lipid layer, rather than being related to the total thickness of the PTF. Thus, unless these issues can be resolved, the biomicroscopic method can be used only for qualitative studies.

The work described herein better evaluates the diversity in the morphology of the PTF that is present in hydrogel lens wear. Furthermore, some possible determinants of PTF morphology in hydrogel lens wear are considered.

The first two studies in this thesis considered the population diversity and temporal variation of the PTF in hydro gel lens wear (chapters three and four). In both studies PTF appearance in specular reflection was categorised as amorphous, coloured, striated or dynamic (variable with blinking), where non-amorphous or patterned morphologies appeared to indicate progressive thinning of the PTF. The first study assessed PTF appearance and lens movement at routine aftercare visits. Patterned appearances were present in 25% of subjects and occurred approximately equally with each type of hydrogel lens that was being worn. Furthermore, patterned PTF appearances were found to be associated with reduced lens movement (Kruskal-Wallis ANOVA, p<0.001).

In the second study, the PTF and blink-induced movement were investigated when thin mid water content lenses were worn over six hours on each of two consecutive days. Patterned PTF appearances were observed in 53 % of subjects during the first 30 minutes of lens wear. Median lens movement was near zero initially and increased within the first hour to 0.34 mm and 0.30 mm on days one and two, respectively. Limited lens movement was related to apparent depletion of the PTF. In some subjects, lens movement and PTF morphology co-varied throughout the lens wearing periods. This relationship strongly indicates that changes in lens movement were accompanied by changes in the thickness or viscosity of the PTF. Since an increase in PTF viscosity would also require a reduction in the aqueous component, this finding provides additional evidence for the production of patterns in specular reflection by optical interference within the full thickness of the PTF.

Three further experiments investigated the determinants of PTF morphology. The effect of lens thickness on PTF morphology, and the role of the PTF in the mechanisms of inferior arcuate staining were investigated with ultrathin and standard thickness high water content hydrogel lenses. Lens dehydration, lens adherence and prelens tear film stability were also measured in order to evaluate their role in inferior arcuate staining. The ultrathin lenses were associated with a higher incidence of both staining and coloured PTF appearances, had greater front surface dehydration, and were more adherent to the eye. These findings indicate that the PTF has a role in the mechanism of inferior arcuate staining with ultrathin lenses. Lenses of greater thickness are associated with a lower incidence of inferior arcuate staining and appear to support a more substantial PTF.

The hypothesis that the PTF could be depleted by instillation of hypotonic saline was investigated in the next experiment. With instillation of the hypotonic solutions the appearance of the PTF in specular reflection changed to a faint coloured or coloured pattern in the majority of subjects. The faint coloured and coloured patterns were associated with lens binding. However, there were no significant changes in measured lens parameters. These findings indicate that lens binding and the development of coloured patterns in specular reflection were due to osmotic depletion of the aqueous component of the PTF.

The hypothesis that increased lens dehydration due to evaporation at the lens front surface may cause PTF depletion was tested in the final experiment. Ambient air flow was associated with an increase in the incidence of PTF patterns. Hydration gradients between the surfaces of the lenses suggested a flow of water from the PTF to the lens front surface during wear. Since patterned PTF appearances are associated with limited lens movement, these findings suggest that environmental factors, such as ambient air flow, may account for some of the variation in the amount of lens movement that can be observed during wear.

This series of studies has shown that PTF morphology can vary throughout a period of wear of currently available hydrogel contact lenses. Several determinants of PTF morphology have been identified; lens wearing time, lens thickness, ambient air flow and solution hypotonicity. In addition several consequences of PTF depletion have been identified; lens movement variation, lens binding, increased lens dehydration and inferior arcuate staining.

This work provides an improved understanding of the PTF in hydrogel lens wear. Since many of the determinants of PTF morphology identified herein are related to the flow of water through the contact lens, future work should assess the hypothesis that lens water transmissibility is a fundamental determinant of PTF morphology. Other future work should evaluate the relationship between PTF thickness and the efficiency of tear exchange and postlens debris expulsion. Ideally, this would be done once a method for the direct measurement of PTF thickness had been developed.