Osmotic determinants of postlens tear film morphology and hydrogel lens movement

Abstract

The determinants of postlens tear film (PTF) composition in hydrogel lens wear are poorly understood, although this layer has important roles in lens movement and corneal integrity. We investigated the hypothesis that the PTF could be depleted by instillation of hypotonic saline, using a randomized, double masked, placebo controlled study design. Solutions of 0.90, 0.60 and 0.45% NaCl were instilled into the eyes of 12 subjects wearing ionic and non-ionic high water content hydrogel lenses. Postlens tear film appearances in specular reflection were categorized as amorphous, faint coloured or coloured, where the coloured patterns represent a progressive thinning of the PTF. With instillation of the hypotonic solutions (0.60 and 0.45% NaCl), the appearance of the PTF in specular reflection changed to a faint coloured or coloured pattern in at least 67% of subjects for each lens type (Friedman ANOVA, P < 0.002). For the 0.45% NaCl solution, median lens movement decreased from 0.50 to 0.10 mm (Friedman ANOVA, P = 0.02); however, there were no significant changes in measured lens parameters and no difference between lens types. Postlens tear film depletion due to a hypotonic shift in tear osmolality, as demonstrated here, may explain the clinically observed phenomenon of lens binding.

Thickness of the Pre- and Post–Contact Lens Tear Film Measured In Vivo by Interferometry

“…Little and Bruce 8 and Bruce and Brennan 32 have developed a method of qualitative assessment of POLTF thickness based on the appearance of the POLTF in white light from a slit lamp. As noted in the introduction, a colored POLTF is due to a white light interference effect, which indicates that the POLTF is less than approximately 1.6 m. In normal wearing conditions, only a minority of POLTFs for hydrogel lenses (Acuvue; Vistakon) yielded colored fringes, 8,9 and this is consistent with our finding that relatively few POLTFs are less than 1.6 m thick (Fig. 8).…”

“…Thickness changes in the POLTF have been monitored by the percentage of subjects who show colored POLTFs. In this way the thickness of the POLTF correlates with lens binding, 33 lens movement, 34 lens thickness, lens wearing time, inferior arcuate staining, 14 ambient air flow, 35 hypo-osmotic solutions, 9 and eye closure. 10 The current method has two advantages for studies of this sort.…”

“…11 If the POLTF is diminished, as it may be during contact lens-related dry eye or the overnight wearing of contact lenses (i.e., lens adherence), the accumulation of debris and inflammatory mediators may pose a significant threat to the ocular surface involving ocular infection, inflammation, or mechanical desiccation. 8,9,12 In fact, depletion of the POLTF is thought to be the mechanism of inferior arcuate staining seen in soft contact lens wearers. 13,14 Investigators have observed colored interference effects from the POLTF when illuminated in white light, indicating that the POLTF is particularly thin in these cases.…”

Osmolality and tear film dynamics

“…[97][98][99] This decrease in tear film osmolality has been regarded as a contributor to post-lens tear film depletion with subsequent lens adherence and as a cause of increased corneal thickness during the adaptation phase to contact lenses. 97,100 The initial decrease in tear film osmolality immediately after lens insertion is often followed by a shift towards hyperosmolality on equilibration of the contact lens. 98,99,101 An inter-eye response has been demonstrated with changes in tear film osmolality not only being observable in the lens-wearing eye but also in the contralateral non-lens-wearing eye, although to a smaller extent.…”

Steady-state diffusion of water through soft-contact-lens materials

“…Water loss may also lead to discomfort and to dryness symptoms [13], and may reduce oxygen transmissibility [14], as well as fitting and corrective power of the lens [7]. Transport of water through a SCL can also be induced by an osmotic gradient between pre-lens and post-lens tear films [15], or by mechanical lid stress [16]. Understanding the importance of these phenomena requires quantitative knowledge of the rate of water transport through a thin polymer membrane.…”