The effects of soft lens and ocular characteristics on the post-lens tear thickness.

Ping Situ is currently a Senior Clinical Scientist at the Centre for Contact Lens Research, University of Waterloo and has become a Research Associate at the Centre since 1995, where she is responsible for conducting clinical research in the areas of contact lenses and dry eye. She originates from China where she was trained as an Ophthalmologist and receives her MSc and PhD in Vision Science from the University of Waterloo, Canada. Her research interests include physiology of the ocular surface and tear film, in particular the sensory properties of the ocular surface and their clinical implications, and visual and clinical performance of contact lenses.

Lin MC, Chen YQ and Polse KA: The effects of soft lens and ocular characteristics on the post-lens tear thickness. Eye & Contact Lens, 2003 Jan;29(1Suppl):S33-6

Adequate tear mixing under a soft contact lens may play an important role in minimizing certain adverse ocular events associated with contact lens wear by reducing the contact time that metabolic by-products, debris and potentially harmful pathogens have with the corneal epithelium. Since the efficiency of tear mixing is depending on post-lens tear thickness (PLTT), understanding factors that may affect post-lens tear film thickness is essential for developing strategies increasing the PLTT thus improving tear mixing and biocompatibility of the soft contact lens with the ocular surface. Although previous work based on rigid lenses on a model eye suggested that base curve radius (BCR) affected PLTT,1 it is unclear whether this is the case for soft lenses. In addition, it is unknown what part that ocular anatomy plays in PLTT and whether there are differences in PLTT between ethnic groups as their eyelid anatomy may be different. In an attempt to answer these questions, Lin et al, examined the effects of soft contact lens BCR, sex, ethnicity, central corneal curvature, and vertical palpebral aperture size (PAS) on PLTT and provided useful information with respect to the influence of lens design and ethnicity on post-lens tear thickness.

Custom-designed lathe-cut soft lenses (Alden 47, polymacon, 35.5% h2O, -2.00 D and 14.0 mm diameter) in three different BCRs: 7.9, 8.3, or 8.7 mm, were fitted in 114 experienced lens wearers. Subjects were randomly assigned to one of three BCR groups. Using an optical pachometer, baseline corneal thickness and the total thickness combining the thickness of the cornea, contact lens and PLTT were measured prior to lens insertion and at 30 minutes after lens insertion, respectively. Contact lens thickness was measured immediately following lens removal using an electronic thickness gauge. The value of PLTT was obtained by subtracting baseline corneal thickness and contact lens thickness from the total thickness.

The results of this experiment showed that the PLTT was decreased with anthe increase of BCR but only the difference between the two extreme BCR groups (7.9 mm and 8.7 mm) was statistically significant. Compared to non-Asian eyes, the Asian eyes were found significantly thinner PLTT, in addition to smaller vertical openings of the yes, flatter cornea in the horizontal meridians and a greater amount of corneal astigmatism. Regression analysis suggested that both lens BCR and PAS were associated with PLTT in non-Asian eyes, while PAS seemed to be the only significant contributor to the changes in PLTT in Asian eyes.

Despite that the value of PLTT in this study was obtained indirectly, the most valuable learning from this study perhaps is the effect of different ethnicity on the post-lens tear thickness. Given that Asian eyes tend to show more positive biomicroscopy findings and greater susceptibility to mechanical irritation of the cornea during lens wear, this information may be useful in designing and fitting lenses to minimize ocular responses to contact lens wear for this ethnic group.

Reference

1. Lin MC, Graham AD, Polse KA, Mandell RB, McNamara NA: Measurement of post contact lens tear thickness: Invest Ophthalmol Vis Sci, Nov. 1999; 40: 2833-2839.