Determination of Oxygen Permeability of Soft Contact Lenses Using Polarographic Lens Stacking and Single-Lens with Various Powers

Abstract

 

Purpose

 

The commonly used lens stacking method has generated mixed oxygen permeability (Dk) results. The purpose of this study is to directly determine and compare the Dk values of silicone hydrogel and conventional hydrogel contact lenses using the lens stacking polarographic method and a novel single-lens with various powers polarographic method.

 

Methods

 

To accurately determine the Dk of contact lenses it is essential to measure different thicknesses of lens material for correction of boundary layer effects. Two methods were used to achieve a range of lens material thickness. The first method, ISO 18369-4: 2006, used stacking of one to five contact lenses. The second method utilized single lenses with various lens powers and thicknesses. Two 8.6-mm radius polarographic cells (Rehder Development Company, CA, USA) and oxygen permeometer Model 201T (Createch, CA, USA) were used. All measurements were conducted in a 99% relative humidity chamber at 35 +/- 0.50 degrees celsius.

 

Results

 

We classified contact lenses into 3 groups according to the manufacturers’ claimed Dk values: Low Dk (<40), Medium Dk (40-60), and High Dk (>60) lenses. Each lens was measured twice with each method, the average results are listed in Table 1. Our data were in good agreement with the published values for Medium and Low Dk contact lenses using the lens stacking method as shown in Table 1. However, the stacking method had a tendency of underestimating the Dk values for High Dk contact lenses; while the various powers method accurately measured High Dk lenses.

 

Conclusions

 

For the Low Dk contact lenses (i.e. conventional hydrogel), the lens stacking technique acquired Dk results that were in good agreement with the previously published values. For the High Dk contact lenses (i.e. silicone hydrogel lenses), the single-lens with various lens powers method provided data that was in very good agreement with the previously published results. For the Medium Dk contact lenses (both silicone and conventional hydrogel), both the lens stacking and various lens powers methods gave satisfactory results within 20% of the nominal published values.

 

 

Table 1: Claimed and Determined Dk Values of 9 Commercially Available Contact Lenses using Lens Stacking and Single-lens with Various Powers

 

Keywords: 477 contact lens • 635 oxygen • 548 hypoxia