The effect of soft contact lens care products on lens modulus

To examine the relative changes in modulus of nine soft contact lens materials when exposed to three lens care solutions.

Measurements of elastic modulus were made using an Instron 3343 tensiometer of lenses which had been pre-soaked in ReNu Multi-Purpose (Bausch & Lomb), OPTI-FREE RepleniSH (Alcon), Clear Care (CIBA Vision) hydrogen peroxide, or a saline control solution for one week. The solutions were changed three times during the week-long soaking period. Following this, the lenses were measured for centre thickness and were cut into two samples of identical width. Young's modulus was measured in the force range of 0.015-0.035 N.

Measurements in saline ranged form 0.31MPa for etafilcon A to 1.76 MPa for lotrafilcon A. All of the lenses, with the exception of senofilcon A, displayed a significant change in modulus with at least one of the solutions. Both etafilcon A and lotrafilcon B showed a significant change in modulus with all solutions. With lotrafilcon B in hydrogen peroxide solution, modulus reduced from 1.32 to 0.93 MPa (P<0.0001). Galyfilcon A decreased from 0.36 to 0.31 MPa in ReNu Multi-Purpose (P<0.0001) and lotrafilcon A decreased from 1.76 to 1.64 MPa in OPTI-FREE RepleniSH (P=0.04). Comfilcon A increased from 0.78 to 0.85 MPa (P<0.0001) in peroxide.

The use of some contact lens-solution combinations results in significant changes in modulus. In the case of the multi-purpose solutions, the changes probably relate to uptake of the formulation components and, in the case of peroxide, to chemical changes to the polymer.