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Resistance of plastic ophthalmic lenses: the effect of base curve on different materials during static load testing
M L Diallo1, P Simonet, B Frenette
1Laboratoire Victor Cohen, Ecole d'Optométrie, Université de Montréal, Canada.
Purpose:
This study was designated to evaluate, through a static load test, the influence of lens base curve on the fracture resistance of three common plastic materials.
Methods:
A JJ Lloyd load cell machine was used to test the fracture resistance of -4.00 D spherical lenses. The samples had a nominal center thickness of 2.0 mm and a base curve distributed in one of five groups (+0.50, +2.50, +4.50, +6.50, and +8.50 D). The lenses were manufactured in CR39, polycarbonate, and TL16, a high refractive index plastic (n = 1.599).
Results:
The lens base curve influenced fracture resistance for all materials. For these materials, resistance increased as the base curve varied from +0.50 to +8.50 D. The resistance of CR39, TL16, and polycarbonate lenses was found to be linearly dependent on lens base curve. The effect is stronger for polycarbonate. Fracture resistance was higher for TL16 than for CR39, and polycarbonate was much more resistant to breakage than the two other materials.
Conclusions:
For a given power, the fracture resistance of an ophthalmic lens is reduced when its base curve has a low value. Consequently, the flattening of ophthalmic lenses for cosmetic purposes is not recommended as far as fracture resistance is concerned.