Of course impact resistance is important, but cr-39 is pretty impact resistant in all powers, and is VERY impact resistant in moderate plus powers. Having been in practice many years, I can't remember seeing any injuries from a shattered cr39 lens, and I don't think I've EVER seen a broken cr39 lens above +2.00 in power. If it were a problem, I'm sure California at least would not allow kids to have cr39. Having said that, I use more and more Trivex every day.
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