At the risk of arousing anyone's disgust, I have used my red radius for nearly ten years. Yes, the same one. The bristles are still intact with very slight curl on the ends.
Bought a couple of drug-store newbies, used once, went back to my fave, grateful I'd not thrown it away, as I couldn't find radius in stores, even our health food store which no longer carries. By far the best toothbrush in the world, and very reasonably priced. Just ordered a new one, green! Thanks for a great product.