I worked my way up to 70# on these for each wrist before with absolutely zero pain. I think you ought look at your warm up for these. Warm them up thourougly like any other isolation exercise first off, second I have had pain before on these, but it was a result of lifting too much weight so you have to start off light and work your way up and I believe in keeping these reps low. I remember I used to do 12 reps maybe on these years ago and I got tingling in my fingers as a result, so at the low position of this movement nerves may be getting pinched so you have to be sure you can handle the weight.
Another note, anytime Ive had sharp pain it was a result of using too high of a weight. everytime