I too switched to a "thumbless" grip. This worked for a little while then I "fingered" it out. I was squeezing the shit out of the bar. My knuckles were going white. Took a few times to get used to but concentrate on a VERY lose grip. Hands become literally, Hooks. Works for me.
Plus think of it. You have 100lb + pounds being supported by a flat wrist at the end of a 12 inch (long) fore arm. That puts the weight a LONG ways from the fulcrum. Try keeping the wrist slightly flexed to remove some of the pressure from the supporting bone stucture.