I would say that the tricep bar you are using is putting undue stress on the wrist/forearm because of the unnatural way you have to hold it. If you hold your fists out in front of you like you are holding the bar then to keep them in that position with weight on the end is all on the top of the forearm. Move your wrist/fist from side to side without moving your arm and see what power is there. Now move your wrist up and down. There is a lot more movement and power this way as the forearm is set up to move this way.

If you use the EZ curl bar palms up, (20 past 8 on a steering wheel) then this is a more natural hold and will move the forearm involvement to something it can cope with more.

HTH's and makes sense!