Reply To: How to change the rotation of the thumb.

Hi Santiago,
I can’t tell exactly from the image but the first thing I would do is make sure that the bone roll is pointing in the correct direction.  
This doesn’t mean just recalculating it, but rather rolling it to point in the right way.

The Z axes should all point towards the direction you want it to bend (I can’t tell exactly from your image)
This includes the MCH bones etc

The Next thing to check is that the transformation constraints are correctly setup on the MCH bones.
Scale Y [0.5, 1.5] extrapolation on –> Rotation X source –> Y [90,-90]

Notice how the minimum is positive and the maximum is negative. That will reverse the direction.

Also make sure you are going from Local to Local space

Failing those ideas, post it as a submission and I will take a look.