Good reads bsms! I love the radiographs! I'm trying to decide what might be upsetting my horse in this D ring as opposed to the kimberwick. Both bits are single jointed, but the kimberwick has straight bars. You would think that would have more of a nutcracker effect on the bars and be more likely to hit the palate than a curved mouth piece, which is what the D has. It also appears, via these radiographs that a more mobile mouth piece gives the horse more of an opportunity to shift the bit away from more painful areas. The kimberwick is very fixed in its position, while the D should be more mobile. Maybe she's just not used to the instability and would actually like a D, egg butt or loose ring if given the chance to get used to it. I also always followed the one or two wrinkles rule and never left the bit hang looser to allow the horse to carry it the way she thinks it's comfortable. Maybe that is worth a try! Our problem could also lie in the thinness of our D ring. It is significantly thinner than the Kimberwick and my horse may just prefer a thicker mouthpiece.
Gah! I just keep talking myself in circles! I really just need to get out there and experiment, but first all the snow and ice in the arena needs to melt!