I did this bridge 19(20)21 in 2000 BL (Before Dr. Lane Ochi). As Dr. Ochi pointed out when one abutment is much smaller than the other the force concentrations will weaken the abutment and it'll fail one way or another. Here #21 fractured horizontally at mid prep. Now all my bridges get a keyway at the distal of the smaller abutment.
I now use keyways on all the bridges I do, placing them at the distal of the smallest abutment tooth to reduce the forces on that tooth and help prevent failure from abutment crown loosening or abutment prep fracturing. Keyways come as inexpensive plastic patterns and all labs have easy access to them. I doubt it's possible to incorporate them into zirconia bridges yet. Hope this helps.