How common is the "domino effect" after a c5-c6 c6-c7 fusion to address radiating pain/weakness due to disc/spur compression?

Disc loading: I do not use terms, "domino effect." The fact is having two levels fused in your neck definitely means adjacent levels are being overworked and have a significant chance of being injured resulting in bulges/herniations. If you gave récent l'y hard fusion as above, try to minimize neck flexion. If you have recurrent neck/arm pain since above fusion--problems. I might be able to help you.