"Breaking your back" often isn't used in it's strictest medical definition, so doesn't necessarily mean breaking a bone. It can mean dislocating one of the bones/joints to the point that it affects or damages the spinal cord. That, in layman's terms, means "something in your back is broken" (aka "spinal injury"), which loosely translates to "breaking your back". Same way a "broken" ankle or wrist can actually just be a dislocated bone. It's just easier/more common to say "It's broken" for most people who don't need an exact medical breakdown/explanation to get their point across