Medically speaking, what exactly is broken/injured if you break your back and is that different than fracturing your spine as the hot Senate candidate apparently did recently?
“Break your back” has no medical meaning. It’s an idiom for working hard.
Spinal fractures can range from minor injuries to permanent disablement if the spinal cord is affected.
I think colloquially it tends to mean a serious spinal fracture.
In anecdotal support, this happened to be in the cycling news today:
ETA: already answered.
And if it’s your mom’s back, it means you stepped on a crack in the concrete. 
People also colloquially refer to spinal cord injuries that don’t involve fractures as “a broken back”
It doesn’t mean anything. It’s a pop culture term, not a diagnosis.
Can you tell us where you are seeing this used in medical reports/parlance/etc?
Um, no, it isn’t. The correct phrase is ‘back breaking work.’
But I have heard and read the phrase ‘broken back’ numerous times by reporters and medical personnel when referring to the injuries someone sustained and no further details were forthcoming.
I would guess that it means the spine has been fractured in some manner, but if that’s the case then why haven’t I heard or seen that phrase “spinal fracture” used?
When my father was in a car accident he fractured several vertebrae. Bad enough to wear a back brace for a good amount of time, not severe enough to need surgery nor disable him.
“Broke his back” seemed about right to describe it.
I’ve wondered the same thing. My mother’s brother (so, my uncle) was a farmer who was a sometime truck driver.
One late night on the road, he hit a deer or a moose and he and his truck crashed upside down into a ditch. I remember my mother getting the phone call and seeing her go pale. “Uncle Rod broke his back,” she told me in a shaky voice.
Though he did mange to recover to a large degree, he never could manage the hard work on his farm as well again and farming became a burden, where it had formerly been his joy. This was attributed to his “bad back from the accident”.
When I got older, I began to wonder what exactly “broke his back” really meant. Unfortunately, none of the adults who were involved are here anymore to ask.