In the event of a spinal injury that displaces a vertebra, what's the best thing to do if the characters don't have any real medical supplies other than a first aid kit or access to a hospital?
Ohhhh man. This is a much more complicated ask than you realized, I think.
The problem isn’t so much the vertebral injury, as in injury to the bone. The problem is with the word displacement. Displacement implies movement.
a look at the anatomy. Here’s a typical vertebra; this one happens to
be from the thoracic spine (connects to a rib), but it’s relatively
common in appearance:
(source) See that big hole in the middle? That’s called the vertebral foramen, which basically means “vertebral hole”. That’s where the spinal cord passes through.
The problem is that vertebrae align vertically, like thusly:
(source) Not only is there a lot of stuff that interlocks, as you can see, but through the middle of all of that is going to run the spinal cord.
imagine one of those vertebrae jutting out of place. Imagine it
sliding, say, anteriorly. While the cord is passing through it. You see
where this is going, right?
This person could have bone compressing their spinal cord.
The pain from this is immense. It can produce numbness, tingling, paresthesia (that feeling where your leg falls asleep, except not necessarily in the leg). It could, in a bad enough injury, actual damage the cord. In a worst-case scenario, it could sever the cord, leaving everything below it paralyzed.
What’s affected will depend on how high up the injury is, but walking is usually difficult (if nothing else, from the pain). Back muscles tend to spasm to protect it, because that’s what back muscles do.
To give you an idea, a partner of mine had a small displacement from her C-spine onto her cord.
As to first aid… well, without access to a hospital, your characters are going to have significant difficulty. Allow the person to lie down. Keep them warm. If they’re paralyzed, they may enter neurogenic shock, where blood vessels below the level of injury lose their innervation and dilate and the legs turn warm and pink but they also fill with fluid and the person doesn’t have enough blood to fill their expanded blood vessels.
People can die that way. And there’s not much to do without pressors and surgery.
I’m so sorry that you got this so late. It’s an ask I saved forever and a day ago and never finished, and I’m a Bad Scripty™. I can only hope it’s better late than never.