indent in my boas spine. help needed

Its a hinged & I put 2 of the cages that house tarantulas on top of it to weigh it down. In the past he has had a habit of squeezing himself between the lid & tank. As for the branch there is nowhere he can wedge between it and the cage.

You need clamps to secure lids on cages.

Even if the cage is 'weighed down' the snake will still push on it and get through and the weight will just hurt them in the end.

I feel like this may be what caused the kink.

But there's no fractures in the spine or ribs, and bones don't just bend, so it can't just be an injury I don't think...

Maybe he fell off the branch and hit his back on his water bowl?

He might of, the water bowl is right under the branch. I didn't even think about that when I put the water bowl under the branch.

He's not heavy enough to cause that kind of injury in a fall inside the cage. I'd have to agree that he had tried squeezing himself out of the cage and got part of himself stuck at some point before getting free. Just because the vet didn't see anything obvious doesn't mean there was no trauma. If there are broken bones, they may be hard to see on an x-ray - the ribs are tiny and will overlap in the image, and it would be tough getting the correct angle to see what's damaged in the spine. Damage to the muscles and cartilage around the bone would be practically impossible to see in an x-ray of such a small snake.

I would highly recommend fixing or replacing the lid so that there is no movement. Any give in the lid will encourage more escape attempts for him (if he makes it) or any other snake you house in there.