I also had a severe back injury - using a full foam roller can overextend your back and cause further injury. My understanding is a half-circle can just provide some lumbar support if you don't have good lumbar support in your desk chair, etc. I've also been told to roll a towel up and use it for lumbar support or lying on.

I've never had a problem with it - I guess you need to find the right sized one. Also, it's a strong stretch and one can build up to it using a rolled-up towel and use it on all parts of the spine. I just go by feeling.

I actually got mine from a chiro - there must be a precise diameter required for the spine - 14.5cm hard foam.