@celia16 According to the ortho I see for my elbow - how's that for specialization - one does elbows and wrists, one does shoulders, another does hips and knees:
PRP SEEMS to be more effective in smaller joints. In their practice, they use it often for elbows, ankles and sometimes shoulders. Less frequently for knees, and even less so for hips. (They don't do spines so don't know about that.)
I think the variability in results is part of the reason that it is not FDA approved this as a treatment. Also, if you choose to try it, make sure it is done by someone who does this daily or weekly as PART of their larger practice, not what he called "an injection mill" where they do only that.
I haven't asked my hip/knee guy for PRP in my knees yet, but we are approaching that stage as I am about done with cortisone. I'll see what he says in the Spring when we meet again.