I didn't answer your questions @scsm! So here are my replies:
Q: How long did your PRP injection work for you? Was it a couple of months or longer?
A: Still working! Lower Back -- Over 2 years and still OK. Knees -- Over 1 year, but I heard a "crack" 2 months ago, as I hyper-extended my left knee, so I am having some pain now (going up the stairs). I fear I have a tear in the lateral meniscus! But didn't get it checked yet.
Q: Were you still having pain two or three months after your PRP injections?
A: Yes, but less pain. It really took 4 months to see progress. From the 4th month, the progress steadily grew for both lower back and knees. At around 20 months or so, my back was painless!
I will add that I fell twice (on steps) in 2018. I thought I would not be able to even move! But nothing like that happened. I had NO back pain, NO issues! My falls were about 23 months following the back procedures. I fell forward and down 3 flights of stairs in my first fall ... Somersaulted down several steps and hit my back several times as I was falling, and ended up in the sitting position ... NO back pain!
Q: What made you want to have stem cell injections instead of continuing the PRP injections?
A: The doctor suggested Stem Cells for my knees right away (in place of PRP), because Stem Cell treatments are a much better AND permanent solution. However, I was and still am taking Statins for my cholesterol. In order for Stem Cells procedures to be effective, you need to be off of Statins for 3+ months. My Primary Doctor did not agree with staying off the statins at that time. He wanted to see if PRP would be sufficient. Now, he agrees I can be off Statins, "IF" the pain, and/or tears in the meniscus reappear.
My doctor put me on certain vitamins that help promote the PRP and Stem Cell healing process. Different vitamins for the two procedures, I am still on the ones he gave me for the PRP!

If you have any additional questions, do send them my way! I will be glad to provide my experience!

--Dee