What is the best filler for knees and elbows?

I’m thin/fit but no matter how much I work out, my elbows & knees are starting to look like an old lady! I don’t have enough loose skin to merit surgery (nor do I want huge scars on my arms/legs). I saw a YouTube vlog about sculptra injections just above the knees & wondered if that (or another filer) was a good option. I’d like whatever filler you suggest to be affordable as I’m guessing much more filler would be needed in these areas than in the face. Any advice?