Welcome. THere is something you can do for vertical jumping - mental training.
Everyday for 10 minutes at least, or 3x a week for 40min-1hour (the numbers don't matter really quality over quantity), Image yourself performing heavy squats, quick jump squats, powercleans, ump squats on a trampoline, and anything else you would do for explosiveness. THis can keep the recruitment sharp.
Good luck with the knee, maybe you could try the smith machine? Definitely easier for those with knee issues.