I had torn knee cartilage (running v v fast intervals, not really properly warmed up, silly me) and I simply had (private/paid for) physiotherapy and a very slow "kneehab" overseen by my personal trainer who works with the physios. It took about 18 months before I felt really safe about doing anything & everything with my knee, and now that side of my body is stronger.
I did a lot of kneehab things, but a lot focussed on strengthening glutes, quads and calves - very slow step ups and step downs, really loading the leg, and eventually onto quite high plyo boxes in the gym, one-legged squats - (supported not pistol squats); squats on the TRX, so the TRX ropes took a lot of the weight - you can do a Spanish squat variation with bands as a substitute.
I stopped running for at least a year, and also stopped ballet class for that time.
I know am back to box jumping, broad jumping, short bursts of running, and ballet class. But I now take extra care getting the knees warm - as well as my standard warm up (30 minute walk to gym, a sequence of bodyweight squats & burpees) I add in about 5-10- minutes static cycling, to really get the sinovial fluid in my knee joints juicy.
My physio kept reassuring me that a) cartilage likes being stressed by things like jumping b) a certain amount of age-related wear & tear is normal, and needs attention in terms of warming up & staying strong, but it shouldn't stop me being insanely active & fit at my age (I was 65 when this happened).
But it's scary - feeling the Ping! of the tissue tearing ...