Hi there,
I am happy to hear that you are doing better.
I more likely to think that damage to the cartilage behind the knee occurs over a longer period of time and is slowly worn down rather than one quick trauma (where damage to the cartilage in other parts of the knee may occur suddenly).
If the kneecap isn't moving smoothly on the bone below it however, this type of damage can occur, so I would probably get a different physio to check you out, they are likely to focus on quads strengthening also but may focus on different parts of the quads in order to ensure that the kneecap bone is moving correctly on the bone below it,
if that makes sense....?
Best of luck !!!