Hi there, a torn meniscus is one of the more common sports injuries over here in OZ, when surgery is necessary they typically do it keyhole which helps minimise recovery time, and developing early degenerative issues.
They get you up and moving almost straight away now, with rehabilitation focusing on early mobilisation of the knee joint, building up quadriceps and hamstring strength.....rehabilitation usually includes walking, bending the legs, and doing exercises that stretch and build up the leg muscles.
All of which is going to be more of a problem for you with the preexisting spasticity and mobility issues, but in a 'normal' person recovery generally ranges anywhere from 3-8 weeks.....i would expect it to take a lot longer for you to get back to your MS normal and you might still require addition knee support ie a brace, daily specific physio exercises you can do at home and exploring the alternative spasticity treatments.
Whilst MS isn't suppose to directly affect the joints, hip and knee pain, joint instability and injury isn't uncommon because of an abnormal gate, falls, spasticity etc being pretty common MS issues in general, so you're definitely not alone in having damaged your leg bad enough to need surgery, there's even surgery specifically for spasticity, though i do not know anything other than it being a last resort type option for when all other treatment options have failed.
Theoretically undergoing surgery isn't suppose to cause you to relapse although like everything MS there isn't any guarantees, we have had others previously mention experiencing pseudo's after surgery and during rehab so it might help to be mindful of your tipping point and not overdo things....
Hope that helps.........JJ
I just had a knee surgery, and have pmed you due to the details I've included :).
Wishing you well!