Sorry for the late reply. I'll say the Orthopaedic and the radiologist
might not be able to identify the minute or remote injury
. It might be a lat meniscus
tear as your orthopaedician says, but to be certain of the diagnosis he's offered you the gold-standard investigation- the Arthroscopy! Its the only option to peek inside the knee, at the same time it offers to rectify the problem in the same sitting (if its a tear of meniscus or ligament whatever). So go right ahead and do it.
Good luck and do ask your surgeon to make a recording of what he does in your knee.