not too much to add except to say good luck with it all and hope it all works out ok
maybe it is a good thing they've spotted something to explain the symptoms and you're at least getting a operation so someone can take a proper look
though rather unrelated I had issues with my knee for some time, joint felt pretty loose + had pain in the tendon first GP dismissed it with a knee support and some aspirin gel... had an xray - unsurprisingly it showed nothing... decided to take advantage of medical insurance with work - got an MRI & saw a sports medicine consultant - patella tendon clearly swollen up to almost double the size in parts on the MRI, had a bunch of sugar solution injections (sounded dubious) + physio sessions - supposed to heal in a year... tendon got better but joint still felt 'loose'.. it wasn't until two years later when I got some more pain in the knee, bit of cartilage damaged - had surgery and thats when I finally found out the cause the surgeon noted that my ACL was damaged and that was why my joint had been loose/unstable... In retrospect, when I started looking into ACL injuries it became apparent that what I'd been describing to my GP in the first place, a couple of years earlier ought to have sounded like a damaged ACL.
Anyway the point of all my waffle was basically to say that if you're really concerned then perhaps it is good someone is physically looking inside, if they find nothing of worry then you'll probably be much more reassured by it.