I have been wearing a removable knee brace for the last seven weeks after an accident which left me with a dislocated patella, torn ligaments and meniscus damage.

Operation was carried out via keyhole surgery, so no kind of plaster etc. involved. Your case sounds the same kind of thing, but I am not an expert.

I flew shorthaul in Y four times before the operation (three weeks ago) and will also be doing so again next week.

My brace was initially restrained to sixty degrees and I managed to fit the leg in reasonably comfortably behind the seat in front of me. My only fear was that the passenger in front of me would (at worst suddenly) decide to recline their seat. So I politely asked them at the beginning of the filght if they would refrain from this, which they agreed to in all cases.

That said, things might be a bit different on long-haul flights, where I recall there is a lot more "reclining in Y".

If you can cover yourself from the risk of the passenger in front of you smashing their seat back on you, then I imagine you would be fine.