I have an Osteoarthritic left knee that came on almost overnight after mowing the lawn too enthusiastically. I am now told I need a replacement and nothing can be done to alleviate the pain of walking. Apparently I've lost most of the cartilage on the inner side of the joint and nothing is really available to provide relief long term. That said, I have good integrity and mobility in the joint, it's just painful to walk on. I manage pain relief with over the counter products (Ibuprofen and Paracetomol) if needed and have some neoprene braces for dog walking etc but I have no discomfort when sitting or flying.
You don't say where you're from but the UK CAA (EASA) take the view that so long as the joint has integrity in strength and won't fold on you then you can still operate. I suspect a class one might be provided with an OML clause but I have my class 2 without any restriction.
As my AME said "If you can climb in and push the pedals, you're good enough to go".
Here's the official protocol from the CAA website.
Musculoskeletal conditions