It looks like significant inflammation with cartilage and joint damage including osteomyelitis which is damaged bone. I think that amputation was not a bad call since this level of destruction is hard to treat. Typically if this is a primary infection there should have been a penetrating wound such as a bite or deep puncture. Fungal disease such as Blastomycosis or Valley Fever normally affects the long bones and not specifically the joint but since those diseases are systemic I would see if your vet can run a blood test (fungal serology) especially if you are in parts of the country where these diseases are more common since he will need to be treated.

Usually synovial sarcoma (cancer of the joint) should have shown up on a biopsy and amputation is curative and since there is no OSA no chemo needed! Overall this is a mystery but perhaps your dog has been cured.

Pam

Your primary vet can run that test. Utah might be on the edge of the area for Valley Fever but your vet should be able to tell you better if your part of the state is at higher risk. 

It is hard to say about synovial cell cancer since those synovial cells are part of a normal joint but sometimes inflammation can mask cancer and there was a lot of inflammation in both his joint and bone.

Pam