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Hi, I can help you with your question today.
What you describe sounds suspicious for a cranial cruciate ligament rupture with a torn meniscus.
Although a ruptured ligament will not be visible on an x-ray, other signs such as joint effusion (fluid in the joint) can support our diagnosis.
This is not something that I would say "goes away on its own."
In a large dog like yours, he will be able to use the leg more quickly and have less arthritis long term if he has surgery on the knee.
I recommend scheduling a consult with an orthopedic surgeon. If you would like assistance finding a surgeon, post back with your city and state.