A. The ACL is a ligament, not a muscle, and it's very common for dogs to tear it, either fully or partially. A tear to this ligament produces chronic lameness that sometimes improves over time, but not addressing the tear usually results in severe arthritis and chronic lameness. If your vet hasn't assessed his knee joint with x-rays and a thorough examination, that should be done. Other possibilities include hip problems, low back problems, or fractures to any of the small bones in the feet. A good exam can localize the pain, and then you can go from there to figure exactly what's wrong and how to treat it.
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