I strongly second the suggestion for an ortho specialist. My newfie girl had a blown ACL that was misdiagnosed for over a year as panosteitis. The fact that the dog will not use the leg is a definite sign of pain, and ACL injuries are very common in dogs. The ortho vets at the veterinary teaching hospital that I took my newf to said that unless a vet has extensive ortho experience, ACL injuries are easy to miss on xrays.
Livin in a Newfie Drool Zone