Basically what Springer said.
The problem is, once there's a tear or partial tear, the joint very quickly begins to become affected by arthritis. A torn ligament is fixable, arthritis is not and can be incredibly debilitating, especially in a large breed dog.
The orthopedic surgeons I work with told me not to waste my money on a brace when my old dog tore her second ligament. The evidence that they do anything just isn't there. In the end, the arthritis is what killed her. She just couldn't get around.
We kept her going for a year by using NSAIDS, tramadol and gabapentin. She ate prescription food j/d and was receiving laser therapy, acupuncture and cartrophen injections.