My Bentley tore both ACLs and needed TPLO surgery on both knees to correct it. Basically, once they blow on knee out, it's a matter a when not if on the other knee. His tear also started out as a partial tear and we gave him Rimadyl and limited his activity (he wasn't overly active in the first place) but the ligament eventually tore...it's very common.
When it tears, the only way to repair it is surgery. It will not ever heal on its own. You can have traditional ACL repair surgery where they basically retie the ligament or you can have TPLO, where they actually reconfigure the leg so the dog does not need an ACL ligament any more. Based on the age and activity level of your dog, that is a decision you would have to make with your vet. When Bentley's ACL finally tore, he was only around 14 months so, knowing reattached ACLs can re-ruputure over time, I elected to have the TPLO.
The good news was he recovered excellent from both surgeries and could run and get around like he was brand new. The bad news was it is pretty expensive and once the one knee goes, the other knee has an 88% chance of going too.
