Has your dog suddenly started limping on one of its back legs?
There are many different reasons for limping in dogs, and knee injuries are among them. One of the most common knee problems seen in dogs is an injury to the cruciate ligament, sometimes called ACL or CCL tear. A ruptured cruciate is often a painful and immobilizing injury.
Although rest and medication may help, the most recommended method of treatment for cruciate ligament injury is a surgical repair.

Signs Your Dog May Have a Cruciate Ligament Injury:
Sudden limping on a back leg
Not bearing weight or "toe-touching"
Sitting with the leg kicked out to the side
Stiffness after rest or difficulty getting up
Swelling around the knee
Decreased activity or reluctance to jump
Picture
Figure shows a ruptured cranial cruciate ligament (in red)