TPLO, or tibial-plateau-leveling osteotomy, is a surgery performed on dogs to stabilize the stifle joint (knee) after ruptures of the cranial cruciate ligament (analogous to the anterior cruciate ligament [ACL] in humans, and sometimes colloquially called the same).
In the vast majority of dogs, the cranial cruciate ligament (CrCL) ruptures as a result of long-term degeneration, whereby the fibres within the ligament weaken over time. We do not know the precise cause of this, but genetic factors are probably most important.