Final rupture is typically due to a traumatic event. However the tendency to rupture, formally known as Cruciate Disease, is in many cases present from birth due to the anatomy of the stifle (knee) joint. The ligament degenerates progressively over time due to ongoing use and the unusual, and perhaps slightly illogical, anatomy of the canine stifle. The ligament may become stretched or partially torn and lameness may be only slight and intermittent, but a process of inflammation, or arthritis, is occurring in the joint at the same time. With continued use of the joint, the condition gradually gets worse until rupture occurs in the course of normal activity. This degeneration over time is due to the steep angle of the tibia causing excessive strain on the ACL when preventing forward movement of the tibia with respect to the femur. This degeneration typically occurs more rapidly in larger breeds and/or obese dogs. The more sedentary lifestyle, including less formal activity and obesity hasten this degeneration of the ACL through lack of supporting musculature. Most studies show that larger breed, younger, more active dogs are particularly at risk simply due to the higher weight and level of use of the ACL over time. This progressive degeneration explains why the traumatic event that causes final rupture can often seem quite minor. This also explains why 35-40% of dogs will rupture their other cruciate within 1-2 years. Dogs’ knees move in very much the same way as human knees, but the top of the tibia (the tibial plateau) slopes backwards in the dog’s knee putting stress on the cruciate ligament and can cause the ligament to rupture with repeated low grade stress. Humans have a perfectly level tibial plateau, which explains why they are not prone to repeated low-level injury. Stepping down off the bed or a small jump can be all it takes to break the ligament. The lameness may be acute but have features of more chronic joint disease or the lameness may simply be a more gradual/chronic problem. Dogs that rupture one cruciate ligament frequently rupture the other one within a year’s time. An owner should be prepared for another surgery in this time frame.