The Anterior cruciate ligament (ACL) is an acute or progressive failure in the cranial cruciate ligament (CCL) which leads your dog to partial or complete instability of the stifle joint. This ligament often ruptures due to excessive internal rotation of tibia when the known joint is partially bent. This can be the result of running or planting hind limbs, which leads to limping and complete loss of the use of limb in critical cases.