Hypothesis 2: Acute cranial cruciate ligament injury Injury during strenuous exercise has been shown to cause cranial cruciate ligament rupture. "Bonnie" certainly experiences activity that puts excessive force on her joints due to her body weight. The cranial cruciate is responsible for limiting tibial rotation and stifle hyperextension. It neutralizes the compressive forces that act to move the tibia cranially, but the tibia is only kept in place if the cranial cruciate is intact. An acute injury to the cranial cruciate can occur if the dog suddenly rotates the femur externally when the stifle is flexed and bearing the dog's weight. This sudden movement causes the internal rotation of the tibia and exceeds the breaking strength of the ligament. The internal rotation causes the cranial cruciate to twist and it may become damaged on the femoral condyle. Therefore, this twisting movement breaks the collagen fibers within the cruciate and the tibia is now free to move cranially (the cranial drawer sign). Excessive force can also be placed on the cranial cruciate when the stifle is hyperextended. This can occur if the dog suddenly stops running and the tibia is held in place while the rest of the body continues moving. Medical meniscal injury is common after cranial cruciate ligament rupture. When this ligament ruptures, the abnormal tibial rotation places an excessive force on the meniscus from the femoral condyle. This occurs because the meniscus is immobile; it is fixed to the joint capsule. The additional compression and rotation placed on the meniscus tears its fibers. It is important to check for a medial meniscal injury during the cruciate repair because failure to repair the injury can decrease the prognosis after surgery due to exacerbation of osteoarthritis.