Hypothesis Two: Cranial cruciate tear due to chronic instability from age and conformation Bonnie is a 6 year old, obese, large breed dog. We donÕt know her history or what conformational problems her parents might have had, so she could have had a minor conformational defect that caused abnormal pressures on the stifle joint. These pressures, over time, could have stretched the cranial cruciate ligament and weakened it enough for it to tear. Normally, congenital problems show up early in the life of the patient. If BonnieÕs deformity was minor enough, itÕs feasible to believe that she showed no abnormal clinical signs until now. Her obesity and high level of activity could have exacerbated this minor problem and led to the ligament damage. When the ligament ruptures, fragments of the damaged ligament and a small amount of hemorrhage are released into the joint space, which then stimulates the phagocytic abilities of the synovial cells. These cells then release degradative enzymes and inflammatory mediators such as cytokines and prostaglandins. The synovial vasculature is affected by these mediators and subsequently become more permeable, causing joint effusion. Sensitivity of nociceptors are increased by prostaglandins, so as the volume of the effusion increases, the fibrous joint capsule is stretched and local nociceptors are stimulated, resulting in pain.