Diagnosis:

Our diagnosis is right cranial cruciate ligament rupture and a possible meniscal tear due to trauma at play.  Our findings on bloodwork indicate that there is a mild inflammatory process associated with a non-septic exudate of the joint space caused by the ligamentous tear.  The radiographic findings indicate a generalized opacity of the joint due to effusion with a loss of dimension of the infrapatellar fat pad, which is associated with an effusion of the joint.  There was no finding of osteosarcoma, an avulsion fracture or definate OCD lesions on radiographs. There is also no history of a past major traumatic event, which could cause a fracture.   The clinical findings of a 4-5mm cranial drawer sign and acute onset of partial weight bearing lameness further support cranial cruciate ligament rupture.  The increase in severity of the lameness leads us to believe that there is also a medial meniscal tear.

We presume that this injury occurred while Bonnie was at play, running and abruptly fixed her tibia, possibly by stepping in a hole, while the rest of her body continued forward. This caused hyperextension or rotation of the tibia.  The stifle was flexed 20-50 degrees as the tibia was forcibly rotated internally as Bonnie rotates her body externally, the cruciate is twisted and the cranial
cruciate ligament ruptured.   As the ligament is ruptured inflammatory mediators are released and a non-septic joint effusion occurs due to recruitment of neutrophils and vasodilation.  Rupture of the cranial cruciate ligament causes a pathologically rolling and sliding of the femur in the stifle joint, which causes partial weight bearing lameness of the right hindlimb.  Up to 80% of cranial cruciate ligament tears result in tears or detachment of the medial meniscus, which cannot follow the pathological motion of the femur due to its attachments to the tibial plateau and thus tears.  Cranial cruciate ligament rupture causes only mild lameness with the animal avoiding full extension of its stifle during motion.  If the medial meniscus is also injured there is resulting moderate to severe lameness, which is what is being seen in Bonnie's case.