Diagnosis: acute cranial cruciate ligament injury

	The hemarthrosis, lack of significant bone pathology and no 
evidence of joint sepsis together with the positive cranial drawer sign 
support the diagnosis of an acute cranial cruciate ligament injury in 
the left stifle.  This injury most likely occurred during one of 
Bonnie's several activities.  Since she is overweight, the forces put 
on her stifle are greater during exercise and predispose it to injury.  
The intact 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.
	Once the cruciate has ruptured, there is intra-articular 
hemorrhage and effusion causing pain.  The joint capsule, which is 
heavily innervated (hence the pain), will eventually limit the effusion 
and hemorrhage since its capacity for distension is limited.  The 
ruptured ligament retracts and fans out and will attain a "mop-end" 
appearance.  Osteophytes develop with time in an effort to stabilize 
the joint.  Inflammatory cells migrate into the joint space in response 
to the cytokines released by the damaged cells.  The inflammation 
increases blood vessel permeability, which leads to the effusion, and 
causes pain due to the release of prostaglandins.

	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.  The possibility of a meniscal tear will be 
investigated during the cruciate repair.