Hypothesis 1: Trauma - Cranial Cruciate Ligament  Rupture

This is a recent (5 days ago) cranial cruciate ligament tear in an older 
overweight dog, Bonnie.  This is a very common injury in dogs, especially 
middle-aged obese animals with poorly developed musculature.  The clinical 
signs are somewhat different than a cranial cruciate ligament tear in a 
younger active dog.  This injury may have occurred while Bonnie was running and 
abruptly fixed her tibia, such as by stepping in a hole, while the rest of 
her body continued forward. This caused either hyperextension or rotation of the 
tibia or both.  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 cranila cruciate ligament ruptured.    Some type of 
meniscal injury is also common in 70-80% of the cases.  This may also lead 
to increased instability of the joint and pain, and possibly more effusion.

In this case Bonnie presents with partial weight bearing left hindlimb 
lameness.  Bonnie has effusion of the left stifle joint and joint capsule 
distention on both sides of her stifle.  There is pain on manipulation of 
the stifle joint.  The joint effusion is due to intra-articular hemorrhage 
from the rupture causing pain as the joint capsule is distended.  Bonnie is 
also presenting the characteristic "toe in hock out" stance due to the 
instability of her stifle joint.  The finding of a 4-5mm positive cranial 
drawer sign under sedation further demonstrates the presence of a ruptured 
cranial cruciate ligament.  A positive response is the ability to move the 
proximal tibia cranial while holding the femur immobile.  You should not be 
able to move the tibia unless there is a rupture of the cranial cruciate 
ligament or the animal is skeletally immature.  A negative result in the 
awake patient does not rule out cranial cruciate ligament rupture.

Due to the recent nature of the injury we do not expect to see fibrosis of 
the joint capsule or associated structures.  There are no signs of 
osteoarthritis, which we might expect to see later as the chronic 
instability leads to degenerative joint disease.

We would like to repeat the orthopedic exam under sedation checking for a 
cranial drawer sign in both flexed and extended positions.  This is to rule 
out a partial cranial cruciate rupture from a complete one.  A partial 
rupture will have minimal movements in the extended position with increased 
cranial drawer in the flexed position.  We would also like to radiograph 
the stifle joint to check for an avulsion of the cranial cruciate ligament. 
  Rarely, an avulsed fragment of bone is seen lateral to the lateral 
condyle of the femur on a craniocaudal view of the stifle joint.  A lateral 
view may show a lucent defect in the lateral condyle, and the fragment may 
be seen overlying the joint.  We might also see intra-articular soft tissue 
swelling and intracapsular swelling displacing adjacent fascial planes.  
However, the absence of evidence of displacement on radiographs should not 
be relied onto exclude a diagnosis of ruptured cranial cruciate ligament.