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 although the etiology is not completely understood.  Degenerative joint changes in a middle-aged dog could cause weakening of the cranial cruciate ligament which is then subject to tear with minor trauma. 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.   Rupture of the cranial cruciate ligament causes a pathologically rolling and sliding of the femur in the stifle joint.  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.  Cranial cruciate ligament rupture alone causes only mild lameness. The animal avoids fully extending its stifle during motion.  If the medial meniscus is also injured it often results in moderate to severe lameness.  In large breed dogs the stifle is swollen.  The stifle joint is has decreased stability and pain with passive manipulation of the joint. 

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.  This makes the contours of the patellar ligament hard to palpate.  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 is no history or clinical 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 confirm a rule out of complete rupture of the cranial cruciate rupture from a partial one.  A partial rupture
will have minimal movements in the extended position with increased cranial drawer in the flexed position.  If the tibia moves one millimeter then the cranial cruciate ligament is only partially torn, of it is displaced more than one millimeter then the CCL is ruptured.  
In light of the fact that we already have a 4-5mm positive cranial drawer sign it is not necessary to perform a tibial compression test, which would also have been beneficial in the diagnosis of cranial cruciate rupture.  In repeating the orthopedic exam we will also listen for an audible click which is indicative of a tear or detachment of the medial meniscus.
We would
also like to radiograph the stifle joint to check for an avulsion of the cranial cruciate ligament.  A ventral dorsal radiograph of the pelvis and a mediolateral radiograph of the stifle should be taken.  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.