Hypothesis 2

Degenerative changes leading to Cranial Cruciate Rupture

Initially, we found Bonnie to be painful. Facts supporting this finding were the dog 
wagged its tail slowly when you entered the room and did not get up.  This response from 
a labrador retriever would be indicative of pain.  A normal lab would stand and greet you 
with a happy wagging tail.  The owner further supported this finding with a history of 
difficulty getting up and down and climbing stairs at home. 

Upon further examination of the dogÕs gait it was discovered that Bonnie was lame in her 
left hind limb and was toe touching lame.  One common cause of difficulty getting up 
and down and climbing stairs is hip dysplasia or joint laxity.  The fact that Bonnie had no 
palpable laxity or pain during full range of motion helped us to rule this out as the 
probable cause of her lameness.  Based on the physical exam (especially that there was 
pain on manipulation of the stifle joint) and the orthopedic examination the source of the 
lameness was isolated to the stifle joint. 

There are many possibilities for lameness that originate from the stifle.  We started 
initially by ruling out things that appeared normal.  Patella luxation was ruled out because 
BonnieÕs patella was s correctly positioned and normally mobile.  Next, both collateral 
ligaments were intact which would rule out collateral ligament rupture. Neoplasia was a 
possibility, but most neoplasms in the stifle, such as synovial cell sarcoma, cause a 
slowly progressive lameness which is inconsistent with the sudden onset of lameness in 
Bonnie.

Upon performing the cranial drawer sign on Bonnie under sedation, it was observed that 
she had a 4-5mm cranial drawer.  Increased cranial movement of the tibia >3-5mm is 
pathognomonic for cranial cruciate rupture.  Other supporting clinical signs of CrCL 
rupture were her signalment:  an obese female 6 year old labrador retriever.   Also her 
partially non-weight bearing lameness, effusion of the stifle, and distention of the joint 
capsule on both sides of the patellar ligament are consistent with cranial cruciate ligament 
rupture.

The next step was deciding why Bonnie had a ruptured cranial cruciate ligament.  Based 
upon her signalment and the fact that she had normal activity resulting in acute lameness, 
we decided to pursue degenerative cranial cruciate rupture.  There are various causes for 
degeneration of the cruciate ligament.  The top two that would be consistent with 
BonnieÕs case are age and weight related degeneration.  Degeneration related to aging 
corresponds with the animalÕs size.  Larger dogs show the most degenerative changes.  
Conformational abnormalities can also lead to degenerative changes of the stifle joint.  It 
was hard to evaluate from the film clip whether or not Bonnie had a true conformational 
abnormality.  She appeared to be Òknock kneedÓ (genu valgum).  The combination of size 
and age accumulate into degenerative changes of the cruciate ligament.  The function of 
the cranial cruciate ligament is to constrain the stifle joint by limiting internal rotation, 
cranial displacement of the tibia relative to the femur, and to prevent hyperextension of 
the joint.  Over time Bonnie has likely placed her cranial cruciate ligament under 
excessive stresses while exercising (as was described in the ownerÕs history).  Her excess 
body weight along with possibly a conformational defect in her hind limbs has damaged 
the cruciate ligaments gradually by stretching.  Each time Bonnie damaged her ligament 
by exercising, there was mild cell damage in the ligament and inflammation and scarring 
followed.  Over time her ligament has become progressively weaker from the scarring.  It 
is likely that Bonnie could have just been walking or exercising normally and simplyt 
rotated her stifle, stepped into a hole, or hyperextended the joint a little more than the 
joint could withstand and caused the ligament to rupture.  Degenerative cruciate 
ligaments rupture more easily than healthy ligaments.  A concern with Bonnie now would 
be that her right cranial cruciate ligament is also likely degenerative and could also 
rupture. Another concern would be possible meniscal injury.