Hypothesis ONE: TRAUMA
With Bonnie's high activity level, there is the 
potential for a great deal of strain 
and pressure on her joints.  Since she is an inside 
dog and the attentive owner does not 
know of any direct trauma, it is likely that the 
injury occurred during normal activity.  
The lameness is unilateral, and localized to the 
left stifle.  This supports the hypothesis of 
trauma.  There are many structures in the hind limb 
that could be affected leading to the 
presenting lameness.  Although it is less likely, 
the femur or tibia could have been 
fractured during strenuous physical activity.  Also 
the tendons of insertion of the muscle 
groups in the hind limb, as well as the muscles 
themselves, may have been ruptured or 
stretched causing pain and instability.  It is a 
partial weight bearing lameness that seems 
to cause the dog more problems when rising and 
climbing stairs than simply walking.  
This would support stifle injury, as pain is felt 
when flexing and extending the leg.  The 
stifle joint is obviously affected, as both joint 
effusion and joint capsule distension (a 
result of inflammation) are present in addition to 
a positive cranial drawer sign.       
In the stifle joint, the ligaments that could be 
affected are the patellar ligament, 
the medial and lateral collateral ligaments, the 
lateral femoropatellar ligament, cranial 
and caudal cruciate ligaments, the meniscofemoral 
ligament, the transverse ligament, and 
the lateral meniscus.  From the history we know 
that the collateral ligaments are intact.  
We can infer that the patellar ligaments are intact 
because the patellas are correctly 
positioned and normally mobile.  Since no cranial-
caudal instability is present we can 
assume that the caudal cruciate ligament is 
undamaged, although it can be difficult to 
differentiate between cranial and caudal cruciate 
rupture.  Often meniscal injuries are 
associated with a popping sound when the dog walks 
or when the stifle is examined.  
Isolated meniscal injuries are rare, although they 
can occur in conjunction with cranial 
cruciate ligament damage.  The likelihood of 
transverse ligament damage is rare, and 
further diagnostic tests may more completely rule 
it out.  
Therefore, the most likely damage to the stifle 
joint occurred to the cranial 
cruciate ligament.  The sudden onset of the 
lameness, and the high activity level of the 
dog all support the possibility of cranial cruciate 
ligament rupture.  Upon initial palpation 
of the stifle joint without sedation, there was a 
negative cranial drawer sign.  However, 
sedation is necessary to negate muscle tension that 
would otherwise mask abnormal 
motility.  When the test was repeated under 
sedation a 4-5mm cranial drawer was 
palpated on the left stifle.  The right stifle had 
2mm of movement, which is within normal 
limits.  Therefore, we can conclude that the left 
cranial cruciate ligament is either 
partially or completely ruptured.
There are two ways to rupture or damage the cranial 
cruciate ligament (CCL).  
The first is associated with violent internal 
rotation.  The function of the ligament is 
constraint of joint motion, limiting internal 
rotation of the tibia.  If violent rotation 
occurs, the caudo-medial edge of the lateral 
femoral condyle can cause damage to the 
ligament as it rotates against the condyle.  This 
type of rotation could occur if Bonnie 
were to spin on a planted limb.  The second way to 
damage the CCL is by hyperextension 
of the stifle.  In this case, the roof of the 
intercondylar notch can act as a knife and sever 
the CCL.  Bonnie could have hyperextended the 
stifle by any excessive motion during 
her normal physical activity.  Running, jumping, 
chasing, fetching and climbing stairs 
could all cause this trauma.
Once the ligament has ruptured, pain, intra-
articular hemorrhage and effusion are 
common sequela.  When the joint has reached maximal 
distension, the joint laxity and 
hemorrhage will be limited.  In Bonnie's case, we 
do see effusion, distension and pain on 
palpation of the joint.  These are typical signs of 
CCL rupture.  If this condition were left 
without repair, chronic instability, and 
osteoarthritis would likely follow.