Hypothesis 3 - Cranial cruciate rupture secondary to other predisposing factors

The effusion found within the joint capsule of the stifle localizes the probable 
cause of the lameness to that joint.  There are numerous things that could cause 
similar signs in the stifle, but the most likely seems to be a fully or 
partially ruptured cranial cruciate ligament in the left stifle.  Considering 
the parts of the orthopedic exam performed so far, only the increased cranial 
drawer sign in the left stifle is abnormal.  The fact that the sign is present 
in the lame leg but not in the sound one also points to it as the probable cause 
of the lameness.  Bonnie presents as the classic cranial cruciate rupture case.  
She is a middle aged, large breed dog, overweight, and very active. 

Numerous conditions can predispose a dog to CCL rupture.  Two common ones are 
obesity and immune mediated disease.  Bonnie is definitely obese.  This is most 
likely due to the excess of calories she has been consuming even though she does 
get fairly regular exercise.  Other endocrine malfunctions, such as Cushing's 
Disease and hypothyroidism, can lead to obesity, but Bonnie is not showing any 
other signs of these so they are not very likely.  They would be mostly things 
to keep in mind if she continued to gain weight even after restricting her 
calories.  Obesity negatively affects the CCL by significantly increasing the 
force that the ligament needs to support.  The increased force constantly works 
against the ligament, gradually weakening it until it finally gives way.  

Immune mediated conditions can also lead to CCL rupture by slowly breaking down 
the ligament to the point where simply the weight of the dog is enough to break 
the ligament.