Hypothesis 2

The dog was born with a congenital malformation of the hind legs, predisposing 
the dog to patellar luxation and other musculoskeletal defects.  When the dog 
jumped off the bed it may have aggravated the problem, leading to the lameness.  
The animal did not show clinical signs until the patella was luxated out of itÕs 
normal position.  The cranial cruciate ligament in the right stifle may have 
ruptured at this time as well, either partially or completely.  Either way, the 
pain response on the left hind leg may be due to the compensation effect, since 
there will be extra stress on the more weight-bearing limb.

Congenital patella luxation commonly causes medial luxation of the stifle in 
Pomeranians, and can cause a chronic, partial-weight-bearing lameness. Medial 
patellar luxation is common in toy, mini, and large breed dogs, but toy and mini 
breeds are 4-8 times more likely to have this problem.  Many of the cases are 
congenital, meaning that the anatomical deformation that causes the luxation is 
present at birth, but the actual luxation is not.  20-25% of cases of medial 
patellar luxation are bilateral.  In mature animals, 15-20% present with 
simultaneous CCLR.  Young to mature animals suffering from medial patellar 
luxation exhibited intermittently abnormal gaits in the past but are presented 
to the veterinarian when the gait becomes drastically abnormal or other more 
severe symptoms develop.

Cranial cruciate ligament rupture could have occurred when the dog jumped off of 
the bed, causing hyperextension followed by rotation and twisting of the 
cruciate ligament.  Often ruptures are associated with jumping off a couch, bed, 
deck, etc.  More often rupture occurs as a chronic event, which is degenerative 
in origin, rather than Òtraumatic.Ó  Partial tears can also occur, which quickly 
progress to complete rupture.  The clinical signs of a cranial cruciate ligament 
rupture for a young, fit, active dog such as this include: acute onset of non-
weight bearing lameness during exercise, capsular distention, non-weight bearing 
for at least a week (this dog was partially weight-bearing), and may use leg 
more with time until sound, which this patient seemed to do.  Lameness recurs 
with stress, overwork, after weather changes, with age, or with meniscal injury.  
The tibia rotates medially, giving a toed in, hock out stance.  Pain is 
insignificant.  Cranial drawer sign is diagnostic.  The right stifle was 
negative, but a negative result in an awake patient does not rule out the 
diagnosis of cranial cruciate ligament, and the left leg exam could not be 
completed.  Another possible reason for a false negative is the fact that the 
injurious event occurred a year ago, allowing time for fibrosis of the joint.  
Often when the CCL ruptures, a meniscal tear is also created, leading to a 
palpable click or pop during flexion and extension, 
and sudden lameness after CCL recovery.