Hypothesis:  Trauma lameness predisposed by medially luxating patella.

Trauma is suspected in this case.  It is suspected that this dog has a congenital medially 
luxating patella.  This is supported by the patella easily luxating to the medial side of the 
trochlear groove.  This chronic luxation of the patella could be from a decreased angle of 
inclination of the femoral neck (coax vera), an atypical position of the quadriceps 
mechanism and patella, and a shallow trochlear sulcus.  Chronic patellar luxation places 
an increased stress on the cranial cruciate ligament because the quadriceps mechanism is 
ineffective in stabilization of the stifle joint.  The patellar luxation would be the initial 
condition and the rupture to tear to a ligament causing lameness (probable cruciate 
ligament rupture/tear) would result from minor trauma because the tibial instability 
created by the luxating patella has chronically stretched the ligaments of the stifle joint 
making it predisposed to a tear or rupture.  A minor trauma such as falling off a bed, as 
this patient did, could have easily ruptured or torn a cruciate ligament.  The buttressing 
noted on the physical exam is fibrosis of the joint capsule.  This is a stress-induced 
fibrosis as the body tries to compensate for the instability of the joint.  

With rupture of the cruciate ligament there is often initial lameness like this patient when 
it fell off the bed.  And then generally within 2-3 weeks there is return to normal.  This 
Pomeranian was normal again within one month.  Also with this type of injury, there is 
often a gradual decline to a sudden decline in the use of the limb after it had initially 
recovered.  This is seen several months after the initial trauma.  In this case the dog was 
initially lame then recovered by one month and became lame again 6 months later.

Trauma is further supported in this case because the dog lives with another dog and plays 
often, so there could have been a trauma associated with this situation.  The dog is also 
overweight making a minor trauma to the stifle even easier.

A negative drawer response does not rule out trauma-induced damage to the cruciate 
ligaments or any other ligaments of the stifle joint.  The test should be repeated under 
anesthesia to confirm the negative results.

Examples of trauma that could have induced this lameness include hyperextension such 
as stepping into a hole while running or forcibly rotating the tibia.  This dog is 
predisposed to trauma such as these because of the joint instability from the luxating 
patella.