Hypothesis 2: Congenital Abnormality

	Congenital patella luxation is one of the most common stifle joint abnormalities; 
medial luxation is particularly common in toy and miniature breeds.  Although the exact 
pathophysiology is controversial, the basic concept involves a congenital abnormality 
disrupting the Òstraight lineÓ placement of the extensor mechanism.  While this initial 
disruption could be any number of things, one good example is medial displacement of 
the quadriceps muscle group.  Upon extension of the stifle joint, the deviation of the 
quadriceps would cause the patella to deviate medially, through the musclesÕ attachment 
to the patella by the quadricepsÕ insertion tendon.  This problem could be further 
complicated as the lack of proper patellar pressure on the trochlear groove may result in a 
shallower groove, thereby making luxation even more likely.  Depending on severity, 
congenital patella luxation is capable of producing an intermittent non-weight bearing 
lameness.