Hypothesis 2:  Congenital 

	Many congenital musculoskeletal abnormalities occur commonly in toy and 
miniature female breed dogs.  Physical exam revealed lameness in the right rear leg.  We 
suspect the cause of the lameness to be isolated to the stifle joint.  For the stifle joint to 
function properly there must be a straight line through the extensor mechanism.  
Congenital malalignment of the quadriceps muscle can occur in dogs.  The quadriceps 
femoris tendon of insertion attaches to the tibia and intercalates the patella.  
Malalignment of this muscle is shifted medially causing subsequent medial luxation of 
the patella.  The quadriceps muscle is the most powerful muscle extensor of the stifle.  If 
there is malalignment of this muscle incongruity of the stifle joint will occur in the right 
rear leg lameness can also be manifested due to the continued luxation of the patella, 
which will lead to degenerative joint disease.  The dogÕs lameness is waxing and waning 
because of the dogÕs activity level and that it is kept indoors.  The medial luxation of the 
patella occurs only during extension of the stifle and the patella returns to the trochlear 
groove normally on flexion.  This further supports the idea of a congenital malalignment 
of the extensor mechanism within the limb.