Congenital problem with the stifle

Toy breed dogs are ten times more likely to be affected with medial patellar 
luxation and 
females are 1.5 times more likely, as well. There are multiple congenital 
abnormalities that 
can cause the luxation of the patella.  In the normal dog, proper anatomic 
alignment  of 
the extensor mechanism is a straight line and is required for the stability of 
the patella.  Any 
abnormality that deviates this from a straight line can cause luxation of the 
patella.  The 
rectus femoris, vastus lateralis, vastus medialis and vastus intermedius muscles 
form the 
muscle group commonly called the quadriceps femoris.  This large muscle group 
converges 
on the patella and provides for extension of the stifle. The muscle group's 
functional insertion 
continues along the patellar ligament to the tibial crest.  A medial 
displacement of the 
quadriceps causes abnormal tension medially throughout growth and development.  
This 
abnormal articulation causes changes to the underlying bone, failing to develop 
a fully 
functional trochlear groove.  These changes can include shallow trochlear 
groove, 
hypoplasia of the medial femoral condyle, a reduced coxofemoral angle (coxa 
vara), lateral 
bowing of the femor, and internal rotation of the tibia which can predipose the 
animal to 
patellar luxation.  Obesity caused increased stress to the joint, which is 
another predisposing 
factor.  Both legs are affected, which is a normal finding for a congenital 
problem.  
Lameness is seen due to a nonfunctional joint from the lack of articulation of 
the patella and 
the trochlear groove.