Diagnosis:
Right medial patellar luxation due to previous traumatic injury.

Due to a fracture in the medial femur that occurred a year ago, the dog was lame on the 
right rear for approximately one month.  During this period, a hematoma forms due to the 
rupture of blood vessels and fills the fracture gap.  Activation of the coagulation cascade 
provides a fibrin mesh which serves as a framework for deposition of fibroblasts and new 
capallaries.  Cytokines activate osteoprogenitor cells which differentiate into osteoblasts 
and osteoclasts.  After one week, the hematoma is organized, the adjacent tissue is being 
prepared for future matrix formation, and the fractured bone is being remodeled.  This is 
called a procallus, and provides some structural integrity.  Osteoblasts form trabeculae of 
woven bone perpendicular to the fracture.  Activated mesechymal cells in the soft tissues 
may also differentiate into chrondroblasts that make fibrocartilage and hyaline cartilage 
surrounding the fracture site.  The repaired tissue should reach its maximum girth at the 
end of the third week which stabilizes the fracture but is not yet strong enough for weight 
bearing purposes.  As the subperiosteal bone approaches the newly formed cartilage, the 
cartilage undergoes endochrondral ossification.  The fracture ends are bridged by a bony 
callus, and as it mineralizes, the strength of the bone is increased to the point that weight 
bearing is allowed.  At this point, the dog appeared clinically normal.  

Over the next five months, the fracture was never stabilized and there was no casting of 
the leg.   This allowed the distal femur to bowing caudally.  This bowing created extra 
tension and stretching of the apparatus when the leg is flexed.  The caudal bowing of the 
distal 1/3 of the femur caused an undue stress on the extensor apparatus.  During this 
period the dog seemed clinically normal because of the formation of a bony callus across 
the fracture site.  This caused increased stability via a clinical union between both aspects 
of the bone.  On extension, there is not enough tension across the extensor apparatus to 
hold the patella in the medial trochlear ridge.  Hence, when the leg is in extension, you 
have patellar luxation because of the distal bowing; but when the leg is flexed, the patella 
returns to normal position.   Over the last six months, the dog has been favoring the right 
leg causing the left leg to have increased stress.  Radiographs showed no arthritic or 
degenerative changes in the left stifle.  This leads us to believe that it is more of a stress 
induced pain.