Diagnosis: 
Intermittent patellar luxation: aberrant healing of femoral fracture.

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 quadriceps muscle group.  This 
large muscle group converges on the patella and provides for extension of the stifle.  
Continuation of the muscle groupÕs tendon (patellar ligament) within the trochlear groove 
of the tibial crest ensures stable, fluent flexion.  A number of situations may alter this 
alignment, from congenital abnormalities to trauma.  In this case, proper alignment of the 
quadriceps muscle group and its distal tendon has been altered by an improper healing of 
a mid-diaphyseal fracture of the right femur.

Interpretation of the radiographs by our group includes observation of a healed mid-
diaphyseal fracture, with a lateral bowing of the right femur.  The fracture most likely 
occurred last year, when the dog jumped off the bed.  The fracture provides a reasonable 
explanation for the non-weight bearing lameness which then followed.  When a fracture 
heals without the support of external and/or interior fixation, the risk for an improperly 
aligned bone is real.  In this case, malalignment of the distal half of the femur laterally (in 
relation to the proximal half) may have resulted in an increased medial (torsional) strain 
on the quadriceps muscle group, and as a result, the patellar ligament, resulting in 
displacement form the trochlear groove of the proximal tibia (luxation).  The severity of 
the luxation is directly related to the magnitude of the strain on the tendon.  In this case, 
the physical exam identified that the patellar luxation was of grade II, indicating a mild 
degree of torsional deformity.