Diagnosis

Our diagnosis, based on signalment, history, physical exam, and specialty tests, is 
that Titan has a congenital medial patellar luxation of the right rear leg.  This was 
worsened by trauma from jumping off the bed and probably by playing with the larger 
dog more recently.  The grade of the luxation is probably a Grade II, based on the fact 
that it is an intermittent non-weight bearing lameness.  A Grade II patellar luxation is 
described as a patella that can dislocate spontaneously or can be pushed out, but can stay 
within the trochlear groove.

Pathophysiology

Patellar luxation usually describes an unstable patella that has a tendency to move 
either medially or laterally over the femoral trochlear ridge.  In a normal dog, the patella 
is well-seated in the trochlear groove.  The normal patella is usually hard to differentiate 
from the ridges of the trochlea.  The longitudinal axis of the quadriceps muscle, patella, 
patellar ligament, trochlea, and tibial tuberosity should be in a straight line.  Patellar 
instability is best evaluated when the stifle joint is in extension.
Congenital medial patellar luxation is a hereditary disease affecting small breed 
dogs most commonly.  The small breeds most affected by this disease are miniature and 
toy Poodles, Pomeranians, Yorkshire Terriers, Pekingese, Boston Terriers, and 
Chihuahuas. 
Medial luxation of the patella is also more common than lateral patellar luxation.  
Congenital luxation is usually evident by 4 to 6 months of age, although minor 
deformities can occur later in life.  Traumatic luxation or grade I and grade II luxations 
can present at any age.  Most dogs with medial patellar luxation have some form of 
related structural deformity and may have a bow-legged appearance.

The pathophysiology of congenital medial patellar luxation is controversial.  
These changes seem to start in the hip, but are reflected throughout the limb.  The 
following paragraph is a list of musculo-skeletal abnormalities associated with medial 
patella luxation. Coxa vara, or a reduction of the inclination (normal inclination angle is 
135), is usually a part of the underlying deformity with medial patellar luxation.  Lateral 
torsion of the distal femur, usually from a deforming force at an angle to an active growth 
plate, can also be associated with patellar luxation.  Medial displacement of the 
quadriceps muscle group can occur from the lateral torsion of the distal femur.  This also 
is associated with patellar luxation.  Lateral bowing of the distal one-third of the femur 
resulting in an S-shaped conformation with tilting of the joint because of abnormal 
pressure.  A shallow trochlear groove, from less patellar pressure than normal on the 
trochlear groove, can cause a failure of the patella to articulate properly with the trochlea.  
Dysplasia of the femoral epiphysis, rotational instability of the stifle joint, tibial 
deformities, and degenerative joint disease are other deformities that may occur from 
abnormal pressure on the growth cartilage of the epiphysis or abnormal wear and tear on 
the articular surfaces, resulting in patellar luxation.
The deformities listed above can range from severe to mild.  The mild cases may 
not always be seen on radiographs, but palpation of the patella should result in manual or 
permanent luxation.  The age of the patient and the permanency of the luxation also play 
a role in the pathophysiology of patellar luxation.  The younger the animal and the less 
often the patella luxates, the milder the case.