Page 359 - Canine Lameness

19.5 Patellar uxation 331

Box 19.2 Grading of Patellar Luxation
Grading of patellar luxation based on location of the patella with typical clinical symptoms
and degree of skeletal deformities (italicized) known to occur:

Grade 1: “In-In” (i.e. the patella is always located in the trochlear groove unless manually
forced and held outside the groove):

Patella does not spontaneously luxate, but it can be manually luxated, however, it returns
●
into the groove upon release of pressure
­o clinical symptoms and no skeletal deformities
●
Grade 2: “In-Out” (i.e. the patella is typically located in the trochlear groove but remains outside
the groove at times even without manipulation):
Patella spontaneously luxates and reduces (which causes the typical “skipping” pelvic limb
●
lameness)
Patella will readily luxate with manual pressure, but reduces on its own (i.e. returns back into the trochlear
●
groove)
Only mild skeletal deformities that may not be readily visible
●
Grade 3: “Out-In” (i.e. the patella is typically located outside of the trochlear groove but can be
manually reduced back into the trochlear groove): STIFLE REGION

Patella is most often in a luxated position (which causes more consistent low-grade lameness, but
●
rare “skipping” pelvic limb lameness may be observed)
Patella can be reduced with manual pressure, but it will reluxate spontaneously upon release of manual
●
pressure
Moderate skeletal deformities that may result in abnormal posture
●
Grade 4: “Out-Out” (i.e. the patella is always located outside of the trochlear groove and cannot
be manually reduced):
Patella is always luxated (which causes a consistent lameness of varying severity)
●
● Severe, obvious skeletal deformities resulting in abnormal posture

As noted above, the degree of lameness can vary but skipping pelvic limb lameness raises suspi-
cion of a Grade 2 patellar luxation. There are other differential diagnoses for skipping pelvic limb
lameness including luxation of the LDE tendon, superficial digital flexor, neurologic disease (e.g.
lateralized disc), and “Happy Jack Skip.” Patient conformation can indicate a propensity for patel-
lar luxation. Dogs with genu varus may be more likely to have MPL; whereas, dogs with genu val-
gus may be more likely to have LPL.
Patellar luxation is graded 1–4 based on physical exam findings (Box 19.2). This categorization is
important as it aids decision-making regarding the course of treatment and the need for surgical
intervention. Some authors have also included the degree of skeletal deformities into this grading
system (Kowaleski et al. 2018); however, this system is more difficult to apply if the degree of
deformity does not match the palpation. Therefore, the grading should be primarily based on the
location and ability to reduce the patella. However, higher-grade luxations are generally associated
with more severe skeletal deformities, more severe clinical symptoms, and more obvious postural
abnormalities. To assess the patella for luxation, the patella needs to be identified and the stifle