Page 360 - Canine Lameness

332 19 Stifle Region

should be moved through ROM. The direction of location should be noted, realizing that bidirectional
luxation is possible. In normal dogs, identification of the patella is straight-forward, but in cases
with substantial periarticular swelling or high-grade luxations it can be difficult. In these cases, the
examiner should identify the tibial tuberosity and follow the patellar ligament proximally.
Some dogs will spontaneously luxate the patella as the stifle flexes and the quadriceps pulls the
patella out of the trochlear groove. Luxation is generally associated with a popping sensation
particularly in low-grade luxations. If the patella does not spontaneously luxate, the examiner
must check to see if it can be manually luxated. Since dogs with MPL tend to have patella alta, the
patella may luxate more easily when the stifle is in full extension (Mostafa et al. 2008). To luxate
the patella medially, the examiner extends the stifle, rotates the distal limb internally, and pushes
the patella medially. It is generally easiest to place the patella between the thumb and index finger
of one hand while using the other hand to manipulate the distal limb. The opposite is performed
for lateral luxation – partial flexion of the stifle, external rotation of the distal limb, and pushing
the patella laterally. Applying pressure through ROM should be performed if the diagnosis is not
obvious. Most animals will display patellar luxation in both standing and recumbent positions;
however, in some patients, the muscle tension will mask a patellar luxation when standing.
Therefore, evaluation should be performed in both positions if there is doubt about the diagnosis.
Similarly, very tense animals may need to be sedated to diagnose mild patellar luxation. Placing
STIFLE REGION ally can also be helpful for diagnosis. Depending on the degree and direction of dynamic patellar
the sedated dog in lateral recumbency with the affected leg down and “pulling” the patella medi-

instability, removing the tension of the quadriceps may make patellar luxation easier, or more
difficult. Therefore, in some animals, sedation or general anesthesia may make it actually more
difficult to palpate the luxation. As noted above, the patella should be reduced (if possible) and
the stifle evaluated for signs of CCLD (including drawer sign). In dogs with Grade 4 luxation, it
can be difficult to differentiate whether mild drawer instability results from the abnormal anat-
omy or from CCLD.

19.5.3 Diagnostics
Radiographs are critical for surgical planning but are also helpful to document osseous anatomy,
document secondary osteoarthritis, and evaluate for concomitant CCLD. However, they cannot be
used to rule out patellar luxation since with Grade 2 and 3 luxations the patella may temporarily
be located within the trochlear groove. In this capacity, radiographs can only be used to determine
that a patient does not have radiographic evidence of patellar luxation (i.e. “not a Grade 4”) or that
there is radiographic evidence of patellar luxation (i.e. “not a Grade 1”).
Standard orthogonal radiographs of the stifle should be obtained. The craniocaudal views are
used to evaluate the position of the patella. A lateral, flexed radiograph is used to assess for evi-
dence of joint effusion and degenerative changes. While both features are observed with patellar
luxation, they are generally mild and substantial changes indicate CCLD (Figure 19.14). The lat-
eral view can also be used to evaluate for patella alta or baja. This is accomplished by measuring
the patellar length (“PL”) and the distance from the distal apex of the patella to the tibial tuberosity
(i.e. the length of the patellar ligament, “PLL”). Ratios of the PLL:PL that are greater than 2.06
indicate patella alta (Mostafa et al. 2008), predisposing the patient to MPL. Similarly, values lesser
than 1.97 indicated patella baja, predisposing the patient to LPL.
If surgical correction is deliberated, particularly in large-breed dogs, additional radiographs of
the femur should be considered. Craniocaudal radiographs of the femur should be performed to
assess whether the degree of femoral varus (for MPL) or valgus (for LPL) requires surgical