Distal Femoral Osteotomy for Patellar Luxation

Pathologic Conformations
Associated with Patellar Luxation
Proper anatomic alignment of the quadriceps/patellar mechanism with the underlying skeleton promotes patellar stability. When the quadriceps muscle contracts, it seeks to form a straight line with the patella positioned between its origin and insertion (Fig. 1A). If the long axis of the quadriceps is not centered over the trochlear sulcus, there is an imbalance in muscular forces favoring patellar luxation (PL). Excessive femoral varus and external femoral torsion are commonly associated with medial patellar luxation (MPL). Distal femoral varus moves the long axis of the quadriceps muscle medial to the trochlear sulcus (Fig. 1B). This discrepancy causes a strong medial tension upon the patella during muscular contraction. The strength and length of the quadriceps muscle in large breed dogs amplifies this discrepancy as compared to small breeds of dogs. Femoral osteotomy is used to treat MPL complicated by excessive femoral varus (Fig. 2).

Radiographic evaluation
To date, no radiographic method for measurement of femoral varus/valgus has been validated in dogs with patellar luxation. Work in our laboratory has shown radiographic measurement of canine femoral varus is repeatable within and between investigators, but does not predict true anatomic varus (at least in clinically normal large breed dogs). In that study, the femoral varus measurements in normal cadavera represent a small range of measurements in which to correlate radiographic femoral varus (RFV) with true anatomic femoral varus (AFV). Therefore, when dogs with excessive femoral varus are added into future study populations, the true accuracy of RFV in predicting AFV should be clarified.
The following reflects our current understanding and will likely be modified as ongoing investigations are completed. Accurate radiographic evaluation of limb alignment requires general anesthesia because subtle alterations in patient position induce markedly erroneous femoral alignment measurements. A craniocaudal (CrCd) radiograph from the hip to the proximal tibia is performed. The femora should be parallel to the spinal axis (neither abducted nor adducted) and to the radiographic cassette (maximal hip extension). In dogs with restricted hip extension proper CrCd positioning requires the patient’s torso be lifted to sitting position. Alternatively, a horizontal beam method is used (when the radiography unit allows for movement of the radiographic tube). Properly positioned radiographs show the tip of the lesser trochanter, the vertical walls of the intercondylar notch distinctly parallel and the fabellae split by the medial and lateral cortical margins of the femur. [...]