Mechanical femoral axis

The mechanical femoral axis is a landmark used to assess angular deformities of the femur and provide guidance for corrections. It is determined in the frontal plane from a ventrodorsal radiograph or multiplanar reconstructed CT images of the hip and femurs. For radiographic evaluation, the dog is placed dorsal recumbency and the pelvic limbs are extended caudally, the femurs are parallel to each other. Positioning is considered satisfactory if :

• the pelvis is symmetrical (iliums and obturator foramina are symmetrical)

• the femur cortex pass through the middle of the sesamoid bones of the gastrocnemius muscle

• the patellas are centred on the trochlea of femur.

A best fit circle outlining the head of femur is drawn and the center of the head of femur is determined. The mechanical femoral axis is determined by a line drawn from the center of the femoral head to the center of the intercondylar fossa.