Mechanical femoral axis

The mechanical femoral axis is a landmark used to assess angular deformities of the femur and provide guidance for corrections. It can be determined in both the frontal and sagittal planes.

In the frontal plane, the mechanical femoral axis is determined from a ventrodorsal radiograph or multiplanar reconstructed CT images of the femur. 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 centered on the trochlea of femur.

In the sagittal plane, the mechanical femoral axis is determined from a medio-lateral radiograph or multiplanar reconstruction of the femur. In radiography, the X-ray beam is centered on the knee joint. The medio-lateral projection is considered acceptable when the medial condyle and lateral condyle are superimposed.

In the frontal plane:

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.

In the sagittal plane :

Method #1 (Peterson et al. 2020)

The mechanical femoral axis is determined by drawing a straight line from the center of the head of femur to the point of intersection between the distal anatomical femoral axis (Peterson et al 2020) and the caudal border of the femoral condyles.

Method #2 (Yasukawa et al. 2016)

The mechanical femoral axis is determined by drawing a straight line between the center of the head of femur and the most caudal aspect of the femoral condyles distally.