Femoral neck axis

The femoral neck axis is a landmark used to assess angular deformations of the femur and guide corrections. It is determined from radiographs and multiplanar reconstructions of the femur. In particular, it is used to determine and measure the femoral inclination angle.

In the frontal plane, the femoral neck axis is determined from a ventrodorsal radiograph of the hips. The dog is positioned in dorsal recumbency, pelvic limbs extended caudally, femurs parallel to each other. Positioning is considered satisfactory if :

• the pelvis is symmetrical: the 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 the femur.

Several methods have been described for determining the femoral neck axis in the frontal plane:

Montavon method

A circle delimiting the head of femur as closely as possible is drawn and the center of the head of femur is determined (A). A point (B) is chosen on the contour of the trochanteric fossa at its junction with the intertrochanteric crest. An arc of a circle centred on A and with a diameter of B is drawn. This arc intersects the medial border of the neck of femur (C). The femoral neck axis is the line starting from A and bisecting the arc BC.

Symax method

A circle delimiting the femoral head as closely as possible is drawn and the center of the femoral head is determined. A second circle is drawn at the proximal end of the femur, so as to touch the edges of the femur in three places (proximal, medial and lateral). The center of this second circle is determined. The femoral neck axis is the line joining the two previously determined centers.

Hauptman A method

The contours of the femoral neck and head are drawn. A line representing the diameter of the femoral head is drawn and the mid-point of this line is marked. The most distal point of the trochanteric fossa is identified. A second line is drawn from this point, perpendicular to the femoral shaft, to the medial cortex of the femur. The middle of this second line is also marked. The femoral neck axis is the axis connecting the previously marked head and neck bisection points. The Hauptman B method is considered less accurate for measuring femoral inclination angle.

In the sagittal plane, the femoral neck axis is determined from a medio-lateral radiograph of the entire femur. Positioning is considered satisfactory if the medial condyle and lateral condyle are superimposed.

A circle delimiting the femoral head as well as possible is drawn and the center of the femoral head is determined. The arc of the circle which cuts the femoral neck in two places is defined and its chord drawn. The femoral neck axis is the line connecting the center of the femoral head and the middle of the chord previously traced.