Femoral neck anteversion angle

The femoral neck anteversion angle (or femoral anteversion, angle of anteversion, or angle of torsion of the femur) is a measurement used to assess femoral conformations, particularly in the study of predisposition to cranial cruciate ligament rupture in dogs. The femoral neck anteversion angle is the angle between the neck of femur and the femoral diaphysis. Radiography is the simplest method for measuring the femoral neck anteversion angle, but radiographs are subject to positioning errors that can affect the measured values. The assessment can therefore be made using a multiplanar reconstruction of the femur.

Several methods have been described for measuring the femoral neck anteversion angle

The first, radiographic method consists in taking two perpendicular views of the femur.

The first view is a ventrodorsal radiograph of the hips and femurs. 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 centered on the trochlea of the femur.

The second view is a radiograph of the femur in medio-lateral incidence. Positioning is considered satisfactory if the medial condyle and lateral condyle are superimposed.

On both views, a best fit circle outlining the head of femur is drawn to define its center (A). The anatomic femoral axes in the sagittal and frontal planes are determined. The distance (a) between the anatomic femoral axis in the sagittal plane and the center of the head of femur and the distance (b) between the anatomic femoral axis in the frontal plane and the center of the head of femur are determined. The femoral neck anteversion angle (alpha) is given by the following equation: tan-1 (a/bF).

The following values for the femoral neck anteversion angle have been published:

Labrador Retrievers 26.0 +/- 5.0 ° (Ragetly et al 2011)

Labrador retriever: 33.7 +/- 4.7° (Mostafa et al. 2018)

Dogs from 14 to 33kg: 31.3 +/-6.2° (18 to 47°) (Montavon et al. 1985)

Immature Serra da Estrela dogs: 29.9 +/-4.8° (19-40°) (Ginja et al. 2006)

The second method uses either radiography in axial projection, or multiplanar reconstruction obtained by CT scan on the femur.

For radiographic assessment, the limb is positioned so that the long axis of the femur is perpendicular to the table and parallel to the X-ray beam. The entire femur is visible, including the head of femur, neck of femur, medial condyle and lateral condyle, with the femoral shaft forming a concentric ring. Under fluoroscopic guidance, positioning can be confirmed and a distoproximal radiographic projection obtained along the long axis of the femur.

For CT evaluation, a transverse plane of the femur is used to visualize the entire femoral shaft as a straight cylinder by scrolling sequential images.

The femoral neck anteversion angle is the angle between the transcondylar axis of the femur and the femoral neck axis.

The following values for femoral neck anteversion angle assessed by CT scan have been published for different breeds of dog:

Combination of different breeds: 19.6 +/- 7.9° (Dudley et al. 2006)

Combination of different breeds: 26.86 +/- 11.46° (Kara et al. 2018)

Toy poodle: 19.8 +/- 4.6° (Yasukawa et al. 2016)

English bulldog: 11.4 +/- 6.4° (Lusetti et al. 2016)

English Staffordshire bull terrier: 26.0 +/- 3.4° (Newman and Voss 2017)

Chihuahua: 29.2 +/- 6.3° (Phetkaew et al. 2018)

Serra da Estrela immature dogs: 30.4 +/-4.2° (20-41°) Scanner (Ginja et al. 2006)