Norberg angle

The Norberg angle is a measurement used to assess the anatomy of the hip joint in dogs. It quantifies the extent of lateral and dorsal coverage of the femoral head and is used to score coxofemoral dysplasia. It is used to measure femoral head subluxation in dogs. Andronescu et al. (2015) showed that the 32-week dorsal acetabular sector angle and Norberg angle combination was the most effective predictor of the presence of osteoarthritis at maturity.

Norberg angle is determined from a ventrodorsal radiograph of the hips. 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.

A best fit circle delimiting the head of femur is drawn and the centre of this circle is positioned. Norberg angle is the angle formed by a horizontal line connecting the centers of the right and left femoral heads and a line connecting each femoral head center to the corresponding craniolateral acetabular margin.

Comhaire and Schoonjans (2011) published the following correlation between the Fédération Cynologique International classification and the Norberg angle in a population of 4372 dogs:

FCI Class : Mean Norberg Angle +/- standard deviation (Reference Interval)

• A1: 107,33 +/- 3,52 (100,4 to 114,2)

• A2:106,09 +/- 3,40 (99,4 ro 112,7)

• B1: 103,70 +/- 3,54 (96,7 to 110,0)

• B2: 103,26 +/- 3,55 (96,2 to 110,2)

• C1: 98,54 +/- 4,25 (90,2 to 107)

• C2: 100,14 +/- 4,98 (90,1 to 110,0)

• D: 94,54 +/- 6,48 (81,6 to 107,4)

• E: 89,14 +/- 6,39 (75,7 to 102,1)