The Quadriceps Angle: Reliability and Accuracy in a Fox (Vulpes vulpes) Pelvic Limb Model


  • Study performed at Department of Small Animal Clinical Sciences, LIFE Faculty, Copenhagen University.

  • Financial support was received from the Danish Kennel Club.

Corresponding Author

James Miles, BVetMed, Universitetshospital for Familiedyr, Dyrlægevej 16, 1870 Frederiksberg C, Denmark




To evaluate the effect of measurement technique and limb positioning on quadriceps (Q) angle measurement, intra- and interobserver reliability, potential sources of error, and the effect of Q angle variation.

Study Design

Cadaveric radiographic study and computer modeling.


Pelvic limbs from red foxes (Vulpes vulpes).


Q angles were measured on hip dysplasia (HD) and whole limb (WL) view radiographs of each limb between the acetabular rim, mid-point (Q1: patellar center, Q2: femoral trochlea), and tibial tuberosity. Errors of 0.5–2.0 mm at measurement landmarks alone and in combination were modeled to identify the effect on Q angle. The effect of measured Q angles on the medial force exerted on the patella (FMEDIAL) was calculated.


The HD position yielded significantly (P < .001) more medial Q angles than the WL position. No significant difference was observed between Q1 and Q2, but Bland–Altman plots indicated they were not equivalent. Intra- and interobserver agreement was substantial. Q2 errors were inherently greater than Q1: the mid-point and tibial tuberosity are the most important sources of Q angle variability. Increasing Q angles significantly increased the exerted FMEDIAL (P < .0001, gradient 1.7%).


Measurements are reliable, but Q2 is more prone to error than Q1, and the 2 measurement techniques are not interchangeable. Positional errors must be kept below 1.3 mm (Q1) or 0.8 mm (Q2).