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The quadriceps angle: reliability and accuracy in a fox (Vulpes vulpes) pelvic limb model
James E Miles1, Jane V Frederiksen, Bente Jensen
1Department of Small Animal Clinical Sciences, Faculty of Life Sciences, University of Copenhagen, Denmark. jami@life.ku.dk
Objectives:
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.
Animals:
Pelvic limbs from red foxes (Vulpes vulpes).
Methods:
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 (F(MEDIAL)) was calculated.
Results:
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 F(MEDIAL) (P < .0001, gradient 1.7%).
Conclusions:
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).
