EFFECTS OF PELVIC AND FEMORAL POSITIONING ON CANINE NORBERG ANGLE MEASUREMENTS AND TEST–RETEST RELIABILITY: A COMPUTED TOMOGRAPHY-BASED SIMULATION STUDY

Abstract

Canine hip dysplasia is a common disease in dogs, often diagnosed by using the Norberg angle (NA), an index for the laxity of the hip joint. Measurement of the NA can be affected by the pelvic and femoral positioning during imaging, the effects and test–retest reliability of which have not been documented. To bridge the gap in knowledge, computed tomography data from 11 Labrador Retriever dogs were obtained and used to generate synthetic ventrodorsal radiographs of the hip for NA measurements via a perspective projection model. Twenty-five synthetic radiographs of the hips were generated at positions defined by combinations of five pelvic tilt angles (-20° to 20° at 10° intervals) and five femoral elevation angles (from full extension to 40° at 10° intervals). For each radiograph, the NA was measured three times by each of the two experienced veterinarian examiners. It was found that both the increase in caudal pelvic tilt and femoral elevation increased the measured NA, although the intra- and inter-examiner reliability was very good for a given hip position. The current results suggest that careful positioning of the pelvis and femur during radiographic imaging is critical for accurately measuring the NA, and thus the laxity of the hip, for the clinical diagnosis of hip dysplasia.