Comparison of measurements of the laxity index with 3 versus 5-point circles on stress radiographs performed with the Vezzoni- modified Badertscher hip distension device

Introduction/Purpose
The laxity index (LI) obtained with Vezzoni-modified Badertscher distension device (VMBDD) presents many advantages compared to the distraction index obtained on the PennHIP distraction radiograph (only 2 X-rays, only a single operator, no legal obligations,…). In a recent study, they described a measurement method of the LI on stressed radiographs performed with VMBDD of which a step consisted to delimit the femoral head by a circle from 5 predefined points. However, the majority of image analysis software package allows 3-point circles with the exception of Digimizer which only the trial version is free. The aim of this study was to compare the LI obtained with measurements made with 3 versus 5-point circles.
Methods
The radiographic images used in this study were acquired on 15 dogs presented to the Veterinary University Clinic in Liège, for obligatory hip screening and lameness from the hips. The 30 LI were measured independently by a last year student and a senior clinician in radiology with 3 and 5-point circles. 5 dogs obtained official LI from the Belgian kennel club SRSH-KMSH (5-point circles). Non-parametric Wilcoxon tests were used to compare the measurements.
Results
The LI measurements performed by the two operators were not significantly different (p=0,45 and 0,1 for the left and right hip, respectively). The LI made from 3-point circles are not significantly different from those to 5 points (p=0,5 for both hips). The measurements were not significantly different from those of the SRSH-KMSH.
Discussion/ Conclusion
These results suggested that usual image analysis software package with 3-point circles allowed correct measurements. They confirmed that a high degree of experience in radiographic assessment is not necessary to perform the LI.