Correlation Between Pelvic Orientation And Acetabular Index Measurement In Children

Objective To investigate the correlation between pelvic orientation and AI measurment by using Digital Reconstructed Radiographs(DRRs)generated from pelvic CT model,and try to find some reliable parameters on plain radiographs to predict pelvic orientation.Methods We retrospectively identified 33 cases who received CT examinations in our department between January 2015 and December 2015 and met the including and excluding criteria.These cases were divided into different groups according to age and acetabular pathology.After importing CT data into post-processing workstation,we reconstructed a virtual pelvic model.Fixing the model in a standard neutral position,we rotated and tilted the model around the axis respectively.DRRs were generated at every 3 degrees of pelvic tilt and rotation.Acetabular index,horizontal diameter(Dh)and vertical diameter(Dv)of bilateral obturator formamen,and the vertical distance between upper boarder of pubic symphysis and H line(h)were measured on each DRR by two independent observers(A,B).Rotation index(Rr=right Dh/left Dh),tilt index(Rt=h/Dv),intraobserver error and interobserver error were calculated afterwards.Results For tilt and rotation up to 12°,the mean AI decreased when pelvis tilted forward and increased when pelvis tilted backward.For rotation,the mean AI increased when the pelvis was rotated towards the hip being assessed,and decreased on the opposite side.The intraobserver error decreased when the pelvis tilted backward.The intraobserver of dysplastic hips was larger than that of normal hips.A pelvis can be predicted to be within 6° of tilt and rotation if the Rt value is between 1.1 and 1.9 for children over 3 years or between 0.9 and 1.5 for children aging from 3 to2 years,or between 0.8 to 1.3 for children younger than 2 years;and at the same time Rr value between 0.7 and 1.5.In this case,AI measurement is influenced by pelvic malposition slightly with an intraobserver error of ±2.0°and interobserver error of ±3.0°.Conclusion Pelvic malposition can affect AI measurment in a certain rule.Acetabular pathology and direction of pelvic movement can influence the errors of AI measurement.Rt is an age-related parameter while Rr is not.On plain radiographs,it is reliable to predict pelvic tilt and rotation by using Rt and Rr respectively.