Digital marker tracing combined with center-of-mass algorithm in measuring pelvic displacement

Objective:To evaluate the reliability of the digital marker tracing method combined with center-of-mass algorithm in measuring human pelvic displacement.Methods: Six cadaveric specimens of the third lumbar vertebra to the proximal 1/3 part of femur were used in present study.The specimens had no structural abnormality and all the soft tissues were dissected,reserving the hip joint capsules and the ligaments of the pelvic ring and floor.Markers with black dots against white background were used to mark the key points of the pelvis.Axial loadings from the proximal lumbar(simulating the two-legged stance) were applied by MTS in the gradient of 0 N to 500 N.Images of the front and lateral views were obtained using two CCD cameras.Using Image J software,we calculated the vertical displacement of S1(the first sacral vertebrae) in the front view and the micro-angular rotation of sacroiliac joint in the lateral view by measuring the marker's movement.Results: There was good correlation between the marked points before and after deformation of the pelvis,with the average correlation coefficient being 0.983.Based on the 768×576 pixels,pixel size(mm) 0.681 5×0.681 5 image,the accuracy of the displacement was about 0.018 pixels and the comparative error could reach 1.11‰.The load-displacement curves obtained in this study accorded well with the clinical results.The pelvic load-displacement curve exhibited approximately a linear behavior;the sacroiliac joint load-angular rotation curve in the sagittal plane exhibited a non-linear behavior.Under a vertical load of 500 N,the average vertical displacement of S1 of the pelvis was(0.835 6±0.283 0) mm and the average micro-angular rotation of sacroiliac joint in lateral view was(0.584±0.221)°.Conclusion: Digital marker tracing method combined with the center-of-mass algorithm is a simple,accurate method for measurement of pelvic displacement,which can be widely used in biomechanical research.