A method of digital image measurement of specimen deformation based on CCD cameras and Image J software was
developed. This method was used to measure the biomechanics behavior of human pelvis. Six cadaveric specimens from
the third lumbar vertebra to the proximal 1/3 part of femur were tested. The specimens without any structural
abnormalities were dissected of all soft tissue, sparing the hip joint capsules and the ligaments of the pelvic ring and
floor. Markers with black dot on white background were affixed to the key regions of the pelvis. Axial loading from the
proximal lumbar was applied by MTS in the gradient of 0N to 500N, which simulated the double feet standing stance.
The anterior and lateral images of the specimen were obtained through two CCD cameras. Based on Image J software,
digital image processing software, which can be freely downloaded from the National Institutes of Health, digital 8-bit
images were processed. The procedure includes the recognition of digital marker, image invert, sub-pixel reconstruction,
image segmentation, center of mass algorithm based on weighted average of pixel gray values. Vertical displacements of
S1 (the first sacral vertebrae) in front view and micro-angular rotation of sacroiliac joint in lateral view were
calculated according to the marker movement. The results of digital image measurement showed as following: marker
image correlation before and after deformation was excellent. The average correlation coefficient was about 0.983.
According to the 768 × 576 pixels image (pixel size 0.68mm × 0.68mm), the precision of the displacement detected in
our experiment was about 0.018 pixels and the comparatively error could achieve 1.11&perthou;. The average vertical
displacement of S1 of the pelvis was 0.8356±0.2830mm under vertical load of 500 Newtons and the average micro-angular
rotation of sacroiliac joint in lateral view was 0.584±0.221°. The load-displacement curves obtained from our optical measure system matched the clinical results. Digital image measurement of specimen deformation based on CCD
cameras and Image J software has good perspective for application in biomechanical research, which has the advantage
of simple optical setup, no-contact, high precision, and no special requirement of test environment.