Pull-out strength of sacral implants using Cotrel-Dubousset fixation devices

Our aim was to determine the bone structure of the sacrum and the optimum location in the sacrum for the insertion of lumbosacral screws in osteosynthesis. Anatomical tests were performed on mortuary specimens at the Medical School in Nice, France, on frozen sections of sacrum and in computed tomographic (CT) sections with 3-dimensional reconstruction. Density assessment of different structures from the first and second sacral vertebrae (S1, S2) was performed and expressed in Hounsfield units. The ala and the lateral part of S1 contained yellow marrow forming a fatty mass bounded by the cortex of the sacroiliac joint at the linea terminalis and by the cancellous bone in the pedicles and the body of S1 and S2. These studies were undertaken prior to the experimental tests. The experimental tests were made using Cotrel-Dubousset instrumentation to determine the pull-out strength of sacral screws with a 7-mm diameter by assessing pull-out strengths of paired screws arranged either unilaterally with one screw in S1 and one in S2 or symmetrically to the left and right in S1. The devices demonstrating the best pull-out strength were the "corporopedicular" screw in S1 and the caudal lateral oblique screw in S1 passing through the sacroiliac joint. The paired screws with the best pull-out strength were those which included a corporopedicular screw in S1.