Initial Stability Analysis of Spine Pedicular Screws Using Modal Analysis Method

Low back pain is a common medical problem. There is no clear cause for the back pain problem, but in most cases, spinal instability can be noted. Lumbar spine fixation is performed to treat some problems of low back pain with or without surgery. The pedicle screw fixation devices are the dominant stabilization systems for a wide variety of defects in human spinal column. Accordingly, pedicle screw design has been the main parameter affecting the fixation strength as measured by the pullout force and insertion torque, sometimes with contradicting results. Five different designs of pedicle screw were used. To reduce the effect of bone quality variations, the bovine tibia cortical bone was chosen. After recording maximum torque during screw insertion in the bone samples, first modal analysis was carried out by recording the sounds of the impact. Then, the conventional pullout test was performed for the comparison purpose. The effect of different designs on the natural frequency derived from the modal analysis and ultimate pullout force were respectively determined. The highest natural frequency of 1653 Hz and pull-out strength of 884 N were all obtained in conical thread screw. In addition, it was confirmed that self-tapping is not a parameter that can affect the fixation quality. Modal analysis was found to be a reliable, non-destructive and accessible method with excellent repeatability pattern which could be considered as a prospective replacement of the pull-out test in the assessment of the pedicle screw fixation stability in vitro and potentially in-vivo.