Effects of sakroiliak fixation to the inferior screw tansion in lumbosakral and lumbopelvic fixation

In modern spinal surgery era, posterior spinal instrumentation with pedicle screw fixation is one of most frequently used technique for various spinal disorders such as scoliosis, kyphosis, or degenerative spine. Pedicle screw fixation improves fixation and facilitates correction in spinal disorders. Long instrumentations that crosses the lumbosacral junction such as lumbosacral or lumbopelvic fixations bear the increased risk of complications like fracture of sacrum, pullout of sacral or iliac screw or pseudoarthrosis. These complications affect the succes of surgery and outcome. We conducted a biomechanical study in long instrumentations that crosses the lumbosacral junction that may be helpful in decreasing the complications and improving fiksation methods. In this study, we analysed the difference in strain at the most caudal screw of posterior spinal instrumentation in a sheep spine model in addition of a sacroiliac joint fixation. Four lower lumbar vertebra together with sacrum and pelvis of fresh freezed sheep spines were used in the study. The specimens were fixed from upper and lower levels to a standardized holders by a cement. There were two groups with 8 specimens in each with posterior instrumentation with pedicle screws. Fixation to lumbar 5-6 and sacral 1 vertebra was done in first group and lumbar 5-6 and iliac bone screw in the second group. Strain gauge with three sensors in each plane was fitted to lower screw in each group. Axial loading was applied and results of strain gauge reading in three planes were recorded. Then sacroiliac fixation with cortical screws were added to each specimen and results were recorded again after axial loading. Results revealed a change in loading of the lower screw, a decrease of loading of iliac screw after sacroiliac fixation. As a result, sacroiliac screw fixation decreases the loading in iliac screw in posterior spinal instrumentations in our biomechanical study. The possible advantage of this finding as a decrease in complication rates in iliac screws may be delineated by further large animal or human cadavar studies studies.