#345 Effect of Spondylotic Deformity on Load Sharing on Screws of a Lumbar Fixation Construct: A Biomechanical Study
Poster Presented by: A. Kiapour
A. Kiapour (1)
J. Aferzon (2)
V. Goel (1)
(1) The University of Toledo, Orthopedic Surgery & Bioengineering, Toledo, OH, USA
(2) Connecticut Spine Institute for Minimally Invasive Surgery, New Britain, CT, USA
Introduction: Lumbar spondylolysis is a common spinal disorder affecting around 3-6% of population worldwide. The symptomatic lumbar spondylosis is often associated with chronic back or leg pain. At the early stages of the disease conservative treatments may be pursued but in the cases which do not respond to long-term conservative treatment methods, surgical intervention is usually required. Spinal fusion (arthrodesis) has been the conven¬tional surgical treatment in such cases. Although studies have evaluated the biomechanics and clinical efficacy of fixation treatment methods for spondylolysis but the data on the load sharing on components of the fixation construct, such as pedicle screws is sparse. This study simulates specific clinical cases of spondylolysis to study hardware loads and hardware failure. Our particular interest is to analyze the effect of the variation in the sacral slope on the loading of the S1 screw.
Methods: A validated finite element (FE) model of ligamentous lumbar spine was used in this study. This model was modified to match sagittal CT and MRI images of a patient with lumbosacral spondylolysis. Intervertebral disc angle and endplate angle with respect to the horizontal transverse plane was measured at each segment. This data was used to alter the geometry of the FE spine to simulate spondylolysis deformity and lumbar lordosis (Fig 1). A posterior screw-rod fixation construct extending from L4 to S1 was implanted into spine with and without deformity and the instrumented spines were subjected to 400N follower pre-compressive load plus a 10Nm bending moment to simulate physiological loadings of flexion, extension, left & right bending and left & right rotation. The axial load and axial torque subjected to screws of lumbosacral level in each loading were computed and compared between the cases.
Results: The axial load on screws of L5 level varied from a push in load of 66N to a pull out load of -38N in different physiological loadings for the spine without deformity; the load ranged changed from 62N (push in)to -42N (pull out) in the spine with deformity. The sacral screws observed an axial load ranging from 80N to -33N at different physiological loads in spine without deformity versus 117N to -36N range in spine with deformity. The peak torque on screws at different loadings achieved the maximum of 0.80Nm (tightening) and a minimum of -0.76 (untightening) at L5 in without deformity and 0.82Nm to -0.78Nm in deformity cases respectively. The corresponding value at sacral screws was 2.1Nm to -2.1Nm in without deformity and 2.2Nm to -2.2Nm in with deformity case.
Conclusion: The axial pull out load and untightening axial torque can determine the risk of screw failure in a rigid fixation construct. This preliminary feasibility analysis showed increased sacral screw loads in deformity model. This model will be extended to study sacral loads through simulation of additional two patients with different sacral slopes.
Fig 1. Intact FE spine & CT of spondylotic spine