Date of Degree
MS (Master of Science)
First Committee Member
Kyung K. Choi
Second Committee Member
Nicole M. Grosland
Third Committee Member
M. L. (Suresh) Raghavan
Fourth Committee Member
The ligamentous human lumbar spine is considered as a long and slender column, which can be buckled when subjected to the axial compressive load even less than 100N. However, previous in vivo study showed that the compressive force acting on the spine predicted by intradiscal pressure was exceed 2600N. Meanwhile, recent experiments suggested that, when the compressive force is subjected to the lumbar spine along the spinal curvature (follower load), the lumbar spine may support up to a compressive force of 1200N without buckling while maintaining its flexibility. Since such a follower load is directed tangential to the curved column over the entire length, the lumbar spine subjected to a follower load should experience only pure compressive force components with zero shear force components.
It is generally agreed that the ligamentous lumbar spine can be stabilized by applying the muscle forces (MFs) in vivo creating follower compressive loads (FCLs). In previous studies, computational model of the lumbar spine showed the feasibility for spinal muscles to stabilize the lumbar spine via the FCL mechanism, which supports the hypothesis of FCLs as normal physiological loads in the spine in-vivo. In addition, the muscle forces of short intrinsic muscles (SIMs), such as interspinales, intertransversarii, and rotatores may increase the stability of the lumbar spine (i.e., deflection of the spinal column or trunk sway) significantly. However, the mechanical roles of SIMs for spinal stability have not been quantified and understood well.
A finite element (FE) model with optimization model of the lumbar spinal system was used in this study. Both models were consisted of 122 pairs of spinal muscle fascicles including 54 SIMs fascicles. The variation of spinal muscle strength was simulated by changing the values of MFCs of long muscles as well as SIMS from zero to 90 N/cm2. Five different MFC conditions of both long muscles and SIMs in the spinal system were investigated in five different postures, which are neutral standing, flexion 40°, extension 5°, left axial rotation 10°, and right lateral bending 30°. The trunk displacement (TD) and joint loads including joint reaction forces (JRFs) and moments (JRMs) predicted from 25 cases of MFC variation were compared in order to investigate the effect of the strength of spinal muscles on the stabilization of the lumbar spine in a given posture.
The results showed that small trunk sways (< 2mm) were predicted when MFCs of both long muscles and SIMs were average or higher regardless of the spinal postures. In contrast, no optimum solution or unstable conditions were predicted in many cases of the weakening of the long muscles, especially in flexion and lateral bending postures. Although the FCLs were created in most of the cases regardless of MFC-S when working with strong long muscles, higher joint loads were predicted as a result of weakening of SIMs. In addition, even if the long muscles were strong, absence of SIMs induced spine buckling in some cases of extension and axial rotation postures.
The results from this study imply that although the effect of MFCs variation of long muscle and/or SIMs was varied depending the spinal postures, the simultaneous use of both SIMs and long muscles is necessary for stabilization of the spine in any physiological posture with minimum joint loads for maximum safety.
Low back pain (LBP) is one of the most prevalent afflictions, which cause billions dollars of healthcare cost in each year. Even though many clinical strategies for LBP treatment have been suggested, current understandings of the basis of LBP problems are limited and insufficient. Out of possible causes of LBP under consideration, the basis of the problem commonly agreed in the field of spine research is the mechanical insufficiency of the spinal column, which is known too flexible to support the upper body weight.
Previously, it was suggested that the normal spinal load could be the compressive forces whose direction is parallel to the curvature of the lumbar spine while the shear forces whose direction is perpendicular to the spinal curvature are abnormal forces. Although those biomechanical loads on the spine are known to be closely associated with the spinal muscle control system, the significant role of back muscles, especially the short intrinsic muscles (SIMs), has not been studied sufficiently. For these reasons, in this study, the effect of variations in the strength and type of spinal muscles for spinal stabilization was investigated in various postures using mathematical and computational methods.
Throughout this study, it was found that the variation of maximum force capacity of spinal muscles affected to the spinal stability as well as the joint loads on the spine, although the magnitude of the effect was varied depending on the muscle types (Long muscles and SIMs) or the spinal postures. Therefore, it can be concluded that the concurrent use of both SIMs and long muscles is necessary for spine stabilization in any physiological posture with minimum joint loads for maximum safety.
xiii, 73 pages
Includes bibliographical references (pages 71-73).
Copyright © 2017 Ino Song
Song, Ino. "The effect of variations in the strength and type of spinal muscles on the stabilization of the lumbar spine via follower compressive load mechanism." MS (Master of Science) thesis, University of Iowa, 2017.