Document Type

Thesis

Date of Degree

Spring 2017

Degree Name

MS (Master of Science)

Degree In

Biomedical Engineering

First Advisor

Tae-Hong Lim

Abstract

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.

Pages

xiii, 73 pages

Bibliography

Includes bibliographical references (pages 71-73).

Copyright

Copyright © 2017 Ino Song

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