Alterations in lumbar spine mechanics due to degenerative disc disease
Ayturk, Ugur Murat
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Puttlitz, Christian MatthewDegenerative disc disease is a major source of low back pain. It is hypothesized to significantly alter the biomechanics of the lumbar spine both at the tissue and motion segment (multi-vertebral) levels. However, explicit correlations between the former and the latter has not been established, and this critical link is only possible through modeling the intervertebral disc tissue behavior within a constitutive framework and implementing it in a finite element model of the lumbar spine. In order to develop a better appreciation of the biomechanics of disc degeneration, the main objectives of this dissertation work were to investigate the degenerative disease related mechanical alterations on lumbar spine through finite element modeling and experimentation, and evaluate the contemporary treatment strategies. To meet this objective, a finite element model of the healthy human lumbar spine was generated based on computed tomography (CT) imagery. Mesh convergence was verified based on strain energy density predictions. Kinematic and mechanical predictions of clinical interest, including range of motion and intradiscal nuclear pressure, were validated under pure moment loading. The mechanical properties of healthy and degenerated annulus fibrosus tissue were quantified using an orthotropic continuum model, with empirical determination of the requisite material coefficients derived from biaxial and uniaxial tension tests. The resultant material models were implemented into the validated finite element model in order to simulate disc degeneration at the L3-L4 level. At the tissue level, degeneration was found to significantly increase the dispersion in the collagen fiber orientation and the nonlinearity of the fiber mechanical behavior. At the motion segment level, degeneration increased the mobility of the spine, with concomitant increases in the local stress predictions in the annulus and facet force transmission. Our results were in good agreement with the clinical findings of instability and injury to the intervertebral disc due to degeneration. Total disc replacement was also considered as a treatment option within the aforementioned finite element framework. The model predictions indicated that single and two-level disc replacement restored motion at the treated levels, while linearizing the kinematic response and increasing the facet force transmission. The data reflect that the successful surgical outcome is most likely obtained when maximum preservation of native disc tissue is achieved during implantation of the prosthetic device.