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Cervical intervertebral disc herniation in response to severe loading in compression and bending
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Journal of Biomechanics 2006, Vol. 39 (Suppl 1)
[2] T. Videman, M. Nurminem. The occurence of anular tears and their relation to lifetime back pain history: A caveric study using barium sulfate discography. Spine 2004; 29: 2668-76. [3] Y. Schroeder, W. Wilson, J.M. Huyghe, F.P.T. Baaijens. Osmoviscoelastic finite element model of the intervertebral disc. Eur. Spine J. 2006; in press. 5863 Tu, 08:30-08:45 (P15) Analysis of anular lesions in the L4/5 intervertebral disc: a hyperelastic model J.P. Little 1, C. Adam 1, J. Evans 1, G. Pettet 2, M. Pearcy 1. 1School ef
Engineering Systems and 2School of Mathematical Science, QUT, Brisbane, Australia Introduction: Low back pain (LBP) affects a large portion of the population and can result from degeneration of the intervertebral discs. Degeneration of the discs may be characterized by a loss of hydration of the nucleus pulposus and the presence of anular lesions which are tears in the anulus fibrosus. As the relationship between LBP and anular lesions has not been established a finite element model (FEM) of an L4/5 intervertebral disc was developed to explore the possible effects of anular lesions on the mechanical behavior of the disc. Particular care was taken to represent the nonlinear nature of the disc materials and geometry and to simulate physiological ranges of motion. Methods: An anatomically accurate transverse profile for the disc FEM was derived from transversely sectioned human cadaveric discs. Model geometries simulated a healthy disc and degenerate discs with either a rim, radial or circumferential lesion. The anulus fibrosus ground substance was represented using a 2nd-order polynomial, hyperelastic strain energy function. For the healthy disc, the nucleus was modeled by a hydrostatic pressure and in the degenerate models this pressure was set to zero. Loading conditions simulating the physiological ranges of flexural motion were applied to the models and the corresponding peak moments compared. Results: Small decreases in peak resistive moment in the degenerate disc models indicated that the mechanics of the disc are minimally affected by the presence of anular lesions. Loss of the nucleus pulposus pressure had a much greater effect on the disc mechanics than the presence of anular lesions alone. Conclusion: These results suggested that were anular lesions to develop prior to the degeneration of the nucleus they would have minimal effect on the mechanics of the whole disc. With the degeneration of the nucleus, the disc stiffness will be greatly reduced and the outer innervated anulus or surrounding osseo-ligamentous structures may become overloaded and thus painful. In particular the strains developed in the vicinity of annular lesions may be especially elevated.
7042 Tu, 08:45-09:00 (P15) On hyperelastic constitutive modeling of annulus fibrosus F.C. Caner 1,2, Z.'~ Guo 3, B. Moran 3, Z.P. Bazant 2, I. Carol 1. 1School ef Civil
Eng., Technical University of Catalonia, Barcelona, Spain, 2Dept. of Civil & Env. Eng., Northwestern University, Evanston, IL, USA, 3Dept. of Mechanical Eng., Northwestern University, Evanston, IL, USA Constitutive modeling of human annulus fibrosus (AF) is of interest to understand the development of pathological behavior and thus to device effective biomechanical treatments which has proven to be challenging. The classical constitutive modeling approach based on strain invariants with discrete fibers has been shown unable to predict experimental data. For example, when such models are calibrated to fit the uniaxial behavior of AF, they often fail in predicting data from biaxial tests or evolution of change of angle between the fibers in uniaxial tests. This study reports two different approaches to constitutive modeling of AF in the elastic range and compares their predictions to the available experimental data. First, a classical model as described above is enriched by providing proper fiber-matrix interaction. Next, a non-classical model, microplane model, is used to predict the experimental data. It is shown that, both of these two distinct approaches are able to represent the experimental data well. In the microplane model, the interaction of distributed fibers through kinematic constraint and subsequent homogenization of the microplane response automatically provides the required additional shear stiffness. We conclude that (i) for classical approaches, in addition to the parallel coupling of fiber and matrix, an adjustable fiber-matrix interaction effect must be considered; (ii) the representation of physical fiber distribution in AF in the modeling becomes essential due to additional shear resistance provided by interaction of distributed fibers.
Oral Presentations
7409 Tu, 09:00-09:15 (P15) Cervical intervertebral disc herniation in response to severe loading in compression and bending P. Pollintine, D. Skrzypiec, P. Dolan, M.A. Adams. Department of Anatomy,
University of Bristol, UK Introduction: The cervical spine can be severely loaded in bending during sporting injuries and "whiplash". Compressive loading could also be high if some advanced warning of impact stimulated vigorous ("protective") contraction of the neck muscles. Combined bending and compression can cause some lumbar discs to herniate in-vitro [1] but the outcome depends on spinal level, and may not be applicable to cervical discs. We test the hypotheses: (a) that cervical discs can prolapse in-vitro, and (b) that prolapse leads to irregular stress distributions inside the disc. Material and Methods: Human cervical "motion segments" (two vertebrae and intervening soft tissues) were obtained from cadavers aged 51-88yrs. Specimens were secured in cups of dental stone and subjected to static compressive loading (150N) for 20s. During this time, the distribution of vertically-acting compressive 'stress' was recorded along the postero-anterior diameter of the disc by pulling a 0.9 mm-diameter pressure transducer through it [2]. Injury was induced by compressing each specimen at 1 mm/s while positioned in 200 flexion, 150 extension or 80 lateral bending. The distribution of compressive stress within the disc was then re-measured. Specimens were sectioned at 2 mm intervals in order to ascertain soft tissue disruption. Results: In all six specimens tested to date, one or both of the apophyseal joint capsules were ruptured by the complex loading. Intervertebral disc prolapse also occurred in all six specimens, with the herniated nucleus appearing on the anterior, posterior and postero-lateral disc surface in extension, flexion and lateral bending respectively. All modes of failure affected intradiscal stresses: on average, nucleus pressure decreased by 75±7%, while stress concentrations in the annulus increased by 130±21%. Discussion: These preliminary results confirm that severe complex loading can cause cervical discs to prolapse. No particular state of disc degeneration is required, provided the loading is sufficiently severe. Indeed, the altered stress distributions suggest that cell-mediated changes would probably follow prolapse. References [1] Adams MA, Hutton WC. Spine 1982; 7(3): 184-91. [2] Adams et al. Spine 1996; 21(4): 434-8.
5507 Tu, 09:15-09:30 (P15) Intradiscal pressures in rat tail discs measured using a miniaturized fiber-optic sensor A.H. Hsieh 1,2, D.A. Ryan 1,2, Z. Chen 2, Y. Liu 2, S.C. Nesson 2, M. Yu 2.
1Bioengineering Graduate Program, University of Maryland, College Park, MD, USA, 2Department of Mechanical Engineering, University of Maryland, College Park, MD, USA Introduction: Rodent tail disc loading models have been extremely useful for studying the role of mechanical load on disc degeneration, but questions have been raised as to whether these discs are biomechanically relevant. While studies have demonstrated that stiffnesses of intact rodent tail motion segments compare well with those of human discs, our recent finite element analyses demonstrated the importance of validating the internal disc mechanics as well. This study utilizes a novel miniaturized fiber-optic sensor, small enough for measuring rat intradiscal pressure, to demonstrate that loadpressure relationships are similar between rat and human discs. Methods: A miniaturized fiber-optic pressure sensor was designed, in which pressure-induced deflections of a flexible diaphragm at the tip of an optical fiber can be measured. Motion segments were isolated from tails of freshly sacrificed rats and immediately frozen. At time of testing, thawed motion segments were mounted on a materials testing system, and maintained hydrated with PBS. A pre-calibrated micro-optical sensor was then inserted into the nucleus pulposus and motion segments subjected to compressive loading. Results: The sensor's small size (<200 ~m in diameter) allowed it to be inserted into rat tail discs without disrupting the annulus fibrosus. Measurements demonstrated the feasibility of using this sensor to quantify external Ioadintradiscal pressure relationships in small animal discs. Discussion: This study demonstrates that a miniaturized fiber-optic pressure sensor can be used to measure intradiscal pressures of rodent tail discs, a capability that has to-date been technologically unachievable due to size constraints. Verification of this relationship by comparing with load-pressure relationships that have been measured in cadaveric porcine and human discs will not only further reaffirm the use of rodent tail discs for studying loadinduced disc degeneration, but will also yield data for validating pressure in finite element models developed in our laboratory.
Journal of Biomechanics 2006, Vol. 39 (Suppl 1)
[2] T. Videman, M. Nurminem. The occurence of anular tears and their relation to lifetime back pain history: A caveric study using barium sulfate discography. Spine 2004; 29: 2668-76. [3] Y. Schroeder, W. Wilson, J.M. Huyghe, F.P.T. Baaijens. Osmoviscoelastic finite element model of the intervertebral disc. Eur. Spine J. 2006; in press. 5863 Tu, 08:30-08:45 (P15) Analysis of anular lesions in the L4/5 intervertebral disc: a hyperelastic model J.P. Little 1, C. Adam 1, J. Evans 1, G. Pettet 2, M. Pearcy 1. 1School ef
Engineering Systems and 2School of Mathematical Science, QUT, Brisbane, Australia Introduction: Low back pain (LBP) affects a large portion of the population and can result from degeneration of the intervertebral discs. Degeneration of the discs may be characterized by a loss of hydration of the nucleus pulposus and the presence of anular lesions which are tears in the anulus fibrosus. As the relationship between LBP and anular lesions has not been established a finite element model (FEM) of an L4/5 intervertebral disc was developed to explore the possible effects of anular lesions on the mechanical behavior of the disc. Particular care was taken to represent the nonlinear nature of the disc materials and geometry and to simulate physiological ranges of motion. Methods: An anatomically accurate transverse profile for the disc FEM was derived from transversely sectioned human cadaveric discs. Model geometries simulated a healthy disc and degenerate discs with either a rim, radial or circumferential lesion. The anulus fibrosus ground substance was represented using a 2nd-order polynomial, hyperelastic strain energy function. For the healthy disc, the nucleus was modeled by a hydrostatic pressure and in the degenerate models this pressure was set to zero. Loading conditions simulating the physiological ranges of flexural motion were applied to the models and the corresponding peak moments compared. Results: Small decreases in peak resistive moment in the degenerate disc models indicated that the mechanics of the disc are minimally affected by the presence of anular lesions. Loss of the nucleus pulposus pressure had a much greater effect on the disc mechanics than the presence of anular lesions alone. Conclusion: These results suggested that were anular lesions to develop prior to the degeneration of the nucleus they would have minimal effect on the mechanics of the whole disc. With the degeneration of the nucleus, the disc stiffness will be greatly reduced and the outer innervated anulus or surrounding osseo-ligamentous structures may become overloaded and thus painful. In particular the strains developed in the vicinity of annular lesions may be especially elevated.
7042 Tu, 08:45-09:00 (P15) On hyperelastic constitutive modeling of annulus fibrosus F.C. Caner 1,2, Z.'~ Guo 3, B. Moran 3, Z.P. Bazant 2, I. Carol 1. 1School ef Civil
Eng., Technical University of Catalonia, Barcelona, Spain, 2Dept. of Civil & Env. Eng., Northwestern University, Evanston, IL, USA, 3Dept. of Mechanical Eng., Northwestern University, Evanston, IL, USA Constitutive modeling of human annulus fibrosus (AF) is of interest to understand the development of pathological behavior and thus to device effective biomechanical treatments which has proven to be challenging. The classical constitutive modeling approach based on strain invariants with discrete fibers has been shown unable to predict experimental data. For example, when such models are calibrated to fit the uniaxial behavior of AF, they often fail in predicting data from biaxial tests or evolution of change of angle between the fibers in uniaxial tests. This study reports two different approaches to constitutive modeling of AF in the elastic range and compares their predictions to the available experimental data. First, a classical model as described above is enriched by providing proper fiber-matrix interaction. Next, a non-classical model, microplane model, is used to predict the experimental data. It is shown that, both of these two distinct approaches are able to represent the experimental data well. In the microplane model, the interaction of distributed fibers through kinematic constraint and subsequent homogenization of the microplane response automatically provides the required additional shear stiffness. We conclude that (i) for classical approaches, in addition to the parallel coupling of fiber and matrix, an adjustable fiber-matrix interaction effect must be considered; (ii) the representation of physical fiber distribution in AF in the modeling becomes essential due to additional shear resistance provided by interaction of distributed fibers.
Oral Presentations
7409 Tu, 09:00-09:15 (P15) Cervical intervertebral disc herniation in response to severe loading in compression and bending P. Pollintine, D. Skrzypiec, P. Dolan, M.A. Adams. Department of Anatomy,
University of Bristol, UK Introduction: The cervical spine can be severely loaded in bending during sporting injuries and "whiplash". Compressive loading could also be high if some advanced warning of impact stimulated vigorous ("protective") contraction of the neck muscles. Combined bending and compression can cause some lumbar discs to herniate in-vitro [1] but the outcome depends on spinal level, and may not be applicable to cervical discs. We test the hypotheses: (a) that cervical discs can prolapse in-vitro, and (b) that prolapse leads to irregular stress distributions inside the disc. Material and Methods: Human cervical "motion segments" (two vertebrae and intervening soft tissues) were obtained from cadavers aged 51-88yrs. Specimens were secured in cups of dental stone and subjected to static compressive loading (150N) for 20s. During this time, the distribution of vertically-acting compressive 'stress' was recorded along the postero-anterior diameter of the disc by pulling a 0.9 mm-diameter pressure transducer through it [2]. Injury was induced by compressing each specimen at 1 mm/s while positioned in 200 flexion, 150 extension or 80 lateral bending. The distribution of compressive stress within the disc was then re-measured. Specimens were sectioned at 2 mm intervals in order to ascertain soft tissue disruption. Results: In all six specimens tested to date, one or both of the apophyseal joint capsules were ruptured by the complex loading. Intervertebral disc prolapse also occurred in all six specimens, with the herniated nucleus appearing on the anterior, posterior and postero-lateral disc surface in extension, flexion and lateral bending respectively. All modes of failure affected intradiscal stresses: on average, nucleus pressure decreased by 75±7%, while stress concentrations in the annulus increased by 130±21%. Discussion: These preliminary results confirm that severe complex loading can cause cervical discs to prolapse. No particular state of disc degeneration is required, provided the loading is sufficiently severe. Indeed, the altered stress distributions suggest that cell-mediated changes would probably follow prolapse. References [1] Adams MA, Hutton WC. Spine 1982; 7(3): 184-91. [2] Adams et al. Spine 1996; 21(4): 434-8.
5507 Tu, 09:15-09:30 (P15) Intradiscal pressures in rat tail discs measured using a miniaturized fiber-optic sensor A.H. Hsieh 1,2, D.A. Ryan 1,2, Z. Chen 2, Y. Liu 2, S.C. Nesson 2, M. Yu 2.
1Bioengineering Graduate Program, University of Maryland, College Park, MD, USA, 2Department of Mechanical Engineering, University of Maryland, College Park, MD, USA Introduction: Rodent tail disc loading models have been extremely useful for studying the role of mechanical load on disc degeneration, but questions have been raised as to whether these discs are biomechanically relevant. While studies have demonstrated that stiffnesses of intact rodent tail motion segments compare well with those of human discs, our recent finite element analyses demonstrated the importance of validating the internal disc mechanics as well. This study utilizes a novel miniaturized fiber-optic sensor, small enough for measuring rat intradiscal pressure, to demonstrate that loadpressure relationships are similar between rat and human discs. Methods: A miniaturized fiber-optic pressure sensor was designed, in which pressure-induced deflections of a flexible diaphragm at the tip of an optical fiber can be measured. Motion segments were isolated from tails of freshly sacrificed rats and immediately frozen. At time of testing, thawed motion segments were mounted on a materials testing system, and maintained hydrated with PBS. A pre-calibrated micro-optical sensor was then inserted into the nucleus pulposus and motion segments subjected to compressive loading. Results: The sensor's small size (<200 ~m in diameter) allowed it to be inserted into rat tail discs without disrupting the annulus fibrosus. Measurements demonstrated the feasibility of using this sensor to quantify external Ioadintradiscal pressure relationships in small animal discs. Discussion: This study demonstrates that a miniaturized fiber-optic pressure sensor can be used to measure intradiscal pressures of rodent tail discs, a capability that has to-date been technologically unachievable due to size constraints. Verification of this relationship by comparing with load-pressure relationships that have been measured in cadaveric porcine and human discs will not only further reaffirm the use of rodent tail discs for studying loadinduced disc degeneration, but will also yield data for validating pressure in finite element models developed in our laboratory.