TY - JOUR
T1 - Extra-fibrillar matrix mechanics of annulus fibrosus in tension and compression
AU - Cortes, Daniel H.
AU - Elliott, Dawn M.
N1 - Funding Information:
Acknowledgments This work was supported with funding from the National Institutes of Health (R01 EB02425) and the Penn Center for Musculoskeletal Disorders.
PY - 2012/7
Y1 - 2012/7
N2 - The annulus fibrosus (AF) of the disk is a highly nonlinear and anisotropic material that undergoes a complex combination of loads in multiple orientations. The tensile mechanical behavior ofAFin the lamellar plane is dominated by collagen fibers and has been accurately modeled using exponential functions. On the other hand, AF mechanics perpendicular to the lamella, in the radial direction, depend on the properties of the ground matrix with little to no fiber contribution. The ground matrix is mainly composed of proteoglycans (PG), which are negatively charged macromolecules that maintain the tissue hydration via osmotic pressure. The mechanical response of the ground matrix can be divided in the contribution of osmotic pressure and an elastic solid part known as extra-fibrillar matrix (EFM). Mechanical properties of the ground matrix have been measured using tensile and confined compression tests. However, EFM mechanics have not been measured directly. The objective of this study was to measure AF nonlinear mechanics of the EFM in tension and compression. To accomplish this, a combination of osmotic swelling and confined compression in disk radial direction, perpendicular to the lamella, was used. For this type of analysis, it was necessary to define a stress-free reference configuration. Thus, a brief analysis on residual stress in the disk and a procedure to estimate the reference configuration are presented. The proposed method was able to predict similar swelling deformations when using different loading protocols and models for the EFM, demonstrating its robustness. The stress-stretch curve of the EFM was linear in the range 0.9 < λ3 < 1.3 with an aggregate modulus of 10.18±3.32 kPa; however, a significant nonlinearity was observed for compression below 0.8. The contribution of the EFMto the total aggregatemodulus of theAF decreased from 70 to 30% for an applied compression of 50% of the initial thickness. The properties obtained in this study are essential for constitutive and finite element models of the AF and disk and can be applied to differentiate between functional degeneration effects such as PG loss and stiffening due to cross-linking.
AB - The annulus fibrosus (AF) of the disk is a highly nonlinear and anisotropic material that undergoes a complex combination of loads in multiple orientations. The tensile mechanical behavior ofAFin the lamellar plane is dominated by collagen fibers and has been accurately modeled using exponential functions. On the other hand, AF mechanics perpendicular to the lamella, in the radial direction, depend on the properties of the ground matrix with little to no fiber contribution. The ground matrix is mainly composed of proteoglycans (PG), which are negatively charged macromolecules that maintain the tissue hydration via osmotic pressure. The mechanical response of the ground matrix can be divided in the contribution of osmotic pressure and an elastic solid part known as extra-fibrillar matrix (EFM). Mechanical properties of the ground matrix have been measured using tensile and confined compression tests. However, EFM mechanics have not been measured directly. The objective of this study was to measure AF nonlinear mechanics of the EFM in tension and compression. To accomplish this, a combination of osmotic swelling and confined compression in disk radial direction, perpendicular to the lamella, was used. For this type of analysis, it was necessary to define a stress-free reference configuration. Thus, a brief analysis on residual stress in the disk and a procedure to estimate the reference configuration are presented. The proposed method was able to predict similar swelling deformations when using different loading protocols and models for the EFM, demonstrating its robustness. The stress-stretch curve of the EFM was linear in the range 0.9 < λ3 < 1.3 with an aggregate modulus of 10.18±3.32 kPa; however, a significant nonlinearity was observed for compression below 0.8. The contribution of the EFMto the total aggregatemodulus of theAF decreased from 70 to 30% for an applied compression of 50% of the initial thickness. The properties obtained in this study are essential for constitutive and finite element models of the AF and disk and can be applied to differentiate between functional degeneration effects such as PG loss and stiffening due to cross-linking.
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U2 - 10.1007/s10237-011-0351-x
DO - 10.1007/s10237-011-0351-x
M3 - Article
C2 - 21964839
AN - SCOPUS:84865627628
SN - 1617-7959
VL - 11
SP - 781
EP - 790
JO - Biomechanics and Modeling in Mechanobiology
JF - Biomechanics and Modeling in Mechanobiology
IS - 6
ER -