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ISSLS Prize Winner: A Detailed Examination of the Elastic Network Leads to a New Understanding of Annulus Fibrosus Organization. Spine (Phila Pa 1976) 2015; 40:1149-57. [PMID: 25893352 DOI: 10.1097/brs.0000000000000943] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
STUDY DESIGN Investigation of the elastic network in disc annulus and its function. OBJECTIVE To investigate the involvement of the elastic network in the structural interconnectivity of the annulus and to examine its possible mechanical role. SUMMARY OF BACKGROUND DATA The lamellae of the disc are now known to consist of bundles of collagen fibers organized into compartments. There is strong interconnectivity between adjacent compartments and between adjacent lamellae, possibly aided by a translamellar bridging network, containing blood vessels. An elastic network exists across the disc annulus and is particularly dense between the lamellae, and forms crossing bridges within the lamellae. METHODS Blocks of annulus taken from bovine caudal discs were studied in either their unloaded or radially stretched state then fixed and sectioned, and their structure analyzed optically using immunohistology. RESULTS An elastic network enclosed the collagen compartments, connecting the compartments with each other and with the elastic network of adjacent lamellae, formed an integrated network across the annulus, linking it together. Stretching experiments demonstrated the mechanical interconnectivities of the elastic fibers and the collagen compartments. CONCLUSION The annulus can be viewed as a modular structure organized into compartments of collagen bundles enclosed by an elastic sheath. The elastic network of these sheaths is interconnected mechanically across the entire annulus. This organization is also seen in the modular structure of tendon and muscle. The results provide a new understanding annulus structure and its interconnectivity, and contribute to fundamental structural information relevant to disc tissue engineering and mechanical modeling. LEVEL OF EVIDENCE N/A.
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Derivation of inter-lamellar behaviour of the intervertebral disc annulus. J Mech Behav Biomed Mater 2015; 48:164-172. [PMID: 25955558 PMCID: PMC4455908 DOI: 10.1016/j.jmbbm.2015.03.028] [Citation(s) in RCA: 40] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2014] [Revised: 03/16/2015] [Accepted: 03/23/2015] [Indexed: 11/30/2022]
Abstract
The inter-lamellar connectivity of the annulus fibrosus in the intervertebral disc has been shown to affect the prediction of the overall disc behaviour in computational models. Using a combined experimental and computational approach, the inter-lamellar mechanical behaviour of the disc annulus was investigated under conditions of radial loading. Twenty-seven specimens of anterior annulus fibrosus were dissected from 12 discs taken from four frozen ovine thoracolumbar spines. Specimens were grouped depending on their radial provenance within the annulus fibrosus. Standard tensile tests were performed. In addition, micro-tensile tests under microscopy were used to observe the displacement of the lamellae and inter-lamellar connections. Finite elements models matching the experimental protocols were developed with specimen-specific geometries and boundary conditions assuming a known lamellar behaviour. An optimisation process was used to derive the interface stiffness values for each group. The assumption of a linear cohesive interface was used to model the behaviour of the inter-lamellar connectivity. The interface stiffness values derived from the optimisation process were consistently higher than the corresponding lamellar values. The interface stiffness values of the outer annulus were from 43% to 75% higher than those of the inner annulus. Tangential stiffness values for the interface were from 6% to 39% higher than normal stiffness values within each group and similar to values reported by other investigators. These results reflect the intricate fibrous nature of the inter-lamellar connectivity and provide values for the representation of the inter-lamellar behaviour at a continuum level.
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Labus KM, Han SK, Hsieh AH, Puttlitz CM. A computational model to describe the regional interlamellar shear of the annulus fibrosus. J Biomech Eng 2015; 136:051009. [PMID: 24599055 DOI: 10.1115/1.4027061] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2013] [Accepted: 03/06/2014] [Indexed: 11/08/2022]
Abstract
Interlamellar shear may play an important role in the homeostasis and degeneration of the intervertebral disk. Accurately modeling the shear behavior of the interlamellar compartment would enhance the study of its mechanobiology. In this study, physical experiments were utilized to describe interlamellar shear and define a constitutive model, which was implemented into a finite element analysis. Ovine annulus fibrosus (AF) specimens from three locations within the intervertebral disk (lateral, outer anterior, and inner anterior) were subjected to in vitro mechanical shear testing. The local shear stress-stretch relationship was described for the lamellae and across the interlamellar layer of the AF. A hyperelastic constitutive model was defined for interlamellar and lamellar materials at each location tested. The constitutive models were incorporated into a finite element model of a block of AF, which modeled the interlamellar and lamellar layers using a continuum description. The global shear behavior of the AF was compared between the finite element model and physical experiments. The shear moduli at the initial and final regions of the stress-strain curve were greater within the lamellae than across the interlamellar layer. The difference between interlamellar and lamellar shear was greater at the outer anterior AF than at the inner anterior region. The finite element model was shown to accurately predict the global shear behavior or the AF. Future studies incorporating finite element analysis of the interlamellar compartment may be useful for predicting its physiological mechanical behavior to inform the study of its mechanobiology.
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Xu JX, Yang SD, Wang BL, Yang DL, Ding WY, Shen Y. Correlative analyses of isolated upper lumbar disc herniation and adjacent wedge-shaped vertebrae. Int J Clin Exp Med 2015; 8:1150-1155. [PMID: 25785106 PMCID: PMC4358561] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2014] [Accepted: 12/05/2014] [Indexed: 06/04/2023]
Abstract
BACKGROUND Upper lumbar disc herniation (ULDH) is easy to be misdiagnosed due to its special anatomical and atypical clinical features. Few studies have identified the relationship between ULDH and adjacent wedge-shaped vertebrae (WSV). HYPOTHESIS WSV may have some indicative relations withULDH. PATIENTS AND METHODS Between January 2003 and October 2013, 47 patients (27 males and 20 females; mean age, 41.2 years) with single-level ULDH (as study group) and 47 sex- and age-matched healthy volunteers (as control group) were studied by radiograph. The two groups were compared with respect to age, sexual proportion, body mass index (BMI), kyphotic angle, and the proportion of WSV. Also, correlative analyses were conducted in the study group to investigate the relation between the kyphotic angle of target vertebrae and other factors including age, BMI, Cobb angle, JOA score and bone mineral density (BMD). RESULTS The average kyphotic angle in the study group was 11° (4°-22°), while the average kyphotic angle in the control group was 2° (0°-7°). Obviously, the mean kyphotic angle in the study group was statistically larger than that in the control group (t=13.797, P<0.001). The proportion of WSV in the study group was significantly larger than that in the control group (x(2)=36.380, P<0.0001). The correlations between kyphotic angles and other items (i.e., age, BMI, BMD, Cobb angle and JOA score) in the study group and the control group were low or uncorrelated. CONCLUSIONS WSV are indicatively associated with adjacent ULDH. Thus, ULDH should be alerted when WSV are first found in radiograph and accompanied by clinical symptoms.
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Affiliation(s)
- Jia-Xin Xu
- Department of Spinal Surgery, The Third Hospital of Hebei Medical UniversityShijiazhuang 050051, China
| | - Si-Dong Yang
- Department of Spinal Surgery, The Third Hospital of Hebei Medical UniversityShijiazhuang 050051, China
| | - Bao-Lin Wang
- Department of Medical Device, The Third Hospital of Hebei Medical UniversityShijiazhuang 050051, China
| | - Da-Long Yang
- Department of Spinal Surgery, The Third Hospital of Hebei Medical UniversityShijiazhuang 050051, China
| | - Wen-Yuan Ding
- Department of Spinal Surgery, The Third Hospital of Hebei Medical UniversityShijiazhuang 050051, China
- Hebei Provincial Key Laboratory of Orthopedic BiomechanicsShijiazhuang 050051, China
| | - Yong Shen
- Department of Spinal Surgery, The Third Hospital of Hebei Medical UniversityShijiazhuang 050051, China
- Hebei Provincial Key Laboratory of Orthopedic BiomechanicsShijiazhuang 050051, China
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Borde B, Grunert P, Härtl R, Bonassar LJ. Injectable, high-density collagen gels for annulus fibrosus repair: An in vitro rat tail model. J Biomed Mater Res A 2014; 103:2571-81. [PMID: 25504661 DOI: 10.1002/jbm.a.35388] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2014] [Revised: 11/22/2014] [Accepted: 12/08/2014] [Indexed: 01/08/2023]
Abstract
A herniated intervertebral disc often causes back pain when disc tissue is displaced through a damaged annulus fibrosus. Currently, the only methods available for annulus fibrosus repair involve mechanical closure of defect, which does little to address biological healing in the damaged tissue. Collagen hydrogels are injectable and have been used to repair annulus defects in vivo. In this study, high-density collagen hydrogels at 5, 10, and 15 mg/mL were used to repair defects made to intact rat caudal intervertebral discs in vitro. A group of gels at 15 mg/mL were also cross-linked with riboflavin at 0.03 mM, 0.07 mM, or 0.10 mM. These cross-linked, high-density collagen gels maintained their presence in the defect under loading and contributed positively to the mechanical response of damaged discs. Discs exhibited increases to 95% of undamaged effective equilibrium and instantaneous moduli as well as up to fourfold decreases in effective hydraulic permeability from the damaged discs. These data suggest that high-density collagen gels may be effective at restoring mechanical function of injured discs as well as potential vehicles for the delivery of biological agents such as cells or growth factors that may aid in the repair of the annulus fibrosus.
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Affiliation(s)
- Brandon Borde
- Department of Biomedical Engineering, Cornell University, Ithaca, New York
| | - Peter Grunert
- Department of Neurological Surgery, Weill Cornell Medical College, New York, New York
| | - Roger Härtl
- Department of Neurological Surgery, Weill Cornell Medical College, New York, New York
| | - Lawrence J Bonassar
- Department of Biomedical Engineering, Cornell University, Ithaca, New York.,Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York
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Griffin DJ, Vicari J, Buckley MR, Silverberg JL, Cohen I, Bonassar LJ. Effects of enzymatic treatments on the depth-dependent viscoelastic shear properties of articular cartilage. J Orthop Res 2014; 32:1652-7. [PMID: 25196502 DOI: 10.1002/jor.22713] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/15/2014] [Accepted: 07/14/2014] [Indexed: 02/04/2023]
Abstract
Osteoarthritis (OA) is a disease that involves the erosion and structural weakening of articular cartilage. OA is characterized by the degradation of collagen and proteoglycans in the extracellular matrix (ECM), particularly at the articular surface by proteinases including matrix metalloproteinases (MMPs) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTSs).(1) Degradation of collagen and proteoglycans is known to alter shear mechanical properties of cartilage, but study of this phenomenon has been focused on bulk tissue properties. The purpose of this study was to assess microscale cartilage damage induced by trypsin or collagenase using a technique to measure the local shear viscoelastic properties. Safranin-O histology revealed a decrease in proteoglycans near the articular surface after collagenase and trypsin digestions, with proteoglycan depletion increasing in time. Similarly, confocal reflectance micrographs showed increasing collagen degradation in collagenase treated samples, although the collagen network remained intact after trypsin treatment. Both treatments induced changes in shear modulus that were confined to a narrow range (∼400µm) near tissue surface. In addition, collagenase altered the total energy dissipation distribution by up to a factor of 100, with longer digestion times corresponding to higher energy dissipation. The ability to detect local mechanical signatures in tissue composition and mechanics is an important tool for understanding the spatially non-uniform changes that occur in articular cartilage diseases such as OA.
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Affiliation(s)
- Darvin J Griffin
- Department of Biomedical Engineering, Cornell University, Ithaca, New York
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Role of biomolecules on annulus fibrosus micromechanics: effect of enzymatic digestion on elastic and failure properties. J Mech Behav Biomed Mater 2014; 40:75-84. [PMID: 25212387 DOI: 10.1016/j.jmbbm.2014.08.012] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2013] [Revised: 08/07/2014] [Accepted: 08/12/2014] [Indexed: 01/07/2023]
Abstract
Uniaxial tension was applied to selectively digested single lamellar human cadaveric annulus fibrosus specimens to investigate the role of different biomolecules in annular biomechanics. Single layered and inter-lamellar annulus fibrosus samples were obtained from 10 isolated cadaveric lumbar intervertebral discs in one of four orientations: longitudinal, transverse, radial, and circumferential. Within each orientation the samples were subjected to a selective enzymatic digestion protocol with collagenase, elastase, chondroitinase ABC, or 1× Phosphate Buffered Saline. Uniaxial tensile tests were performed to failure at a strain rate of 0.005s(-1). Failure stress and strain, and elastic moduli were compared among the digested conditions. The collagenase- and elastase-treated groups had the most significant effect on the mechanical properties among the orientation groups, decreasing the failure stress for both interlaminar and intralaminar groups. Collagenase-treated groups showed an increase in the failure strain following enzymatic digestion for the intralaminar groups and one interlaminar testing direction (circumferential). The chondroitinase ABC-treated group only had a significant impact on the single layer orientations, decreasing the failure stress and strain (intralaminar group). The digested properties described provide insights into the laminar mechanical behavior and the role of the molecular components to the annular mechanical behavior. Understanding annular mechanics may prove insightful in diagnosis, prevention and repair of debilitating intervertebral disc disorders and manufacturing of tissue-engineered annulus.
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Abstract
STUDY DESIGN Mechanical study on cadaver motion segments. OBJECTIVE To determine whether high gradients of compressive stress within the intervertebral disc are associated with progressive disc degeneration. SUMMARY OF BACKGROUND DATA Mechanical loading can initiate disc degeneration but may be unimportant in disease progression, because degenerative changes cause the disc to be increasingly "stress-shielded" by the neural arch. However, the most typical feature of advanced disc degeneration (delamination and collapse of the annulus) may not depend on absolute values of compressive stress but on gradients of compressive stress that act to shear annulus lamellae. METHODS A total of 191 motion segments (T7-T8 to L5-S1) were dissected from 42 cadavers aged 19 to 92 years. Each was subjected to approximately 1 kN compression, while intradiscal stresses were measured by pulling a pressure transducer along the disc's midsagittal diameter. "Stress gradients" in the annulus were quantified as the average rate of increase in compressive stress (MPa/mm) between the nucleus and the region of maximum stress in the anterior or posterior annulus. Measurements were repeated before and after creep loading and in simulated flexed and erect postures. Disc degeneration was assessed macroscopically on a scale of 1 to 4. RESULTS As grade of disc degeneration increased from 2 to 4, nucleus pressure decreased by an average 68%, and maximum compressive stress in the annulus decreased by 48% to 64%, depending on location and posture. In contrast, stress gradients in the annulus increased by an average 75% in the anterior annulus (in flexed posture) and by 108% in the posterior annulus (in erect posture). Spearman rank correlation showed that these increases were statistically significant. CONCLUSION Despite stress-shielding by the neural arch, gradients of compressive stress increase with increasing grade of disc degeneration. Stress gradients act to shear adjacent lamellae and can explain progressive annulus delamination and collapse. LEVEL OF EVIDENCE N/A.
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Buckley MR, Bonassar LJ, Cohen I. Localization of viscous behavior and shear energy dissipation in articular cartilage under dynamic shear loading. J Biomech Eng 2014; 135:31002. [PMID: 24231813 DOI: 10.1115/1.4007454] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2012] [Accepted: 08/27/2012] [Indexed: 11/08/2022]
Abstract
Though remarkably robust, articular cartilage becomes susceptible to damage at high loading rates, particularly under shear. While several studies have measured the local static and steady-state shear properties of cartilage, it is the local viscoelastic properties that determine the tissue's ability to withstand physiological loading regimens. However, measuring local viscoelastic properties requires overcoming technical challenges that include resolving strain fields in both space and time and accurately calculating their phase offsets. This study combined recently developed high-speed confocal imaging techniques with three approaches for analyzing time- and location-dependent mechanical data to measure the depth-dependent dynamic modulus and phase angles of articular cartilage. For sinusoidal shear at frequencies f = 0.01 to 1 Hz with no strain offset, the dynamic shear modulus |G*| and phase angle δ reached their minimum and maximum values (respectively) approximately 100 μm below the articular surface, resulting in a profound focusing of energy dissipation in this narrow band of tissue that increased with frequency. This region, known as the transitional zone, was previously thought to simply connect surface and deeper tissue regions. Within 250 μm of the articular surface, |G*| increased from 0.32 ± 0.08 to 0.42 ± 0.08 MPa across the five frequencies tested, while δ decreased from 12 deg ± 1 deg to 9.1 deg ± 0.5 deg. Deeper into the tissue, |G*| increased from 1.5 ± 0.4 MPa to 2.1 ± 0.6 MPa and δ decreased from 13 deg ± 1 deg to 5.5 deg ± 0.2 deg. Viscoelastic properties were also strain-dependent, with localized energy dissipation suppressed at higher shear strain offsets. These results suggest a critical role for the transitional zone in dissipating energy, representing a possible shift in our understanding of cartilage mechanical function. Further, they give insight into how focal degeneration and mechanical trauma could lead to sustained damage in this tissue.
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60
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Szczesny SE, Edelstein RS, Elliott DM. DTAF dye concentrations commonly used to measure microscale deformations in biological tissues alter tissue mechanics. PLoS One 2014; 9:e99588. [PMID: 24915570 PMCID: PMC4051763 DOI: 10.1371/journal.pone.0099588] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2014] [Accepted: 05/16/2014] [Indexed: 11/18/2022] Open
Abstract
Identification of the deformation mechanisms and specific components underlying the mechanical function of biological tissues requires mechanical testing at multiple levels within the tissue hierarchical structure. Dichlorotriazinylaminofluorescein (DTAF) is a fluorescent dye that is used to visualize microscale deformations of the extracellular matrix in soft collagenous tissues. However, the DTAF concentrations commonly employed in previous multiscale experiments (≥2000 µg/ml) may alter tissue mechanics. The objective of this study was to determine whether DTAF affects tendon fascicle mechanics and if a concentration threshold exists below which any observed effects are negligible. This information is valuable for guiding the continued use of this fluorescent dye in future experiments and for interpreting the results of previous work. Incremental strain testing demonstrated that high DTAF concentrations (≥100 µg/ml) increase the quasi-static modulus and yield strength of rat tail tendon fascicles while reducing their viscoelastic behavior. Subsequent multiscale testing and modeling suggests that these effects are due to a stiffening of the collagen fibrils and strengthening of the interfibrillar matrix. Despite these changes in tissue behavior, the fundamental deformation mechanisms underlying fascicle mechanics appear to remain intact, which suggests that conclusions from previous multiscale investigations of strain transfer are still valid. The effects of lower DTAF concentrations (≤10 µg/ml) on tendon mechanics were substantially smaller and potentially negligible; nevertheless, no concentration was found that did not at least slightly alter the tissue behavior. Therefore, future studies should either reduce DTAF concentrations as much as possible or use other dyes/techniques for measuring microscale deformations.
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Affiliation(s)
- Spencer E. Szczesny
- Department of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Rachel S. Edelstein
- Department of Biomedical Engineering, University of Delaware, Newark, Delaware, United States of America
| | - Dawn M. Elliott
- Department of Biomedical Engineering, University of Delaware, Newark, Delaware, United States of America
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Szczesny SE, Elliott DM. Interfibrillar shear stress is the loading mechanism of collagen fibrils in tendon. Acta Biomater 2014; 10:2582-90. [PMID: 24530560 DOI: 10.1016/j.actbio.2014.01.032] [Citation(s) in RCA: 107] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2013] [Revised: 12/04/2013] [Accepted: 01/30/2014] [Indexed: 12/15/2022]
Abstract
Despite the critical role tendons play in transmitting loads throughout the musculoskeletal system, little is known about the microstructural mechanisms underlying their mechanical function. Of particular interest is whether collagen fibrils in tendon fascicles bear load independently or if load is transferred between fibrils through interfibrillar shear forces. We conducted multiscale experimental testing and developed a microstructural shear lag model to explicitly test whether interfibrillar shear load transfer is indeed the fibrillar loading mechanism in tendon. Experimental correlations between fascicle macroscale mechanics and microscale interfibrillar sliding suggest that fibrils are discontinuous and share load. Moreover, for the first time, we demonstrate that a shear lag model can replicate the fascicle macroscale mechanics as well as predict the microscale fibrillar deformations. Since interfibrillar shear stress is the fundamental loading mechanism assumed in the model, this result provides strong evidence that load is transferred between fibrils in tendon and possibly other aligned collagenous tissues. Conclusively establishing this fibrillar loading mechanism and identifying the involved structural components should help develop repair strategies for tissue degeneration and guide the design of tissue engineered replacements.
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Affiliation(s)
- Spencer E Szczesny
- Department of Bioengineering, University of Pennsylvania, 240 Skirkanich Hall, 210 South 33rd St, Philadelphia, PA 19104, USA
| | - Dawn M Elliott
- Department of Biomedical Engineering, University of Delaware, 125 East Delaware Avenue, Newark, DE 19716, USA.
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Effects of secreted factors in culture medium of annulus fibrosus cells on microvascular endothelial cells: elucidating the possible pathomechanisms of matrix degradation and nerve in-growth in disc degeneration. Osteoarthritis Cartilage 2014; 22:344-54. [PMID: 24361793 PMCID: PMC3952937 DOI: 10.1016/j.joca.2013.12.008] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/20/2013] [Revised: 11/29/2013] [Accepted: 12/10/2013] [Indexed: 02/02/2023]
Abstract
OBJECTIVE To test whether the interaction between annulus fibrosus cells (AFCs) and endothelial cells (ECs) disrupts matrix homeostasis and stimulates production of innervation mediators. METHODS Human microvascular ECs were cultured in the conditioned media of AF cell culture derived from degenerated human surgical specimen. Matrix-metalloproteinases (MMPs) and platelet-derived growth factor (PDGF) of ECs of this culture were analyzed by qRT-PCR, Western, and immunofluorescence. Vascular endothelial growth factor (VEGF), Interleukin-8 (IL-8), and nerve growth factor (NGF) in the media of this cell culture were assayed by ELISA. To determine the effects of ECs on AFCs, qRT-PCR was performed to determine mRNA levels of collagen I, II and aggrecan in AFCs cultured in EC conditioned media. RESULTS Compared to ECs cultured in naïve media, ECs exposed to AFC conditioned media expressed higher mRNA and protein levels of key biomarkers of invasive EC phenotype, MMP-2 (2×), MMP-13 (4×), and PDGF-B (1.5-2×), and NGF (24.9 ± 15.2 pg/mL vs 0 in naïve media). Treatment of AF cells with EC culture conditioned media decreased collagen type II expression two fold. Considerable quantities of pro-angiogenic factors IL-8 (396.7 ± 302.0 pg/mL) and VEGF (756.2 ± 375.9 pg/mL) were also detected in the conditioned media of untreated AF cell culture. DISCUSSION AFCs from degenerated discs secreted factors which stimulated EC production of factors known to induce matrix degradation, angiogenesis, and innervation. IL-8 and VEGF maybe the secreted factors from AFCs which mediate a pro-angiogenic stimulus often implicated in the development of disc degeneration.
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Reutlinger C, Bürki A, Brandejsky V, Ebert L, Büchler P. Specimen specific parameter identification of ovine lumbar intervertebral discs: On the influence of fibre-matrix and fibre-fibre shear interactions. J Mech Behav Biomed Mater 2013; 30:279-89. [PMID: 24361932 DOI: 10.1016/j.jmbbm.2013.11.019] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2013] [Revised: 11/18/2013] [Accepted: 11/25/2013] [Indexed: 10/25/2022]
Abstract
Numerical models of the intervertebral disc, which address mechanical questions commonly make use of the difference in water content between annulus and nucleus, and thus fluid and solid parts are separated. Despite this simplification, models remain complex due to the anisotropy and nonlinearity of the annulus and regional variations of the collagen fibre density. Additionally, it has been shown that cross-links make a large contribution to the stiffness of the annulus. Because of this complex composite structure, it is difficult to reproduce several sets of experimental data with one single set of material parameters. This study addresses the question to which extent the ultrastructure of the intervertebral disc should be modelled so that its moment-angle behaviour can be adequately described. Therefore, a hyperelastic constitutive law, based on continuum mechanical principles was derived, which does not only consider the anisotropy from the collagen fibres, but also interactions among the fibres and between the fibres and the ground substance. Eight ovine lumbar intervertebral discs were tested on a custom made spinal loading simulator in flexion/extension, lateral bending and axial rotation. Specimen-specific geometrical models were generated using CT images and T2 maps to distinguish between annulus fibrosus and nucleus pulposus. For the identification of the material parameters the annulus fibrosus was described with two scenarios: with and without fibre-matrix and fibre-fibre interactions. Both scenarios showed a similar behaviour on a load displacement level. Comparing model predictions to the experimental data, the mean RMS of all specimens and all load cases was 0.54±0.15° without the interaction and 0.54±0.19° when the fibre-matrix and fibre-fibre interactions were included. However, due to the increased stiffness when cross-links effects were included, this scenario showed more physiological stress-strain relations in uniaxial and biaxial stress states. Thus, the present study suggests that fibre-matrix and fibre-fibre interactions should be considered in the constitutive law when the model addresses questions concerning the stress field of the annulus fibrosus.
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Affiliation(s)
- Christoph Reutlinger
- Institute for Surgical Technology and Biomechanics, University of Bern, Switzerland.
| | - Alexander Bürki
- Institute for Surgical Technology and Biomechanics, University of Bern, Switzerland
| | | | - Lars Ebert
- Institute of Forensic Medicine, University of Zurich, Switzerland
| | - Philippe Büchler
- Institute for Surgical Technology and Biomechanics, University of Bern, Switzerland
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Sharifi S, van Kooten TG, Kranenburg HJC, Meij BP, Behl M, Lendlein A, Grijpma DW. An annulus fibrosus closure device based on a biodegradable shape-memory polymer network. Biomaterials 2013; 34:8105-13. [DOI: 10.1016/j.biomaterials.2013.07.061] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2013] [Accepted: 07/18/2013] [Indexed: 01/21/2023]
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65
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Oldfield MJ, Burrows C, Kerl J, Frasson L, Parittotokkaporn T, Beyrau F, Rodriguez y Baena F. Highly resolved strain imaging during needle insertion: Results with a novel biologically inspired device. J Mech Behav Biomed Mater 2013; 30:50-60. [PMID: 24231189 DOI: 10.1016/j.jmbbm.2013.10.016] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2013] [Revised: 10/08/2013] [Accepted: 10/15/2013] [Indexed: 11/16/2022]
Abstract
Percutaneous needle insertions are a common part of minimally invasive surgery. However, the insertion process is necessarily disruptive to the substrate. Negative side effects are migration of deep-seated targets and trauma to the surrounding material. Mitigation of these effects is highly desirable, but relies on a detailed understanding of the needle-tissue interactions, which are difficult to capture at a sufficiently high resolution. Here, an adapted Digital Image Correlation (DIC) technique is used to quantify mechanical behaviour at the sliding interface, with resolution of measurement points which is better than 0.5mm, representing a marked improvement over the state of the art. A method for converting the Eulerian description of DIC output to Lagrangian displacements and strains is presented and the method is validated during the simple insertion of a symmetrical needle into a gelatine tissue phantom. The needle is comprised of four axially interlocked quadrants, each with a bevel tip. Tests are performed where the segments are inserted into the phantom simultaneously, or in a cyclic sequence taking inspiration from the unique insertion strategy associated to the ovipositor of certain wasps. Data from around the needle-tissue interface includes local strain variations, material dragged along the needle surface and relaxation of the phantom, which show that the cyclic actuation of individual needle segments is potentially able to mitigate tissue strain and could be used to reduce target migration.
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Affiliation(s)
- M J Oldfield
- Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW72AZ, United Kingdom
| | - C Burrows
- Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW72AZ, United Kingdom
| | - J Kerl
- Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW72AZ, United Kingdom
| | - L Frasson
- Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW72AZ, United Kingdom
| | - T Parittotokkaporn
- Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW72AZ, United Kingdom
| | - F Beyrau
- Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW72AZ, United Kingdom
| | - F Rodriguez y Baena
- Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW72AZ, United Kingdom.
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Region specific response of intervertebral disc cells to complex dynamic loading: an organ culture study using a dynamic torsion-compression bioreactor. PLoS One 2013; 8:e72489. [PMID: 24013824 PMCID: PMC3755972 DOI: 10.1371/journal.pone.0072489] [Citation(s) in RCA: 65] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2013] [Accepted: 07/10/2013] [Indexed: 11/19/2022] Open
Abstract
The spine is routinely subjected to repetitive complex loading consisting of axial compression, torsion, flexion and extension. Mechanical loading is one of the important causes of spinal diseases, including disc herniation and disc degeneration. It is known that static and dynamic compression can lead to progressive disc degeneration, but little is known about the mechanobiology of the disc subjected to combined dynamic compression and torsion. Therefore, the purpose of this study was to compare the mechanobiology of the intervertebral disc when subjected to combined dynamic compression and axial torsion or pure dynamic compression or axial torsion using organ culture. We applied four different loading modalities [1. control: no loading (NL), 2. cyclic compression (CC), 3. cyclic torsion (CT), and 4. combined cyclic compression and torsion (CCT)] on bovine caudal disc explants using our custom made dynamic loading bioreactor for disc organ culture. Loads were applied for 8 h/day and continued for 14 days, all at a physiological magnitude and frequency. Our results provided strong evidence that complex loading induced a stronger degree of disc degeneration compared to one degree of freedom loading. In the CCT group, less than 10% nucleus pulposus (NP) cells survived the 14 days of loading, while cell viabilities were maintained above 70% in the NP of all the other three groups and in the annulus fibrosus (AF) of all the groups. Gene expression analysis revealed a strong up-regulation in matrix genes and matrix remodeling genes in the AF of the CCT group. Cell apoptotic activity and glycosaminoglycan content were also quantified but there were no statistically significant differences found. Cell morphology in the NP of the CCT was changed, as shown by histological evaluation. Our results stress the importance of complex loading on the initiation and progression of disc degeneration.
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Baldit A, Ambard D, Cherblanc F, Royer P. Experimental analysis of the transverse mechanical behaviour of annulus fibrosus tissue. Biomech Model Mechanobiol 2013; 13:643-52. [DOI: 10.1007/s10237-013-0524-x] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2012] [Accepted: 08/10/2013] [Indexed: 11/24/2022]
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Seol D, Choe H, Ramakrishnan PS, Jang K, Kurriger GL, Zheng H, Lim TH, Martin JA. Organ culture stability of the intervertebral disc: rat versus rabbit. J Orthop Res 2013; 31:838-46. [PMID: 23456659 DOI: 10.1002/jor.22285] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/02/2012] [Accepted: 11/05/2012] [Indexed: 02/04/2023]
Abstract
There is a need to develop mechanically active culture systems to better understand the role of mechanical stresses in intervertebral disc (IVD) degeneration. Motion segment cultures that preserve the native IVD structure and adjacent vertebral bodies are preferred as model systems, but rapid ex vivo tissue degeneration limits their usefulness. The stability of rat and rabbit IVDs is of particular interest, as their small size makes them otherwise suitable for motion segment culture. The goal of this study was to determine if there are substantial differences in the susceptibility of rat and rabbit IVDs to culture-induced degeneration. Lumbar IVD motion segments were harvested from young adult male Sprague-Dawley rats and New Zealand White rabbits and cultured under standard conditions for 14 days. Biochemical assays and safranin-O histology showed that while glycosaminoglycan (GAG) loss was minimal in rabbit IVDs, it was progressive and severe in rat IVDs. In the rat IVD, GAG loss was concomitant with the loss of notochordal cells and the migration of endplate (EP) cells into the nucleus pulposus (NP). None of these changes were evident in the rabbit IVDs. Compared to rabbit IVDs, rat IVDs also showed increased matrix metalloproteinase-3 (MMP-3) and sharply decreased collagen type I and II collagen expression. Together these data indicated that the rabbit IVD was dramatically more stable than the rat IVD, which showed culture-related degenerative changes. Based on these findings we conclude that the rabbit motion segments are a superior model for mechanobiologic studies.
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Affiliation(s)
- Dongrim Seol
- Departments of Orthopedics and Rehabilitation, University of Iowa, Iowa City, Iowa 52242, USA
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69
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Driscoll TP, Nakasone RH, Szczesny SE, Elliott DM, Mauck RL. Biaxial mechanics and inter-lamellar shearing of stem-cell seeded electrospun angle-ply laminates for annulus fibrosus tissue engineering. J Orthop Res 2013; 31:864-70. [PMID: 23335319 DOI: 10.1002/jor.22312] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/08/2012] [Accepted: 12/22/2012] [Indexed: 02/04/2023]
Abstract
The annulus fibrosus (AF) of the intervertebral disk plays a critical role in vertebral load transmission that is heavily dependent on the microscale structure and composition of the tissue. With degeneration, both structure and composition are compromised, resulting in a loss of AF mechanical function. Numerous tissue engineering strategies have addressed the issue of AF degeneration, but few have focused on recapitulation of AF microstructure and function. One approach that allows for generation of engineered AF with appropriate (+/-)30° lamellar microstructure is the use of aligned electrospun scaffolds seeded with mesenchymal stem cells (MSCs) and assembled into angle-ply laminates (APL). Previous work indicates that opposing lamellar orientation is necessary for development of near native uniaxial tensile properties. However, most native AF tensile loads are applied biaxially, as the disk is subjected to multi-axial loads and is constrained by its attachments to the vertebral bodies. Thus, the objective of this study was to evaluate the biaxial mechanical response of engineered AF bilayers, and to determine the importance of opposing lamellar structure under this loading regime. Opposing bilayers, which replicate native AF structure, showed a significantly higher modulus in both testing directions compared to parallel bilayers, and reached ∼60% of native AF biaxial properties. Associated with this increase in biaxial properties, significantly less shear, and significantly higher stretch in the fiber direction, was observed. These results provide additional insight into native tissue structure-function relationships, as well as new benchmarks for engineering functional AF tissue constructs.
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Affiliation(s)
- Tristan P Driscoll
- McKay Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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Iatridis JC, Nicoll SB, Michalek AJ, Walter BA, Gupta MS. Role of biomechanics in intervertebral disc degeneration and regenerative therapies: what needs repairing in the disc and what are promising biomaterials for its repair? Spine J 2013; 13:243-62. [PMID: 23369494 PMCID: PMC3612376 DOI: 10.1016/j.spinee.2012.12.002] [Citation(s) in RCA: 219] [Impact Index Per Article: 19.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/11/2011] [Revised: 07/26/2012] [Accepted: 12/09/2012] [Indexed: 02/03/2023]
Abstract
BACKGROUND CONTEXT Degeneration and injuries of the intervertebral disc (IVD) result in large alterations in biomechanical behaviors. Repair strategies using biomaterials can be optimized based on the biomechanical and biological requirements of the IVD. PURPOSE To review the present literature on the effects of degeneration, simulated degeneration, and injury on biomechanics of the IVD, with special attention paid to needle puncture injuries, which are a pathway for diagnostics and regenerative therapies and the promising biomaterials for disc repair with a focus on how those biomaterials may promote biomechanical repair. STUDY DESIGN A narrative review to evaluate the role of biomechanics on disc degeneration and regenerative therapies with a focus on what biomechanical properties need to be repaired and how to evaluate and accomplish such repairs using biomaterials. Model systems for the screening of such repair strategies are also briefly described. METHODS Articles were selected from two main PubMed searches using keywords: intervertebral AND biomechanics (1,823 articles) and intervertebral AND biomaterials (361 articles). Additional keywords (injury, needle puncture, nucleus pressurization, biomaterials, hydrogel, sealant, tissue engineering) were used to narrow the articles down to the topics most relevant to this review. RESULTS Degeneration and acute disc injuries have the capacity to influence nucleus pulposus (NP) pressurization and annulus fibrosus (AF) integrity, which are necessary for an effective disc function and, therefore, require repair. Needle injection injuries are of particular clinical relevance with the potential to influence disc biomechanics, cellularity, and metabolism, yet these effects are localized or small and more research is required to evaluate and reduce the potential clinical morbidity using such techniques. NP replacement strategies, such as hydrogels, are required to restore the NP pressurization or the lost volume. AF repair strategies including cross-linked hydrogels, fibrous composites, and sealants offer promise for regenerative therapies to restore AF integrity. Tissue engineered IVD structures, as a single implantable construct, may promote greater tissue integration due to the improved repair capacity of the vertebral bone. CONCLUSIONS IVD height, neutral zone characteristics, and torsional biomechanics are sensitive to specific alterations in the NP pressurization and AF integrity and must be addressed for an effective functional repair. Synthetic and natural biomaterials offer promise for NP replacement, AF repair, as an AF sealant, or whole disc replacement. Meeting mechanical and biological compatibilities are necessary for the efficacy and longevity of the repair.
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Affiliation(s)
- James C. Iatridis
- Leni and Peter W. May Department of Orthopaedics, Mount Sinai School of Medicine, New York, NY
| | - Steven B. Nicoll
- Department of Biomedical Engineering, The City College of New York, New York, NY
| | - Arthur J. Michalek
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT
| | - Benjamin A. Walter
- Leni and Peter W. May Department of Orthopaedics, Mount Sinai School of Medicine, New York, NY,Department of Biomedical Engineering, The City College of New York, New York, NY
| | - Michelle S. Gupta
- Department of Biomedical Engineering, The City College of New York, New York, NY
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Abstract
Tissue engineering (TE) has promise as a biological solution and a disease modifying treatment for arthritis. Although cartilage can be generated by TE, substantial inter- and intra-donor variability makes it impossible to guarantee optimal, reproducible results. TE cartilage must be able to perform the functions of native tissue, thus mechanical and biological properties approaching those of native cartilage are likely a pre-requisite for successful implantation. A quality-control assessment of these properties should be part of the implantation release criteria for TE cartilage. Release criteria should certify that selected tissue properties have reached certain target ranges, and should be predictive of the likelihood of success of an implant in vivo. Unfortunately, it is not currently known which properties are needed to establish release criteria, nor how close one has to be to the properties of native cartilage to achieve success. Achieving properties approaching those of native cartilage requires a clear understanding of the target properties and reproducible assessment methodology. Here, we review several main aspects of quality control as it applies to TE cartilage. This includes a look at known mechanical and biological properties of native cartilage, which should be the target in engineered tissues. We also present an overview of the state of the art of tissue assessment, focusing on native articular and TE cartilage. Finally, we review the arguments for developing and validating non-destructive testing methods for assessing TE products.
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Affiliation(s)
- Joseph M. Mansour
- Skeletal Research Center, Department of Biology Case Western Reserve University Cleveland, OH, 44106
| | - Jean F. Welter
- Skeletal Research Center, Department of Biology Case Western Reserve University Cleveland, OH, 44106
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Abstract
STUDY DESIGN In vitro study of the biological response of the intervertebral disc (IVD) to cyclic torsion by using bovine caudal IVDs. OBJECTIVE To evaluate the biological response of the IVD to repetitive cyclic torsion of varying magnitudes at a physiological frequency. SUMMARY OF BACKGROUND DATA Mechanical loading is known to be a risk factor for disc degeneration (DD) but the role of torsion in DD is controversial. It has been suggested that a small magnitude of spinal rotation decreases spinal pressure, increases spinal length, and enhances nutrition exchange in the IVD. However, athletes who participate actively in sports involving torsional movement of the spine are frequently diagnosed with DD and/or disc prolapse. METHODS Bovine caudal discs with end plates were harvested and kept in custom-made chambers for in vitro culture and mechanical stimulation. Torsion was applied to the explants for 1 hour/day over four consecutive days by using a servohydraulic testing machine. The biological response was evaluated by cell viability, metabolic activity, gene expression, glycosaminoglycan content, and histological evaluation. RESULTS A significantly higher cell viability was found in the inner annulus of the 2˚ torsion group than in the static control group. A trend of decreasing metabolic activity in the nucleus pulposus with increasing torsion magnitude was observed. Apoptotic activity in the nucleus pulposus significantly increased with 5˚ torsion. No statistical significant difference in gene expression was found between the three torsion angles. No visible change in matrix organization could be observed by histological evaluation. CONCLUSION The IVD can tolerate short-term repetitive cyclic torsion, as tested in this study. A small angle of cyclic torsion can be beneficial to the IVD in organ culture, possibly by improving nutrition and waste exchange, whereas large torsion angle may cause damage to disc in the long term.
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73
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Walter B, Korecki C, Purmessur D, Roughley P, Michalek A, Iatridis J. Complex loading affects intervertebral disc mechanics and biology. Osteoarthritis Cartilage 2011; 19:1011-8. [PMID: 21549847 PMCID: PMC3138834 DOI: 10.1016/j.joca.2011.04.005] [Citation(s) in RCA: 143] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/04/2011] [Revised: 04/12/2011] [Accepted: 04/13/2011] [Indexed: 02/02/2023]
Abstract
BACKGROUND Complex loading develops in multiple spinal motions and in the case of hyperflexion is known to cause intervertebral disc (IVD) injury. Few studies have examined the interacting biologic and structural alterations associated with potentially injurious complex loading, which may be an important contributor to chronic progressive degeneration. OBJECTIVE This study tested the hypothesis that low magnitudes of axial compression loading applied asymmetrically can induce IVD injury affecting cellular and structural responses in a large animal IVD ex-vivo model. METHODS Bovine caudal IVDs were assigned to either a control or wedge group (15°) and placed in organ culture for 7 days under static 0.2MPa load. IVD tissue and cellular responses were assessed through confined compression, qRT-PCR, histology and structural and compositional measurements, including Western blot for aggrecan degradation products. RESULTS Complex loading via asymmetric compression induced cell death, an increase in caspase-3 staining (apoptosis), a loss of aggrecan and an increase in aggregate modulus in the concave annulus fibrosis. While an up-regulation of MMP-1, ADAMTS4, IL-1β, and IL-6 mRNA, and a reduced aggregate modulus were induced in the convex annulus. CONCLUSION Asymmetric compression had direct deleterious effects on both tissue and cells, suggesting an injurious loading regime that could lead to a degenerative cascade, including cell death, the production of inflammatory mediators, and a shift towards catabolism. This explant model is useful to assess how injurious mechanical loading affects the cellular response which may contribute to the progression of degenerative changes in large animal IVDs, and results suggest that interventions should address inflammation, apoptosis, and lamellar integrity.
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Affiliation(s)
- B.A. Walter
- College of Engineering and Mathematical Sciences, University of Vermont, Burlington, VT, USA, Orthopaedic Research Laboratories, Mount Sinai School of Medicine, New York, NY, USA
| | - C.L. Korecki
- College of Engineering and Mathematical Sciences, University of Vermont, Burlington, VT, USA
| | - D. Purmessur
- College of Engineering and Mathematical Sciences, University of Vermont, Burlington, VT, USA, Orthopaedic Research Laboratories, Mount Sinai School of Medicine, New York, NY, USA
| | - P.J. Roughley
- Shriners Hospital for Children, Montreal, QC, Canada
| | - A.J. Michalek
- College of Engineering and Mathematical Sciences, University of Vermont, Burlington, VT, USA
| | - J.C. Iatridis
- College of Engineering and Mathematical Sciences, University of Vermont, Burlington, VT, USA, Orthopaedic Research Laboratories, Mount Sinai School of Medicine, New York, NY, USA
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74
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Abstract
STUDY DESIGN In vitro and in vivo rat tail model to assess effects of torsion on intervertebral disc biomechanics and gene expression. OBJECTIVE Investigate effects of torsion on promoting biosynthesis and producing injury in rat caudal intervertebral discs. SUMMARY OF BACKGROUND DATA Torsion is an important loading mode in the disc and increased torsional range of motion is associated with clinical symptoms from disc disruption. Altered elastin content is implicated in disc degeneration, but its effects on torsional loading are unknown. Although effects of compression have been studied, the effect of torsion on intervertebral disc gene expression is unknown. METHODS In vitro biomechanical tests were performed in torsion on rat tail motion segments subjected to 4 treatments: elastase, collagenase, genipin, control. In vivo tests were performed on rats with Ilizarov-type fixators implanted to caudal motion segments with five 90 minute loading groups: 1 Hz cyclic torsion to ± 5 ± 15° and ± 30°, static torsion to + 30°, and sham. Anulus and nucleus tissues were separately analyzed using qRT-PCR for gene expression of anabolic, catabolic, and proinflammatory cytokine markers. RESULTS In vitro tests showed decreased torsional stiffness following elastase treatment and no changes in stiffness with frequency. In vivo tests showed no significant changes in dynamic stiffness with time. Cyclic torsion upregulated elastin expression in the anulus fibrosus. Up regulation of TNF-α and IL-1β was measured at ±30°. CONCLUSION We conclude that strong differences in the disc response to cyclic torsion and compression are apparent with torsion increasing elastin expression and compression resulting in a more substantial increase in disc metabolism in the nucleus pulposus. Results highlight the importance of elastin in torsional loading and suggest that elastin remodels in response to shearing. Torsional loading can cause injury to the disc at excessive amplitudes that are detectable biologically before they are biomechanically.
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75
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O’Connell GD, Vresilovic EJ, Elliott DM. Human intervertebral disc internal strain in compression: the effect of disc region, loading position, and degeneration. J Orthop Res 2011; 29:547-55. [PMID: 21337394 PMCID: PMC3428014 DOI: 10.1002/jor.21232] [Citation(s) in RCA: 116] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/11/2010] [Accepted: 06/28/2010] [Indexed: 02/04/2023]
Abstract
The primary function of the disc is mechanical; therefore, degenerative changes in disc mechanics and the interactions between the annulus fibrosus (AF) and nucleus pulposus (NP) in nondegenerate and degenerate discs are important to functional evaluation. The disc experiences complex loading conditions, including mechanical interactions between the pressurized NP and the surrounding fiber-reinforced AF. Our objective was to noninvasively evaluate the internal deformations of nondegenerate and degenerate human discs under axial compression with flexion, neutral, and extension positions using magnetic resonance imaging and image correlation. The side of applied bending (e.g., anterior AF in flexion) had higher tensile radial and compressive axial strains, and the opposite side of bending exhibited tensile axial strains even though the disc was loaded under axial compression. Degenerated discs exhibited higher compressive axial and tensile radial strains, which suggest that load distribution through the disc subcomponents are altered with degeneration, likely due to the depressurized NP placing more of the applied load directly on the AF. The posterior AF exhibited higher compressive axial and higher tensile radial strains than the other AF regions, and the strains were not correlated with degeneration, suggesting this region undergoes high strains throughout life, which may predispose it to failure and tears. In addition to understanding internal disc mechanics, this study provides important new data into the changes in internal strain with degeneration, data for validation of finite element models, and provides a technique and baseline data for evaluating surgical treatments.
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Affiliation(s)
- Grace D. O’Connell
- Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia PA
| | | | - Dawn M. Elliott
- Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia PA
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Hollingsworth NT, Wagner DR. Modeling shear behavior of the annulus fibrosus. J Mech Behav Biomed Mater 2011; 4:1103-14. [PMID: 21783119 DOI: 10.1016/j.jmbbm.2011.03.019] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2010] [Revised: 02/24/2011] [Accepted: 03/15/2011] [Indexed: 11/25/2022]
Abstract
Modeling the mechanical properties of the annulus fibrosus has two distinct challenges: the complex loading state experienced in vivo and the anisotropic, nonlinear nature of the tissue. Previous efforts to model the annulus fibrosus have not considered shear data in the analysis, yet the shear response may be critical to understanding tissue behavior and damage. In this study, we compared four hyperelastic constitutive models fitted to uniaxial and biaxial tension, confined compression, and shear experiments from the literature. Models were either directly based on Spencer's formulation for a fiber-reinforced composite material with two equivalent fiber families or represented the annulus as two transversely isotropic materials. Each model was composed of additive strain energy terms that represent specific constituents of the annulus fibrosus (proteoglycan matrix, collagen fibers, and collagen crosslinks). Additionally, we investigated the effect of restricting the fibers such that they supported tensile loads only. Best fit coefficients for these models were calculated both including and excluding shear data from the regression. All of the models fit the data well when shear data was excluded from the regression; when shear data was included in the regression, two models that were based on Spencer's formulation performed better than the others. None of the models could consistently predict data that was not included in the regression. Restricting the fibers to support only tensile loads had only a modest effect on the fit of the models, but did alter which constituent carried the majority of the strain energy in shear deformations. Our study suggests that a single hyperelastic model may capture the anisotropic behavior of the annulus fibrosus for multiple loading cases, including shear. However, care must be taken when extrapolating these models to additional deformations outside of the training dataset.
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Affiliation(s)
- Nathaniel T Hollingsworth
- Soft Tissue Research Laboratory, Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, United States
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Jacobs NT, Smith LJ, Han WM, Morelli J, Yoder JH, Elliott DM. Effect of orientation and targeted extracellular matrix degradation on the shear mechanical properties of the annulus fibrosus. J Mech Behav Biomed Mater 2011; 4:1611-9. [PMID: 22098863 DOI: 10.1016/j.jmbbm.2011.03.016] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2011] [Revised: 02/28/2011] [Accepted: 03/08/2011] [Indexed: 10/18/2022]
Abstract
The intervertebral disc experiences combinations of compression, torsion, and bending that subject the disc substructures, particularly the annulus fibrosus (AF), to multidirectional loads and deformations. Combined tensile and shear loading is a particularly important loading paradigm, as compressive loads place the AF in circumferential hoop tension, and spine torsion or bending induces AF shear. Yet the anisotropy of AF mechanical properties in shear, as well as important structure-function mechanisms governing this response, are not well-understood. The objective of this study, therefore, was to investigate the effects of tissue orientation and enzymatic degradation of glycosaminoglycan (GAG) and elastin on AF shear mechanical properties. Significant anisotropy was found: the circumferential shear modulus, Gθz, was an order of magnitude greater than the radial shear modulus, Grθ. In the circumferential direction, prestrain significantly increased the shear modulus, suggesting an important role for collagen fiber stretch in shear properties for this orientation. While not significant and highly variable, ChABC treatment to remove GAG increased the circumferential shear modulus compared to PBS control (p=0.15). Together with the established literature for tensile loading of fiber-reinforced GAG-rich tissues, the trends for changes in shear modulus with ChABC treatment reflect complex, structure-function relationships between GAG and collagen that potentially occur over several hierarchical scales. Elastase digestion did not significantly affect shear modulus with respect to PBS control; further contributing to the notion that circumferential shear modulus is dominated by collagen fiber stretch. The results of this study highlight the complexity of the structure-function relationships that govern the mechanical response of the AF in radial and circumferential shear, and provide new and more accurate data for the validation of material models and tissue-engineered disc replacements.
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Affiliation(s)
- Nathan T Jacobs
- Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, PA 19104-6081, United States
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Gregory DE, Veldhuis JH, Horst C, Wayne Brodland G, Callaghan JP. Novel lap test determines the mechanics of delamination between annular lamellae of the intervertebral disc. J Biomech 2011; 44:97-102. [DOI: 10.1016/j.jbiomech.2010.08.031] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/02/2009] [Revised: 08/21/2010] [Accepted: 08/24/2010] [Indexed: 11/17/2022]
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The effects of needle puncture injury on microscale shear strain in the intervertebral disc annulus fibrosus. Spine J 2010; 10:1098-105. [PMID: 20971041 PMCID: PMC2991597 DOI: 10.1016/j.spinee.2010.09.015] [Citation(s) in RCA: 73] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/11/2010] [Revised: 08/26/2010] [Accepted: 09/23/2010] [Indexed: 02/03/2023]
Abstract
BACKGROUND CONTEXT Needle puncture of the intervertebral disc (IVD) is required for delivery of therapeutic agents to the nucleus pulposus and for some diagnostic procedures. Needle puncture has also been implicated as an initiator of disc degeneration. It is hypothesized that needle puncture may initiate IVD degeneration by altering microscale mechanical behavior in the annulus fibrosus (AF). PURPOSE Quantify the changes in AF microscale strain behavior resulting from puncture with a hypodermic needle. STUDY DESIGN Cadaveric IVD tissue explant study. METHODS Annulus fibrosus explants from bovine caudal IVDs that had been punctured radially with hypodermic needles were loaded in dynamic sinusoidal shear while being imaged with a confocal microscope. Digital image analysis was used to quantify local tissue strain and damage propagation with repeated shearing. RESULTS Needle puncture changed the distribution of microscale shear strains in the AF under load from homogenous (equal to far field) to a distinct pattern of high (4× far field) and low (0.25× far field) strain areas. Repeated loading did not cause further growth of the disruption beyond the second cycle. CONCLUSIONS Needle puncture results in a drastic alteration of microscale strain behavior in the AF under load. This alteration may directly initiate disc degeneration by being detrimental to tissue-cell mechanotransduction.
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80
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Lai JH, Levenston ME. Meniscus and cartilage exhibit distinct intra-tissue strain distributions under unconfined compression. Osteoarthritis Cartilage 2010; 18:1291-9. [PMID: 20633686 PMCID: PMC3268046 DOI: 10.1016/j.joca.2010.05.020] [Citation(s) in RCA: 56] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/25/2010] [Revised: 05/02/2010] [Accepted: 05/29/2010] [Indexed: 02/02/2023]
Abstract
OBJECTIVE To examine the functional behavior of the surface layer of the meniscus by investigating depth-varying compressive strains during unconfined compression. DESIGN Pairs of meniscus and articular cartilage explants (n=12) site-matched at the tibial surfaces were subjected to equilibrium unconfined compression at 5, 10, 15, and 20% compression under fluorescence imaging. Two-dimensional (2D) deformations were tracked using digital image correlation (DIC). For each specimen, local compressive engineering strains were determined in 200 μm layers through the depth of the tissue. In samples with sharp strain transitions, bilinear regressions were used to characterize the surface and interior tissue compressive responses. RESULTS Meniscus and cartilage exhibited distinct depth-dependent strain profiles during unconfined compression. All cartilage explants had elevated compressive engineering strains near the surface, consistent with previous reports. In contrast, half of the meniscus explants tested had substantially stiffer surface layers, as indicated by surface engineering strains that were ∼20% of the applied compression. In the remaining samples, surface and interior engineering strains were comparable. 2D Green's strain maps revealed highly heterogeneous compressive and shear strains throughout the meniscus explants. In cartilage, the maximum shear strain appeared to be localized at 100-250 μm beneath the articular surface. CONCLUSIONS Meniscus was characterized by highly heterogeneous strains during compression. In contrast to cartilage, which consistently had a compliant surface region, meniscal explants were either substantially stiffer near the surface or had comparable compressive stiffness through the depth. The relatively compliant interior may allow the meniscus to maintain a consistent surface contour while deforming during physiologic loading.
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Affiliation(s)
| | - Marc E. Levenston
- Department of Mechanical Engineering, Stanford University, Stanford, CA 94305-4038,Correspondence to: Marc E. Levenston, Ph.D., Department of Mechanical Engineering, 233 Durand Building, Stanford University, Stanford, CA 94305-4038, 650.723.9464 (P), 650.725.1587 (F),
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81
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Quinn KP, Winkelstein BA. Full field strain measurements of collagenous tissue by tracking fiber alignment through vector correlation. J Biomech 2010; 43:2637-40. [DOI: 10.1016/j.jbiomech.2010.05.008] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2010] [Revised: 05/10/2010] [Accepted: 05/10/2010] [Indexed: 11/30/2022]
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82
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Noailly J, Planell JA, Lacroix D. On the collagen criss-cross angles in the annuli fibrosi of lumbar spine finite element models. Biomech Model Mechanobiol 2010; 10:203-19. [DOI: 10.1007/s10237-010-0227-5] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2009] [Accepted: 05/17/2010] [Indexed: 11/28/2022]
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Barbir A, Michalek AJ, Abbott RD, Iatridis JC. Effects of enzymatic digestion on compressive properties of rat intervertebral discs. J Biomech 2010; 43:1067-73. [PMID: 20116063 DOI: 10.1016/j.jbiomech.2009.12.005] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2009] [Revised: 12/17/2009] [Accepted: 12/17/2009] [Indexed: 01/07/2023]
Abstract
Enzymatic treatments were applied to rat motion segments to establish structure-function relationships and determine mechanical parameters most sensitive to simulated remodeling and degeneration. Rat caudal and lumbar disc biomechanical behaviors were evaluated to improve knowledge of their similarities and differences due to their frequent use during in vivo models. Caudal motion segments were assigned to four groups: soaked (control), genipin treated, elastase treated, and collagenase treated. Fresh lumbar and caudal discs were also compared. The mechanical protocol involved five force-controlled loading stages: equilibration, cyclic compression-tension, quasi-static compression, frequency sweep, and creep. Crosslinking was found to have the greatest effect on IVD properties at resting stress. Elastin's role was greatest in tension and at higher force conditions, where GAG content was also a contributing factor. Collagenase treatment caused tissue compaction, which impacted mechanical properties at both high and low force conditions. Equilibration creep and cyclic compression-tension tests were the mechanical tests most sensitive to alterations in specific matrix constituents. Caudal and lumbar motion segments had many similarities but biomechanical differences suggested some distinctions in collagenous structure and water transport characteristics in addition to the geometric differences. Results provide a basis for interpreting biomechanical changes observed in animal model studies of degeneration and remodeling, and underscore the need to maintain and/or repair collagen integrity in IVD health and disease.
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Affiliation(s)
- Ana Barbir
- College of Engineering and Mathematical Sciences, University of Vermont, 33 Colchester Avenue, 207 Perkins Hall, Burlington, VT 05405, USA.
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Affiliation(s)
- James C. Iatridis
- School of Engineering and in the Department of Orthopaedics & Rehabilitation, University of Vermont, 33 Colchester Avenue, 201 Perkins Hall, Burlington, Vermont 05405, USA.
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Buckley MR, Bergou AJ, Fouchard J, Bonassar LJ, Cohen I. High-resolution spatial mapping of shear properties in cartilage. J Biomech 2009; 43:796-800. [PMID: 19896130 DOI: 10.1016/j.jbiomech.2009.10.012] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2009] [Revised: 10/01/2009] [Accepted: 10/05/2009] [Indexed: 10/20/2022]
Abstract
Structural properties of articular cartilage such as proteoglycan content, collagen content and collagen alignment are known to vary over length scales as small as a few microns (Bullough and Goodfellow, 1968; Bi et al., 2006). Characterizing the resulting variation in mechanical properties is critical for understanding how the inhomogeneous architecture of this tissue gives rise to its function. Previous studies have measured the depth-dependent shear modulus of articular cartilage using methods such as particle image velocimetry (PIV) that rely on cells and cell nuclei as fiducial markers to track tissue deformation (Buckley et al., 2008; Wong et al., 2008a). However, such techniques are limited by the density of trackable markers, which may be too low to take full advantage of optical microscopy. This limitation leads to noise in the acquired data, which is often exacerbated when the data is manipulated. In this study, we report on two techniques for increasing the accuracy of tissue deformation measurements. In the first technique, deformations were tracked in a grid that was photobleached on each tissue sample (Bruehlmann et al., 2004). In the second, a numerical technique was implemented that allowed for accurate differentiation of optical displacement measurements by minimizing the propagated experimental error while ensuring that truncation error associated with local averaging of the data remained small. To test their efficacy, we employed these techniques to compare the depth-dependent shear moduli of neonatal bovine and adult human articular cartilage. Using a photobleached grid and numerical optimization to gather and analyze data led to results consistent with those reported previously (Buckley et al., 2008; Wong et al., 2008a), but with increased spatial resolution and characteristic coefficients of variation that were reduced up to a factor of 3. This increased resolution allowed us to determine that the shear modulus of neonatal bovine and adult human tissue both exhibit a global minimum at a depth z of around 100 microm and plateau at large depths. The consistency of the depth dependence of |G*|(Z) for adult human and neonatal bovine tissue suggests a functional advantage resulting from this behavior.
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Affiliation(s)
- Mark R Buckley
- Department of Physics, Clark Hall C7, Cornell University, Ithaca, NY 14853, USA.
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