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PubMed1993

Mechanotransduction across the cell surface and through the cytoskeleton.

Mechanical stresses were applied directly to cell surface receptors with a magnetic twisting device. The extracellular matrix receptor, integrin beta 1, induced focal adhesion formation and supported a force-dependent stiffening response, whereas nonadhesion receptors did not. The cytoskeletal stiffness (ratio of stress to strain) increased in direct proportion to the applied stress and required intact microtubules and intermediate filaments as well as microfilaments. Tensegrity models that incorporate mechanically interdependent struts and strings that reorient globally in response to a localized stress mimicked this response. These results suggest that integrins act as mechanoreceptors and transmit mechanical signals to the cytoskeleton. Mechanotransduction, in turn, may be mediated simultaneously at multiple locations inside the cell through force-induced rearrangements within a tensionally integrated cytoskeleton.

Actin Cytoskeleton↗
PubMed2001

Pulse pressure, endothelium function, and arterial stiffness in spontaneously hypertensive rats.

In rats, removal of the carotid arterial or abdominal aortic endothelium results in an acute increase of diameter and compliance. In humans, acute local administration of a specific NO synthase inhibitor increases radial artery compliance but not the diameter. The purpose of this review is to determine whether in spontaneously hypertensive rats (SHR), a cause-and-effect relationship may be observed between endothelial function and arterial stiffness with possible consequences on pulse pressure (PP) control. The study is based on a comparative time-dependent analysis of the following in young and old SHR: aortic blood pressure measurements and reactivity, ultrasonographic arterial stiffness assessment, aortic histomorphometry and staining, and molecular biology with evaluation of endothelium function. In young SHR, aortic mean blood pressure and PP increase proportionally, whereas isobaric arterial stiffness is unchanged or poorly modified. The endothelial NO response to norepinephrine is normal or upregulated as a response to predominant vasoconstrictive influences. In contrast, in old SHR, PP and mean blood pressure change disproportionately with age, together with an enhanced isobaric arterial stiffness. The endothelial NO response to norepinephrine is abolished, in association with endothelium-dependent heightened norepinephrine reactivity and enhanced accumulation of vessel extracellular matrix. In this latter case, exogenous NO acutely and selectively lowers the increased PP. Thus, during SHR aging, a negative feedback may be observed between NO bioactivity and PP through changes in arterial structure and function. Whether this alteration contributes to the development of systolic hypertension in old populations remains to be determined.

Aging↗
PubMed2001

Mechanical properties and cell cultural response of polycaprolactone scaffolds designed and fabricated via fused deposition modeling.

A number of different processing techniques have been developed to design and fabricate three-dimensional (3D) scaffolds for tissue-engineering applications. The imperfection of the current techniques has encouraged the use of a rapid prototyping technology known as fused deposition modeling (FDM). Our results show that FDM allows the design and fabrication of highly reproducible bioresorbable 3D scaffolds with a fully interconnected pore network. The mechanical properties and in vitro biocompatibility of polycaprolactone scaffolds with a porosity of 61 +/- 1% and two matrix architectures were studied. The honeycomb-like pores had a size falling within the range of 360 x 430 x 620 microm. The scaffolds with a 0/60/120 degrees lay-down pattern had a compressive stiffness and a 1% offset yield strength in air of 41.9 +/- 3.5 and 3.1 +/- 0.1 MPa, respectively, and a compressive stiffness and a 1% offset yield strength in simulated physiological conditions (a saline solution at 37 degrees C) of 29.4 +/- 4.0 and 2.3 +/- 0.2 MPa, respectively. In comparison, the scaffolds with a 0/72/144/36/108 degrees lay-down pattern had a compressive stiffness and a 1% offset yield strength in air of 20.2 +/- 1.7 and 2.4 +/- 0.1 MPa, respectively, and a compressive stiffness and a 1% offset yield strength in simulated physiological conditions (a saline solution at 37 degrees C) of 21.5 +/- 2.9 and 2.0 +/- 0.2 MPa, respectively. Statistical analysis confirmed that the five-angle scaffolds had significantly lower stiffness and 1% offset yield strengths under compression loading than those with a three-angle pattern under both testing conditions (p < or = 0.05). The obtained stress-strain curves for both scaffold architectures demonstrate the typical behavior of a honeycomb structure undergoing deformation. In vitro studies were conducted with primary human fibroblasts and periosteal cells. Light, environmental scanning electron, and confocal laser microscopy as well as immunohistochemistry showed cell proliferation and extracellular matrix production on the polycaprolactone surface in the 1st culturing week. Over a period of 3-4 weeks in a culture, the fully interconnected scaffold architecture was completely 3D-filled by cellular tissue. Our cell culture study shows that fibroblasts and osteoblast-like cells can proliferate, differentiate, and produce a cellular tissue in an entirely interconnected 3D polycaprolactone matrix.

Biocompatible Materials↗
PubMed2001

Age-associated changes in cardiac matrix and integrins.

The progressive shift from young age to senescence is characterized by structural and functional changes in the cardiac extracellular matrix (ECM), which supports and aligns myocytes and blood vessels, and maintains myocardial mass, structure and function. As cardiac function declines with advancing age, ECM collagen and fibronectin influence diastolic stiffness. ECM binding to membrane-bound receptors, or integrins, directly links ECM to cardiac muscle and fibroblast cells, affording it the permissive role to modulate heart function. To better understand the ECM structure-function relationship in the old heart, we studied the relative protein content of these ECM proteins and integrins across three age groups. Old Balb-c mice (20 months) exhibit biventricular, cardiac hypertrophy, and greater left ventricular (LV) collagen, fibronectin, alpha 1 and alpha 5 integrin protein than middle-aged (12 months) or young (2 months) LV (P<0.05). beta1 integrin protein content is lower in old LV (P<0.05). These data show that advancing age is associated with greater collagen, fibronectin, alpha 1 and alpha 5 integrin content, suggesting that these matrix proteins undergo coordinated regulation in the aging heart. The differential integrin and ECM protein content suggests that there is regulatory signaling to the fibroblasts, which maintain the cardiac ECM.

Aging↗
PubMed2006

Medial collateral ligament healing in macrophage metalloelastase (MMP-12)-deficient mice.

Medial collateral ligament (MCL) injuries heal by a wound repair scar response controlled by a complex cellular and cytokine environment. Many enzymes participate in wound repair, particularly the matrix metalloproteinases. We hypothesize macrophage metalloelastase (MME/MMP-12) deficiency results in impaired healing of MCL injury. One hundred fifty MME-deficient and 150 WT (MME+/+) mice underwent knee MCL transection with the opposite knee as a sham operated control. Mice were sacrificed at 3, 7, 28, 42, and 56 days. At each of the five time points, 15 mice were utilized for biological and 15 were utilized for biomechanical testing. Outcome measures were the presence of macrophages to represent the inflammatory phase of wound healing, collagen synthesis to assay for matrix repair, and biomechanical testing for repair strength. Immunohistochemistry demonstrated significantly fewer macrophages in cut MCLs from MME-deficient mice versus wild-type (WT) mice at 3, 7, 28, and 42 days (all p<or=0.04). In situ hybridization to Col1a1 mRNA in the MME-deficient cut MCLs at 7, 28, and 42 day time points showed a decreased level of type I pro-collagen mRNA compared to the WT cut MCLs (p<0.05). Biomechanical testing revealed cut ligaments from MME-deficient mice had significantly lower ultimate force and stiffness compared to cut ligaments from WT mice (p<0.001), with maximal differences of 40% at 7 days for ultimate force and 28 days for stiffness (p<0.05 by Tukey post hoc test). We conclude MME is important in the multifactorial cascade of knee MCL injury healing, showing significant differences in both the early inflammatory and in the matrix tissue synthesis phases.

Animals↗
PubMed2005

Analysis of stored osteochondral allografts at the time of surgical implantation.

BACKGROUND: To date, the morphological, biochemical, and biomechanical characteristics of articular cartilage in osteochondral allografts that have been stored have not been fully described. HYPOTHESIS: Osteochondral allografts procured and stored commercially for a standard period as determined by tissue banking protocol will have compromised chondrocyte viability but preserved extracellular matrix quality. STUDY DESIGN: Controlled laboratory study. METHODS: Unused cartilage from 16 consecutive osteochondral allografts was sampled during surgery after tissue bank processing and storage. Ten grafts were examined for cell viability and viable cell density using confocal microscopy, proteoglycan synthesis via 35SO4 uptake, and glycosaminoglycan content and compared with fresh cadaveric articular cartilage. Biomechanical assessment was performed on the 6 remaining grafts by measuring the indentation stiffness of the cartilage. RESULTS: The mean storage time for the transplanted specimens was 20.3 +/- 2.9 days. Chondrocyte viability, viable cell density, and 35SO4 uptake were significantly lower in allografts at implantation when compared to fresh, unstored controls, whereas matrix characteristics, specifically glycosaminoglycan content and biomechanical measures, were unchanged. In addition, chondrocyte viability in the stored allografts was preferentially decreased in the superficial zone of cartilage. CONCLUSION: Human osteochondral allografts stored for a standard period (approximately 3 weeks) before implantation undergo decreases in cell viability, especially in the critically important superficial zone, as well as in cell density and metabolic activity, whereas matrix and biomechanical characteristics appear conserved. The exact clinical significance of these findings, however, is unknown, as there are no prospective studies examining clinical outcomes using grafts stored for extended periods. CLINICAL RELEVANCE: Surgeons who perform this procedure should understand the cartilage characteristics of the graft after 21 days of commercial storage in serum-free media.

Cartilage, Articular↗
PubMed2001

Mechanical consequences of bone loss in cancellous bone.

The skeleton is continuously being renewed in the bone remodeling process. This prevents accumulation of damage and adapts the architecture to external loads. A side effect is a gradual decrease of bone mass, strength, and stiffness with age. We investigated the effects of bone loss on the load distribution and mechanical properties of cancellous bone using three-dimensional (3D) computer models. Several bone loss scenarios were simulated. Bone matrix was removed at locations of high strain, of low strain, and random throughout the architecture. Furthermore, resorption cavities and thinning of trabeculae were simulated. Removal of 7% of the bone mass at highly strained locations had deleterious effects on the mechanical properties, while up to 50% of the bone volume could be removed at locations of low strain. Thus, if remodeling would be initiated only at highly strained locations, where repair is likely needed, cancellous bone would be continuously at risk of fracture. Thinning of trabeculae resulted in relatively small decreases in stiffness; the same bone loss caused by resorption cavities caused large decreases in stiffness and high strain peaks at the bottom of the cavities. This explains that a reduction in the number and size of resorption cavities in antiresorptive drug treatment can result in large reductions in fracture risk, with small increases in bone mass. Strains in trabeculae surrounding a cavity increased by up to 1,000 microstrains, which could lead to bone apposition. These results give insight in the mechanical effects of bone remodeling and resorption at trabecular level.

Bone Remodeling↗
PubMed2005

Therapeutic effects of PG201, an ethanol extract from herbs, through cartilage protection on collagenase-induced arthritis in rabbits.

In order to assess the therapeutic effects of PG201 (an ethanol extract from herbs) on osteoarthritis, we investigated whether PG201 could suppress the disease progression of collagenase-induced arthritis (CNIA) in rabbits. The right knees of rabbits were injected intra-articularly with collagenase, and the rabbits were orally treated with distilled water (DW), PG201 (200 mg/kg) or diclofenac (DCF, 10 mg/kg) once a day for 8 weeks. Oral administration of PG201 significantly suppressed the stiffness and bone space narrowing. Cartilage erosion and GAG release (p<0.01) were considerably reduced in the knee joints. As well, the mRNA expression of matrix degradation enzymes including MMP-1, -3, and -13 was decreased. On the contrary, the concentrations of TIMP-2 in the synovial fluids were considerably amplified in the PG201 treated group (p<0.01), but not in the DCF treated group. The pathologic inflammatory molecules involved in cartilage destruction such as IL-1beta, PGE2, and NO were also diminished by PG201. Taken together, these results indicate that PG201 has therapeutic effects on CNIA through the prominent protection of cartilage. PG201 indeed has great potential as a form of treatment for osteoarthritis.

Animals↗
PubMed2005

Collagen fibril form and function.

The majority of collagen in the extracellular matrix is found in a fibrillar form, with long slender filaments each displaying a characteristic approximately 67?nm D-repeat. Here they provide the stiff resilient part of many tissues, where the inherent strength of the collagen triple helix is translated through a number of hierarchical levels to endow that tissue with its specific mechanical properties. A number of collagen types have important structural roles, either comprising the core of the fibril or decorating the fibril surface to give enhanced functionality. The architecture of subfibrillar and suprafibrillar structures (such as microfibrils), lateral crystalline and liquid crystal ordering, interfibrillar interactions, and fibril bundles is described. The fibril surface is recognized as an area that contains a number of intimate interactions between different collagen types and other molecular species, especially the proteoglycans. The interplay between molecular forms at the fibril surface is discussed in terms of their contribution to the regulation of fibril diameter and their role in interfibrillar interactions.

Animals↗
PubMed1997

[Experimental protocol for mechanical characterization of a femoral implant of carbon-Peek composite hip prosthesis in fatigue].

This study concerns the fatigue behavior of a C/Peek hip implant. It is now well-established that the extent of bone loss around a total hip arthroplasty stem is related to stress shielding process. Due to a modulus mismatch between the bone and the implant material, the load transfer to the stem decreases the mechanical stimulus needed by the bone to maintain its structure. Because of its low modulus of elasticity and its good resistance to fatigue in aeronautical applications, the Fiber Carbon/Peek composite could potentially replace some of the metal alloys used in hip stem implant. After a literature survey on biomechanical performances of some fiber carbon composites, including AS4/Peek, experimental quasi-static and fatigue compression tests have been performed on AS4/Peek hip implants. The structural and mechanical characterization of the injection moulded composite material has been realized. The prosthesis compression and fatigue behaviour have been studied with a joint-stimulating apparatus immersed in a physiological solution temperature controlled. Instead of the low specimen homogeneity, no fatigue damage has been revealed either by X-ray observations of stiffness measurements, till ten millions of cycles. The quasi-static compressive fracture morphology has been analyzed by S.E.M. and have shown a good fiber matrix bonding. This mechanical results would suggest that AS4/Peek hip stem are worthy of further investigation as implantable prostheses.

Biomechanical Phenomena↗
PubMed2003

Osteoarthritis-like changes and decreased mechanical function of articular cartilage in the joints of mice with the chondrodysplasia gene (cho).

OBJECTIVE: To investigate whether heterozygosity for a loss-of-function mutation in the gene encoding the alpha1 chain of type XI collagen (Col11a1) in mice (chondrodysplasia, cho) causes osteoarthritis (OA), and to understand the biochemical and biomechanical effects of this mutation on articular cartilage in knee and temporomandibular (TM) joints. METHODS: Articular cartilage from the knee and TM joints of mice heterozygous for cho (cho/+) and their wild-type littermates (+/+) was examined. The morphologic properties of cartilage were evaluated, and collagen fibrils were examined by transmission electron microscopy. Immunohistochemical staining was performed to examine the protein expression levels of matrix metalloproteinase 3 (MMP-3) and MMP-13 in knee joints. In 6-month-old animals, fixed-charge density was determined using a semiquantitative histochemical method, and tensile stiffness was determined using an osmotic loading technique. RESULTS: The diameter of collagen fibrils in articular cartilage of knee joints from heterozygous cho/+ mice was increased relative to that in control cartilage, and histologic analysis showed OA-like degenerative changes in knee and TM joints, starting at age 3 months. The changes became more severe with aging. At 3 months, protein expression for MMP-3 was increased in knee joints from cho/+ mice. At 6 months, protein expression for MMP-13 was higher in knee joints from cho/+ mice than in joints from their wild-type littermates, and negative fixed-charge density was significantly decreased. Moreover, tensile stiffness in articular cartilage of knee joints from cho/+ mice was moderately reduced and was inversely correlated with the increase in articular cartilage degeneration. CONCLUSION: Heterozygosity for a loss-of-function mutation in Col11a1 results in the development of OA in the knee and TM joints of cho/+ mice. Morphologic and biochemical evidence of OA appears to precede significant mechanical changes, suggesting that the cho mutation leads to OA through a mechanism that does not initially involve mechanical factors.

Animals↗
PubMedDate not supplied

A mechanism for the decrease in stiffness of bioprosthetic heart valve tissues after cross-linking.

It has been reported that the elastic modulus of glutaraldehyde treated bioprosthetic valve materials is lower than that of fresh tissues. This finding appears paradoxic, because cross-linked tissues are usually stiffer than fresh ones. To investigate this phenomenon, the stiffness of 12 each of fresh and glutaraldehyde treated strips of porcine aortic valve cusps was measured at different levels of applied preload. To eliminate shrinkage effects, tissues were fixed while constrained at the gauge length. From paired comparisons of fresh versus fixed, it was found that glutaraldehyde fixed materials had a consistently lower stiffness than fresh tissue at low loads (0.39 N/m for treated vs 0.54 N/m for fresh at a 150 N/m preload), but no difference was found at high loads (1.09 N/m for treated vs 1.07 N/m for fresh at 600 N/ m preload). It is concluded that the observed drop in elastic modulus at low loads is a phenomenon associated with cross-linking of the collagen fiber matrix. Because a cross-linked matrix offers greater resistance to stretch, fixed tissue generates higher tensions at lower strains, at a point on the loading curve where the collagen fibers are more crimped and the slope of the curve is lower. At high loads, however, the stiffness of fixed tissues is equivalent to that of fresh. Measuring stiffness at a given stress or tension, rather than at a particular state of collagen fiber crimp, leads to the apparent lower stiffness of fixed tissues.

Animals↗
PubMed1986

A compliant interface for total knee arthroplasty.

Low pressure sensitive Fujifilm was used to measure the load distribution between the resected tibial surface and a tibial component at axial loads up to 3,000 N for a rigid interface, a compliant interface of dacron double-sided velour, and a cemented interface. The pressure patterns consisted of a multitude of small red dots, generally reflecting the slight irregularities of the cut surface and the stiffness of the cancellous bone at the surface. The pressure patterns were photographed with high-contrast film and input into a computer using a photodiode matrix camera. The data were analyzed to yield the number of contact points for each sample. The velour was more effective in distribution of load to the proximal tibia than the rigid and cemented interfaces, while there was no significant difference between the cemented interface and the rigid interface. A second series of tests showed significant increases in contact points from rigid to one layer to two layers of velour. Cyclic axial loading tests were performed to study the characteristics of rigid and compliant interfaces in a model of in vitro subsidence. Static pressure patterns taken at regular intervals showed that subsidence occurred in vitro in up to 1/3 of the tibias, and that the regions of load transfer could change with time. A model of subsidence was proposed and it was suggested that a velour layer could inhibit the subsidence.

Biomechanical Phenomena↗
PubMed1999

The mineralization density of iliac crest bone from children with osteogenesis imperfecta.

We studied iliac crest biopsy cores taken from young individuals with osteogenesis imperfecta of several types, and from age-matched normals; the same samples had been used in prior studies using conventional light microscopic histomorphometric procedures. The PMMA blocks were micro-milled to a fine finish, carbon coated, and imaged using backscattered electrons (BSE) in an automated digital scanning electron microscope (SEM). For comparison of BSE signal levels between samples, microscope operation parameters were standardized by reference to halogenated dimethacrylate standards, and recording data from stereological arrays of 512*512 nonoverlapping pixels at 3.5 micrometer separation. All OI types showed higher average mineralization densities than age- and site-matched normals. This is interpreted as the result of the failure in matrix assembly, such that it has a higher water volume fraction available for mineral deposition. Added to the net deficit in bone quantity, the predicted higher stiffness of the more mineralized bone will account for much of the observed 'brittleness' that characterizes this class of genetic disease. The mean mineralization density, which was higher in types III, IV, and V than in type I, appears to be correlated with disease severity.

Adolescent↗
PubMed1997

Optical and mechanical determination of Poisson's ratio of adult bovine humeral articular cartilage.

The equilibrium stiffness of articular cartilage is controlled by flow-independent elastic properties (Young's modulus, ES, and Poisson's ratio, v(s)) of the hydrated tissue matrix. In the current study, an optical (microscopic) method has been developed for the visualization of boundaries of cylindrical bovine humeral head articular cartilage disks (n = 9), immersed in physiological solution, and compressed in unconfined geometry. This method allowed a direct, model-independent estimation of Poisson's ratio of the tissue at equilibrium, as well as characterization of the shape changes of the sample during the nonequilibrium dynamic phase. In addition to optical analyses, the equilibrium behavior of cartilage disks in unconfined and confined ramp-stress relaxation tests provided a direct estimation of the aggregate modulus, H(a) and Young's modulus and, indirectly, Poisson's ratio for the articular cartilage. The mean value for Poisson's ratio obtained from the optical analysis was 0.185 +/- 0.065 (mean +/- S.D., n = 9). Values of elastic parameters obtained from the mechanical tests were 0.754 +/- 0.198 MPa, 0.677 +/- 0.223 MPa, and 0.174 +/- 0.106 for H(a), ES, and v(s), respectively (mean +/- S.D., n = 7). The similar v(s)-values obtained with optical and mechanical techniques imply that, at equilibrium for these two tests, the isotropic model is acceptable for mechanical analysis. However, the microscopic technique revealed that the lateral expansion, especially during the initial phase of relaxation, was inhomogeneous through the tissue depth. The superficial cartilage zone expanded less than the radial zone. The zonal differences in expansion were attributed to the known zonal differences in the fibrillar collagen architecture and proteoglycan concentration.

Analysis of Variance↗
PubMed2006

Sloppy-model universality class and the Vandermonde matrix.

In a variety of contexts, physicists study complex, nonlinear models with many unknown or tunable parameters to explain experimental data. We explain why such systems so often are sloppy: the system behavior depends only on a few "stiff" combinations of the parameters and is unchanged as other "sloppy" parameter combinations vary by orders of magnitude. We observe that the eigenvalue spectra for the sensitivity of sloppy models have a striking, characteristic form with a density of logarithms of eigenvalues which is roughly constant over a large range. We suggest that the common features of sloppy models indicate that they may belong to a common universality class. In particular, we motivate focusing on a Vandermonde ensemble of multiparameter nonlinear models and show in one limit that they exhibit the universal features of sloppy models.

Algorithms↗
PubMed2005

Prestrain decreases cartilage susceptibility to injury by ramp compression in vitro.

BACKGROUND: Injurious mechanical loading of articular cartilage can be an initiating factor in the development of degenerative joint disease. The tissue response to compression depends on the loading conditions and matrix mechanical properties. The short-term loading history of cartilage can affect its water content and microstructural organization, and may thereby modify its susceptibility to injury. We therefore examined the role of prestrain on the response of articular cartilage to injurious compression. METHODS: The full-thickness cartilage of bovine osteochondral explants was subjected to prestrains of 0, 5, 10, 25 or 50% before application of injurious ramp compression characterized by a strain rate of 7x10(-2) or 7x10(-3)s-1 and peak stress of 3.5 or 14 MPa. Effects of prestrain were evaluated in terms of fluid exudation, tissue mechanical stiffening, and the tissue response to injurious compression as characterized by macroscopic crack formation, cell viability and glycosaminoglycan release to culture media. RESULTS: Macroscopic crack formation due to injurious compression decreased with increasing prestrain in association with lower cell mortality. Significantly decreased susceptibility to injury was already evident for 10% prestrain. In contrast, explant mechanical stiffness was unchanged up to 25% prestrain. CONCLUSION: Findings demonstrate that compressive strains due to the short-term loading history of cartilage may strongly reduce its susceptibility to mechanical injury. Conversely, matrix swelling may render cartilage more vulnerable to injury. The cartilage response to injurious compression is therefore strongly influenced by matrix fluid content, and possibly also by other structural parameters such as collagen fiber orientation.

Animals↗
PubMed2003

Interstitial protein alterations in rabbit vocal fold with scar.

Fibrous and interstitial proteins compose the extracellular matrix of the vocal fold lamina propria and account for its biomechanic properties. Vocal fold scarring is characterized by altered biomechanical properties, which create dysphonia. Although alterations of the fibrous proteins have been confirmed in the rabbit vocal fold scar, interstitial proteins, which are known to be important in wound repair, have not been investigated to date. Using a rabbit model, interstitial proteins decorin, fibromodulin, and fibronectin were examined immunohistologically, two months postinduction of vocal fold scar by means of forcep biopsy. Significantly decreased decorin and fibromodulin with significantly increased fibronectin characterized scarred vocal fold tissue. The implications of altered interstitial proteins levels and their affect on the fibrous proteins will be discussed in relation to increased vocal fold stiffness and viscosity, which characterizes vocal fold scar.

Animals↗