Measurement of the spatial redistribution of water in rabbit Achilles tendon in response to static tensile loading

The redistribution of water in response to static tensile loading was investigated in rabbit Achilles tendons in vitro. The distribution of water was measured along a radially oriented line, using a one-dimensional proton-density map created from fits to diffusion-weighted magnetic resonance (MR) data. Water movements were measured during application of tensile loads of 5N (N=7) and 10N (N=6). Water distribution along the line was measured before loading and up to 42 min after load application. Static loading with either 5 or 10N loads caused a steady increase in proton density in the outside edge (rim) of the tendon. The 10N load lowered the proton density in the core of the tendon, but did so in a single step that was observed when the load was applied. The 5N load caused no change in proton density in the core region. The immediate redistribution from the core was statistically significant for the 10N load, but not the 5N load application. Statistically significant within-group proton-density increases were observed in the rim after 42 min postload for all tendons irrespective of load condition. The rate of proton-density postload increase at the rim region did not depend upon load. The rate for the 5N load case was 0.010 +/- 0.002 min(-1) and 0.007 +/- 0.002 min(-1) in the 10N case. Thus, while generally consistent with an extrusion model, the data show other features that argue for a more complex model.     

Influence of a single loading episode on gene expression in healing rat Achilles tendons

Mechanical loading stimulates tendon healing via mechanisms that are largely unknown. Genes will be differently regulated in loaded healing tendons, compared with unloaded, just because of the fact that healing processes have been changed. To avoid such secondary effects and study the effect of loading per se, we therefore studied the gene expression response shortly after a single loading episode in otherwise unloaded healing tendons. The Achilles tendon was transected in 30 tail-suspended rats. The animals were let down from the suspension to load their tendons on a treadmill for 30 min once, 5 days after tendon transection. Gene expression was studied by Affymetrix microarray before and 3, 12, 24, and 48 h after loading. The strongest response in gene expression was seen 3 h after loading, when 150 genes were up- or downregulated (fold change ≥2, P ≤ 0.05). Twelve hours after loading, only three genes were upregulated, whereas 38 were downregulated. Fewer than seven genes were regulated after 24 and 48 h. Genes involved in the inflammatory response were strongly regulated at 3 and 12 h after loading; this included upregulation of iNOS, PGE synthase, and IL-1β. Also genes involved in wound healing/coagulation, angiogenesis, and production of reactive oxygen species were strongly regulated by loading. Microarray results were confirmed for 16 selected genes in a repeat experiment (N = 30 rats) using real-time PCR. It was also confirmed that a single loading episode on day 5 increased the strength of the healing tendon on day 12. In conclusion, the fact that there were hardly any regulated genes 24 h after loading suggests that optimal stimulation of healing requires a mechanical loading stimulus every day.     

The relation between collagen fibril kinematics and mechanical properties in the mitral valve anterior leaflet

We have recently demonstrated that the mitral valve anterior leaflet (MVAL) exhibited minimal hysteresis, no strain rate sensitivity, stress relaxation but not creep (Grashow et al., 2006, Ann Biomed Eng., 34(2), pp. 315-325; Grashow et al., 2006, Ann Biomed. Eng., 34(10), pp. 1509-1518). However, the underlying structural basis for this unique quasi-elastic mechanical behavior is presently unknown. As collagen is the major structural component of the MVAL, we investigated the relation between collagen fibril kinematics (rotation and stretch) and tissue-level mechanical properties in the MVAL under biaxial loading using small angle X-ray scattering. A novel device was developed and utilized to perform simultaneous measurements of tissue level forces and strain under a planar biaxial loading state. Collagen fibril D-period strain (epsilonD) and the fibrillar angular distribution were measured under equibiaxial tension, creep, and stress relaxation to a peak tension of 90 N/m. Results indicated that, under equibiaxial tension, collagen fibril straining did not initiate until the end of the nonlinear region of the tissue-level stress-strain curve. At higher tissue tension levels, epsilonD increased linearly with increasing tension. Changes in the angular distribution of the collagen fibrils mainly occurred in the tissue toe region. Using epsilonD, the tangent modulus of collagen fibrils was estimated to be 95.5+/-25.5 MPa, which was approximately 27 times higher than the tissue tensile tangent modulus of 3.58+/-1.83 MPa. In creep tests performed at 90 N/m equibiaxial tension for 60 min, both tissue strain and epsilonD remained constant with no observable changes over the test length. In contrast, in stress relaxation tests performed for 90 min epsilonD was found to rapidly decrease in the first 10 min followed by a slower decay rate for the remainder of the test. Using a single exponential model, the time constant for the reduction in collagen fibril strain was 8.3 min, which was smaller than the tissue-level stress relaxation time constants of 22.0 and 16.9 min in the circumferential and radial directions, respectively. Moreover, there was no change in the fibril angular distribution under both creep and stress relaxation over the test period. Our results suggest that (1) the MVAL collagen fibrils do not exhibit intrinsic viscoelastic behavior, (2) tissue relaxation results from the removal of stress from the fibrils, possibly by a slipping mechanism modulated by noncollagenous components (e.g. proteoglycans), and (3) the lack of creep but the occurrence of stress relaxation suggests a "load-locking" behavior under maintained loading conditions. These unique mechanical characteristics are likely necessary for normal valvular function.     

Tendon exhibits complex poroelastic behavior at the nanoscale as revealed by high-frequency AFM-based rheology

Tendons transmit load from muscle to bone by utilizing their unique static and viscoelastic tensile properties. These properties are highly dependent on the composition and structure of the tissue matrix, including the collagen I hierarchy, proteoglycans, and water. While the role of matrix constituents in the tensile response has been studied, their role in compression, particularly in matrix pressurization via regulation of fluid flow, is not well understood. Injured or diseased tendons and tendon regions that naturally experience compression are known to have alterations in glycosaminoglycan content, which could modulate fluid flow and ultimately mechanical function. While recent theoretical studies have predicted tendon mechanics using poroelastic theory, no experimental data have directly demonstrated such behavior. In this study, we use high-bandwidth AFM-based rheology to determine the dynamic response of tendons to compressive loading at the nanoscale and to determine the presence of poroelastic behavior. Tendons are found to have significant characteristic dynamic relaxation behavior occurring at both low and high frequencies. Classic poroelastic behavior is observed, although we hypothesize that the full dynamic response is caused by a combination of flow-dependent poroelasticity as well as flow-independent viscoelasticity. Tendons also demonstrate regional dependence in their dynamic response, particularly near the junction of tendon and bone, suggesting that the structural and compositional heterogeneity in tendon may be responsible for regional poroelastic behavior. Overall, these experiments provide the foundation for understanding fluid-flow-dependent poroelastic mechanics of tendon, and the methodology is valuable for assessing changes in tendon matrix compressive behavior at the nanoscale.     

The rigidity of repaired flexor tendons increases following ex vivo cyclic loading

Transected flexor tendons are typically treated by suture repair followed by rehabilitation that generates repetitive tendon loading. Recent results in an in vivo canine model indicate that during the first 10 days after injury and repair, there is an increase in the rigidity of the tendon repair site. Our objective was to determine whether or not ex vivo cyclic loading of repaired flexor tendons causes a similar increase in repair-site rigidity. We simulated 10 days of rehabilitation by applying 6000 loading cycles to repaired canine flexor tendons ex vivo at force levels generated during passive motion rehabilitation; we then evaluated their tensile mechanical properties. High-force (peak force, 17 N) cyclic loading increased repair-site rigidity by 100% and decreased repair-site strain by 50%, whereas low-force (5 N) loading did not change the properties of the repair site. This mechanical conditioning effect may explain, in part, the changes in tensile properties observed after only 10 days of healing in vivo. Mechanical conditioning of repaired flexor tendons by repetitive forces applied during rehabilitation may lead to increases in repair-site rigidity and decreases in strain, thereby altering the mechanical loading environment of tissues and cells at the repair site.     

Backbone dynamics of barstar: a (15)N NMR relaxation study

Backbone dynamics of uniformly (15)N-labeled barstar have been studied at 32 degrees C, pH 6.7, by using (15)N relaxation data obtained from proton-detected 2D (1)H-(15)N NMR spectroscopy. (15)N spin-lattice relaxation rate constants (R(1)), spin-spin relaxation rate constants (R(2)), and steady-state heteronuclear (1)H-(15)N NOEs have been determined for 69 of the 86 (excluding two prolines and the N-terminal residue) backbone amide (15)N at a magnetic field strength of 14.1 Tesla. The primary relaxation data have been analyzed by using the model-free formalism of molecular dynamics, using both isotropic and axially symmetric diffusion of the molecule, to determine the overall rotational correlation time (tau(m)), the generalized order parameter (S(2)), the effective correlation time for internal motions (tau(e)), and NH exchange broadening contributions (R(ex)) for each residue. As per the axially symmetric diffusion, the ratio of diffusion rates about the unique and perpendicular axes (D( parallel)/D( perpendicular)) is 0.82 +/- 0.03. The two results have only marginal differences. The relaxation data have also been used to map reduced spectral densities for the NH vectors of these residues at three frequencies: 0, omega(H), and omega(N), where omega(H),(N) are proton and nitrogen Larmor frequencies. The value of tau(m) obtained from model-free analysis of the relaxation data is 5.2 ns. The reduced spectral density analysis, however, yields a value of 5.7 ns. The tau(m) determined here is different from that calculated previously from time-resolved fluorescence data (4.1 ns). The order parameter ranges from 0.68 to 0.98, with an average value of 0.85 +/- 0.02. A comparison of the order parameters with the X-ray B-factors for the backbone nitrogens of wild-type barstar does not show any considerable correlation. Model-free analysis of the relaxation data for seven residues required the inclusion of an exchange broadening term, the magnitude of which ranges from 2 to 9.1 s(-1), indicating the presence of conformational averaging motions only for a small subset of residues.     

Pulse NMR study of structural characteristics of temperature-sensitive hydrogel

The proton relaxation behavior of a temperature-sensitive gel, observed through using a pulse NMR technique, was related to the structural characteristics of the gel. As the temperature increased, the pore size of the gel decreased gradually while the relaxation time constants, T and T, showed different patterns of change. It is believed that the proton relaxation was affected by a direct temperature effect and an indirect, temperature-related structural effect. The response of the protons to these effects varied depending on where the protons resided. A sudden increase in several NMR parameter values occurred at around 30°C, the lower critical solution temperature (LCST). This may be related to a transition state of the gel undergoing dramatic local non-uniform changes around the LCST.     

Different regions of bovine deep digital flexor tendon exhibit distinct elastic,but not viscous,mechanical properties under both compression and shear loading

Tendons in different locations function in unique, and at times complex, invivo loading environments. Specifically, some tendons are subjected to compression, shear and/or torsion in addition to tensile loading, which play an important role in regulating tendon properties. To date, there have been few studies evaluating tendon mechanics when loaded in compression and shear, which are particularly relevant for understanding tendon regions that experience such non-tensile loading during normal physiologic function. The objective of this study was to evaluate mechanical responses of different regions of bovine deep digital flexor tendons (DDFT) under compressive and shear loading, and correlate structural characteristics to functional mechanical properties. Distal and proximal regions of DDFT were evaluated in a custom-made loading system via three-step incremental stress-relaxation tests. A two-relaxation-time solid linear model was used to describe the viscoelastic response. Results showed large differences in the elastic behavior between regions: distal region stresses were 4–5 times larger than proximal region stresses during compression and 2–3 times larger during shear. Surprisingly, the viscous (i.e., relaxation) behavior was not different between regions for either compression or shear. Histological analysis showed that collagen and proteoglycan in the distal region distributed differently from the proximal region. Results demonstrate mechanical differences between two regions of DDFT under compression and shear loading, which are attributed to variations of composition and microstructural organization. These findings deepen our understanding of structure–function relationships of tendon, particularly for tissues adapted to supporting combinations of tension, compression, and shear in physiological loading environments.     

Mechanical stimulation induces pp125(FAK) and pp60(src) activity in an in vivo model of trabecular bone formation.

Utilizing an in vivo model of trabecular bone formation, we demonstrated the temporal and spatial activation of pp125(FAK) in response to specific mechanical load stimuli. Bone chambers equipped with hydraulic actuators were aseptically inserted into each proximal tibial metaphysis of adult, male dogs under general anesthesia. The load stimulus consisted of a trapezoidal waveform, with a maximum compressive load of 17.8 N, loading rate of 89 N/s, at 1 Hz frequency. One chamber was loaded for 2 (120 cycles), 15 (900 cycles), or 30 min (1,800 cycles), whereas the contralateral chamber served as unloaded control. Bone chambers were biopsied at postload time points of 0, 15, and 45 min. Load-induced activation of FAK was rapid, and the duration of activation was dependent on the number of applied load cycles. Mechanical stimulation increased the association of FAK with Src and the time course of complex formation paralleled the temporal activation of FAK. Evaluation of cryosections revealed prominent FAK immunoreactivity among marrow fibroblasts and stromal cells.     

Modulation of neonatal growth plate development by ex vivo intermittent mechanical stress

Although growth plate response to mechanical stress has been increasingly studied, our understanding of mechanical modulation of neonatal growth plate is incomplete, especially concerning biochemical changes. This study was designed to explore the cellular and biochemical responses of the cranial base growth plate (CBGP) explant upon cyclic loading. The growth plate with subchondral bone was aseptically isolated from each of 24 neonatal rabbits and fixated in an organ culture system. Cyclic loading was applied to growth plate explants at 200 mN and 1 Hz for 60 min (N=12), whereas control explants were immersed in organ culture for 60 min without mechanical loading (N=12). Computerized image analysis revealed that cyclic loading induced significantly more proliferating chondrocytes than unloaded controls (p<0.001), as well as significantly higher growth plate height at 856+/-30 microm than the unloaded controls at 830+/-36 microm (p<0.05). Immunoblotting with monoclonal antibodies (mAb) disclosed that the average mAb binding area for chondroitin sulfate was significantly higher in the loaded specimens than the unloaded controls at (p<0.001). The average mAb binding area for keratan sulfate was also significantly higher in the loaded specimens than the unloaded controls (p<0.01). Biochemical analysis showed that the average total hyaluronan content of loaded specimens at 0.25+/-0.06 microg/microg DNA was significantly higher than the unloaded controls at 0.09+/-0.05 microg/microg DNA (p<0.01). Taken together, these data suggest that brief doses of cyclic, intermittent forces activate cellular and molecular responses in the CBGP ex vivo. Whether hyaluronan-mediated pathway is involved in the biological responses of growth plate to mechanical loading warrants additional investigations.     

Mechanical effects of load speed on the human colon

The aim of this study was to examine the mechanical behavior of the colon using tensile tests under different loading speeds.Specimens were taken from different locations of the colonic frame from refrigerated cadavers. The specimens were submitted to uniaxial tensile tests after preconditioning using a dynamic load (1 m/s), intermediate load (10 cm/s), and quasi-static load (1 cm/s).A total of 336 specimens taken from 28 colons were tested. The stress-strain analysis for longitudinal specimens indicated a Young’s modulus of 3.17 ± 2.05 MPa under dynamic loading (1 m/s), 1.74 ± 1.15 MPa under intermediate loading (10 cm/s), and 1.76 ± 1.21 MPa under quasi-static loading (1 cm/s) with p < 0.001. For the circumferential specimen, the stress-strain curves indicated a Young’s modulus of 3.15 ± 1.73 MPa under dynamic loading (1 m/s), 2.14 ± 1.3 MPa under intermediate loading (10 cm/s), and 0.63 ± 1.25 MPa under quasi-static loading (1 cm/s) with p < 0.001. The curves reveal two types of behaviors of the colon: fast break behavior at high speed traction (1 m/s) and a lower break behavior for lower speeds (10 cm/s and 1 cm/s). The circumferential orientation required greater levels of stress and strain to obtain lesions than the longitudinal orientation. The presence of taeniae coli changed the mechanical response during low-speed loading.Colonic mechanical behavior varies with loading speeds with two different types of mechanical behavior: more fragile behavior under dynamic load and more elastic behavior for quasi-static load.     

Collagen fibril diameter distribution does not reflect changes in the mechanical properties of in vitro stress-deprived tendons

The purpose of this study was to determine if an association exists between the tensile properties and the collagen fibril diameter distribution in in vitro stress-deprived rat tail tendons. Rat tail tendons were paired into two groups of 21 day stress-deprived and 0 time controls and compared using transmission electron microscopy (n = 6) to measure collagen fibril diameter distribution and density, and mechanical testing (n =6) to determine ultimate stress and tensile modulus. There was a statistically significant decrease in both ultimate tensile strength (control: 17.95+/-3.99 MPa, stress-deprived: 6.79+/-3.91 MPa) and tensile modulus (control: 312.8+/-89.5 MPa, stress-deprived: 176.0+/-52.7 MPa) in the in vitro stress-deprived tendons compared to controls. However, there was no significant difference between control and stress-deprived tendons in the number of fibrils per tendon counted, mean fibril diameter, mean fibril density, or fibril size distribution. The results of this study demonstrate that the decrease in mechanical properties observed in in vitro stress-deprived rat tail tendons is not correlated with the collagen fibril diameter distribution and, therefore, the collagen fibril diameter distribution does not, by itself, dictate the decrease in mechanical properties observed in in vitro stress-deprived rat tail tendons.     

Effect of resistive loading on ventilatory response to hypoxia.

Ventilatory responses to hypoxia, with and without an inspiratory resistive load, were measured in eight normal subjects, using a rebreathing technique. During the studies, the end-tidal P-CO2 was kept constant at mixed venous level (Pv-CO2) by drawing expired gas through a variable CO2-absorbing bypass. The initial bag O2 concentration was 24% and rebreathing was continued until the O2 concentration in the bag fell to 6% or the subject's arterial oxygen saturation (Sa-O2), monitored continuously by ear oximetry, fell to 70%. Studies with and without the load were performed in a formally randomized order for each subject. Linear regressions for rise in ventilation against fall in Sa-O2 were calculated. The range of unloaded responses was 0.78-3.59 1/min per 1% fall in Sa-O2 and loaded responses 0.37-1.68 1/min per 1% fall in Sa-O2. In each subject, the slope of the response curve during loading fell by an almost constant fraction of the unloaded response, such that the ratio of loaded to unloaded slope in all subjects ranged from 0.41 to 0.48. However, the extrapolated intercept of the response curve on the Sa-O2 axis did not alter significantly indicating that the P-CO2 did not alter between experiments. These results suggest that the change in ventilatory response to hypoxia during inspiratory resistive loading is related to the mechanical load applied, with the loaded slope being directly proportional to the unloaded one.     

Cell wall mechanical properties as measured with bacterial thread made from Bacillus subtilis.

Engineering approaches used in the study of textile fibers have been applied to the measurement of mechanical properties of bacterial cell walls by using the Bacillus subtilis bacterial thread system. Improved methods have been developed for the production of thread and for measuring its mechanical properties. The best specimens of thread produced from cultures of strain FJ7 grown in TB medium at 20 degrees C varied in diameter by a factor of 1.09 over a 30-mm thread length. The stress-strain behavior of cell walls was determined over the range of relative humidities between 11 and 98%. Measurements of over 125 specimens indicated that cell wall behaved like other viscoelastic polymers, both natural and man-made, exhibiting relaxation under constant elongation and recovery upon load removal. This kinetic behavior and also the cell wall strength depended greatly on humidity. The recovery from extension observed after loading even up to a substantial fraction of the breaking load indicated that the properties measured were those of cell wall material rather than of behavior of the thread assemblage. Control experiments showed that neither drying of thread nor the length of time it remained dry before testing influenced the mechanical properties of the cell walls. Specimens drawn from TB medium and then washed in water and redrawn were found to be stiffer and stronger than controls not washed. However, tensile properties were not changed by exposure of cells to lysozyme before thread production. This suggests that glycan backbones are not arranged along the length of the cell cylinder. The strength of the cell wall in vivo was estimated by extrapolation to 100% relative humidity to be about 3 N/mm2. Walls of this strength would be able to bear a turgor pressure of 6 atm (ca. 607.8 kPa), but if the increase in strength of water-washed threads was appropriate, the figure could be 24 atm (ca. 2,431.2 kPa).     

Effects of stress shielding on the transverse mechanical properties of rabbit patellar tendons

With the aim of studying mechanisms of the remodeling of tendons and ligaments, the effects of stress shielding on the rabbit patellar tendon were studied by performing tensile and stress relaxation tests in the transverse direction. The tangent modulus, tensile strength, and strain at failure of non-treated, control patellar tendons in the transverse direction were 1272 kPa, 370 kPa, and 40.5 percent, respectively, whereas those of the tendons stress-shielded for 1 week were 299 kPa, 108 kPa, and 40.4 percent, respectively. Stress shielding markedly decreased tangent modulus and tensile strength in the transverse direction, and the decreases were larger than those in the longitudinal direction, which were determined in our previous study. For example, tensile strength in the transverse and longitudinal direction decreased to 29 and 50 percent of each control value, respectively, after 1 week stress shielding. In addition, the stress relaxation in the transverse direction of stress-shielded patellar tendons was much larger than that of nontreated, control ones. In contrast to longitudinal tensile tests for the behavior of collagen, transverse tests reflect the contributions of ground substances such as proteoglycans and mechanical interactions between collagen fibers. Ground substances provide lubrication and spacing between fibers, and also confer viscoelastic properties. Therefore, the results obtained from the present study suggest that ground substance matrix, and interfiber and fiber-matrix interactions have important roles in the remodeling response of tendons to stress.     

Experimental Evaluation of Fiber Orientation Based Material Properties of Skeletal Muscle in Tension

Biomechanical researches are essential to develop new techniques to improve the clinical relevance. Skeletal muscle generates the force which results in the motion of human body, so it is essential to study the mechanical and structural properties of skeletal muscle. Many researchers have carried out mechanical study of skeletal muscle with in-vivo testing. This work aims to examine anisotropic mechanical behavior of skeletal muscle with in vitro test (tensile test). It is important to understand the mechanical and structural behavior of skeletal muscle when it is subjected to external loading; the research aims to determine the structural properties of skeletal muscle by tensile testing. Tensile testing is performed on 5 samples of skeletal muscle of a goat at the rate of 1mm/min with fiber orientation along the length and 45° inclined to the length. It is found that muscle is stiffer in the direction parallel to the muscle fiber than at 45° to the muscle fibers. The tensile strength of the skeletal muscle along the fiber direction is 0.44 MPa at maximum load of 110 N and for direction 45° inclined to the muscle fibers, the strength is 0.234 MPa at max load 43 N. The displacement of Muscle sample against the maximum load is small along the length of the muscle fiber i.e. under longitudinal elongation [15.257 mm] as compared to 45° inclined to the length of skeletal muscle [17.775 mm] and under cross fiber elongation [19.7291mm by FEA]. The testing is not performed for 90° fiber orientation due to unavailability of soft tissue in cross fiber direction of the required specification, but finite element analysis is done on the skeletal muscle for the cross fiber orientation. As the fiber orientation within skeletal muscle differs with respect to the length of the muscle, the stiffness of skeletal muscle is also changing effectively. Hence skeletal muscle exhibits the anisotropic mechanical behavior.     

Mechanical properties of collagen fascicles from the rabbit patellar tendon

Tensile and viscoelastic properties of collagen fascicles of approximately 300 microns in diameter, which were obtained from rabbit patellar tendons, were studied using a newly designed micro-tensile tester. Their cross-sectional areas were determined with a video dimension analyzer combined with a CCD camera and a low magnification microscope. There were no statistically significant differences in tensile properties among the fascicles obtained from six medial-to-lateral locations of the patellar tendon. Tangent modulus, tensile strength, and strain at failure of the fascicles determined at about 1.5 percent/s strain rate were 216 +/- 68 MPa, 17.2 +/- 4.1 MPa, and 10.9 +/- 1.6 percent (mean +/- S.D.), respectively. These properties were much different from those of bulk patellar tendons; for example, the tensile strength and strain at failure of these fascicles were 42 percent and 179 percent of those of bulk tendons, respectively. Tangent modulus and tensile strength of collagen fascicles determined at 1 percent/s strain rate were 35 percent larger than those at 0.01 percent/s. The strain at failure was independent of strain rate. Relaxation tests showed that the reduction of stress was approximately 25 percent at 300 seconds. These stress relaxation behavior and strain rate effects of collagen fascicles differed greatly from those of bulk tendons. The differences in tensile and viscoelastic properties between fascicles and bulk tendons may be attributable to ground substances, mechanical interaction between fascicles, and the difference of crimp structure of collagen fibrils.     

Disc mechanics with trans-endplate partial nucleotomy are not fully restored following cyclic compressive loading and unloaded recovery

Mechanical function of the intervertebral disc is maintained through the interaction between the hydrated nucleus pulposus, the surrounding annulus fibrosus, and the superior and inferior endplates. In disc degeneration the normal transfer of load between disc substructures is compromised. The objective of this study was to explore the mechanical role of the nucleus pulposus in support of axial compressive loads over time. This was achieved by measuring the elastic slow ramp and viscoelastic stress-relaxation mechanical behaviors of cadaveric sheep motion segments before and after partial nucleotomy through the endplate (keeping the annulus fibrosus intact). Mechanics were evaluated at five conditions: Intact, intact after 10,000 cycles of compression, acutely after nucleotomy, following nucleotomy and 10,000 cycles of compression, and following unloaded recovery. Radiographs and magnetic resonance images were obtained to examine structure. Only the short time constant of the stress relaxation was altered due to nucleotomy. In contrast, cyclic loading resulted in significant and large changes to both the stiffness and stress relaxation behaviors. Moreover, the nucleotomy had little to no effect on the disc mechanics after cyclic loading, as there were no significant differences comparing mechanics after cyclic loading with or without the nucleotomy. Following unloaded recovery the mechanical changes that had occurred as a consequence of cyclic loading were restored, leaving only a sustained change in the short time constant due to the trans-endplate nucleotomy. Thus the swelling and redistribution of the remaining nucleus pulposus was not able to fully restore mechanical behaviors. This study reveals insights into the role of the nucleus pulposus in disc function, and provides new information toward the potential role of altered nucleus pulpous function in the degenerative cascade.     

A visco-hyperelastic constitutive approach for modeling polyvinyl alcohol sponge

This study proposes the quasi-linear viscoelastic (QLV) model to characterize the time dependent mechanical behavior of poly(vinyl alcohol) (PVA) sponges. The PVA sponges have implications in many viscoelastic soft tissues, including cartilage, liver, and kidney as an implant. However, a critical barrier to the use of the PVA sponge as tissue replacement material is a lack of sufficient study on its viscoelastic mechanical properties. In this study, the nonlinear mechanical behavior of a fabricated PVA sponge is investigated experimentally and computationally using relaxation and stress failure tests as well as finite element (FE) modeling. Hyperelastic strain energy density functions, such as Yeoh and Neo-Hookean, are used to capture the mechanical behavior of PVA sponge at ramp part, and viscoelastic model is used to describe the viscose behavior at hold part. Hyperelastic material constants are obtained and their general prediction ability is verified using FE simulations of PVA tensile experiments. The results of relaxation and stress failure tests revealed that Yeoh material model can define the mechanical behavior of PVA sponge properly compared with Neo-Hookean one. FE modeling results are also affirmed the appropriateness of Yeoh model to characterize the mechanical behavior of PVA sponge. Thus, the Yeoh model can be used in future biomechanical simulations of the spongy biomaterials. These results can be utilized to understand the viscoelastic behavior of PVA sponges and has implications for tissue engineering as scaffold.     

Local strain measurement reveals a varied regional dependence of tensile tendon mechanics on glycosaminoglycan content

Proteoglycans (PG) and their associated glycosaminoglycan (GAG) side chains are known to play a key role in the bearing of compressive loads in cartilage and other skeletal connective tissues. In tendons and connective tissues that are primarily loaded in tension, the influence of proteoglycans on mechanical behavior is debated due to conflicting experimental evidence that alternately supports or controverts a functional role of proteoglycans in bearing tensile load. In this study we sought to better reconcile these conflicting data by investigating the possibility that GAG content is differentially related to tensile tendon mechanics depending upon the anatomical subregion one considers. To test this hypothesis, we quantified the mechanical consequences of proteoglycan disruption within specific tendon anatomical subregions using an optical–mechanical measurement approach.Achilles tendons from adult mice were treated with chondroitinase ABC to obtain two groups consisting of native tendons and GAG-depleted tendons. All the tendons were mechanically tested and imaged with high-resolution digital video in order to optically quantify tendon strains. Tendon surface strains were locally analyzed in three main subregions: the central midsubstance, and the proximal and distal midsubstance near the muscle and bone insertions, respectively. Upon GAG digestion, the tendon midsubstance softened appreciably near the bone insertion, while elastic modulus in the central and proximal thirds was unchanged. Thus the contribution of PGs to tensile tendon mechanics is not straightforward and points to a heterogeneous and complex structure–function relationship in tendon. This study further highlights the importance of performing local strain analysis with regard to tensile tendon mechanics.