Dynamic Compressive Stiffness Research Articles

Repetitive strikes loading organ culture model to investigate the biological and biomechanical responses of the intervertebral disc.

Disc degeneration is associated with repetitive violent injuries. This study aims to explore the impact of repetitive strikes loading on the biology and biomechanics of intervertebral discs (IVDs) using an organ culture model. IVDs from the bovine tail were isolated and cultured in a bioreactor, with exposure to various loading conditions. The control group was subjected to physiological loading, while the model group was exposed to either one strike loading (compression at 38% of IVD height) or repetitive one strike loading (compression at 38% of IVD height). Disc height and dynamic compressive stiffness were measured after overnight swelling and loading. Furthermore, histological morphology, cell viability, and gene expression were analyzed on Day 32. Glycosaminoglycan (GAG) and nitric oxide (NO) release in conditioned medium were also analyzed. The repetitive one strike group exhibited early disc degeneration, characterized by decreased dynamic compression stiffness, the presence of annulus fibrosus clefts, and degradation of the extracellular matrix. Additionally, this group demonstrated significantly higher levels of cell death (p < 0.05) and glycosaminoglycan (GAG) release (p < 0.05) compared to the control group. Furthermore, upregulation of MMP1, MMP13, and ADAMTS5 was observed in both nucleus pulposus (NP) and annulus fibrosus (AF) tissues of the repetitive one strike group (p < 0.05). The one strike group exhibited annulus fibrosus clefts but showed no gene expression changes compared to the control group. This study shows that repetitive violent injuries lead to the degeneration of a healthy bovine IVDs, thereby providing new insights into early-stage disc degeneration.

Programmed NP Cell Death Induced by Mitochondrial ROS in a One-Strike Loading Disc Degeneration Organ Culture Model.

Increasing evidence has indicated that mitochondrial reactive oxygen species (ROS) play critical roles in mechanical stress-induced lumbar degenerative disc disease (DDD). However, the detailed underlying pathological mechanism needs further investigation. In this study, we utilized a one-strike loading disc degeneration organ culture model to explore the responses of intervertebral discs (IVDs) to mechanical stress. IVDs were subjected to a strain of 40% of the disc height for one second and then cultured under physiological loading. Mitoquinone mesylate (MitoQ) or other inhibitors were injected into the IVDs. IVDs subjected to only physiological loading culture were used as controls. Mitochondrial membrane potential was significantly depressed immediately after mechanical stress (P < 0.01). The percentage of ROS-positive cells significantly increased in the first 12 hours after mechanical stress and then declined to a low level by 48 hours. Pretreatment with MitoQ or rotenone significantly decreased the proportion of ROS-positive cells (P < 0.01). Nucleus pulposus (NP) cell viability was sharply reduced at 12 hours after mechanical stress and reached a stable status by 48 hours. While the levels of necroptosis- and apoptosis-related markers were significantly increased at 12 hours after mechanical stress, no significant changes were observed at day 7. Pretreatment with MitoQ increased NP cell viability and alleviated the marker changes by 12 hours after mechanical stress. Elevated mitochondrial ROS levels were also related to extracellular matrix (ECM) degeneration signs, including catabolic marker upregulation, anabolic marker downregulation, increased glycosaminoglycan (GAG) loss, IVD dynamic compressive stiffness reduction, and morphological degradation changes at the early time points after mechanical stress. Pretreatment with MitoQ alleviated some of these degenerative changes by 12 hours after mechanical stress. These changes were eliminated by day 7. Taken together, our findings demonstrate that mitochondrial ROS act as important regulators of programmed NP cell death and ECM degeneration in IVDs at early time points after mechanical stress.

One strike loading organ culture model to investigate the post-traumatic disc degenerative condition.

SummaryBackgroundAcute trauma on intervertebral discs (IVDs) is thought to be one of the risk factors for IVD degeneration. The pathophysiology of IVD degeneration induced by single high impact mechanical injury is not very well understood. The aim of this study was using a post-traumatic IVD model in a whole organ culture system to analyze the biological and biomechanical consequences of the single high-impact loading event on the cultured IVDs.MethodsIsolated healthy bovine IVDs were loaded with a physiological loading protocol in the control group or with injurious loading (compression at 50% of IVD height) in the one strike loading (OSL) group. After another 1 day (short term) or 8 days (long term) of whole organ culture within a bioreactor, the samples were collected to analyze the cell viability, histological morphology and gene expression. The conditioned medium was collected daily to analyze the release of glycosaminoglycan (GAG) and nitric oxide (NO).ResultsThe OSL IVD injury group showed signs of early degeneration including reduction of dynamic compressive stiffness, annulus fibrosus (AF) fissures and extracellular matrix degradation. Compared to the control group, the OSL model group showed more severe cell death (P ​< ​0.01) and higher GAG release in the culture medium (P ​< ​0.05). The MMP and ADAMTS families were up-regulated in both nucleus pulposus (NP) and AF tissues from the OSL model group (P ​< ​0.05). The OSL injury model induced a traumatic degenerative cascade in the whole organ cultured IVD.ConclusionsThe present study shows a single hyperphysiological mechanical compression applied to healthy bovine IVDs caused significant drop of cell viability, altered the mRNA expression in the IVD, and increased ECM degradation. The OSL IVD model could provide new insights into the mechanism of mechanical injury induced early IVD degeneration.The translational potential of this articleThis model has a high potential for investigation of the degeneration mechanism in post-traumatic IVD disease, identification of novel biomarkers and therapeutic targets, as well as screening of treatment therapies.

Development of an ex vivo cavity model to study repair strategies in loaded intervertebral discs.

The aim of this study was to compare two approaches for the delivery of biomaterials to partially nucleotomised intervertebral discs in whole organ culture under loading. Such models can help to bridge the gap between in vitro and in vivo studies by assessing (1) suitability of biomaterial delivery and defect closure methods, (2) effect of mechanical loading and (3) tissue response. Mechanical performance of bovine discs filled with a hyaluronan-based thermoreversible hydrogel delivered through the annulus fibrosus (AF) or the bony endplate (EP) was evaluated under cyclic axial loading in a bioreactor. The loading protocol was optimised to achieve physiological disc height changes in nucleotomised discs. A loading regime of 0.06±0.02MPa, 0.1Hz, 6h daily was applied on the nucleotomised discs. Disc height and stiffness were tracked for 5days, followed by histological analyses. Creation of a defect is less demanding for AF approach, while sealing is superior with the EP approach. Dynamic compressive stiffness is reduced following nucleotomy, with no significant difference between the two approaches. Disc height loss was higher, disc height recovery was lower and region around the defect with reduced cell viability was smaller for AF-approached than EP-approached discs. Two alternative methods for biomaterial testing in whole organ culture under loading were developed. Such models bring insights on the ability of the biomaterial to restore the mechanical behaviour of the discs. From a clinical perspective, the cavity models can simulate treatment of nucleotomy after disc herniation in young patients, whereby the remaining nucleus pulposus is still functional and therefore at high risk of re-herniation, though the defect may differ from the clinical situation.

Polyurethane scaffold with in situ swelling capacity for nucleus pulposus replacement

Nucleus pulposus (NP) replacement offers a minimally invasive alternative to spinal fusion or total disc replacement for the treatment of intervertebral disc (IVD) degeneration. This study aimed to develop a cytocompatible NP replacement material, which is feasible for non-invasive delivery and tunable design, and allows immediate mechanical restoration of the IVD. A bi-phasic polyurethane scaffold was fabricated consisting of a core material with rapid swelling property and a flexible electrospun envelope. The scaffold was assessed in a bovine whole IVD organ culture model under dynamic load for 14 days. Nucleotomy was achieved by incision through the endplate without damaging the annulus fibrosus. After implantation of the scaffold and in situ swelling, the dynamic compressive stiffness and disc height were restored immediately. The scaffold also showed favorable cytocompatibility for native disc cells. Implantation of the scaffold in a partially nucleotomized IVD down-regulated catabolic gene expression, increased proteoglycan and type II collagen intensity and decreased type I collagen intensity in remaining NP tissue, indicating potential to retard degeneration and preserve the IVD cell phenotype. The scaffold can be delivered in a minimally invasive manner, and the geometry of the scaffold post-hydration is tunable by adjusting the core material, which allows individualized design.

Biomimetic Nucleus Pulposus Replacement for the Treatment of Degenerative Disc Disease

Introduction Nucleus pulposus (NP) replacement offers an alternative minimally invasive treatment for traditional spinal fusion or total disc replacement for degenerative disc disease (DDD). Recently, a novel polyurethane scaffold (PUS) with swelling capability in situ was developed to restore the mechanical functions of the intervertebral disc (IVD). In addition, a fibrinogen–hyaluronic acid (FBG–HA) conjugate-based hydrogel was manufactured to mimic the native NP extracellular matrix. The aim of this study is to evaluate the PUS and the FBG–HA hydrogel, with/without transglutaminase crosslinked bone morphogenetic protein (TG-BMP) 2/7 heterodimer in an organ culture system under dynamic load. Materials and Methods Discoid/ravioli-shaped polyurethane scaffolds were manufactured with a PU hydrogel-based core with swelling capacity in between two electrospun nanofiber envelope sheets. In addition, FBG–HA conjugate solution was synthesized with 235 kDa HA at FBG/HA w/w ratio of 17:1. FBG–HA hydrogels were prepared by mixing ⅔ volume of FBG–HA conjugate solution with ⅓ volume of thrombin solution (5.2 U/mL). Bovine IVDs with endplates were nucleotomized by incision through the endplate and refilled with either (1) PUS, (2) PUS surrounded by 50 to 80 µL FBG–HA or (3) PUS surrounded by 50 to 80µL FBG–HA containing 5,000 ng/mL TG–BMP 2/7. The endplate defect was closed with an endogenous endplate stopper and sealed with polymethyl methacrylate. Empty discs served as negative controls. To assess the mechanical compatibility of the different implants, dynamic compressive stiffness modulus (DCSM) was measured for each disc at different time points: intact, after nucleotomy, after refilling with biomaterial and free swelling recovery, after 3-hour dynamic load at 0 to 0.1 MPa, 0.1 Hz within a bioreactor system, and after free swelling recovery overnight. Moreover, biomaterials were evaluated in an organ culture system under dynamic load for 14 days at 0 to 0.1 MPa, 0.1 Hz for 3 hours/d. Disc height was recorded after load and recovery at day 1, 7, and 14. After 14 days, disc tissue was harvested and gene expression analyzed using real-time PCR. Glycosaminoglycan- (GAG) and collagen-content of the disc tissue was assessed and proteoglycan synthesis was analyzed by Sulfur-35 (35S) incorporation measurement. Histology was performed using Safranin O/Fast Green staining. One-way ANOVA was used to determine the statistical significance. Results After dynamic load, all three implant groups maintained their disc height, while it dropped by 7% in the empty controls ( p &lt; 0.001 vs. implant groups). The PUS and addition of FBG–HA hydrogel caused immediate restoration of DCSM (PUS: 73 ± 21%; p &lt; 0.05 vs. empty control: 25 ± 5%). While addition of FBG–HA hydrogel surrounding the PUS delayed the stiffness restoring effect, stiffness also increased to 69 ± 16% after dynamic load and free swelling recovery ( p &lt; 0.05 vs. empty control: 27 ± 5%). GAG-/collagen-content was maintained in all three implant groups. Aggrecan and collagen-2 gene expression were upregulated in NP cells by TG-BMP2/7 and FBG–HA hydrogel, respectively, although not significantly. Conclusion Results indicate that PUS is able to restore the mechanical functions of nucleotomized discs. Addition of FBG–HA hydrogel does not affect the swelling capacity of PUS in situ. FBG–HA hydrogel and TG-BMP2/7 may further support the biological repair of NP tissue. Acknowledgment Funded by the European Commission under the FP7–NMP Project NPMimetic.

AN INNOVATIVE PROCEDURE FOR ESTIMATING CONTACT FORCE DURING IMPACT

Certain types of damage generated by the impact of a solid object are correlated with the amount of force that is localized around the area of contact between the impactor and the surface of the target. The stiffer the impactor, the higher the contact force when the amount of impulse delivered by the impact is held constant. Thus, realistic simulations of the contact force by finite element analysis (FEA) require representative, and detailed, information of the dynamic compressive properties of both the impactor and the target. In situations where relevant properties of the impactor material have not been documented, the estimation of contact force is filled with uncertainties. In addressing this challenge an innovative experimental–simulation calibration procedure involving the use of a custom made (inexpensive) tubular device is presented in this paper for measuring the dynamic compressive stiffness properties of impactor objects. Given the calibrated values the amount of contact force generated by the same impactor material in projected scenarios could be simulated with good accuracies for predicting damage.

Nucleus Pulposus Replacement: Hydrogel or Scaffold?—An Organ Culture Study under Dynamic Load

IntroductionNucleus pulposus (NP) replacement offers a minimally invasive disc regeneration treatment alternative to traditional fusion or total disc replacement. An ideal NP replacement material s...

Cartilage-like mechanical properties of poly (ethylene glycol)-diacrylate hydrogels

Hydrogels prepared from poly-(ethylene glycol) (PEG) have been used in a variety of studies of cartilage tissue engineering. Such hydrogels may also be useful as a tunable mechanical material for cartilage repair. Previous studies have characterized the chemical and mechanical properties of PEG-based hydrogels, as modulated by precursor molecular weight and concentration. Cartilage mechanical properties vary substantially, with maturation, with depth from the articular surface, in health and disease, and in compression and tension. We hypothesized that PEG hydrogels could mimic a broad range of the compressive and tensile mechanical properties of articular cartilage. The objective of this study was to characterize the mechanical properties of PEG hydrogels over a broad range and with reference to articular cartilage. In particular, we assessed the effects of PEG precursor molecular weight (508 Da, 3.4 kDa, 6 kDa, and 10 kDa) and concentration (10–40%) on swelling property, equilibrium confined compressive modulus (HA0), compressive dynamic stiffness, and hydraulic permeability (kp0) of PEG hydrogels in static/dynamic confined compression tests, and equilibrium tensile modulus (Eten) in tension tests. As molecular weight of PEG decreased and concentration increased, hydrogels exhibited a decrease in swelling ratio (31.5–2.2), an increase in HA0 (0.01–2.46 MPa) and Eten (0.02–3.5 MPa), an increase in dynamic compressive stiffness (0.055–42.9 MPa), and a decrease in kp0 (1.2 × 10−15 to 8.5 × 10−15 m2/(Pa s)). The frequency-dependence of dynamic compressive stiffness amplitude and phase, as well as the strain-dependence of permeability, were typical of the time- and strain-dependent mechanical behavior of articular cartilage. HA0 and Eten were positively correlated with the final PEG concentration, accounting for swelling. These results indicate that PEG hydrogels can be prepared to mimic many of the static and dynamic mechanical properties of articular cartilage.

Identifying the dynamic compressive stiffness of a prospective biomimetic elastomer by an inverse method

Soft elastomeric materials that mimic real soft human tissues are sought to provide realistic experimental devices to simulate the human body's response to blast loading to aid the development of more effective protective equipment. The dynamic mechanical behavior of these materials is often measured using a Kolsky bar because it can achieve both the high strain rates (>100s−1) and the large strains (>20%) that prevail in blast scenarios. Obtaining valid results is challenging, however, due to poor dynamic equilibrium, friction, and inertial effects. To avoid these difficulties, an inverse method was employed to determine the dynamic response of a soft, prospective biomimetic elastomer using Kolsky bar tests coupled with high-speed 3D digital image correlation. Individual tests were modeled using finite elements, and the dynamic stiffness of the elastomer was identified by matching the simulation results with test data using numerical optimization. Using this method, the average dynamic response was found to be nearly equivalent to the quasi-static response measured with stress–strain curves at compressive strains up to 60%, with an uncertainty of ±18%. Moreover, the behavior was consistent with the results in stress relaxation experiments and oscillatory tests although the latter were performed at lower strain levels.

Effects of dynamic compressive loading on chondrocyte biosynthesis in self-assembling peptide scaffolds

Dynamic mechanical loading has been reported to affect chondrocyte biosynthesis in both cartilage explant and chondrocyte-seeded constructs. In this study, the effects of dynamic compression on chondrocyte-seeded peptide hydrogels were analyzed for extracellular matrix synthesis and retention over long-term culture. Initial studies were conducted with chondrocyte-seeded agarose hydrogels to explore the effects of various non-continuous loading protocols on chondrocyte biosynthesis. An optimized alternate day loading protocol was identified that increased proteoglycan (PG) synthesis over control cultures maintained in free-swelling conditions. When applied to chondrocyte-seeded peptide hydrogels, alternate day loading stimulated PG synthesis up to two-fold higher than that in free-swelling cultures. While dynamic compression also increased PG loss to the medium throughout the 39-day time course, total PG accumulation in the scaffold was significantly higher than in controls after 16 and 39 days of loading, resulting in an increase in the equilibrium and dynamic compressive stiffness of the constructs. Viable cell densities of dynamically compressed cultures differed from free-swelling controls by less than 20%, demonstrating that changes in PG synthesis were due to an increase in the average biosynthesis per viable cell. Protein synthesis was not greatly affected by loading, demonstrating that dynamic compression differentially regulated the synthesis of PGs. Taken together, these results demonstrate the potential of dynamic compression for stimulating PG synthesis and accumulation for applications to in vitro culture of tissue engineered constructs prior to implantation.

Effects of harvest and selected cartilage repair procedures on the physical and biochemical properties of articular cartilage in the canine knee.

This study utilizes a canine model to quantify changes in articular cartilage 15-18 weeks after a knee joint is subjected to surgical treatment of isolated chondral defects. Clinical and experimental treatment of articular cartilage defects may include implantation of matrix materials or cells, or both. Three cartilage repair methods were evaluated: microfracture, microfracture and implantation of a type-II collagen matrix, and implantation of an autologous chondrocyte-seeded collagen matrix. The properties of articular cartilage in other knee joints subjected to harvest of articular cartilage from the trochlear ridge (to obtain cells for the cell-seeded procedure) were also evaluated. Physical properties (thickness, equilibrium compressive modulus, dynamic compressive stiffness, and streaming potential) and biochemical composition (hydration, glycosaminoglycan content, and DNA content) of the cartilage from sites distant to the surgical treatment were compared with values measured for site-matched controls in untreated knee joints. No significant differences were seen in joints subjected to any of the three cartilage repair procedures. However, a number of changes were induced by the harvest operation. The largest changes (displaying up to 3-fold increases) were seen in dynamic stiffness and streaming potential of patellar groove cartilage from joints subjected to the harvest procedure. Whether the changes reported will lead to osteoarthritic degeneration is unknown, but this study provides evidence that the harvest procedure associated with autologous cell transplantation for treatment of chondral defects may result in changes in the articular cartilage in the joint.

Molded neoprene liner pads for the poseidon missile launcher

AbstractNeoprene formulations and mechanical designs have been developed to meet special operational requirements for the Poseidon Missile Launch System liner. The liner supports the missile in an aligned position, provides shock mitigation, reacts properly with the missile during launch, and damps vibrations. Operational requirements include static and dynamic compressive stiffness, damping, and water drainability. A mechanical design was evolved that utilized notched, prebuckled struts as supporting members. Compression–deflection characteristics were modified by simultaneous changes in geometrical design and polymer formulation. A study of the effects of variations in polymer compounding and mechanical design on the compressive stiffness, rate sensitivity of the compression–deflection characteristics, and vibrational damping is described. Mold release agents and their role in successful production are discussed.