Compressive Mechanical Loading Research Articles

Mechanical loading recovers bone but not muscle lost during unloading

Space travel and prolonged bed rest are examples of mechanical unloading that induce significant muscle and bone loss. The compromised structure and function of bone and muscle owing to unloading make the reloading period a high risk for injury. To explore interactions between skeletal bone and muscle during reloading, we hypothesized that acute external mechanical loading of bone in combination with re-ambulation facilitates the proportional recovery of bone and muscle lost during hind limb suspension (HLS) unloading. Adult male C57Bl/6J mice were randomly assigned to a HLS or time-matched ground control (GC) group. After 2-weeks of HLS, separate groups of mice were studied at day 14 (no re-ambulation), day 28 (14 days re-ambulation) and day 56 (42 days re-ambulation); throughout the re-ambulation period, one limb received compressive mechanical loading and the contralateral limb served as an internal control. HLS induced loss of trabecular bone volume (BV/TV; −51 ± 2%) and muscle weight (−15 ± 2%) compared to GC at day 14. At day 28, the left tibia (re-ambulation only) of HLS mice had recovered approximately 20% of BV/TV lost during HLS, while the right tibia (re-ambulation and acute external mechanical loading) recovered to GC values of BV/TV (~100% recovery). At day 56, the right tibia continued to recover bone for some outcomes (trabecular BV/TV, trabecular thickness), while the left limb did not. Cortical bone displayed a delayed response to HLS, with a 10% greater decrease in BV/TV at day 28 compared to day 14. In contrast to bone, acute external mechanical loading during the re-ambulation period did not significantly increase muscle mass or protein synthesis in the gastrocnemius, compared to re-ambulation alone. Our results suggest acute external mechanical loading facilitates the recovery of bone during reloading following HLS unloading, but this does not translate to a concomitant recovery of muscle mass.

Maturity-dependent cartilage cell plasticity and sensitivity to external perturbation

ObjectiveArticular cartilage undergoes biological and morphological changes throughout maturation. The prevalence of osteoarthritis in the aged population suggests that maturation predisposes cartilage to degradation and/or impaired regeneration, but this process is not fully understood. Therefore, the objective of this study was to characterize the cellular and genetic profile of cartilage, as well as biological plasticity in response to mechanical and culture time stimuli, as a function of animal maturity. Methods/designPorcine articular cartilage explants were harvested from stifle joints of immature (2–4 weeks), adolescent (5–6 months), and mature (1–5 years) animals. Half of all samples were subjected to a single compressive mechanical load. Loaded samples were paired with unloaded controls for downstream analyses. Expression of cartilage progenitor cell markers CD105, CD44, and CD29 were determined via flow cytometry. Expression of matrix synthesis genes Col1, Col2, Col10, ACAN, and SOX9 were determined via qPCR. Tissue morphology and matrix content were examined histologically. Post-loading assays were performed immediately and following 7 days in culture. ResultsCD105 and CD29 expression decreased with maturity, while CD44 expression was upregulated in cartilage from mature animals. Expression of matrix synthesis genes were generally upregulated in cartilage from mature animals, and adolescent animals showed the lowest expression of several matrix synthesizing genes. Culture time and mechanical loading analyses revealed greater plasticity to mechanical loading and culture time in cartilage from younger animals. Histology confirmed distinct structural and biochemical profiles across maturity. ConclusionThis study demonstrates differential, nonlinear expression of chondroprogenitor markers and matrix synthesis genes as a function of cartilage maturity, as well as loss of biological plasticity in aged tissue. These findings have likely implications for age-related loss of regeneration and osteoarthritis progression.

Transversely isotropic freeze-dried PVA hydrogels: Theoretical modelling and experimental characterization

Polyvinyl alcohol (PVA) hydrogels have recently emerged as suitable bio-mimetic materials for various applications such as scaffold engineering, controlled drug delivery and cell culture. In this paper, PVA hydrogels with different concentrations of PVA have been synthesized using uni-directional freeze-drying technique. Unidirectional freeze-dried hydrogels exhibit transverse isotropy. The microstructurally aligned porous architecture of the hydrogels has been confirmed from Scanning Electron Microscope (SEM) images. Experiments have been conducted to characterize their transversely isotropic and viscoplastic behavior. These materials have been observed to behave non-linearly when subjected to compressive mechanical loads. Theoretical models are proposed to predict the mechanical behavior of the transversely isotropic microstructurally aligned hydrogels. The first model is a visco-elasto-plastic model that incorporates the effect of non-Newtonian behavior effectively using a fractal dashpot. The second model is an anisotropic finite viscoplastic model. The predictions of both the models are found to be in close agreement with the experimental results.

Mechanics of 1–3 piezocomposites subjected to creep–fatigue loads

1–3 piezocomposites are extensively used for sensor, actuator, naval and aerospace applications owing to their enhanced electromechanical coupling, wider bandwidth and reliability. However, their performance gets adversely affected when they are subjected to combined static and dynamic loads that initiate both creep and fatigue damage, simultaneously. 1–3 piezocomposites undergo damage due to creep–fatigue interactions because of the passive and viscoelastic polymer matrix and inherent domain switching mechanisms. In this work, experiments are conducted to obtain the electromechanical response of 1–3 piezocomposites subjected to electrical fatigue and mechanical compressive creep loads. It is observed that, the electromechanical response degrades with increase in number of electrical fatigue cycles, matrix volume fraction and mechanical compressive pre-stress. A nonlinear phenomenological combined creep–fatigue damage model is developed within a thermodynamic framework to account for the creep–fatigue interactions in 1–3 piezocomposites. The model predictions are found to be in agreement with the experimental observations.

Experimental and Finite Element Analysis of the Open-Cells Porous Materials Subjected to Compression Mechanical Loading

Progress in Additive Manufacturing (AM) technology enables the fabrication of complex structures that could not be obtained with traditional manufacturing methods. One AM research area is the development and use of lightweight products with cellular structures, containing complex lattices and pores, which give improved performance and functionality. It is well known that there is a strong link between mechanical properties and architecture of samples with cellular structures. This paper presents a comparison and validation of Finite Element Analysis (FEA) simulations of cellular structures with experimental data obtained from compression tests, and degradation behaviour under load compression. The specimens, with spherical open-cells, were produced in VeroClear RGD810 photopolymer resin. Mechanical compression tests were performed to investigate the compressive behaviour and the mechanical response was registered in the form of compressive stress-strain curves. Also, using the specimens� CAD data and compression test parameters, a Finite Element Analysis (FEA) was performed. A macroscopic analysis of the specimens� structure and microhardness tests before and after compression tests were also carried out.

Axial Compressive Strength Experimental and Numerical Studies of Full-Scale Specimens Simulating GFRP Composite Bushing Column

To study the axial compressive performance of glass fiber-reinforced plastic (GFRP) composite structure, a full-scale axial compression experiment was conducted on GFRP composite bushing (GFRP-CB) column specimens. The axial compressive mechanical properties, failure mode, and load–displacement curves were obtained and analyzed. It is found that brittle fracture occurs in the GFRP-CB column specimens. Diagonal cracks as the main failure mode appear in the upper part of the GFRP pipe. Moreover, the maximum stress in specimens is less than the compressive strength of the GFRP composite material. The steel casing remains in the elastic state during the entire loading process. Based on the strength theory of transversely orthotropic material, the finite element method was used to scrutinize the influence of eccentricity and diameter–thickness ratios on strength of specimens. Numerical results prove that the ultimate bearing capacity of the eccentric compression specimens decreases with the eccentricity ratio. Reduction in diameter–thickness ratio can improve the ultimate bearing capacity of the specimens. As the diameter–thickness ratio decreases by 20%, the ultimate bearing capacity increases by 20%. Finally, an equation for calculating the ultimate bearing capacity of the GFRP-CB column is proposed according to test results. The calculated and test results are fairly consistent, which indicates the effectiveness and accuracy of the proposed equation.

Post-buckling analysis of FGM plates under in-plane mechanical compressive loading by using a mesh-free approximation

This study introduces an accurate and effective mesh-free approximation based on the radial point interpolation method (RPIM) to predict the post-buckling responses of FGM plates in mechanical edge compression. In the RPIM, a new radial basis function is presented in a compactly supported form to build the shape functions without any fitting parameters. The equilibrium and governing equations for the plate are derived by using the higher-order shear deformation theory in which a new hybrid type transverse shear function is incorporated in order to better represent the displacement fields. A von Karman type nonlinear equation which accounts for both the geometric nonlinearity and the initial geometric imperfection is constructed. A solution procedure based on the total Lagrangian formulation to trace the post-buckling path, which utilizes the modified Newton–Raphson method, is designed. The numerical results illustrate the accuracy of the proposed meshless method for predicting the post-buckling behavior of FGM plates.

Influence of mechanical compressive loads on microstructurally aligned PVA xerogels

Xerogels are attractive candidates for scaffolds in tissue engineering and drug delivery. Polyvinyl alcohol xerogels are extensively used for biomedical applications owing to their ease of synthesis, biocompatibility and biodegradability. However, these materials are challenged in terms of their weak mechanical properties and structural instabilities. This work is aimed to look into the morpho-mechanical response of transversely isotropic freeze-dried PVA xerogels under mechanical compressive loads. Microstructurally aligned PVA xerogels are synthesized using unidirectional freeze-drying technique. Uniaxial compressive tests are performed alongside viscoelastic characterization. The effect of mechanical compressive loads upon the xerogel microstructure is studied. Microbuckling of material fibers, creasing and wrinkling of the bulk xerogels are some of the microstructural and surface instabilities discussed.

Polymeric composite reinforced by non‐crimp fabric: Effects of anisotropy and configuration on tensile and compressive behaviors

Glass fiber‐reinforced plastics offer a wide range of application possibilities, especially regarding the use of new reinforcement configurations developed by the industry worldwide. Concerning these reinforcing materials, highlights are given to high performance fabrics such as hybrid and non‐crimp fabrics (NCFs). However, the study of the mechanical behavior of composite laminates based on this type of reinforcement finds itself difficult due to the effects of hybridization (different forms of coupled fabrics), as well as anisotropy (influence of the orientation of the fibers relative to the loading direction) present in these fabrics. In this sense, this work aims to develop a composite laminate reinforced by four layers of multiaxial E‐glass fiber NCF using epoxy vinyl ester resin commercially named Derakane Momentum 411–350 as matrix. The main objective is to analyze its strength and stiffness from the perspective of the effects of loading type (tensile and compressive tests), laminate configuration (stacking sequence), and anisotropy (orientation of the fibers/loading direction). Comparative studies were developed, in addition to macroscopic characterization of the mechanical fracture according to ASTM standards. The results showed that the laminate configuration have different influences, depending on the loading type (tensile or compressive loading). In terms of anisotropy, the test specimens aligned at ±45° presented a great reduction in the mechanical properties, both in tensile and compressive loading. Also, in comparison with the literature, it is observed that the LNCF laminate presented similar or higher mechanical behavior, validating the results obtained in this work. POLYM. COMPOS., 40:2231–2243, 2019. © 2018 Society of Plastics Engineers

Nonlinear stability and vibration of pre/post-buckled multilayer FG-GPLRPC rectangular plates

• Proposing the nonlinear equations of parabolic shear deformable FG-GPLRPC plates. • Prediction of nonlinear stability and vibration of pre- and post-buckled FG-GPLRPC plates. • Investigating the impacts of GPL distribution scheme and weight fraction. • Exploring the impacts of the GPLs’ and plate's geometry and boundary conditions. The present study examines the nonlinear stability and free vibration features of multilayer functionally graded graphene platelet-reinforced polymer composite (FG-GPLRPC) rectangular plates under compressive in-plane mechanical loads in pre/post buckling regimes. The GPL weight fractions layer-wisely vary across the lateral direction. Furthermore, GPLs are uniformly dispersed in the polymer matrix of each layer. The effective Young's modulus of GPL-reinforced nanocomposite is assessed via the modified Halpin–Tsai technique, while the effective mass density and Poisson's ratio are attained by the rule of mixture. Taking the von Kármán-type nonlinearity into account for the large deflection of the FG-GPLRPC plate, as well as utilizing the variational differential quadrature (VDQ) method and Lagrange equation, the system of discretized coupled nonlinear equations of motions is directly achieved based upon a parabolic shear deformation plate theory; taking into account the impacts of geometric nonlinearity, in-plane loading, rotary inertia and transverse shear deformation. Afterwards, first, by neglecting the inertia terms, the pseudo-arc length approach is used in order to plot the equilibrium postbuckling path of FG-GPLRPC plates. Then, supposing a time-dependent disturbance about the postbuckling equilibrium status, the frequency responses of pre/post-buckled FG-GPLRC plate are obtained in terms of the compressive in-plane load. The influences of various vital design parameters are discussed through various parametric studies.

Snap-through buckling of fly ash cenosphere/epoxy syntactic foams under thermal environment

Experimental investigation on deflection behaviour of fly ash cenosphere/epoxy syntactic foam at room temperature and under thermal environment (three different heating conditions) is investigated. Influence of fly ash cenosphere volume fraction and nature of temperature variation on deflection behaviour of syntactic foam beam is discussed elaborately. Results reveal that the syntactic foam beam experience snap-through buckling under thermal environment and is reflected by two bifurcation points in temperature-deflection plot. It is observed that the time duration for which the foam beam stays in the first buckled position increases with increase in cenosphere content. Thermal environment induces compressive stresses in the samples causing such snap-through buckling. However, such phenomenon is not observed when mechanical compressive loads are applied under room temperature conditions. Temperature variation across the beam strongly influences snap-through buckling in syntactic foams in addition to volume fraction of filler content.

Transformation induced plasticity effect under tensile and compression stresses in nanostructured bainite

TRIP effect and mechanical stability of retained austenite are important factors which must be considered when talking about the ductility in nanostructured bainite. Meanwhile, different parameters affect austenite mechanical stability among those the deformation mode is of considerable importance. This article aims to compare the capability of high carbon retained austenite to transform to martensite when being subjected to external tensile and compressive mechanical loads in those materials. Scanning electron microscopy were used to investigate the microstructural features and interrupted tensile and compression tests were implemented to evaluate austenite stability. X-ray analyses were carried out to evaluate the austenite to martensite evolution during straining the samples and three different mathematical approaches were also implemented to express the austenite mechanical stability by mathematical constants. Results indicated that higher yield strength, enhanced true plastic strength and higher ductility could be obtained in compression deformation state. Moreover, tensile tests strongly simulated the martensite formation while compressive deformation mode opposed the volume expansion associated with austenite to martensite transformation. Accordingly, austenite was mechanically more stable during compression and it transformed to martensite more gradually comparing to that of tension.

Vertebral compression fractures associated with yoga: a case series.

The importance of exercise in skeletal health is increasingly recognized by both patients and providers. However, the safety of prescribed or recreational exercise in at-risk populations remains under-reported and under-publicized. Yoga has gained widespread popularity due to its physical and psychological benefits. When practiced in a population at increased fracture risk, however, some yoga poses may increase fracture risk, particularly at the spine, rather than increasing BMD as noted in recent popular press reports. Nine subjects (8 women) with a median age of 66 years (range 53-87), developed vertebral compression fracture (VCF) one month to six years after initiating yoga-associated spinal flexion exercises (SFE). VCF presented with back pain and occurred in the thoracicspine (N.=6), lumbar-spine (N.=4) and cervical-spine (N.=1). Four patients had osteoporosis by BMD criteria prior to VCF and 2 had osteopenia (median T-score -2.35; range -3.3 to +2.0). Interestingly, all patients had their lowest T-scores at the spine. Three patients had a history of fragility fracture prior to the index VCF. While one patient had primary hyperparathyroidism and another was treated with high dose prednisone, no other risk factors for bone loss including medications or secondary osteoporosis causes were identified in the other patients. This study identified patients in whom increased torsional and compressive mechanical loading pressures occurring during yoga SFE resulted in de novo VCF. Despite the need for selectivity in yoga poses in populations at increased fracture risk, both scientific and media reports continue to advertise yoga as a bone protective activity. Accordingly, yoga is misconceived as a 'onesize-fits-all' prescription. Instead, the appropriate selection of patients likely to benefit from yoga must be a cornerstone of fracture prevention.

Flexible Capacitive Piezoelectric Sensor with Vertically Aligned Ultralong GaN Wires.

We report a simple and scalable fabrication process of flexible capacitive piezoelectric sensors using vertically aligned gallium nitride (GaN) wires as well as their physical principles of operation. The as-grown N-polar GaN wires obtained by self-catalyst metal-organic vapor phase epitaxy are embedded into a polydimethylsiloxane (PDMS) matrix and directly peeled off from the sapphire substrate before metallic electrode contacting. This geometry provides an efficient control of the wire orientation and an additive contribution of the individual piezoelectric signals. The device output voltage and efficiency are studied by finite element calculations for compression mechanical loading as a function of the wire geometrical growth parameters (length and density). We demonstrate that the voltage output level and sensitivity increases as a function of the wire length and that a conical shape is not mandatory for potential generation as it was the case for horizontally assembled devices. The optimal design to improve the overall device response is also optimized in terms of wire positioning inside PDMS, wire density, and total device thickness. Following the results of these calculations, we have fabricated experimental devices exhibiting outputs of several volts with a very good reliability under cyclic mechanical excitation.

BIO alleviated compressive mechanical force-mediated mandibular cartilage pathological changes through Wnt/β-catenin signaling activation.

Osteoarthritis induced by compressive mechanical force is characterized by decreased chondrocyte proliferation and degradation of the ECM. To examine underlying mechanisms of the pathological changes of mandibular cartilage induced by compressive mechanical force, an established animal model was used to examine Wnt signaling activation by glycogen synthase kinase-3 beta (GSK3β) inhibitor 6-Bromoindirubin-3'-oxime (BIO) injection in vivo. Histological changes in mandibular cartilage were assessed via hematoxylin & eosin (HE), masson, and alcian blue staining. Immunohistochemistry and real-time PCR were performed to evaluate activation of the Wnt signaling pathway and chondrocytes proliferation markers. Chondrocytes apoptosis was examined by TUNEL staining. During the compressive mechanical force loading-mediated process, Wnt signaling was largely inhibited, which showed the inhibited expression of β-catenin and the increased expression of GSK-3β. The expression of chondrocytes proliferation markers Ki67, and proliferating cell nuclear antigen (PCNA) also decreased. With BIO injection, the Wnt signaling was restored and the proliferation of mandibular chondrocytes was also increased in the late stage (7 days) of compressive mechanical force loading. Finally, the decreasing mandibular cartilage thickness, the degradation of extracellular matrix, and the erosion of bone trabecula were subsequently restored. Also, the changes of extracellular matrix markers such as collagen II and collagen X, matrix metalloproteases, and inflammatory cytokines were reversed followed by the injection of BIO. In summary, compressive mechanical force decreased endogenously Wnt signaling, leading to impaired proliferation in chondrocytes and degradation in cartilage matrix. Restoration of Wnt signaling largely recovered the proliferation defects and alleviated the pathological changes of mandibular cartilage. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1228-1237, 2018.

Nonlocal strain gradient shell model for axial buckling and postbuckling analysis of magneto-electro-elastic composite nanoshells

Abstract The present study deals with the size-dependent nonlinear buckling and postbuckling characteristics of magneto-electro-elastic cylindrical composite nanoshells incorporating simultaneously the both of hardening-stiffness and softening-stiffness size effects. To accomplish this purpose, the nonlocal strain gradient elasticity theory is applied to the classical shell theory. Via the virtual work's principle, the size-dependent governing differential equations are constructed including the coupling terms between the axial mechanical compressive load, external magnetic potential and external electrical potential. The nonlinear prebuckling deformations and the large postbuckling deflections are taken into consideration based upon the boundary layer theory of shell buckling. Finally, an improved perturbation technique is employed to achieve explicit analytical expressions for nonlocal strain gradient stability curves of magneto-electro-elastic nanoshells under various surface electric and magnetic voltages. It is seen that a positive electric potential and a negative magnetic potential cause to increase both of the nonlocality and strain gradient size dependencies in the nonlinear instability behavior of axially loaded magneto-electro-elastic composite nanoshells, while a negative electric potential and a positive magnetic potential play an opposite role.

Decorin-containing collagen hydrogels as dimensionally stable scaffolds to study the effects of compressive mechanical loading on angiogenesis.

Angiogenesis is a critical component during wound healing, and the process is sensitive to mechanical stimuli. Current in vitro culture environments used to investigate three-dimensional microvascular growth often lack dimensional stability and the ability to withstand compression. We investigated the ability of decorin, a proteoglycan known to modulate collagen fibrillogenesis, incorporated into a collagen hydrogel to increase construct dimensional stability while maintaining vascular growth. Decorin did not affect microvascular growth parameters, while increasing the compressive modulus of collagen gels and significantly reducing the contraction of 3% collagen gels after 16 days in culture.

Characterization process to measure the electrical contact resistance of Gas Diffusion Layers under mechanical static compressive loads

Recent research has identified the mechanical properties of the fuel cell internal components (in particular, the Gas Diffusion Layers – GDLs) as key-parameters to obtain high final performances of the generator. The mechanical compression modulus of these components, the stability of their mechanical properties with respect to temperature and humidity, and their ability to interact with water have an impact on the electrical contact resistances in the stack and, by consequence, on the overall performance of the electric generator. Reducing the losses by contact resistance is an objective necessary to optimize the fuel cells in operation. The study of GDL electrical behavior under various internal operating conditions provides a suitable database to better understand their effects on the overall stack performance.This paper describes an experimental method for measuring the electrical contact resistance versus the static mechanical pressure applied to the GDLs. A nonlinear behavior of the electrical contact resistance versus the mechanical stress is observed. The PTFE and MPL additions modify the electrical contact resistance.

Cyclic compression stimulates osteoblast differentiation via activation of the Wnt/β-catenin signaling pathway.

It is widely accepted that mechanical stress is an important factor in bone associated cell differentiation, including that of mesenchymal stem cells, osteoblasts and osteocytes. The present study aimed to determine the effect of mechanical cyclic compressive load on osteoblast differentiation, and whether this was associated with activation of the wingless‑type (Wnt)/β-catenin signaling pathway. Using a 3D scaffold model, MC3T3‑E1 cells were exposed to cyclic compressive loading via the Flexcell‑5000C™ Compression system. Sinusoidal wave magnitudes of 0.33, 0.5and1MPa were applied for 4, 6 and 8h, at 1Hz frequency. Expression levels of genes associated with osteoblast differentiation were enhanced following compression, including alkaline phosphatase, osteocalcin, runt‑related transcription factor2 and osterix. Optimal compression was observed using a magnitude of 0.5MPa for 6h, whereas a magnitude of 1MPa had no effect on osteoblast differentiation, and had a negative effect when applied for prolonged time periods. Compressive loading additionally enhanced the mRNA expression levels of the Wnt/β‑catenin signaling pathway component, low density lipoprotein receptor‑related protein 5, and the protein expression levels of Wnt1, disheveled segment polarity protein‑2 (DVL2) and β-catenin. By contrast, mRNA expression levels of sclerostin and the inactive form of β-catenin (phosphorylated at Ser33/37/Thr41) were reduced following compressive loading. Following compressive loading of cells, dickkopf-related protein1 (DKK‑1), an inhibitor of the Wnt signaling pathway, increased protein expression levels of the inactive form of the Wnt‑associated protein, phosphorylated‑β‑catenin, compared with compression alone. However, DVL2 and Wnt1 protein expression levels were unaffected, suggesting that the loading‑induced activation of Wnt/β‑catenin signaling decreased however, it was not prevented by DKK‑1 treatment. In conclusion, the present study demonstrated that cyclic compressive load promoted osteoblast differentiation and may be dependent on the Wnt/β-catenin signaling pathway in regard to magnitude and duration.

Elucidating the role of compression waves and impact duration for generating mild traumatic brain injury in rats

Background: In total, 3.8 million concussions occur each year in the US leading to acute functional deficits, but the underlying histopathologic changes that occur are relatively unknown. In order to improve understanding of acute injury mechanisms, appropriately designed pre-clinical models must be utilized.Methods:The clinical relevance of compression wave injury models revolves around the ability to produce consistent histopathologic deficits. Mild traumatic brain injuries activate similar neuroinflammatory cascades, cell death markers and increases in amyloid precursor protein in both humans and rodents. Humans, however, infrequently succumb to mild traumatic brain injuries and, therefore, the intensity and magnitude of impacts must be inferred. Understanding compression wave properties and mechanical loading could help link the histopathologic deficits seen in rodents to what might be happening in human brains following concussions.Results: While the concept of linking duration and intensity of impact to subsequent histopathologic deficits makes sense, numerical modelling of compression waves has not been performed in this context. In this interdisciplinary work, numerical simulations were performed to study the creation of compression waves in an experimental model.Conclusion: This work was conducted in conjunction with a repetitive compression wave injury paradigm in rats in order to better understand how the wave generation correlates with histopathologic deficits.