Time-dependent properties of human umbilical fascia

This investigation is devoted to the study of the viscoelastic behavior of human abdominal fascia from the umbilical region. Seventeen samples 10 mm wide and up to 70 mm long were cut along the primary fiber direction (group FL) or perpendicular to it (group FT) and subjected to relaxation tests. The viscoelastic response of the tissue at three different strain levels (4%, 5%, and 6%) was investigated. The relaxation curves were fitted using a two-stage decaying exponential form. The following parameters were determined: initial stress σ(0), relaxation times τ(1) and τ(2), stress reduction Δσ, initial relaxation modulus E and equilibrium relaxation modulus E(eq), as well as the ratio E/E(eq). Fiber orientation and strain levels were varied to determine their influence on the viscoelastic properties of fascia. The results highlight the inherent viscoelastic mechanical properties of umbilical fascia. The values of the viscoelastic parameters determined for the longitudinal and transverse directions varied markedly. Significant differences were found between the two groups FL and FT for the initial stress at 5% and 6% strain (p < 0.038) and for the initial and equilibrium moduli at the 6% strain level (p < 0.046). The stress reduction in samples from the FL group (45-55%) was less than that in samples from the FT group (37-54%), but this difference was not significant (p > 0.388). The influence of strain level on the parameter values was not statistically significant (p > 0.121). The nonlinear response of the tissue was demonstrated over the chosen strain range.     

Time-Dependent Properties of Human Umbilical Fascia

This investigation is devoted to the study of the viscoelastic behavior of human abdominal fascia from the umbilical region. Seventeen samples 10 mm wide and up to 70 mm long were cut along the primary fiber direction (group FL) or perpendicular to it (group FT) and subjected to relaxation tests. The viscoelastic response of the tissue at three different strain levels (4%, 5%, and 6%) was investigated. The relaxation curves were fitted using a two-stage decaying exponential form. The following parameters were determined: initial stress σ₀, relaxation times τ₁ and τ₂, stress reduction Δσ, initial relaxation modulus E and equilibrium relaxation modulus E_eq, as well as the ratio E/E_eq. Fiber orientation and strain levels were varied to determine their influence on the viscoelastic properties of fascia. The results highlight the inherent viscoelastic mechanical properties of umbilical fascia. The values of the viscoelastic parameters determined for the longitudinal and transverse directions varied markedly. Significant differences were found between the two groups FL and FT for the initial stress at 5% and 6% strain (p < 0.038) and for the initial and equilibrium moduli at the 6% strain level (p < 0.046). The stress reduction in samples from the FL group (45–55%) was less than that in samples from the FT group (37–54%), but this difference was not significant (p > 0.388). The influence of strain level on the parameter values was not statistically significant (p > 0.121). The nonlinear response of the tissue was demonstrated over the chosen strain range.     

Mechanical characterization and biocompatibility of a novel reinforced fascia patch for rotator cuff repair

To provide mechanical augmentation for rotator cuff repair, it is necessary (though perhaps not sufficient) that scaffolds have tendon-like material and suture retention properties, be applied to the repair in a surgically appropriate manner, and maintain their mechanical properties for an acceptable period of time following surgery. While allograft fascia lata has material, structural, and biochemical properties similar to tendon tissue, its poor suture retention properties abrogates its potential as an augmentation device. The goal of this work was to design a novel reinforced fascia patch with suture retention and stiffness properties adequate to provide mechanical augmentation for rotator cuff repair. Fascia was reinforced by stitching with PLLA or PLLA/PGA polymer braids. Reinforced fascia patches had a maximum construct load greater than (or equal to) the suture retention properties of human rotator cuff tendon (～250N) at time zero and after in vivo implantation for 12 weeks in a rat subcutaneous model. The patches were able to withstand the 2500 loading cycles projected for the early post-operative period. The patches also demonstrated biocompatibility with the host using a rat abdominal wall defect model. These studies suggest the potential use of reinforced fascia patches to provide mechanical augmentation, minimize tendon retraction and possibly reduce the incidence of rotator cuff repair failure.     

模拟雄性大鼠老龄骨质疏松力学性质实验研究

研究了正常与老龄骨质疏松(雄性)大鼠骨的力学性质.选用280～360g、5～6个月龄Wistar雄性大鼠40只,随机分为正常对照组20只,骨质疏松模型组20只.对模型组大鼠于0周摘除睾丸,对照组与模型组大鼠饲养14周后,以腹主动脉放血法处死大鼠.取大鼠肱骨进行拉伸、压缩实验,取大鼠股骨进行弯曲、剪切实验,取大鼠胫骨进行扭转、冲击实验.大鼠肱骨拉伸、压缩最大载荷、最大应力、最大位移、最大应变及弹性模量,得出了正常对照组和模型组大鼠股骨三点弯曲最大载荷、最大弯矩、最大应力、弹性模量及剪切最大载荷、最大应力,得出了正常对照组和模型组大鼠胫骨扭转最大扭矩、扭转角、扭转剪应力、最大冲击功、冲击韧性指标,并对实验结果进行分析讨论.     

Mechanical behaviour and rupture of normal and pathological human ascending aortic wall

The mechanical properties of aortic wall, both healthy and pathological, are needed in order to develop and improve diagnostic and interventional criteria, and for the development of mechanical models to assess arterial integrity. This study focuses on the mechanical behaviour and rupture conditions of the human ascending aorta and its relationship with age and pathologies. Fresh ascending aortic specimens harvested from 23 healthy donors, 12 patients with bicuspid aortic valve (BAV) and 14 with aneurysm were tensile-tested in vitro under physiological conditions. Tensile strength, stretch at failure and elbow stress were measured. The obtained results showed that age causes a major reduction in the mechanical parameters of healthy ascending aortic tissue, and that no significant differences are found between the mechanical strength of aneurysmal or BAV aortic specimens and the corresponding age-matched control group. The physiological level of the stress in the circumferential direction was also computed to assess the physiological operation range of healthy and diseased ascending aortas. The mean physiological wall stress acting on pathologic aortas was found to be far from rupture, with factors of safety (defined as the ratio of tensile strength to the mean wall stress) larger than six. In contrast, the physiological operation of pathologic vessels lays in the stiff part of the response curve, losing part of its function of damping the pressure waves from the heart.     

Effects of Creep and Cyclic Loading on the Mechanical Properties and Failure of Human Achilles Tendons

The Achilles tendon is one of the most frequently injured tendons in humans, and yet the mechanisms underlying its injury are not well understood. This study examines the ex vivo mechanical behavior of excised human Achilles tendons to elucidate the relationships between mechanical loading and Achilles tendon injury. Eighteen tendons underwent creep testing at constant stresses from 35 to 75 MPa. Another 25 tendons underwent sinusoidal cyclic loading at 1 Hz between a minimum stress of 10 MPa and maximum stresses of 30–80 MPa. For the creep specimens, there was no significant relationship between applied stress and time to failure, but time to failure decreased exponentially with increasing initial strain (strain when target stress is first reached) and decreasing failure strain. For the cyclically loaded specimens, secant modulus decreased and cyclic energy dissipation increased over time. Time and cycles to failure decreased exponentially with increasing applied stress, increasing initial strain (peak strain from first loading cycle), and decreasing failure strain. For both creep and cyclic loading, initial strain was the best predictor of time or cycles to failure, supporting the hypothesis that strain is the primary mechanical parameter governing tendon damage accumulation and injury. The cyclically loaded specimens failed faster than would be expected if only time-dependent damage occurred, suggesting that repetitive loading also contributes to Achilles tendon injuries. © 2003 Biomedical Engineering Society. PAC2003: 8719Rr     

Correlations between age, prestrain, diameter and atherosclerosis in the male abdominal aorta

The longitudinal prestrain of arteries facilitates their physiological function. Remodeling, adaptation and aging result in an age-dependent magnitude of the pretension. Although the phenomenon is known, detailed statistics, especially for human arteries, are lacking. This study was designed to propose the regression model capable of estimating the prestrain of the human abdominal aorta. The length of the abdominal aorta before, l, and after excision from the body, L, the diameter, heart weight, thickness of left ventricle and degree of atherosclerosis were collected in autopsies of 156 male cadavers of known age. Longitudinal prestrain was quantified by means of the stretch ratio λ=l/L. Statistical analysis revealed significant dependence between age, prestrain, diameter and atherosclerosis, which were best fitted to the power law equation. Longitudinal prestretch reduced with age significantly; λmean=1.30±0.07 for age<30 (n=29), whereas λmean=1.06±0.03 for age>59 (n=31) with p-value<0.0001. Raw data gave linear correlation coefficients as follows: λ-age (R=-0.842); l-age (R=0.023); L-age (R=0.476); (l-L)-age (R=-0.811). It was concluded that longitudinal prestrain decreases nonlinearly with age and both age and diameter are suitable predictors of the prestrain. Data suggests that unloaded length elongates with age in contrast to the elastic retraction.     

Influence of anisotropy on the mechanical behaviour of bioprosthetic heart valves

Chemically modified pericardium is commonly used in the fabrication of bioprosthetic heart valves. This material exhibits non-linear elastic behaviour and, as for most other biological soft tissues, it is orthotropic in its extensibility. The influence of the natural orthotropy of pericardium on the mechanical behaviour of pericardial heart valves during the whole cardiac cycle has been studied, using the finite element method. A model of the leaflet of a bicuspid valve has been created, defining the material of the tissue as orthotropic non-linear elastic. Two preferential orthogonal orientations of the tissue have been analysed (axial and circumferential). The results show that even a small amount of orthotropy (an orthotropy index of 1.5 has been used) can significantly affect the mechanical behaviour of the valve, and that an appropriate orientation of the fibres can contribute to optimizing the stress distribution in the leaflets.     

胰周筋膜及筋膜间隙CT影像的解剖观察及临床意义

目的　探讨腹部CT影像胰周筋膜及胰周的筋膜间隙与新鲜成人尸体标本胰周筋膜与筋膜间隙相互对应关系,从而对微创胰腺外科相关的手术提供指导意义。 方法　本研究选取惠东县人民医院经临床确诊的急性胰腺炎病例156例,对其腹部CT扫描的胰周影像特点进行归纳总结。同时选取南方医科大学解剖教研室提供的5具新鲜成人尸体标本进行解剖,明确腹部CT影像胰周筋膜及筋膜间隙与尸体标本的对应关系。 结果　在少数急性胰腺炎患者腹部CT扫描影像上和新鲜成人尸体标本上可同时观察到胰腺前筋膜、胰腺后筋膜、肾前筋膜,左右肾前筋膜是相互延续的。胰前间隙位于胰腺实质和其前方的胰腺前筋膜之间,胰后间隙位于胰腺实质和其后方的胰腺后筋膜之间,胰腺后融合筋膜间隙位于胰腺后筋膜和肾前筋膜之间。 结论　少数急性胰腺炎患者的腹部CT扫描影像上能观察到胰周筋膜与胰周的筋膜间隙,与新鲜成人尸体标本存在着相互对应关系。     

Analytically derived material properties of multilaminated extracellular matrix devices using the ball-burst test

Xenogeneic extracellular matrices (ECMs) have been shown to be effective as naturally occurring scaffolds for soft-tissue repair. As acellular tissue substitutes at the time of surgical implantation, ECMs are subjected to the mechanical forces and micro-environmental conditions representative of the anatomical location in which they are placed. Ideally such natural scaffolds would possess mechanical properties that allow for normal tissue function in and around the implant site. The ball-burst test was used to simulate biaxial forces and to determine the strength of the ECM scaffold under a relevant physiological loading condition. The ball-burst test, in itself, does not quantify intrinsic mechanical properties and therefore a methodology was developed to determine the maximum stress resultant tangent modulus (MSRTM) or the maximum stress tangent modulus (MSTM), stress to failure (sigma(f)), failure stress resultant (N(f)), ball-burst pressure (P), and maximum elongation (lambda(max)) from the raw ball-burst data obtained at a constant-rate of transverse. The analytical methodology was compared to finite element simulations and showed good correlation with the analytical solution presented. The proposed approximations were used to compute biaxial failure properties for a variety of multilaminate ECM devices with varying number of layers, disinfection and sterilization, and organ origin.     

Mechanical and biochemical analyses of tibial compartment fascia in chronic compartment syndrome

Increases in compartment pressure associated with chronic compartment syndrome (CCS) may be due to changes in the mechanical properties and/or thickness of fascia (4,22). To explore this possibility, we compared the mechanical and biochemical characteristics (stiffness, thickness, time-dependent response, collagen content, and collagen crosslinking) of fascia from patients with symptomatic anterior compartment syndrome to fascia from adjacent collateral compartments. We tested 43 specimens harvested from 20 individuals during surgical fasciectomy. Properties of normal (lateral)-compartment (NC) and pathological (anterior)-compartment (PC) fascia were mechanically tested in the axial and transverse directions forming four groups. An external control group (EX) of six specimens of anterior and lateral-compartment fascia harvested from amputated legs was also included in the study. PC fascia was found to be thicker and structurally stiffer (elastic modulus times thickness) in the axial direction than was NC fascia (p≤0.05). No significant differences were found between NC and PC time-dependent response, although significant differences between percent relaxation in the pooled axial and transverse direction specimens were observed. No differences were found in the collagen content, as measured by hydroxyproline (Hyp) concentration, between NC and PC fascia. PC fascia was found to have less collagen crosslinking by hydroxylyslpyridinoline (HP) concentration. In conclusion, although this study does not elucidate etiological factors in CCS, the changes found in PC fascia suggest that fascial mechanical properties contribute to the pathology.     

Effect of orientation and targeted extracellular matrix degradation on the shear mechanical properties of the annulus fibrosus

The intervertebral disc experiences combinations of compression, torsion, and bending that subject the disc substructures, particularly the annulus fibrosus (AF), to multidirectional loads and deformations. Combined tensile and shear loading is a particularly important loading paradigm, as compressive loads place the AF in circumferential hoop tension, and spine torsion or bending induces AF shear. Yet the anisotropy of AF mechanical properties in shear, as well as important structure-function mechanisms governing this response, are not well-understood. The objective of this study, therefore, was to investigate the effects of tissue orientation and enzymatic degradation of glycosaminoglycan (GAG) and elastin on AF shear mechanical properties. Significant anisotropy was found: the circumferential shear modulus, Gθz, was an order of magnitude greater than the radial shear modulus, Grθ. In the circumferential direction, prestrain significantly increased the shear modulus, suggesting an important role for collagen fiber stretch in shear properties for this orientation. While not significant and highly variable, ChABC treatment to remove GAG increased the circumferential shear modulus compared to PBS control (p=0.15). Together with the established literature for tensile loading of fiber-reinforced GAG-rich tissues, the trends for changes in shear modulus with ChABC treatment reflect complex, structure-function relationships between GAG and collagen that potentially occur over several hierarchical scales. Elastase digestion did not significantly affect shear modulus with respect to PBS control; further contributing to the notion that circumferential shear modulus is dominated by collagen fiber stretch. The results of this study highlight the complexity of the structure-function relationships that govern the mechanical response of the AF in radial and circumferential shear, and provide new and more accurate data for the validation of material models and tissue-engineered disc replacements.     

Full-field bulge test for planar anisotropic tissues: Part I – Experimental methods applied to human skin tissue

The nonlinear anisotropic properties of human skin tissue were investigated using bulge testing. Full-field displacement data were obtained during testing of human skin tissues procured from the lower back of post-mortem human subjects using 3-D digital image correlation. To measure anisotropy, the dominant fiber direction of the tissue was determined from the deformed geometry of the specimen. Local strains and stress resultants were calculated along both the dominant fiber direction and the perpendicular direction. Variation in anisotropy and stiffness was observed between specimens. The use of stress resultants rather than the membrane stress approximation accounted for bending effects, which are significant for a thick nonlinear tissue. Of the six specimens tested, it was observed that specimens from older donors exhibited a stiffer and more isotropic response than those from younger donors. It was seen that the mechanical response of the tissue was negligibly impacted by preconditioning or the ambient humidity. The methods presented in this work for skin tissue are sufficiently general to be applied to other planar tissues, such as pericardium, gastrointestinal tissue, and fetal membranes. The stress resultant–stretch relations will be used in a companion paper to obtain material parameters for a nonlinear anisotropic hyperelastic model.     

Investigation of three home-applied bleaching agents on enamel structure and mechanical properties: an in situ study

The safety of at-home tooth bleaching, based upon carbamide peroxide (CP) or hydrogen peroxide (HP) as the active agent, has been questioned. The aim of the present study was to investigate the effects of three differently concentrated home-applied bleaching agents on human enamel under in situ conditions. Sixty specimens were divided randomly into four groups and treated with 10% CP, 15% CP, 20% CP, and distilled water, respectively. Raman spectroscopy, attenuated total reflectance-infrared (ATR-IR) spectroscopy, atomic force microscopy (AFM), microhardness, and fracture toughness (FT) measurements were conducted to determine variations on enamel structure and mechanical properties before and after the bleaching process. Raman revealed little variation of Raman relative intensity after treatment with CP, which was consistent with the results of ATR-IR, AFM, and microhardness analyses. In addition, laser-induced fluorescence (LIF) intensity, and FT showed significant decreases on CP-treated specimens. These findings suggested there were minimal demineralization effects of the three at-home bleaching agents on enamel in situ. However, the decrease of LIF intensity and FT on enamel seemed to be inevitable.     

Influence of anisotropy on the mechanical behaviour of bioprosthetic heart valves

Chemically modified pericardium is commonly used in the fabrication of bioprosthetic heart valves. This material exhibits non-linear elastic behaviour and, as for most other biological soft tissues, it is orthotropicin its extensibility. The influence of the natural orthotropy of pericardium on the mechanical behaviour of pericardial heart valves during the whole cardiac cycle has been studied, using the finiteelement method. A model of the leaflet of a bicuspid valve has been created, defining the material of the tissue as orthotropic non-linear elastic. Two preferential orthogonal orientations of the tissue have been analysed (axial and circumferential). The results show that even a small amount of orthotropy (an orthotropyindex of 1.5 has been used) can significantly affect the mechanical behaviour of the valve, and that an appropriate orientation of the fibres can contribute to optimizing the stress distribution in the leaflets.     

High mechanical efficiency of left ventricular arrhythmic contractions during atrial fibrillation

We analyzed the frequency distribution of the left ventricular (LV) mechanical efficiency of individual arrhythmic beats during electrically induced atrial fibrillation (AF) in normal canine hearts. This efficiency is the fraction of the external mechanical work (EW) in the total mechanical energy measured by the systolic pressure-volume area (PVA). The mean, median, and mode of this efficiency (EW/PVA) were as high as 78%, 80%, and 81%, respectively, on average in six hearts. These high efficiencies were comparable to that of the regular beats in these hearts. The frequency distribution of the EW/PVA during AF tended to skew to the higher side in all the hearts. Since the EW/PVA is directly related to both the ventriculo-arterial (or afterload) coupling ratio (E(a)/E(max); E(a) = effective arterial elastance, E(max) = end-systolic ventricular elastance) and the ejection fraction on a per-beat basis, we also analyzed their frequency distributions. We found them to skew enough to account for the rightward skewed frequency distribution of the EW/PVA during AF with the unexpectedly high mean EW/PVA. These results indicate that the LV arrhythmia during AF per se does not directly suppress the mean level of LV mechanical efficiency in normal canine hearts.     

Investigation of the mechanical properties of the human crural fascia and their possible clinical implications

The mechanical properties of deep fasciae strongly affect muscular actions, development of pathologies, such as acute and chronic compartment syndromes, and the choice of the various fascial flaps. Actually, a clear knowledge of the mechanical characterization of these tissues still lacks. This study focuses attention on experimental tests of different regions of human crural fascia taken from an adult frozen donor. Tensile tests along proximal–distal and medial–lateral direction at a strain rate of 120 %/s were performed at the purpose of evaluating elastic properties. Viscous phenomena were investigated by applying incremental relaxation tests at total strain of 7, 9 and 11 % and observing stress decay for a time interval of 240 s. The elastic response showed that the fascia in the anterior compartment is stiffer than in the posterior compartment, both along the proximal–distal and medial–lateral directions. This result can explain why the compartment syndromes are more frequent in this compartment with respect to posterior one. Furthermore, the fascia is stiffer along the proximal–distal than along medial–lateral direction. This means that the crural fascia can adapt to the muscular variation of volume in a transversal direction, while along the main axis it could be considered as a structure that contributes to transmitting the muscular forces at a distance and connecting the different segments of the limb. The stress relaxation tests showed that the crural fascia needs 120 s to decrease stress of 40 %, suggesting a similar time also in the living so that the static stretching could have an effect on the fascia.     

Muscle stiffness in human ankle dorsiflexors: intrinsic and reflex components

1. The purpose of this study was to evaluate the mechanical response to stretch in normal human ankle dorsiflexors at different levels of voluntary contraction. In an active muscle, the total mechanical response is the sum of the intrinsic response from the contractile apparatus, the response from passive tissues, and the reflex mediated response. Each of these components was investigated. 2. The total incremental stiffness was defined as the ratio between the torque increment and the amplitude of the stretch. In 14 subjects the total stiffness increased from approximately 0.6 N.m/deg to approximately 2.5 N.m/deg at 50% of MVC and remained constant (+/- 10%) from 30 to 80% of MVC. 3. The contribution to incremental stiffness from intrinsic muscle properties was measured during electrical stimulation of the deep peroneal nerve at 7-50 Hz. Intrinsic stiffness increased linearly with torque from approximately 0.5 N.m/deg to approximately 2.5 N.m/deg at 80% of MVC. 4. The reflex component (total minus intrinsic stiffness) had a maximum of 0.5-1.5 N.m/deg at 30-50% of MVC and was approximately zero at no and maximal contraction. For intermediate levels of contraction the reflex increased the stiffness with 40-100% of the intrinsic stiffness in this flexor muscle. 5. The reflex contribution to total stiffness began approximately 50 ms after onset of stretch and peaked 150-300 ms after onset of stretch. 6. Total, intrinsic, and reflex mediated stiffness were all nearly independent of the amplitude of stretch in the range from 2 to 7 degrees. The higher stiffness observed for 1 degree stretches could be due to "short range stiffness" of the cross bridges. 7. Stretching of a contracting muscle generates large force increments even for moderate amplitudes of stretch. Approximately half of this force increment is due to the stretch reflex, which makes the muscle stiffer than predicted from the intrinsic stiffness. These findings in human flexor muscles are surprisingly similar to previous findings in extensor muscles of the decerebrate cat.     

Selective glycogen depletion pattern in human muscle fibres after exercise of varying intensity and at varying pedalling rates

1. Glycogen depletion pattern in human skeletal muscle fibres was studied after bicycle exercise of varying intensity performed at different pedalling rates. Work intensities studied were equivalent to 30-150% of V(O) (2) max. with pedalling rates of 30-120 rev/min.2. Glycogen depletion increased dramatically with increasing exercise intensity; depletion was 2.7 and 7.4 times greater respectively at workloads demanding 64 and 84% V(O) (2) max. than at workloads calling for 31% V(O) (2) max. Even greater rates of glycogen utilization occurred at supramaximal loads.3. Slow twitch, high oxidative (ST) fibres were the first to lose glycogen (reduced PAS staining) at all workloads below V(O) (2) max. Progressive glycogen depletion occurred in fast twitch (FT) fibres as work continued. Large quantities of glycogen remained in the muscle after 3 hr of exercise at low exercise intensity. This was almost exclusively found in FT fibres. At workloads exceeding maximal aerobic power, there was an initial depletion of glycogen in both fibre types. Varying the pedalling rate and, thus, the total force exerted in each pedal thrust had no effect on the pattern of glycogen depletion in the fibres.4. Results point to primary reliance upon ST fibres during submaximal endurance exercise, FT fibres being recruited after ST fibres are depleted of glycogen. During exertion requiring energy expenditure greater than the maximal aerobic power, both fibre types appeared to be continuously involved in carrying out the exercise.