Maximum Stiffness Values Research Articles

Shear wave elastography parameters of normal soft tissues of the neck.

Shear wave elastography is a relatively new method of quantitative measurement of tissue elasticity. Assuming that malignant lesions are stiffer than benign ones, elastography may provide supplementary information for their discrimination. However, potential confounding factors impacting tissue stiffness should be investigated first. The objective of this study was to measure the stiffness of selected tissues of the head and neck in a normal population and to evaluate its relationship to age, sex, and body mass index. Stiffness of the thyroid, submandibular and parotid glands, masseter and sternocleidomastoid muscles, and cervical lymph nodes was measured bilaterally in 128 healthy volunteers (83 female and 45 male). At least 20 subjects in each decade of life (20-29, 30-39‥, 70+) were enrolled. Shear wave elastography was performed by a single radiologist in all the subjects. The stiffnesses obtained were correlated with age, sex, and body mass index. The mean stiffness was 9.5 ± 3.6 kPa for the thyroid, 9.5 ± 4.6 kPa for the lymph node, 11.0 ± 3.4 kPa for the submandibular gland, 9.0 ± 3.5 kPa for the parotid gland, 9.9 ± 4.1 kPa for the sternocleidomastoid, and 10.0 ± 4.3 kPa for the masseter muscle. A slight general decrease in stiffness with increasing age was found. BMI and weight had a small impact on the minimum and maximum stiffness values. The sex of the subject did not affect elasticity. The mean stiffness of healthy head and neck organs has a relatively narrow distribution around 11 kPa. The changes of stiffness with age, BMI, and weight that were identified are too small to have clinical impact.

Impact of Image Orientation on Measurements of Thyroid Nodule Stiffness Using Shear Wave Elastography.

To assess the effect of orientation on the stiffness measurement by shear wave elastography (SWE) within a thyroid nodule. Forty-five patients with 50 thyroid nodules were enrolled in this study. After a short sonographic examination, SWE images were acquired in transverse and longitudinal orientations. Multiple elastograms of each nodule were obtained in both orientations. Mean and maximum stiffness values were recorded from each region of interest. The overall mean and maximum stiffness values of each nodule were calculated for both orientations separately. The concordance correlation coefficient was used to assess overall agreement between measurements. A receiver operating characteristic curve analysis was conducted to assess diagnostic performance for benign versus malignant nodules. The 45 patients included 6 men and 39 women (mean age, 53 years; range, 23-84 years). The mean stiffness values ± SD were 19.1 ± 18.4 and 20.9 ± 21.5 kPa for transverse and longitudinal measurements, respectively, with a small overall bias between the orientations (1.8 kPa; P = .09). Overall agreement was good (concordance correlation coefficient, 0.93) although moderated somewhat after the stiffness values were log transformed to reduce skewness (concordance correlation coefficient, 0.76). Of the 50 nodules, 6 were classified as malignant. The area under the curve based on the mean stiffness value from longitudinal images tended to be better than for transverse images, but the difference was not statistically significant (0.93 versus 0.83; P = .17). Our study has shown that good diagnostic performance can be achieved by using SWE in longitudinal and transverse orientations. Moderate to good agreement in mean and maximum stiffness values for both orientations was observed, with only a small bias. Diagnostic performance for benign and malignant nodules was similar. The similar performance of both orientations gives flexibility in imaging patients with anatomic or physical limitations.

Assessing the Relative Contributions of Active Ankle and Knee Assistance to the Walking Mechanics of Transfemoral Amputees Using a Powered Prosthesis.

Powered knee-ankle prostheses are capable of providing net-positive mechanical energy to amputees. Yet, there are limitless ways to deliver this energy throughout the gait cycle. It remains largely unknown how different combinations of active knee and ankle assistance affect the walking mechanics of transfemoral amputees. This study assessed the relative contributions of stance phase knee swing initiation, increasing ankle stiffness and powered plantarflexion as three unilateral transfemoral amputees walked overground at their self-selected walking speed. Five combinations of knee and ankle conditions were evaluated regarding the kinematics and kinetics of the amputated and intact legs using repeated measures analyses of variance. We found eliminating active knee swing initiation or powered plantarflexion was linked to increased compensations of the ipsilateral hip joint during the subsequent swing phase. The elimination of knee swing initiation or powered plantarflexion also led to reduced braking ground reaction forces of the amputated and intact legs, and influenced both sagittal and frontal plane loading of the intact knee joint. Gradually increasing prosthetic ankle stiffness influenced the shape of the prosthetic ankle plantarflexion moment, more closely mirroring the intact ankle moment. Increasing ankle stiffness also corresponded to increased prosthetic ankle power generation (despite a similar maximum stiffness value across conditions) and increased braking ground reaction forces of the amputated leg. These findings further our understanding of how to deliver assistance with powered knee-ankle prostheses and the compensations that occur when specific aspects of assistance are added/removed.

Can the gracilis replace the anterior cruciate ligament in the knee? A biomechanical study.

The purpose of this study was to determine whether a four-strand gracilis-only construct possesses the biomechanical properties needed to act as an anterior cruciate ligament (ACL) reconstruction graft. This was a pilot study with 32 cadaver specimens. The biomechanical properties of three types of grafts were determined using validated tensile testing methods: patellar tendon (BTB), both hamstring tendons together (GST4) and gracilis alone (G4). The maximum load at failure of the G4 was 416.4N (±187.7). The GST4 and BTB had a maximum load at failure of 473.5N (±176.9) and 413.3N (±120.4), respectively. The three groups had similar mean maximum load and stiffness values. The patellar tendon had significantly less elongation at failure than the other two graft types. The biomechanical properties of a four-strand gracilis construct are comparable to the ones of standard grafts. This type of graft would be useful in the reconstruction of the anteromedial bundle in patients with partial ACL ruptures.

Bioreactor and scaffold design for the mechanical stimulation of anterior cruciate ligament grafts

Various studies have shown that physical stimuli modulate cell function and this has motivated the development of a bioreactor to engineer tissues in vitro by exposing them to mechanical loads. Here, we present a bioreactor for the physical stimulation of anterior cruciate ligament (ACL) grafts, whereby complex multi-dimensional strain can be applied to the matrices. Influences from environmental conditions to the behavior of different cells on our custom-made silk scaffold can be investigated since the design of the bioreactor allows controlling these parameters precisely. With the braided design of the presented silk scaffold we achieve maximum loads and stiffness values matching those of the human ACL. Thus, the existent degummed and wet silk scaffolds absorb maximum loads of 2030±109 N with stiffness values of 336±40 N/mm.

Bony Incorporation of Soft Tissue Anterior Cruciate Ligament Grafts in an Animal Model

Background: The effect of low-dose gamma irradiation on healing of soft tissue allografts remains largely unknown. Hypothesis: The authors hypothesized that soft tissue allograft healing to bone would be delayed compared with that of autograft tissue and that low-dose (1.2 Mrad) gamma irradiation would not affect the healing response of allograft tissue after anterior cruciate ligament (ACL) reconstruction. Study Design: Controlled laboratory study. Methods: Forty-eight New Zealand White rabbits underwent bilateral ACL reconstructions with semitendinosus tendon graft. Sixteen rabbits were reconstructed with autografts and the remainder with allografts. The 32 allograft rabbits each received 1 irradiated allograft (1.2 Mrad), with the contralateral leg receiving a nonirradiated allograft. Animals were euthanized at 2 weeks or 8 weeks postoperatively. Tensile stiffness, maximum load, and displacement at maximum load were measured. Tibial and femoral segments were sectioned perpendicular to the tunnel axis allowing for histologic and histomorphometric analyses at the tendon-bone interface. Results: There were no significant differences between the maximum load or stiffness values among all groups at 8 weeks. At 2 weeks, autograft exhibited significantly (P < .01) lower maximum load than did the nonirradiated grafts. Regarding histology, at both 2- and 8-week time points, autograft tendon displayed more advanced degenerative and remodeling processes in comparison with irradiated allograft and nonirradiated allograft. Discussion: The maximum load and stiffness of a healing tendon allograft in ACL reconstruction appear to be unaltered by low-dose (1.2 Mrad) irradiation. At 8 weeks, there were no biomechanical differences in tendon-bone healing of allografts when compared with autograft controls. Histologic analyses suggested a faster remodeling response in autograft specimens in comparison with allografts at both time points. Clinical Relevance: The findings support the contention that low-dose gamma irradiation is safe for sterilization of ACL soft tissue allografts without compromise of graft properties at early time points.

Validation of a switchable stiffness system for shock isolation.

The idea of a switchable stiffness device intended to provide improved shock isolation is presented. A control strategy in two stages, namely, the reduction of stiffness during a shock and the later suppression of residual vibration switching between minimum and maximum stiffness values is analyzed. Experimental validation is presented, where a switchable stiffness device based on magnetic forces is developed. It is found that the implementation of switchable stiffness can yield better shock isolation performance.

Flexural Stiffness of Steel-Prestressed Concrete Composite Beam

For continuous composite beams that in normal working stage, due to concrete cracks at negative moment region are restricted by transverse tendon reinforcements at the slab, it would be reasonable to consider concrete contribution to the bending capacity of such composite beams. Based on this, a variable stiffness method is proposed in this paper to calculate the composite beam deflection. Before concrete slab cracks, the flexural stiffness (the maximum value) is calculated according to the reduced transformed beam section, with the consideration of concrete contribution; After concrete cracks, this stiffness value will be decreased as per the increase of the concrete crack, and at final stage, when concrete contribution to the flexural stiffness reach zero, the stiffness value reach it’s minimum value. The loads capacity at such maximum and minimum stiffness values could be considered as the beam capacity before concrete cracking as per current code, and the elastic moment capacity after concrete cracking respectively, while the stiffness between these maximum and minimum values could be determined by linear interpolation. The bending stiffness derived from this method has a higher precision than the method presented in current China specification in which the concrete contribution is totally neglected. Especially, this procedure would be more useful for those prestressed concrete composite beams, as they have higher crack- resistance capability.

Fiber Alignment in Directed Carbon Fiber Preforms — A Feasibility Study

The directed carbon fiber preforming (DCFP) process has the potential to create discontinuous fiber architectures with significant levels of fiber alignment. This article investigates the achievable levels of alignment and identifies the compromises inherent in the production of aligned preforms. The tensile properties of laminates produced from 6 and 24 K filament counts are compared at three different fiber lengths (28, 58, and 115mm). Experimental characterization indicates that up to 94% of fiber bundles can be aligned within ±10° using the current DCFP alignment method. Consequently, tensile stiffness and strength can be increased by 206 and 234%, respectively, over the random fiber case, as a result of the high concentration of aligned fibers in the loading direction. These alignment properties equate to maximum stiffness and strength retention values of 83 and 31% compared to continuous unidirectional material.

Deterioration of bone quality by long-term magnetic field with extremely low frequency in rats

The aim of our study was to investigate the effects of long-term (45 days) magnetic field (50 Hz, 1 mT; MF) on femur biomechanical parameters of rats. Bone mineral density (BMD) and histological investigation were also evaluated. For this purpose, twenty-four 8-week-old, Wistar–Albino female and male rats were assigned randomly to female control (FC) and MF-exposed rats (F-MF); male control (MC) and MF-exposed rats (M-MF). BMD was measured by dual-energy X-ray absorptiometry. Cross-sectional area of the femoral shaft was evaluated by computerized tomography. Biomechanical measurements were performed at the mid-diaphysis of the left femur with tensile test. Maximum load, displacement, stiffness, energy absorption capacity (structural properties); ultimate stress, ultimate strain, elastic modulus and toughness (material properties) were calculated. Diaphysial cortical bone thickness was measured by using histological method from the right femur. In respect to the cortical thickness of the rats' femurs, there was statistically significant interaction between the gender and group ( P < 0.05), while the BMD and cortical area were not ( P > 0.05). The BMD, cortical thickness and area values of the femurs of MF-exposed rats (F-MF, M-MF) were significantly decreased in comparison to that of the controls (FC, MC) ( P < 0.05). There were no statistically significant differences between the control and the MF-exposed rats in respect to the femur length ( P > 0.05). There were no statistically significant interaction between the gender and group with regard to the maximum load, displacement, stiffness, ultimate stress, ultimate strain, elastic modulus and toughness endpoints, while the energy absorption capacity was significant ( P < 0.05). Maximum load, displacement and stiffness values of MF-exposed rats were significantly decreased compared to that of the controls ( P < 0.05). Femurs of M-MF rats had less absorbed energy than that in controls ( P < 0.05). Ultimate stress and elastic modulus parameters were significantly decreased in MF-exposed rats in comparison to that of the controls ( P < 0.05). Ultimate strain was higher in MF-exposed rats than that in controls ( P > 0.05). The mean of toughness was decreased in MF-exposed rats compared to that of the controls ( P > 0.05). In conclusion, the bone quality of rats is decreased by magnetic field exposure.

Laboratory Characterization of Recycled Concrete for Use as Pavement Base Material

The purpose of this research was to characterize both demolition and haul-back sources of recycled concrete material (RCM) available in Utah County, Utah, for use as pavement base material and to investigate the self-cementing behaviors of each. A telephone survey was conducted to assess the state of the practice with respect to the local use of RCM, and extensive laboratory testing was performed to evaluate the strength and durability properties of both sources of RCM. Laboratory testing included determinations of particle-size distributions, Atterberg limits, specific gravity values, absorption characteristics, moisture–density relationships, California bearing ratio (CBR), unconfined compressive strength (UCS), stiffness, resistance to freeze–thaw cycling, tube suction test (TST) moisture susceptibility classifications, and Los Angeles abrasion losses. The results of the testing indicate that marked stiffening and strength gain of the materials occur within a 7-day curing period, although the rate and extent of the increases are dependent on the material source. The higher fines content of the haul-back material corresponded to higher absorption, optimum moisture content, maximum dry density, CBR, UCS, and stiffness values compared with the demolition material, presumably because of the exposure of greater quantities of unhydrated cement in the production of the haul-back RCM. However, the haul-back RCM was characterized by a greater loss in stiffness after freeze–thaw cycling and received a marginal moisture-susceptibility rating in the TST, while the demolition RCM was rated as non-moisture-susceptible. Further research is needed to more definitively investigate the self-cementing mechanism of RCM.

Perceived Instability of Virtual Haptic Texture. II. Effect of Collision-Detection Algorithm

This article reports the second study in a series that investigates perceived instability—unrealistic sensations associated with virtual objects—of virtual haptic texture. Our first study quantified the maximum stiffness values under which virtual haptic textures were perceived to be stable (Choi &amp; Tan, 2004). The present study investigated the effect of the collision-detection algorithm by removing the step changes in force magnitude that could have contributed to perceived instability in the first study. Our results demonstrate a significant increase in the maximum stiffness for stable haptic texture rendering. We also report a new type of perceived instability, aliveness, that is characterized by a pulsating sensation. We discuss the possible cause of aliveness and show that it is not always associated with control instability. Our results underscore the important roles played by environment modeling and human haptic perception, as well as control stability, in ensuring a perceptually stable virtual haptic environment.

Stiffness m-value and the Low Temperature Relaxation Properties of Asphalt Binders

ABSTRACT The development of the Strategic Highway Research Program (SHRP) low temperature binder specifications was based on the observation that asphalt mixture creep stiffness at 2 hours correlated well with instances of transverse cracking. Extending the correlation to asphalt binder stiffness and using time-temperature superposition, a maximum stiffness value was imposed on the binders and a limiting temperature at which the stiffness at 60 seconds reached the limiting value was selected as a specification criterion. The slope at 60 seconds of the stiffness vs. time curve on a double logarithmic scale, the so-called “m-value” was introduced as an additional criterion based on the idea that a low m-value meant a slower relaxation of the thermal stresses, which was detrimental for the performance. This paper investigates the use of the “m-value” as a specification parameter for asphalt binder physical characterization. A comparison between m and the corresponding slope for the relaxation modulus is performed. The influence of m-value on thermal stress calculation performed in accordance to AASHTO MP1a is also investigated.

Role of Bending Beam Rheometer Parameters in Thermal Stress Calculations

The development of the Strategic Highway Research Program lowtemperature binder specifications was based on the observation that asphalt mixture creep stiffness at 2 h correlated well with instances of transverse cracking. With the correlation extended to asphalt binder stiffness and with time-temperature superposition, a maximum stiffness value was imposed on the binders, and a limiting temperature at which the stiffness at 60 s reached the limiting value was selected as a specification criterion. In addition, the slope at 60 s of the stiffness-versus-time curve on a double logarithmic scale, the so-called m-value, was introduced as an additional criterion. The role played by the stiffness and the m-value in the development of thermal stresses in asphalt pavements was investigated. The analysis showed that thermal stress magnitude was strongly related to the stiffness magnitude. At similar stiffness levels, higher m-values resulted in the faster development of thermal stresses.

Stiffness m-value and the Low Temperature Relaxation Properties of Asphalt Binders

ABSTRACT The development of the Strategic Highway Research Program (SHRP) low temperature binder specifications was based on the observation that asphalt mixture creep stiffness at 2 hours correlated well with instances of transverse cracking. Extending the correlation to asphalt binder stiffness and using time-temperature superposition, a maximum stiffness value was imposed on the binders and a limiting temperature at which the stiffness at 60 seconds reached the limiting value was selected as a specification criterion. The slope at 60 seconds of the stiffness vs. time curve on a double logarithmic scale, the so-called “m-value” was introduced as an additional criterion based on the idea that a low m-value meant a slower relaxation of the thermal stresses, which was detrimental for the performance. This paper investigates the use of the “m-value” as a specification parameter for asphalt binder physical characterization. A comparison between m and the corresponding slope for the relaxation modulus is performed. The influence of m-value on thermal stress calculation performed in accordance to AASHTO MP1a is also investigated.

Effect of pressure on bone cement stiffness: Anin vitro study

Twenty-two canine femora were injected with PMMA cement at pressures varying between 0.2 and 1.3 MPa. Sections cut from the femora were tested in bending in several planes and under axial loading. Cement penetration was determined for each section. The bending and axial stiffness values were correlated with the cement penetration values. Both bending and axial stiffness were related linearly to penetration and nonlinearly to the cement intrusion pressure. The stiffness values increased with the intrusion pressure: slowly at low pressures, rapidly at medium pressures, slowly again at higher pressures, and leveling off at about 1.2 MPa. The 75 per cent of the maximum bending and axial stiffness values were achieved with a cement intrusion pressure of 0.55 MPa.