Low-magnitude Loading Research Articles

Comparative study of the effect of loading increments on the mesophilic codigestion of waste activated sludge and food waste: Reactor performance, stability analysis, and microbial community

The performance and stability of mesophilic codigestion of waste activated sludge (WAS) and food waste (FW) were compared in two parallel, continuously stirred tank reactors using high- and low-magnitude loading increments for the loading regimes. The results indicated that a high methane (CH4) production of 6.98 L L−1·d−1 was realized without volatile fatty acid accumulation via low-magnitude loading increments at a high loading of 26.5 g-COD·L−1·d−1, and this system was more stable and achieved a higher efficiency than the codigestion system that used high-magnitude loading increments at similar loading and operating conditions. Furthermore, higher CH4 yields of 258–334 mL-CH4·g-COD−1, TCOD removal efficiencies of 64–79%, conversion ratios of 62–88%, and methanogenic activities of 0.37–0.40 g-CH4-COD·g-VS−1·d−1 were consistently maintained via the low-magnitude loading increments during the high-rate period. High abundances of the phyla Firmicutes (63.3%) and genus Methanosarcina (94.5%) contributed to the high rates and stable operating conditions of the mesophilic system for WAS and FW codigestion using low-magnitude loading increments.

Gene Expression in Response to High Frequency, Low Magnitude Loading on the Anterior Cruciate Ligament

IntroductionPrevention of Anterior Cruciate Ligament (ACL) injuries is important, as ACL injuries are associated with the development of knee osteoarthritis, increased pain, functional limitations, and decreased quality of living. Despite the focus on numerous ACL injury prevention mechanisms, ACL injury rates are still on the rise. Investigating a way to strengthen the ACL may be an alternative method to prevent ACL tears. Traditionally, tissue strengthening would occur through a high magnitude load, applied a limited number of times. However, as ligaments are designed to restrict unhealthy joint movement, high magnitude loading of ligaments is likely non‐desirable, making high frequency, low magnitude loading an intriguing possibility. Previous studies have shown bone’s ability to adapt and strengthen in response to high frequency, low magnitude loading. Therefore, we hypothesized ligaments, the connective tissue connecting bones together, would also exhibit similar adaptive properties. However, there is limited research on a typical in vivo ligament response to mechanical loading.ObjectiveThe purpose of this study was to identify what genes were altered in expression in response to high frequency, low magnitude loading.MethodsSeven New Zealand white rabbits were loaded utilizing a custom device built to apply a high frequency, low magnitude load to the rabbit’s shank. While anesthetized, rabbits were placed into the loading device with the left leg secured in a cast and attached to the load cell with a non‐strain cord (Figure 1). To ensure the loading occurred along the longitudinal axis of the ACL, the hip was extended, knee positioned at 30° knee flexion, with the device pulling on the tibia at 35‐40° to apply tibial anterior translation. Loads were applied oscillating between 2‐12 N, at 15 Hz for 20 minutes. Four hours post loading, both of the rabbit’s ACLs were harvested (loaded ACL & internal control). Three rabbits served as external controls and received no loading. Total RNA was extracted from the ACLs using a Qiagen RNAeasy kit following manufacturer’s instructions. Next Generation Sequencing RNAseq detected gene expression differences between the loaded ACL and internal control ACL, and the loaded ACL and the external control ACL using a Sleuth Wald test during data analysis.ResultsIn response to mechanical loading, there were three genes differentially expressed between the loaded and the internal control ACL, none of which were annotated within the rabbit genome. Between the loaded and external control ACL, there were 121 genes differentially expressed. Grouping these genes by function, there were 15 related to mechanotransduction, five to metabolic pathways, four to interleukin response and tissue repair, three to actin cytoskeleton regulation, two relating to protein transport, and one to collagen synthesis.ConclusionsThis is the first data demonstrating a ligament adaptive response to in vivohigh frequency, low magnitude loading. Additionally, these results shed light on the possible mechanotransductive response pathway in ligament. Future studies should characterize optimal loading parameters (i.e., loading frequency, magnitude, and duration), and investigate the long‐term effects of mechanical loading on a ligament.

Modeling cortical bone adaptation using strain gradients.

Cyclic and low-magnitude loading promotes osteogenesis (i.e. new bone formation). Normal strain, strain energy density and fatigue damage accumulation are typically considered as osteogenic stimuli in computer models to predict site-specific new bone formation. These models however had limited success in explaining osteogenesis near the sites of minimal normal strain, for example, neutral axis of bending. Other stimuli such as fluid motion or strain gradient also stimulate bone formation. In silico studies modeled the new bone formation as a function of fluid motion, however, computation of fluid motion involves complex mathematical calculations. Strain gradients drive fluid flow and thus can also be established as the stimulus. Osteogenic potential of strain gradients is however not well established. The present study establishes strain gradients as osteogenic stimuli. Bending-induced strain gradients are computed at cortical bone cross-sections reported in animal loading in vivo studies. Correlation analysis between strain gradients and site of osteogenesis is analyzed. In silico model is also developed to test the osteogenic potential of strain gradients. The model closely predicts in vivo new bone distribution as a function of strain gradients. The outcome establishes strain gradient as computationally easy and robust stimuli to predict site-specific osteogenesis. The present study may be useful in the development of biomechanical approaches to mitigate bone loss.

High-rate mesophilic co-digestion with food waste and waste activated sludge through a low-magnitude increasing loading regime: Performance and microorganism characteristics

To achieve a high-rate operation of co-digestion with waste activated sludge (WAS) and food waste (FW) for biogas production, a low-magnitude loading regime was investigated in a mesophilic continuous stirred tank reactor (CSTR) over long-term operation for approximately 600 days. The results showed that high-rate mesophilic CSTR co-digestion was realized using the low-magnitude loading regime. A maximum methane production of 12.9 L/L/day was achieved in the mesophilic CSTR co-digestion at an organic loading rate (OLR) of 48.1 g-COD/L/day. Moreover, high-efficiency and stable mesophilic CSTR co-digestion can still be performed at OLR of 50.8–52.1 g-COD/L/day and solid retention time (SRT) of 2.56–2.63 days without volatile fatty acid (VFA) accumulation. A high methane yield, hydrolysis conversion ratio, and methanogenic activity and the key anaerobic digestion enzymes were all maintained during the high-rate operation period. 16S rRNA gene sequencing results indicated that the relative abundance of the class Clostridia and genus Methanosarcina could reach 85.0% and 97.3%, respectively, corresponding to a high hydrolysis rate and VFA conversion rate. The metabolic capability of the genus Methanosarcina was the main reason for the highly efficient and stable operation of the mesophilic CSTR co-digestion. Using metagenomic analysis, Methanosarcina barkeri and Methanosarcina flavescens were established as the main methane-producing species during high-rate mesophilic CSTR co-digestion. The enrichment of the genus Methanosarcina through a low-magnitude loading regime is a promising method for realizing the highly efficient and stable operation of co-digestion with WAS and FW for biogas production at low retention times.

Effect of Vancomycin Soaking on Anterior Cruciate Ligament Graft Biomechanics

PurposeTo evaluate the effect of soaking of anterior cruciate ligament (ACL) grafts in vancomycin solution on graft biomechanical properties at the time of implantation. MethodsThe central third of patellar tendons was harvested from mature bovine knees and prepared as a tendon-only graft or a bone-tendon-bone (BTB) graft. Tendons were wrapped in gauze soaked in vancomycin solution (VS) (5 mg/mL) or normal saline (NS) and left to stand for 30 minutes at room temperature, simulating graft exposure times in the operating room during ACL reconstruction. Tensile testing was carried out on a materials testing system with (1) low-magnitude loading (60 N at 3 mm/s) with repeated testing of tendon-only grafts; and (2) high-magnitude loading (600 N at 10 mm/min) of BTB grafts. For tendon-only grafts, specimens were first wrapped in NS-soaked gauze and underwent testing, with repeated testing performed after wrapping in gauze soaked in VS or buffered VS (pH 7.0). For BTB grafts, specimens were randomly assigned to treatment with VS or NS. ResultsFor tendon-only grafts, there was no difference in Young’s modulus (YM) after soaking with VS soaking (baseline, 12.69 MPa; treatment, 16.07 ± 4.44 MPa; P = .99) or buffered VS (baseline, 12.45 ± 4.55 MPa; treatment, 15.56 ± 2.83 MPa; P = .99). For BTB grafts, there were no differences in elongation strain (VS, 46.8% ± 7.0%; NS, 31.5% ± 13.5%, P = .19) or YM (VS, 158.4 ± 15.8 MPa; NS, 158.5 ± 23.3 MPa, P = .99). ConclusionsAccording to controlled biomechanical tests, vancomycin soaking of patellar tendon grafts does not adversely affect time-zero material properties. Clinical RelevanceThis study suggests that vancomycin wrapping has no immediate adverse effects on the biomechanical properties of ACL grafts. Randomized controlled trials are warranted to validate the widespread use of vancomycin soaking of tendon grafts for infection prophylaxis during ACL reconstruction.

Single-component force balance for the measurement of low-magnitude mean aerodynamic loads

A novel force balance is developed for the measurement of low-magnitude mean aerodynamic forces. The measurement of such small loads is often desired for low Reynolds number wind tunnel experiments, however commercially available options for force measurement do not have high enough sensitivity or accuracy. A force balance is designed to measure forces that range from mN to hundreds of milli-Newtons in magnitude. The sensitivity of the balance is derived theoretically, and the actual value is determined from calibration. The force balance demonstrates both a high degree of linearity, and low hysteresis over the desired load range. The measurement capability of the force balance is validated using measurements of the drag force on a circular cylinder, and the lift force on a square cylinder for Reynolds numbers between 1100 and 10 000. In each case, the force measured by the newly developed force balance shows good agreement with data from literature, and a level of accuracy currently not possible with commercially available sensors for such low-magnitude loads.

Establishing the relationship between loading parameters and bone adaptation

Cyclic and low-magnitude loading is considered effective in arresting the bone loss as it promotes osteogenesis (i.e. new bone formation) at the sites of elevated normal strain magnitude. In silico models assumed normal strain as the stimulus to predict the sites of new bone formation. These models, however, may fail to fit the amount of newly formed bone. Loading parameters such as strain, frequency, and loading cycle decide the amount of new bone formation. The models did not incorporate this information. In fact, there is no unifying relationship to quantify the amount of new bone formation as a function of loading parameters. Therefore, the present work aims to establish an empirical relationship between loading parameters and a new bone formation parameter i.e. mineral apposition rate (MAR). A neural network model is used to serve the purpose. Loading parameters are supplied as input, whereas, MAR served as output. The model is trained and tested with experimental data. The model establishes an empirical relationship to estimate MAR as a function of loading parameters. The model's predictions of MAR align with in vivo experimental results. The model's response is analyzed which indicates that the bone adaptation characteristics are successfully captured in the relationship. The relationship established may be incorporated further to improve qualitative and quantitative prediction capabilities of computer models. These findings can be extended in future to design and develop effective biomechanical strategies such as prophylactic exercise to cure bone loss.

Low-Magnitude Forces for Bone Modeling and Remodeling in Dentofacial Orthopedics

To examine the evidence in support of light continuous forces for enhancing bone adaptation (modeling and remodeling) in orthodontics and dentofacial orthopedics. Clinical evidence suggests that light continuous orthodontic force can achieve physiologic expansion of the maxillary arch, but the long-term stability and the biological effects of the procedure are unclear. Compared to conventional orthodontic appliances that deliver heavy interrupted forces for tooth movement, the application of low-magnitude forces in animal models leads to anabolic modeling and remodeling of the alveolar bone in the path of orthodontic tooth movement. This results in dental translation and expansion of the alveolar process. Light continuous forces are preferable to heavy forces for more physiologic dentofacial orthopedics. The interaction of low-magnitude loads with soft tissue posture achieves therapeutic adaptation of the craniofacial skeleton. The increasing emphasis on genomic medicine and personalized treatment planning should focus on low-magnitude loads in orthodontics and dentofacial orthopedics.

In silico modeling of bone adaptation to rest-inserted loading: Strain energy density versus fluid flow as stimulus

In vivo studies suggest that cyclic and low-magnitude loading can be useful over pharmaceutical drugs in normalizing bone loss as it encourages osteogenesis (i.e. new bone formation) at the sites of elevated strain magnitude. In silico models assumed normal strain or strain energy density (SED) as the stimulus to predict loading-induced osteogenesis, however, these models may have limited success in fitting the in vivo new bone formation at several instances. For example, rest-inserted cyclic loading amplifies the new bone formation as compared to continuous-cyclic loading even though similar strain magnitude were induced in both the cases. It is also believed that loading-induced interstitial fluid flow can also be a potential stimulus of osteogenesis. The present study hypothesizes that fluid motion as osteogenic stimulus may explain the afore-mentioned anomalies. Accordingly, this work studies osteogenesis as functions of SED and canalicular fluid motion using an in silico model. Therefore, the new bone formation is considered roughly proportional to stimuli above their osteogenic thresholds. This model attempts to simulate in vivo new bone formation noticed in rest-inserted cantilever loading studies. The model's prediction of site-specific new bone formation improves when fluid flow is considered as the stimulus. It is also noticed that fluid motion as the stimulus closely fits the new bone formation for another in vivo study where the effects of aging on osteogenesis were examined. These attempts to establish fluid flow as a potential osteogenic stimulus can be useful in the prediction of site-specific new bone formation. The findings will ultimately be useful in designing biomechanical interventions such as prophylactic exercises to cure bone loss.

Aberrant Gait Biomechanics Are Not Associated with Aberrant Landing Biomechanics in Those with ACL Reconstruction

High knee joint loading during walking gait and jump-landing (JL) may influence the development of post-traumatic osteoarthritis (PTOA) and increase the risk of a second anterior cruciate ligament (ACL) injury, respectively, following anterior cruciate ligament reconstruction (ACLR). It remains unknown if individuals who demonstrate higher lower extremity loading during walking gait also demonstrate higher loading during JL. PURPOSE: To determine associations between peak vertical ground reaction force (vGRF) and vGRF instantaneous loading rate during walking gait and JL in individuals with an ACLR. Secondarily, we sought to determine if limb symmetry indices (LSI = ACLR limb/uninvolved limb) for the kinetics variables associated between the gait and JL tasks. METHODS: Thirty-five individuals (74% female, 45.5±38 months post-ACLR, 22.1±3.4 years old, 169.4±10.8 cm, 73.4±17.9kg) with a unilateral ACLR were recruited for this cross-sectional study. Participants performed 5-trials of self-selected walking gait (over 6m walkway) and JL (30 cm box placed 50% of height from force plates), respectively. Kinetics were extracted from the first 50% of the stance phase of walking gait and the first 100ms of landing for JL. Pearson product-moment (r) and Spearman’s Rho (ρ) analyses were used to determine associations between the same outcome measures collected during walking gait and JL. Significance was set a priori at P≤ 0.05. RESULTS: Greater vGRF instantaneous loading rate during gait (51.27±12.56% bodyweight per second (BW/s)) associated with greater vGRF instantaneous loading rate during JL (180.12±119.27 BW/s; ρ=0.389, P=0.021) in the uninvolved limb. All other associations between walking gait and JL kinetics and LSI were negligible (range ρ= -0.289 to 0.209) and non-significant. CONCLUSION: No systematic associations were found between gait and JL kinetics for either limb or LSI’s in people with unilateral ACLR. Individuals who demonstrate high or low loading during dynamic activities may not be the same people that demonstrate high or low magnitude loading during gait. Therefore, individuals who may be at risk for PTOA onset, due to aberrant repetitive lower extremity loading, may not be the same people who are at risk for a second ACL injury, which is more likely to occur during a dynamic movement.

An in vitro scratch tendon tissue injury model: effects of high frequency low magnitude loading

ABSTRACTThe healing process of ruptured tendons is suboptimal, taking months to achieve tissue with inferior properties to healthy tendon. Mechanical loading has been shown to positively influence tendon healing. However, high frequency low magnitude (HFLM) loads, which have shown promise in maintaining healthy tendon properties, have not been studied with in vitro injury models. Here, we present and validate an in vitro scratch tendon tissue injury model to investigate effects of HFLM loading on the properties of injured rat tail tendon fascicles (RTTFs). A longitudinal tendon tear was simulated using a needle aseptically to scratch a defined length along individual RTTFs. Tissue viability, biomechanical, and biochemical parameters were investigated before and 7 days after culture . The effects of static, HFLM (20 Hz), and low frequency (1 Hz) cyclic loading or no load were also investigated. Tendon viability was confirmed in damaged RTTFs after 7 days of culture, and the effects of a 0.77 ± 0.06 cm scratch on the mechanical property (tangent modulus) and tissue metabolism in damaged tendons were consistent, showing significant damage severity compared with intact tendons. Damaged tendon fascicles receiving HFLM (20 Hz) loads displayed significantly higher mean tangent modulus than unloaded damaged tendons (212.7 ± 14.94 v 92.7 ± 15.59 MPa), and damaged tendons receiving static loading (117.9 ± 10.65 MPa). HFLM stimulation maintained metabolic activity in 7-day cultured damaged tendons at similar levels to fresh tendons immediately following damage. Only damaged tendons receiving HFLM loads showed significantly higher metabolism than unloaded damaged tendons (relative fluorescence units —7021 ± 635.9 v 3745.1 ± 641.7). These validation data support the use of the custom-made in vitro injury model for investigating the potential of HFLM loading interventions in treating damaged tendons.

Use of a new model allowing controlled uniaxial loading to evaluate tendon healing in a bone tunnel.

The optimal mechanical loading regimen for the healing of a tendon graft in a bone tunnel is unknown. We developed a rat model that directly tensions a healing tendon graft, without the use of confounding joint motion. Fifty cycles of either 0, 3, or 6 N of tension were applied to groups daily for 3 or 6 weeks. At 3 weeks the low load (3 N) group had the highest failure load (p = 0.009), but by 6 weeks there were no differences in failure load among groups. At 3 weeks the high load (6 N) group had greater osteoclast activity compared to the immobilized (0 N) group (p < 0.05), and by 6 weeks there were significantly more osteoclasts in the high load group compared to the low load group (p = 0.01). Bone volume fraction was higher in the immobilized group compared to the 3 N load group at 3 weeks (p = 0.014) and 6 weeks (p = 0.007). At 6 weeks, the immobilized group had greater trabecular number compared to both loading groups (p < 0.05). In conclusion, low magnitude loading had a beneficial early effect but continued loading led to poorer new bone formation over time and no beneficial effect at 6 weeks, perhaps due to delayed maturation from cumulative loads. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:852-859, 2016.

The Role of Mechanical Stimulation in Recovery of Bone Loss-High versus Low Magnitude and Frequency of Force.

Musculoskeletal pathologies associated with decreased bone mass, including osteoporosis and disuse-induced bone loss, affect millions of Americans annually. Microgravity-induced bone loss presents a similar concern for astronauts during space missions. Many pharmaceutical treatments have slowed osteoporosis, and recent data shows promise for countermeasures for bone loss observed in astronauts. Additionally, high magnitude and low frequency impact such as running has been recognized to increase bone and muscle mass under normal but not microgravity conditions. However, a low magnitude and high frequency (LMHF) mechanical load experienced in activities such as postural control, has also been shown to be anabolic to bone. While several clinical trials have demonstrated that LMHF mechanical loading normalizes bone loss in vivo, the target tissues and cells of the mechanical load and underlying mechanisms mediating the responses are unknown. In this review, we provide an overview of bone adaptation under a variety of loading profiles and the potential for a low magnitude loading as a way to counteract bone loss as experienced by astronauts.

A novel in vitro loading system for high frequency loading of cultured tendon fascicles

Tendons are known to adapt to their mechanical environment, however high frequency low magnitude (HFLM) loading regimes (10–50Hz), which are effective in promoting bone anabolic effects, have not been investigated in controlled conditions in tendon. In vitro loading systems (IVLS) enable precise characterisation of the link between their controlled mechanical environment and cultured tissue biological response.We report a novel IVLS design using an applied magnetic field to produce time varying loading in cultured rat tail tendon fascicles (RTTF). The design was validated through magnetic flux, load cell and viability measurements, and we report the results of preliminary experiments testing the hypothesis that an HFLM loading regime will maintain the biochemical and mechanical properties of fresh RTTF in culture over 7 days.Tissue viability was maintained for 7 days under all loading conditions, and the average peak load applied to RTTFs using the IVLS at 20Hz was 0.125N. RTTFs cultured for 7 days with HFLM loading showed a trend for a higher tangent modulus than fresh tissue, and significantly higher modulus than unloaded RTTFs. GAG content of HFLM cultured RTTFs was not significantly changed from that of fresh RTTFs.This novel, validated IVLS will provide new knowledge of tendon mechanobiology and has already shown the potential of clinically relevant HFLM loading for influencing tendon biology.

Enhancement of Implant Osseointegration by High-Frequency Low-Magnitude Loading

BackgroundMechanical loading is known to play an important role in bone remodelling. This study aimed to evaluate the effect of high- and low-frequency axial loading, applied directly to the implant, on peri-implant bone healing and implant osseointegration.MethodologyTitanium implants were bilaterally installed in rat tibiae. For every animal, one implant was loaded (test) while the other one was not (control). The test implants were randomly divided into 8 groups according to 4 loading regimes and 2 experimental periods (1 and 4 weeks). The loaded implants were subject to an axial displacement. Within the high- (HF, 40 Hz) or low-frequency (LF, 8 Hz) loading category, the displacements varied 2-fold and were ranked as low- or high-magnitude (LM, HM), respectively. The strain rate amplitudes were kept constant between the two frequency groups. This resulted in the following 4 loading regimes: 1) HF-LM, 40 Hz-8 µm; 2) HF-HM, 40 Hz-16 µm; 3) LF-LM, 8 Hz-41 µm; 4) LF-HM, 8 Hz-82 µm. The tissue samples were processed for resin embedding and subjected to histological and histomorphometrical analyses. Data were analyzed statistically with the significance set at p<0.05.Principal FindingsAfter loading for 4 weeks, HF-LM loading (40 Hz-8 µm) induced more bone-to-implant contact (BIC) at the level of the cortex compared to its unloaded control. No significant effect of the four loading regimes on the peri-implant bone fraction (BF) was found in the 2 experimental periods.ConclusionsThe stimulatory effect of immediate implant loading on bone-to-implant contact was only observed in case of high-frequency (40 Hz) low-magnitude (8 µm) loading. The applied load regimes failed to influence the peri-implant bone mass.

TRABECULAR BONE ADAPTATION TO LOW-MAGNITUDE HIGH-FREQUENCY LOADING AT MICRO-GRAVITY

Exposure to microgravity causes loss of lower body bone mass in some astronauts. Low-magnitude high-frequency loading can stimulate bone formation on earth. Here we hypothesized that low-magnitude high-frequency loading will also stimulate bone formation under microgravity conditions. Two groups of six bovine cancellous bone explants were cultured at microgravity on a Russian Foton-M3 spacecraft and were either loaded dynamically using a sinusoidal curve or experienced only a static load. Comparable reference groups were investigated at normal gravity. Bone structure was assessed by histology, and mechanical competence was quantified using mCT and FE modelling; bone remodelling was assessed by fluorescent labelling and secreted bone turnover markers. Statistical analyses on morphometric parameters and apparent stiffness did not reveal significant differences between the treatment groups. The release of bone formation marker from the groups cultured at normal gravity increased significantly from the first to the second week of the experiment by 90.4% and 82.5% in response to static and dynamic loading, respectively. Bone resorption markers decreased significantly for the groups cultured at microgravity by 7.5% and 8.0% in response to static and dynamic loading, respectively. We found low strain magnitudes to drive bone turnover when applied at high frequency, and this to be valid at normal as well as at microgravity. In conclusion, we found the effect of mechanical loading on trabecular bone to be regulated mainly by an increase of bone formation at normal gravity and by a decrease in bone resorption at microgravity. Additional studies with extended experimental time and increased samples number appear necessary for a further understanding of the anabolic potential of dynamic loading on bone quality and mechanical competence. Citation: Torcasio A, Jahn K, Van Guyse M, Spaepen P, Tami AE, et al. (2014) Trabecular Bone Adaptation to Low-Magnitude High-Frequency Loading in Microgravity. PLoS ONE 9(5): e93527. doi:10.1371/journal.pone.0093527 Editor: Ali Al-Ahmad, University Hospital of the Albert-Ludwigs-University Freiburg, Germany Received May 22, 2013; Accepted March 7, 2014; Published May 2, 2014 Copyright: 2014 Torcasio et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was financially supported by grant C90346 (Belgium PRODEX-9 project) and the ESA MAP grant #AO99-122. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: harry.vanlenthe@mech.kuleuven.be

Accuracy of finite element predictions in sideways load configurations for the proximal human femur

Subject-specific finite element models have been used to predict stress-state and fracture risk in individual patients. While many studies analysed quasi-axial loading configurations, only few works simulated sideways load configurations, such as those arising in a fall. The majority among these latter directly predicted bone strength, without assessing elastic strain prediction accuracy. The aim of the present work was to evaluate if a subject-specific finite element modelling technique from CT data that accurately predicted strains in quasi-axial loading configurations is suitable to accurately predict strains also when applying low magnitude loads in sideways configurations. To this aim, a combined numerical–experimental study was performed to compare finite element predicted strains with strain-gauge measurements from three cadaver proximal femurs instrumented with sixteen strain rosettes and tested non-destructively under twelve loading configurations, spanning a wide cone (0–30° for both adduction and internal rotation angles) of sideways fall scenarios. The results of the present study evidenced a satisfactory agreement between experimentally measured and predicted strains (R2 greater than 0.9, RMSE% lower than 10%) and displacements. The achieved strain prediction accuracy is comparable to those obtained in state of the art studies in quasi-axial loading configurations. Still, the presence of the highest strain prediction errors (around 30%) in the lateral neck aspect would deserve attention in future studies targeting bone failure.

Mesomechanical modelling of SnAgCu solder joints in flip chip

In modern microelectronic packages (considered here as a mesoscale), the size of microstructural features of an alloy is compatible with the scale of an entire element that can contain only one or a few grains. In this case, the mechanical behaviour of the element deviates from isotropic/homogenous character at the macroscopic scale of a bulk specimen, comprising a large number of randomly oriented grains. Generally, a crystal-plasticity model, which is based on dislocation sliding in certain slip systems, is applied to describe a local lattice-induced anisotropic behaviour. However, even at a room temperature, the movement of dislocations is not a single mechanism of the inelastic behaviour of eutectic SnAgCu solder due to its low melting point. Under a low-magnitude loading condition, creep also has an effect due to a movement of vacancies. At high temperatures, this creep can become a dominant mechanism for the inelastic behaviour, diminishing the role of the crystal-plasticity model. This paper accounts for the creep component of deformation and unites it with the traditional crystal-plasticity model. In addition, deformation due to thermal expansion is introduced into the constitutive equation to capture the major mechanisms of the mechanical behaviour of a SnAgCu solder micro-joint used in electronics.

Mice lacking thrombospondin 2 show an atypical pattern of endocortical and periosteal bone formation in response to mechanical loading

Thrombospondin 2 (TSP2) is an extracellular matrix (ECM) protein localized to bone. Since mice with a targeted disruption of the TSP2 gene (TSP2-null) have increased bone formation, we hypothesized that mice lacking TSP2 would show an enhanced osteogenic response to mechanical loading. We addressed our hypothesis by subjecting wild-type (WT) and TSP2-null mice to mechanical loading using the non-invasive murine tibia loading device, and statistical comparisons were made between loaded and unloaded bones within genotype, between genotypes, and between the periosteal and endocortical surfaces within genotype. Right tibiae of WT and TSP2-null mice received 5 days of a low-magnitude loading protocol. This low-magnitude loading (inducing ∼900 and 500 με at periosteal and endocortical surfaces of WT bones, respectively) affected neither periosteal nor endocortical bone formation rate (BFR/BS) when comparing loaded to intact bones in either WT or TSP2-null mice, nor did it result in any significant differences between WT and TSP2-null. As well, there was no difference between loaded endocortical and periosteal surfaces in WT mice; however, endocortical BFR/BS in TSP2-null loaded tibia was significantly elevated relative to the periosteal BFR/BS—despite peak periosteal strains being significantly greater than endocortical strains in TSP2-null mice (690 versus 460 με). To confirm this counterintuitive surface-specific response in TSP2-null mice and to induce significant periosteal bone formation, osteogenic potency of the loading protocol was amplified by doubling the number of loading bouts (10 loading days) and loading magnitude (1 Hz, resulting in 1400 and 900 με peak strain at the periosteal and endocortical surfaces, respectively). Under load, both WT and TSP2-null mice showed significantly increased periosteal mineralizing surface (by nearly three-fold and five-fold, respectively), but mineral apposition rate (MAR) was not statistically changed. The increased MS/BS resulted in a five-fold increase in WT periosteal BFR/BS, but the TSP2-null periosteal BFR/BS was unchanged. Furthermore, this increase in WT loaded periosteal BFR/BS was statistically greater than the WT endocortical BFR/BS. At the endocortical surface of WT mice, loading did not significantly increase bone formation parameters (versus intact). In contrast, at the endocortical surface of TSP2-null mice, loading induced a significant two-fold increase in BFR/BS (versus intact), that was also significantly greater than the endocortical BFR/BS of loaded WT mice. Thus, exogenous loading of TSP2-null mice resulted in highly variable responses that did not reflect the induced strains at the periosteal and endocortical surfaces. While in WT mice, loading resulted in increased periosteal BFR/BS that was greater than the endocortical BFR/BS, in TSP2-null mice loading resulted in endocortical (not periosteal) BFR/BS that was elevated. This reversal in envelope-specific bone formation in TSP2-null mice occurred despite periosteal strains being significantly greater than endocortical (1290 versus 775 με) and strain distributions being similar to that of WT. These results show that the disruption of a single gene can lead to a reversal in normal pattern of load induced bone formation, and more specifically, that the functional interaction of TSP2 with mechanical loading is highly contextual and specific to the cortical bone envelope examined.

Evaluation of subchondral bone mineral density associated with articular cartilage structure and integrity in healthy equine joints with different functional demands

To determine and correlate subchondral bone mineral density and overlying cartilage structure and tensile integrity in mature healthy equine stifle (low magnitude loading) and metacarpophalangeal (high magnitude loading) joints. 8 healthy horses, 2 to 3 years of age. Osteochondral samples were acquired from the medial femoral condyle (FC) and medial trochlear ridge (TR) of the stifle joint and from the dorsal (MC3D) and palmar (MC3P) aspects of the distal medial third metacarpal condyles of the metacarpophalangeal joint. Articular cartilage surface fibrillation (evaluated via India ink staining) and tensile biomechanical properties were determined. The volumetric bone mineral density (vBMD) of the underlying subchondral plate was assessed via dual-energy x-ray absorptiometry. Cartilage staining (fibrillation), tensile moduli, tensile strength, and vBMD were greater in the MC3D and MC3P locations, compared with the FC and TR locations, whereas tensile strain at failure was less in MC3D and MC3P locations than FC and TR locations. Cartilage tensile moduli correlated positively with vBMD, whereas cartilage staining and tensile strain at failure correlated negatively with vBMD. In areas of high joint loading, the subchondral bone had high vBMD and the articular cartilage surface layer had high tensile stiffness but signs of structural wear (fibrillation and low failure strain). The site-dependent variations and relationships in this study support the concept that articular cartilage and subchondral bone normally adapt to physiologic loading in a coordinated way.