Effect of mechanical fatigue on commercial bioprosthetic TAVR valve mechanical and microstructural properties

Valvular structural deterioration is of particular concern for transcatheter aortic valve replacements due to their suspected shorter longevity and increasing use in younger patient populations. In this work we investigated the mechanical and microstructural changes in commercial TAVR valves composed of both glutaraldehyde fixed bovine and porcine pericardium (GLBP and GLPP) following accelerated wear testing (AWT) as outlined in ISO 5840 standards. This provided greater physiological relevance to the loading compared to previous studies and by utilizing digital image correlation we were able to obtain strain contours for each leaflet pre and post fatigue and identify sites of fatigue damage. The areas of greatest change in mechanical strain for each leaflet were then further probed using biaxial tensile testing, confocal microscopy, and electron microscopy. It was observed that overall strain decreased in the GLPP valves following AWT of 200 million cycles while the GLBP valve showed an increase in overall strain. Biaxial tensile testing showed a statistically significant reduction in stress for GLPP while no significant changes were seen for GLBP. Both confocal and electron microscopy showed a disruption to the gross collagen organization and fibrillar structure, including fragmentation, for GLPP but only the former for GLBP. However, further test data is required to confirm these findings and to provide a better understanding of this fatigue pathway is required such that it can be incorporated into both valve design and selection processes to improve overall longevity for both GLPP and GLBP devices.

Effect of density grading on the mechanical behaviour of advanced functionally graded lattice structures

Lattice structures have found significant applications in the biomedical field due to their interesting combination of mechanical and biological properties. Among these, functionally graded structures sparked interest because of their potential of varying their mechanical properties throughout the volume, allowing the design of biomedical devices able to match the characteristics of a graded structure like human bone. The aim of this works is the study of the effect of the density grading on the mechanical response and the failure mechanisms of a novel functionally graded lattice structure, namely Triply Arranged Octagonal Rings (TAOR). The mechanical behaviour was compared with the same lattice structures having constant density ratio. Electron Beam Melting technology was used to manufacture titanium alloy specimens with global relative densities from 10% to 30%. Functionally graded structures were obtained by increasing the relative density along the specimen, by individually designing the lattice's layers. Scanning electron and a digital microscopy were used to evaluate the dimensional mismatch between actual and designed structures. Compressive tests were carried out to obtain the mechanical properties and to evaluate the collapse modes of the structures in relation to their average relative density and lattice grading. Open-source Digital Image Correlation algorithm was applied to evaluate the deformation behaviour of the structures and to calculate their elastic moduli. The results showed that uniform density structures provide higher mechanical properties than functionally graded ones. The Digital Image Correlation results showed the possibility of effectively designing the different layers of functionally graded structures selecting desired local mechanical properties to mimic the different characteristics of cortical and cancellous bone.

The importance of mechanical and biological cues of tympanic membrane grafts to ensure optimal regeneration

Chronic suppurative otitis media (CSOM) is often associated with permanent tympanic membrane (TM) perforation and conductive hearing loss. The current clinical gold standard, using autografts and allografts, suffers from several drawbacks. Artificial replacement materials can help to overcome these drawbacks. Therefore, scaffolds fabricated through digital light processing (DLP) were herein created to support TM regeneration. Various UV-curable printing inks, including gelatin methacryloyl (GelMA), gelatin-norbornene-norbornene (GelNBNB) (crosslinked with thiolated gelatin (GelSH)) and alkene-functionalized poly-ε-caprolactone (E-PCL) (crosslinked with pentaerythritol tetrakis(3-mercaptopropionate) (PETA4SH)) were optimized regarding photo-initiator (PI) and photo-absorber (PA) concentrations through viscosity characterization, photo-rheology and the establishment of working curves for DLP. Our material platform enabled the development of constructs with a range of mechanical properties (plateau storage modulus varying between 15 and 119 kPa). Excellent network connectivity for the GelNBNB and E-PCL constructs was demonstrated (gel fractions >95 %) whereas a post-crosslinking step was required for the GelMA constructs. All samples showed excellent biocompatibility (viability >93 % and metabolic activity >88 %). Finally, in vivo and ex vivo assessments, including histology, vibration and deformation responses measured through laser doppler vibrometry and digital image correlation respectively, were performed to investigate the effects of the scaffolds on the anatomical and physiological regeneration of acute TM perforations in rabbits. The data showed that the most efficient healing with the best functional quality was obtained when both mechanical (obtained with the PCL-based resin) and biological (obtained with the gelatin-based resins) material properties were taken into account.

Design of dissimilar material joint for defect-free multi-material additive manufacturing via laser-directed energy deposition

Additive manufacturing technology has advanced beyond creating optimized features, from strengthening materials to make them lightweight to fabricating multi-material combinations that offer functionalities beyond the capabilities of individual materials. In this study, a lamination method for laser-directed energy deposition (LDED) is developed to achieve dense multi-material features, and a design that combines different and dissimilar materials is developed. To evaluate these novel developments, two materials-AISI 316L stainless steel and Inconel 625-are introduced. Tensile specimens, fabricated via multi-material additive manufacturing using LDED, are subjected to tensile tests that are recorded on video for digital image correlation. After the tests, fracture surface analyses of the fractured specimens are performed via scanning electron microscopy, and optical monitoring analyses are performed on the specimens that are not subjected to the tensile tests. The results indicate that the specimens demonstrate varied mechanical properties due to the influence of lamination direction and order, which affect the formation of critical cracks and pores.

Subchondral bone fatigue injury in the parasagittal condylar grooves of the third metacarpal bone in thoroughbred racehorses elevates site-specific strain concentration

Condylar stress fracture of the distal end of the third metacarpal/metatarsal (MC3/MT3) bones is a major cause of Thoroughbred racehorse injury and euthanasia worldwide. Functional adaptation to exercise and fatigue damage lead to structural changes in the subchondral bone that include increased modeling (resulting in sclerotic bone tissue) and targeted remodeling repair (resulting in focal resorption spaces in the parasagittal groove). Whether these focal structural changes, as detectable by standing computed tomography (sCT), lead to elevated strain at the common site of condylar stress fracture has not been demonstrated. Therefore, the goal of the present study was to compare full-field three-dimensional (3D) strain on the distopalmar aspect of MC3 bone specimens with and without focal subchondral bone injury (SBI). Thirteen forelimb specimens were collected from racing Thoroughbreds for mechanical testing ex vivo and underwent sCT. Subsequently, full-field displacement and strain at the joint surface were determined using stereo digital image correlation. Strain concentration was observed in the parasagittal groove (PSG) of the loaded condyles, and those with SBI in the PSG showed higher strain rates in this region than control bones. PSG strain rate in condyles with PSG SBI was more sensitive to CT density distribution in comparison with condyles with no sCT-detectable injury. Findings from this study help to interpret structural changes in the subchondral bone due to fatigue damage and to assess risk of incipient stress fracture in a patient-specific manner.

Validation of a double-semicircular notched configuration for mechanical testing of orthodontic thermoplastic aligner materials

The potential of using specimens with a double-semicircular-notched configuration for performing tensile tests of orthodontic thermoplastic aligner materials was explored. Unnotched and double-semicircular-notched specimens were loaded in tension using a universal testing machine to determine their tensile strength, while finite element analysis (FEA) and digital image correlation (DIC) were used to estimate stress and strain, respectively. The shape did affect the tensile strength, demonstrating the importance of unifying the form of the specimen. During the elastic phase under tension, double-semicircular-notched specimens showed similar behavior to unnotched specimens. However, great variance was observed in the strain patterns of the unnotched specimens, which exhibited greater chance of end-failure, while the strain patterns of the double-semicircular-notched specimens showed uniformity. Considerable agreement between the theoretical (FEA) and practical models (DIC) further confirmed the validity of the double-semicircular-notched models.

Raw dataset of compression tests on a vegetable oil-based polyurethane foam exposed to different ageing conditions

The current dataset brings raw compression test information of a vegetable-based polyurethane foam (PUF) exposed to different temperatures over different periods of time. Such experimental dataset can provide researchers with important information in the application of numerical and data-driven simulations. Also, it saves money and time once the experimental part is already available. At total, 90 compression tests were done following the ASTM D1621-16 standard with pictures for digital image correlation (DIC) being simultaneously acquired. The 90 specimens were divided in nine different ageing conditions. The foam was considered transversely isotropic, thus, 10 specimens for each condition were divided in two groups, five specimens for direction 1 and five for direction 3, where direction 3 is the foam expansion direction. The 3D DIC results show longitudinal and transverse strains from virtual extensometers. The results are available in .TRA and .csv files for the tests and DIC outputs, respectively. Also, the dataset brings the pictures used for DIC in .TIF format. It also brings the dimensions of each specimen prior to the test in .txt format. These results provide information for the calculation of major mechanical properties that can be freely used in finite element models for different and creative ways to simulate the ageing process of a vegetable-based PUF.

The biomechanics of wounds at physiologically relevant levels: Understanding skin's stress-shielding effect for the quantitative assessment of healing

Wounds are responsible for the decrease in quality of life of billions of people around the world. Their assessment relies on subjective parameters which often delays optimal treatments and results in increased healthcare costs. In this work, we sought to understand and quantify how wounds at different healing stages (days 1, 3, 7 and 14 post wounding) change the mechanical properties of the tissues that contain them, and how these could be measured at clinically relevant strain levels, as a step towards quantitative wound tracking technologies. To achieve this, we used digital image correlation and mechanical testing on a mouse model of wound healing to map the global and local tissue strains. We found no significant differences in the elastic and viscoelastic properties of wounded vs unwounded skin when samples were measured in bulk, presumably as these were masked by the protective mechanisms of skin, which redistributes the applied loads to mitigate high stresses and reduce tissue damage. By measuring local strain values and observing the distinct patterns they formed, it was possible to establish a connection between the healing phase of the tissue (determined by the time post-injury and the observed histological features) and the overall mechanical behaviour. Importantly, these parameters were measured from the surface of the tissue, using physiologically relevant strains without increasing the tissue's damage. Adaptations of these approaches for clinical use have the potential to aid in the identification of skin healing problems, such as excessive inflammation or lack of mechanical progression over time. An increase, decrease, or lack of change in the elasticity and viscoelasticity parameters, can be indicative of wound state, thus ultimately leading to improved diagnostic outcomes.

Dataset on the tested and simulated response of thick cold-formed circular hollow sections under cyclic loading

This article describes a dataset used to calibrate a finite element model of a thick circular hollow section (CHS) with varying d/t (diameter to thickness) ratio under cyclic loading which may be used as a computational model validation benchmark by researchers working on similar problems in structural and mechanical engineering. The test data consists of seven cold-formed S335J2H steel CHS tube specimens tested to buckling failure in low-cycle fatigue under a three-point bending arrangement, instrumented with discrete strain gauges, displacement transducers and string potentiometers together with continuous surface deformation fields obtained by two pairs of digital image correlation (DIC) cameras. 'Half-cycle' material data from the uniaxial tensile testing of dog-bone coupons is also provided. Comparisons between measured and simulated entities such as midspan forces, moments, displacements and mean curvatures can be obtained with MATLAB processing scripts. Complete ABAQUS model input files are also provided to aid in benchmarking.

Investigations on the effect of natural veined calcite on the mechanical properties of limestone

The damage behavior of limestone rock masses containing calcite mineral filling under uniaxial compression experimental conditions is unclear, and the fracture mechanism of the rock masses needs to be further explored. In this study, uniaxial compression tests were conducted on limestone rock specimens containing veined calcite by combining acoustic emission and digital image correlation techniques. The effects of veined calcite on the generation and development of cracks on the surface of the specimens until the formation of macroscopic penetration and the strength properties of the rock mass were analyzed. The results showed that the transversely distributed veined calcite caused significant stress concentrations in the rock specimens. The longitudinally distributed veined calcite caused cracks in the specimens or influenced the expansion path of the longitudinal principal cracks. The final damage pattern of the specimens didn't differ significantly from that of conventional rock masses due to the presence of veined calcite. The presence of the veined calcite had effect on the uniaxial compressive capacity of the rock, but the load variation process of the specimen with time still conformed to the load variation pattern during the uniaxial compressive test of conventional rocks.

Study of fatigue damage behavior in off-axis CFRP composites using digital image correlation technology

This study investigates the fatigue behavior of off-axis carbon fiber reinforced polymer (CFRP) composites under varying stress levels, with a focus on both tensile-tensile and compressive-compressive loading modes. We conduct a comprehensive analysis of energy dissipation and stiffness across various loading conditions, highlighting the critical role of fiber deflection effects in the recovery of tensile-tensile fatigue properties. Utilizing digital image correlation (DIC) technology, we identify both commonalities and distinctions in crack propagation pathways and failure mechanisms between these two fatigue scenarios. In the case of tensile-tensile fatigue, the predominant damage mechanism involves the development of multiple interlaminar shear cracks. These cracks initiate debonding at the fiber/resin interface, propagating from macrocracks at the edges to microcracks at the center, ultimately culminating in fiber pull-out failure. Conversely, in compressive fatigue, damage occurs centrally in the form of intralaminar shear cracks. As damage accumulates, these cracks progressively propagate along the fibers towards the edges, accompanied by localized fiber buckling, ultimately resulting in compressive failure. Furthermore, we determine a critical compressive strain threshold, which serves as a pivotal indicator of failure in compressive-compressive fatigue testing for off-axis CFRP composites.

Design, simulation and experimental analysis of a monolithic bending section for enhanced maneuverability of single use laparoscopic devices

Standard laparoscopes, which are widely used in minimally invasive surgery, have significant handling limitations due to their rigid design. This paper presents an approach for a bending section for laparoscopes based on a standard semi-finished tube made of Nitinol with laser-cut flexure hinges. Flexure hinges simply created from a semi-finished product are a key element for realizing low-cost compliant structures with minimal design space. Superelastic materials such as Nitinol allow the reversible strain required for this purpose while maintaining sufficient strength in abuse load cases. This paper focuses on the development of a bending section for single use laparoscopic devices (OD 10 mm) with a bending angle of 100°, which enables the application of 100 µm diameter Nitinol actuator wires. For this purpose, constructive measures to realise a required bending curvature and Finite Element Analysis for determining the strain distribution in the flexural region are applied and described for the design of the flexure hinges. In parallel, the influence of the laser-based manufacturing process on the microstructure is investigated and evaluated using micrographs. The deformation behavior of the bending section is experimentally determined using Digital Image Correlation. The required actuation forces and the failure load of the monolithic bending section is measured and compared to a state of the art riveted bending section made of stainless steel. With the developed monolothic bending section the actuation force could be reduced by 50% and the available inner diameter could be increased by 10% while avoiding the need of any assembly step.

Effect of the apron in the mechanical characterisation of hyperelastic materials by means of biaxial testing: A new method to improve accuracy

Biological soft tissues and polymers used in biomedical applications (e.g. in the cardiovascular area) are hyperelastic incompressible materials that commonly operate under multi-axial large deformation fields. Their characterisation requires biaxial tensile testing. Due to the typically small sample size, the gripping of the specimens commonly relies on rakes or sutures, where the specimen is punctured at the edges of the gauge area. This approach necessitates of an apron, excess of material around the gauge region. This work analyses the apron influence on the estimated mechanical response of biaxial tests performed by using a rakes gripping system, with the aim of verifying the test accuracy and propose improved solutions. In order to isolate the effect of the apron, avoiding the influence of anisotropy and inhomogeneity typical of most soft tissues, homogeneous and isotropic hyperplastic samples made from a uniform sheet of casted silicone were tested. The stress-strain response of specimens with different apron sizes/shapes was measured experimentally by means of biaxial testing and digital image correlation. Tests were replicated numerically, to interpret the experimental findings. The apron surrounding the gauge area acts as an additional annular constraint which stiffens the system, resulting in a significant overestimate in the stress values. This error can be avoided by introducing specific cuts in the apron. The study quantifies, for the first time, the correlation between the apron size/shape and the experimental stress overestimation, proposing a research protocol which, although identified on homogeneous hyperelastic materials, can be useful in providing more accurate characterisation of both, synthetic polymers and soft tissues.

Biaxial hyperelastic and anisotropic behaviors of the corneal anterior central stroma along the preferential fibril orientations. Part I: Measurement and calibration of personalized stress-strain curves

Lacking specimens is the biggest limitation of studying the mechanical behaviors of human corneal. Extracting stress-strain curves is the crucial step in investigating hyperelastic and anisotropic properties of human cornea. 15 human corneal specimens extracted from the small incision lenticule extraction (SMILE) surgery were applied in this study. To accurately measure the personalized true stress-strain curve using corneal lenticules, the digital image correlation (DIC) method and finite element method were used to calibrate the stress and the strain of the biaxial extension test. The hyperelastic load-displacement curves obtained from the biaxial extension test were performed in preferential fibril orientations, which are arranged along the nasal-temporal (NT) and the superior-inferior (SI) directions within the anterior central stroma. The displacement and strain fields were accurately calibrated and calculated using the digital image correlation (DIC) method. A conversion equation was given to convert the effective engineering strain to the true strain. The stress field distribution, which was simulated using the finite element method, was verified. Based on this, the effective nominal stress with personalized characteristics was calibrated. The personalized stress-strain curves containing individual characteristic, like diopter and anterior surface curvature, was accurately measured in this study. These results provide an experimental method using biaxial tensile test with corneal lenticules. It is the foundation for investigating the hyperelasticity and anisotropy of the central anterior stroma of human cornea.

Full-field in vivo experimental study of the strains of a breathing human abdominal wall with intra-abdominal pressure variation

The presented study aims to assess the mechanical behaviour of the anterior abdominal wall based on an in vivo experiment on humans. Full-field measurement of abdominal wall displacement during changes of intra-abdominal pressure is performed using a digital image correlation (DIC) system. Continuous measurement in time enables the observation of changes in the strain field during breathing. The understanding of the mechanical behaviour of a living human abdominal wall is important for the proper design of surgical meshes used for ventral hernia repair, which was also a motivation for the research presented below. The research refers to the strain field of a loaded abdominal wall and presents the evolution of principal strains and their directions in the case of 12 subjects, 8 male and 4 female. Peritoneal dialysis procedure allows for the measurement of intra-abdominal pressure after fluid introduction. High variability among patients is observed, also in terms of principal strain direction. Subjects exhibit intra-abdominal pressure of values from 11 to 21 cmH2O. However, the strain values are not strongly correlated with the pressure value, indicating variability of material properties.

Experimental and numerical analysis of the influence of intramedullary nail position on the cut-out phenomenon

BACKGROUND AND OBJECTIVE

Proximal femur fractures, colloquially known as hip fractures, are a common pathology with increasing incidence in the last years due to the enhanced ageing population. Regarding the extracapsular fracture, the treatment for this pathology consists of a fixation of the fragments using an osteosynthesis device, mainly the intramedullary nail. This repairing method implies several complications, which may include the failure of the fixation device, frequently occurring due to the "cut-out" mechanism. The present work focuses on the study of how the position of the cephalic screw, which should be fixed during surgery, affects the cut-out risk. Through experimental tests and numerical models some variables that can be critical for the cut-out phenomenon are analysed.

METHODS

This study has been carried out through a numerical model based on the finite element method and experimental tests. The digital image correlation technique has been used in experimental tests to measure displacements on the femoral surface with the objective of numerical model validation. Some basic daily activities with different intramedullary nail positions have been analysed through the numerical model, considering variables that can induce the cut-out complication.

RESULTS

The results show how the intramedullary nail position clearly influences the cut-out risk, showing that displacements in the upper, anterior and posterior direction increase the cut-out risk, while displacement in the lower direction endangers the intramedullary nail itself. Thus, the centred position is the one which reduces the cut-out risk.

CONCLUSIONS

This work supposes an improvement in the knowledge of the cut-out phenomenon thanks to the combination of experimental testing and validated numerical models. The effects of different intramedullary nail positions in the femoral head are studied, including a novelty variable as torque, which is critical for the structural integrity of the fixation. The main conclusion of the work is the determination of the central intramedullary nail position as the most favourable one for decreasing the cut-out risk.

Prestrain in the eardrum investigated using laser-ablation perforation: A proof of principle study on the New Zealand white rabbit

While the presence of residual stress (also called prestress) in the tympanic membrane (TM) was hypothesized more than 150 years ago by von Helmholtz (1869), little experimental data exists to date. In this paper, a novel approach to study residual stress is presented. Using a pulsed laser, the New Zealand white rabbit TM is perforated at seven predefined locations. The subsequent retraction of the membrane around the holes is computed using digital image correlation (DIC). The amount of retraction is the so-called prestrain, which is caused by the release of prestress due to the perforation. By measuring the prestrain using DIC, we show that residual stress is clearly present over the entire rabbit TM surface. In total, fourteen TMs have been measured in this work. An automated approach allows tracking the holes' deformation during the measurement process and enables a more robust analysis than was previously possible. We find similar strains (around 5%) as reported in previous work, in which slits were created manually using flattened surgical needles. However, the new approach greatly reduces measurement time, which minimizes dehydration artifacts. To investigate the effect of perforation location on the TM, the spatial decrease of the prestrain (α) around the perforation was quantified. Perforations inferior to the umbo showed the least negative α values, i.e., the most gradual decrease around the hole, and were the most consistent. Perforations on other locations showed more negative α values, i.e., steeper decrease in strain, but were less consistent across samples. We also investigated the effect of the holes' creation sequence but did not observe a significant change in the results. Overall, the presented method allows for consistent residual stress measurements over the TM surface. The findings contribute to our fundamental knowledge of the mechanics of the rabbit TM and provide a basis for future work on human TMs.

Advanced tensile testing as a new tool to quantify properties of food

Mechanical properties of food products are regularly analysed by tensile tests. The aim of this study was to demonstrate the potential of using advanced tensile testing techniques to better understand the mechanical properties of anisotropic food products, such as meat analogues and certain dairy products. The effects of various tensile testing parameters, including tensile gauge length and deformation rate, on the interpretation of mechanical properties of meat analogues was studied. Additionally, digital image correlation, an image analysis technique, was used for true distance recording and analysis of fracturing behaviour of the products. An isotropic product was prepared from solely soy protein isolate, and an anisotropic product was prepared from soy protein isolate and pectin using the shear cell technology. The tensile properties of the products were studied with four different moulds with varying gauge lengths of 17.5, 15, 11.5, and 8.5 mm, and at three deformation rates of 46.2, 23.1, and 11.6 mm/min. A smaller gauge length and slower deformation rate improved visualization and interpretation of the multi-stage descending branch in force - distance curves of anisotropic products. Additionally, tensile parameters, specifically toughness, proved to be more accurate at small gauge length and slow deformation rate, because overestimation due to rapid crack propagation was prevented. True distance data obtained with digital image correlation further improved the interpretation of the fracturing behaviour of the products. Inhomogeneous strain distribution in anisotropic products was shown with digital image correlation, in contrast to the homogeneous strain distribution observed in isotropic products. Furthermore, the Poisson's ratio, obtained through digital image correlation, explained inherent differences in structure and plasticity between isotropic and anisotropic meat analogues. This study shows the importance of careful selection of testing parameters and techniques. Moreover, it advises the use of digital image correlation for better measurement of fracture mechanics and strain distribution.

Applying a homogeneous pressure distribution to the upper vertebral endplate: Validation of a new loading system, pilot application to human vertebral bodies, and finite element predictions of DIC measured displacements and strains

Image-based personalized Finite Element Models (pFEM) could detect alterations in physiological deformation of human vertebral bodies, but their accuracy has been seldom reported. Meaningful validation experiments should allow vertebral endplate deformability and ensure well-controlled boundary conditions. This study aimed to (i) validate a new loading system to apply a homogeneous pressure on the vertebral endplate during vertebral body compression regardless of endplate deformation; (ii) perform a pilot study on human vertebral bodies measuring surface displacements and strains with Digital Image Correlation (DIC); (iii) determine the accuracy of pFEM of the vertebral bodies. Homogeneous pressure application was achieved by pressurizing a fluid silicone encased in a rubber silicone film acting on the cranial endplate. The loading system was validated by comparing DIC-measured longitudinal strains and lower-end contact pressures, measured on three homogeneous pseudovertebrae of constant transversal section at 2.0 kN, against theoretically calculated values. Longitudinal strains and contact pressures were rather homogeneous, and their mean values close to theoretical calculations (5% underestimation). DIC measurements of surface longitudinal and circumferential displacements and strains were obtained on three human vertebral bodies at 2.0 kN. Complete displacement and strain maps were achieved for anterolateral aspects with random errors ≤0.2 μm and ≤30 μstrain, respectively. Venous plexus and double curvatures limited the completeness and accuracy of DIC data in posterior aspects. pFEM of vertebral bodies, including cortical bone mapping, were built from computed tomography images. In anterolateral aspects, pFEM accuracy of the three vertebrae was: (i) comparable to literature in terms of longitudinal displacements (R2>0.8); (ii) extended to circumferential displacements (pooled data: R2>0.9) and longitudinal strains (zero median error, 95% error: <27%). Circumferential strains were overestimated (median error: 39%). The new methods presented may permit to study how physiological and pathologic conditions influence the ability of vertebral endplates/bodies to sustain loads.

Ex Vivo Uniaxial Tensile Properties of Rat Uterosacral Ligaments

This manuscript presents new experimental methods for testing the ex vivo tensile properties of the uterosacral ligaments (USLs) in rats. The USL specimens ([Formula: see text]) were carefully dissected to preserve their anatomical attachments, and they were loaded along their main in vivo loading direction (MD) using a custom-built uniaxial tensile testing device. During loading, strain maps in both the MD and the perpendicular direction (PD) were collected using the digital image correlation technique. The mean (± S.E.M.) maximum load and displacement at the maximum load were [Formula: see text] N and [Formula: see text] mm, respectively. The USLs were found to be highly heterogeneous structures, with some specimens experiencing strains in the MD that were lower than [Formula: see text] and others reaching strains that were up to [Formula: see text] in the intermediate region. At 0.5 kPa stress, a value reached by all the specimens, the mean strain in the MD was [Formula: see text] while at 5 kPa stress, a value achieved only by 9 out of the 21 specimens, the mean strain increased to [Formula: see text]. Under uniaxial loading, the specimens also elongated in the PD, with strains that were one order of magnitude lower than the strains in the MD; at the 0.5 kPa stress, the mean strain in the PD was recorded to be [Formula: see text] and, at the 5 kPa stress, the strain in the PD was [Formula: see text]. The directions of maximum principal strains remained almost unchanged with the increase in stress, indicating that little microstructural re-organization occurred due to uniaxial loading. This study serves as a springboard for future investigations on the supportive function of the USLs in the rat model by offering guidelines on testing methods that capture their complex mechanical behavior.

Strain evaluation of axially loaded collateral ligaments: a comparison of digital image correlation and strain gauges

The response of soft tissue to loading can be obtained by strain assessment. Typically, strain can be measured using electrical resistance with strain gauges (SG), or optical sensors based on the digital image correlation (DIC), among others. These sensor systems are already established in other areas of technology. However, sensors have a limited range of applications in medical technology due to various challenges in handling human soft materials. The aim of this study was to compare directly attached foil-type SG and 3D-DIC to determine the strain of axially loaded human ligament structures. Therefore, the medial (MCL) and lateral (LCL) collateral ligaments of 18 human knee joints underwent cyclic displacement-controlled loading at a rate of 20 mm/min in two test trials. In the first trial, strain was recorded with the 3D-DIC system and the reference strain of the testing machine. In the second trial, strain was additionally measured with a directly attached SG. The results of the strain measurement with the 3D-DIC system did not differ significantly from the reference strain in the first trial. The strains assessed in the second trial between reference and SG, as well as between reference and 3D-DIC showed significant differences. This suggests that using an optical system based on the DIC with a given unrestricted view is an effective method to measure the superficial strain of human ligaments. In contrast, directly attached SGs provide only qualitative comparable results. Therefore, their scope on human ligaments is limited to the evaluation of changes under different conditions.

In-vivo high-speed biomechanical imaging of the cornea using Corvis ST and digital image correlation

In-vivo corneal biomechanical characterization has gained significant clinical relevance in ophthalmology, especially in the early diagnosis of eye disorders and diseases (e.g. keratoconus). In clinical medicine, the air-puff-based tonometers such as Ocular Response Analyzer (ORA) and Corvis ST have been used in the in-vivo biomechanical testing. In the test, the high-speed dynamic deformation of the cornea under air-puff excitation is analyzed to identify the abnormities in the morphological and biomechanical properties of the cornea. While most existing measurements reflect the overall corneal biomechanical properties, in-vivo high-speed strain and strain rate fields at the tissue level have not been assessed. In this study, 20 subjects were classified into two different groups: the normal (NORM, N = 10) group and the keratoconus (KC, N = 10) group. Image sequences of the horizontal cross-section of the human cornea under air puff were captured by the Corvis ST tonometer. The macroscale mechanical response of the cornea was determined through image analysis. The high-speed evolution of full-field corneal displacement, strain, velocity, and strain rate was reconstructed using the incremental digital image correlation (DIC) approach. Differences in the parameters between the NORM and KC groups were statistically analyzed and compared. Statistical results indicated that compared with the NORM group, the KC corneas absorbed more energy (KC: 8.98 ± 2.76 mN mm; NORM: 4.79 ± 0.62 mN mm; p-value <0.001) with smaller tangent stiffness (KC: 22.49 ± 2.62 mN/mm; NORM: 24.52 ± 3.20 mN/mm; p-value = 0.15) and larger maximum deflection (KC: 0.99 ± 0.07 mN/mm; NORM: 0.92 ± 0.06 mN/mm; p-value <0.05) on the macro scale. Further, we also observed that The maximum displacement (KC: 1.17 ± 0.06 mm; NORM: 1.06 ± 0.07 mm; p-value <0.005), velocity (KC: 236 ± 29 mm/s; NORM: 203 ± 17 mm/s; p-value <0.01), shear strain (KC: 24.43 ± 2.59%; NORM: 20.26 ± 1.54%; p-value <0.001), and shear strain rate (KC: 69.74 ± 11.99 s-1; NORM: 54.84 ± 3.03 s-1; p-value <0.005) in the KC group significantly increased at the tissue level. This is the first time that the incremental DIC method was applied to the in-vivo high-speed corneal deformation measurement in combination with the Corvis ST tonometer. Through the image registration using incremental DIC analysis, spatiotemporal dynamic strain/strain rate maps of the cornea can be estimated at the tissue level. This is constructive for the clinical recognition and diagnosis of keratoconus at a more underlying level.

Short-term vancomycin and buffer soaking does not change rabbit achilles tendon tensile material properties

BACKGROUND

Allograft tendons are commonly used during orthopedic surgery to reconstruct tissue that is severely damaged. Soaking the tendon in an antibiotic solution, specifically vancomycin, has been shown to lower the risk of post-operative infections. While some material properties of tendon and ligament after antibiotic soaking have previously been characterized, extensive sub-failure allograft tendon material properties after soaking in antibiotic solutions have not.

METHODS

Forty tendons were dissected from rabbits and soaked in either a phosphate buffered saline (PBS) only solution or vancomycin and PBS solution for five or 30 min. Immediately after soaking, quasi-static tensile experiments were performed in a materials testing system.

FINDINGS

Tissue nominal stress, Lagrange strain, toe-region properties and elastic modulus were characterized. For all forty tendons, the average elastic modulus was found to be 455 ± 37 MPa, the average transition strain (from toe-region to linear elastic region) was 0.0487 ± 0.0035, and the average transition stress was 9.71 ± 0.79 MPa. No statistically significant differences in any of these material properties were found across soaking medium or soaking time.

INTERPRETATION

From these results, we conclude that soaking an allograft tendon in antibiotic solution for up to 30 min prior to implantation does not change the tensile material properties of tendons, supporting current clinical practice.

An Improved Finite Element Model of Temporomandibular Joint in Maxillofacial System: Experimental Validation

Finite element (FE) analysis has become a popular method of exploring the biomechanical characteristics of temporomandibular joint (TMJ). However, the FE model should be improved and its reliability should be verified further. This study developed a complete maxillofacial model by cone-beam computed tomography (CBCT) and magnetic resonance imaging (MRI). The integrity and physiological environment of TMJ were considered. Then the FE model and corresponding 3D printed model were developed and loaded under the same conditions. The strains on the mandible and upper surface of the left articular disc were measured on the experimental model and compared with the FE model. The differences of the strains on the mandible were less than 6%. The strain distributions on the disc were also approximate between the experimental and simulated results. It indicated that the strains calculated from the improved FE model were reliable on the mandible and inside the TMJ.

Dataset for the hybrid non-toughened and toughened epoxy adhesive properties

In this article, the manufacturing and toughening effects on the material properties of epoxy adhesives used in wind turbine rotor blades are presented. Different adhesive materials are developed by combining SPABOND™ 820HTA (non-toughened) and SPABOND™ 840HTA (toughened) adhesives with the machine and manual mixing methods. Firstly, the manufacturing quality are compared between the two methods, in terms of void percentage and void volume using micro-computed tomography. Dynamical Mechanical analysis, uniaxial tensile testing, V-notch shear testing and single-edge-notch beam testing are carried out to evaluate the manufacturing and toughening effects. In these experiments, the digital image correlation technique is exploited to obtain the displacement and strain data. Origin Pro^Ⓡ and MATLAB R2021b^Ⓡ are utilized for technical data analysis, plotting, smoothing, filtering, and averaging. The obtained data could be used to select the adhesive material based on the strength and stiffness requirements, develop failure criteria, and predict the thick adhesive joint behavior by finite element modeling.

The Influence of Static Load and Sideways Impact Fall on Extramedullary Bone Plates Used to Treat Intertrochanteric Femoral Fracture: A Preclinical Strength Assessment

Hip fracture accounts for a large number of hospitalizations, thereby causing substantial economic burden. Majority (> 90%) of all hip fractures are associated to sideways fall. Studies on sideways fall usually involve loading at quasi-static or at constant displacement rate, which neglects the physics of actual fall. Understanding femur resonance frequency and associated mode shapes excited by dynamic loads is also critical. Two commercial extramedullary implants, proximal femoral locking plate (PFLP) and variable angle dynamic hip screw (VA-DHS), were chosen to carry out the preclinical assessments on a simulated Evans-I type intertrochanteric fracture. In this study, we hypothesized that the behavior of the implant depends on the loading types-axial static and transverse impact-and a rigid implanted construct will absorb less impact energy for sideways fall. The in silico models were validated using experimental measurements of full-field strain data obtained from a 2D digital image correlation (DIC) study. Under peak axial load of 3 kN, PFLP construct predicted greater axial stiffness (1.07 kN/mm) as opposed to VA-DHS (0.85 kN/mm), although the former predicted slightly higher proximal stress shielding. Further, with greater mode 2 frequency, PFLP predicted improved performance in resisting bending due to sideways fall as compared to the other implant. Overall, the PFLP implanted femur predicted the least propensity to adverse stress intensities, suggesting better structural rigidity and higher capacity in protecting the fractured femur against fall.

In-plane and out-of-plane deformations of gilt utero-sacral ligaments

The uterosacral ligaments (USLs) are supportive structures of the uterus and apical vagina. The mechanical function of these ligaments within the pelvic floor is crucial not only in normal physiological conditions but also in reconstructive surgeries for pelvic organ prolapse. Discrepancies in their anatomical and histological description exist in the literature, but such discrepancies are likely due to large variations of these structures. This makes mechanical testing very challenging, requiring the development of advanced methods for characterizing their mechanical properties. This study proposes the use of planar biaxial testing, digital image correlation (DIC), and optical coherence tomography (OCT) to quantify the deformations of the USLs, both in-plane and out-of-plane. Using the gilts as an animal model, the USLs were found to deform significantly less in their main direction (MD) of in vivo loading than in the direction perpendicular to it (PD) at increasing equibiaxial stresses. Under constant equibiaxial loading, the USLs deform over time equally, at comparable rates in both the MD and PD. The thickness of the USLs decreases as the equibiaxial loading increases but, under constant equibiaxial loading, the thickness increases in some specimens and decreases in others. These findings could contribute to the design of new mesh materials that augment the support function of USLs as well as noninvasive diagnostic tools for evaluating the integrity of the USLs.

Computationally efficient model to predict the deformations of a cellular foot orthotic

BACKGROUND

Foot orthotics (FOs) are frequently prescribed to provide comfortable walking for patients. Finite element (FE) simulation and 3D printing pave the way to analyse, optimize and fabricate functionally graded lattice FOs where the local stiffness can vary to meet the therapeutic needs of each individual patient. Explicit FE modelling of lattice FOs with converged 3D solid elements is computationally prohibitive. This paper presents a more computationally efficient FE model of cellular FOs.

METHOD

The presented FE model features shell elements whose mechanical properties were computed from the numerical homogenization technique. To verify the results, the predictions of the homogenized models were compared to the explicit model's predictions when the FO was under a static pressure distribution of a foot. To validate the results, the predictions were also compared with experimental measurements when the FO was under a vertical displacement at the medial longitudinal arch.

RESULTS

The verification procedure showed that the homogenized model was 46 times faster than the explicit model, while their relative difference was less than 8% to predict the local minimum of out-of-plane displacement. The validation procedure showed that both models predicted the same contact force with a relative difference of less than 1%. The predicted force-displacement curves were also within a 90% confidence interval of the experimental measurements having a relative difference smaller than 10%. In this case, using the homogenized model reduced the computational time from 22 h to 22 min.

CONCLUSION

The presented homogenized model can be therefore employed to speed up the FE simulation to predict the deformations of the cellular FOs.

Examining lung mechanical strains as influenced by breathing volumes and rates using experimental digital image correlation

BACKGROUND

Mechanical ventilation is often employed to facilitate breathing in patients suffering from respiratory illnesses and disabilities. Despite the benefits, there are risks associated with ventilator-induced lung injuries and death, driving investigations for alternative ventilation techniques to improve mechanical ventilation, such as multi-oscillatory and high-frequency ventilation; however, few studies have evaluated fundamental lung mechanical local deformations under variable loading.

METHODS

Porcine whole lung samples were analyzed using a novel application of digital image correlation interfaced with an electromechanical ventilation system to associate the local behavior to the global volume and pressure loading in response to various inflation volumes and breathing rates. Strains, anisotropy, tissue compliance, and the evolutionary response of the inflating lung were analyzed.

RESULTS

Experiments demonstrated a direct and near one-to-one linear relationship between applied lung volumes and resulting local mean strain, and a nonlinear relationship between lung pressures and strains. As the applied air delivery volume was doubled, the tissue surface mean strains approximately increased from 20 to 40%, and average maximum strains measured 70-110%. The tissue strain anisotropic ratio ranged from 0.81 to 0.86 and decreased with greater inflation volumes. Local tissue compliance during the inflation cycle, associating evolutionary strains in response to inflation pressures, was also quantified.

CONCLUSION

Ventilation frequencies were not found to influence the local stretch response. Strain measures significantly increased and the anisotropic ratio decreased between the smallest and greatest tidal volumes. Tissue compliance did not exhibit a unifying trend. The insights provided by the real-time continuous measures, and the kinetics to kinematics pulmonary linkage established by this study offers valuable characterizations for computational models and establishes a framework for future studies to compare healthy and diseased lung mechanics to further consider alternatives for effective ventilation strategies.

Biomechanics of the parasite-host interaction of the European mistletoe

The European mistletoe (Viscum album) is an epiphytic hemiparasite that attaches to its host by an endophytic system. Two aspects are essential for its survival: the structural integrity of the host-parasite interface must be maintained during host growth and the functional integrity of the interface must be maintained during ontogeny and under mechanical stress. We investigated the mechanical properties of the mistletoe-host interaction. Intact and sliced mistletoe-host samples, with host wood as reference, were subjected to tensile tests up to failure. We quantified the rough fractured surface by digital microscopy and analysed local surface strains by digital image correlation. Tensile strength and deformation energy were independent of mistletoe age but exhibited markedly lower values than host wood samples. Cracks initiated at sites with a major strain of about 30%, especially along the mistletoe-host interface. The risk of sudden failure was counteracted by various sinkers and a lignification gradient that smooths the differences in the mechanical properties between the two species. Our results improve the understanding of the key mechanical characteristics of the host-mistletoe interface and show that the mechanical connection between the mistletoe and its host is age-independent. Thus, functional and structural integrity is ensured over the lifetime of the mistletoe.

In vivo skin anisotropy dataset from annular suction test

To characterize the anisotropic and viscoelastic behaviors of the skin, we conducted an experimental campaign of in-vivo suction tests using the CutiScan®CS100 device from Courage and Khazaka electronics. In this data paper, we present the raw acquired data of the tests and their respective treated data. The tests were performed 30 times on the anterior forearm of a 28-year-old Caucasian male at different pressure set-points, ranging from 100 to 500 mbar with an increment of 20 mbar, at ambient temperature in a windowless room. The primary dataset consists of videos recorded by a probe camera associated with the CutiScan® device during the tests. After data treatment with DIC (Digital Image Correlation) technique and based on a homemade Python program, we have obtained secondary data tables and 2D displacement for all mapped grid nodes.

Depth-dependent patterns in shear modulus of temporomandibular joint cartilage correspond to tissue structure and anatomic location

To fully understand TMJ cartilage degeneration and appropriate repair mechanisms, it is critical to understand the native structure-mechanics relationships of TMJ cartilage and any local variation that may occur in the tissue. Here, we used confocal elastography and digital image correlation to measure the depth-dependent shear properties as well as the structural properties of TMJ cartilage at different anatomic locations on the condyle to identify depth-dependent changes in shear mechanics and structure. We found that samples at every anatomic location showed qualitatively similar shear modulus profiles as a function of depth. In every sample, four distinct zones of mechanical behavior were observed, with shear modulus values spanning 3-5 orders of magnitude across zones. However, quantitative characteristics of shear modulus profiles varied by anatomic location, particularly zone size and location, with the most significant variation in zonal width occurring in the fibrocartilage surface layer (zone 1). This anatomic variation suggests that different locations on the TMJ condyle may play unique mechanical roles in TMJ function. Furthermore, zones identified in the mechanical data corresponded on a sample-by-sample basis to zones identified in the structural data, indicating the known structural zones of TMJ cartilage may also play unique mechanical roles in TMJ function.

Global and local characterization explains the different mechanisms of failure of the human ribs

Knowledge of the mechanics and mechanistic reasons inducing rib fracture is fundamental for forensic investigations and for the design of implants and cardiopulmonary resuscitation devices. A mechanical rationale to explain the different rib mechanisms of failure is still a challenge. The aim of this work was to experimentally characterize human ribs to test the hypothesis that a correlation exists between the ribs properties and the mechanism of failure. 89 ribs were tested in antero-posterior compression. The full-field strain distribution was measured through Digital Image Correlation. The fracture load ranged 7-132 N. Two main different mechanisms of failure were observed: brittle and buckling. The strain analysis showed that the direction of principal strains was either aligned with the ribs, or oblique, around 45°, with a rather uniform direction in the most strained area. The maximum principal strains were in the range between 1000 and 30000 microstrain and the minimum principal strain between -30000 and -800 microstrain. The ribs undergoing brittle fracture had significantly thicker cortical bone than those undergoing buckling. Also, larger tensile strains were observed in the specimens with brittle fracture than in the buckling ones. These findings support the focus of cortical thickness modelling which could help in sharpening computational models for the aforesaid purposes.

Type, size, and position of metastatic lesions explain the deformation of the vertebrae under complex loading conditions

BACKGROUND

Bone metastases may lead to spine instability and increase the risk of fracture. Scoring systems are available to assess critical metastases, but they lack specificity, and provide uncertain indications over a wide range, where most cases fall. The aim of this work was to use a novel biomechanical approach to evaluate the effect of lesion type, size, and location on the deformation of the metastatic vertebra.

METHOD

Vertebrae with metastases were identified from 16 human spines from a donation programme. The size and position of the metastases, and the Spine Instability Neoplastic Score (SINS) were evaluated from clinical Quantitative Computed Tomography images. Thirty-five spine segments consisting of metastatic vertebrae and adjacent healthy controls were biomechanically tested in four different loading conditions. The strain distribution over the entire vertebral bodies was measured with Digital Image Correlation. Correlations between the features of the metastasis (type, size, position and SINS) and the deformation of the metastatic vertebrae were statistically explored.

RESULTS

The metastatic type (lytic, blastic, mixed) characterizes the vertebral behaviour (Kruskal-Wallis, p = 0.04). In fact, the lytic metastases showed more critical deformation compared to the control vertebrae (average: 2-fold increase, with peaks of 14-fold increase). By contrast, the vertebrae with mixed or blastic metastases did not show a clear trend, with deformations similar or lower than the controls. Once the position of the lytic lesion with respect to the loading direction was taken into account, the size of the lesion was significantly correlated with the perturbation to the strain distribution (r² = 0.72, p < 0.001). Conversely, the SINS poorly correlated with the mechanical evidence, and only in case of lytic lesions (r² = 0.25, p < 0.0001).

CONCLUSION

These results highlight the relevance of the size and location of the lytic lesion, which are marginally considered in the current clinical scoring systems, in driving the spinal biomechanical instability. The strong correlation with the biomechanical evidence indicates that these parameters are representative of the mechanical competence of the vertebra. The improved explanatory power compared to the SINS suggests including them in future guidelines for the clinical practice.

Nonlinearity of the coefficient of thermal expansion in brain tissue

The coefficient of thermal expansion (CTE) in biological tissues is an integral parameter behind the application of electromagnetic energy to biomedical technologies; however, its behavior is far from being fully characterized. In this study, we apply digital image correlation (DIC) to non-invasively measure the microscale thermal expansions of recently excised embryonic E18 rodent brain tissue slices. Although the CTE has been measured previously in soft tissues, the literature surrounding the expansion of brain tissue remains sparse. Previous work in measuring the thermal expansion behavior of soft tissue often simplifies the results into a single measurement of a linear CTE parameter and fails to convey the temperature-dependent nonlinearity that exists. In this work, we demonstrate that: (1) the coefficient of brain tissue is more similar to fat than blood, and (2) there exists a significant nonlinear increase in CTE at physiologically-relevant temperatures. This suggests some limitations with the interpretation of previously reported values of the CTE, which are often measured at room temperature.

Tensile and compressive strain evolutions of bovine compact bone under four-point bending fatigue loading

Bones are biological composite materials with multiscale structures. Bone fatigue damage is commonly characterized by an increase in strain that is accompanied by microdamage at different scales. This study investigated the damage evolutions of bone specimens under four-point bending fatigue loading using neutral axis migration. Tensile and compressive strains during the fatigue process were simultaneously measured using a digital image correlation technique. The compressive strain of the bone specimen increased rapidly at first and then proceeded slowly while the tensile strain decreased during fatigue loading. Consequently, the neutral axis shifted downward as the damage accumulated. A positive correlation exists between the downward offset of the neutral axis and the number of cycles. The variation in compressive strain is larger than that in tensile strain in this situation.

Tear propagation in vaginal tissue under inflation

Vaginal tearing at childbirth is extremely common yet understudied despite the long-term serious consequences on women's health. The mechanisms of vaginal tearing remain unknown, and their knowledge could lead to the development of transformative prevention and treatment techniques for maternal injury. In this study, whole rat vaginas with pre-imposed elliptical tears oriented along the axial direction of the organs were pressurized using a custom-built inflation setup, producing large tear propagation. Large deformations of tears through propagation were analyzed, and nonlinear strains around tears were calculated using the digital image correlation technique. Second harmonic generation microscopy was used to examine collagen fiber organization in mechanically untested and tested vaginal specimens. Tears became increasingly circular under pressure, propagating slowly up to the maximum pressure and then more rapidly. Hoop strains were significantly larger than axial strains and displayed a region- and orientation-dependent response with tear propagation. Imaging revealed initially disorganized collagen fibers that aligned along the axial direction with increasing pressure. Fibers in the near-regions of tear tips aligned toward the hoop direction, hampering tear propagation. Changes in tear geometry, regional strains, and fiber orientation revealed the inherent toughening mechanisms of the vaginal tissue. STATEMENT OF SIGNIFICANCE: Women's reproductive health has historically been understudied despite alarming maternal injury and mortality rates in the world. Maternal injury and disability can be reduced by advancing our limited understanding of the large deformations experienced by women's reproductive organs. This manuscript presents, for the first time, the mechanics of tear propagation in vaginal tissue and changes to the underlying collagen microstructure near to and far from the tear. A novel inflation setup capable of maintaining the in vivo tubular geometry of the vagina while propagating a pre-imposed tear was developed. Toughening mechanisms of the vagina to propagation were examined through measurements of tear geometry, strain distributions, and reorientation of collagen fibers. This research draws from current advances in the engineering science and mechanics fields with the goal of improving maternal health care.

Complementary time-lapse datasets of x-ray computed tomography and real-time strain mapping for an ex-situ study of non-crimp glass fibre composites under fatigue loading

Data published in this paper corresponds to a time-lapse ex-situ experiment aimed at analyzing the tension-tension fatigue damage in non-crimp glass-epoxy composites by multi-scale x-ray computed tomography (XCT) of the damage features and their timeline. This is then correlated with the strain fields obtained through digital image correlation (DIC). The XCT - DIC datasets by is acquired by interrupting mechanical fatigue tests at three time-steps, after the material has undergone 0 cycles, 70,000 cycles, 80,000 cycles, and 120,000 cycles. This is one of the first multi-modally correlated datasets available for these types of non-crimp glass fibre composites, which explore the structure-property relationship in a time-dependent behavior. This dataset can be used to explore glass-fibre composites microstructure under a progressive damage scheme and can be used to test and train a plethora of image processing and analysis techniques. This dataset can also be used as an attempt to model the fatigue behavior of quasi-unidirectional non-crimp fibre composites by image-based simulations.

Femoral strength and strains in sideways fall: Validation of finite element models against bilateral strain measurements

Low impact falls to the side are the main cause of hip fractures in elderly. Finite element (FE) models of the proximal femur may help in the assessment of patients at high risk for a hip fracture. However, extensive validation is essential before these models can be used in a clinical setting. This study aims to use strain measurements from bilateral digital image correlation to validate an FE model against ex vivo experimental data of proximal femora under a sideways fall loading condition. For twelve subjects, full-field strain measurements were available on the medial and lateral side of the femoral neck. In this study, subject-specific FE models were generated based on a consolidated procedure previously validated for stance loading. The material description included strain rate dependency and separate yield and fracture strain limits in tension and compression. FE predicted fracture force and experimentally measured peak forces showed a strong correlation (R² = 0.92). The FE simulations predicted the fracture initiation within 3 mm distance of the experimental fracture line for 8/12 subjects. The predicted and measured strains correlated well on both the medial side (R² = 0.87) and the lateral side (R² = 0.74). The lower correlation on the lateral side is attributed to the irregularity of the cortex and presence of vessel holes in this region. The combined validation against bilateral full-field strain measurements and peak forces has opened the door for a more elaborate qualitative and quantitative validation of FE models of femora under sideways fall loading.

Finite element simulation of fixed dental prostheses made from PMMA -Part I: Experimental investigation under quasi-static loading and chewing velocities

Material properties are of high clinical relevance, even though in vitro laboratory setups may differ from clinical conditions. Therefore, the aim of the present study was to investigate the fracture behavior of three-unit bridge restoration (Telio CAD) with different test velocities (1.0 mm/min International Organization for Standardization (ISO) standard speed/ 130 mm/s mean chewing velocity) and to provide crucial validation experiments for the upcoming Part 2 of our study, in which FEA on such temporary restorations will be conducted. Local strains were detected using digital image correlation (DIC). The material exhibited significantly different responses at different test velocities, and the forces at fracture were found to be much smaller at chewing velocity (130 mm/s) than in the quasi-static test. Overall, the results of the present study show that characteristics pertaining to material behavior can change significantly with increasing chewing velocity, and that fracture forces decrease with increasing test velocity.

Digital image correlation in dental materials and related research: A review

OBJECTIVE

Digital image correlation (DIC) is a non-contact image processing technique for full-field strain measurement. Although DIC has been widely used in engineering and biomechanical fields, it is in the spotlight only recently in dental materials. Therefore, the purpose of this review paper is introducing the working principle of the DIC technique with some modifications and providing further potential applications in various dental materials and related fields.

METHODS

The accuracy of the algorithm depending on the environmental characteristics of the DIC technique, as well as the advantages and disadvantages of strain measurement using optical measurements, have been elaborated in dental materials and related fields. Applications to those researches have been classified into the following categories: shrinkage behavior of light-cured resin composite, resin-tooth interface, mechanical properties of tooth structure, crack extension and elastic properties of dental materials, and deformation of dental restoration and prosthesis. This classification and discussion were performed using literature survey and review based on numerous papers in the international journals published over the past 20 years. The future directions for predicting the precise deformation of dental materials under various environments, as well as limitations of the DIC technique, was presented in this review.

RESULTS

The DIC technique was demonstrated as a more effective tool to measure full-field polymerization shrinkage of composite resin, even in a simulated clinical condition over the existing methods. Moreover, the DIC combined with other technologies can be useful to evaluate the mechanical behavior of material-tooth interface, dentine structure and restorative and prosthetic materials with high accuracy. Three-dimensional DIC using two cameras extended the measurement range in-plane to out-of-plane, enabling measure of the strain directly on the surface of dental restorations or prosthesis.

SIGNIFICANCE

DIC technique is a potential tool for measuring and predicting the full-field deformation/strain of dental materials and actual prostheses in diverse clinical conditions. The versatility of DIC can replace the existing complex sensor devices in those studies.

No effect in primary stability after increasing interference fit in cementless TKA tibial components

Cementless total knee arthroplasty (TKA) implants rely on interference fit to achieve initial stability. However, the optimal interference fit is unknown. This study investigates the effect of using different interference fit on the initial stability of tibial TKA implants. Experiments were performed on human cadaveric tibias using a low interference fit of 350 μm of a clinically established cementless porous-coated tibial implant and a high interference fit of 700 μm. The Orthoload peak loads of gait and squat were applied to the specimens with a custom-made load applicator. Micromotions and gaps opening/closing were measured at the bone-implant interface using Digital Image Correlation (DIC) in 6 regions of interest (ROIs). Two multilevel linear mixed-effect models were created with micromotions and gaps as dependent variables. The results revealed no significant differences for micromotions between the two interference fits (gait p = 0.755, squat p = 0.232), nor for gaps opening/closing (gait p = 0.474, squat p = 0.269). In contrast, significant differences were found for the ROIs in the two dependent variables (p < 0.001), where more gap closing was seen in the posterior ROIs than in the anterior ROIs during both loading configurations. This study showed that increasing the interference fit from 350 to 700 μm did not influence initial stability.

Effects of cyclic loading on the mechanical properties and failure of human patellar tendon

Patellar tendinopathy is a common overuse injury in sports such as volleyball, basketball, and long-distance running. Microdamage accumulation, in response to repetitive loading of the tendon, plays an important role in the pathophysiology of patellar tendinopathy. This damage presents mechanically as a reduction in Young's modulus and an increase in residual strain. In this study, 19 human patellar tendon samples underwent cyclic testing in load control until failure, segmented by four ramped tests where digital image correlation (DIC) was used to assess anterior surface strain distributions. Ramped tests were performed prior to cyclic testing and at timepoints corresponding to 10%, 20%, and 30% of cyclic stiffness reduction. Young's modulus significantly decreased and cyclic energy dissipation significantly increased over the course of cyclic testing. The DIC analysis illustrated a heterogeneous strain distribution, with strain concentrations increasing in magnitude and size over the course of cyclic testing. Peak stress and initial peak strain magnitudes significantly correlated with the number of cycles to failure (r² = 0.65 and r² = 0.57, respectively, p < 0.001); however, the rates of peak cyclic strain and modulus loss displayed the highest correlations with the number of cycles to failure (r² = 96% and r² = 86%, respectively, p < 0.001). The high correlation between the rates of peak cyclic strain and modulus loss suggest that non-invasive methods to continuously monitor tendon strain may provide meaningful predictions of overuse injury in the patellar tendon.

Open cell polyurethane foam compression failure characterization and its relationship to morphometry

Open cell polyurethane foams are often used as cancellous bone surrogates because of their similarities in morphology and mechanical response. In this work, open cell polyurethane foams of three different densities are characterized from morphometric and mechanical perspectives. The analysis of micro-computed tomography images has revealed that the high density foams present the greatest inhomogeneities. Those inhomogeneities promoted the failure location. We have used the finite element models as a tool to estimate elastic and failure properties that can be used in numerical modeling. Furthermore, we have assessed the anisotropic mechanical response of the foams, whose differences are related to the morphometric inhomogeneities. We found significant relationships between morphometry and the elastic and failure response. The detailed information about morphometry, elastic constants and strength limits provided in this work can be of interest to researchers and practitioners that often use these polyurethane foams in orthopedic implants and cement augmentation evaluations.

Decreased stress shielding with a PEEK femoral total knee prosthesis measured in validated computational models

Due to their high stiffness, metal femoral implants in total knee arthroplasty may cause stress shielding of the peri-prosthetic bone, which can lead to loss of bone stock. Using a polymer (PEEK) femoral implant reduces the stiffness mismatch between implant and bone, and therefore has the potential to decrease strain shielding. The goal of the current study was to evaluate this potential benefit of PEEK femoral components in cadaveric experiments. Cadaveric femurs were loaded in a materials testing device, while a 3-D digital image correlation set-up captured strains on the surface of the intact femurs and femurs implanted with PEEK and CoCr components. These experimental results were used to validate specimen-specific finite element models, which subsequently were used to assess the effect of metal and PEEK femoral components on the bone strain energy density. The finite element models showed strain maps that were highly comparable to the experimental measurements. The PEEK implant increased strain energy density, relative to the preoperative bone and compared to CoCr. This was most pronounced in the regions directly under the implant and near load contact sites. These data confirm the hypothesis that a PEEK femoral implant can reduce peri-prosthetic stress shielding.

Is the 0.2%-Strain-Offset Approach Appropriate for Calculating the Yield Stress of Cortical Bone?

The 0.2% strain offset approach is mostly used to calculate the yield stress and serves as an efficient method for cross-lab comparisons of measured material properties. However, it is difficult to accurately determine the yield of the bone. Especially when computational models require accurate material parameters, clarification of the yield point is needed. We tested 24 cortical specimens harvested from six bovine femora in three-point bending mode, and 11 bovine femoral cortical specimens in the tensile mode. The Young's modulus and yield stress for each specimen derived from the specimen-specific finite element (FE) optimization method was regarded as the most ideal constitutive parameter. Then, the strain offset optimization method was used to find the strain offset closest to the ideal yield stress for the 24 specimens. The results showed that the 0 strain offsets underestimated (- 25%) the yield stress in bending and tensile tests, while the 0.2% strain offsets overestimated the yield stress (+ 65%) in three-point bending tests. Instead, the yield stress determined by 0.007 and 0.05% strain offset for bending and tensile loading respectively, can effectively characterize the biomechanical responses of the bone, thereby helping to build an accurate FE model.

Integrated correction of optical distortions for global HR-EBSD techniques

Optical distortions caused by camera lenses affect the accuracy of the elastic strains and lattice rotations measured by high-angular resolution techniques. This article introduces an integrated correction of optical distortions for global HR-EBSD/HR-TKD approaches. The digital image correlation analysis is directly applied to optically distorted patterns, avoiding the pattern pre-processing step conducted so far while preserving the numerical efficiency of the Gauss-Newton algorithm. The correction implementation is first described and its numerical cost is assessed considering a homography-based HR-EBSD approach. The correction principle is validated numerically for various levels of first-order radial distortion over a wide range of disorientation angles (0 to 14°) and elastic strain (0 to 5×10^-2). The errors induced when neglecting such distortions as well as the influence of both the radial distortion coefficient and the pattern centre and optical centre locations are quantified. Even when both reference and target patterns are distorted, the correction appears necessary whatever the disorientation between those patterns. The required accuracy on the true distortion parameters for an effective correction is consequently determined.

Evaluation of experimental, analytical, and computational methods to determine long-bone bending stiffness

Methods used to evaluate bone mechanical properties vary widely depending on the motivation and environment of individual researchers, clinicians, and industries. Further, the innate complexity of bone makes validation of each method difficult. Thus, the purpose of the present research was to quantify methodological error of the most common methods used to predict long-bone bending stiffness, more specifically, flexural rigidity (EI). Functional testing of a bi-material porcine bone surrogate, developed in a previous study, was conducted under four-point bending test conditions. The bone surrogate was imaged using computed tomography (CT) with an isotropic voxel resolution of 0.625 mm. Digital image correlation (DIC) of the bone surrogate was used to quantify the methodological error between experimental, analytical, and computational methods used to calculate EI. These methods include the application of Euler Bernoulli beam theory to mechanical testing and DIC data; the product of the bone surrogate composite bending modulus and second area moment of inertia; and finite element analysis (FEA) using computer-aided design (CAD) and CT-based geometric models. The methodological errors of each method were then compared. The results of this study determined that CAD-based FEA was the most accurate determinant of bone EI, with less than five percent difference in EI to that of the DIC and consistent reproducibility of the measured displacements for each load increment. CT-based FEA was most accurate for axial strains. Analytical calculations overestimated EI and mechanical testing was the least accurate, grossly underestimating flexural rigidity of long-bones.

Novel expandable architected breathing tube for improving airway securement in emergency care

Life-saving interventions utilize endotracheal intubation to secure a patient's airway, but performance of the clinical standard of care endotracheal tube (ETT) is inadequate. For instance, in the current COVID-19 crisis, patients can expect prolonged intubation. This protracted intubation may produce health complications such as tracheal stenosis, pneumonia, and necrosis of tracheal tissue, as current ETTs are not designed for extended use. In this work, we propose an improved ETT design that seeks to overcome these limitations by utilizing unique geometries which enable a novel expanding cylinder. The mechanism provides a better distribution of the contact forces between the ETT and the trachea, which should enhance patient tolerability. Results show that at full expansion, our new ETT exerts pressures in a silicone tracheal phantom well within the recommended standard of care. Also, preliminary manikin tests demonstrated that the new ETT can deliver similar performance in terms of air pressure and air volume when compared with the current gold standard ETT. The potential benefits of this new architected ETT are threefold, by limiting exposure of healthcare providers to patient pathogens through streamlining the intubation process, reducing downstream complications, and eliminating the need of multiple size ETT as one architected ETT fits all.

Effect of age on the failure properties of human meniscus: High-speed strain mapping of tissue tears

The knee meniscus is a soft fibrous tissue with a high incidence of injury in older populations. The objective of this study was to determine the effect of age on the failure behavior of human knee meniscus when applying uniaxial tensile loads parallel or perpendicular to the primary circumferential fiber orientation. Two age groups were tested: under 40 and over 65 years old. We paired high-speed video with digital image correlation to quantify for the first time the planar strains occurring in the tear region at precise time points, including at ultimate tensile stress, when the tissue begins losing load-bearing capacity. On average, older meniscus specimens loaded parallel to the fiber axis had approximately one-third less ultimate tensile strain and absorbed 60% less energy to failure within the tear region than younger specimens (p < 0.05). Older specimens also had significantly reduced strength and material toughness when loaded perpendicular to the fibers (p < 0.05). These age-related changes indicate a loss of collagen fiber extensibility and weakening of the non-fibrous matrix with age. In addition, we found that when loaded perpendicular to the circumferential fibers, tears propagated near the planes of maximum tensile stress and strain. Whereas when loaded parallel to the circumferential fibers, tears propagated oblique to the loading axis, closer to the planes of maximum shear stress and strain. Our experimental results can assist the selection of valid failure criteria for meniscus, and provide insight into the effect of age on the failure mechanisms of soft fibrous tissue.