Influence of test paradigm on loading dynamics during proximal femur fracture tests simulating sideways falls

Fall-related hip fractures are a serious public health issue in older adults. As most mechanistic hip fracture risk prediction models incorporate tissue tolerance, test methods that can accurately characterize the fracture force of the femur (and factors that influence it) are imperative. While bone possesses viscoelastic properties, experimental characterization of rate-dependencies has been inconsistent in the whole-femur literature. The goal of this study was to investigate the influence of experimental paradigm on loading rate and fracture force (both means and variability) during mechanical tests simulating lateral fall loadings on the proximal femur. Six pairs of matched femurs were split randomly between two test paradigms: a 'lower rate' materials testing system (MTS) with a constant displacement rate of 60 mm/s, and a hip impact test system (HIT) comprised of a custom-built vertical drop tower utilizing an impact velocity of 4 m/s. The loading rate was 88-fold higher for the HIT (mean (SD) = 2465.49 (807.38) kN/s) compared to the MTS (27.78 (10.03) kN/s) paradigm. However, no difference in fracture force was observed between test paradigms (mean (SD) = 4096.4 (1272.6) N for HIT, and 3641.3 (1285.8) N for MTS). Within-paradigm variability was not significantly different across paradigms for either loading rate or fracture force (coefficients of variation ranging from 0.311 to 0.361). Within each test paradigm, significant positive relationships were observed between loading rate and fracture force (HIT adjusted R2 = 0.833, p = 0.007; MTS adjusted R2 = 0.983, p < 0.0001). Overall, this study provides evidence that energy-based impact simulators can be a valid method to measure femoral bone strength in the context of fall-related hip fractures. This study motivates future research to characterize potential non-linear relationships between loading rate and fracture threshold at both macro and microscales.

Biomechanical properties of artificial bones made by Sawbones: A review

Biomedical engineers and physicists frequently use human or animal bone for orthopaedic biomechanics research because they are excellent approximations of living bone. But, there are drawbacks to biological bone, like degradation over time, ethical concerns, high financial costs, inter-specimen variability, storage requirements, supplier sourcing, transportation rules, etc. Consequently, since the late 1980s, the Sawbones® company has been one of the world's largest suppliers of artificial bones for biomechanical testing that counteract many disadvantages of biological bone. There have been many published reports using these bone analogs for research on joint replacement, bone fracture fixation, spine surgery, etc. But, there exists no prior review paper on these artificial bones that gives a comprehensive and in-depth look at the numerical data of interest to biomedical engineers and physicists. Thus, this paper critically reviews 25 years of English-language studies on the biomechanical properties of these artificial bones that (a) characterized unknown or unreported values, (b) validated them against biological bone, and/or (c) optimized different design parameters. This survey of data, advantages, disadvantages, and knowledge gaps will hopefully be useful to biomedical engineers and physicists in developing mechanical testing protocols and computational finite element models.

Simulating Prophylactic Fixation Methods for Osteoporotic Femoral Neck Fracture Prevention

INTRODUCTION

Geriatric patients who suffer femoral neck fractures have high morbidity and mortality. Prophylactic fixation of the femoral neck is a potential avenue to reduce the incidence of femoral neck fractures. We studied 3 different implants traditionally used to stabilize the femoral neck: 6.5 mm cannulated screws (CANN), the femoral neck system (FNS) (Depuy Synthes), and the dynamic hip screw (DHS) (Depuy Synthes).

MATERIALS AND METHODS

Five osteoporotic Sawbone femurs were used for each model and a control group. Two scenarios were investigated: single leg stance to measure construct stiffness and lateral impact to measure construct stiffness, energy to fracture, and qualitative examination of fracture patterns. Stiffness for each femur and energy to fracture for the lateral impact scenario were calculated and compared between groups using one-way ANOVA.

RESULTS

DHS showed significantly higher stiffness than the other 2 implants and the control in single leg stance. In the lateral impact scenario, the DHS and CANN were significantly stiffer FNS and the control. Femurs implanted with CANN tended to fracture at the greater trochanter while FNS fractured in a transverse subtrochanteric pattern, and DHS fractured obliquely in the subtrochanteric region.

DISCUSSION

FNS and DHS experienced fracture patterns less amenable to surgical correction. CANN and DHS proved better able to resist external forces in the lateral fall scenario. CANN also proved better able to resist external forces in the single leg stance scenario and experienced a more amenable fracture pattern in the lateral fall scenario.

CONCLUSIONS

FNS was less able to resist external forces compared with the other implants. This work informs the potential implications between the choice of implants that, although historically have not been used prophylactically, may be considered in the future for prophylactic stabilization of the femoral neck. Cadaveric study and clinical trials are recommended for further study.

Improved virtual extensometer measurement method in complex multi-fracture situation

To overcome the limitation of the virtual extensometer method in measuring the crack opening displacement (COD) in the process of complex multi-crack propagation of rock, the measurement error of Digital Image Correlation (DIC) local deformation is theoretically analyzed. An improved virtual extensometer method for measuring the COD is proposed, which considers the temporal and spatial characteristics of crack development in the process of complex crack propagation. The accuracy of the proposed method is verified by the strain localization band numerical simulation test and indoor single crack simulation test. Furthermore, the method is applied to the two-dimensional similarity simulation test of simulating complex multi-fractures in rock stratum. The COD obtained by the traditional and improved methods is compared with the measured COD. The results show that in the case of multiple complex cracks, to obtain the COD accurately, the relative distance between the virtual extensometer measuring point and the crack should be greater than half of the sum of the width of the crack strain localization zone and the subset size. With the development of the crack, the relative distance between the virtual extensometer measuring point and the crack should increase with the increase of the width of the crack strain localization zone. The error of the COD measured by the traditional method increases with fracture development, and the maximum is 21.20%. The maximum relative error between the COD measured by the improved method and the measured crack opening is 3.61%. The research results improve the accuracy of the virtual extensometer in measuring the COD under complex multi-crack conditions.

A novel specimen shape for measurement of linear strain fields by means of digital image correlation

Strains on the surface of engineering structures or biological tissues are non-homogeneous. These strain fields can be captured by means of Digital Image Correlation (DIC). However, DIC strain field measurements are prone to noise and filtering of these fields influences measured strain gradients. This study aims to design a novel tensile test specimen showing two linear gradients, to measure full-field linear strain measurements on the surface of test specimens, and to investigate the accuracy of DIC strain measurements globally (full-field) and locally (strain gauges' positions), with and without filtering of the DIC strain fields. Three materials were employed for this study: aluminium, polymer, and bovine bone. Normalized strain gradients were introduced that are load independent and evaluated at two local positions showing 3.6 and 6.9% strain change per mm. Such levels are typically found in human bones. At these two positions, two strain gauges were applied to check the experimental strain magnitudes. A third strain gauge was applied to measure the strain in a neutral position showing no gradient. The accuracy of the DIC field measurement was evaluated at two deformation stages (at [Formula: see text] 500 and 1750 μstrain) using the root mean square error (RMSE). The RMSE over the two linear strain fields was less than 500 μstrain for both deformation stages and all materials. Gaussian low-pass filter (LPF) reduced the DIC noise between 25% and 64% on average. As well, filtering improved the accuracy of the local normalized strain gradients measurements with relative difference less than 20% and 12% for the high- and low-gradient, respectively. In summary, a novel specimen shape and methodological approach are presented which are useful for evaluating and improving the accuracy of the DIC measurement where non-homogeneous strain fields are expected such as on bone tissue due to their hierarchical structure.

A systematic approach to the scale separation problem in the development of multiscale models

Throughout engineering there are problems where it is required to predict a quantity based on the measurement of another, but where the two quantities possess characteristic variations over vastly different ranges of time and space. Among the many challenges posed by such 'multiscale' problems, that of defining a 'scale' remains poorly addressed. This fundamental problem has led to much confusion in the field of biomedical engineering in particular. The present study proposes a definition of scale based on measurement limitations of existing instruments, available computational power, and on the ranges of time and space over which quantities of interest vary characteristically. The definition is used to construct a multiscale modelling methodology from start to finish, beginning with a description of the system (portion of reality of interest) and ending with an algorithmic orchestration of mathematical models at different scales within the system. The methodology is illustrated for a specific but well-researched problem. The concept of scale and the multiscale modelling approach introduced are shown to be easily adaptable to other closely related problems. Although out of the scope of this paper, we believe that the proposed methodology can be applied widely throughout engineering.

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.

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.

Numerical analysis of hip fracture due to a sideways fall

The primary purpose of this paper is to outline a methodology for evaluating the likelihood of cortical bone fracture in the proximal femur in the event of a sideways fall. The approach includes conducting finite element (FE) analysis in which the cortical bone is treated as an anisotropic material, and the admissibility of the stress field is validated both in tension and compression regime. In assessing the onset of fracture, two methodologies are used, namely the Critical Plane approach and the Microstructure Tensor approach. The former is employed in the tension regime, while the latter governs the conditions at failure in compression. The propagation of localized damage is modeled using a constitutive law with embedded discontinuity (CLED). In this approach, the localized deformation is described by a homogenization procedure in which the average properties of cortical tissue intercepted by a macrocrack are established. The key material properties governing the conditions at failure are specified from a series of independent material tests conducted on cortical bone samples tested at different orientations relative to the loading direction. The numerical analysis deals with simulations of experiments involving the sideways fall, and the results are compared with the experimental data. This includes both the evolution of fracture pattern and the local load-displacement characteristics. The proposed approach is numerically efficient, and the results do not display a pathological mesh-dependency. Also, in contrast to the XFEM approach, the analysis does not require any extra degrees of freedom.

Periprosthetic femoral fractures in sideways fall configuration: comparative numerical analysis of the influence of femoral stem design

INTRODUCTION

The aim of this study was to investigate the mechanisms of periprosthetic fractures occurring as a result of a sideways fall in total hip arthroplasty patients, and to compare the predictions of numerical models in terms of load distribution on the implanted femur with clinical data.

MATERIALS AND METHODS

3 numerical models were built: 1 for intact femur and 2 for implanted femur with a straight stem (resembling PBF, Permedica) and with an anatomical stem (resembling ABG II, Stryker). 4 loading configurations were simulated; 1 simulates a vertical load, and 3 simulate a fall with impact on the greater trochanter in different directions. Stress state calculated in the implanted femur was compared for the 2 models with reference to the intact case. These were compared with clinical data collected at a single centre (Istituto Ortopedico Gaetano Pini, Milan, Italy) where 41 patients were investigated after periprosthetic fracture: 26 patients had a straight uncemented stem and 15 an anatomical uncemented stem.

RESULTS

The maximum calculated strain in compression in the case of ABG II implanted femur was 2 times higher than in the presence of PBF stem in the vertical loading configuration. For configurations of sideways fall, in both models, there was a progressive increase of stress state in the bone with increasing angle. Simulations of sideways fall elicited results in accordance with clinical observations: due to the peculiar stem design and consequent state of stress in the bone, anatomical stems seem to induce trochanteric fractures more frequently, while for straight stems type B fractures are more likely to occur.

CONCLUSIONS

Clinical findings confirmed numerical model predictions: stem design seems to highly influence distribution of stress in the bone and consequent localisation of the fracture site.

Leg Joint Stiffness Affects Dynamics of Backward Falling From Standing Height: A Simulation Work

Falling backward can lead to injuries including hip fracture, back injury, and traumatic brain impact among older adults. A loss of consciousness is associated with falling backward and accounts for about 13% of all falls among older adults. Little is known about the dynamics of backward falls, such as the falling duration, the impact severity, and how the fall dynamics are affected by the biomechanical properties of the lower limb joints, particularly the rotational stiffness. The purpose of this study was to investigate the influence of the stiffness of individual leg joints on the dynamics of backward falls after losing consciousness in terms of the falling duration and impact velocities. Based on a 15-segment human model, we simulated the process of falling backward by sweeping the parameter space of ankle, knee, and hip's stiffnesses varying from 0 to 8.73 N·m·deg-1 (or 500 N·m·rad-1). The results revealed that the falling duration and impact speeds of the head and hip ranged from 0.27 to 0.63 s, 2.65 to 7.88 m·s-1, and 0.35 to 3.36 m·s-1, respectively, when the stiffness of the leg joints changed within their limits. Overall, the influence of the joint stiffness on the falling dynamics (falling duration and impact speed) is comparable between hip and knee joints, whereas ankle stiffness showed little influence on the backward falling dynamics. Our findings could provide references for designing protective devices to prevent impact-induced injuries after a backward fall.

Comparison of femur strain under different loading scenarios: Experimental testing

Bone fracture, formation and adaptation are related to mechanical strains in bone. Assessing bone stiffness and strain distribution under different loading conditions may help predict diseases and improve surgical results by determining the best conditions for long-term functioning of bone-implant systems. In this study, an experimentally wide range of loading conditions (56) was used to cover the directional range spanned by the hip joint force. Loads for different stance configurations were applied to composite femurs and assessed in a material testing machine. The experimental analysis provides a better understanding of the influence of the bone inclination angle in the frontal and sagittal planes on strain distribution and stiffness. The results show that the surface strain magnitude and stiffness vary significantly under different loading conditions. For the axial compression, maximal bending is observed at the mid-shaft, and bone stiffness is also maximal. The increased inclination leads to decreased stiffness and increased magnitude of maximum strain at the distal end of the femur. For comparative analysis of results, a three-dimensional, finite element model of the femur was used. To validate the finite element model, strain gauges and digital image correlation system were employed. During validation of the model, regression analysis indicated robust agreement between the measured and predicted strains, with high correlation coefficient and low root-mean-square error of the estimate. The results of stiffnesses obtained from multi-loading conditions experiments were qualitatively compared with results obtained from a finite element analysis of the validated model of femur with the same multi-loading conditions. When the obtained numerical results are qualitatively compared with experimental ones, similarities can be noted. The developed finite element model of femur may be used as a promising tool to estimate proximal femur strength and identify the best conditions for long-term functioning of the bone-implant system in future study.

A novel sideways fall simulator to study hip fractures ex vivo

Falls to the side are the leading cause of hip fractures in the elderly. The load that a person experiences during a fall cannot be measured with volunteers for ethical reasons. To evaluate injurious loads, while considering relevant energy input and body posture for a sideways fall, a subject-specific cadaveric impact experiment was developed. Full cadaveric femur-pelvis constructs (N = 2) were embedded in surrogate soft tissue material and attached to metallic surrogate lower limbs. The specimens were then subjected to an inverted pendulum motion, simulating a fall to the side with an impact to the greater trochanter. The load at the ground and the deformation of the pelvis were evaluated using a 6-axis force transducer and two high-speed cameras. Post-test, a trauma surgeon (PG) evaluated specimen injuries. Peak ground contact forces were 7132 N and 5641 N for the fractured and non-fractured specimen, respectively. We observed a cervical fracture of the femur in one specimen and no injuries in a second specimen, showing that the developed protocol can be used to differentiate between specimens at high and low fracture risk.

Damage Identification on Vertebral Bodies During Compressive Loading Using Digital Image Correlation

MINI: Identifying fracture is important for understanding vertebral mechanics. Isolated cadaveric thoracolumbar vertebrae were compressed, and surface strains were measured using digital image correlation. Fracture locations from video analysis were qualitatively similar to the locations of high compressive strains and local damage occurred before the maximum force was reached.

STUDY DESIGN

Ex vivo compression experiments on isolated cadaveric vertebrae.

OBJECTIVE

To qualitatively compare the fracture locations identified in video analysis with the locations of high compressive strain measured with digital image correlation (DIC) on vertebral bodies and to evaluate the timing of local damage to the cortical shell relative to the global yield force.

SUMMARY OF BACKGROUND DATA

In previous ex vivo experiments, cortical bone fracture has been identified using various methods including acoustic emission sensors, strain gages, video analysis, or force signals. These methods are, however, limited in their ability to detect the location and timing of fracture. We propose use of DIC, a noncontact optical technique that measures surface displacement, to quantify variables related to damage.

METHODS

Isolated thoracolumbar human cadaveric vertebral bodies (n = 6) were tested in compression to failure at a quasi-static rate, and the force applied was measured using a load cell. The surface displacement and strain were measured using DIC. Video analysis was performed to identify fractures.

RESULTS

The location of fractures identified in the video corresponded well with the locations of high compressive strain on the bone. Before reaching the global yield force, more than 10% of the DIC measurements reached a minimum principal strain of 1.0%, a previously reported threshold for cortical bone damage.

CONCLUSION

DIC measurements provide an objective measure that can be used to identify the location and timing of fractures during ex vivo vertebral experiments. This is important for understanding fracture mechanics and for validating vertebral computational models that incorporate failure.

LEVEL OF EVIDENCE

N /A.

Development and in vitro validation of a simplified numerical model for the design of a biomimetic femoral stem

BACKGROUND

Dense and stiff metallic femoral stems implanted into femurs for total hip arthroplasties produce a stress shielding effect since they modify the original load sharing path in the bony structure. Consequently, in the long term, the strain adaptive nature of bones leads to bone resorption, implant loosening, and the need for arthroplasty revision. The design of new cementless femoral stems integrating open porous structures can reduce the global stiffness of the stems, allowing them a better match with that of bones and provide their firm fixation via bone ingrowth, and, thus reduce the risk of implantation failure.

METHODS

This paper aims to develop and validate a simplified numerical model of stress shielding, which calculates the levels of bone resorption or formation by comparing strain distributions on the surface of the intact and the implanted femurs subjected to a simulated biological loading. Two femoral stems produced by laser powder-bed fusion using Ti-6Al-4V alloy are employed: the first is fully dense, while the second features a diamond cubic lattice structure in its core. The validation consists of a comparison of the numerically calculated force-displacement diagrams, and displacement and strain fields with their experimental equivalents obtained using the digital image correlation technique.

RESULTS AND CONCLUSIONS

The numerical models showed reasonable agreement between the force-displacement diagrams. Also, satisfactory results for the correlation analyses of the total displacement and equivalent strain fields were obtained. The stress shielding effect of the implant was assessed by comparing the equivalent strain fields of the implanted and intact femurs. The results obtained predicted less bone resorption in the femur implanted with the porous stem than with its dense counterpart.

Prophylactic augmentation of the osteoporotic proximal femur-mission impossible?

The high incidence of secondary hip fractures and the associated markedly increased mortality call for preventive actions that could help to avoid these injuries. By providing immediate strengthening and not relying on patient compliance, internal prophylactic augmentation of the osteoporotic proximal femur may overcome the main limitations of systemic bone drugs and wearable protective pads. However, such a method would have to provide sufficient and reliable strengthening effect with minimal risks and side effects to justify the need of an invasive treatment. The requirements for an internal reinforcement approach are thus strict and include mechanical, biological, clinical, ethical and financial criteria. Here we first attempt to describe the properties of an ideal augmentation method. Previously published methodologies and techniques developed at our research institute, including approaches using cements, metals, other materials or combined approaches, are then reviewed and evaluated according to these aspects. We conclude that none of the discussed methodologies appears to be able to deliver a sufficiently high gain-versus-risk ratio that could justify the clinical application and thus augmentation of the osteoporotic proximal femur remains a challenge. Finally, we provide suggestions for the development and evaluation of future strategies.

Contribution of fluid in bone extravascular matrix to strain-rate dependent stiffening of bone tissue - A poroelastic study

Osteoporotic fractures represent an increasing cost to society, and its diagnosis methods based on bone density still lack accuracy in identifying risk of fracture. This is why a better understanding of mechanical behavior of bone tissue is of importance, especially when it comes to relating experimental observations to realistic physiological fall loading conditions. This study aims at exploring the stiffening effect of pore fluid in bone extravascular matrix subject to high strain rate loading that is more realistic to simulate a physiological fall. A computational approach is used, where bone tissue microstructure extracted from micro-CT images is modeled using finite elements. The solid phase of bone tissue is modeled as a poroelastic material, a porous matrix filled with fluid. When the extravascular matrix experiences certain volumetric deformation, the fluid in pores presents load carrying capacity, which consequently varies the apparent stiffness of bone tissue. It is shown that effects of fluid stiffening in bone can be significant, depending on the chosen material properties, the amount of volumetric strain in tissue and the loading rate with respect to hydraulic conductivity and drainage conditions. It is also shown that such stiffening effect is influenced by bone microstructure, and is more significant in cortical bone than in trabecular bone.

Comparison of Strain Rosettes and Digital Image Correlation for Measuring Vertebral Body Strain

Strain gages are commonly used to measure bone strain, but only provide strain at a single location. Digital image correlation (DIC) is an optical technique that provides the displacement, and therefore strain, over an entire region of interest on the bone surface. This study compares vertebral body strains measured using strain gages and DIC. The anterior surfaces of 15 cadaveric porcine vertebrae were prepared with a strain rosette and a speckled paint pattern for DIC. The vertebrae were loaded in compression with a materials testing machine, and two high-resolution cameras were used to image the anterior surface of the bones. The mean noise levels for the strain rosette and DIC were 1 με and 24 με, respectively. Bland–Altman analysis was used to compare strain from the DIC and rosette (excluding 44% of trials with some evidence of strain rosette failure or debonding); the mean difference ± 2 standard deviations (SDs) was −108 με ± 702 με for the minimum (compressive) principal strain and −53 με ± 332 με for the maximum (tensile) principal strain. Although the DIC has higher noise, it avoids the relatively high risk we observed of strain gage debonding. These results can be used to develop guidelines for selecting a method to measure strain on bone.

Full-field in vitro measurements and in silico predictions of strain shielding in the implanted femur after total hip arthroplasty

Alterations in bone strain as a result of implantation may contribute towards periprosthetic bone density changes after total hip arthroplasty. Computational models provide full-field strain predictions in implant–bone constructs; however, these predictions should be verified using experimental models wherever it is possible. In this work, finite element predictions of surface strains in intact and implanted composite femurs were verified using digital image correlation. Relationships were sought between post-implantation strain states across seven defined Gruen zones and clinically observed longer-term bone density changes. Computational predictions of strain distributions in intact and implanted femurs were compared to digital image correlation measurements in two regions of interest. Regression analyses indicated a strong linear correlation between measurements and predictions (R = 0.927 intact, 0.926 implanted) with low standard error (standard error = 38 µε intact, 26 µε implanted). Pre- to post-operative changes in measured and predicted surface strains were found to relate qualitatively to clinically observed volumetric bone density changes across seven Gruen zones: marked proximal bone density loss corresponded with a 50%−64% drop in surface strain, and slight distal density changes corresponded with 4%−14% strain increase. These results support the use of digital image correlation as a pre-clinical tool for predicting post-implantation strain shielding, indicative of long-term bone adaptations.

Strain distribution in the proximal Human femur during in vitro simulated sideways fall

This study assessed: (i) how the magnitude and direction of principal strains vary for different sideways fall loading directions; (ii) how the principal strains for a sideways fall differ from physiological loading directions; (iii) the fracture mechanism during a sideways fall. Eleven human femurs were instrumented with 16 triaxial strain gauges each. The femurs were non-destructively subjected to: (a) six loading configurations covering the range of physiological loading directions; (b) 12 configurations simulating sideways falls. The femurs were eventually fractured in a sideways fall configuration while high-speed cameras recorded the event. When the same force magnitude was applied, strains were significantly larger in a sideways fall than for physiological loading directions (principal compressive strain was 70% larger in a sideways fall). Also the compressive-to-tensile strain ratio was different: for physiological loading the largest compressive strain was only 30% larger than the largest tensile strain; but for the sideways fall, compressive strains were twice as large as the tensile strains. Principal strains during a sideways fall were nearly perpendicular to the direction of principal strains for physiological loading. In the most critical regions (medial part of the head-neck) the direction of principal strain varied by less than 9° between the different physiological loading conditions, whereas it varied by up to 17° between the sideways fall loading conditions. This was associated with a specific fracture mechanism during sideways fall, where failure initiated on the superior-lateral side (compression) followed by later failure of the medially (tension), often exhibiting a two-peak force-displacement curve.

Full-Field Strain Measurement During Mechanical Testing of the Human Femur at Physiologically Relevant Strain Rates

Understanding the mechanical properties of human femora is of great importance for the development of a reliable fracture criterion aimed at assessing fracture risk. Earlier ex vivo studies have been conducted by measuring strains on a limited set of locations using strain gauges (SGs). Digital image correlation (DIC) could instead be used to reconstruct the full-field strain pattern over the surface of the femur. The objective of this study was to measure the full-field strain response of cadaver femora tested at a physiological strain rate up to fracture in a configuration resembling single stance. The three cadaver femora were cleaned from soft tissues, and a white background paint was applied with a random black speckle pattern over the anterior surface. The mechanical tests were conducted up to fracture at a constant displacement rate of 15 mm/s, and two cameras recorded the event at 3000 frames per second. DIC was performed to retrieve the full-field displacement map, from which strains were derived. A low-pass filter was applied over the measured displacements before the crack opened in order to reduce the noise level. The noise levels were assessed using a dedicated control plate. Conversely, no filtering was applied at the frames close to fracture to get the maximum resolution. The specimens showed a linear behavior of the principal strains with respect to the applied force up to fracture. The strain rate was comparable to the values available in literature from in vivo measurements during daily activities. The cracks opened and fully propagated in less than 1 ms, and small regions with high values of the major principal strains could be spotted just a few frames before the crack opened. This corroborates the hypothesis of a strain-driven fracture mechanism in human bone. The data represent a comprehensive collection of full-field strains, both at physiological load levels and up to fracture. About 10,000 points were tracked on each bone, providing superior spatial resolution compared to ∼15 measurements typically collected using SGs. These experimental data collection can be further used for validation of numerical models, and for experimental verification of bone constitutive laws and fracture criteria.