Response of Thoraco-Abdominal Tissue in High-Rate Compression.

Body armor is used to protect the human from penetrating injuries, however, in the process of defeating a projectile, the back face of the armor can deform into the wearer at extremely high rates. This deformation can cause a variety of soft and hard tissue injuries. Finite element modeling (FEM) represents one of the best tools to predict injuries from this high-rate compression mechanism. However, the validity of a model is reliant on accurate material properties for biological tissues. In this study, we measured the stress-strain response of thoraco-abdominal tissue during high-rate compression (1000 and 1900 s-1) using a split Hopkinson pressure bar (SHPB). High-rate material properties of porcine adipose, heart, spleen, and stomach tissue were characterized. At a strain rate of 1000 s-1, adipose (E = 4.7 MPa) had the most compliant stress-strain response, followed by spleen (E = 9.6 MPa), and then heart tissue (E = 13.6 MPa). At a strain rate of 1900 s-1, adipose (E = 7.3 MPa) had the most compliant stress-strain response, followed by spleen (E = 10.7 MPa), heart (E = 14.1 MPa), and stomach (E = 32.6 MPa) tissue. Only adipose tissue demonstrated a consistent rate dependence for these high strain rates, with a stiffer response at 1900 s-1 compared to 1000 s-1. However, comparison of all these tissues to previously published quasi-static and intermediate dynamic experiments revealed a strong rate dependence with increasing stress response from quasi-static to dynamic to high strain rates. Together, these findings can be used to develop a more accurate finite element model of high-rate compression injuries.

Super-resolution of clinical CT: Revealing microarchitecture in whole bone clinical CT image data.

Osteoporotic fractures, prevalent in the elderly, pose a significant health and economic burden. Current methods for predicting fracture risk, primarily relying on bone mineral density, provide only modest accuracy. If better spatial resolution of trabecular bone in a clinical scan were available, a more complete assessment of fracture risk would be obtained using microarchitectural measures of bone (i.e. trabecular thickness, trabecular spacing, bone volume fraction, etc.). However, increased resolution comes at the cost of increased radiation or can only be applied at small volumes of distal skeletal locations. This study explores super-resolution (SR) technology to enhance clinical CT scans of proximal femurs and better reveal the trabecular microarchitecture of bone. Using a deep-learning-based (i.e. subset of artificial intelligence) SR approach, low-resolution clinical CT images were upscaled to higher resolution and compared to corresponding MicroCT-derived images. SR-derived 2-dimensional microarchitectural measurements, such as degree of anisotropy, bone volume fraction, trabecular spacing, and trabecular thickness were within 16 % error compared to MicroCT data, whereas connectivity density exhibited larger error (as high as 1094 %). SR-derived 3-dimensional microarchitectural metrics exhibited errors <18 %. This work showcases the potential of SR technology to enhance clinical bone imaging and holds promise for improving fracture risk assessments and osteoporosis detection. Further research, including larger datasets and refined techniques, can advance SR's clinical utility, enabling comprehensive microstructural assessment across whole bones, thereby improving fracture risk predictions and patient-specific treatment strategies.

Probabilistic Finite Element Analysis of Human Rib Biomechanics: A Framework for Improved Generalizability.

In dynamic impact events, thoracic injuries often involve rib fractures, which are closely related to injury severity. Previous studies have investigated the behavior of isolated ribs under impact loading conditions, but often neglected the variability in anatomical shape and tissue material properties. In this study, we used probabilistic finite element analysis and statistical shape modeling to investigate the effect of population-wide variability in rib cortical bone tissue mechanical properties and rib shape on the biomechanical response of the rib to impact loading. Using the probabilistic finite element analysis results, a response surface model was generated to rapidly investigate the biomechanical response of an isolated rib under dynamic anterior-posterior load given the variability in rib morphometry and tissue material properties. The response surface was used to generate pre-fracture force-displacement computational corridors for the overall population and a population sub-group of older mid-sized males. When compared to the experimental data, the computational mean response had a RMSE of 4.28N (peak force 94N) and 6.11N (peak force 116N) for the overall population and sub-group respectively, whereas the normalized area metric when comparing the experimental and computational corridors ranged from 3.32% to 22.65% for the population and 10.90% to 32.81% for the sub-group. Furthermore, probabilistic sensitivities were computed in which the contribution of uncertainty and variability of the parameters of interest was quantified. The study found that rib cortical bone elastic modulus, rib morphometry and cortical thickness are the random variables that produce the largest variability in the predicted force-displacement response. The proposed framework offers a novel approach for accounting biological variability in a representative population and has the potential to improve the generalizability of findings in biomechanical studies.

The I-PREDICT 50th Percentile Male Warfighter Finite Element Model: Development and Validation of the Thoracolumbar Spine.

Military personnel are commonly at risk of lower back pain and thoracolumbar spine injury. Human volunteers and postmortem human subjects have been used to understand the scenarios where injury can occur and the tolerance of the warfighter to these loading regimes. Finite element human body models (HBMs) can accurately simulate the mechanics of the human body and are a useful tool for understanding injury. In this study, a HBM thoracolumbar spine was developed and hierarchically ï»¿validated as part of the Incapacitation Prediction for Readiness in Expeditionary Domains: an Integrated Computational Tool (I-PREDICT) program. Constitutive material models were sourced from literature and the vertebrae and intervertebral discs were hexahedrally meshed from a 50th percentile male CAD dataset. Ligaments were modeled through attaching beam elements at the appropriate anatomical insertion sites. 94 simulations were replicated from experimental PMHS tests at the vertebral body, functional spinal unit (FSU), and regional lumbar spine levels. The BioRank (BRS) biofidelity ranking system was used to assess the response of the I-PREDICT model. At the vertebral body level, the I-PREDICT model showed good agreement with experimental results. The I-PREDICT FSUs showed good agreement in tension and compression and had comparable stiffness values in flexion, extension, and axial rotation. The regional lumbar spine exhibited "good" biofidelity when tested in tension, compression, extension, flexion, posterior shear, and anterior shear (BRS regional average = 1.05). The validated thoracolumbar spine of the I-PREDICT model can be used to better understand and mitigate injury risk to the warfighter.

Investigating the Impact of Blunt Force Trauma: A Probabilistic Study of Behind Armor Blunt Trauma Risk.

Evaluating Behind Armor Blunt Trauma (BABT) is a critical step in preventing non-penetrating injuries in military personnel, which can result from the transfer of kinetic energy from projectiles impacting body armor. While the current NIJ Standard-0101.06 standard focuses on preventing excessive armor backface deformation, this standard does not account for the variability in impact location, thorax organ and tissue material properties, and injury thresholds in order to assess potential injury. To address this gap, Finite Element (FE) human body models (HBMs) have been employed to investigate variability in BABT impact conditions by recreating specific cases from survivor databases and generating injury risk curves. However, these deterministic analyses predominantly use models representing the 50th percentile male and do not investigate the uncertainty and variability inherent within the system, thus limiting the generalizability of investigating injury risk over a diverse military population. The DoD-funded I-PREDICT Future Naval Capability (FNC) introduces a probabilistic HBM, which considers uncertainty and variability in tissue material and failure properties, anthropometry, and external loading conditions. This study utilizes the I-PREDICT HBM for BABT simulations for three thoracic impact locations-liver, heart, and lower abdomen. A probabilistic analysis of tissue-level strains resulting from a BABT event is used to determine the probability of achieving a Military Combat Incapacitation Scale (MCIS) for organ-level injuries and the New Injury Severity Score (NISS) is employed for whole-body injury risk evaluations. Organ-level MCIS metrics show that impact at the heart can cause severe injuries to the heart and spleen, whereas impact to the liver can cause rib fractures and major lacerations in the liver. Impact at the lower abdomen can cause lacerations in the spleen. Simulation results indicate that, under current protection standards, the whole-body risk of injury varies between 6 and 98% based on impact location, with the impact at the heart being the most severe, followed by impact at the liver and the lower abdomen. These results suggest that the current body armor protection standards might result in severe injuries in specific locations, but no injuries in others.

Computational anatomy in the study of bone structure.

Osteoporosis is a major public health threat for millions of Americans with billions of dollars per year of national direct costs for osteoporotic fractures. Osteoporosis results in a decrease in overall bone mass and subsequent increase in the risk of bone fracture. Bone strength arises from the combination of bone size and shape, the distribution of bone mass throughout the structure, and the quality of the bone material. Advances in medical imaging have enabled a comprehensive assessment of bone structure through the analysis of high-resolution scans of relevant anatomical sites, eg, the proximal femur. However, conventional imaging analysis techniques use predefined regions of interest that do not take full advantage of such scans. Recently, computational anatomy, a set of imaging-based analysis algorithms, has emerged as a promising technique in studies of osteoporosis. Computational anatomy enables analyses that are not biased to one particular region and provide a more complete assessment of the whole structure. In this article, we review studies that have used computational anatomy to investigate the structure of the proximal femur in relation to age, fracture, osteoporotic treatment, and spaceflight effects.

Dendritic processes of osteocytes are mechanotransducers that induce the opening of hemichannels.

Osteocytes with long dendritic processes are known to sense mechanical loading, which is essential for bone remodeling. There has been a long-standing debate with regard to which part(s) of osteocyte, the cell body versus the dendritic process, acts as a mechanical sensor. To address this question experimentally, we used a transwell filter system that differentiates the cell body from the dendritic processes. Mechanical loading was applied to either the cell body or the dendrites, and the osteocyte's response was observed through connexin 43 hemichannel opening. The hemichannels located on the cell body were induced to open when mechanical loading was applied to either the dendritic processes or the cell body. However, no significant hemichannel activity in the dendrites was detected when either part of the cell was mechanically stimulated. Disruption of the glycocalyx by hyaluronidase on the dendrite side alone is sufficient to diminish a dendrite's ability to induce the opening of hemichannels on the cell body, while hyaluronidase has no such effect when applied to the cell body. Importantly, hyaluronidase treatment to the dendrite side resulted in formation of poor integrin attachments with the reduced ability of the dendrites to form integrin attachments on the underside of the transwell filter. Together, our study suggests that the glycocalyx of the osteocyte dendritic process is required for forming strong integrin attachments. These integrin attachments probably serve as the mechanotransducers that transmit the mechanical signals to the cell body leading to the opening of hemichannels, which permits rapid exchange of factors important for bone remodeling.

A comparative analysis of dimensionality reduction surrogate modeling techniques for full human body finite element impact simulations.

Fast-running surrogate computational models (simpler computational models) have been successfully used to replace time-intensive finite element models. However, it is unclear how well they perform in accurately and efficiently replicating complex, full human body finite element models. Here we survey several surrogate modeling techniques and assess their accuracy in predicting full strain fields of tissues of interest during a highly dynamic behind armor blunt trauma impact to the liver. We found that coupling dimensionality reduction on the high-dimensional output space (principal component analysis or autoencoders) with machine learning techniques (Gaussian Process Regression or multi-output neural networks) provides a framework capable of accurately and efficiently replacing complex full human body models. It was found that these surrogate models can successfully predict the strain fields (<10% average strain error) of select tissues during a nonlinear impact event but careful consideration should be given to element parsing and modeling technique.

Biomechanical characteristics of proficient free-throw shooters-markerless motion capture analysis.

The winning game outcome in basketball is partially contingent on the team's ability to secure and make more free-throw shooting attempts, especially close to the end of the game. Thus, the purpose of the present study was to perform a comprehensive biomechanical analysis of the free-throw shooting motion to examine differences between (a) proficient (≥70%) and non-proficient shooters (<70%) and (b) made and missed free-throw shoots within the proficient group of shooters. Thirty-four recreationally active males with previous basketball playing experience attempted ten consecutive free-throw shots (4.57 m), with a 10-15 s rest interval between each shot. An innovative three-dimensional markerless motion capture system (SwRI Enable, San Antonio, TX, USA) composed of nine high-definition cameras recording at 120 Hz was used to capture and analyze the biomechanical parameters of interest. Independent t-tests and Mann-Whitney U tests were used to examine a presence of statistically significant differences. The findings of the present study reveal that proficient free-throw shooters performed the shooting motion in a more controlled manner by having significantly lower knee and center of mass peak and mean angular velocities. Also, proficient shooters attained a significantly greater release height and had less forward trunk lean when compared to non-proficient shooters at the time point of the ball release. Moreover, despite being beneficial for improvements in shooting accuracy, our findings suggest that overemphasizing the release height may be in certain instances counterproductive, as it may lead to more missed than made free-throw shots within the proficient group of shooters.

Mobility Solutions After a Lower Extremity Fracture and Applicability to Battlefield and Wilderness Medicine.

The potential for delayed evacuation of injured Service members from austere environments highlights the need to develop solutions that can stabilize a wound and enable mobility during these prolonged casualty care (PCC) scenarios. Lower extremity fractures have traditionally been treated by immobilization (splinting) followed by air evacuation - a paradigm not practical in PCC scenarios. In the civilian sector, treatment of extremity injuries sustained during remote recreational activities have similar challenges, particularly when adverse weather or terrain precludes early ground or air rescue. This review examines currently available fracture treatment solutions to include splinting, orthotic devices, and biological interventions and evaluates their feasibility: 1) for prolonged use in austere environments and 2) to enable patient mobilization. This review returned three common types of splints to include: a simple box splint, pneumatic splints, and traction splints. None of these splinting techniques allowed for ambulation. However, fixed facility-based orthotic interventions that include weight-bearing features may be combined with common splinting techniques to improve mobility. Biologically-focused technologies to stabilize a long bone fracture are still in their infancy. Integrating design features across these technologies could generate advanced treatments which would enable mobility, thus maximizing survivability until patient evacuation is feasible.

Biglycan and chondroitin sulfate play pivotal roles in bone toughness via retaining bound water in bone mineral matrix.

Recent in vitro evidence shows that glycosaminoglycans (GAGs) and proteoglycans (PGs) in bone matrix may functionally be involved in the tissue-level toughness of bone. In this study, we showed the effect of biglycan (Bgn), a small leucine-rich proteoglycan enriched in extracellular matrix of bone and the associated GAG subtype, chondroitin sulfate (CS), on the toughness of bone in vivo, using wild-type (WT) and Bgn deficient mice. The amount of total GAGs and CS in the mineralized compartment of Bgn KO mouse bone matrix decreased significantly, associated with the reduction of the toughness of bone, in comparison with those of WT mice. However, such differences between WT and Bgn KO mice diminished once the bound water was removed from bone matrix. In addition, CS was identified as the major subtype in bone matrix. We then supplemented CS to both WT and Bgn KO mice to test whether supplemental GAGs could improve the tissue-level toughness of bone. After intradermal administration of CS, the toughness of WT bone was greatly improved, with the GAGs and bound water amount in the bone matrix increased, while such improvement was not observed in Bgn KO mice or with supplementation of dermatan sulfate (DS). Moreover, CS supplemented WT mice exhibited higher bone mineral density and reduced osteoclastogenesis. Interestingly, Bgn KO bone did not show such differences irrespective of the intradermal administration of CS. In summary, the results of this study suggest that Bgn and CS in bone matrix play a pivotal role in imparting the toughness to bone most likely via retaining bound water in bone matrix. Moreover, supplementation of CS improves the toughness of bone in mouse models.

Hip load capacity cut-points for Astronaut Skeletal Health NASA Finite Element Strength Task Group Recommendations.

Concerns raised at a 2010 Bone Summit held for National Aeronautics and Space Administration Johnson Space Center led experts in finite element (FE) modeling for hip fracture prediction to propose including hip load capacity in the standards for astronaut skeletal health. The current standards for bone are based upon areal bone mineral density (aBMD) measurements by dual X-ray absorptiometry (DXA) and an adaptation of aBMD cut-points for fragility fractures. Task Group members recommended (i) a minimum permissible outcome limit (POL) for post-mission hip bone load capacity, (ii) use of FE hip load capacity to further screen applicants to astronaut corps, (iii) a minimum pre-flight standard for a second long-duration mission, and (iv) a method for assessing which post-mission physical activities might increase an astronaut's risk for fracture after return. QCT-FE models of eight astronaut were analyzed using nonlinear single-limb stance (NLS) and posterolateral fall (NLF) loading configurations. QCT data from the Age Gene/Environment Susceptibility (AGES) Reykjavik cohort and the Rochester Epidemiology Project were analyzed using identical modeling procedures. The 75th percentile of NLS hip load capacity for fractured elderly males of the AGES cohort (9537N) was selected as a post-mission POL. The NLF model, in combination with a Probabilistic Risk Assessment tool, was used to assess the likelihood of exceeding the hip load capacity during post-flight activities. There was no recommendation to replace the current DXA-based standards. However, FE estimation of hip load capacity appeared more meaningful for younger, physically active astronauts and was recommended to supplement aBMD cut-points.

Measurements of mobile and bound water by nuclear magnetic resonance correlate with mechanical properties of bone.

Since clinical measures of bone mineral density do not necessarily predict whether a person will fracture a bone without an intervention, there is a need to find supplementary tools for assessing bone quality. Presently, we hypothesized that measures of mobile and bound water by a Nuclear Magnetic Resonance (NMR) technique are correlated with bone strength and toughness, respectively. To test this, bending specimens from the mid-diaphysis of 18 human femurs were collected from 18 male donors and divided into middle aged and elderly groups. After NMR measurements of each hydrated specimen, an inversion technique was used to convert the free induction decay data into a distribution of spin-spin (T2) relaxation rates. Then, the distribution resolved into three distinct components that likely represent solid hydrogen, water bound to bone tissue, and mobile water that occupy microscopic pores within the bone specimen. The integrated signal intensities of the bound and mobile components were normalized by the wet mass of the specimen. Following NMR measurements, three point bending tests were conducted to determine the modulus of elasticity, flexure strength, and work to fracture of each specimen. Next, the porosity, mineral-to-collagen ratio, and pentosidine concentration were measured. In this sample of human cortical bone, there was no age-related difference in the amount of mobile water, but the decrease in the amount of bound water with increasing age was statistically significant. Moreover, bound water was associated with both strength and work to fracture of bone, while mobile water was correlated with modulus of elasticity and appeared to quantify the level of microscopic pores within bone. On the other hand, bound water was correlated with the concentration of non-enzymatic collagen cross-links. The results of this study indicate that quantifying mobile and bound water with magnetic resonance techniques could potentially serve as indicators of bone quality.

Validity and reliability of an accelerometer-based player tracking device.

This study aimed to determine the intra- and inter-device accuracy and reliability of wearable athletic tracking devices, under controlled laboratory conditions. A total of nineteen portable accelerometers (Catapult OptimEye S5) were mounted to an aluminum bracket, bolted directly to an Unholtz Dickie 20K electrodynamic shaker table, and subjected to a series of oscillations in each of three orthogonal directions (front-back, side to side, and up-down), at four levels of peak acceleration (0.1g, 0.5g, 1.0g, and 3.0g), each repeated five times resulting in a total of 60 tests per unit, for a total of 1140 records. Data from each accelerometer was recorded at a sampling frequency of 100Hz. Peak accelerations recorded by the devices, Catapult PlayerLoad™, and calculated player load (using Catapult's Cartesian formula) were used for the analysis. The devices demonstrated excellent intradevice reliability and mixed interdevice reliability. Differences were found between devices for mean peak accelerations and PlayerLoad™ for each direction and level of acceleration. Interdevice effect sizes ranged from a mean of 0.54 (95% CI: 0.34-0.74) (small) to 1.20 (95% CI: 1.08-1.30) (large) and ICCs ranged from 0.77 (95% CI: 0.62-0.89) (very large) to 1.0 (95% CI: 0.99-1.0) (nearly perfect) depending upon the magnitude and direction of the applied motion. When compared to the player load determined using the Cartesian formula, the Catapult reported PlayerLoad™ was consistently lower by approximately 15%. These results emphasize the need for industry wide standards in reporting validity, reliability and the magnitude of measurement errors. It is recommended that device reliability and accuracy are periodically quantified.

Determination of sex differences of human cadaveric mandibular condyles using statistical shape and trait modeling.

The objective of this study was to elucidate sex differences in the anatomy of human temporomandibular joint mandibular condyles using a statistical shape and trait model (SSTM). Mandibles were obtained from 16 human cadavers (79±13years). The condyles were scanned using micro-computed tomography with 27µm resolution. An image processing algorithm was used to segment the bone, determine the border of the entire mandibular condyle and trabecular bone compartments, and create triangulated meshes of the compartments. One subject was chosen as the template and was registered to the other individuals using a coherence point drift algorithm. This process positioned all vertices at corresponding anatomic locations. For the trabecular bone region, around each vertex position, the average bone image intensity, which is proportional to bone density, and microstructural traits, including trabecular bone volume fraction, thickness, separation, connectivity, and connectivity density were calculated. For the entire mandibular condyle mesh, the surface vertices were extracted to represent the overall anatomy of the condyle. Using a SSTM, the shape and trait information was reduced to a small set of independent and uncorrelated variables for each individual. Wilcoxon rank sum tests were used to test for differences in the variables between sexes. A lasso approach was used to determine a set of variables that differentiate between sexes. Male condyles were on average larger than female condyles, with complex differences in the microstructural traits. Two out of 15 principal components were statistically different between males and females (p<0.1). The lasso approach determined a set of 7 principal components that fully described the complex shape and trait differences between males and females. A SSTM was able to determine sex-dependent differences in the shape of the mandibular condyle. These differences may alter the biomechanics of the joint and contribute to the development of temporomandibular joint disease.

Tissue strain amplification at the osteocyte lacuna: a microstructural finite element analysis.

A parametric finite element model of an osteocyte lacuna was developed to predict the microstructural response of the lacuna to imposed macroscopic strains. The model is composed of an osteocyte lacuna, a region of perilacunar tissue, canaliculi, and the surrounding bone tissue. A total of 45 different simulations were modeled with varying canalicular diameters, perilacunar tissue material moduli, and perilacunar tissue thicknesses. Maximum strain increased with a decrease in perilacunar tissue modulus and decreased with an increase in perilacunar tissue modulus, regardless of the thickness of the perilacunar region. An increase in the predicted maximum strain was observed with an increase in canalicular diameter from 0.362 to 0.421 microm. In response to the macroscopic application of strain, canalicular diameters increased 0.8% to over 1.0% depending on the perilacunar tissue modulus. Strain magnification factors of over 3 were predicted. However, varying the size of the perilacunar tissue region had no effect on the predicted perilacunar tissue strain. These results indicate that the application of average macroscopic strains similar to strain levels measured in vivo can result in significantly greater perilacunar tissue strains and canaliculi deformations. A decrease in the perilacunar tissue modulus amplifies the perilacunar tissue strain and canaliculi deformation while an increase in the local perilacunar tissue modulus attenuates this effect.

Sex dependent mechanical properties of the human mandibular condyle.

The mandibular condyle consists of articular cartilage and subchondral bone that play an important role in bearing loads at the temporomandibular joint (TMJ) during static occlusion and dynamic mastication. The objective of the current study was to examine effects of sex and cartilage on 1) static and dynamic mechanical analysis (DMA) based dynamic energy storage and dissipation for the cartilage-subchondral bone construct of the human mandibular condyle, and 2) their correlations with the tissue mineral density and trabecular morphological parameters of subchondral bone. Cartilage-subchondral bone constructs were obtained from 16 individual human cadavers (9 males, 7 females, 79.00±13.10 years). After scanning with micro-computed tomography, the specimens were subjected to a non-destructive compressive static loading up to 7N and DMA using a cyclic loading profile (-5±2N at 2Hz). After removing the cartilage from the same specimen, the series of loading experiments were repeated. Static stiffness (K) and energy dissipation (W), and dynamic storage (K'), loss (K'') stiffness, and energy dissipation (tan δ) were assessed. Gray values, which are proportional to degree of bone mineralization, and trabecular morphological parameters of the subchondral bone were also measured. After removal of the cartilage, static energy dissipation significantly decreased (p<0.009) but dynamic energy dissipation was not influenced (p>0.064). Many subchondral bone properties were significantly correlated with the overall mechanical behavior of the cartilage-subchondral bone constructs for males (p<0.047) but not females (p>0.054). However, after removal of cartilage from the constructs, all of the significant correlations were no longer found (p>0.057). The current findings indicate that the subchondral bone is responsible for bearing static and dynamic loading in males but not in females. This result indicates that the female condyle may have a mechanically disadvantageous TMJ loading environment.

The effect of three-dimensional shape optimization on the probabilistic response of a cemented femoral hip prosthesis.

Probabilistic analyses allow the effect of uncertainty in system parameters on predicted model performance measures to be determined. Furthermore, using performance functions to describe a failure event, the probability of failure can be quantified. The effect of three-dimensional prosthesis shape optimization on the probabilistic response and failure probability of a cemented hip prosthesis system is investigated. Random variables include joint and muscle loading, cortical and cancellous bone and PMMA bone cement elastic properties, and strength parameters describing failure of the bone cement and the prosthesis-bone cement interface. Several performance functions describing the bone cement and prosthesis-cement interface are used to compute the probability of failure. When evaluated deterministically, most performance functions indicated a safe design, with the exception of interface tensile failure. However, when evaluated probabilistically, finite probabilities of failure were computed, some significant. The most likely mode of failure before shape optimization was prosthesis-bone cement interface tensile failure with a predicted probability of failure of 97.9%. Deterministic prosthesis shape optimization reduced the probability of failure for all performance functions and reduced prosthesis-bone cement interface tensile failure by 31.7%. Probability sensitivity factors indicate that the uncertainty in the joint loading, cement strength, and implant-cement interface strength have the greatest effect on the computed probability of failure. Implant shape optimization results in a more robust implant design that is less sensitive to uncertainties in joint loading, which cannot be easily controlled, and more sensitive to cement and interface properties, which are easier to modify.

Osteocyte lacunae tissue strain in cortical bone.

Current theories suggest that bone modeling and remodeling are controlled at the cellular level through signals mediated by osteocytes. However, the specific signals to which bone cells respond are still unknown. Two primary theories are: (1) osteocytes are stimulated via the mechanical deformation of the perilacunar bone matrix and (2) osteocytes are stimulated via fluid flow generated shear stresses acting on osteocyte cell processes within canaliculi. Recently, much focus has been placed on fluid flow theories since in vitro experiments have shown that bone cells are more responsive to analytically estimated levels of fluid shear stress than to direct mechanical stretching using macroscopic strain levels measured on bone in vivo. However, due to the complex microstructural organization of bone, local perilacunar bone tissue strains potentially acting on osteocytes cannot be reliably estimated from macroscopic bone strain measurements. Thus, the objective of this study was to quantify local perilacunar bone matrix strains due to macroscopically applied bone strains similar in magnitude to those that occur in vivo. Using a digital image correlation strain measurement technique, experimentally measured bone matrix strains around osteocyte lacunae resulting from macroscopic strains of approximately 2000 microstrain are significantly greater than macroscopic strain on average and can reach peak levels of over 30,000 microstrain locally. Average strain concentration factors ranged from 1.1 to 3.8, which is consistent with analytical and numerical estimates. This information should lead to a better understanding of how bone cells are affected by whole bone functional loading.