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Osteoporosis is characterized by loss of bone mineral density and increased fracture risk. Reduction of hip fracture incidence is of major clinical importance. Hip protectors aim to attenuate the impact force transmitted to the femur upon falling, however different conclusions on their efficacy have been reported; some authors suggest this may be due to differences in compliance. The aim of this study was to apply an In Silico trial methodology to predict the effectiveness of hip protectors and its dependence on compliance. A cohort of 1044 virtual patients (Finite Element models of proximal femur) were generated. A Markov chain process was implemented to predict fracture incidence with and without hip protectors, by simulating different levels of compliance. At each simulated follow-up year, a Poisson distribution was randomly sampled to determine the number of falls sustained by each patient. Impact direction and force were stochastically sampled from a range of possible scenarios. The effect of wearing a hip protector was simulated by applying attenuation coefficients to the impact force (12.9 %, 19 % and 33.8 %, as reported for available devices). A patient was considered fractured when impact force exceeded the femur strength. Without hip protector, virtual patients experienced 66 ± 5 fractures in 10 years. Wearing the three devices, fracture incidence was reduced to 43 ± 4, 35 ± 4 and 17 ± 2 respectively, at full compliance. As expected, effectiveness was dependent on compliance. This In Silico trial technology can be applied in the future to test multiple interventions, optimise intervention strategies, improve clinical trial design and drug development., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Articular cartilage exhibits site-specific biomechanical properties. However, no study has comprehensively characterized site-specific cartilage properties from the same knee joints at different stages of osteoarthritis (OA). Cylindrical osteochondral explants (n = 381) were harvested from donor-matched lateral and medial tibia, lateral and medial femur, patella, and trochlea of cadaveric knees (N = 17). Indentation test was used to measure the elastic and viscoelastic mechanical properties of the samples, and Osteoarthritis Research Society International (OARSI) grading system was used to categorize the samples into normal (OARSI 0-1), early OA (OARSI 2-3), and advanced OA (OARSI 4-5) groups. OA-related changes in cartilage mechanical properties were site-specific. In the lateral and medial tibia and trochlea sites, equilibrium, instantaneous and dynamic moduli were higher (p < 0.001) in normal tissue than in early and advanced OA tissue. In lateral and medial femur, equilibrium, instantaneous and dynamic moduli were smaller in advanced OA, but not in early OA, than in normal tissue. The phase difference (0.1-0.25 Hz) between stress and strain was significantly smaller (p < 0.05) in advanced OA than in normal tissue across all sites except medial tibia. Our results indicated that in contrast to femoral and patellar cartilage, equilibrium, instantaneous and dynamic moduli of the tibia and trochlear cartilage decreased in early OA. These may suggest that the tibia and trochlear cartilage degrades faster than the femoral and patellar cartilage. The information is relevant for developing site-specific computational models and engineered cartilage constructs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

This work investigates how changes in cortical bone microstructure alter the risk of fragility fractures. The secondary osteons of non-osteoporotic (by DXA) women with fragility fractures have reduced lamellar width and greater areas of birefringent brightness in transverse sections, a pathological condition. We used hierarchical finite element (FE) models of the proximal femur of two women aged 67 and 88 (younger and older) during one-legged stance. At specific locations of the anterior-inferior neck (ROI), we analyzed micro-models containing osteons comprised of alternating birefringent extinct and bright lamellae. The plane of lamellar isotropy (XY) was transverse to the osteon longitudinal axis (Z) which was parallel to the femoral neck axis. To evaluate changes in fracture risk with changes in microstructure, we investigated principal and von Mises stresses, and planar stress measures that accounted for transverse isotropy. For both younger and older femurs, 48% to 100% of stress measures were larger in models with healthy architecture than in models with pathological architecture, while controlling for type of lamella and osteon. These findings suggest that bone adaptation reduces stress at most pathological lamellar sites. However, in the bright lamellae of the younger femur, the pathological tensile, compressive and distortional stresses in the transverse plane and distortional stress in the longitudinal planes were larger than the non-negligible corresponding stresses in 6 of the 28 osteon models with healthy architecture, in 5 of the 7 locations. Therefore, a minority of sites with pathological architecture present greater stress, and therefore, greater fracture risk., Competing Interests: Declaration of Competing Interest None of the authors has conflicts to disclose., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Impact force induced in sideways falls is an important determinant of hip fracture risk. While body parameters may differently affect the magnitude of impact force in men and women, the effect of sex is not considered in existing impact force predictors. The objective of this study was to construct a female-specific equation to predict the fall-induced impact force applied to the hip and evaluate whether it could improve hip facture risk assessment. A previously developed human-body dynamic model was used to simulate falling of 80 women and determine the hip impact force. Results were then used to derive a female-specific equation between impact force and body parameters. The proposed female-specific equation and available non-sex-specific impact force predictors in the literature were integrated with a finite element model to discriminate hip fracture patients among 393 women (99 hip fractures; 294 non-fracture controls). Results of the implemented methods were compared to evaluate whether considering the effect of sex could improve the discrimination of females with and without a hip fracture. The area under the curve (AUC) and odds ratio (OR) for the assessed hip fracture risk by the proposed female-specific method (AUC = 0.799, OR = 4.22) were significantly (p<0.001) greater than those of non-sex-specific methods (the most accurate: AUC =0.750, OR =3.62). This study indicates that the proposed equation may be useful to improve hip fracture risk assessment for women., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Hindlimb suspension is the most used model for inducing bone loss from unloading but requires a separate ground control group. This control group cannot be used for genetic studies involving outbred mice. In this study, we evaluated a single limb immobilization (SLI) model for inducing bone loss from unloading, with the contralateral limb from the same animal used as a control. Male 10-week old C57Bl/6J mice had one limb immobilized for one, two, or three weeks. Subsequently, an additional group of male 16-week old C57Bl/6J mice had one limb immobilized for three weeks. SLI resulted in decreased tibial trabecular BV/TV, Tb Th, and Tb N compared to contralateral limbs in young mice. Femoral trabecular BV/TV, Tb Th, Tb N, and femoral cortical area fraction were also decreased. Mechanical properties were not affected after three weeks. In adult mice, femoral trabecular BV/TV, Tb Th, and Tb N were decreased. Femoral stiffness, ultimate stress, and Young's modulus were decreased. Bone properties decreased by SLI were also decreased by hindlimb suspension previously. The results suggest SLI can be an effective model for inducing bone loss in growing and adult mice after three weeks of immobilization., (Copyright © 2018 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Sideways falls impose high stress on the thin superolateral cortical bone of the femoral neck, the region regarded as a fracture-prone region of the hip. Exercise training is a natural mode of mechanical loading to make bone more robust. Exercise-induced adaptation of cortical bone along the femoral neck has been previously demonstrated. However, it is unknown whether this adaption modulates hip fracture behavior. The purpose of this study was to investigate the influence of specific exercise loading history on fall-induced hip fracture behavior by estimating fracture load and location with proximal femur finite element (FE) models created from magnetic resonance images (MRI) of 111 women with distinct exercise histories: 91 athletes (aged 24.7 ± 6.1 years, >8 years competitive career) and 20 women as controls (aged 23.7 ± 3.8 years). The athletes were divided into five groups based on typical loading patterns of their sports: high-impact (H-I: 9 triple-jumpers and 10 high jumpers), odd-impact (O-I: 9 soccer and 10 squash players), high-magnitude (H-M: 17 power-lifters), repetitive-impact (R-I: 18 endurance runners), and repetitive non-impact (R-NI: 18 swimmers). Compared to the controls, the H-I, O-I, and R-I groups had significantly higher (11-26%, p < 0.05) fracture loads. Also, the fracture location in the H-I and O-I groups was significantly more proximal (7-10%) compared to the controls. These results suggest that an exercise loading history of high impacts, impacts from unusual directions, or repetitive impacts increases the fracture load and may lower the risk of fall-induced hip fracture., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Background: While several studies describe kinematics of healthy and osteoarthritic knees using the accurate imaging and computer modelling now possible, no systematic review exists to synthesise these data., Method: A systematic review extracted quantitative observational, quasi-experimental and experimental studies from PubMed, Scopus, Medline and Web of Science that examined motion of the bony or articular surfaces of the tibiofemoral joint during any functional activity. Studies using surface markers, animals, and in vitro studies were excluded., Results: 352 studies were screened to include 23 studies. Dynamic kinematics were recorded for gait, step-up, kneeling, squat and lunge and quasi-static squat, knee flexion in side-lying or supine leg-press. Kinematics were described using a diverse range of measures including six degrees of freedom kinematics, contact patterns or the projection of the femoral condylar axis above the tibia. Meta-analysis of data was not possible since no three papers recorded the same activity with the same measures. Visual evaluation of data revealed that knees with osteoarthritis maintained a more adducted position and showed less posterior translation of the lateral femoral condylar axis than healthy knees. Variability in activities and in recording measures produced greater variation in kinematics, than did knee osteoarthritis., Conclusion: Differences in kinematics between osteoarthritic and healthy knees were observed, however, these differences were more subtle than expected. The synthesis and progress of this research could be facilitated by a consensus on reference systems for axes and kinematic reporting., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

A design concept was formulated for implants to treat medial osteoarthritis of the knee, using a metal plate resurfacing of the tibia plateau and a plastic bearing embedded in the distal end of the femur. A finite element analysis was carried out to determine whether a metal backing would be needed for the femoral component, and to what extent the stress and strain distribution in the trabecular bone surrounding the implant would match the normal intact condition. The CT scans from three knees scheduled for unicompartmental replacement were selected to generate computer models with variable bone densities in each element to cover a range of density patterns. Loading conditions were defined for a range of flexion angles, from loads at the center to the end of the component. A 2-peg fixation design was analyzed for both an all-plastic and a metal-backed construction. For the metal-backed, the interface von Mises stresses were close to intact values at the same level in the bone, although there was a 34 percent increase for loading at the end of the component. However, the all-plastic gave stresses elevated up to 109 percent. The maximum principal strain values for metal-backed in the trabecular bone below the implant were variable between specimens but close to intact under all conditions. In contrast the all-plastic showed strains up to 81 percent increased. The metal pegs showed load transfer, but the loads transmitted by the plastic pegs was small, as evidenced by the low interface stresses. The conclusion was that metal-backing was necessary to avoid excessive bone stresses and strains, while metal peg fixation was evidently an advantage., (Copyright © 2016. Published by Elsevier Ltd.)

The determination of femoral strain in post-menopausal women is important for studying bone fragility. Femoral strain can be calculated using a reference musculoskeletal model scaled to participant anatomies (referred to as scaled-generic) combined with finite-element models. However, anthropometric errors committed while scaling affect the calculation of femoral strains. We assessed the sensitivity of femoral strain calculations to scaled-generic anthropometric errors. We obtained CT images of the pelves and femora of 10 healthy post-menopausal women and collected gait data from each participant during six weight-bearing tasks. Scaled-generic musculoskeletal models were generated using skin-mounted marker distances. Image-based models were created by modifying the scaled-generic models using muscle and joint parameters obtained from the CT data. Scaled-generic and image-based muscle and hip joint forces were determined by optimisation. A finite-element model of each femur was generated from the CT images, and both image-based and scaled-generic principal strains were computed in 32 regions throughout the femur. The intra-participant regional RMS error increased from 380 με (R2=0.92, p<0.001) to 4064 με (R2=0.48, p<0.001), representing 5.2% and 55.6% of the tensile yield strain in bone, respectively. The peak strain difference increased from 2821 με in the proximal region to 34,166 με at the distal end of the femur. The inter-participant RMS error throughout the 32 femoral regions was 430 με (R2=0.95, p<0.001), representing 5.9% of bone tensile yield strain. We conclude that scaled-generic models can be used for determining cohort-based averages of femoral strain whereas image-based models are better suited for calculating participant-specific strains throughout the femur., (Crown Copyright © 2015. Published by Elsevier Ltd. All rights reserved.)

Finite helical axis (FHA) measures of the knee joint during weight-bearing tasks may capture dynamic knee stability following Anterior Cruciate Ligament (ACL) injury. The aim was to investigate dynamic knee stability during two-leg squat (TLS) and one-leg side hop (SH) in a long-term follow-up of ACL injury, and to examine correlations with knee laxity (KT-1000), osteoarthritis (OA, Kellgren-Lawrence) and knee function (Lysholm score). Participants were injured 17-28 years ago and then treated with surgery (n=33, ACLR) or physiotherapy only (n=37, ACLPT) and healthy-knee controls (n=33) were tested. Movements were registered with an optical motion capture system. We computed three FHA inclination angles, its' Anterior-Posterior (A-P) position, and an index quantifying directional changes (DI), during stepwise knee flexion intervals of ∼15°. Injured knees were less stable compared to healthy controls' and to contralateral non-injured knees, regardless of treatment: the A-P intersection was more anterior (indicating a more anterior positioning of tibia relative to femur) positively correlating with high laxity/low knee function, and during SH, the FHA was more inclined relative to the flexion-extension axis, possibly due to reduced rotational stability. During the TLS, A-P intersection was more anterior in the non-injured knee than the injured, and DI was higher, probably related to higher load on the non-injured knee. ACLR had less anterior A-P intersection than ACLPT, suggesting that surgery enhanced stability, although rotational stability may remain reduced. More anterior A-P intersection and greater inclination between the FHA and the knee flexion-extension axis best revealed reduced dynamic stability ∼23 years post-injury., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

In the United States, approximately eight million osseous fractures are reported annually, of which 5-10% fail to create a bony union. Osteoblast-specific deletion of the gene Pten in mice has been found to stimulate bone growth and accelerate fracture healing. Healing rates at four weeks increased in femurs from Pten osteoblast conditional knock-out mice (Pten-CKO) compared to wild-type mice (WT) of the same genetic strain as measured by an increase in mechanical stiffness and failure load in four-point bending tests. Preceding mechanical testing, each femur was imaged using a Skyscan 1172 micro-computed tomography (μCT) scanner (Skyscan, Kontich, Belgium). The present study used µCT image-based analysis to test the hypothesis that the increased femoral fracture force and stiffness in Pten-CKO were due to greater section properties with the same effective material properties as that of the WT. The second moment of area and section modulus were computed in ImageJ 1.46 (National Institutes of Health) and used to predict the effective flexural modulus and the stress at failure for fourteen pairs of intact and callus WT and twelve pairs of intact and callus Pten-CKO femurs. For callus and intact femurs, the failure stress and tissue mineral density of the Pten-CKO and WT were not different; however, the section properties of the Pten-CKO were more than twice as large 28 days post-fracture. It was therefore concluded, when the gene Pten was conditionally knocked-out in osteoblasts, the resulting increased bending stiffness and force to fracture were due to increased section properties., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Impact mechanics theory suggests that peak loads should decrease with increase in system energy absorption. In light of the reduced hip fracture risk for persons with high body mass index (BMI) and for falls on soft surfaces, the purpose of this study was to characterize the effects of participant BMI, gender, and flooring surface on system energy absorption during lateral falls on the hip with human volunteers. Twenty university-aged participants completed the study with five men and five women in both low BMI (<22.5 kg/m(2)) and high BMI (>27.5 kg/m(2)) groups. Participants underwent lateral pelvis release experiments from a height of 5 cm onto two common floors and four safety floors mounted on a force plate. A motion-capture system measured pelvic deflection. The energy absorbed during the initial compressive phase of impact was calculated as the area under the force-deflection curve. System energy absorption was (on average) 3-fold greater for high compared to low BMI participants, but no effects of gender were observed. Even after normalizing for body mass, high BMI participants absorbed 1.8-fold more energy per unit mass. Additionally, three of four safety floors demonstrated significantly increased energy absorption compared to a baseline resilient-rolled-sheeting system (% increases ranging from 20.7 to 28.3). Peak system deflection was larger for high BMI persons and for impacts on several safety floors. This study indicates that energy absorption may be a common mechanism underlying the reduced risk of hip fracture for persons with high BMI and for those who fall on soft surfaces., (Crown Copyright © 2014. Published by Elsevier Ltd. All rights reserved.)

To improve bone strength prediction beyond limitations of assessment founded solely on the bone mineral component, we investigated the effect of hyperlipidemia, present in more than 40% of osteoporotic patients, on multiscale structure of murine bone. Our overarching purpose is to estimate bone strength accurately, to facilitate mitigating fracture morbidity and mortality in patients. Because (i) orientation of collagen type I affects, independently of degree of mineralization, cortical bone׳s micro-structural strength; and, (ii) hyperlipidemia affects collagen orientation and μCT volumetric tissue mineral density (vTMD) in murine cortical bone, we have constructed the first multiscale finite element (mFE), mouse-specific femoral model to study the effect of collagen orientation and vTMD on strength in Ldlr(-/-), a mouse model of hyperlipidemia, and its control wild type, on either high fat diet or normal diet. Each µCT scan-based mFE model included either element-specific elastic orthotropic properties calculated from collagen orientation and vTMD (collagen-density model) by experimentally validated formulation, or usual element-specific elastic isotropic material properties dependent on vTMD-only (density-only model). We found that collagen orientation, assessed by circularly polarized light and confocal microscopies, and vTMD, differed among groups and that microindentation results strongly correlate with elastic modulus of collagen-density models (r(2)=0.85, p=10(-5)). Collagen-density models yielded (1) larger strains, and therefore lower strength, in simulations of 3-point bending and physiological loading; and (2) higher correlation between mFE-predicted strength and 3-point bending experimental strength, than density-only models. This novel method supports ongoing translational research to achieve the as yet elusive goal of accurate bone strength prediction., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Hip fracture remains a major health problem for the elderly. Clinical studies have assessed fracture risk based on bone quality in the aging population and cadaveric testing has quantified bone strength and fracture loads. Prior modeling has primarily focused on quantifying the strain distribution in bone as an indicator of fracture risk. Recent advances in the extended finite element method (XFEM) enable prediction of the initiation and propagation of cracks without requiring a priori knowledge of the crack path. Accordingly, the objectives of this study were to predict femoral fracture in specimen-specific models using the XFEM approach, to perform one-to-one comparisons of predicted and in vitro fracture patterns, and to develop a framework to assess the mechanics and load transfer in the fractured femur when it is repaired with an osteosynthesis implant. Five specimen-specific femur models were developed from in vitro experiments under a simulated stance loading condition. Predicted fracture patterns closely matched the in vitro patterns; however, predictions of fracture load differed by approximately 50% due to sensitivity to local material properties. Specimen-specific intertrochanteric fractures were induced by subjecting the femur models to a sideways fall and repaired with a contemporary implant. Under a post-surgical stance loading, model-predicted load sharing between the implant and bone across the fracture surface varied from 59%:41% to 89%:11%, underscoring the importance of considering anatomic and fracture variability in the evaluation of implants. XFEM modeling shows potential as a macro-level analysis enabling fracture investigations of clinical cohorts, including at-risk groups, and the design of robust implants., (© 2013 Published by Elsevier Ltd.)

This study compared the capabilities of micro-computed tomography (micro-CT) and dental cone-beam computed tomography (CBCT) in assessing trabecular bone parameters and cortical bone strength. Micro-CT and CBCT scans were applied to 28 femurs from 14 rats to obtain independent measurements of the volumetric cancellous bone mineral density (vCanBMD) in the femoral head, volumetric cortical bone mineral density (vCtBMD) in the femoral diaphysis, cross-sectional moment of inertia (CSMI), and bone strength index (BSI) (=CSMI×vCtBMD). Five structural parameters of the trabecular bone of the femoral head were calculated from micro-CT images. A three-point bending test was then conducted to measure the fracture load of each femur. Bivariate linear Pearson analysis was conducted to calculate the correlation coefficients (r values) of the micro-CT, dental CBCT, and three-point bending measurements. The statistical analyses showed a strong correlation between vCanBMD values obtained using micro-CT and dental CBCT (r=0.830). There were strong or moderate correlation between vCanBMD measured using dental CBCT and five parameters of trabecular structure measured using micro-CT. Additionally, the results were satisfactory regardless of whether micro-CT or dental CBCT was used to measure the femoral diaphysis vCtBMD (r=0.733 and 0.680, respectively), CSMI (r=0.756 and 0.726, respectively), or BSI (r=0.846 and 0.847, respectively) to predict fracture loads. This study has yielded a new method for using dental CBCT to evaluate bone parameters and bone strength; however, further studies are necessary to validate the use of dental CBCT on humans., (© 2013 Elsevier Ltd. All rights reserved.)

Finite element (FE) analysis based on quantitative computed tomography (QCT) images is an emerging tool to estimate bone strength in a specific patient or specimen; however, it is limited by the computational power required and the associated time required to generate and solve the models. Thus, our objective was to develop a fast, validated method to estimate whole bone structural stiffness and failure load in addition to a sensitivity analysis of varying boundary conditions. We performed QCT scans on twenty fresh-frozen proximal femurs (age: 77±13 years) and mechanically tested the femurs in a configuration that simulated a sideways fall on the hip. We used custom software to generate the FE models with boundary conditions corresponding to the mechanical tests and solved the linear models to estimate bone structural stiffness and estimated failure load. For the sensitivity analysis, we varied the internal rotation angle of the femoral neck from -30° to 45° at 15° intervals and estimated structural stiffness at each angle. We found both the FE estimates of structural stiffness (R(2)=0.89, p<0.01) and failure load (R(2)=0.81, p<0.01) to be in high agreement with the values found by mechanical testing. An important advantage of these methods was that the models of approximately 500,000 elements took less than 11 min to solve using a standard desktop workstation. In this study we developed and validated a method to quickly and accurately estimate proximal femur structural stiffness and failure load using QCT-driven FE methods., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Fluoroscopic evaluation of total knee arthroplasty (TKA) has reported sudden anterior translation of the femur relative to the tibia (paradoxical anterior motion) for some cruciate-retaining designs. This motion may be tied to abrupt changes in the femoral sagittal radius of curvature characteristic of traditional TKA designs, as the geometry transitions from a large load-bearing distal radius to a smaller posterior radius which can accommodate femoral rollback. It was hypothesized that a gradually reducing radius may attenuate sudden changes in anterior-posterior motion that occur in mid-flexion with traditional discrete-radius designs. A combined experimental and computational approach was employed to test this hypothesis. A previously developed finite element (FE) model of the Kansas knee simulator (KKS), virtually implanted with multiple implant designs, was used to predict the amount of paradoxical anterior femoral slide during a simulated deep knee bend. The model predicted kinematics demonstrated that incorporating a gradually reducing radius in mid-flexion reduced the magnitude of paradoxical anterior translation between 21% and 68%, depending on the conformity of the tibial insert. Subsequently, both a dual-radius design and a modified design incorporating gradually reducing radii were tested in vitro in the KKS for verification. The model-predicted and experimentally observed kinematics exhibited good agreement, while the average experimental kinematics demonstrated an 81% reduction in anterior translation with the modified design. The FE model demonstrated sufficient sensitivity to appropriately differentiate kinematic changes due to subtle changes in implant design, and served as a useful pre-clinical design-phase tool to improve implant kinematics., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

There is a growing interest in studying the fracture behavior of bones, primarily due to the increasing societal burden of osteoporotic fractures. In addition, bone is one of the most important biological materials whose fracture behavior is not yet well understood. This is partly due to the fact that bone is a complex hierarchical material, and exhibits heterogeneous, anisotropic, and viscoelastic mechanical behavior. Understanding the fracture behavior of such a complex material requires application of a full-field strain measurement technique. Digital image correlation (DIC) is a relatively new full-field strain measurement technique that can be used for measurement of 3D surface strains during mechanical testing of different types of bones. In this study, we use the DIC technique to measure the surface strains during compression testing of two groups of rat femora. The first group of femora was harvested from young animals (12 weeks), while the second group was harvested from more mature animals (26 weeks). The surface strains are measured both in the linear range and close to the fracture. Using the measured data, we assess two strain-based fracture prediction criteria, namely equivalent strain fracture criterion and fracture limit diagram, to determine whether they can consistently predict the onset of fracture. The maximum load is measured to be 296±22 N (mean±SD) for young animals and 670±123 N for mature animals. It is shown that fracture in the vast majority of cases occurs in the area of maximum tensile strain. The equivalent strain fracture criterion predicts that the fracture occurs when the equivalent strain reaches 1.04±0.02% (average±SD) for young animals and 1.39±0.24% for mature animals. The fracture limit diagram predicts that the fracture occurs once the sum of major and minor principal surface strains reaches 0.63±0.23% for young animals and -0.63±0.30% for mature animals. Based on these numbers and consistency of the criteria with the strain values recorded at the fracture locations, it is concluded that the equivalent strain fracture criterion tends to be more consistent among the tested specimens., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Reports that knee cartilage health is sensitive to kinematic changes, combined with reports of extension loss following ACL reconstruction, underscores the importance of restoring ambulatory knee extension in the context of preventing premature osteoarthritis. The purpose of this study was to test the relationship between individual variations in peak knee extension at heel-strike of walking and the anterior-posterior location of thickest cartilage in the medial and lateral femoral condyles of healthy contralateral and ACL reconstructed knees. In vivo gait analysis and knee MR images were collected from 29 subjects approximately 2 years after unilateral ACL reconstruction. Knee extension was measured at heel-strike of walking and 3-D femoral cartilage thickness models were reconstructed from MR images. The ACL reconstructed knees had significantly reduced knee extension (-1.5±4.2°) relative to the contralateral knees (-4.6±3.4°) at heel-strike of walking but did not have side-to-side differences in the anterior-posterior location or magnitude of thickest medial and lateral femoral cartilage. The anterior-posterior location of the thickest medial femoral cartilage was correlated with knee extension at heel-strike in both the healthy contralateral (R(2)=0.356, p<0.001) and reconstructed (R(2)=0.234, p=0.008) knees. These results suggest that ACL reconstruction can impair terminal extension at periods of ambulatory loading known to be related to cartilage morphology in healthy joints. The fact that the femoral cartilage thickness distribution had not changed at 2 years post-op, even in the subset of subjects with extension loss, suggests that loads may be shifted to thinner cartilage regions, which could have important implications on long-term joint health., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Total Knee Arthroplasty (TKA) is a very successful surgical procedure but clinical outcomes were reported to be affected by implant design, ligament balancing, alignment or patient-related anatomical factors. It was recently demonstrated that malpositioning of the TKA components and patient related anatomical factors can considerably alter tibio-femoral (TF) and patellofemoral maximum contact forces. However, up to now, how a component malpositioning and different soft-tissue anatomy changes TF knee kinematics was not yet fully investigated. The goal of this study was to evaluate how sensitive TF kinematics are to these factors during a simulated loaded squat for different TKA designs. Four TKA types (a fixed bearing, posterior stabilized prosthesis; a high flexion fixed bearing guided motion prosthesis; a mobile bearing prosthesis and a hinge prosthesis) were virtually implanted on the same virtual cadaver leg model which underwent a loaded squat between 0° and 120°. The reference models were then modified to simulate either component malpositioning in several directions or changes in ligaments geometry by change in the collateral ligament insertions. The results showed that, for all implant designs, TF kinematics were affected by changes in implant positioning and anatomical factors. While the ranges of motion predicted for all tested configurations were generally similar to the reference configuration for each type of TKA, the modifications resulted in shifts in the maximum and minimum values for the TF rotations and translations., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Low-magnitude high-frequency (LMHF) loading has recently received attention for its anabolic effect on bone. The mechanism of transmission of the anabolic signal is not fully understood, but evidence indicates that it is not dependent on bone matrix strain. One possible source of signaling is mechanostimulation of the cells in the bone marrow. We hypothesized that the magnitude of the fluid shear stress in the marrow during LMHF loading is in the mechanostimulatory range. As such, the goal of this study was to determine the range of shear stress in the marrow during LMHF vibration. The shear stress was estimated from computational models, and its dependence on bone density, architecture, permeability, marrow viscosity, vibration amplitude and vibration frequency were examined. Three-dimensional finite element models of five trabecular bone samples from different anatomic sites were constructed, and a sinusoidal velocity profile was applied to the models. In human bone models during axial vibration at an amplitude of 1 g, more than 75% of the marrow experienced shear stress greater than 0.5Pa. In comparison, in vitro studies indicate that fluid induced shear stress in the range of 0.5 to 2.0Pa is anabolic to a variety of cells in the marrow. Shear stress at the bone-marrow interface was as high as 5.0Pa. Thus, osteoblasts and bone lining cells that are thought to reside on the endosteal surfaces may experience very high shear stress during LMHF loading. However, a more complete understanding of the location of the various cell populations in the marrow is needed to quantify the effects on specific cell types. This study suggests the shear stress within bone marrow in real trabecular architecture during LMHF vibration could provide the mechanical signal to marrow cells that leads to bone anabolism., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Currently available implants for direct attachment of prosthesis to the skeletal system after transfemoral amputation (OPRA system, Integrum AB, Sweden and ISP Endo/Exo prosthesis, ESKA Implants AG, Germany) show many advantages over the conventional socket fixation. However, restraining biomechanical issues such as considerable bone loss around the stem and peri-prosthetic bone fractures are present. To overcome these limiting issues a new concept of the direct intramedullary fixation was developed. We hypothesize that the new design will reduce the peri-prosthetic bone failure risk and adverse bone remodeling by restoring the natural load transfer in the femur. Generic CT-based finite element models of an intact femur and amputated bones implanted with 3 analyzed implants were created and loaded with a normal walking and a forward fall load. The strain adaptive bone remodeling theory was used to predict long-term bone changes around the implants and the periprosthetic bone failure risk was evaluated by the von Mises stress criterion. The results show that the new design provides close to physiological distribution of stresses in the bone and lower bone failure risk for the normal walking as compared to the OPRA and the ISP implants. The bone remodeling simulations did not reveal any overall bone loss around the new design, as opposed to the OPRA and the ISP implants, which induce considerable bone loss in the distal end of the femur. This positive outcome shows that the presented concept has a potential to considerably improve safety of the rehabilitation with the direct fixation implants., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

This study uses a nanoindentation technique to examine variations in the local mechanical properties of porcine femoral cortical bone under hydrated conditions. Bone specimens from three age groups (6, 12 and 42 months), representing developing bone, ranging from young to mature animals, were tested on the longitudinal and transverse cross-sectional surfaces. Elastic modulus and hardness of individual lamellae within bone's microstructure: laminar bone, interstitial bone, and osteons, were measured. Both the elastic modulus and hardness increased with age. However, the magnitudes of these increases were different for each microstructural component. The longitudinal moduli were higher than the transverse moduli. Dehydrated samples were also tested to allow a comparison with hydrated samples and these resulted in higher moduli and hardness than the hydrated samples. Again, the degree of variation was different for each microstructural component. These results indicate that the developmental changes in bone have different rates of mechanical change within each microstructural component., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The forces generated by the muscles with origin on the human femur play a major role in transtibial amputee gait, as they are the most effective of the means that the body can use for propulsion. By estimating the forces generated by the thigh muscles of transtibial amputees, and comparing them to the forces generated by the thigh muscles of normal subjects, it is possible to better estimate the energy output needed from prosthetic devices. The purpose of this paper is to obtain the forces generated by the thigh muscles of transtibial amputees and compare these with forces obtained from the same muscles in the case of normal subjects. Two transtibial amputees and four normal subjects similar in size to the amputees were investigated. Level ground walking was chosen as the movement to be studied, since it is a common activity that most amputees engage in. Inverse dynamics and a muscle recruitment algorithm (developed by AnyBody Technology(®)) were used for generating the muscle activation patterns and for computing the muscle forces. The muscle forces were estimated as two sums: one for all posterior muscles and one for the anterior muscles, based on the position of the muscles of the thigh relative to the frontal plane of the human body. The results showed that a significantly higher force is generated by the posterior muscles of the amputees during walking, leading to a general increase of the metabolic cost necessary for one step., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Since the late 1980s, computational analysis of total hip arthroplasty (THA) prosthesis components has been completed using macro-level bone remodeling algorithms. The utilization of macro-sized elements requires apparent bone densities to predict cancellous bone strength, thereby, preventing visualization and analysis of realistic trabecular architecture. In this study, we utilized a recently developed structural optimization algorithm, design space optimization (DSO), to perform a micro-level three-dimensional finite element bone remodeling simulation on the human proximal femur pre- and post-THA. The computational simulation facilitated direct performance comparison between two commercially available prosthetic implant stems from Zimmer Inc.: the Alloclassic and the Mayo conservative. The novel micro-level approach allowed the unique ability to visualize the trabecular bone adaption post-operation and to quantify the changes in bone mineral content by region. Stress-shielding and strain energy distribution were also quantified for the immediate post-operation and the stably fixated, post-remodeling conditions. Stress-shielding was highest in the proximal region and remained unchanged post-remodeling; conversely, the mid and distal portions show large increases in stress, suggesting a distal shift in the loadpath. The Mayo design conserves bone mass, while simultaneously reducing the incidence of stress-shielding compared to the Alloclassic, revealing a key benefit of the distinctive geometry. Several important factors for stable fixation, determined in clinical evaluations from the literature, were evident in both designs: high levels of proximal bone loss and distal bone densification. The results suggest this novel computational framework can be utilized for comparative hip prosthesis shape, uniquely considering the post-operation bone remodeling as a design criterion., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Numerical models commonly account for elastic inhomogeneity in cortical bone using power-law scaling relationships with various measures of tissue density, but limited experimental data exists for anatomic variation in elastic anisotropy. A recent study revealed anatomic variation in the magnitude and anisotropy of elastic constants along the entire femoral diaphysis of a single human femur (Espinoza Orías et al., 2009). The objective of this study was to confirm these trends across multiple donors while also considering possible confounding effects of the anatomic quadrant, apparent tissue density, donor age, and gender. Cortical bone specimens were sampled from the whole femora of 9 human donors at 20%, 50%, and 80% of the total femur length. Elastic constants from the main diagonal of the reduced fourth-order tensor were measured on hydrated specimens using ultrasonic wave propagation. The tissue exhibited orthotropy overall and at each location along the length of the diaphysis (p < 0.0001). Elastic anisotropy increased from the mid-diaphysis toward the epiphyses (p < 0.05). The increased elastic anisotropy was primarily caused by a decreased radial elastic constant (C(11)) from the mid-diaphysis toward the epiphyses (p < 0.05), since differences in the circumferential (C(22)) and longitudinal (C(33)) elastic constants were not statistically significant (p > 0.29). Anatomic variation in intracortical porosity may account for these trends, but requires further investigation. The apparent tissue density was positively correlated with the magnitude of each elastic constant (p < 0.0001, R(2) > 0.46), as expected, but was only weakly correlated with C(33)/C(11) (p < 0.05, R(2) = 0.04) and not significantly correlated with C(33)/C(22) and C(11)/C(22)., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Recent studies show that minor and major centroidal axes (CA) of long bone cross sections provide valuable predictions of prevailing loading patterns in age and treatment matched cohorts of animals. Furthermore, using CA, we recently showed that the degree of mineralization and area of woven bone laid down in the first two weeks after creation of a critical sized bone defect relate inversely and correlate significantly to loading patterns. Here, we aim to determine how closely independent measures of the spatial distribution of bone apposition determined using the major and minor CA as reference points correlate to those using anatomically defined axes as reference points. In histological sections from the previous study, we found no statistically significant difference between the anatomical and centroidal axes with respect to the centroid location or axis rotation, but there is a significant albeit small difference in the average distance between centroids. Outcome measures calculated in areas of bone defined by 15°, 30°, 45°, 60°, or 90° sectors when using the CA differ less than 5% from those calculated using anatomical axes as reference points. Hence, the major and minor CA provide objective reference points for comparison of mechanobiological outcome measures between animals in matched cohorts. Calculation of major and minor CA is automated, which reduces the potential for observer bias. A major advantage of using the major and minor CA as reference points is that it allows for direct relation of outcome measures to loading patterns in age and treatment matched cohorts, ultimately providing a tool to test mechanobiological hypotheses on histological cross sections of bone., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

This investigation of microstructure in the human proximal femur probes the relationship between the parameters of the FRAX index of fracture risk and the parameters of bone microstructure. The specificity of fracture sites at the proximal femur raises the question of whether trabecular parameters are site-specific during post-menopause, before occurrence of fragility fracture. The donated proximal femurs of sixteen post-menopausal women in the sixth and seventh decades of life, free of metabolic pathologies and therapeutic interventions that could have altered the bone tissue, constituted the material of the study. We assessed bone mineral density of the proximal femurs by dual energy X-ray absorptiometry and then sectioned the femurs through the center of the femoral head and along the femoral neck axis. For each proximal femur, morphometry of trabeculae was conducted on the plane of the section divided into conventional regions and sub-regions consistent with the previously identified trabecular families that provide regions of relatively homogeneous microstructure. Mean trabecular width and percent bone area were calculated at such sites. Our findings indicate that each of mean trabecular width and percent bone area vary within each proximal femur independently from each other, with dependence on site. Both trabecular parameters show significant differences between pairs of sites. We speculate that a high FRAX index at the hip corresponds to a reduced percent bone area among sites that gives a more homogeneous and less site-specific quality to the proximal femur. This phenomenon may render the local tissue less able to carry out the expected mechanical function., (Published by Elsevier Ltd.)

Long-term clinical follow-up studies have shown that radiolucent lines at the cement interfaces of total hip replacement femoral components develop gradually, ultimately leading to loosening. In this experimental study, 32 synthetic femurs implanted with cemented femoral components were cyclically loaded with a dynamic joint reaction force, torque, and muscle force, to assess the relative effects of surface finish and collars on interface fixation. Four each of four otherwise identical straight femoral stems, varying only in surface finish and presence or lack of collars were used. Specimens were tested under two conditions: (1) with intact interfaces simulating immediate post-operative conditions and (2) with a thin-film at the stem-cement interface, simulating conditions several weeks to months post-operative when fibrous tissue has formed with the implant still stable. Micromotion was measured at both interfaces in three directions. Surface finish had a larger relative effect than collars, regardless of whether or not a thin-film was present. For example, a proximal grit-blasted finish enhanced fixation at the stem-cement interface by 7-12 μm per-cycle (p<0.05) and decreased early cement mantle loosening by 7-13 μm. For straight stems, rougher surfaces provided greater stability than polished, even with a thin film at the stem-cement interfaces, contradicting the theory that once debonded, rough stems are less stable than polished at the stem-cement interface. The findings of this experimental study exemplify the need to take advantage of all available tools for the preclinical evaluation of orthopaedic implants, including long-term clinical observations of related devices, analytical and numeric models, and experimental bench-top simulations., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Since 2005 reports have been published describing unusual femoral shaft fractures primarily in postmenopausal women treated for prolonged periods with a bisphosphonate drug for osteoporosis. In some patients pain develops in the femur prior to a completed fracture. Bilateral fractures have occurred in some patients. It is unclear whether oversuppression of bone cell activity is a major factor in the pathogenesis of the fractures, or whether these are a rare manifestation of the underlying bone disease. Such fractures do occur in other metabolic bone disorders in which there are marked abnormalities of bone structure., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

High-energy synchrotron X-ray scattering (>60 keV) allows noninvasive quantification of internal strains within bone. In this proof-of-principle study, wide angle X-ray scattering maps internal strain vs position in cortical bone (murine tibia, bovine femur) under compression, specifically using the response of the mineral phase of carbonated hydroxyapatite. The technique relies on the response of the carbonated hydroxyapatite unit cells and their Debye cones (from nanocrystals correctly oriented for diffraction) to applied stress. Unstressed, the Debye cones produce circular rings on the two-dimensional X-ray detector while applied stress deforms the rings to ellipses centered on the transmitted beam. Ring ellipticity is then converted to strain via standard methods. Strain is measured repeatedly, at each specimen location for each applied stress. Experimental strains from wide angle X-ray scattering and an attached strain gage show bending of the rat tibia and agree qualitatively with results of a simplified finite element model. At their greatest, the apatite-derived strains approach 2500 με on one side of the tibia and are near zero on the other. Strains maps around a hole in the femoral bone block demonstrate the effect of the stress concentrator as loading increased and agree qualitatively with the finite element model. Experimentally, residual strains of approximately 2000 με are present initially, and strain rises to approximately 4500 με at 95 MPa applied stress (about 1000 με above the strain in the surrounding material). The experimental data suggest uneven loading which is reproduced qualitatively with finite element modeling., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

The fracture resistance of bone arises from the composition, orientation, and distribution of the primary constituents at each hierarchical level of organization. Therefore, to establish the relevance of Raman spectroscopy (RS) in identifying differences between strong or tough bone and weak or brittle bone, we investigated whether Raman-derived properties could explain the variance in biomechanical properties at both the whole bone and the tissue-level, and do so independently of traditional measurements of mineralization. We harvested femurs from wild-type mice and mice lacking matrix metalloproteinase 2 because the mutant mice have a known reduction in mineralization. Next, RS quantified compositional properties directly from the intact diaphysis followed by micro-computed tomography to quantify mineralization density (Ct.TMD). Correlations were then tested for significance between these properties and the biomechanical properties as determined by the three-point bending test on the same femurs. Harvested tibia were embedded in plastic, sectioned transversely, and polished in order to acquire average Raman properties per specimen that were then correlated with average nanoindentation properties per specimen. Dividing the ν(1) phosphate by the proline peak intensity provided the strongest correlation between the mineral-to-collagen ratio and the biomechanical properties (whole bone modulus, strength, and post-yield deflection plus nanoindentation modulus). Moreover, the linear combination of ν(1) phosphate/proline and Ct.TMD provided the best explanation of the variance in strength between the genotypes, and it alone was the best explanatory variable for brittleness. Causal relationships between Raman and fracture resistance need to be investigated, but Raman has the potential to assess fracture risk., (Published by Elsevier Ltd.)

Insufficiency fracture following radiation therapy (RTx) is a challenging clinical problem and typical bone mass measures fail to predict these fractures. The goals of this research were to develop a mouse model that results in reduced bone strength following focal irradiation, quantify morphological and strength changes occurring over time, and determine if a positive correlation between bone morphology and strength is retained after irradiation. Right hind limbs of 13 week-old female Balb/c mice were irradiated (5 or 20 Gy) using a therapeutic X-ray unit. Left limbs served as control. Animals were euthanized at 2, 6, 12, or 26 weeks. Axial compression tests of the distal femur were used to quantify whole bone strength. Specimen-specific, non-linear finite element (FE) analyses of the mechanical tests were performed using voxel-based meshes with two different material failure models: a linear bone density-strength relationship and a non-linear 'embrittled' relationship. Radiation resulted in a dose dependent increase in cortical bone density and marked loss of trabecular bone, measured using micro-CT. An early (2 week) increase in bone volume was associated with an increase in bone strength following irradiation; at 12 weeks there was a loss of bone strength despite higher bone volume for irradiated limbs. There was a positive correlation between bone volume bone and strength in control (r²=0.63) but not irradiated femora (r²=0.08). FE analysis with a constant strain failure model resulted in improved prediction of bone strength for irradiated limbs (r²=0.34) and this was improved further with the embrittled material model (r²=0.46). In summary, focal irradiation leads to substantial changes in bone morphology and strength with time, where there is a decreased bone strength following irradiation in the face of increasing bone mass; FE models with a non-linear embrittled material model were most successful in simulating these experimental findings., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

In using musculoskeletal models, researchers can calculate muscle forces, and subsequently joint contact forces, providing insight into joint loading and the progression of such diseases as osteoarthritis (OA). The purpose of this study was to estimate the knee contact force (KCF) in patients with varying degrees of OA severity using muscle forces and joint reaction forces derived from OpenSim. Walking data was obtained from healthy individuals (n=14) and those with moderate (n=10) and severe knee OA (n=2). For each subject, we generated 3D, muscle-actuated, forward dynamic simulations of the walking trials. Muscle forces that reproduced each subject's gait were calculated. KCFs were then calculated using the vector sum of the muscle forces and joint reaction forces along the longitudinal axis of the femur. Moderate OA subjects exhibited a similar KCF pattern to healthy subjects, with lower second peaks (p=0.021). Although subjects with severe OA had similar initial peak KCF to healthy and moderate OA subjects (more than 4 times BW), the pattern of the KCF was very different between groups. After an initial peak, subjects with severe OA continually unloaded the joint, whereas healthy and moderate OA subjects reloaded the knee during late stance. In subjects with symmetric OA grades, there appears to be differences in loading between OA severities. Similar initial peaks of KCF imply that reduction of peak KCF may not be a compensatory strategy for OA patients; however, reducing duration of high magnitude loads may be employed.

Methods of producing relevant and quantifiable load alterations in vivo with which to study load-induced cartilage degeneration analogous to osteoarthritis are limited. An animal model was used to investigate the effects of increased chronic loads on articular cartilage. Mature rabbits were randomized into one of three experimentally loaded groups and a fourth unoperated control group. A mechanical-loading device was skeletally fixed to the hind limb of animals in the loaded groups. Engaging the device resulted in an additional load of 0%, +22% or +44% body weight to the medial compartment of the experimental knee, while allowing normal joint function. Following a 12-week loading protocol, a creep-indentation test and needle probe test were used to determine the biphasic material properties and thickness of the cartilage at four locations of each femoral and tibial condyle of the experimental and contralateral limbs. Analyses of covariance were performed to compare outcome measures across the treatment groups. The effect of increased load was site and load-level specific with alterations of material properties and thickness most prominent in the posterior region of the medial compartment of the tibia. At this site, permeability increased 128% and thickness increased 28% in the +44% body weight group relative to the 0% body weight group. This model of altered chronic loading initiated changes in the material properties to the articular cartilage at the sites of increased load over 12-weeks that were consistent with early degenerative changes suggesting that increased tibio-femoral loading may be responsible for the alterations. This work begins to elucidate the chronic-load threshold and the time course of cartilage degeneration at different levels of altered loading., (2010 Elsevier Ltd. All rights reserved.)

The knee is often a site of injury that can often lead to a chronic disease known as osteoarthritis (OA). The disease may be initiated, in part, by acute injuries to joint cartilage and its cells. In a recent study by this laboratory, using Flemish Giant rabbits, an impact compressive load on the tibial femoral joint was shown to cause significant levels of acute damage to chondrocytes in cartilage of the medial and lateral tibial plateaus. In the current study, using the same model, histological and mechanical data from the plateaus were documented at 6 and 12 months post impact, and compared to the unimpacted control limbs and a limb from unimpacted, control animals. The mechanical properties of cartilage were measured with indentation relaxation tests on the medial and lateral plateaus in regions covered and uncovered by the meniscus. The histological studies on impacted limbs showed surface lesions on both plateaus, thickening of the underlying subchondral bone at 12 months and numerous occult microcracks at the calcified cartilage-subchondral bone interface at 6 and 12 months, without significant changes in cartilage thickness or its mechanical properties versus controls. Yet, there was an increase in both the matrix and fiber moduli and a decrease in the permeability of uncovered, medial plateau cartilage in both limbs of impacted animals between 6 and 12 months post impact that was not documented in control animals., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)

In the present study, a probabilistic finite element tool was assessed using an uncemented total hip replacement model. Fully bonded and frictional interfaces were investigated for combinations of three proximal femurs and two implant designs, the Proxima short stem and the IPS hip stem prostheses. The Monte Carlo method was used with two performance indicators: the percentage of bone volume that exceeded specified strain limits and the maximum nodal micromotion. The six degrees of freedom of bone-implant relative position, magnitude of the hip contact force (L), and spatial direction of L were the random variables. The distal portion of the proximal femurs was completely constrained and some of the main muscle forces acting in the hip were applied. The coefficients of the linear approximation between the random variables and the output were used as the sensitivity values. In all cases, bone-implant position related parameters were the most sensitive parameters. The results varied depending on the femur, the implant design and the interface conditions. Values of maximum nodal micromotion agreed with results from previous studies, confirming the robustness of the implemented computational tool. It was demonstrated that results from a single model study should not be generalised to the entire population of femurs and that bone variability is an important factor that should be investigated in such analyses., (Copyright 2009 Elsevier Ltd. All rights reserved.)

Patellofemoral pain syndrome (PFPS) is a disorder of the patellofemoral (PF) joint in which abnormal tracking is often cited as a factor in pain development. PF tracking is partially dependent on passive stabilizers (ex: PF geometry). Relations amongst PFPS, PF tracking, and contact mechanics are poorly understood. In-vivo investigation of passive PF joint stabilizers including PF tracking, contact mechanics, cartilage thickness, and patellar shape will allow structural characterization of the PF joint and may highlight differences associated with PFPS. This study examined the role that passive stabilizers play in PFPS (n=10) versus healthy subjects (n=10). PF tracking (contact area centroid migration), cartilage thickness, shape, congruence, and contact patterns were quantified using magnetic resonance imaging during isometric loading at 15 degrees , 30 degrees , and 45 degrees of knee flexion. Distinct relationships were identified between patellar shape and tracking and contact, particularly at low flexion (15-30 degrees ). Healthy subjects exhibited distinct PF tracking and contact patterns related to Type I patella shape (80%) with increasing total contact area (p<0.001) and proximal centroid migration (15-30 degrees p=0.012; 30-45 degrees p<0.001) for increasing knee angles. PFPS subjects deviated from these patterns at low flexion, demonstrating higher total contact area than healthy subjects (p=0.046 at 15 degrees ), lack of proximal centroid migration (15-30 degrees ), and more Type II (30%) and III (20%) patella shapes. This study highlights a new finding that patellar shape combined with low degrees of flexion (15-30 degrees ) may be important to consider, as this is where PFPS tracking and contact patterns deviate from healthy.

Experimental models can be used for pre-clinical testing of cemented and other type of hip replacements. Total hip replacement (THR) failure scenarios include, among others, cement damage accumulation and the assessment of accurate stress and strain magnitudes at the cement mantle interfaces (stem-cement and cement-bone) can be used to predict mechanical failure. The aseptic loosening scenario in cemented hip replacements is currently not fully understood, and methods of evaluating medical devices must be developed to improve clinical performance. Different results and conclusions concerning the cement micro-cracking mechanism have been reported. The aim of this study was to verify the in vitro behavior of two cemented femoral stems with respect to fatigue crack formation. Fatigue crack damage was assessed at the medial, lateral, anterior and posterior sides of the Lubinus SPII and Charnley stems. All stems were loaded and tested in stair climbing fatigue loading during one million cycles at 2 Hz. After the experiments each implanted synthetic femur was sectioned and analyzed. We observed more damage (cracks per area) for the Lubinus SPII stem, mainly on the proximal part of the cement mantle. The micro-cracking formation initiated in the stem-cement interface and grew towards the direction of cortical bone of the femur. Overall, the cement-bone interface seems to be crucial for the success of the hip replacement. The Charnley stem provoked more damage on the cement-bone interface. A failure index (maximum length of crack/maximum thickness of cement) considered was higher for the cement-stem interface of the Lubinus SPII stem. For a cement mantle thickness higher than 5 mm, cracking initiated at the cement-bone interface and depended on the opening canal process (reaming procedure and instrumentation). The analysis also showed that fatigue-induced damage on the cement mantle, increasing proximally, and depended on the axial position of the stem. The cement thickness is an important factor for the success of THR and this study evidenced that cement thickness higher than 2 mm apparently does not affect the mechanical behavior of the cement mantel and induce more crack formation on the cement-bone interface.

Results of recent imaging studies and theoretical models suggest that the superior femoral neck is a location of local weakness due to an age-related thinning of the cortex, and thus the site of hip fracture initiation. The purpose of this study was to experimentally determine the spatial and temporal characteristics of the macroscopic failure process during a simulated hip fracture that would occur as a result of a sideways fall. Twelve fresh frozen human cadaveric femora were used in this study. The femora were fractured in an apparatus designed to simulate a fall on the greater trochanter. Image sequences of the surface events related to the fractures were captured using two high-speed video cameras at 9111 Hz. The videos were analyzed with respect to time and load to determine the location and sequence of these events occurring in the proximal femur. The mean failure load was 4032 N (SD 370 N). The first surface events were identified in the superior femoral neck in eleven of the twelve specimens. Nine of these specimens fractured in a clear two-step process that initiated with a failure in the superior femoral neck, followed by a failure in the inferior femoral neck. This cadaveric model of hip fracture empirically confirms hypotheses that suggested that hip fractures initiate with a failure in the superior femoral neck where stresses are primarily compressive during a sideways fall impact, followed by a failure in the inferior neck where stresses are primarily tensile. Our results confirm the superolateral neck of the femur as an important region of interest for future hip fracture screening, prevention and treatment research.

Injuries due to backward fall apart from sideways fall are a major health problem, particularly among the aged populations. The objectives of this study was to evaluate the responses to changing body configurations (angle between the trunk and impacting floor as 0 degrees, 15 degrees, 45 degrees and 80 degrees) during backward fall, based on a previously developed CT-scan-derived 3D non-linear and non-homogeneous finite element (FE) model of pelvis-femur-soft tissue complex with simplified biomechanical representation of the whole body. Under constant impact energy, these FE models evaluated the pelvic injury situations on the basis of peak impact force (7.64-16.74 kN) and peak principal compressive strain (more than 1.5%), consistent with the clinically observed injuries (sacral insufficiency, coccydynia). Also the change in location of peak strain and increase in peak impact force for changing configurations from 0 degrees to 80 degrees indicated the effect of whole body inertia during backward fall. It was also concluded that the inclusion of sacro-iliac and acetabular cartilages in the above FE models will further reduce above findings marginally (9.2% for 15 degrees fall). These quantifications would also be helpful for a better design and development of safety structures such as safety floor for the nursing home or home for the aged persons.

Anterior cruciate ligament (ACL) injury is a major problem worldwide and prevails during high-impact activities. It is not well-understood how the extent and distribution of cartilage damage will arise from repetitive landing impact loads that can lead to ACL failure. This study seeks to investigate the sole effect of repetitive incremental landing impact loads on the induction of ACL failure, and extent and distribution of tibiofemoral cartilage damage in cadaveric knees. Five cadaveric knees were mounted onto a material testing system at 70 degrees flexion to simulate landing posture. A motion-capture system was used to track rotational and translational motions of the tibia and femur, respectively. Each specimen was compressed at a single 10Hz haversine to simulate landing impact. The compression trial was successively repeated with increasing actuator displacement till a significant compressive force drop was observed. All specimens underwent ACL failure, which was confirmed via magnetic resonance scans and dissection. Volume analysis, thickness measurement and histological techniques were employed to assess cartilage lesion status. For each specimen, the highest peak compressive force (1.9-7.8kN) was at the final trial in which ACL failure occurred; corresponding posterior femoral displacement (7.6-18.0mm) and internal tibial rotation (0.6 degrees -4.7 degrees ) were observed. Significant compressive force drop (79.8-90.9%) was noted upon ACL failure. Considerable cartilage deformation and damage were found in exterior, posterior and interior femoral regions with substantial volume reduction in lateral compartments. Repeated application of incremental landing impact loads can induce both ACL failure and cartilage damage, which may accelerate the risk of developing osteoarthritis.

The elastic behaviour of trabecular bone is a function not only of bone volume and architecture, but also of tissue material properties. Variation in tissue modulus can have a substantial effect on the biomechanical properties of trabecular bone. However, the nature of tissue property variation within a single trabecula is poorly understood. This study uses nanoindentation to determine the mechanical properties of bone tissue in individual trabeculae. Using an ovariectomised ovine model, the modulus and hardness distribution across trabeculae were measured. In both normal and ovariectomised bone, the modulus and hardness were found to increase towards the core of the trabeculae. Across the width of the trabeculae, the modulus was significantly less in the ovariectomised bone than in the control bone. However, in contrast to this hardness was found not to differ significantly between the two groups. This study provides valuable information on the variation of mechanical material properties in healthy and diseased trabecular bone tissue. The results of the current study will be useful in finite element modelling where more accurate values of trabecular bone modulus will enable the prediction of the macroscale behaviour of trabecular bone.

The knee is one of the most frequently injured joints in the human body. Approximately 91% of ACL injuries occur during sporting activities, usually from a non-contact event. The most common kinetic scenarios related with ACL injuries are internal twisting of the tibia relative to the femur or combined torque and compression during a hard landing. The hypothesis of this study was that the magnitudes and types of motion observed after ACL rupture would significantly change from the relative joint displacements present just before ACL injury. Compression or torsion experiments were conducted on 7 pairs of knee joints with repetitive tests at increasing intensity until catastrophic failure. ACL injury was documented in all cases at 5.4+/-2kN of TF compression or 33+/-13Nm of internal tibial torque. The femur displaced posteriorly relative to the tibia in pre-failure and with a higher magnitude in failure tests under both loading conditions. In compression experiments there was internal rotation of the tibia in pre-failure tests, but external rotation of the tibia after the ACL failed. In torsion experiments, failure occurred at 58+/-19 degrees of internal tibial rotation, and valgus rotation of the femur increased significantly after ACL injury. These new data show that the joint motions can vary in magnitude and direction before and after failure of the ACL. Video-based studies consistently document external rotation of the tibia combined with valgus knee bending as the mechanism of ACL injury although these motions could be occurring after ACL rupture.

Patellofemoral complications are among the important reasons for revision knee arthroplasty. Femoral component malposition has been implicated in patellofemoral maltracking, which is associated with anterior knee pain, subluxation, fracture, wear, and aseptic loosening. Rotating-platform mobile bearings compensate for malrotation between the tibial and femoral components and may, therefore, reduce any associated patellofemoral maltracking. To test this hypothesis, we developed a dynamic model of quadriceps-driven open-kinetic-chain extension in a knee implanted with arthroplasty components. The model was validated using tibiofemoral and patellofemoral kinematics and forces measured in cadaver knees. Knee kinematics and patellofemoral forces were measured after simulating malrotation (+/-3 degrees ) of the femoral component. Rotational alignment of the femoral component affected tibial rotation near full extension and tibial adduction at higher flexion angles. External rotation of the femoral component increased patellofemoral lateral tilt, lateral shift, and lateral shear forces. Up to 21 degrees of bearing rotation relative to the tibia was noted in the rotating-bearing condition. However, the rotating bearing had minimal effect in reducing the patellofemoral maltracking or shear induced by femoral component rotation. The rotating platform does not appear to be forgiving of malalignment of the extensor mechanism resulting from femoral component malrotation. These results support the value of improving existing methodologies for accurate femoral component alignment in total knee arthroplasty.

Osteogenesis imperfecta (OI) is a heritable disease, which results from an abnormal amount or structure of Type I collagen. Bisphosphonates, a class of synthetic antiresorptive drugs, used in osteoporosis management, are also used to decrease fracture incidence and improve quality of life in children with OI. In this study, we used the oim mouse to test the hypotheses that pamidronate treatment during active growth (1) produces larger, stronger, stiffer long bone diaphyses without altering bone material properties, and (2) negatively impacts longitudinal bone growth. Our results indicate that femoral cross-sectional moment of inertia in the distal metaphysis tended to increase with pamidronate treatment and that the treated bones are thicker and structurally stiffer, but shorter than their control-dose counterparts.

Individuals with peripheral neuropathy (PN) may compensate for decreased somatosensation by reducing walking speed. Predisposition to falls may therefore arise from an inability to adapt to challenging walking speeds. The purpose of this study was to examine the effects of PN on the magnitude of variability and local instability on walking at different speeds. Twelve individuals with PN and 12 controls completed a 6-min walk test to determine fast walking speed (FWS). Sagittal plane hip, knee, and ankle joint angles were then calculated during 3 min of treadmill walking at 100%, 80%, and 60% FWS. The magnitudes of stride duration variability (SDvar), joint angle variability (JTvar), and both short- and long-term Lyapunov exponents (used to estimate local instability) were calculated. The PN group walked slower than the control group (p<.001). With groups combined, walking faster led to increased local instability and increased variability (p<.001). The PN group exhibited increased variability (SDvar, p=.02; JTvar, p=.01) over all speeds, and exaggerated local instability (p<.05) when walking at the fastest speed. PN leads to increased walking variability and local instability, particularly when walking at challenging speeds. These results are important to consider in future patient education and rehabilitation programs.

In the present investigation, the effects of acute and chronic dose of alcohol were evaluated on mechanical properties of long bones of Sprague Dawley rats. In "acute study", 18 animals were divided into three groups containing six animals each, i.e. Group A: control animals, normal saline was given to them intraperitoneally for the period of 5 days; Group B: treated animals, given 20% (v/v) absolute alcohol and Group C: treated animals, given 30% (v/v) absolute alcohol, by same route and time duration. In "chronic study", also, 18 animals were divided into three groups containing six animals each, i.e. Group A: control animals, normal saline was given to them intraperitoneally for the period of 6 weeks; Group B: treated animals, given 20% (v/v) absolute alcohol and Group C: treated animals, given 30% (v/v) absolute alcohol by same route and time duration. A significant increase was observed in bone weight of animals taking 20% alcohol but there was decrease in the same for 30% alcohol in case of acute study. For chronic study, there was a decrease in bone weight for both treated groups. During acute study, breaking strength of bone was increased in case of 20% alcohol administration but a slight decrease was shown in the same for 30% alcohol group as compared to control animals. Breaking strength of long bone in the case of chronic study was decreased in case of both groups taking alcohol, i.e. 20% and 30%. The present document is useful in understanding the functional load carrying capacity of bone during alcoholism.

The Theory of Critical Distances (TCD) is a method used to study the failure of material in situations where stress concentrations, such as holes and notches, are present. This method uses two material constants: a critical length and a critical stress. The elastic stress field close to the stress concentration is examined, applying a fracture criterion. The TCD has been applied to predict brittle fracture in various different materials and various types of notches but it has not previously been applied to bone. Since bone fails by brittle fracture with limited plasticity, it is expected that the TCD will be applicable. Experimental data were obtained from the literature on the effects of sharp notches and holes loaded in various ways (tension, torsion and bending). These tests were modelled using finite element analysis. It was found that the TCD could be successfully applied to predict the load required for brittle fracture as a function of the type and size of the stress concentration feature. The critical distance was found to be almost constant, about 0.3-0.4mm, for all types of bone studied: the critical stress was found to be related to the material's ultimate tensile strength by a constant factor of T=1.33. The results of this study will be of practical value in the assessment of stress concentration features introduced during surgery and of naturally occurring bone defects.