A novel in silico method to quantify primary stability of screws in trabecular bone

Insufficient primary stability of screws in bone leads to screw loosening and failure. Unlike conventional continuum finite‐element models, micro‐CT based finite‐element analysis (micro‐FE) is capable of capturing the patient‐specific bone micro‐architecture, providing accurate estimates of bone stiffness. However, such in silico models for screws in bone highly overestimate the apparent stiffness. We hypothesized that a more accurate prediction of primary implant stability of screws in bone is possible by considering insertion‐related bone damage. We assessed two different screw types and loading scenarios in 20 trabecular bone specimens extracted from 12 cadaveric human femoral heads (N = 5 for each case). In the micro‐FE model, we predicted specimen‐specific Young's moduli of the peri‐implant bone damage region based on morphometric parameters such that the apparent stiffness of each in silico model matched the experimentally measured stiffness of the corresponding in vitro specimen as closely as possible. The standard micro‐FE models assuming perfectly intact peri‐implant bone overestimated the stiffness by over 330%. The consideration of insertion related damaged peri‐implant bone corrected the mean absolute percentage error down to 11.4% for both loading scenarios and screw types. Cross‐validation revealed a mean absolute percentage error of 14.2%. We present the validation of a novel micro‐FE modeling technique to quantify the apparent stiffness of screws in trabecular bone. While the standard micro‐FE model overestimated the bone‐implant stiffness, the consideration of insertion‐related bone damage was crucial for an accurate stiffness prediction. This approach provides an important step toward more accurate specimen‐specific micro‐FE models. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2415–2424, 2017.

     

Evaluation of orthogonal mechanical properties and density of human trabecular bone from the major metaphyseal regions with materials testing and computed tomography

We evaluated the orthogonal mechanical properties of human trabecular bone from the major metaphyseal regions with materials testing and quantitative computed tomography (CT). The proximal tibia, distal femur, proximal femur, distal radius, and proximal humerus from fresh cadaver specimens between the ages of 55 and 70 years were excised and prepared for experimentation. The bones were embedded and scanned at 1 or 1.5 mm intervals on a Technicare HPS 1440 and GE 9800 CT scanner. After scanning, the bones were sectioned, producing 8-mm cubes of trabecular bone which were mechanically tested in uniaxial compression at a strain rate of 1%. The testing sequence consisted of preyield tests in two of the three orthogonal directions and failure in the third. After testing, the cubes were evaluated for apparent density and ash weight. The results of the study show that the strength and stiffness of trabecular bone varies significantly within metaphyseal regions and from metaphysis to metaphysis. The power and significance of relationships between density and modulus varied as a function of metaphyseal location. Both linear and nonlinear models were significant, suggesting that trabecular deformation occurs in response to both axial and bending loads. Finally, the need for architectural measures of trabecular bone to predict mechanical properties is emphasized.     

High-intensity exercise induces structural, compositional and metabolic changes in cuboidal bones--findings from an equine athlete model

Fatigue fracture of cuboidal bones occurs in the human foot as well as the equine carpus. The racehorse provides a naturally-occurring model to study the effects of high-intensity exercise on the morphology and metabolism of cuboidal bones. We studied both the mineral and the collagenous matrix of the third (C(3)) and radial (C(r)) carpal bones of raced and non-raced Thoroughbred (TB) horses. We hypothesised that racehorses would show increases in the mineral component of these bones and post-translational modifications of the collagenous matrix alongside changes in markers of collagen remodelling and bone formation. C(3) and C(r) carpal bones were retrieved from raced TB horses (n=14) and non-raced TB horses (n=11). Standardised proximal-distal sections were taken from each bone and these were sliced transversely to study the proximal-distal differences in bone metabolism from the subchondral plate through to trabecular bone. Histomorphometry and bone mineral density measurements were performed in parallel with biochemical analyses including total collagen, collagen synthesis and cross-links, matrix metalloproteinases-2 and 9 and their inhibitors, calcium and phosphate, and bone alkaline phosphatase. The results of this study show that, while there is a net increase in bone formation in the racehorses, there is additionally an increase in bone collagen synthesis and remodelling, particularly within the trabecular regions of the bone. The increase in bone density would lead to greater stiffness, particularly in the cortical bone, and failure of this 'stiffer' cortical bone may result from its lack of support from the rapidly remodelling and structurally weakened underlying trabecular bone.     

Initial effect of collarless stem stiffness on femoral bone strain

Stress shielding resulting from a stiffness mismatch between bone and femoral prosthesis stems (leading to bone resorption in the proximal femur) is believed to contribute to failure in total hip arthroplasty. In this study, strains were measured under compressive femoral head loads both in the intact femur and after implanting first a collarless steel stem and then a geometrically identical fiber-reinforced polymer composite stem 64% less stiff. Decreasing stem stiffness would be expected increase load transfer from the stem to the proximal medial femur, decreasing the degree of stress shielding. The authors found that proximal medial bone strains were significantly lower with either the steel or composite stem implanted than in the intact case. However, there were no significant differences in strain patterns between the steel and composite stem cases. This apparent insensitivity to prosthesis stiffness may result from factors related to implant geometry and fit.     

Importance of individual rods and plates in the assessment of bone quality and their contribution to bone stiffness.

Local morphometry based on the assessment of individual rods and plates was applied to 42 human vertebral trabecular bone samples. Results showed that multiple linear regression models based on local morphometry as a measure for bone microstructure helped improving our understanding of the role of local structural changes in the determination of bone stiffness as assessed from direct and computational biomechanics. INTRODUCTION: In a recent study, we proposed a method for local morphometry of trabecular bone, i.e., morphometry as applied to individual rods and plates. In this study, we used this method to study the relative importance of local morphometry in the assessment of bone architecture and its relative contribution to the stiffness of human vertebral bone. MATERIALS AND METHODS: We extracted 42 human trabecular bone autopsies from nine intact spinal columns. The cylindrical samples were imaged with muCT to assess bone microstructure. From these images, global and local morphometric indices were derived and related to Young's modulus as assessed by experimental uniaxial compression testing (Emeas) and computational finite element analysis (EFE). RESULTS: We found the best single predictor for Young's modulus to be apparent bone volume density (BV/TV), which explained 89% of the variance in EFE when fitted with a power law. A multiple linear regression model combining mean trabecular spacing (Tb.Sp), mean slenderness of the rods (), and the relative amount of rod volume to total bone volume (Ro.BV/BV) was able to explain 90% of the variance in EFE. This model could not be improved by adding BV/TV as an independent variable. Furthermore, we found that mean trabecular thickness of the rods was significantly related to EFE (r2 = 0.42), whereas mean trabecular thickness of plates had no correlation to Young's modulus. Because the globally determined trabecular thickness does not discriminate between rods and plates, this index had only a poor predictive power for EFE (r2 = 0.09), showing the importance of local analysis of individual rods and plates. CONCLUSIONS: From these results, we conclude that models based on local morphometry help improving our understanding of the relative importance of local structural changes in the determination of the stiffness of bone. Separate analysis of individual rods and plates may help to better predict age and disease-related fractures as well as to shed new light on the effect of pharmaceutical intervention in the prevention of such fractures beyond BMD.     

Effekte der frühen Östrogenersatztherapie auf die Knochenstabilität von ovarektomierten Ratten

Postmenopausal osteoporosis leads to a significant increase in bone fragility. In this study we used the rat tibia plateau fracture model to investigate the efficiency of estrogen replacement therapy (ERT) to mitigate the post-ovariectomy decrease in fracture load. A total of 73 virgin Sprague Dawley rats had been ovariectomized and 26 animals underwent sham operation. The ovariectomized animals were either untreated (n = 35) or treated with estrogen injections (10 micrograms/kg per day 3 days a week until sacrifice), starting treatment at either 0, 5, 8, or 13 days post surgery. Before starting ERT and at 50 days post surgery, the trabecular structure of the right proximal tibial metaphysis of each animal was imaged non-invasively using high resolution X-ray topography. The animals were then sacrificed and the right knee from each animal was harvested and mounted into a servo-hydraulic materials testing system so that the distal femoral condyle could be forced into the proximal tibial plateau until fracture occurred. The failure load (F) of the ovariectomized group without estrogen administration was significantly less than that for the sham group. The mean stiffness (K) of the ovariectomized group was 22 percent less than that of the sham group, though this difference did not reach statistical significance. Across all groups, the failure load and stiffness were significantly correlated with the trabecular bone volume. Our data suggest that prompt ERT can increase the fracture load and stiffness of trabecular bone by allowing bone formation to continue in previously activated bone remodeling units while suppressing the production of new remodeling units. This may be the mechanism by which estrogen and other antiresorptive agents increase bone mass, and thereby reduce the risk of osteoporotic fractures in postmenopausal women.     

Contribution of Trabecular and Cortical Components to Biomechanical Behavior of Human Vertebrae: An Ex Vivo Study

Whereas there is clear evidence for a strong influence of bone quantity (i.e., bone mass or bone mineral density) on vertebral mechanical behavior, there are fewer data addressing the relative influence of cortical and trabecular bone microarchitecture. The aim of this study was to determine the relative contributions of bone mass, trabecular microarchitecture, and cortical thickness and curvature to the mechanical behavior of human lumbar vertebrae. Thirty‐one L3 vertebrae (16 men, 15 women, aged 75 ± 10 years and 76 ± 10 years, respectively) were obtained. Bone mineral density (BMD) of the vertebral body was assessed by lateral dual energy X‐ray absorptiometry (DXA), and 3D trabecular microarchitecture and anterior cortical thickness and curvature was assessed by micro‐computed tomography (µCT). Then compressive stiffness, work to failure, and failure load were measured on the whole vertebral body. BMD was correlated with compressive stiffness (r = 0.60), failure load (r = 0.70), and work to failure (r = 0.55). Except for the degree of anisotropy, all trabecular and cortical parameters were correlated with mechanical behavior (r = 0.36 to 0.58, p = .05 to .001, and r = 0.36 to 0.61, p = .05 to .0001, respectively). Stepwise and multiple regression analyses indicated that the best predictor of (1) failure load was the combination of BMD, structural model index (SMI), and trabecular thickness (Tb.Th) (R = 0.80), (2) stiffness was the combination of BMD, Tb.Th, and curvature of the anterior cortex (R = 0.82), and (3) work to failure was the combination of anterior cortical thickness and BMD (R = 0.68). Our data imply that measurements of cortical thickness and curvature may enhance prediction of vertebral fragility and that therapies that improve both vertebral cortical and trabecular bone properties may provide a greater reduction in fracture risk. © 2010 American Society for Bone and Mineral Research     

Deterioration of trabecular plate-rod and cortical microarchitecture and reduced bone stiffness at distal radius and tibia in postmenopausal women with vertebral fractures

Postmenopausal women with vertebral fractures have abnormal bone microarchitecture at the distal radius and tibia by HR-pQCT, independent of areal BMD. However, whether trabecular plate and rod microarchitecture is altered in women with vertebral fractures is unknown. This study aims to characterize the abnormalities of trabecular plate and rod microarchitecture, cortex, and bone stiffness in postmenopausal women with vertebral fractures. HR-pQCT images of distal radius and tibia were acquired from 45 women with vertebral fractures and 45 control subjects without fractures. Trabecular and cortical compartments were separated by an automatic segmentation algorithm and subjected to individual trabecula segmentation (ITS) analysis for measuring trabecular plate and rod morphology and cortical bone evaluation for measuring cortical thickness and porosity, respectively. Whole bone and trabecular bone stiffness were estimated by finite element analysis. Fracture and control subjects did not differ according to age, race, body mass index, osteoporosis risk factors, or medication use. Women with vertebral fractures had thinner cortices, and larger trabecular area compared to the control group. By ITS analysis, fracture subjects had fewer trabecular plates, less axially aligned trabeculae and less trabecular connectivity at both the radius and the tibia. Fewer trabecular rods were observed at the radius. Whole bone stiffness and trabecular bone stiffness were 18% and 22% lower in women with vertebral fractures at the radius, and 19% and 16% lower at the tibia, compared with controls. The estimated failure load of the radius and tibia were also reduced in the fracture subjects by 13% and 14%, respectively. In summary, postmenopausal women with vertebral fractures had both trabecular and cortical microstructural deterioration at the peripheral skeleton, with a preferential loss of trabecular plates and cortical thinning. These microstructural deficits translated into lower whole bone and trabecular bone stiffness at the radius and tibia. Our results suggest that abnormalities in trabecular plate and rod microstructure may be important mechanisms of vertebral fracture in postmenopausal women.     

In vivo diffuse damage in human vertebral trabecular bone

Accumulation of microdamage in vivo may lead to loss of bone quality. Until recently, linear microcracks were the only known form of in vivo microdamage, but through the use of confocal microscopy an additional level of damage (diffuse damage) has been identified. In this study, in vivo diffuse damage was characterized and quantified in human vertebral trabecular bone as a function of tissue morphology, age, race, gender, and previously quantified in vivo linear microcracks. Presence of diffuse damage in human vertebral tissue was confirmed and validated by simultaneous use of polarized, ultraviolet, and laser confocal microscopy. Diffuse damage was found to occur preferentially within trabecular packets rather than in interstitial bone (p < 0.05). It was consistently higher in men compared with women (p < 0.05), but was not different by race or age group. Diffuse damage did not correlate with linear microcracks, but both exhibited the same probability distribution in which the percentage of individuals having a particular amount of damage decreased exponentially as damage content increased. These findings suggest that diffuse damage accumulation and repair are governed by the same biological phenomena as microcracks, but diffuse damage contributes independently to the microdamage content of bone.     

Effects of intermittent administration of pamidronate on the mechanical properties of canine cortical and trabecular bone

The mechanical properties of cortical and trabecular bones from beagles treated with the bisphosphonate pamidronate (administered intermittently 1 week every month for 3 months, at a dosage of 0.45 μmol/kg/day) were assessed. The mechanical properties of cortical bone were measured by four-point bending tests on femoral quadrants, in order to measure their elastic modulus and ultimate stress. The structural properties of whole tibias were measured in torsion to determine the torsional stiffness and failure torque. The elastic modulus and maximum compressive stress of the trabecular bone samples were measured by compression tests of trabecular cores. Intermittent treatment with pamidronate did not change the pattern of mechanical properties that occurs naturally around the femur or the torsional stiffness and failure torque of the tibias. By contrast, pamidronate did significantly increase the modulus of elasticity (by 19%) and maximum compressive stress (by 33%) of vertically aligned cylindrical trabecular specimens taken from the vertebrae of the beagles.     

High‐resolution peripheral quantitative computed tomography can assess microstructural and mechanical properties of human distal tibial bone

High‐resolution peripheral quantitative computed tomography (HR‐pQCT) is a newly developed in vivo clinical imaging modality. It can assess the 3D microstructure of cortical and trabecular bone at the distal radius and tibia and is suitable as an input for microstructural finite element (µFE) analysis to evaluate bone's mechanical competence. In order for microstructural and image‐based µFE analyses to become standard clinical tools, validation with a current gold standard, namely, high‐resolution micro‐computed tomography (µCT), is required. Microstructural measurements of 19 human cadaveric distal tibiae were performed for the registered HR‐pQCT and µCT images, respectively. Next, whole bone stiffness, trabecular bone stiffness, and elastic moduli of cubic subvolumes of trabecular bone in both HR‐pQCT and µCT images were determined by µFE analysis. The standard HR‐pQCT patient protocol measurements, derived bone volume fraction (BV/TV^d), trabecular number (Tb.N*), trabecular thickness (Tb.Th), trabecular spacing (Tb.Sp), and cortical thickness (Ct.Th), as well as the voxel‐based direct measurements, BV/TV, Tb.N*, Tb.Th*, Tb.Sp*, Ct.Th, bone surface‐to‐volume ratio (BS/BV), structure model index (SMI), and connectivity density (Conn.D), correlated well with their respective gold standards, and both contributed to µFE‐predicted mechanical properties in either single or multiple linear regressions. The mechanical measurements, although overestimated by HR‐pQCT, correlated highly with their gold standards. Moreover, elastic moduli of cubic subvolumes of trabecular bone predicted whole bone or trabecular bone stiffness in distal tibia. We conclude that microstructural measurements and mechanical parameters of distal tibia can be efficiently derived from HR‐pQCT images and provide additional information regarding bone fragility. © 2010 American Society for Bone and Mineral Research     

Osteocytes and bone lining cells: Which are the best candidates for mechano-sensors in cancellous bone?

Previously, we have investigated the possible role of osteocytes as mechano-sensors, and mediators of bone turnover. It was found that the proposed regulatory mechanism produced morphologies of trabecular bone, under particular loading conditions, which were consistent with morphogenesis and adaptation as seen in reality. The main objective of this study was to descern whether lining cells or osteoblasts could possibly play a similar role as effectively with regard to their capacity for self-optimization of the trabecular architecture, in terms of a low apparent mass to stiffness ratio. For that purpose the earlier analyses with osteocytes as mechano-sensors, distributed throughout the bone, were repeated for mechano-sensors located at bone surfaces only. Compared to the osteocyte model, the surface cell remodelling algorithm was reluctant to change its architecture, which implies that it is less sensitive to changes in the loading pattern. This resulted in less efficient bone adaptation, which was reflected by a considerably higher relative mass for a similar apparent stiffness in the loading direction. In other words, more mass is needed to obtain an equally stiff structure, at the apparent level, with respect to the externally applied loads. Furthermore, stresses and strains at the tissue level vary across a much wider range, relative to the osteocyte model, where the higher incidence of elevated strains indicates an increased failure risk. Therefore, we conclude that mechanical information at the bone surface may not be sufficient to adequately regulate functional bone adaptation.     

Bone Microarchitecture Assessed by HR‐pQCT as Predictor of Fracture Risk in Postmenopausal Women: The OFELY Study

Several cross‐sectional studies have shown that impairment of bone microarchitecture contributes to skeletal fragility. The aim of this study was to prospectively investigate the prediction of fracture (Fx) by bone microarchitecture assessed by high‐resolution peripheral computed tomography (HR‐ pQCT) in postmenopausal women. We measured microarchitecture at the distal radius and tibia with HR‐pQCT in the OFELY study, in addition to areal BMD with dual‐energy X‐ray absorptiometry (DXA) in 589 women, mean ± SD age 68 ± 9 years. During a median [IQ] 9.4 [1.0] years of follow‐up, 135 women sustained an incident fragility Fx, including 81 women with a major osteoporotic Fx (MOP Fx). After adjustment for age, women who sustained Fx had significantly lower total and trabecular volumetric densities (vBMD) at both sites, cortical parameters (area and thickness at the radius, vBMD at the tibia), trabecular number (Tb.N), connectivity density (Conn.D), stiffness, and estimated failure load at both sites, compared with control women. After adjustment for age, current smoking, falls, prior Fx, use of osteoporosis‐related drugs, and total hip BMD, each quartile decrease of several baseline values of bone microarchitecture at the radius was associated with significant change of the risk of Fx (HR of 1.39 for Tb.BMD [p = 0.001], 1.32 for Tb.N [p = 0.01], 0.76 for Tb.Sp.SD [p = 0.01], 1.49 [p = 0.01] for Conn.D, and 1.27 for stiffness [p = 0.02]). At the tibia, the association remained significant for stiffness and failure load in the multivariate model for all fragility Fx and for Tt.BMD, stiffness, and failure load for MOP Fx. We conclude that impairment of bone microarchitecture—essentially in the trabecular compartment of the radius—predict the occurrence of incident fracture in postmenopausal women. This assessment may play an important role in identifying women at high risk of fracture who could not be adequately detected by BMD measurement alone, to benefit from a therapeutic intervention. © 2017 American Society for Bone and Mineral Research.     

Contribution of the advanced glycation end product pentosidine and of maturation of type I collagen to compressive biomechanical properties of human lumbar vertebrae

Collagen characteristics contribute to bone biomechanical properties. Yet, few studies have analyzed the independent contributions of bone mineral density (BMD) and post-translational modifications of type I collagen to whole bone strength. Thus, the aim of this study was to determine the relative contributions of BMD and both enzymatic and non-enzymatic collagen crosslink concentration to the biomechanical properties of human vertebrae. Nineteen L3 vertebrae were collected after necropsy (age 26-93; 10 males, 9 females). BMD of the vertebral body was measured by DXA, and the vertebrae were compressed to failure to assess the stiffness, failure load and work to fracture. After mechanical testing, the concentration of both enzymatic crosslinks pyridinoline (PYD), and deoxypyridinoline (DPD) as well as, and the non-enzymatic crosslinks pentosidine (PEN) were analyzed in trabecular and cortical bone by reversed-phase HPLC. The extent of aspartic acid isomerization of type I collagen C telopeptide (CTX) was evaluated by ELISA of native (alpha CTX) and isomerized (beta CTX) forms. BMD was significantly positively related with stiffness (R(2) = 0.74; P < 0.0001), failure load (R(2) = 0.69; P < 0.0001) and work to fracture (R(2) = 0.44; P = 0.002). Bivariate regression analysis showed no association between collagen traits and biomechanical properties. However, in a multiple regression model, BMD and trabecular PEN were both significantly associated with failure load and work to fracture (multiple R(2) = 0.83, P = 0.001 and R(2) = 0.67, P = 0.001, respectively). Similarly, BMD and trabecular alpha/beta CTX ratio were both associated with stiffness (multiple R(2) = 0.83, P = 0.015). These findings indicate that post-translational modifications of type I collagen have an impact on skeletal fragility.     

Volumetric quantitative computed tomography of the proximal femur: relationships linking geometric and densitometric variables to bone strength. Role for compact bone

Introduction In assessing cervical fractures of the proximal femur, this in vitro quantitative computed tomography (QCT) study had three objectives: to compare QCT to dual-energy X-ray absorptiometry (DXA) for predicting the failure load of the proximal femur, to compare the contributions of density and geometry to bone failure load, and to compare the contributions of cortical and trabecular bone to bone failure load. A novel three-dimensional (3D) analysis tool [medical image analysis framework (MIAF-Femur)] was used to analyze QCT scans. Methods The proximal ends of 28 excised femurs were studied (1) using QCT to separately measure bone mineral density (BMD) and geometric variables of trabecular and cortical bone, (2) using mechanical tests to failure in a stance configuration, and (3) using DXA to measure BMD. The variables were described with mean, standard deviation, and range. Correlation matrix and multivariate linear models were computed. Results Among correlations, cortical thicknesses of the femoral neck were significantly correlated with femoral failure load, especially of the inferoanterior quadrant (r ²=0.41; p<0.001), as was cortical volume at the “extended neck“ (r ²=0.41; p<0.001). Femoral failure load variance was best explained by a combination of QCT variables. Combining densitometric and geometric variables measured by QCT explained 76% of femoral failure load variance compared with 69% with the DXA model. Geometric variables (measured by QCT) explained 43% of femoral failure load variance compared with 72% for densitometric variables (measured by QCT). A model including only trabecular variables explained 52% of femoral failure load variance compared with 59% for a model including only cortical variables. Conclusion The QCT-MIAF reported here provides analysis of both geometric and densitometric variables characterizing cortical and trabecular bone. Confirmation of our results in an independent sample would suggest that QCT may better explain failure load variance for cervical fracture than the gold standard DXA-provided BMD. This work was supported in part by grants from EU, contract number: QLK6-CT-2002-02440-3DQCT     

Intratrabecular distribution of tissue stiffness and mineralization in developing trabecular bone

The purpose of this study was to investigate the relation between bone tissue stiffness and degree of mineralization distribution and to examine possible changes during prenatal development. Understanding this may provide insight into adaptation processes and into deformation mechanisms of the bone microstructure. Mandibular condyles from four fetal and newborn pigs were used. Tissue stiffness was measured using nanoindentation, the degree of mineralization with microCT. Eight indents were made over the trabecular width of 15 trabeculae in each specimen, leading to a total of 960 indents. Subsequently, the degree of mineralization of these locations was determined. Intratrabecular variations in bone tissue stiffness and degree of mineralization showed a similar pattern; low at trabecular surfaces and higher in the cores. A strong correlation was found between the two variables, which remained unchanged during development. It was concluded that bone tissue in fetal and newborn trabecular cores resembles adult trabecular bone tissue properties and is distributed in a regular radial pattern in trabeculae. For the first time, it was shown that the intratrabecular tissue stiffness develops along the same path as the degree of mineralization. Knowledge regarding intratrabecular tissue stiffness and mineralization results in a better understanding of trabecular bone mechanical behavior on a structural and tissue level.     

Relative roles of microdamage and microfracture in the mechanical behavior of trabecular bone.

Compared to trabecular microfracture, the biomechanical consequences of the morphologically more subtle trabecular microdamage are unclear but potentially important because of its higher incidence. A generic three-dimensional finite element model of the trabecular bone microstructure was used to investigate the relative biomechanical roles of these damage categories on reloading elastic modulus after simulated overloads to various strain levels. Microfractures of individual trabeculae were modeled using a maximum fracture strain criterion, for three values of fracture strain (2%, 8%, and 35%). Microdamage within the trabeculae was modeled using a strain-based modulus reduction rule based on cortical bone behavior. When combining the effects of both microdamage and microfracture, the model predicted reductions in apparent modulus upon reloading of over 60% at an applied apparent strain of 2%, in excellent agreement with previously reported experimental data. According to the model, up to 80% of the trabeculae developed microdamage at 2% apparent strain, and between 2% and 10% of the trabeculae were fractured, depending on which fracture strain was assumed. If microdamage could not occur but microfracture could, good agreement with the experimental data only resulted if the trabecular hard tissue had a fracture strain of 2%. However, a high number of fractures (10% of the trabeculae) would need to occur for this case, and this has not been observed in published damage morphology studies. We conclude therefore that if the damage behavior of trabecular hard tissue is similar to that of cortical bone, then extensive microdamage is primarily responsible for the large loss in apparent mechanical properties that can occur with overloading of trabecular bone.     

Creep dominates tensile fatigue damage of the cement-bone interface.

Fatigue damage from activities of daily living has been considered to be a major cause of aseptic loosening in cemented total hip arthroplasty. The cement-bone interface is one region where loosening could occur, but to date the fatigue response of the interface has not been examined. Cement-bone specimens were prepared from fresh frozen human cadaver tissue using simulated in vivo conditions. Tensile fatigue tests to failure were performed in an environmental chamber. Loss of specimen stiffness (stiffness damage) and permanent displacement after unloading (creep damage) were found in all specimens. At failure, creep damage accounted for the majority (79.9+/-10.6%) of the total strain damage accumulation at failure (apparent strain, epsilon=0.0114+/-0.00488). A power law relationship between strain-damage rate and time-to-failure showed that the strain-damage rate was an excellent predictor of the fatigue life of the cement-bone interface. The S-N response of the interface was obtained as a function of the applied stress ratio and the initial apparent strain. The total motion between cement and bone (72.2+/-29.8 microm) prior to incipient failure due to both stiffness and creep fatigue damage may be sufficient to result in fibrous tissue formation and contribute to eventual clinical loosening.