Post-yield Mechanical Properties Research Articles

Water bound to hydroxyproline is correlated with post-yield mechanical properties of human cortical bone

Water bound to hydroxyproline is correlated with post-yield mechanical properties of human cortical bone

Post-yield mechanical properties of bovine trabecular bone – Relationships with bone volume fraction and strain rate

Bone finite element models, which are increasingly used to assess fracture risk in patients with degenerative bone diseases, require extensive experimental data to adequately model the material properties being assigned. Previous studies have demonstrated that bone elastic mechanical properties are dependent on density as well as strain rate, however, the effect these same variables on the post-yield behaviour of trabecular bone has not been thoroughly studied. For this study, bovine femoral samples were tested at several strain rates using three different tests devices: a conventional constant strain rate apparatus for low strain rates, a Drop Weight Impact Test device for the intermediate regime and a Split Hopkinson Pressure Bar System with magnesium bar and a PVCA pulse shaper for high strain rates. Ultimate and plateau stress, as well as absorbed energy were recorded. The results of this experimentation show that post-yield mechanical properties of trabecular bone are strain rate and volume fraction dependant. Two constitutive material models describing the Ultimate strength, Plateau stress and absorbed energy are proposed based on published models valid for elastic properties of trabecular bone.

Effect of carbon black distribution on the properties of polyethylene pipes - Part 1: Degradation of post yield mechanical properties and fracture surface analyses

In this study, we investigated the effect of carbon black distribution on the degradation of mechanical properties of high-density polyethylene in the form of plastic pipes used in water distribution networks. Polyethylene pipes with similar carbon black concentrations but different carbon black distributions were produced with industrial scale compounding and extrusion equipment. Tensile specimens were directly prepared from extruded pipe samples and elongated to fracture at different strain rates. Carbon black distributions of bulk samples and fracture surfaces were investigated using stereo and scanning electron microscopy (SEM). It was found that the carbon black distributions, fracture surfaces and fracture modes were significantly different in these pipes. Although the yield properties were similar, the post-yield properties of samples were significantly different, dramatically decreasing with the increasing inhomogeneity of carbon black distribution. Pipes with a certain level of heterogeneity in the carbon black distribution showed ductile and brittle fractures in the same fracture plane, whereas homogenous black and natural polyethylene (without carbon black) showed ductile fractures only.

The effect of ribose pre-treatment of cortical bone on γ-irradiation sterilization effectiveness.

Reconstruction of large skeletal defects is a significant and challenging issue. Tissue banks often use γ-irradiation (15-35kGy) to sterilize bone allografts, which, however, drastically impairs the post-yield mechanical properties. In previous studies, we reported the development of a method that protects human bone collagen connectivity through ribose crosslinking while still undergoing γ-irradiation. Given these promising results, the next step was to determine if the presence of ribose within the bone tissue would interfere with the effectiveness of the γ-irradiation sterilization against bacteria. This study had two stages. The aim of the first stage was to assess the protective effect of ribose in solution using a Bacillus pumilus spore strip model. The aim of the second stage was to assess the protective effect of ribose (R) on spores within a human cortical bone model in comparison to conventionally irradiated bone (I). Treatment of B. pumilus spore strips with ribose in solution led to temperature-dependent effects on spore viability versus spore strips treated with PBS alone. Ribose solution at 60°C led to a notable two logs decrease in spore count relative to PBS at 60°C. In the human bone model, the number of spores in the I and R groups were greatly decreased in comparison to the non-irradiated N group. No spore colonies were detected in the R group (n=4) whereas two of the four plates of group I formed colonies. This study provides evidence that the method of pre-treating bone with ribose crosslinking prior to irradiation sterilization, while improving irradiation sterilized bone allograft quality, also may improve the effectiveness of the sterilization process.

Intermittent Parathyroid Hormone After Prolonged Alendronate Treatment Induces Substantial New Bone Formation and Increases Bone Tissue Heterogeneity in Ovariectomized Rats.

Postmenopausal osteoporosis is often treated with bisphosphonates (eg, alendronate, [ALN]), but oversuppression of bone turnover by long-term bisphosphonate treatment may decrease bone tissue heterogeneity. Thus, alternate treatment strategies after long-term bisphosphonates are of great clinical interest. The objective of the current study was to determine the effect of intermittent parathyroid hormone (PTH) following 12 weeks of ALN (a bisphosphonate) treatment in 6-month-old, ovariectomized (OVX) rats on bone microarchitecture, bone remodeling dynamics, and bone mechanical properties at multiple length scales. By using in vivo μCT and 3D in vivo dynamic bone histomorphometry techniques, we demonstrated the efficacy of PTH following ALN therapy for stimulating new bone formation, and increasing trabecular thickness and bone volume fraction. In healthy bone, resorption and formation are coupled and balanced to sustain bone mass. OVX results in resorption outpacing formation, and subsequent bone loss and reduction in bone tissue modulus and tissue heterogeneity. We showed that ALN treatment effectively reduced bone resorption activity and regained the balance with bone formation, preventing additional bone loss. However, ALN treatment also resulted in significant reductions in the heterogeneity of bone tissue mineral density and tissue modulus. On the other hand, PTH treatment was able to shift the bone remodeling balance in favor of formation, with or without a prior treatment with ALN. Moreover, by altering the tissue mineralization, PTH alleviated the reduction in heterogeneity of tissue material properties induced by prolonged ALN treatment. Furthermore, switching to PTH treatment from ALN improved bone's postyield mechanical properties at both the whole bone and apparent level compared to ALN alone. The current findings suggest that intermittent PTH treatment should be considered as a viable treatment option for patients with prior treatment with bisphosphonates. © 2017 American Society for Bone and Mineral Research.

Novel Raman Spectroscopic Biomarkers Indicate That Postyield Damage Denatures Bone's Collagen.

Raman spectroscopy has become a powerful tool in the assessment of bone quality. However, the use of Raman spectroscopy to assess collagen quality in bone is less established than mineral quality. Because postyield mechanical properties of bone are mostly determined by collagen rather than the mineral phase, it is essential to identify new spectroscopic biomarkers that help infer the status of collagen quality. Amide I and amide III bands are uniquely useful for collagen conformational analysis. Thus, the first aim of this work was to identify the regions of amide bands that are sensitive to thermally induced denaturation. Collagen sheets and bone were thermally denatured to identify spectral measures that change significantly following denaturation. The second aim was to assess whether mechanical damage denatures the collagen phase of bone, as reflected by the molecular spectroscopic biomarkers identified in the first aim. The third aim was to assess the correlation between these new spectroscopic biomarkers and postyield mechanical properties of cortical bone. Our results revealed five peaks whose intensities were sensitive to thermal and mechanical denaturation: ∼1245, ∼1270, and ∼1320 cm(-1) in the amide III band, and ∼1640 and ∼1670 cm(-1) in the amide I band. Four peak intensity ratios derived from these peaks were found to be sensitive to denaturation: 1670/1640, 1320/1454, 1245/1270, and 1245/1454. Among these four spectral biomarkers, only 1670/1640 displayed significant correlation with all postyield mechanical properties. The overall results showed that these peak intensity ratios can be used as novel spectroscopic biomarkers to assess collagen quality and integrity. The changes in these ratios with denaturation may reflect alterations in the collagen secondary structure, specifically a transition from ordered to less-ordered structure. The overall results clearly demonstrate that this new spectral information, specifically the ratio of 1670/1640, can be used to understand the involvement of collagen quality in the fragility of bone. © 2015 American Society for Bone and Mineral Research.

Raman spectral classification of mineral- and collagen-bound water's associations to elastic and post-yield mechanical properties of cortical bone

Water that is bound to bone's matrix is implied as a predictor of fracture resistance; however, bound water is an elusive variable to be measured nondestructively. To date, the only nondestructive method used for studying bone hydration status is magnetic resonance variants (NMR or MRI). For the first time, bone hydration status was studied by short-wave infrared (SWIR) Raman spectroscopy to investigate associations of mineral-bound and collagen-bound water compartments with mechanical properties. Thirty cortical bone samples were used for quantitative Raman-based water analysis, gravimetric analysis, porosity measurement, and biomechanical testing. A sequential dehydration protocol was developed to replace unbound (heat drying) and bound (ethanol treatment) water in bone. Raman spectra were collected serially to track the OH-stretch band during dehydration. Four previously identified peaks were investigated: I3220/I2949, I3325/I2949 and I3453/I2949 reflect status of organic-matrix related water (mostly collagen-related water) compartments and collagen portion of bone while I3584/I2949 reflects status of mineral-related water compartments and mineral portion of bone. These spectroscopic biomarkers were correlated with elastic and post-yield mechanical properties of bone. Collagen–water related biomarkers (I3220/I2949 and I3325/I2949) correlated significantly and positively with toughness (R2=0.81 and R2=0.79; p<0.001) and post-yield toughness (R2=0.65 and R2=0.73; p<0.001). Mineral-water related biomarker correlated significantly and negatively with elastic modulus (R2=0.78; p<0.001) and positively with strength (R2=0.46; p<0.001). While MR-based techniques have been useful in measuring unbound and bound water, this is the first study which probed bound-water compartments differentially for collagen and mineral-bound water. For the first time, we showed an evidence for contributions of different bound-water compartments to mechanical properties of wet bone and the reported correlations of Raman-based water measurements to mechanical properties underline the necessity for enabling approaches to assess these new biomarkers noninvasively in vivo to improve the current diagnosis of those who may be at risk of bone fracture due to aging and diseases.

Within subject heterogeneity in tissue-level post-yield mechanical and material properties in human trabecular bone

The ability to determine patient-specific mechanical properties of trabecular bone is needed for a reliable estimation of fracture risks. Tissue mechanics and material composition are important factors that contribute to trabecular bone performance, but only a few studies have investigated the post-yield behaviour of human trabecular bone, and limited knowledge for modelling is available about ultimate properties needed.Aim of this paper was to investigate absolute values and deviation of mechanical and material properties of human trabecular bone at the tissue level, in a healthy and osteoporotic donor. A combination of tensile and bending tests of single trabeculae up to failure, μCT measurement of sample geometry and finite element analysis were incorporated to determine mechanical properties. The samples were analysed with Raman spectroscopy to evaluate the material composition. High within-subject variability was found, for both the healthy and osteoporotic donor. Nevertheless, the two donors could be separated by analysing the ultimate strain and post-yield work, as well as two of the material parameters (B-type carbonate substitution ratio and collagen cross-link ratio). It indicates that tissue level properties seem to be relevant also for macroscopic mechanical behaviour. These findings also suggest that the mechanical variability for the inelastic region at the tissue level may be associated with varying material properties, while until yielding occurs our data does not suggest any connection between the mechanical and the investigated material. Finally, a set of mechanical properties of human bone have been reported that are a relevant reference for computational studies and FE analysis.

Novel method to analyze post-yield mechanical properties at trabecular bone tissue level

Tissue level mechanics is a key factor to be investigated to improve the knowledge of how the overall trabecular structure reacts to loading and overloading. The aim of this study was to develop a new device for measuring the mechanical competence of single trabeculae in the post-yield region for both tensile and bending tests, characterized by high accuracy and precision, and to assess the effect of testing mode, donor age and material composition. A novel approach for measuring the displacement and deformation was developed (accuracy error of 0.3% and a precision of 2.7%). A total of 30 samples from two bovine femora of different ages (from <3-year-old and 14-year-old cows) were tested in tension or bending, while average material properties have been acquired by means of Raman spectroscopy. A group of trabeculae was tested in bending after treatment for collagen degradation. As a result, a complete set of post-yield properties has been reported. The results highlight significant differences between tensile and bending groups, with higher values for the bending test mode for yield strain, ultimate strain and post-yield work and lower for the elastic modulus. Significant higher values were found for the old donor (differences in the range of 30–60%) for elastic modulus, yield stress and ultimate stress as well as for material properties measured by Raman spectroscopy. We quantified that changes in materials properties induced by collagen degradation corresponded to a substantial decrease (up to 120% for post-yield work) of mechanical competence, both in the elastic and inelastic region.

Heterogeneous Glycation of Cancellous Bone and Its Association with Bone Quality and Fragility

Non-enzymatic glycation (NEG) and enzymatic biochemical processes create crosslinks that modify the extracellular matrix (ECM) and affect the turnover of bone tissue. Because NEG affects turnover and turnover at the local level affects microarchitecture and formation and removal of microdamage, we hypothesized that NEG in cancellous bone is heterogeneous and accounts partly for the contribution of microarchitecture and microdamage on bone fragility. Human trabecular bone cores from 23 donors were subjected to compression tests. Mechanically tested cores as well as an additional 19 cores were stained with lead-uranyl acetate and imaged to determine microarchitecture and measure microdamage. Post-yield mechanical properties were measured and damaged trabeculae were extracted from a subset of specimens and characterized for the morphology of induced microdamage. Tested specimens and extracted trabeculae were quantified for enzymatic and non-enzymatic crosslink content using a colorimetric assay and Ultra-high Performance Liquid Chromatography (UPLC). Results show that an increase in enzymatic crosslinks was beneficial for bone where they were associated with increased toughness and decreased microdamage. Conversely, bone with increased NEG required less strain to reach failure and were less tough. NEG heterogeneously modified trabecular microarchitecture where high amounts of NEG crosslinks were found in trabecular rods and with the mechanically deleterious form of microdamage (linear microcracks). The extent of NEG in tibial cancellous bone was the dominant predictor of bone fragility and was associated with changes in microarchitecture and microdamage.

Fabric-mechanical property relationships of trabecular bone allografts are altered by supercritical CO2 treatment and gamma sterilization

Tissue grafts are implanted in orthopedic surgery every day. In order to minimize infection risk, bone allografts are often delipidated with supercritical CO₂ and sterilized prior to implantation. This treatment may, however, impair the mechanical behavior of the bone graft tissue. The goal of this study was to determine clinically relevant mechanical properties of treated/sterilized human trabecular bone grafts, e.g. the apparent modulus, strength, and the ability to absorb energy during compaction. They were compared with results of identical experiments performed previously on untreated/fresh frozen human trabecular bone from the same anatomical site (Charlebois, 2008). We tested the hypothesis that the morphology-mechanical property relationships of treated cancellous allografts are similar to those of fresh untreated bone. The morphology of the allografts was determined by μCT. Subsequently, cylindrical samples were tested in unconfined and confined compression. To account for various morphologies, the experimental data was fitted to phenomenological mechanical models for elasticity, strength, and dissipated energy density based on bone volume fraction (BV/TV) and the fabric tensor determined by MIL. The treatment/sterilization process does not appear to influence bone graft stiffness. However, strength and energy dissipation of the bone grafts were found to be significantly reduced by 36% to 47% and 66% to 81%, respectively, for a broad range of volume fraction (0.14<BV/TV<0.39) and degree of anisotropy (1.24<DA<2.18). Since the latter properties are strongly dominated by BV/TV, the clinical consequences of this reduction can be compensated by using grafts with lower porosity. The data of this study suggests that an increase of 5-10% in BV/TV is sufficient to compensate for the reduced post-yield mechanical properties of treated/sterilized bone in monotonic compression. In applications where graft stiffness needs to be matched and strength is not a concern, treated allograft with the same BV/TV as an appropriate fresh bone graft may be used.

Effects of three different preservation methods on the mechanical properties of human and bovine cortical bone

Background and purpose In the development of new strategies for fracture fixation, new methods have to be tested biomechanically under in vitro conditions before clinical trials can be performed. The gold standard for laboratory evaluations is fresh-frozen specimen. As the availability of fresh-frozen specimens is limited and since their use bears infectious risks, specimens treated with various chemical embalming fluids are also used. These preservation methods may alter the mechanical properties of the specimens used. Therefore, the aims of the present study were to determine the effects of three different preservation methods (formalin fixation (FO), Thiel-fixation (TH), and alcohol–glycerine fixation (AG)) on the elastic and postyield mechanical properties of cortical bone and to compare these properties to those of fresh-frozen (FF) specimens. Materials and methods Cylindrical cortical specimens (diameter 3 mm, length 60 mm) were obtained from human femurs ( n = 48) and bovine tibiae ( n = 40). Before specimen immersion in different fixation fluids, bone mineral density (BMD) as well as the initial Young's modulus was determined. The Young's modulus was determined in a nondestructive bending test, and measurements were repeated after 6 months of immersion in fixative solution. Subsequent to the nondestructive test, a destructive 3-point bending test was conducted to assess the postyield and fracture properties. Results The BMD as well as the initial Young's modulus showed no significant differences between the four test groups. After 6 months in fixative solution, the Young's modulus was significantly lowered in human Thiel specimens and only showed minor changes in formalin- and alcohol–glycerine-treated specimens. The plastic energy absorption of human and bovine specimens was altered significantly. Formalin as well as alcohol–glycerine fixation yielded a significant decrease in plastic energy absorption, whereas Thiel fixation significantly increased the plastic energy absorption. Discussion/conclusion Because of the significantly altered plastic mechanical properties of cortical bone, the use fresh-frozen bone specimens is recommended in biomechanical studies investigating failure loads of orthopaedic implants. The use of embalmed specimens should be restricted to pilot tests.

Short-Term Exercise in Mice Increases Tibial Post-Yield Mechanical Properties While Two Weeks of Latency Following Exercise Increases Tissue-Level Strength

We have previously shown that exercise during growth increases post-yield deformation in C57BL6/129 (B6;129) male tibiae at the expense of reduced pre-yield deformation and structural and tissue strength. Other research in the literature indicates that increased mineral content, cross-sectional geometry and structural strength due to exercise can be maintained or increased after exercise ends for as long as 14 weeks. It was therefore hypothesized that after our exercise protocol ended, effects of exercise on mechanical properties would persist, resulting in increased post-yield behavior and rescued strength versus age-matched control mice. Beginning at 8 weeks of age, exercise consisted of running on a treadmill (30 min/day, 12 m/min, 5 degrees incline) for 21 consecutive days. At the end of running and 2 weeks later, in the cortical bone of the tibial mid-diaphyses of B6;129 male mice, changes due to exercise and latency following exercise were assayed by mechanical tests and analyses of cross-sectional geometry. Exercise increased structural post-yield deformation compared with weight-matched control mice, without changes in bone size or shape, suggesting that exercised-induced changes in pre-existing bone quality were responsible. Over the 2-week latency period, no growth-related changes were noted in control mice, but exercise-induced changes resulted in increased tissue stiffness and strength versus mice sacrificed immediately after exercise ended. Our data indicate that periods of exercise followed by latency can alter strength, stiffness, and ductility of bone independent of changes in size or shape, suggesting that exercise may be a practical way to increase the quality of the bone extracellular matrix.

Grain size, strain rate, and temperature dependence of flow stress in ultra-fine grained and nanocrystalline Cu and Al: Synthesis, experiment, and constitutive modeling

For the first time, high quality bulk nanocrystalline (nc) fcc metals, with least amounts of imperfections, exhibiting high strength and ductility at room and different temperatures, under quasi-static and dynamic types of loading, were prepared and a comprehensive study on their post-yield mechanical properties was performed. This investigation included study of the effect of temperature on stress–strain responses of mechanically milled bulk nc Cu and Al. The samples after preparation through mechanical milling and consolidation processes were subjected to uniaxial compressive loading at quasi-static and dynamic strain rates of 10 −2 s −1 and 1840–3105 s −1, respectively, at temperatures ranging from 223 to 523 K. In both materials strong dependency of flow stress to temperature was observed; this dependency was rather more pronounced when the materials were tested at the quasi-static strain rate. Further, a new grain size and temperature dependent viscoplastic phenomenological constitutive equation, Khan–Liang–Farrokh (KLF) model was developed based on the Khan–Huang–Liang (KHL) constitutive equation. The model was featured to correlate different characteristic behaviors of polycrystalline materials in the plastic regime, as the result of grain refinement. In addition, the viscoplastic responses of bulk Cu and Al of different grain sizes (from sub-micron to nanometer range), and those from bulk nc Cu and Al at different strain rates (quasi-static to dynamic), recently published ( Khan et al., 2008a, 2008b), were simulated using the newly developed equation. The results confirmed reasonable capability of the developed model to correlate a wide spectrum of the viscoplastic responses of these fcc metals.

Proline and Processing of Spider Silks

Major ampullate (MAA) silks from a variety of spider species were collected by artificial silking that adjusted the samples to have similar breaking strains. Those silks are highly comparable in post-yield mechanical properties, but their supercontraction behaviors and initial moduli vary in large ranges and both correlate with the content of one amino acid, proline. These relationships, in combination with protein sequence data, support the hypothesis that the proline-related motif, that is, GPGXX, may play a key role in silk. This also explains the interspecific variability of spider dragline silk. Moreover, MAA silks from three representative species were prepared in a range of processing conditions and their mechanical properties were compared. Our results indicate how chemical compositions, coupled with processing conditions, shape the mechanical properties of the spider silk.

Apparent viscoelastic anisotropy as measured from nondestructive oscillatory tests can reflect the presence of a flaw in cortical bone.

There is evidence that damage, viscoelastic stiffness properties, and postyield mechanical properties are related in bone tissue. Our objective was to test whether presence of a flaw would have an influence on the apparent viscoelastic properties of bone. Examining the effect of flaw orientation on apparent viscoelastic properties and utilization of dynamic mechanical analysis (DMA) as a nondestructive means for detection of damage were our secondary objectives. Cortical bone beams (2 x 2 x 19 mm) machined from the cranial cortex of the radii of six Warhill sheep were used. The specimens were placed in a DMA machine and baseline measurements of storage modulus (E1) and loss factor (tan delta), once for loads in the craniocaudal and once in the mediolateral directions, were performed using a three-point bending configuration for a frequency range of 1-10 Hz. Craniocaudal/mediolateral measurement ratio was calculated as a measure of anisotropy for tan delta and E1. After cutting a thin through-thickness macroscopic notch on the caudal surface at the center of each beam, oscillatory tests were repeated. Two-way repeated measures analysis of variance followed by Tukey's test was used with group (craniocaudal, mediolateral, notched craniocaudal, and notched mediolateral measurements) and frequency as factors. Regression analysis and analysis of covariance were used for examining the relationship between viscoelastic parameters and frequency. Tan delta and E1 were not different between craniocaudal and mediolateral measurements before the flaw was introduced (p > 0.8 and p = 1, respectively). In the presence of the flaw, tan delta was significantly increased (p < 0.003) whereas E1 was significantly reduced (p < 0.001) for craniocaudal measurements. Tan delta and E1 were nearly isotropic in the tested directions before the introduction of a flaw into the bone tissue. Introduction of a flaw resulted in increased tan delta and E1 anisotropy. Presence of a notch resulted in a significant increase in tan delta anisotropy with increasing frequency. In conclusion, we have demonstrated that cortical bone tissue exhibits a different apparent viscoelastic behavior in the presence of a flaw and depending on the flaw's orientation. Our finding that the presence of a notch and its orientation can be detected by nondestructive DMA suggests that in vivo techniques may be developed for detection of cortical bone damage.

Stiff and strong compressive properties are associated with brittle post-yield behavior in equine compact bone material

Our hypothesis was that post-yield mechanical behavior of compact bone material in compression, defined as the stress, strain, or energy absorbed between 0.2% strain-offset and the point of maximum stress, is correlated with material density, modulus, strength, histomorphometric evidence of remodeling, and post-failure gross specimen morphology. Post-yield behavior of compact bone material from the third metacarpal bone of 10 horses, ages 5 months to 20 years, was investigated using single-load compression-to-failure. The post-yield stress, strain, and absorbed energy were compared with the compressive elastic modulus, yield stress, ash density, post-failure macroscopic appearance of the specimen, and histologic evidence of remodeling. High values of elastic modulus, yield stress, and ash density were associated with low values of post-yield mechanical properties (stress, strain, and absorbed energy). Macroscopic post-failure morphology was associated with post-yield mechanical behavior, in that specimens displaying fractures were associated with lower post-yield mechanical properties, and that those without evidence of frank fracture were associated with higher post-yield mechanical properties. Microscopic evidence of remodeling activity was associated with high post-yield mechanical properties, but not with gross post-failure morphology. There was an abrupt change from relatively high values to extremely low values of post-yield mechanical properties at intermediate levels of ash density. This feature may serve as a functional upper limit to the maximization of bone material stiffness and strength.

The effect of hole diameter on the torsional mechanical properties of the equine third metacarpal bone.

The torsional monotonic structural material properties of equine metacarpi with or without, either a 5/16 inch or 3/8 inch diameter bicortical lateromedial middiaphyseal hole were assessed to determine the effect of a hole on metacarpal strength. Torsional stiffness was not significantly effected by the presence of a bicortical hole, whereas yield and failure angles, torques and energies of metacarpi with a hole were 51% to 97% of those of intact bones. Significant differences were not apparent for yield and failure mechanical properties between metacarpi with a 5/16 inch diameter hole and metacarpi with a 3/8 inch diameter hole; however, postyield mechanical properties were lower for metacarpi with a 3/8 inch hole. Whereas some metacarpi with a 5/16 inch hole were capable of plastic deformation before failure, metacarpi with a 3/8 inch diameter hole appeared to have sufficient stress concentration to propagate complete fracture on structural yield.