Micro-hardness measurements on single haversian systems in bone

Validity of the use of porcine bone in forensic cut mark studies

Porcine bone is often used as a substitute for human bone in forensic trauma studies, but little has been published on its comparative mechanical behavior. The factors affecting mechanical properties and therefore selection of bone models are complex and include the age of the animal at death, and physiological loading conditions, the latter being of particular relevance when using a quadrupedal animal as a human substitute. The regional variation in hardness of adult and infant porcine bones was investigated using Vickers' indentation tests and compared to published data for human limb bones to relate differences to inherent genetic effects and loading influences, and to examine the validity of the porcine-human model. Significant differences in hardness were observed both along and around the adult porcine humerus and femur, but no significant differences were found along the length of the infant bones. Significant differences were found between the forelimb and hindlimb, but only in the infant specimens. The hardness values for porcine adult cortical bone from the femur (52.23 ± 1.00 kg mm^-2 ) were comparable to those reported in the literature for adult human cortical bone from the fibula, ilium, and calcaneus. These data will help inform subject selection in terms of both species and bone type for use in future trauma studies.

Respective roles of organic and mineral components of human cortical bone matrix in micromechanical behavior: an instrumented indentation study

Bone is a multiscale composite material made of both a type I collagen matrix and a poorly crystalline apatite mineral phase. Due to remodeling activity, cortical bone is made of Bone Structural Units (BSUs) called osteons. Since osteon represents a fundamental level of structural hierarchy, it is important to investigate the relationship between mechanical behavior and tissue composition at this scale for a better understanding of the mechanisms of bone fragility. The aim of this study is to analyze the links between ultrastructural properties and the mechanical behavior of bone tissue at the scale of osteon. Iliac bone biopsies were taken from untreated postmenopausal osteoporotic women, embedded, sectioned and microradiographed to assess the degree of mineralization of bone (DMB). On each section, BSUs of known DMB were indented with relatively high load (~500 mN) to determine local elastic modulus (E), contact hardness (H(c)) and true hardness (H) of several bone lamellae. Crystallinity and collagen maturity were measured by Fourier Transform InfraRed Microspectroscopy (FTIRM) on the same BSUs. Inter-relationships between mechanical properties and ultrastructural components were analyzed using multiple regression analysis. This study showed that elastic deformation was only explained by DMB whereas plastic deformation was more correlated with collagen maturity. Contact hardness, reflecting both elastic and plastic behaviors, was correlated with both DMB and collagen maturity. No relationship was found between crystallinity and mechanical properties at the osteon level.

Material property variation of mandibular symphyseal bone in colobine monkeys

The anterior mandibular corpus of anthropoid primates is routinely subjected to masticatory loads that result in relatively high local levels of stress and strain. While structural morphological responses to these loads have been extensively explored, relatively little is known about material property variation in mandibular bone of nonhuman primates. Consequently, the role of regional and local variation in bone stiffness in conditioning stress and strain gradients is poorly understood. We sampled elastic modulus variation in the bone of the anterior mandibular corpus in two species (N = 3 each) of sympatric colobine monkeys, Procolobus badius and Colobus polykomos. These monkeys were chosen for comparison owing to their distinctive dietary regimens, as P. badius rarely includes hard objects in its diet while C. polykomos habitually processes obdurate items during feeding. Elastic modulus is determined through bone hardness data obtained via microindentation, which enables the description of stiffness variation on sub-millimeter scales. Labial bone stiffness exceeds that of lingual bone in the sample overall. Female mandibular bone is generally stiffer than that found in males, and overall Procolobus mandibular bone is stiffer than that in Colobus. These results, interpreted collectively, suggest that the material response to elevated masticatory stress is increased compliance of the affected bone.

The role of mineralization and organic matrix in the microhardness of bone tissue from controls and osteoporotic patients

Degree of mineralization of bone (DMB) is a major intrinsic determinant of bone strength at the tissue level but its contribution to the microhardness (Vickers indentation) at the intermediary level of organization of bone tissue, i.e., Bone Structural Units (BSUs), has never been assessed. The purpose of this study was to analyze the relationship between the microhardness, the DMB and the organic matrix, measured in BSUs from human iliac bone biopsies. Iliac bone samples from controls and osteoporotic patients (men and women), embedded in methyl methacrylate, were used. Using a Vickers indenter, microhardness (kg/mm2) was measured, either globally on surfaced blocks or focally on 100 microm-thick sections from bone samples (load of 25 g applied during 10 sec; CV=5%). The Vickers indenter was more suited than the Knoop indenter for a tissue like bone in which components are diversely oriented. Quantitative microradiography performed on 100 microm-thick sections, allowed measurement of parameters reflecting the DMB (g/cm3). Assessed on the whole bone sample, both microhardness and DMB were significantly lower (-10% and -7%, respectively) in osteoporotic patients versus controls (p<0.001). When measured separately at the BSU level, there were significant positive correlations between microhardness and DMB in controls (r2=0.36, p<0.0001) and osteoporotic patients (r2=0.43, p<0.0001). Mineralization is an important determinant of the microhardness, but did not explain all of its variance. To highlight the role of the organic matrix in bone quality, microhardness of both osteoid and adjacent calcified matrix were measured in iliac samples from subjects with osteomalacia. Microhardness of organic matrix is 3-fold lower than the microhardness of calcified tissue. In human calcanei, microhardness was significantly correlated with DMB (r2=0.33, p=0.02) and apparent Young's modulus (r2=0.26, p=0.03). In conclusion, bone microhardness measured by Vickers indentation is an interesting methodology for the evaluation of bone strength and its determinants at the BSU level. Bone microhardness is linked to Young's modulus of bone and is strongly correlated to mineralization, but the organic matrix accounts for about one third of its variance.

Mechanical properties of femoral cortical bone following cemented hip replacement

Femoral bone remodeling following total hip replacement is a big concern and has never been examined mechanically. In this study, six goats underwent unilateral cemented hip hemiarthroplasty with polymethyl methacrylate (PMMA) bone cement. Nine months later animals were sacrificed, and the femoral cortical bone slices at different levels were analysed using microhardness testing and microcomputed tomography (micro-CT) scanning. Implanted femurs were compared to contralateral nonimplanted femurs. Extensive bone remodeling was demonstrated at both the proximal and middle levels, but not at the distal level. Compared with the nonimplanted side, significant decreases were found in the implanted femur in cortical bone area, bone mineral density, and cortical bone hardness at the proximal level, as well as in bone mineral density and bone hardness at the middle level. However, no significant difference was observed in either variable for the distal level. In addition, similar proximal-to-distal gradient changes were revealed both in cortical bone microhardness and bone mineral density. From the mechanical point of view, the results of the present study suggested that stress shielding is an important mechanical factor associated with bone adaptation following total hip replacement.

Mechanical property determination of bone through nano- and micro-indentation testing and finite element simulation

Measurement of the mechanical properties of bone is important for estimating the stresses and strains exerted at the cellular level due to loading experienced on a macro-scale. Nano- and micro-mechanical properties of bone are also of interest to the pharmaceutical industry when drug therapies have intentional or non-intentional effects on bone mineral content and strength. The interactions that can occur between nano- and micro-indentation creep test condition parameters were considered in this study, and average hardness and elastic modulus were obtained as a function of indentation testing conditions (maximum load, load/unload rate, load-holding time, and indenter shape). The results suggest that bone reveals different mechanical properties when loading increases from the nano- to the micro-scale range (microN to N), which were measured using low- and high-load indentation testing systems. A four-parameter visco-elastic/plastic constitutive model was then applied to simulate the indentation load vs. depth response over both load ranges. Good agreement between the experimental data and finite element model was obtained when simulating the visco-elastic/plastic response of bone. The results highlight the complexity of bone as a biological tissue and the need to understand the impact of testing conditions on the measured results.

An application of nanoindentation technique to measure bone tissue Lamellae properties

Measuring the microscopic mechanical properties of bone tissue is important in support of understanding the etiology and pathogenesis of many bone diseases. Knowledge about these properties provides a context for estimating the local mechanical environment of bone related cells thait coordinate the adaptation to loads experienced at the whole organ level. The objective of this study was to determine the effects of experimental testing parameters on nanoindentation measures of lamellar-level bone mechanical properties. Specifically, we examined the effect of specimen preparation condition, indentation depth, repetitive loading, time delay, and displacement rate. The nanoindentation experiments produced measures of lamellar elastic moduli for human cortical bone (average value of 17.7 +/- 4.0 GPa for osteons and 19.3 +/- 4.7 GPa for interstitial bone tissue). In addition, the hardness measurements produced results consistent with data in the literature (average 0.52 +/- 0.15 GPa for osteons and 0.59 +/- 0.20 GPa for interstitial bone tissue). Consistent modulus values can be obtained from a 500-nm-deep indent. The results also indicated that the moduli and hardnesses of the dry specimens are significantly greater (22.6% and 56.9%, respectively) than those of the wet and wet and embedded specimens. The latter two groups were not different. The moduli obtained at a 5-nm/s loading rate were significantly lower than the values at the 10- and 20-nm/s loading rates while the 10- and 20-nm/s rates were not significantly different. The hardness measurements showed similar rate-dependent results. The preliminary results indicated that interstitial bone tissue has significantly higher modulus and hardness than osteonal bone tissue. In addition, a significant correlation between hardness and elastic modulus was observed.

Heterogeneity of bone lamellar-level elastic moduli

Advances in our ability to assess fracture risk, predict implant success, and evaluate new therapies for bone metabolic and remodeling disorders depend on our understanding of anatomically specific measures of local tissue mechanical properties near and surrounding bone cells. Using nanoindentation, we have quantified elastic modulus and hardness of human lamellar bone tissue as a function of tissue microstructures and anatomic location. Cortical and trabecular bone specimens were obtained from the femoral neck and diaphysis, distal radius, and fifth lumbar vertebra of ten male subjects (aged 40-85 years). Tissue was tested under moist conditions at room temperature to a maximum depth of 500 nm with a loading rate of 10 nm/sec. Diaphyseal tissue was found to have greater elastic modulus and hardness than metaphyseal tissues for all microstructures, whereas interstitial elastic modulus and hardness did not differ significantly between metaphyses. Trabecular bone varied across locations, with the femoral neck having greater lamellar-level elastic modulus and hardness than the distal radius, which had greater properties than the fifth lumbar vertebra. Osteonal, interstitial, and primary lamellar tissues of compact bone had greater elastic moduli and hardnesses than trabecular bone when comparing within an anatomic location. Only femoral neck interstitial tissue had a greater elastic modulus than its osteonal counterpart, which suggests that microstructural distinctions can vary with anatomical location and may reflect differences in the average tissue age of cortical bone or mineral and collagen organization.

Age, gender, and bone lamellae elastic moduli

To enhance preventative and therapeutic strategies for metabolic bone diseases and bone fragility disorders, we began to explore the physical properties of bone tissue at the cellular level. Proximal femurs were harvested from 27 cadavera (16 male and 11 female) for in vitro measurement of the mechanical properties. We measured the variations in lamellar-level elastic modulus and hardness in human bone as a function of age and gender to identify microstructural properties responsible for age and gender-related reductions in the mechanical integrity. The lateral femoral necks were examined, and age, gender, height, body mass, and body mass index were not found to correlate with lamellar-level elastic modulus or hardness. This result was consistent for osteonal, interstitial, and trabecular tissue. These data suggest that increased bone mass maintenance, known to occur in heavier individuals, is not accompanied by increases in the lamellar-level elastic modulus or hardness. The independence of elastic modulus and hardness from age and gender suggests that age and gender-related decreases in mechanical integrity do not involve alterations in elastic modulus or hardness of the extracellular matrix. Lamellar-level ultimate, fatigue, and fracture toughness properties should also be investigated. Other factors, such as tissue mass and organization, may also contribute to age and gender-related decreases in the mechanical integrity.

Microhardness and anisotropy of the vital osseous interface and endosseous implant supporting bone

Limited information is available on the mechanical properties of the rapidly remodeling bone that surrounds endosseous implants. Fifteen implant-bone blocks were obtained from the mid-femoral diaphyses of three mature male hounds 12 weeks after placement of the implants. To evaluate the microhardness and cortical anisotropy of bone, the implants were sectioned along their long axes. In this process, the femurs were sectioned transversely. Knoop microhardness measurements (HK) were made with a 50 g force on cortical bone and a 25 g force on periosteal callus, endocortical callus, and circumferential lamellar bone. The long diagonal of the indenter was placed parallel to the implant (in the radial bone direction). Measurements were made in cortical bone at 200, 400, 600, 800, 1,000, 1,500, 2,000, and 2,500 microm from both sides of the implant. To detect cortical anisotropy in the radial compared with the tangential direction, a second set of indentations was made perpendicular to the first. Microhardness of periosteal callus and endocortical callus and anisotropy of circumferential lamellar bone near the endocortical surfaces of the femur were also evaluated. Repeated measures analysis of variance showed significantly (p < 0.05) lower microhardness values (30.6 +/- 0.8 HK [mean +/- SEM]) for cortical bone at 200 microm than at any other location (range: 40.3-46.6 HK). Microhardness anisotropy was not detected in cortical bone. Furthermore, within 200 microm of the implant surface, the Knoop microhardness values were significantly lower for periosteal and endocortical calluses than for cortical bone. These data provide information about the mechanical properties of bone adjacent to endosseous implants at a microstructural level. The results are consistent with the high rate of remodeling seen adjacent to endosseous implants at 12 weeks after implantation.

Microstructure-microhardness relations in parallel-fibered and lamellar bone

Understanding the mechanical function of bone material in relation to its structure is a fascinating but very complicated problem to resolve. Part of the complexity arises from the hierarchical structural organization of bone. Microhardness measurements, initially on relatively simply structured parallel-fibered bone, show a marked anisotropy in three orthogonal directions. This may, in part, be due to the highly anisotropic structure of the basic building block of bone, the mineralized collagen fibril. Microhardness measurements made face-on to the layers of crystals and collagen triple helical molecules, show much lower values than those made edge-on to these layers. Microhardness measurements of the much more complex "rotated-plywood" structure of lamellar bone, reveal the well-known general tendency toward anisotropy in relation to the long axis of the bone. A detailed examination of microhardness-microstructure relations of lamellar bone, however, shows that only in certain orientations can microhardness values be related directly to a specific attribute of the lamellar structure. Clearly, the gradual tilting and rotating of the mineralized collagen fibrils that form this structure produce a material that tends toward having isotropic microhardness properties, even though its basic building block is highly anisotropic. This may be an important structural attribute that allows lamellar bone to withstand a variety of mechanical challenges.

Hardness, an indicator of the mechanical competence of cancellous bone

Hardness and calcium content in compact bone are strongly related. Variation in Young's modulus is produced mainly by variations in mineralisation. Therefore, there should be a relationship between hardness and Young's modulus. We demonstrate this. The calcium content of cancellous bone and adjacent compact bone in several species shows little difference, the cancellous bone having approximately 10% less calcium. The hardness of cancellous bone in Bos is approximately 12% less than that of adjacent compact bone, and the calcium is approximately 2% less. These lines of evidence make it unlikely that the Young modulus of cancellous bone material is much different from that of compact bone. Similar evidence suggests that the yield stress of cancellous bone is similar to that of adjacent compact bone.

Effects of a low calcium maternal and weaning diet on the thickness and microhardness of rat incisor enamel and dentine

The deposition and mineralization of incisor hard tissues have been studied in rat pups nursed by mothers on a low calcium diet or weaned with the maternal diet. Animals were killed at 30 days (control and low calcium diets; maternally fed) or 60 days (after 30 days weaning on maternal diet). The degree of mineralization of enamel and dentine was evaluated by a microhardness method on thick transverse sections. The enamel and dentine thickness, and the diameters of the incisor sections and pulp cavity were measured on microradiographs from the sections. Microhardness values of enamel were similar in groups killed after 30 days maternal feeding, but the microhardness of root enamel was 73-74% less in the low calcium-diet weaned group. Peripulpar dentine had mean microhardness values lower than controls in the group fed maternally for 30 days, whereas the whole root dentine appeared significantly less hard in the low calcium-diet weaned group than in the controls. A significant reduction of the incisor bucco-lingual diameter was observed only in this last experimental group. Enamel thickness was significantly lower in the roots of both experimental groups and in the necks of the low calcium weaned group. The reduction in dentine thickness was greater (from -30 to -56%); in the root it was more evident on the lingual aspect. Thus calcium deficiency in the mother's diet did not influence either the deposition or the mineralization of the pup's incisor enamel and dentine. However, when the offspring were weaned with the maternal calcium-deficient diet, mineralization of the tooth hard tissue was retarded.

Relationship between morphologic features and hardness of the subchondral bone of the medial tibial condyle in the normal state and in osteoarthritis and rheumatoid arthritis

In osteoarthritis and rheumatoid arthritis the hardness of the subchondral bone of the medial tibial plateau is lower than in normals. In order to further analyse this study of the morphologic characteristics in bone from the mentioned region was carried out in 22 normals, 14 osteoarthritis and 12 rheumatoid arthritis. Specimens from these groups were subjected to a radiologic assay, a light microscopic investigation and an evaluation of the occurrence of intraosseous lipids. The normals showed a remarkable integrity of the subchondral trabecular network with advancing age only with slight osteoporosis and occasional sclerosis. In osteoarthritis there were osteoporosis, osteolysis, sclerosis and osteophytes all in good correlation to the grade of osteoarthritis present. In rheumatoid arthritis there were areas of rarefaction, fractures of the trabeculae, sclerosis and invasion of granulation tissue. The radiologic appearance corresponded well with the morphologic observation. No abnormal presence of lipids was encountered. This investigation supports the concept that the hardness of the subchondral bone of the medial tibial plateau much depends on the morphologic structure of the bone.

Structural changes of cortical bone in secondary hyperparathyroidism: replacement of lamellar bone by woven bone

In femoral cortical bone of 16 uremic patients with long standing renal insufficiency an increased fraction of woven bone was found both in Haversian and in interstitial bone. Either partly resorbed Haversian systems were replaced by non lamellar woven bone or single Haversian systems showed partly well organized lamellar bone and partly disorganized non lamellar texture without signs of antecedent resorption. The replacement of lamellar bone by woven bone was measured morphometrically in undecalcified ground sections. Woven bone was defined by its lack of structural birefringence under polarized light. In advanced cases of secondary hyperparathyroidism more than 60% of cortical bone were composed of woven bone. The substitution of immature less organized woven bone for mature well orfanized lamellar bone has important implications for the biomechanical properties of the skeleton.