Estimation of mechanical properties of cortical bone by computed tomography

It is difficult to assess from conventional x-rays the amount of loading that a bone can tolerate. The question therefore was asked whether the mechanical properties of cortical bone could be estimated by using a computed tomography (CT) system typically employed in the clinical setting. In vitro cross sectional diaphyseal scans of adult human tibiae were made using a GE 9800 scanner and linear attenuation coefficients determined in several regions of the central cross sections. Samples from the mid-diaphyses of these tibiae were harvested, tested in three-point bending to failure, and mechanical properties as well as density and ash fraction determined. The respective relationships between CT measurements, mechanical properties, and physical properties were calculated using regression analysis. In addition, a solid calibration phantom (tricalciumphosphate) was scanned to evaluate the variability of CT measurements. The physical parameters measured in this study were found to be comparable with data from other authors but correlations were moderate to weak. Linear regression revealed the following correlation coefficients with CT data: r = 0.55 (Young's modulus), r = 0.50 (strength), r = 0.65 (apparent density) and r = 0.46 (ash fraction). The correlation coefficients of these regressions for both linear and power fits were not significantly different. A high linear correlation (r = 0.99) was found between the chamber densities and the measured attenuation coefficients, but accuracy varied between 2 and 6%. The small range of specimen mechanical properties as well as the limitations inherent with the methods employed may explain these results. We conclude that clinical equipment as used in this study is not sufficient to accurately estimate the mechanical properties of cortical bone.     

Mechanical behavior of human trabecular bone after overloading.

With the etiology of osteoporotic fractures as motivation, the goal of this study was to characterize the mechanical behavior of human trabecular bone after overloading. Specifically, we quantified the reductions in modulus and strength and the development of residual deformations and determined the dependence of these parameters on the applied strain and apparent density. Forty cylindrical specimens of human L1 vertebral trabecular bone were destructively loaded in compression at 0.5% strain per second to strains of up to 3.0% and then immediately unloaded to zero stress and reloaded. (An ancillary experiment on more readily available bovine bone had been performed previously to develop this testing protocol.) In general, the reloading stress-strain curve had a short initial nonlinear region with a tangent modulus similar to Young's modulus. This was followed by an approximately linear region spanning to 0.7% strain, with a reduced residual modulus. The reloading curve always approached the extrapolated envelope of the original loading curve. Percent modulus reduction (between Young's and residual), a quantitative measure of mechanical damage, ranged from 5.2 to 91.0% across the specimens. It increased with increasing plastic strain (r2 = 0.97) but was not related to modulus or apparent density. Percent strength reduction, in the range of 3.6-63.8%, increased with increasing plastic strain (r2 = 0.61) and decreasing apparent density (r2 = 0.23). The residual strains of up to 1.05% depended strongly on applied strain (r2 = 0.96). Statistical comparisons with previous data for bovine tibial bone lend substantial generality to these trends and provide an envelope of expected behavior for other sites. In addition to providing a basis for biomechanical analysis of the effects of damage in trabecular bone at the organ level, these findings support the concept that occasional overloads may increase the risk of fracture by substantially degrading the mechanical properties of the underlying trabecular bone.     

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 tipper limit to the maximization of bone material stiffness and strength.     

The influence of bone volume fraction and ash fraction on bone strength and modulus

Although bone strength and modulus are known to be influenced by both volume fraction and mineral content (ash fraction), the relative influence of these two parameters remains unknown. Single-parameter power law functions are used widely to relate bone volume or ash fraction to bone strength and elastic modulus. In this study we evaluate the potential for predicting bone mechanical properties with two-parameter power law functions of bone volume fraction (BV/TV) and ash fraction (alpha) of the form y = a(BV/TV)(b) alpha(c) (where y is either ultimate strength or elastic modulus). We derived an expression for bone volume fraction as a function of apparent density and ash fraction to perform a new analysis of data presented by Keller in 1994. Exponents b and c for the prediction of bone strength were found to be 1.92 +/- 0.02 and 2.79 +/- 0.09 (mean +/- SE), respectively, with r(2) = 0.97. The value of b was found to be consistent with that found previously, whereas the value of c was lower than values previously reported. For the prediction of elastic modulus we found b and c to be 2.58 +/- 0.02 and 2.74 +/- 0.13, respectively, with r(2) = 0.97. The exponent related to ash fraction was typically larger than that associated with bone volume fraction, suggesting that a change in mineral content will, in general, generate a larger change in bone strength and stiffness than a similar change in bone volume fraction. These findings are important for interpreting the results of antiresorptive drug treatments that can cause changes in both ash and bone volume fraction.     

Predictive value of bone mineral density and morphology determined by peripheral quantitative computed tomography for cancellous bone strength of the proximal femur

Peripheral quantitative computed tomography (pQCT) is an established diagnostic method for assessment of bone mineral density in the diagnosis of osteoporosis. However, the capacity of structural parameters of cancellous bone measured by high-resolution computed tomography remains to be explored. In 33 patients, bone mineral density (BMD) of the proximal femur was measured in vitro by pQCT using cylindrical biopsies from the intertrochanteric region harvested before the implantation of an artificial hip joint. By digital image analysis of CT scans, parameters derived from histomorphometry describing the microarchitecture of cancellous bone were measured. The biopsies were also loaded to failure by an uniaxial compression test to determine the biomechanical parameters, Young's modulus, strength, and maximum energy absorption (E(max)). Strong correlations were found for BMD vs. mechanical parameters (r = 0.73 for Young's modulus, r = 0.82 for strength, and r = 0.79 for E(max); p < 0.001, n = 29). The morphological parameters, bone volume per trabecular volume (BV/TV), apparent trabecular thickness (app.Tb.Th), apparent trabecular separation (app.Tb.Sp), and trabecular number (Tb.N), correlated significantly with all mechanical parameters. The combination of morphological parameters with BMD in a multivariate regression model led to an overall, but only moderate, increase in R(2) in all cases. Our data confirm the high predictive value of BMD for the mechanical competence of cancellous bone of the intertrochanteric region. However, quantification of cancellous bone structure by image analysis of CT scans may provide additional qualitative information for the analysis of bone strength.     

Load distribution and the predictive power of morphological indices in the distal radius and tibia by high resolution peripheral quantitative computed tomography

A 3D high resolution peripheral quantitative computed tomography scanner (HR-pQCT) (XtremeCT, Scanco Medical, voxel size 82 microm) has been recently developed that can perform in vivo human measurements on peripheral sites, including the wrist and tibia. The goals of this study were to use HR-pQCT measurements to determine the ability of morphological and density measurements to predict bone apparent stiffness and apparent Young's modulus in the distal radius and tibia, to determine the relative importance of cortical and trabecular bone in carrying load in the human distal radius and tibia. Furthermore, the ability of a sub-volume of trabecular bone apparent Young's modulus to predict the Young's modulus of a whole radius and tibia section was determined. A total of 25 measurements of the radius and 12 measurements of the tibia were used for morphological and finite element analyses of sections, and sub-volume cubes of trabecular bone from the distal radius and tibia. The subjects were chosen to obtain a large variation in age ranges and bone architecture and density. By combining multiple measurements, a strong ability to predict bone apparent stiffness and apparent Young's modulus was found for morphological and density measurements in the radius and tibia (R(2)>0.80). The relative importance of the trabecular and cortical bone in carrying load was also found to vary consistently with location in the sample for both the radius and the tibia. This indicates that measurements of the cortical and trabecular bone are required for assessing fracture risk. A cubic section of trabecular bone was found to be insufficient to accurately represent the apparent bone Young's modulus of a radius or tibia section. Morphological and density measurements of the distal radius and tibia have been shown in this study to predict bone apparent Young's modulus and apparent stiffness, and may indicate when a more time consuming finite element analysis is warranted. It should be noted that these results may be an overestimation of the predictive ability of structural parameters, as the influence of bone density is removed from the finite element analyses, and the results were only influenced by bone structure. A measurement of bone apparent Young's modulus is independent of subject size (as opposed to reaction force), and may provide the ability to distinguish between two patients that have similar mean morphological and density measurements; but different overall structures, and therefore, different fracture risk.     

Bone density does not reflect mechanical properties in early-stage arthrosis

Subchondral cancellous bone specimens were removed from 10 human postmortem early-stage arthrotic proximal tibiae (mean age 73 (63-81) years) and 10 age- and gender-matched normal proximal tibiae. The early-stage arthrosis was confirmed histologically and the specimens were divided into 4 groups: medial arthrosis, lateral control, normal medial and normal lateral controls. The specimens were tested in compression to determine mechanical properties and then physical/compositional properties. Compared to the normal medial control, we found reductions in ultimate stress, Young's modulus, and failure energy, and an increase in ultimate strain of arthrotic cancellous bone. Bone volume fraction, apparent density, apparent ash density, and collagen density were higher in cancellous bone with arthrosis, but no differences were found in tissue density, mineral and collagen concentrations between arthrotic cancellous bone and the 3 controls. None of the mechanical properties of arthrotic cancellous bone could be predicted by the physical/compositional properties measured. The increase in bone tissue in early-stage arthrotic cancellous bone did not make up for the loss of mechanical properties, which suggests a deterioration in the quality of arthrotic cancellous bone.     

Regional differences in mechanical and material properties of femoral head cancellous bone in health and osteoarthritis

Osteoarthritis (OA) is a debilitating condition common among the aging population. In this study we have determined mechanical and material properties of cancellous bone cores from two differently loaded regions of femoral heads obtained from healthy subjects and those with end-stage osteoarthritis. Densitometric properties were determined prior to compression testing for Young's modulus (EC) and yield strength (sigma(y)), after which bones were powdered for analysis of collagen and mineral content. In both OA and normal cancellous bone, volumetric bone mineral density (BMDv), apparent density (rhoA), E(C), and sigma(y) were systematically greater in the superior than in the inferior region (P<0.05). In the OA inferior region, median BMDv (0.434 g-cm(-3)) and rA (0.426 g-cm(-3)) were significantly greater than in normals (0.329 and 0.287 g-cm(-3), respectively, both P<0.05) reflecting an increased amount of tissue. The mineral:collagen ratio was decreased in OA, but this was only significant in the superior region (P<0.008). Relationships between EC and both BMDv and rho(A) were weaker in OA bone cores (r(2) = 0.66 and r(2) = 0.59) than in normals (r(2) = 0.86 and r(2) = 0.77, respectively). Likewise, sigma(y) and both BMDv and rho(A) were weaker in OA (r(2) = 0.74 and r(2) = 0.70) than in normals (r(2) = 0.83 and r(2) = 0.77, respectively). For the same value of density measure, EC and sigma(y) tended to be lower in OA bone when compared with normal bone. In conclusion, femoral head cancellous bone mass in end-stage osteoarthritis is increased but undermineralized, and is neither stiffer nor stronger than normal cancellous bone.     

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.     

Relationship Among MRTA, DXA, and QUS Revisited

Inexpensive, commercially produced devices that directly measure bone strength in vivo are not currently available. Mechanical response tissue analysis (MRTA), a unique prototype device, is an in vivo vibrational test that measures transverse bending stiffness (a measure of whole bone strength expressed as the product of estimated Young's modulus of elasticity and cross-sectional moment of inertia, EI, Nm2) at ulna midshaft. We compared speed of sound (SOS; [m/s]) in ulna cortical bone using a commercially available axial transmission quantitative ultrasound (QUS) device with EI using MRTA. Dual-energy X-ray absorptiometry (DXA) was used to provide an estimate of ulna size (cm2), bone mineral content (BMC; [g/cm]) and areal bone mineral density (BMD; [g/cm2]). The objective of the study was to determine if ulna SOS--alone or in combination with BMD from DXA--was correlated with ulna EI, thus becoming a surrogate measure of transverse bending stiffness, and thus whole bone strength. Data were collected from 138 female volunteers (18-86 yr). EI and SOS were significantly correlated, r = +0.218, p = 0.01, but r2 was very low, 4.8%. SOS and total ulna BMD were combined to estimate elastic modulus, which correlated with EI, r = +0.377, p < 0.0001; however, the correlation was not significantly better than with SOS alone. We conclude that axial transmission QUS is not a strong surrogate in vivo technique for estimating transverse bending stiffness.     

Influence of physiological effort of growth and chemical composition on antler bone mechanical properties

Antler is a good model to study bone biology both because it is accessible and because it grows and is shed every year. Previous studies have shown that chemical composition changes as the antler is grown, implying constraints in mineral availability and the physiological effort made to grow it. This study aimed at examining antler mechanical properties to assess whether they reflect physiological effort and whether they are associated with precise mineral bone composition rather than just ash content, which is usually the main factor affecting mechanical properties. We examined Young's modulus of elasticity (E), strength, and work to maximum load, as well as bone mineral composition, along the antler shaft. Then we compared trends between antlers from two populations: captive, well-fed, health-managed deer (n=15), and free-ranging deer with lower food quality and no health treatment (n=10). Greater E, strength and work were found for better fed and health managed deer. In addition, antler chemical composition of both populations differed in Na, Mg, K, Fe and Si, and marginally in Zn, but not in ash or Ca content. Significant and clear divergent trends in mechanical properties supporting greater physiological exhaustion in free-ranging deer were found for all mechanical variables. Detailed models showed that, in addition to ash content, independent factors extracted from principal component analyses on composition affected E and strength, but not work to maximum load. The results suggest that there is an association between bone chemical composition and mechanical properties independently of ash content.     

Bone density does not reflect mechanical properties in early-stage arthrosis

Subchondral cancellous bone specimens were removed from 10 human postmortem early-stage arthrotic proximal tibiae (mean age 73 (63-81) years) and 10 age- and gender-matched normal proximal tibiae. The early-stage arthrosis was confirmed histologically and the specimens were divided into 4 groups: medial arthrosis, lateral control, normal medial and normal lateral controls. The specimens were tested in compression to determine mechanical properties and then physical/compositional properties. Compared to the normal medial control, we found reductions in ultimate stress, Young's modulus, and failure energy, and an increase in ultimate strain of arthrotic cancellous bone. Bone volume fraction, apparent density, apparent ash density, and collagen density were higher in cancellous bone with arthrosis, but no differences were found in tissue density, mineral and collagen concentrations between arthrotic cancellous bone and the 3 controls. None of the mechanical properties of arthrotic cancellous bone could be predicted by the physical/compositional properties measured. The increase in bone tissue in early-stage arthrotic cancellous bone did not make up for the loss of mechanical properties, which suggests a deterioration in the quality of arthrotic cancellous bone.     

Local expression of human growth hormone in bone results in impaired mechanical integrity in the skeletal tissue of transgenic mice

The effect of local production of human growth hormone on murine cortical bone was investigated using a transgenic mouse model. Femora and humeri from human growth hormone transgenic mice and littermate control mice were obtained, and the geometrical, biomechanical, compositional, and histomorphometric properties of all specimens were determined. The goals were to investigate the effects of local expression of human growth hormone on skeletal integrity, including the functional geometry of long bone and its related structural and mechanical behavior, as well as tissue composition and integrity. As expected, local production of human growth hormone by osteoblasts indeed resulted in longer femora with significantly greater mid-diaphyseal cross-sectional geometry in the transgenic mice (16% increase in cross-sectional area and 29% increase in bending moments of inertia). However, the significant increase in geometry was not associated with a proportional increase in bending stiffness and other structural properties, which suggested that the mechanical properties of the cortical bone tissue may have been inferior. Microspecimen bending tests verified this prediction, given that transgenic cortical bone tissue had significantly lower apparent elastic modulus and ultimate strength (52 and 68%, respectively, of control values). These defects in the whole bone structural and tissue mechanical properties of transgenic specimens were associated with a smaller fraction of ash, larger fractions of woven bone and cartilage islands, and greater porosity in the mid-diaphyseal cortices. These results suggest that local production of human growth hormone by osteoblasts is indeed anabolic for bone, but at the expense of bone tissue integrity.     

Properties of growing trabecular ovine bone. Part I: mechanical and physical properties

Our aim was to determine the relationship between age and the mechanical and physical properties of trabecular bone, to describe the patterns in which the variations in these properties take place, and to investigate the influence of the physical properties on the mechanical characteristics of trabecular bone during growth. We used 30 lambs in three age groups and 20 sheep in two age groups. Cubes of subchondral bone were cut from the proximal tibia according to a standardised protocol. We performed non-destructive compression tests of the specimens in three orthogonal directions and compression tests to failure in the axial direction. The physical properties of the specimens were also determined. The data were correlated with age and compared in skeletally immature and mature animals. Multiple regression analyses were performed between the mechanical and the physical properties. Age correlated positively with elastic modulus, bone strength, energy absorption to failure, elastic energy, mechanical anisotropy ratio, tissue density, apparent density, apparent ash density, and bone mineral content, and inversely with ultimate strain, viscoelastic energy absorption, relative energy loss, the collagen content of bone and the percentage porosity. The values of all variables were significantly different in the skeletally mature and immature groups. The apparent density of trabecular bone tissue was found to be the major predictor of its compressive mechanical properties. Together with the content of bone muscle and bone collagen, the apparent density could explain 84% of the variation in the elastic modulus, whereas only a small portion of the variation in ultimate strain could be explained by the variation in apparent density.     

Prediction of mechanical properties of the human calcaneus by broadband ultrasonic attenuation

Broadband ultrasonic attenuation (dB MHz cm⁻¹, nBUA) was determined for specimens from 20 human calcanei, along with apparent density, elasticity (Young's modulus), and compressive strength. The calcanei were modified to provide “whole” (only soft tissue removed), “core” (mediolateral cores corresponding to in vivo measurement region), “can” (cortical end plates removed from core), and “der” (core defatted) samples. The nBUA values for the various modifications were highly correlated. The presence of the cortical endplates creates a significant nBUA, probably due to complex phase interactions. nBUA_can was a good predictor of elasticity (R² = 75.7%) and strength (R² = 73.6%). Apparent density was a better predictor of the mechanical variables than nBUA, with R² values of 88.5% for elasticity and 87.6% for strength. The morphological anisotropy defined by “fabric” for the specimens was extremely uniform. The coefficient of variation in nBUA (40.5%) and compressive strength (64.4%) was significantly greater than for apparent density (23.5%) and fabric (6.7%). It is well known that a power law relationship exists between apparent density and elasticity or strength in cancellous bone. An interesting finding in this work is that there also appears to be a power law relationship between nBUA and apparent density, with an exponent of approximately 2, which, in the light of clinical implications, warrants further investigation.     

Compressive behaviour of child and adult cortical bone

In this study, cortical bone tissue from children was investigated. It is extremely difficult to obtain human child tissue. Therefore, the only possibility was to use bone tissue, free from any lesion, collected from young bone cancer patients.The compressive mechanical behaviour of child bone tissue was compared to the behaviour of adult tissue. Moreover, two hypotheses were tested: 1) that the mechanical behaviour of both groups is correlated to ash density; 2) that yield strain is an invariant.Small parts of the diaphysis of femora or tibiae from 12 children (4–15 years) and 12 adults (22–61 years) were collected. Cylindrical specimens were extracted from the cortical wall along the longitudinal axis of the diaphysis. A total of 107 specimens underwent compressive testing (strain rate: 0.1 s^{− 1}). Only the specimens showing a regular load–displacement curve (94) were considered valid and thereafter reduced to ash.It was found that the child bone tissue had significant lower compressive Young's modulus (− 34%), yield stress (− 38%), ultimate stress (− 33%) and ash density (− 17%) than the adult tissue. Conversely, higher compressive ultimate strain was found in the child group (+ 24%). Despite specimens extracted from both children and adults, ash density largely described the variation in tissue strength and stiffness (R² = in the range of 0.86–0.91). Furthermore, yield strain seemed to be roughly an invariant to subject age and tissue density.These results confirm that the mechanical properties of child cortical bone tissue are different from that of adult tissue. However, such differences are correlated to differences in tissue ash density. In fact, ash density was found to be a good predictor of strength and stiffness, also for cortical bone collected from children. Finally, the present findings support the hypothesis that compressive yield strain is an invariant.     

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.     

Effect of microstructure on the mechanical properties of Haversian cortical bone

The mechanical properties of cortical bone have been extensively studied at the macrostructural scale. However, knowledge of the macroscopic mechanical properties is not sufficient to predict local phenomena, such as damage or bone remodeling, both of which are dependent on local mechanical behavior. The objective of this study is to quantify the mechanical properties of cortical bone at several length scales, with emphasis on the microstructure of Haversian systems. Samples of mature bovine cortical bone, with a Haversian microstructure, were obtained from the posterior area of the mid-femoral diaphysis. A nanoindentation technique was used to measure the local Young's modulus. The distribution of the bone mineral content was obtained by backscattered electron imaging using a scanning electron microscope. A novel compression device employing microextensometry techniques was developed to quantify local strains. Digital image correlation was performed on the microstructure imaged by optical microscopy during compression tests. This study demonstrated that the local Young's modulus and strain were heterogeneous at the scale of an osteon. For both properties, the ratio between the maximum and minimum values was approximately two. The local Young's modulus and bone-mineral content were reasonably correlated (r2 = 0.75; P < 0.0001), but this was not the case for the distribution of local strains versus bone mineral content (r2 = 0.395; P < 0.0001). Hence, local strains cannot be described simply in terms of the bone mineral content, as the Haversian canal and osteonal microstructure have a major influence on these properties. In conclusion, the microstructure must be considered in evaluating the local strain and stress fields of cortical bone.     

Shock wave treatment shows dose-dependent enhancement of bone mass and bone strength after fracture of the femur

Shock wave treatment is believed to improve bone healing after fracture. The purpose of this study was to evaluate the effect of shock wave treatment on bone mass and bone strength after fracture of the femur in a rabbit model. A standardized closed fracture of the right femur was created with a three-point bending method in 24 New Zealand white rabbits. Animals were randomly divided into three groups: (1) control (no shock wave treatment), (2) low-energy (shock wave treatment at 0.18 mJ/mm2 energy flux density with 2000 impulses), and (3) high-energy (shock wave treatment at 0.47 mJ/mm2 energy flux density with 4000 impulses). Bone mass (bone mineral density (BMD), callus formation, ash and calcium contents) and bone strength (peak load, peak stress and modulus of elasticity) were assessed at 12 and 24 weeks after shock wave treatment. While the BMD values of the high-energy group were significantly higher than the control group (P = 0.021), the BMD values between the low-energy and control groups were not statistically significant (P = 0.358). The high-energy group showed significantly more callus formation (P < 0.001), higher ash content (P < 0.001) and calcium content (P = 0.003) than the control and low-energy groups. With regard to bone strength, the high-energy group showed significantly higher peak load (P = 0.012), peak stress (P = 0.015) and modulus of elasticity (P = 0.011) than the low-energy and control groups. Overall, the effect of shock wave treatment on bone mass and bone strength appears to be dose dependent in acute fracture healing in rabbits.     

Bisphosphonate treatment affects trabecular bone apparent modulus through micro-architecture rather than matrix properties.

Bisphosphonates are emerging as an important treatment for osteoporosis. But whether the reduced fracture risk associated with bisphosphonate treatment is due to increased bone mass, improved trabecular architecture and/or increased secondary mineralization of the calcified matrix remains unclear. We examined the effects of bisphosphonates on both the trabecular architecture and matrix properties of canine trabecular bone. Thirty-six beagles were divided into a control group and two treatment groups, one receiving risedronate and the other alendronate at 5-6 times the clinical dose for osteoporosis treatment. After one year, the dogs were killed, and samples from the first lumbar vertebrae were examined using a combination of micro-computed tomography, finite element modeling, and mechanical testing. By combining these methods, we examined the treatment effects on the calcified matrix and trabecular architecture independently. Conventional histomorphometry and microdamage data were obtained from the second and third lumbar vertebrae of the same dogs [Bone 28 (2001) 524]. Bisphosphonate treatment resulted in an increased apparent Young's modulus, decreased bone turnover, increased calcified matrix density, and increased microdamage. We could not detect any change in the effective Young's modulus of the calcified matrix in the bisphosphonate treated groups. The observed increase in apparent Young's modulus was due to increased bone mass and altered trabecular architecture rather than changes in the calcified matrix modulus. We hypothesize that the expected increase in the Young's modulus of the calcified matrix due to the increased calcified matrix density was counteracted by the accumulation of microdamage.