Arena surface vertical impact forces vary with surface compaction.

Mechanical properties of arena surfaces are extrinsic factors for musculoskeletal injury. Vertical impact forces of harrowed and compacted cushion were measured at five locations on 12 arena surfaces (five dirt, seven synthetic [dirt and fiber]). Eight variables related to impact force, displacement, and acceleration were calculated. Surface temperature, cushion depth and moisture content were also measured. The effects of surface material type (dirt/synthetic) and cushion compaction (harrowed/compacted) on vertical impact properties were assessed using an analysis of variance. Relationships of manageable surface properties with vertical impact forces were examined through correlations. Compacted cushion exhibited markedly higher vertical impact force and deceleration with lower vertical displacement than harrowed cushion (P < 0.001), and the effect was greater on dirt than synthetic surfaces (P = 0.039). Vertical displacement (P = 0.021) and soil rebound (P = 0.005) were the only variables affected by surface type. Surface compaction (harrowed, compacted) had a significantly greater effect on vertical impact forces than surface type (dirt, synthetic). By reducing surface compaction through harrowing, extrinsic factors related to musculoskeletal injury risk are reduced. These benefits were more pronounced on dirt than synthetic surfaces. These results indicate that arena owners should regularly harrow surfaces, particularly dirt surfaces.

Shear ground reaction force variation among equine arena surfaces.

Shear forces at the surface-hoof interface affect hoof slide, surface grip, forces transferred to the limb, and injury risk. However, the variation in shear forces among surfaces with different compositions have not been quantified. Shear ground reaction forces were measured on five dirt and seven synthetic arena surfaces. Cohesion/adhesion and angle of internal friction/coefficient of friction were calculated. Surface composition, surface temperature, cushion depth, and moisture content were also measured. The effects of surface material (dirt/synthetic) on shear properties were assessed using analysis of variance (ANOVA; P < 0.05). The relationships between surface composition or management properties and shear properties were analyzed using linear correlation. Shear properties were not different between dirt and synthetic surface categories; however, surface fiber content was correlated with adhesion and coefficient of friction. These correlations predict that more fiber will decrease soil adhesion (r = -0.75; P < 0.01) and increase the coefficient of friction (r = 0.81; P < 0.01). Furthermore, maximum shear force was significantly correlated with cushion depth (r = 0.61; P < 0.01) and moisture content (r = 0.57; P < 0.01), where shear force was greater on surfaces with thicker cushion layers or higher moisture content. The findings suggest that shear mechanical behavior is more dependent on surface composition than surface material categories (dirt/synthetic) and also indicate that arena owners can influence shear forces by adjusting either surface composition or management.

Modelling the effect of race surface and racehorse limb parameters on in silico fetlock motion and propensity for injury.

BACKGROUND: The metacarpophalangeal joint (fetlock) is the most commonly affected site of racehorse injury, with multiple observed pathologies consistent with extreme fetlock dorsiflexion. Race surface mechanics affect musculoskeletal structure loading and injury risk because surface forces applied to the hoof affect limb motions. Race surface mechanics are a function of controllable factors. Thus, race surface design has the potential to reduce the incidence of musculoskeletal injury through modulation of limb motions. However, the relationship between race surface mechanics and racehorse limb motions is unknown. OBJECTIVE: To determine the effect of changing race surface and racehorse limb model parameters on distal limb motions. STUDY DESIGN: Sensitivity analysis of in silico fetlock motion to changes in race surface and racehorse limb parameters using a validated, integrated racehorse and race surface computational model. METHODS: Fetlock motions were determined during gallop stance from simulations on virtual surfaces with differing average vertical stiffness, upper layer (e.g. cushion) depth and linear stiffness, horizontal friction, tendon and ligament mechanics, as well as fetlock position at heel strike. RESULTS: Upper layer depth produced the greatest change in fetlock motion, with lesser depths yielding greater fetlock dorsiflexion. Lesser fetlock changes were observed for changes in lower layer (e.g. base or pad) mechanics (nonlinear), as well as palmar ligament and tendon stiffness. Horizontal friction and fetlock position contributed less than 1° change in fetlock motion. MAIN LIMITATIONS: Simulated fetlock motions are specific to one horse's anatomy reflected in the computational model. Anatomical differences among horses may affect the magnitude of limb flexion, but will likely have similar limb motion responses to varied surface mechanics. CONCLUSIONS: Race surface parameters affected by maintenance produced greater changes in fetlock motion than other parameters studied. Simulations can provide evidence to inform race surface design and management to reduce the incidence of injury.

Stress-whitening occurs in demineralized bone.

INTRODUCTION: The incidence of age-related bone fracture is increasing with average population age. Bone scatters more light (stress-whitens) during loading, immediately prior to failure, in a manner visually similar to polymer crazing. We wish to understand the stress-whitening process because of its possible effect on bone toughness. The goals of this investigation were a) to establish that stress-whitening is a property of the demineralized organic matrix of bone rather than only a property of mineralized tissue and that stress whitening within the demineralized bone is dependent upon both b) hydrogen bonding and, c) the orientation of loading. METHODS: Demineralized cortical bone specimens were loaded in tension to failure (0.08 strain/s). The effect of hydrogen bonding on mechanical properties and the stress-whitening process was probed by altering the Hansen's hydrogen bonding parameter (δh) of the immersing solution. RESULTS: Stress-whitening occurred in the demineralized bone. Stress-whitening was negatively correlated with δh (R(2)=0.81, p<0.0001). Stress-whitening was significantly lower (p<0.0001) in specimens loaded orthogonally compared to those loaded parallel to the long (strong) axis. CONCLUSION: The stress-whitening observed was consistent with increased Mie scattering. We suggest that the change in Mie scattering was due to collagen fibril dehydration driven by the externally applied stress. The presence of stress-whitening in demineralized bone suggests that this process may be a property of the collagenous matrix and hence may be present in other collagenous tissues rather than an emergent property of the bone composite.

Do stress-whitening and optical clearing of collagenous tissue occur by the same mechanism?

Bone is biphasic with an organic matrix and an inorganic mineral component. As we age bone's susceptibility to fracture increases. It has been shown that there is no change in mean mineralization with aging, but bone nevertheless becomes less tough. This aging effect is therefore likely related to the organic phase. Under mechanical loading, immediately prior to failure, bone has been observed to visually become more opaque and has been termed stress-whitening. Stress-whitening is known to make materials tougher. The goal of this investigation was to investigate stress-whitening in the collagenous matrix of bone. Hydrogen bonds play a key role in collagen stability and we hypothesize that changes in hydrogen bonding will significantly affect matrix stiffness, toughness and stress whitening. Demineralized bone specimens were loaded in tension and stress-whitening was monitored. The effect of hydrogen bonding on mechanical properties and stress-whitening process was probed by altering the Hansen's hydrogen bonding parameter (δh) of the immersing solution. The Hansen's hydrogen bonding parameter of the immersing fluid affected the morphology, mechanical properties and stress whitening of specimens. Specimens were visually whiter in the absence of mechanical load in low δh solvents (the specimens solvent-whitened). Both the observed stress-whitening and solvent-whitening were reversible and repeatable processes. The observed solvent-whitening that occurred without the presence of load was consistent with solvent-induced optical clearing (the opposite of whitening) in skin caused by collagen fibril swelling. Stress whitening and solvent whitening can be explained by a common mechanism, collagen fibril densification and thinning, leading to an increased distinction between the collagen fibrillar phase and immersing fluid, ultimately leading to more scattering. Bones may be at a greater risk for fracture as we age because solubility of the matrix changes, thus making the collagen less hydrated (and more brittle) even in the same solvent.

The importance of intrinsic damage properties to bone fragility: a finite element study.

As the average age of the population has increased, the incidence of age-related bone fracture has also increased. While some of the increase of fracture incidence with age is related to loss of bone mass, a significant part of the risk is unexplained and may be caused by changes in intrinsic material properties of the hard tissue. This investigation focused on understanding how changes to the intrinsic damage properties affect bone fragility. We hypothesized that the intrinsic (µm) damage properties of bone tissue strongly and nonlinearly affect mechanical behavior at the apparent (whole tissue, cm) level. The importance of intrinsic properties on the apparent level behavior of trabecular bone tissue was investigated using voxel based finite element analysis. Trabecular bone cores from human T12 vertebrae were scanned using microcomputed tomography (µCT) and the images used to build nonlinear finite element models. Isotropic and initially homogenous material properties were used for all elements. The elastic modulus (E(i)) of individual elements was reduced with a secant damage rule relating only principal tensile tissue strain to modulus damage. Apparent level resistance to fracture as a function of changes in the intrinsic damage properties was measured using the mechanical energy to failure per unit volume (apparent toughness modulus, W(a)) and the apparent yield strength (σ(ay), calculated using the 0.2% offset). Intrinsic damage properties had a profound nonlinear effect on the apparent tissue level mechanical response. Intrinsic level failure occurs prior to apparent yield strength (σ(ay)). Apparent yield strength (σ(ay)) and toughness vary strongly (1200% and 400%, respectively) with relatively small changes in the intrinsic damage behavior. The range of apparent maximum stresses predicted by the models was consistent with those measured experimentally for these trabecular bone cores from the experimental axial compressive loading (experimental: σ(max) = 3.0-4.3 MPa; modeling: σ(max) = 2-16 MPa). This finding differs significantly from previous studies based on nondamaging intrinsic material models. Further observations were that this intrinsic damage model reproduced important experimental apparent level behaviors including softening after peak load, microdamage accumulation before apparent yield (0.2% offset), unload softening, and sensitivity of the apparent level mechanical properties to variability of the intrinsic properties.

Polymer Mechanics as a Model for Short-Term and Flow-Independent Cartilage Viscoelasticity.

Articular cartilage is the load bearing soft tissue that covers the contacting surfaces of long bones in articulating joints. Healthy cartilage allows for smooth joint motion, while damaged cartilage prohibits normal function in debilitating joint diseases such as osteoarthritis. Knowledge of cartilage mechanical function through the progression of osteoarthritis, and in response to innovative regeneration treatments, requires a comprehensive understanding of the molecular nature of interacting extracellular matrix constituents and interstitial fluid. The objectives of this study were therefore to (1) examine the timescale of cartilage stress-relaxation using different mechanistic models and (2) develop and apply a novel (termed "sticky") polymer mechanics model to cartilage stress-relaxation based on temporary binding of constituent macromolecules. Using data from calf cartilage samples, we found that different models captured distinct timescales of cartilage stress-relaxation: monodisperse polymer reptation best described the first second of relaxation, sticky polymer mechanics best described data from ∼1-100 seconds of relaxation, and a model of inviscid fluid flow through a porous elastic matrix best described data from 100 seconds to equilibrium. Further support for the sticky polymer model was observed using experimental data where cartilage stress-relaxation was measured in either low or high salt concentration. These data suggest that a complete understanding of cartilage mechanics, especially in the short time scales immediately following loading, requires appreciation of both fluid flow and the polymeric behavior of the extracellular matrix.

Compliance calibration for fracture testing of anisotropic biological materials.

The compliance technique has been used to monitor crack length during fracture and fatigue testing of materials. Difficulties arise when this technique is applied to anisotropic biological materials such as bone. In this tutorial, two different methods of analyzing compliance calibration data are described: the standard ASTM method and a new approach developed by the authors specifically for anisotropic materials. An example is given showing how data from equine cortical bone can be analyzed. In this example, calibration tests were conducted on thirty-six three point bend specimens machined from the mid-diaphysis of six pairs of equine third metacarpal bones. Cracks were propagated in three orientations with respect to the long axis of the bone: transverse, longitudinal, and radial. Specimen compliance was determined for a crack range of 0.30 to 0.65 times the specimen width from load vs. crack opening displacement data. The results demonstrate that the ASTM method is not applicable to anisotropic biomaterials such as bone. Rather, it is necessary to develop separate compliance calibration equations for each crack propagation orientation investigated.

Cartilage stress-relaxation is affected by both the charge concentration and valence of solution cations.

OBJECTIVE: Understanding the mechanical functions of specific cartilage molecules such as aggrecan is important for understanding both healthy cartilage and disease progression. Cartilage is primarily composed of chondrocytes and an extracellular matrix consisting of multiple biopolymers, ions, and water. Aggrecan is one matrix biopolymer which consists of a core protein and multiple anionic glycosaminoglycans. Previous research has demonstrated that the stiffness of extracted aggrecan decreases under increased solution cation concentration, and the purpose of this study was to determine whether changes in solution ion concentration resulted in changes in tissue-level viscoelastic properties. METHODS: Middle-zone explants of bovine calf patellofemoral cartilage were harvested and cultured overnight before mechanical testing. Repeated stress-relaxation and cyclical loading tests were performed after equilibration in solutions of 0.15 M and 1 M NaCl and 0.075 M and 0.5 M CaCl(2). A stretched exponential model was fit to the stress-relaxation data. Storage and loss moduli were determined from the cyclical loading data. RESULTS: Changes in ionic strength and species affected both stress-relaxation and cyclical loading of cartilage. Stress-relaxation was faster under higher ionic strength. CaCl(2) concentration increases resulted in decreased peak stress, while NaCl increases resulted in decreased equilibrium stress. Storage and loss moduli were affected differently by NaCl and CaCl(2). CONCLUSIONS: These results show that cartilage stress-relaxation proceeds faster under higher concentrations of solution cations, consistent with the theory of polymer dynamics. These data demonstrate the complexity of cartilage mechanical properties and suggest that aggrecan stiffness may be important in tissue-level cartilage viscoelastic properties.

Patient-specific knee joint finite element model validation with high-accuracy kinematics from biplane dynamic Roentgen stereogrammetric analysis.

Little is known about in vivo menisci loads and displacements in the knee during strenuous activities. A new method that combines high-speed kinematics measured with biplane dynamic Roentgen stereogrammetric analysis (DRSA) and a subject-specific finite element (FE) model for studying in vivo meniscal behavior is presented here. Further model calibration in a very controlled uniaxial low and high-rate compression loading condition is presented by comparing the model behavior against the measured high-accuracy menisci DRSA kinematics and direct tibio-femoral pressure measurement from a K-scan sensor. It is apparent that certain model aspects such as removing of the pressure sensor from the model can result in relatively large errors (14%) in contact parameters that are not reflected in the change of the measured meniscal kinematics. Changing mesh size to 1mm by 1mm elements increased the magnitude of all but one of the contact variables by up to 45%. This local validation using accurate localized patient-specific geometry and meniscal kinematics was needed to enhance model fidelity at the level of contact between menisci and cartilage.

Volume effects on yield strength of equine cortical bone.

Volume effects are a fundamental determinant of structural failure. A material exhibits a volume effect if its failure properties are dependent on the specimen volume. Many brittle ceramics exhibit volume effects due to loading a structure in the presence of "critical" flaws. The number of flaws, their locations, and the effect of stress field within the stressed volume play a role in determining the structure's failure properties. Since real materials are imperfect, structures composed of large volumes of material have higher probabilities of containing a flaw than do small volumes. Consequently, large material volumes tend to fail at lower stresses compared to smaller volumes when tested under similar conditions. Volume effects documented in brittle ceramic and composite structures have been proposed to affect the mechanical properties of bone. We hypothesized that for cortical bone material, (1) small volumes have greater yield strengths than large volumes and (2) that compared to microstructural features, specimen volume was able to account for comparable amounts of variability in yield strength. In this investigation, waisted rectangular, equine third metacarpal diaphyseal specimens (n=24) with nominal cross sections of 3 x 4 mm and gage lengths of either 10.5, 21, or 42 mm, were tested monotonically in tension to determine the effect of specimen volume on their yield strength. Yield strength was greatest in the smallest volume group compared to the largest volume group. Within each group of specimens the logarithm of yield strength was positively correlated with the cumulative failure probability, indicating that the data follow the two-parameter Weibull distribution. Additionally, log yield strength was negatively correlated with log volume, supporting the hypothesis that small stressed volumes of cortical bone possess greater yield strength than similarly tested large stressed volumes.

Volume effects on fatigue life of equine cortical bone.

Materials, including bone, often fail due to loading in the presence of critical flaws. The relative amount, location, and interaction of these flaws within a stressed volume of material play a role in determining the failure properties of the structure. As materials are generally imperfect, larger volumes of material have higher probabilities of containing a flaw of critical size than do smaller volumes. Thus, larger volumes tend to fail at fewer cycles compared with smaller volumes when fatigue loaded to similar stress levels. A material is said to exhibit a volume effect if its failure properties are dependent on the specimen volume. Volume effects are well documented in brittle ceramics and composites and have been proposed for bone. We hypothesized that (1) smaller volumes of cortical bone have longer fatigue lives than similarly loaded larger volumes and (2) that compared with microstructural features, specimen volume was able to explain comparable amounts of variability in fatigue life. In this investigation, waisted rectangular specimens (n=18) with nominal cross-sections of 3x4 mm and gage lengths of 10.5, 21, or 42 mm, were isolated from the mid-diaphysis of the dorsal region of equine third metacarpal bones. These specimens were subjected to uniaxial load controlled fatigue tests, with an initial strain range of 4000 microstrain. The group having the smallest volume exhibited a trend of greater log fatigue life than the larger volume groups. Each volume group exhibited a significant positive correlation between the logarithm of fatigue life and the cumulative failure probability, indicating that the data follow the two-parameter Weibull distribution. Additionally, log fatigue life was negatively correlated with log volume, supporting the hypothesis that smaller stressed volumes of cortical bone possess longer fatigue lives than similarly tested larger stressed volumes.

On sampling bones for microcracks.

This paper addresses the problem of designing experiments to measure microcrack density in cortical bone. Microcracks are relatively scarce in bone cross-sections, and their size requires microscope settings having small fields of view. Thus, substantial time is required to count cracks in each cross-section. Consequently, most studies evaluate a relatively small cross-sectional area from each specimen, the chance of finding a crack in any given field is small, and there is a significant chance of not finding even one crack in the specimens representing a particular subject. Therefore, a statistical model for microcrack counting was created to develop guidelines for sampling bones for microcracks. Three questions were addressed. 1) What are the relationships of sample size to variability in microcrack density results and the probability of crackless specimens? 2) How can sample size be chosen a priori so as to reduce the probability of crackless specimens and the associated variability in the data to an acceptable level? 3) What are the confidence intervals for the mean density of microcracks measured using microscopic counting? Using a Poisson model for the distribution of microcracks within microscope fields the total area (mm(2)) that should be examined for each specimen is given by A(s)=-ln(F)/Cr.Dn, where Cr.Dn is the expected microcrack density for an individual sample and F is the desired probability (expressed as a fraction) that the individual sample will contain no microcracks. This equation is validated against 8 results from three different experiments.

Comparison of the linear finite element prediction of deformation and strain of human cancellous bone to 3D digital volume correlation measurements.

The mechanical properties of cancellous bone and the biological response of the tissue to mechanical loading are related to deformation and strain in the trabeculae during function. Due to the small size of trabeculae, their motion is difficult to measure. To avoid the need to measure trabecular motions during loading the finite element method has been used to estimate trabecular level mechanical deformation. This analytical approach has been empirically successful in that the analytical models are solvable and their results correlate with the macroscopically measured stiffness and strength of bones. The present work is a direct comparison of finite element predictions to measurements of the deformation and strain at near trabecular level. Using the method of digital volume correlation, we measured the deformation and calculated the strain at a resolution approaching the trabecular level for cancellous bone specimens loaded in uniaxial compression. Smoothed results from linearly elastic finite element models of the same mechanical tests were correlated to the empirical three-dimensional (3D) deformation in the direction of loading with a coefficient of determination as high as 97% and a slope of the prediction near one. However, real deformations in the directions perpendicular to the loading direction were not as well predicted by the analytical models. Our results show, that the finite element modeling of the internal deformation and strain in cancellous bone can be accurate in one direction but that this does not ensure accuracy for all deformations and strains.

Summary--Measuring "bone quality".

The idea of bone quality is well-established in the literature and represents a real conundrum in the treatment of osteoporosis. On the one hand, there are measurements for patients that predict fracture risk for the population as a whole, but between individual patients, one will fracture but another will not, despite the fact that all of the technical measurements we use to predict fracture risk are the same. There are, of course, many aspects of bone mechanical properties that cannot yet be measured in patients. The session began with a discussion of what bone quality is, then the speakers presented work on novel aspects of bone properties that could help explain why fracture prediction in vivo is inexact.

Long-term ovariectomy decreases ovine compact bone viscoelasticity.

Changes in bone mineral density associated with estrogen depletion in humans do not account for all of the associated change in fracture risk, and it is possible that some of this variation may lie in changes of other aspects of bone quality. The purpose of this study was to investigate changes in viscoelastic behavior of compact bone that may be associated with estrogen depletion. Changes in compact bone viscoelastic properties associated with three years of ovariectomy were investigated with dynamic mechanical analysis (low-amplitude 3-point bending at frequencies of 1-20 Hz) using beams milled from the diaphysis of the ovine radius. The viscoelastic storage modulus was significantly (5.2%) lower at the higher frequencies for the ovariectomized animals. The general anatomic variation in storage modulus, in which cranial sectors had higher values than caudal sectors, did not change with ovariectomy. The loss tangent (tandelta, a measure of damping) was also greatly decreased (up to 83%) at high frequencies in the ovariectomized animals. Anatomic variation in tandelta at low (6-12 Hz) frequencies (cranial and caudal sectors having higher values than lateral or medial sectors) was enhanced with ovariectomy. Changes in viscoelastic properties associated with long-term estrogen depletion could be responsible for a significant reduction in the toughness or strength of a bone without concomitant changes in screening modalities used to evaluate bone quality (e.g., DXA, QCT, QUA).

Matrix concentration of insulin-like growth factor I (IGF-I) is negatively associated with biomechanical properties of human tibial cancellous bone within individual subjects.

Insulin-like growth factor-I (IGF-I), abundant in bone matrix, is believed to play an important role during bone development and remodeling. To our knowledge, however, few studies have addressed the relationship between the concentration of IGF-I in bone matrix and the biomechanical properties of bone tissue. In this study, forty-five cylindrical specimens of cancellous bone were harvested from six human tibiae and scanned using micro-computed tomography (microCT). The bone volume fraction (BV/TV) was calculated from three-dimensional (3D) microCT images. Mechanical tests were then performed on a servohydraulic testing system to determine the strength and stiffness of cancellous bone. Following mechanical testing, the concentration of IGF-I in bone matrix was measured by using an enzyme-linked immunoabsorbent assay (ELISA). Within each subject, the concentration of IGF-I in bone matrix had significant (P<0.01) negative correlations with the bone volume fraction, strength, and stiffness of cancellous bone. In particular, the anterior quadrant of the proximal tibia was significantly (P<0.02) greater in IGF-I matrix concentration and marginally significantly lower in strength (P=0.053) and stiffness (P=0.059) than the posterior quadrant. The negative correlations between the cancellous bone matrix concentration of IGF-I and cancellous bone biomechanical properties within subjects found in this study may help us understand the variation of the biomechanical properties of cancellous bone in proximal human tibiae.

Determinants of ovine compact bone viscoelastic properties: effects of architecture, mineralization, and remodeling.

Significant decreases in ovine compact bone viscoelastic properties (specifically, stress-rate sensitivity, and damping efficiency) are associated with three years of ovariectomy and are particularly evident at higher frequencies [Proc. Orthop. Res. Soc. 27 (2002) 89]. It is unclear what materials or architectural features of bone are responsible for either the viscoelastic properties themselves, or for the changes in those properties that were observed with estrogen depletion. In this study, we examined the relationship between these viscoelastic mechanical properties and features involving bone architecture (BV/TV), materials parameters (ash density, %mineralization), and histologic evidence of remodeling (%remodeled, cement line interface). The extent of mineralization was inversely proportional to the material's efficiency in damping stress oscillations. The damping characteristics of bone material from ovariectomized animals were significantly more sensitive to variation in mineralization than was bone from control animals. At low frequencies (6 Hz or less), increased histologic evidence of remodeling was positively correlated with increased damping efficiency. However, the dramatic decreases in stress-rate sensitivity that accompanied 3-year ovariectomy were seen throughout the bone structure and occurred even in areas with little or no secondary Haversian remodeling as well as in areas of complete remodeling. Taken together, these data suggest that, while the mineral component may modify the viscoelastic behavior of bone, the basic mechanism underlying bone viscoelastic behavior, and of the changes in that behavior with estrogen depletion, reside in a non-mineral component of the bone that can be significantly altered in the absence of secondary remodeling.

Polymer dynamics as a mechanistic model for the flow-independent viscoelasticity of cartilage.

The initial, rapid, flow independent, apparent stress relaxation of articular cartilage disks deformed by unconfined compressive displacement is shown to be consistent with the theory of polymer dynamics. A relaxation function for polymers based upon a mechanistic model of molecular interaction (reptation) appropriately approximated early, flow independent relaxation of stress. It is argued that the theory of polymer dynamics, with its reliance on mechanistic models of molecular interaction, is an appropriate technique for application to and the understanding of rapid, flow independent, stress relaxation in cartilage.

Effects of vertebral bone fragility and bone formation rate on the mineralization levels of cancellous bone from white females.

Back-scattered electron microscopy was used to study mineralization levels of human iliac cancellous bone of white females (N = 49). Mineralization levels were assessed by converting bone pixel grayscale levels to atomic number (Z) using known calibration standards. The data set consisted of bone biopsies from normal and vertebral fracture subjects that had either high or low values for bone formation rate (BFR(s)) within their respective groups (fracture/low BFR(s), N = 12; fracture/high BFR(s), N = 10; normal/low BFR(s), N = 12; normal/high BFR(s), N = 15). The following three measures of mineralization were quantitatively determined for each specimen: an overall mean mineralization (Z(mean)), the mineralization of trabecular packets deep within the interior of trabeculae (Z(deep)), and the mineralization of superficial exterior packets (Z(superficial)). Two-way analysis of variance revealed that the high BFR(s) group had a significantly lower Z(superficial) than the low BFR(s) group [mean (SD) 10.383 (0.270) vs. 10.563 (0.289)], and there was no significant interaction. BFR(s) had no effect on Z(mean) or Z(deep). For the pooled data, Z(deep) was significantly higher than Z(superficial) [10.866 (0.242) vs. 10.471 (0.291)]. There was no significant difference in Z(mean), Z(deep), or Z(superficial) between normals and those with vertebral fracture, but the standard deviations of the mineralization measures in the fracture group were at least double that of the normal group. Frequency histograms show that the two groups have fundamentally different mineralization distributions. The normal group demonstrates typical Gaussian distributions centered around the mean, and the distributions of the fracture group are bimodal, with peaks occurring at either the high or low tails of the distributions of the normal group. We hypothesize that both low and high patterns of mineralization might detrimentally affect bone material properties, with low mineralization levels causing reduced stiffness and strength and high mineralization resulting in reduced fracture toughness. The degree to which the mineralization differences may affect strength and stiffness of individual elements is estimated. The higher standard deviations of mineralization measures in the fracture group may reflect an inability to properly regulate trabecular level stress and strain. Forward stepwise regression analysis showed significant relationships between Ob.S/OS and both Z(superficial) and Z(mean), suggesting that the osteoblast may play an important role in regulating mineralization.