Biomedical subjects

J H Kinney

Publications and source records attributed to J H Kinney.

At least 19 recordsLinked to original sources

PubMed2006

Role of alcohol in the fracture resistance of teeth.

Healthy dentin, the mineralized tissue that makes up the bulk of the tooth, is naturally hydrated in vivo; however, it is known that various chemical reagents, including acetone and ethanol, can induce dehydration and thereby affect its properties. Here, we sought to investigate this in light of the effect of alcohol on the mechanical properties of dentin, specifically by measuring the stiffness, strength, and toughness of dentin in simulated body fluid and Scotch whisky. Results indicated that chemical dehydration induced by the whisky had a significant beneficial effect on the elastic modulus, strength, and fracture toughness of dentin. Although this made teeth more resistant to fracture, the change in properties was fully reversible upon rehydration. This effect is considered to be associated with increased cross-linking of the collagen molecules from intermolecular hydrogen-bonding, where water is replaced with weaker hydrogen-bond-forming solvents such as alcohol.

Acetone↗

PubMed2005

Effects of polar solvents on the fracture resistance of dentin: role of water hydration.

Although healthy dentin is invariably hydrated in vivo, from a perspective of examining the mechanisms of fracture in dentin, it is interesting to consider the role of water hydration. Furthermore, it is feasible that exposure to certain polar solvents, e.g., those found in clinical adhesives, can induce dehydration. In the present study, in vitro deformation and fracture experiments, the latter involving a resistance-curve (R-curve) approach (i.e., toughness evolution with crack extension), were conducted in order to assess changes in the constitutive and fracture behavior induced by three common solvents-acetone, ethanol and methanol. In addition, nanoindentation-based experiments were performed to evaluate the deformation behavior at the level of individual collagen fibers and ultraviolet Raman spectroscopy to evaluate changes in bonding. The results indicate a reversible effect of chemical dehydration, with increased fracture resistance, strength, and stiffness associated with lower hydrogen bonding ability of the solvent. These results are analyzed both in terms of intrinsic and extrinsic toughening phenomena to further understand the micromechanisms of fracture in dentin and the specific role of water hydration.

Animals↗

PubMed2005

Mechanistic aspects of fracture and R-curve behavior in human cortical bone.

An understanding of the evolution of toughness is essential for the mechanistic interpretation of the fracture of cortical bone. In the present study, in vitro fracture experiments were conducted on human cortical bone in order to identify and quantitatively assess the salient toughening mechanisms. The fracture toughness was found to rise linearly with crack extension (i.e., rising resistance- or R-curve behavior) with a mean crack-initiation toughness, K0 of approximately 2 MPa square root m for crack growth in the proximal-distal direction. Uncracked ligament bridging, which was observed in the wake of the crack, was identified as the dominant toughening mechanism responsible for the observed R-curve behavior. The extent and nature of the bridging zone was examined quantitatively using multi-cutting compliance experiments in order to assess the bridging zone length and estimate the bridging stress distribution. Additionally, time-dependent cracking behavior was observed at stress intensities well below those required for overload fracture; specifically, slow crack growth occurred at growth rates of approximately 2 x 10(-9) m/s at stress intensities approximately 35% below the crack-initiation toughness. In an attempt to measure slower growth rates, it was found that the behavior switched to a regime dominated by time-dependent crack blunting, similar to that reported for dentin; however, such blunting was apparent over much slower time scales in bone, which permitted subcritical crack growth to readily take place at higher stress intensities.

Adult↗

PubMed2005

Mechanistic aspects of in vitro fatigue-crack growth in dentin.

Although the propagation of fatigue cracks has been recognized as a problem of clinical significance in dentin, there have been few fracture mechanics-based studies that have investigated this issue. In the present study, in vitro cyclic fatigue experiments were conducted over a range of cyclic frequencies (1-50 Hz) on elephant dentin in order to quantify fatigue-crack growth behavior from the perspective of understanding the mechanism of fatigue in dentin. Specifically, results obtained for crack extension rates along a direction parallel to the dentinal tubules were found to be well described by the stress-intensity range, DeltaK, using a simple Paris power-law approach with exponents ranging from 12 to 32. Furthermore, a frequency dependence was observed for the crack-growth rates, with higher growth rates associated with lower frequencies. By using crack-growth experiments involving alternate cyclic and static loading, such fatigue-crack propagation was mechanistically determined to be the result of a "true" cyclic fatigue mechanism, and not simply a succession of static fracture events. Furthermore, based on the observed frequency dependence of fatigue-crack growth in dentin and observations of time-dependent crack blunting, a cyclic fatigue mechanism involving crack-tip blunting and re-sharpening is proposed. These results are deemed to be of importance for an improved understanding of fatigue-related failures in teeth.

Animals↗

PubMed2005

Aspects of in vitro fatigue in human cortical bone: time and cycle dependent crack growth.

Although fatigue damage in bone induced by cyclic loading has been recognized as a problem of clinical significance, few fracture mechanics based studies have investigated how incipient cracks grow by fatigue in this material. In the present study, in vitro cyclic fatigue experiments were performed in order to quantify fatigue-crack growth behavior in human cortical bone. Crack-growth rates spanning five orders of magnitude were obtained for the extension of macroscopic cracks in the proximal-distal direction; growth-rate data could be well characterized by the linear-elastic stress-intensity range, using a simple (Paris) power law with exponents ranging from 4.4 to 9.5. Mechanistically, to discern whether such behavior results from "true" cyclic fatigue damage or is simply associated with a succession of quasi-static fracture events, cyclic crack-growth rates were compared to those measured under sustained (non-cyclic) loading. Measured fatigue-crack growth rates were found to exceed those "predicted" from the sustained load data at low growth rates ( approximately 3 x 10(-10) to 5 x 10(-7) m/cycle), suggesting that a "true" cyclic fatigue mechanism, such as alternating blunting and re-sharpening of the crack tip, is active in bone. Conversely, at higher growth rates ( approximately 5 x 10(-7) to 3 x 10(-5) m/cycle), the crack-growth data under sustained loads integrated over the loading cycle reasonably predicts the cyclic fatigue data, indicating that quasi-static fracture mechanisms predominate. The results are discussed in light of the occurrence of fatigue-related stress fractures in cortical bone.

Adult↗

PubMed2005

Age-related transparent root dentin: mineral concentration, crystallite size, and mechanical properties.

Many fractures occur in teeth that have been altered, for example restored or endodontically repaired. It is therefore essential to evaluate the structure and mechanical properties of these altered dentins. One such altered form of dentin is transparent (sometimes called sclerotic) dentin, which forms gradually with aging. The present study focuses on differences in the structure and mechanical properties of normal versus transparent dentin. The mineral concentration, as measured by X-ray computed microtomography, was significantly higher in transparent dentin, the elevated concentration being consistent with the closure of the tubule lumens. Crystallite size, as measured by small angle X-ray scattering, was slightly smaller in transparent dentin, although the importance of this finding requires further study. The elastic properties were unchanged by transparency; however, transparent dentin, unlike normal dentin, exhibited almost no yielding before failure. In addition, the fracture toughness was lowered by roughly 20% while the fatigue lifetime was deleteriously affected at high stress levels. These results are discussed in terms of the altered microstructure of transparent dentin.

Aged↗

PubMed2005

Fracture in human cortical bone: local fracture criteria and toughening mechanisms.

Micromechanical models for fracture initiation that incorporate local failure criteria have been widely developed for metallic and ceramic materials; however, few such micromechanical models have been developed for the fracture of bone. In fact, although the fracture event in "hard" mineralized tissues such as bone is commonly believed to be locally strain-controlled, only recently has there been experimental evidence (using double-notched four-point bend testing) to support this widely held belief. In the present study, we seek to shed further light on the nature of the local cracking events that precede catastrophic fracture in human cortical bone, and to define their relationship to the microstructure. Specifically, numerical computations are reported that demonstrate that the stress and strain states ahead of such a notch are qualitatively similar irrespective of the deformation mechanism (pressure-insensitive plasticity vs. pressure-sensitive microcracking). Furthermore, we use the double-notched test to examine crack-microstructure interactions from a perspective of determining the salient toughening mechanisms in bone and to characterize how these may affect the anisotropy in fracture properties. Based on preliminary micromechanical models of these processes, the relative contributions of various toughening mechanisms are established. In particular, crack deflection and uncracked-ligament bridging are identified as the major mechanisms of toughening in cortical bone.

Adult↗

PubMed2004

Effect of aging on the toughness of human cortical bone: evaluation by R-curves.

Age-related deterioration of the fracture properties of bone, coupled with increased life expectancy, is responsible for increasing incidence of bone fracture in the elderly, and hence, an understanding of how its fracture properties degrade with age is essential. The present study describes ex vivo fracture experiments to quantitatively assess the effect of aging on the fracture toughness properties of human cortical bone in the longitudinal direction. Because cortical bone exhibits rising crack-growth resistance with crack extension, unlike most previous studies, the toughness is evaluated in terms of resistance-curve (R-curve) behavior, measured for bone taken from wide range of age groups (34-99 years). Using this approach, both the ex vivo crack-initiation and crack-growth toughness are determined and are found to deteriorate with age; the initiation toughness decreases some 40% over 6 decades from 40 to 100 years, while the growth toughness is effectively eliminated over the same age range. The reduction in crack-growth toughness is considered to be associated primarily with a degradation in the degree of extrinsic toughening, in particular, involving crack bridging in the wake of the crack.

Adult↗

PubMed2004

Resonant ultrasound spectroscopy measurements of the elastic constants of human dentin.

The technique of resonant ultrasound spectroscopy (RUS) was used to measure the second-order elastic constants of hydrated human dentin. Specimens were placed between two transducers, and the resonant frequencies of vibration were measured between 0.5 and 1.4 MHz. The elastic constants determined from the measured resonant frequencies in hydrated dentin exhibited slight hexagonal anisotropy, with the stiffest direction being perpendicular to the axis of the tubules (E11 = 25.1GPA) This hexagonal anisotropy was small (E33/E11 = 0.92), and almost disappeared when the specimens were dried. In addition, there was a pronounced anisotropy in the Poisson's ratio of wet dentin: v21 = 0.45; v31 = 0.29. With drying in air, this anisotropy vanished: v21 = v31 = 0.29. The isotropic Young's modulus of dried dentin was 28.1 GPa. RUS shows promise for determining the elastic constants in mineralized tissues.

Anisotropy↗

PubMed2004

On the in vitro fatigue behavior of human dentin: effect of mean stress.

Human dentin is susceptible to failure under repetitive cyclic-fatigue loading. This investigation seeks to address the paucity of data that reliably quantify this phenomenon. Specifically, the effect of alternating vs. mean stresses, characterized by the stress- or load-ratio R (ratio of minimum-to-maximum stress), was investigated for three R values (-1, 0.1, and 0.5). Dentin was observed to be prone to fatigue failure under cyclic stresses, with susceptibility varying, depending upon the stress level. The "stress-life" (S/N) data obtained are discussed in the context of constant-life diagrams for fatigue failure. The results provide the first fatigue data for human dentin under tension-compression loading and serve to map out safe and unsafe regimes for failure over a wide range of in vitro fatigue lives (< 10(3) to > 10(6) cycles).

Algorithms↗

PubMed2003

On the fracture of human dentin: is it stress- or strain-controlled?

Despite substantial clinical interest in the fracture resistance of human dentin, there is little mechanistic information in archival literature that can be usefully used to model such fracture. In fact, although the fracture event in dentin, akin to other mineralized tissues like bone, is widely believed to be locally strain-controlled, there has never been any scientific proof to support this belief. The present study seeks to address this issue through the use of a novel set of in vitro experiments in Hanks' balanced salt solution involving a double-notched bend test geometry, which is designed to discern whether the critical failure events involved in the onset of fracture are locally stress- or strain-controlled. Such experiments are further used to characterize the notion of "plasticity" in dentin and the interaction of cracks with the salient microstructural features. It is observed that fracture in dentin is indeed locally strain-controlled and that the presence of dentinal tubules does not substantially affect this process of crack initiation and growth. The results presented are believed to be critical steps in the development of a micromechanical model for the fracture of human dentin that takes into consideration the influence of both the microstructure and the local failure mode.

Biocompatible Materials↗

PubMed2003

In vitro fracture toughness of human dentin.

The in vitro fracture toughness of human dentin has been reported to be of the order of 3 MPa (square root)m. This result, however, is based on a single study for a single orientation, and furthermore involves notched, rather than fatigue precracked, test samples. The present study seeks to obtain an improved, lower-bound, value of the toughness, and to show that previously reported values may be erroneously high because of the absence of a sharp crack as the stress concentrator. Specifically, the average measured critical stress intensity, K(c), for the onset of unstable fracture along an orientation perpendicular to the long axis of the tubules in dentin is found to be 1.8 MPa (square root)m in simulated body fluid (Hanks' balanced salt solution), when tested in a three-point bending specimen containing a (nominally) atomically sharp precrack generated during prior fatigue cycling. This is to be compared with a value of 2.7 MPa (square root)m measured under identical experimental conditions except that the bend specimen contained a sharp machined notch (of root radius 30-50 microm). The effect of acuity of the precrack on the fracture toughness of human dentin is discussed in the context of these data.

Apatites↗

PubMed2003

In vitro fatigue behavior of human dentin with implications for life prediction.

Although human dentin is known to be susceptible to failure under repetitive cyclic fatigue loading, there are few reports in the literature that reliably quantify this phenomenon. This study seeks to address the paucity of fatigue data through a systematic investigation of the effects of prolonged cyclical loading on human dentin in an environment of ambient temperature Hank's balanced salt solution (HBSS) at cyclic frequencies of 2 and 20 Hz. The "stress-life" (S/N) data thus obtained are discussed in the context of possible mechanisms of fatigue damage and failure in this material. In addition, stiffness loss data collected in situ during the S/N tests are used to deduce crack velocities and the thresholds for such cracking. These results are presented in a fracture mechanics context as plots of fatigue-crack propagation rates (da/dN) as a function of the stress-intensity range (Delta K). Such S/N and da/dN-Delta K data are discussed in light of the development of a framework for a fracture-mechanics-based methodology for the prediction of the fatigue life of teeth. It is concluded that the presence of small (on the order of 250 microm) incipient flaws in human teeth will not radically affect their useful life.

Apatites↗

PubMed2003

Basic fibroblast growth factor forms new trabeculae that physically connect with pre-existing trabeculae, and this new bone is maintained with an anti-resorptive agent and enhanced with an anabolic agent in an osteopenic rat model.

Osteoporosis is a disease of excess bone fragility that results from both the loss of bone mass and trabecular bone microarchitecture, thereby creating a very fragile skeleton. The purpose of this study was to determine whether treatment of ovariectomized (OVX) osteopenic rats with basic fibroblast growth factor (bFGF) would stimulate the production of new trabeculae, and whether the newly formed trabeculae would make physical connections with the pre-existing trabeculae after prolonged estrogen deficiency. Six-month-old Sprague Dawley rats were OVXed or sham-operated and were left untreated until day 60 post-OVX. A high resolution microscopic scan (XTM) of the right proximal tibia was performed on groups 1 and 2 on day 1 post-OVX, and was repeated in all animals on day 60 post-OVX. At day 60 groups 1 and 2 were treated with vehicle and groups 3 to 6 were injected with bFGF 200 microg/kg/d intravenously for 15 days. At day 82, all animals obtained another in vivo XTM scan of the right tibia; then group 4 were treated with 17B estradiol 10 microg/kg/3x a week, group 5 were treated with hPTH (1-34) at 80 microg/kg/d for 35 days, group 6 were sacrificed, and groups 1 and 2 were treated with vehicle injections for 35 days. At day 110, all remaining animals were sacrificed, and repeat ex vivo XTM scans of the right proximal tibia were performed. Trabecular bone structural variables-including trabecular bone volume, connectivity, number, and thickness-were obtained from all XTM scans. Biochemical markers of bone turnover were also obtained 24 hours before each XTM scan (osteocalcin and deoxypyridinoline), and analyzed by ELISA. Animals OVXed and treated with vehicle had decreased trabecular bone volume, connectivity and number compared to sham-operated animals at both day 60 and day 110. Animals treated with bFGF from day 60-75 post-OVX had evidence of new trabeculae that physically connected with pre-existing trabeculae and also of increased trabecular bone volume seven days after the injections were discontinued. Biochemical markers of bone formation had a small and insignificant increase over baseline levels during the bFGF injections. Bone resorption markers were significantly reduced during the injection period, but returned to baseline levels after the injections were stopped. In addition, we also demonstrated that these newly formed trabecular connections could be maintained or added to with either estrogen or hPTH (1-34) treatments. Thirty-five days after ending the bFGF treatment, trabecular bone volume and connectivity was 25-80% higher in the estrogen and hPTH (1-34) treated animals compared to the untreated animals ( p<0.01). These results support continued development of bFGF as a potential treatment for severely osteoporotic individuals.

Animals↗

PubMed2003

Effect of orientation on the in vitro fracture toughness of dentin: the role of toughening mechanisms.

Toughening mechanisms based on the presence of collagen fibrils have long been proposed for mineralized biological tissues like bone and dentin; however, no direct evidence for their precise role has ever been provided. Furthermore, although the anisotropy of mechanical properties of dentin with respect to orientation has been suggested in the literature, accurate measurements to support the effect of orientation on the fracture toughness of dentin are not available. To address these issues, the in vitro fracture toughness of dentin, extracted from elephant tusk, has been characterized using fatigue-precracked compact-tension specimens tested in Hank's balanced salt solution at ambient temperature, with fracture paths perpendicular and parallel to the tubule orientations (and orientations in between) specifically being evaluated. It was found that the fracture toughness was lower where cracking occurred in the plane of the collagen fibers, as compared to crack paths perpendicular to the fibers. The origins of this effect on the toughness of dentin are discussed primarily in terms of the salient toughening mechanisms active in this material; specifically, the role of crack bridging, both from uncracked ligaments and by individual collagen fibrils, is considered. Estimates for the contributions from each of these mechanisms are provided from theoretical models available in the literature.

Animals↗

PubMed2003

Crack blunting, crack bridging and resistance-curve fracture mechanics in dentin: effect of hydration.

Few studies have focused on a description of the fracture toughness properties of dentin in terms of resistance-curve (R-curve) behavior, i.e., fracture resistance increasing with crack extension, particularly in light of the relevant toughening mechanisms involved. Accordingly, in the present study, fracture mechanics based experiments were conducted on elephant dentin in order to determine such R-curves, to identify the salient toughening mechanisms and to discern how hydration may affect their potency. Crack bridging by uncracked ligaments, observed directly by microscopy and X-ray tomography, was identified as a major toughening mechanism, with further experimental evidence provided by compliance-based experiments. In addition, with hydration, dentin was observed to display significant crack blunting leading to a higher overall fracture resistance than in the dehydrated material. The results of this work are deemed to be of importance from the perspective of modeling the fracture behavior of dentin and in predicting its failure in vivo.

Animals↗

PubMed2003

On the importance of geometric nonlinearity in finite-element simulations of trabecular bone failure.

The finite element method, which has been successfully applied to studies of the elastic properties of trabecular bone, is now being used to simulate its failure. These simulations have used a geometrically linear (linear kinematic) approximation to the total stiffness matrix; nonlinear terms in the total stiffness matrix have been excluded from the computation in order to achieve efficiency. Because trabecular bone appears to be a slender (i.e., geometrically nonlinear) structure, we studied the validity of the linear kinematic approximation for simulating its failure. Two cases, designed to bracket the extremes of stability behavior, were explored: a single representative spicule of trabecular bone (case 1) and a volume of trabecular bone consisting of relatively low aspect ratio members (case 2). For case 1, geometrically linear (GL) and nonlinear (GNL) analyses were performed with two different materials models: a plastic damage model and a brittle damage model. When GNL terms were included in the total stiffness matrix, we found that load-path bifurcation preceded tissue failure regardless of the form of the damage model. This bifurcation was the result of a complex coupling between material yield and structural instability. The nature of this coupling was highly sensitive to the form of the damage model. None of these behaviors was observed in the linear analyses, where failure was insensitive to the form of the damage model and where structural instabilities were prevented from occurring. For case 2, compressive loading of a volume of trabecular bone, geometric nonlinear effects were pronounced. There was a bifurcation in load response that resulted in large apparent strain to failure. The GL simulations, on the other hand, precluded this bifurcation. We hypothesize that trabecular bone is a geometric nonlinear structure; nonlinear terms must be included in the total stiffness matrix to accurately simulate its failure.

Biomechanical Phenomena↗

PubMed2003

Mechanistic fracture criteria for the failure of human cortical bone.

A mechanistic understanding of fracture in human bone is critical to predicting fracture risk associated with age and disease. Despite extensive work, a mechanistic framework for describing how the microstructure affects the failure of bone is lacking. Although micromechanical models incorporating local failure criteria have been developed for metallic and ceramic materials, few such models exist for biological materials. In fact, there is no proof to support the widely held belief that fracture in bone is locally strain-controlled, as for example has been shown for ductile fracture in metallic materials. In the present study, we provide such evidence through a novel series of experiments involving a double-notch-bend geometry, designed to shed light on the nature of the critical failure events in bone. We examine how the propagating crack interacts with the bone microstructure to provide some mechanistic understanding of fracture and to define how properties vary with orientation. It was found that fracture in human cortical bone is consistent with strain-controlled failure, and the influence of microstructure can be described in terms of several toughening mechanisms. We provide estimates of the relative importance of these mechanisms, such as uncracked-ligament bridging.

Bone and Bones↗