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.     

Transition behavior in fatigue of human dentin: structure and anisotropy

The influence of tubule orientation on the transition from fatigue to fatigue crack growth in human dentin was examined. Compact tension (CT) and rectangular beam specimens were prepared from the coronal dentin of molars with three unique tubule orientations (i.e., 0 degrees , 45 degrees and 90 degrees). The CT specimens (N=25) were used to characterize fatigue crack initiation and steady-state cyclic extension, whereas the rectangular beams (N=132) were subjected to 4-pt flexure and used in quantifying the stress-life fatigue response. The transition behavior was analyzed using both the Kitagawa-Takahashi and El Haddad approaches. Results showed that both the fatigue crack growth and stress-life responses were dependent on the tubule orientation. The average Paris Law exponent for crack growth perpendicular (90 degrees) to the tubules (m=13.3+/-1.1) was significantly greater (p<0.05) than that for crack growth oblique (45 degrees) to the tubules (m=11.5+/-1.87). Similarly, the fatigue strength of dentin with 90 degrees tubule orientation was significantly lower (p<0.05) than that for the other two orientations, regardless of the range of cyclic stress. The apparent endurance strengths of specimens with 0 degrees (44MPa) and 45 degrees (53MPa) orientations were nearly twice that of the 90 degrees (24MPa) orientation. Based on these results, human dentin exhibits the largest degree of anisotropy within the stress-life regime and the transition from fatigue to fatigue crack growth occurs under the lowest cyclic stress range when the tubules are aligned with the cyclic normal stress (90 degrees orientation).     

Tubule orientation and the fatigue strength of human dentin

In this study the influence of tubule orientation on the strength of human dentin under static and cyclic loads was examined. Rectangular beams were sectioned from the coronal dentin of virgin extracted molars (N=83) and then loaded in quasi-static 4-point flexure or 4-point flexural fatigue to failure. The flexure strength, energy to fracture and fatigue strength were evaluated for specimens with the dentin tubules aligned parallel (theta=0 degrees ) and perpendicular (theta=90 degrees ) to the plane of maximum normal stress. Results from monotonic loading showed that both the flexural strength and energy to fracture of dentin specimens with theta=0 degrees were significantly greater than those with theta=90 degrees . Furthermore, the apparent endurance strength of dentin with theta=0 degrees (44MPa) was significantly greater than that of the dentin with theta=90 degrees (24MPa). The ratio of apparent endurance strength (for fully reversed loading) to the flexure strength for theta=0 degrees and theta=90 degrees was 0.41 and 0.28, respectively. Although the influence of tubule orientation was most important to mechanical behavior, the flexure strength and energy to fracture also decreased with an increase in tubule density. According to differences in the fatigue strength with tubule orientation, restorative practices promoting large cyclic normal stresses perpendicular to the tubules would be more likely to facilitate fatigue failure in dentin with cyclic loading.     

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.     

Hydration and dynamic fatigue of dentin

An experimental investigation on the dynamic fatigue response of dentin was conducted to examine the influence of stress rate on the strength and energy to fracture. Rectangular beams were prepared from the coronal dentin of bovine maxillary molars and subjected to four-point flexure to failure. The dentin beams were examined in the fully hydrated and dehydrated condition at stress rates (sigma) ranging from 0.01 to 100 MPa/s. Results for the hydrated dentin showed that the flexure strength, energy to fracture, and flexure modulus all increased with increasing stress rate; the flexure strength increased from 100 MPa ((sigma) = 0.01 MPa/s) to 250 MPa ((sigma) = 100 MPa/s). In contrast, the elastic modulus and strength of the dehydrated dentin decreased with increasing stress rate; the flexural strength of the dehydrated dentin deceased from 170 MPa ((sigma) = 0.01 MPa/s) to 100 MPa ((sigma) = 100 MPa/s). While the hydrated dentin behaved more like a brittle material at low stress rates, the strain to fracture was found to be nearly independent of (sigma). According to the experimental results, restorative conditions that cause development of static stresses within the tooth could promote a decrease in the damage tolerance of dentin.     

Adhesive performance of dentin bonding agents applied in vivo and in vitro. Effect of intrapulpal pressure and dentin depth

The purpose of this study was to evaluate the influence of intrapulpal pressure and dentin depth on bond strengths of an etch-and-rinse and a self-etching bonding agent to dentin in vitro and in vivo. Twenty-four pairs of premolars were randomly divided into four groups (n = 6) according to the dentin bonding agent, Single Bond and Clearfil SE Bond, and intrapulpal pressure, null or positive. Each tooth of the pair was further designated to be treated in vivo or in vitro. The intrapulpal pressure was controlled in vivo by the delivery of local anesthetics containing or not a vasoconstrictor, while in vitro, it was achieved by keeping the teeth under hydrostatic pressure. Class I cavities were prepared and the dentin bonding agents were applied followed by incremental resin restoration. For the teeth treated in vitro, the same restorative procedures were performed after a 6 month-storage period. Beams with 1 mm(2) cross-sectional area were prepared and microtensile tested. Clearfil SE Bond was not influenced by any of the variables of the study, while bond strengths produced in vitro were significantly higher for Single Bond. Overall, lower bond strengths were produced in deep dentin, which reached statistical significance when Single Bond was applied under physiological or simulated intrapulpal pressure. In conclusion, in vitro bonding may overestimate the immediate adhesive performance of more technique-sensitive dentin bonding systems. The impact of intrapulpal pressure on bond strength seems to be more adhesive dependent than dentin morphological characteristics related to depth.     

An examination of the electromyographic fatigue threshold test

Summary The purpose of this investigation was to examine times to exhaustion at various percentages of the electromyographic fatigue threshold (EMG_FT). Eight adult males [mean (SD), 21 (1) years] volunteered for the investigation. EMG_FT was derived by determining the rate of rise in the electrical activity of the vastus lateralis [using integrated electromyography (iEMG)] over time (iEMG slope) for four fatiguing power outputs during cycle ergometry. The four power outputs were then plotted as a function of the four iEMG slope coefficients. The y-intercept of the power output versus iEMG slope coefficient graph was defined as the EMG_FT. The intraclass correlation for repeated EMG_FT tests was R=0.65 (SEE=7 W) and there was no significant (P>0.05) difference between the mean (SD) values for test [260 (11) W] versus retest [262 (32) W]. Actual times to exhaustion were determined for work bouts at power outputs equal to 85, 100, 115, 130, and 145% of EMG_FT. The mean (SD) times to exhaustion for these work bouts were 495 (231), 225 (72), 135 (35), 94 (17), and 72 (14) s, respectively. A power curve was derived using the mean power outputs and mean times to exhaustion from the five rides at various percentages of EMG_FT. The power curve provided estimates of the power outputs which could be maintained for 30 and 60 min. There were significant (P<0.05) differences between the mean EMG_FT (260 W) and the power outputs which could be maintained for 30 (151 W) and 60 (125 W) min. EMG_FT overpredicted the estimated power outputs that could be maintained for 30 and 60 min by 42% and 52%, respectively.     

Contributions of aging to the fatigue crack growth resistance of human dentin

An evaluation of the fatigue crack resistance of human dentin was conducted to identify the degree of degradation that arises with aging and the dependency on tubule orientation. Fatigue crack growth was achieved in specimens of coronal dentin through application of Mode I cyclic loading and over clinically relevant lengths (0 ≤ a ≤ 2 mm). The study considered two directions of cyclic crack growth in which the crack was either in-plane (0°) or perpendicular (90°) to the dentin tubules. Results showed that regardless of tubule orientation, aging of dentin is accompanied by a significant reduction in the resistance to the initiation of fatigue crack growth, as well as a significant increase in the rate of incremental extension. Perpendicular to the tubules, the fatigue crack exponent increased significantly (from m=14.2 ± 1.5 to 24.1 ± 5.0), suggesting an increase in brittleness of the tissue with age. For cracks extending in-plane with the tubules, the fatigue crack growth exponent does not change significantly with patient age (from m=25.4 ± 3.03 to 22.9 ± 5.3), but there is a significant increase in the incremental crack growth rate. Regardless of age, coronal dentin exhibits the lowest resistance to fatigue crack growth perpendicular to the tubules. While there are changes in the cyclic crack growth rate and mechanisms of cyclic extension with aging, this tissue maintains its anisotropy.     

Kitagawa-Takahashi diagrams define the limiting conditions for cyclic fatigue failure in human dentin

As cyclic fatigue is considered to be a major cause of clinical tooth fractures, achieving a comprehensive understanding of the fatigue behavior of dentin is of importance. In this note, the fatigue behavior of human dentin is examined in the context of the Kitagawa-Takahashi diagram to define the limiting conditions for fatigue failure. Specifically, this approach incorporates two limiting threshold criteria for fatigue: (i) a threshold stress for fatigue failure, specifically the smooth-bar (unnotched) fatigue endurance strength, at small crack sizes and (ii) a threshold stress-intensity range for fatigue-crack growth at larger crack sizes. The approach provides a "bridge" between the traditional fatigue life and fracture mechanics based damage-tolerant approaches to fatigue-life estimation, and as such defines a "failure envelope" of applied stresses and flaw sizes where fatigue failure is likely in dentin This approach may also be applied to fatigue failure in human cortical bone (i.e. clinical "stress fractures"), which exhibits similar fatigue behavior characteristics, and in principle may aid clinicians in making quantitative evaluations of the risk of fractures in mineralized tissues.     

An examination of four models predicting fatigue in multiple sclerosis.

Fatigue is a common symptom of multiple sclerosis (MS) that is purported to cause significant distress and have detrimental effects on daily functioning, social and occupational obligations, and overall well-being. The prevalence of fatigue in MS is high, with 53-87% of patients reporting significant problems with fatigue across different studies reported in the literature. The cause of fatigue in MS is still poorly understood. Some researchers have suggested that fatigue is a direct consequence of the MS disease process, but several studies have failed to find a relationship between disease severity and MS fatigue. A number of investigations have reported that depression and sleep are significantly related to fatigue in MS, as well as to one another. The purpose of the present investigation was to examine the relationships among disease severity, depression, and sleep disturbance in MS, and their possible role in predicting fatigue. Four models were proposed to explore these relationships. The best fitting model showed that all three were significant independent contributors to fatigue in MS, accounting for 43% of the variance, with sleep disturbance reigning as the largest contributor. Furthermore, although disease severity predicted fatigue in our sample, both depression and sleep disturbance emerged as stronger predictors. These findings suggest that, beyond core physical/neurological MS symptomatology, there are other factors that contribute to fatigue in MS, namely, depression and sleep disturbance.     

Interaction patterns between dentin and adhesive on prepared class V cavities in vitro and in vivo

The interface between dentin and an acetone-based single-component adhesive system (Prime&Bond 2.1, DeTrey Dentsply, Germany) was morphologically investigated by scanning electron microscopy (SEM). Interaction patterns of human teeth were correlated in vivo and in vitro. The SEM examination proved that the formation of a hybrid and an adhesive layer, the peri- and intratubular adhesive penetration, as well as hiatus and nanoleakage formation were no different on vital and nonvital dentin within the limitation of the experimental arrangement of this study.     

Age, dehydration and fatigue crack growth in dentin

A preliminary study of the effects from age and dehydration on fatigue crack growth in human dentin was conducted. Compact tension (CT) fatigue specimens of coronal dentin were prepared from extracted molars and subjected to high cycle fatigue (10(5)相似文献   

Biological responses of silver-coated thermosets: An in vitro and in vivo study

Bisphenol A glycidylmethacrylate (BisGMA)/triethyleneglycol dimethacrylate (TEGDMA) thermosets are biomaterials commonly employed for orthopedic and dental applications; for both these fields, bacterial adhesion to the surface of the implant represents a major issue for the outcome of the surgical procedures. In this study, the antimicrobial properties of a nanocomposite coating formed by polysaccharide 1-deoxylactit-1-yl chitosan (Chitlac) and silver nanoparticles (nAg) on methacrylate thermosets were studied. The Chitlac–nAg system showed good anti-bacterial and anti-biofilm activity although its biocidal properties can be moderately, albeit significantly, inhibited by serum proteins. In vitro studies on the silver release kinetic in physiological conditions showed a steady metal release associated with a gradual loss of antimicrobial activity. However, after 3 weeks there was still effective protection against bacterial colonization which could be accounted for by the residual silver. This time-span could be considered adequate to confer short-term protection from early peri-implant infections. Preliminary in vivo tests in a mini-pig animal model showed good biological compatibility of Chitlac–nAg-coated materials when implanted in bony tissue. The comparison was made with implants of titanium Ti6Al4V alloy and with a Chitlac-coated thermoset. Bone healing patterns and biocompatibility parameters observed for nAg-treated material were comparable with those observed for control implants.     

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.     

Factors influencing human IgE synthesis in vitro and in vivo

Several pitfalls may affect studies on human IgE synthesis in vitro. In this paper, the requirement for stringent specificity of the anti-IgE antibodies used and for assessment not only of IgE detectable in culture supernatants but also as cell-associated IgE is emphasized. The use of cycloheximide-treated cultures as controls also leaves wishes open. Activated, human T cells and T cell hybridomas produce IgE-binding factors, which may be detected by a sensitive in vitro test and which may apparently also become the endeavour of synthesis by molecular biological techniques. Although the evidence available in rodents for the role of IgE-binding factors in modulating IgE synthesis has not yet been fully reproduced by us in man, the fact that classical IgE-enhancing procedures in rodents (e.g. radiotherapy, T cell suppression) also affect IgE production in man leads to believe that similar immunoregulation mechanisms apply to various mammalian species studied so far.