Fatigue testing of biomaterials and their interfaces

Abstract

Objective: The objective of this article is to describe the importance of fatigue to the success of restorative dentistry, with emphasis on the methods for evaluating the fatigue properties of materials in this field, and the durability of their bonded interfaces.

Methods: The stress-life fatigue and fatigue crack growth approaches for evaluating the fatigue resistance of dental biomaterials are introduced. Emphasis is placed on in vitro studies of the hard tissue foundation, restorative materials and their bonded interfaces. The concept of durability is then discussed, including the effects of conventional "mechanical" fatigue combined with pervasive threats of the oral environment, including variations in pH and the activation of endogenous dentin proteases.

Results: There is growing evidence that fatigue is a principal contributor to the failure of restorations and that measures of static strength, used in qualifying new materials and practices, are not reflective of the fatigue performance. Results of selected studies show that the fundamental steps involved in the placement of restorations, including the cutting of preparations and etching, cause a significant reduction to the fatigue strength of the hard tissue foundation. In regards to the bonded interface, results of studies focused on fatigue resistance highlight the importance of the hybridization of resin tags, and that a reduction in integrity of the dentin collagen is detrimental to the durability of dentin bonds.

Significance: Fatigue should be a central concern in the development of new dental materials and in assessing the success of restorative practices. A greater recognition of contributions from fatigue to restoration failures, and the development of approaches with closer connection to in vivo conditions, will be essential for extending the definition of lifelong oral health.

Keywords: Bonded interface; Cracks; Dentin; Durability; Fatigue; Fracture.

Fig. 1

Schematic description of a material subjected to a cyclic stress (S) that has (a) small randomly distributed intrinsic defects, and (b) small intrinsic defects and a well-defined flaw. In the case of (b), the larger defect could develop through growth of the smaller defects in response to cyclic loading into one of larger size, or as a result of steps involved in the restoration process that cause a larger flaw.

Fig. 2

Characterizing the fatigue behavior of dentin and importance of age using two different approaches. The tissue used in these studies was coronal dentin from 3rd molars and the two age groups are defined as young (age ≤ 35) and old (55 ≤ age). (a) A comparison of the fatigue life diagrams for dentin within the two age groups. Data points represent beams that failed and data points with arrows indicate testing that was stopped after roughly 1.2 million cycles because the specimen did not fail. Basquin-type power law models are presented, which defines the mean fatigue strength over the entire fatigue life distribution. The apparent fatigue limit (Se) is also highlighted, which defines the cyclic stress amplitude below which fatigue failure does not occur. (b) A comparison of the fatigue crack growth resistance for dentin within the two age groups. This data is obtained from multiple specimens, and each data point represents a measurement of the average incremental growth of the crack per cycle (da/dN) for a specific stress intensity range (ΔK). The (+) and (−) markers highlight regions of better and worse fatigue crack growth resistance as the top right signifies higher fatigue crack growth rates at low stress intensity range.

Fig. 3

A comparison of fatigue life diagrams for coronal dentin after specific steps involved in the placement of a restoration. The “control” consists of dentin beams that were prepared with conventional diamond slicing equipment and that resulted in an average surface roughness of less than 0.2 μm. These specimens are considered free of flaws. (a) comparison of fatigue life distributions for the control and dentin beams subjected to bur treatment. Cutting was performed with a 6-flute tungsten carbide straight fissure bur (Model FG 57, SS White, Lakewood NJ, USA) and commercial air turbine (Midwest Quiet Air-L High Speed Handpiece, Dentsply, York, PA, USA) with water spray irrigation. (b) comparison of fatigue life distributions for the control and dentin beams subjected to bur treatment followed by a 15 s etch with 37.5% gel (Kerr). In both (a) and (b), each data point corresponds to fatigue testing and failure of a single dentin beam. Data points with arrows identify beams that did not fail and the test was discontinued. The R² accompanying each empirical equation of best fit represents the coefficient of determination. Note that the average flexure strength of dentin beams prepared with these three conditions (control, BT and BT + ET) was approximately 150 MPa and there was no apparent influence of the treatments. Data is from Lee et al. [46].

Fig. 4

Fatigue crack growth responses for mid-coronal dentin (i.e. midway between the pulp and dentin-enamel junction) after being subjected to cyclic loading in either a neutral (control, pH = 7) or acidic (lactic acid solution with pH = 5) environments. These distributions are for cracks extending in-plane and perpendicular to the tubules [37]. (a) A comparison of growth rates within neutral and acidic environments. Cyclic crack growth within the acidic environments occurs at a significantly greater rate (Z = −6.447, p = 0.0005). (b) A comparison of fatigue crack growth for dentin with open lumens (Acid) and after sealing the lumens with resin adhesive (Acid + Sealed). There was no significant difference in the responses between the dentin with open and sealed lumens (Z = 0.4832, p = 0.629). Therefore, the acid attack and degree of degradation was equivalent.

Fig. 5

A comparison of fatigue responses for the resin-dentin bonded interface. (a) Fatigue life distributions for the bonded interface specimens prepared with a commercial two-step (SB) and three-step (SBMP) resin adhesive and comparison with the distributions for dentin and the resin composite (Z100). Note that the data points with arrows represent those specimens that reached at least 1.2 × 10⁶ cycles and the test was discontinued. (b) Fatigue crack growth distributions for the same materials evaluated in (a). As expected, the fatigue strength and fatigue crack growth resistance of the bonded interfaces are significantly lower than those of the resin composite and dentin. However, note that while the interface prepared with two-step resin adhesives shows inferior fatigue strength, it exhibit superior resistance to fatigue crack growth. Results for the interfaces are from Zhang et al. [73]. Results for dentin are for fatigue crack growth perpendicular to the tubules and were reported in Ivancik et al. [37].

Fig. 6

(a) A comparison of the fatigue responses for the SB and SBMP interfaces with results for resin-dentin bonds prepared with SE Bond. Data is from Mutluay et al. [70]. Results for all three systems were obtained using the twin bonded interface approach. Note the value of generating the fatigue life diagram towards understanding the complete fatigue behavior of the bonded interfaces. (b) The influence of cariogenic protocols on the degradation in fatigue strength of dentin [90].

Fig. 7

An evaluation of the fatigue properties of resin-dentin bonded interfaces prepared with a commercial three-step adhesive (SBMP). These specimens were prepared, stored in simulated saliva at 37 °C, and then evaluated after 0, 3 and 6 months. (a) reduction in the fatigue strength distribution with aging. (b) decrease in the fatigue crack growth resistance with aging. According to the Wilcoxon Sum Rank test, the reductions in fatigue strength and fatigue crack growth resistance were both significant at both the 3 and 6 month periods.

Fig. 8

Importance of cross-linking on the fatigue crack growth resistance of resin-dentin bonded interface prepared with three-step adhesive (SBMP). (a) A comparison of the fatigue crack growth responses after 6 months aging for the control specimens (shown in Fig. 6) and specimens treated with EDC for 60 s before application of the adhesive. The fatigue crack growth rate is significantly lower in the EDC treated samples. (b) high magnification SEM view of the interface between the resin tags and interpenetrating collagen fibrils of the intertubular dentin for a control specimen after 6 months of aging. Scale bar represents 2.5 μm. Arrows are used to highlight fractured fibrils (black) and degraded fibrils (white). (c) stained, demineralized section of a CT specimen incubated in water for 6 months before crack propagation through the interface. E indicates the adhesive. This high magnification view shows large voids (asterisk) that have developed amongst the degraded collagen fibrils. Some collagen fibrils showed reduced diameter degradation (pointer). Data is from [107,108].