Full length articleOn the importance of aging to the crack growth resistance of human enamel
Graphical abstract
Introduction
Due to many improvements in oral health care, the population of partially- and fully-dentate seniors is increasing [1], [2]. There are new challenges associated with this growing group of patients. One concern is that tooth fractures occur more frequently in senior patients than in young adults [3], [4]. Indeed, biological aging is inevitable and it causes detrimental changes to nearly all tissues of the body, including those of teeth [5]. Hence, a better understanding of aging in dental tissues is of critical importance to the field of dentistry.
The importance of aging to the mechanical behavior of hard tissues is not a new topic. In bone, there is a significant reduction in the fracture resistance with increasing age [6], [7], [8], [9]. Dentin, the major hard tissue of the tooth, also undergoes a reduction in the fatigue strength and fracture resistance with patient age [5], [10], [11], [12], [13], [14]. The degradation occurring to these hard tissues has been attributed to changes over various length scales, and involves both the mineral content and the organic matrix. Bone and dentin are collagenous tissues and consist of nearly equal parts of mineral and organic components by volume. In contrast, tooth enamel is the most highly mineralized hard tissue of the body and does not contain a collagenous matrix. Therefore, the changes in mechanical behavior of this tissue with age could be quite different from that of bone and dentin.
Enamel consists of approximately 96% carbonated hydroxyapatite, 3% water and 1% protein content by weight. The individual crystallites are combined with a spatially varying amount of organic substance and coalesce into columnar rods, also known as the enamel prisms [15]. The crystal orientation within the rods varies spatially as well. Within the head region the crystals are aligned with the longitudinal axis, whereas those in the tail (or inter-rod) region are oriented obliquely [16]. The interface between adjacent rods is separated by a narrow zone sometimes referred to as the rod sheath, and consists of crystallites with less well-defined orientation and a slightly higher degree of organic content. Overall, the proteins serve as the cohesive material between adjacent crystallites and, to a greater extent, at the rod interfaces [17]. There is a change in the distribution of rods across the thickness of enamel. Close to the occlusal surface the rods are aligned and parallel to one another, whereas in the middle and inner layers of enamel they follow a complex undulating path known as decussation. In evaluations using optical microscopy rod decussation leads to the appearance of alternating bands of reflected light, which are known as “Hunter Schreger bands” [18]. Enamel rod decussation bestows toughness to this tissue and prevents chipping and tooth fractures [19], [20], [21].
Due to compositional variations that result from prolonged exposure to the oral environment, human enamel undergoes a reduction in permeability and discoloration with aging [17]. In addition, visual observations of enamel often show that the density of cracks and craze lines increases substantially with aging. Indeed, the hardness and elastic modulus of enamel increase with age [22], [23] and they cause an increase in indentation brittleness [24]. While important, previous studies concerning the importance of aging to the mechanical behavior of enamel have been limited to the use of indentation approaches. This study utilizes a combination of experiments and numerical analysis to quantify the contributions of biological aging to the crack growth resistance of human enamel for the first time.
Section snippets
Materials and methods
Caries free human third molars were obtained from participating clinics in the state of Maryland according to an approved protocol (Y04DA23151) issued by the Institutional Review Board of the University of Maryland Baltimore County. All of the teeth were obtained from donors between 17 and 83 years of age and stored after extraction in Hanks Balanced Salt Solution (HBSS) with record of age and gender. Each tooth was inspected at receipt, and any with signs of visible damage or decay were
Results
Typical load vs load-line displacement curves for quasi-static crack growth within representative specimens of young and old enamel are shown in Fig. 3; both of these responses were obtained for longitudinal crack growth. In general, the responses exhibited two distinctive regions of behavior. Region I consists of pre-loading and an increase in the elastic energy prior to advancement of the crack. At the onset of crack extension there was a distinct drop in the magnitude of opening load. Region
Discussion
Following the investigation reported by Hassan et al. [36], the fracture behavior of human tooth enamel has been studied for a period of more than three decades. Nevertheless, the importance of aging to the mechanical behavior of enamel has received limited attention. The present investigation is the first to explore the contribution of biological aging to the fracture resistance of this tissue using a conventional fracture mechanics approach. Results demonstrated that the crack growth
Conclusions
Based on experimental results and a complementary finite element analysis of fracture in human enamel, there is a reduction in the crack growth resistance of this tissue with aging. For cracks extending along the axis of the rods, the fracture toughness of old enamel was 1.38 ± 0.35 MPa m0.5, which was more than a 30% reduction from that of enamel from young adult teeth. For cracks extending transverse to the rods, the average apparent fracture toughness was 0.37 ± 0.15 MPa m0.5, which was 70% lower
Acknowledgements
The authors acknowledge that this study was supported in part by the National Institutes of Dental and Craniofacial Research through grant (R01 DE016904) and the National Science Foundation (NSF DMR 1337727). The authors would also like to thank Ultradent Products Inc for supplying the Vit-l-escence resin composite.
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