Bovine Enamel and Human Enamel
The findings in this study revealed two main characteristics of the microstructure of bovine enamel that possessed a complicated spatial-relationship between prisms and interprisms (Figs. 3–5) and a larger size of fiber-like enamel crystals (Fig. 9). The interprisms of bovine enamel were arranged into continuous plate-like structures between rows of enamel prisms (Fig. 3b,c). The orientations of the interprism crystals were quite different from the orientations of prism crystals at the prism/interprism decussating planes, which were perpendicular to the planes of decussating prism groups as shown in Fig. 3b. In contrast, the interprism crystals of human enamel only slightly deviated from the enamel prism crystals, and appeared to “lock” the prisms in place (Popowics et al., 2004). Therefore, there were no prism/interprism decussating planes in human enamel (Popowics et al., 2004).
Bovine enamel and human enamel possess HSBs consisting of decussating prism groups, however, the arrangements of prisms through the HSBs were in a different pattern (Popowics et al., 2004) (Fig. 6). Popowics et al. (2004) found that the groups of the cross-sectioned prisms of HSBs in the human enamel were highly unified in orientation within a band, and the prisms of diazones were nearly perpendicular to the prisms of parazones. However, bovine enamel prisms in a band were not unified in orientation (Fig. 3c), and prism orientations of two adjacent bands slightly deviated from each other (Fig. 3b).
The different arrangements of prisms and interprisms between bovine enamel and human enamel might result in some different outcomes in certain studies using bovine enamel (Cadwell and Johannessen, 1971; Barkmeier and Erickson, 1994; Oesterle et al., 1998; Saleh and Taymour, 2003; Abuabara et al., 2004; Bechtle et al., 2010). Previous research indicated that bovine enamel possessed a lower fracture-resistant value than human enamel (Bechtle et al., 2010). This might help to explain the easy propagation of cracks within the bovine enamel interprisms. In bovine enamel, the crack only followed a single interprism/prism decussating plane, whereas in the human enamel, the crack should follow curvatures of the prism/interprism boundaries (Rasmussen et al., 1976; Xu et al., 1998). Furthermore, a greater difference in the orientations of decussating enamel prisms of human enamel means more loss of energy in redirecting the crack path, thus impeding the progression of the crack (Bajaj and Arola, 2009). Therefore, the enamel of bovine incisors would not be a suitable substitute for human enamel used to study enamel crack propagation.
Moreover, prisms parallel to the adhesive interfaces may result in lower bond strengths than prisms perpendicular to the adhesive interfaces (Ikeda et al., 2002; Gamborgi et al., 2007). Therefore, a large number of plate-like interprisms of the bovine enamel parallel to the adhesive interfaces might be one of the reasons why bovine enamel possesses lower bond strengths than the human enamel (Cadwell and Johannessen, 1971; Barkmeier and Erickson, 1994; Oesterle et al., 1998; Saleh and Taymour, 2003).
The SEM finding in this study indicated that bovine enamel crystals have a thickness and length of approximately 80 and 600 nm, respectively (Fig. 9). They are larger, in both thickness and length, than reported for human enamel crystals (Xie et al., 2008; Hannig and Hannig, 2010). The larger crystals of the bovine enamel might be the result of a more rapid development during tooth formation (Fridell et al., 1988). Bovine enamel is more porous than human enamel which may be attributable to the larger crystals found in bovine enamel (Arends and Jongebloed, 1978). This could help to explain why demineralization of bovine enamel progresses about three times faster than human enamel (Flim and Arends, 1977; Featherstone and Mellberg, 1981). In addition, the calcium and phosphorus contents of human enamel are slightly higher than those of bovine enamel. However, bovine enamel has a significantly higher microhardness value than reported for human enamel does (Davidson et al., 1973; Souza-Gabriel et al., 2010). Attin et al. (2007) found that losses of enamel in human teeth were significant less than that of bovine teeth after exposure to erosion and erosion-abrasion challenges. Bovine enamel is still used as a substitute for evaluating demineralization and remineralization of enamel because bovine enamel and human enamel react similarly to acidic challenges and remineralization conditions (Mellberg, 1992). In addition, some previous research demonstrated that bovine enamel produced more extensive microleakage (Abuabara et al., 2004). Abuabara et al. (2004) found that the microleakage pattern could be affected by the bovine enamel substrate, allowing a higher marginal leakage than a human enamel substrate. This might be attributable to the greater porosity and larger crystal size of bovine enamel compared to human enamel (Fig. 9; Arends and Jongebloed, 1978; Abuabara et al., 2004; Xie et al., 2008; Hannig and Hannig, 2010). Thus, bovine enamel may not be suitable for microleakage studies when used as a substitute for human enamel.
Microstructure and Mechanical Properties
Enamel with parallel prisms in the incisal ridge could provide an efficient edge for incising hard or tough foods like plants and roots (Macho and Berner, 1993). Parallel prisms in the outer enamel of the incisal ridge ensure suitable abrasion to keep enamel edges sharp, because parallel prisms are less resistant to abrasion when compared with the HSBs (Bechtle et al., 2010). Prisms become more resistant when they intersect the occlusal surface perpendicularly (Bechtle et al., 2010). In the inner enamel of the incisal ridge, the parallel prisms that have an undulating structure from the EDJ to the outer enamel (Fig. 8a) could resist the occlusal loads effectively (Macho and Berner, 1993). In addition, parallel prisms in the outer enamel could ensure a suitable structure of the bovine teeth (Bechtle et al., 2010).
The HSBs of the bovine enamel provide structural support and biomechanical advantages in the teeth (Shimizu and Macho, 2008; Bajaj and Arola, 2009; Chai et al., 2009). The directions of the bovine enamel prisms changed from the EDJ to the OES as the prisms passed through the HSBs (Figs. 3 and 4). Thus, the prisms could intersect the enamel surface at a suitable angle to offer the greatest resistance to abrasion (Xie et al., 2009).
Mammalian HSBs are reported to be species specific (Osborn, 1965; Hanaizumi et al., 1998; Lyngstadaas et al., 1998; Lucas et al., 2009). A band of HSBs in the bovine enamel was composed of rows of the prism groups, each having 8–10 prisms and plate-like interprisms at intervals (Fig. 3c). The HSBs of bovine enamel are less developed than those of human enamel because of the existence of interprism/prism decussating planes (Popowics et al., 2004). Because the planes result in a weak combination of prisms, bovine enamel becomes less fracture-resistant (Rensberger and Pfretzschner, 1992; Bechtle et al., 2010). Furthermore, the gradual change of prism orientations in the adjacent bands makes bovine enamel more prone to breakage than human enamel. This is likely because less energy during propagation of a crack would be dissipated throughout the continuous interprism planes between prisms in bovine enamel.
The prism/interprism decussating planes also exist in the pig and the warthog (Koenigswald Wv, 1991; Pfretzschner, 1992). The combined decussation of enamel prisms and interprisms of swine teeth fortify the enamel against crack propagation in multiple directions (Popowics et al., 2004). Therefore, it could be inferred that interprism/prism decussating planes of the bovine enamel might enhance the fracture resistance of the bovine enamel in multiple directions.