Enamel microstructure and microstrain in the fracture of human and pig molar cusps

The role of microstructure in enamel strain and breakage was investigated in human molar cusps and those of the pig, Sus scrofa. Rosette strain gauges were affixed to cusp surfaces (buccal human M3, n=15, and lingual pig M1, n=13), and a compressive load was applied to individual cusps using an MTS materials testing machine. Load and strain data were recorded simultaneously until cusp fracture, and these data were used to estimate enamel stresses, principal strains, and stiffness. Fractured and polished enamel fragments were examined in multiple planes using scanning electron microscopy (SEM). Human cusp enamel showed greater stiffness than pig enamel (P=0.02), and tensile stress at yield was higher (17.9 N/mm2 in humans versus 8.9 N/mm2 in pigs, P=0.06). SEM revealed enamel rod decussation in both human and pig enamel; however, only pig enamel showed a decussation plane between rod and inter-rod crystallites. Human inter-rod enamel was densely packed between rods, whereas in pig enamel, inter-rod enamel formed partitions between rows of enamel rods. Overall, human enamel structure enabled molar cusps to withstand horizontal tensile stress during both elastic and plastic phases of compressive loading. In contrast, pig cusp enamel was less resistant to horizontal tensile stresses, but appeared to fortify the enamel against crack propagation in multiple directions. These structural and biomechanical differences in cusp enamel are likely to reflect species-level differences in occlusal function.

The fracture behaviour of human and pig molar cusps

Masticatory efficiency depends upon the ability of the molar cusps to apply concentrated bite forces to food particles and simultaneously to withstand the dental stresses that may cause enamel fracture. This study investigated how low-crowned molar cusps in omnivorous mammals, specifically humans, Homo sapiens, and pigs, Sus scrofa, resist fracture under compressive load. A uniaxial compressive load was applied to individual molar cusps with a materials testing machine. The progressive loading and deformation of the cusps were recorded for interrupted and continuous tests. In interrupted tests, the appearance of progressive cusp fracture was recorded. Stiffness and fracture stresses were calculated from continuous test results. Pig cusps responded to both interrupted and continuous loads with greater deformation; progressive crumbling of the cusp tip resulted in new occlusal contacts on enamel lophs. Conversely, human cusps showed minimal breakage before failure. Continuous compressive tests demonstrated the greater stiffness of human cusps, as well as the capacity to sustain higher cusp tip stresses. The greater stiffness and high fracture resistance of human cusps may be attributed to the thickness of enamel. Test results reflected fundamentally different means of crown stress management that correspond with phylogenetic differences in masticatory function.

Fine structure of bone in dinosaurs, birds and mammals

After observation of detailed structural evidence for the origin of birds from dinosaurs, and in light of evidence that dinosaur bone tissue resembles the histology in mammals, the histology of bone has become one of the focal points in discussions of the physiology of dinosaurs and Mesozoic birds. Most of this microstructural information has focused on features related to the vascular organization and the amount of remodelled bone around vascular canals. However, the finer structures have received less attention, although differences in such structures have been observed among modern vertebrates. Here we present evidence that canaliculi--the submicrometre-sized channels that interconnect bone cells and vascular canals--and the collagen fibre bundles in bone are differently organized among certain dinosaur lineages. Ornithomimid dinosaurs are more like birds than mammals in these features. In canalicular structure, and to some extent in fibre bundle arrangement, ornithischian dinosaurs are more like mammals. These differences in both canalicular and lamellar structure are probably linked to differences in the process and rate of bone formation.

Stereological analysis of bone architecture in the pig zygomatic arch

BACKGROUND

Stereological analysis of trabecular bone structure may reveal information about regional variations in stress distribution, especially in areas like the zygomatic arch in which those variations are difficult to assess mechanically. This study investigates regional differences in trabecular orientation, thickness, and density in the zygomatic and squamosal bones of pigs.

METHODS

Zygomatic arches were serially sectioned frontally (n = 4), horizontally (n = 4), or parasagittally (n = 4), at a thickness of 0.8 mm. Sections were viewed under a stereomicroscope; video-images were digitized and analyzed with an automated program.

RESULTS

All regions were anisotropic. Predominant orientation of trabeculae differed between and within bones. Three main patterns were seen. Anteriorly, zygomatic trabeculae were mainly arranged vertically and anteroposteriorly (relative to the occlusal plane). Posteriorly, including the jaw joint region, the squamosal featured primarily mediolateral trabeculae. In the midsection of the arch, where the two bones overlap, the trabeculae displayed a predominantly anteroposterior orientation with a secondary mediolateral peak. Trabeculae were typically 0.3-0.4 mm wide and occupied 40-50% of the area of the sections with few regional variations.

CONCLUSIONS

Trabecular bone in the pig zygomatic arch is arranged orthogonally, relative to the occlusal plane. In conjunction with information from strain gauge recording, these data suggest that the zygomatic bone is bent in the parasagittal plane whereas the squamosal is bent out-of-plane. The mediolateral trabeculae in the posterior regions are consistent with a cantilever effect at the jaw joint.

Enamel structure in astrapotheres and its functional implications

Astrapotheres, large extinct ungulates of South America, share with rhinoceroses vertical prism decussation in the cheek tooth enamel. The similarity extends beyond merely the direction of the planes of decussation. The vertical decussation in astrapotheres is confined to the inner part of the enamel and has uniformly well-defined zones in which the prism direction differs by nearly 90 degrees and the zones are separated by narrow transitional borders of intermediate prism direction. The outer enamel consists of predominantly occlusally and outwardly directed prisms. Within the outer enamel is a region of horizontally decussating prisms; here the angle of decussation is usually smaller than that of the inner vertically decussating prisms. Except for the horizontal decussation in the outer enamel, these conditions match structures that have been described for rhinocerotoids. These features, together with the similarity in premortem crack direction and gross shape of the cheek teeth, imply that astrapotheres and rhinocerotoids shared essentially the same system of cheek tooth mechanics. However, the microstructure of the canine enamel in the astrapotheres is distinct. The lower canine enamel of the Oligocene Parastrapotherium exhibits a form of vertical decussation modified by a wavelike bending of prism zones, whereas the decussation in the rhinocerotoid canine is horizontal. The lower canine in Parastrapotherium was subjected to different loading conditions, judging from multiple sets of premortem crack directions. The modified vertical decussation would in theory resist cracking under different directions of tensile stresses. This is confirmed by the sinuous paths of cracks that run in directions differing by up to 90 degrees. That diverse stresses were generated in the enamel during life is confirmed by the pattern of premortem cracks in Parastrapotherium. The enamel in the upper canine of a late Miocene astrapothere lacks decussation but may have resisted cracking under varied loading conditions by virtue of a 3-dimensional wavelike bending of the prisms.

Microscopic effects of predator digestion on the surfaces of bones and teeth

Concentrations of small fossil mammals are frequently encountered in Cenozoic deposits, but the causes for such accumulations have seldom been determined. In many cases the tooth, jaw, and limb fragments appear to be well-preserved under light microscopy, and it is difficult to differentiate damage due to predator digestion from breakage and abrasion due to physical agents. In order to find more specific evidence of predator digestion, we used a scanning electron microscope (SEM) to examine the surface microstructure of bones and teeth consumed by Bubo virginianus (great horned owl) and Canis latrans (coyote), which prey upon similar species. Effects of digestion were found on all the digested bones and teeth examined. The effects on bone include distinctive sets of pits and fissures, dissolution, and physical polishing. The pits and fissures are apparently caused by solution that commences in canals beneath the surface of the bone. The most conspicuous effects on teeth are island-like pillars of dentin surrounded by deep solution fissures. The effects of digestion by coyote and owl are fundamentally the same but differ in degree of development. Bone digested by the owl shows a greater degree of polishing and rounding of edges but has less extensive fissuring. Wide variation in the degree of surface damage occurs in bones digested by the coyote, even within a single fecal pellet.

Changes in the tooth enamel of early Paleocene mammals allowing increased diet diversity

Hunter-Schreger bands (HSB) are seen in teeth that are composed of crossed sets of enamel prisms. They are present in the teeth of man and many other mammals, but absent in most insectivores and multituberculates. It has been suggested that the presence of HSB makes the tooth enamel less likely to split and is associated with chewing ability. We have traced the occurrence of HSB back to the arctonic condylarths of the early Palaeocene (Puercan) age; this must be close to the first appearance of the bands in placental mammals. Our data indicate that the teeth of almost all large mammals since the early Palaeocene have contained these bands, in an orientation that is optimal for limiting the propagation of vertical fractures. The appearance of the bands is associated with the differentiation of herbivores and carnivores from insectivores and our data indicate that their development was critical to the diversification of mammals because it allowed the use of new types of foods.

Cracks in fossil enamels resulting from premortem vs. postmortem events

Vertebrate enamel preserves a record of fracture-producing strain. Fracturing during the life of the individual is potentially a source of selection for stronger enamel in the course of evolution. To determine if it is possible to recognize such fractures in fossil enamel, cracks in a variety of fossil materials, including enamel-covered holostean scales, crocodilian teeth, theropod and hadrosaurid dinosaur teeth, and mammalian teeth were examined. Cracks that occurred during the life of the individual could be recognized by abrasive wear on edges exposed at the surface of the enamel in areas worn by oral or locomotor abrasion. Certain distinctive crack patterns were identified as results of specific stress states occurring during life. Transverse cracks on the anterior parts of Lepisosteus scales were probably caused by external loading. Hertzian cracks and shallow, arcuate, lateral cracks on the occlusal edges of tooth enamel appear to be caused by stress concentrating impacts. Horizontal cracks arranged asymmetrically on the sides of conical teeth were reproduced in models subjected to bending stresses. Oblique cracks near the tips of conical fossil teeth were produced in models by oblique loads near the tip. Vertical cracks around cylindrical or conical tooth surfaces may be caused by several different sources of stress, including lateral "wind" loads and vertical "snow" loads. Of the postmortem causes of fracturing of fossil enamel, drying cracks seem to be the most important. Experimental drying produced from 25% to 50% of the cracks in dry teeth.