Unravelling the functional biomechanics of dental features and tooth wear

Abstract

Most of the morphological features recognized in hominin teeth, particularly the topography of the occlusal surface, are generally interpreted as an evolutionary functional adaptation for mechanical food processing. In this respect, we can also expect that the general architecture of a tooth reflects a response to withstand the high stresses produced during masticatory loadings. Here we use an engineering approach, finite element analysis (FEA), with an advanced loading concept derived from individual occlusal wear information to evaluate whether some dental traits usually found in hominin and extant great ape molars, such as the trigonid crest, the entoconid-hypoconulid crest and the protostylid have important biomechanical implications. For this purpose, FEA was applied to 3D digital models of three Gorillagorilla lower second molars (M2) differing in wear stages. Our results show that in unworn and slightly worn M2s tensile stresses concentrate in the grooves of the occlusal surface. In such condition, the trigonid and the entoconid-hypoconulid crests act to reinforce the crown locally against stresses produced along the mesiodistal groove. Similarly, the protostylid is shaped like a buttress to suffer the high tensile stresses concentrated in the deep buccal groove. These dental traits are less functional in the worn M2, because tensile stresses decrease physiologically in the crown with progressing wear due to the enlargement of antagonistic contact areas and changes in loading direction from oblique to nearly parallel direction to the dental axis. This suggests that the wear process might have a crucial influence in the evolution and structural adaptation of molars enabling to endure bite stresses and reduce tooth failure throughout the lifetime of an individual.

Figure 1. Digital reconstruction of the gorilla specimen ZMB-31626 (lower left second molar – LM₂).

The three dental traits examined in this study (protostylid, trigonid crest, entoconid-hypoconulid crest) are highlighted both in the crown (top) and in the enamel-dentine junction (bottom).

Figure 2. Basic steps to create the volumetric mesh and to recognize the contact areas for specimen ZMB-31435.

A, dental tissues and supporting structures for the lower left second molar (LM₂) of specimen ZMB-31435. B, collision detection for specimen ZMB-31435 in the Occlusal Fingerprint Analyser (OFA) software during maximum intercuspation contact situation; the LM¹-LM² are transparent to show the collision (red spots) in the occlusal surface of the LM₂ (see also Video S1). C, the FE mesh of specimen ZMB-31435 consisting of 2,482,913 ten-nodded tetrahedral elements. PDL = periodontal ligament; B = buccal; D = distal; L = lingual; M = mesial.

Figure 3. Loading position and direction for specimen ZMB-31435, ZMB-31626 and ZMB-83551.

For each lower left second molar (LM₂) only the volumetric mesh of the enamel is displayed. The load (red arrows) was distributed proportionally according to the occlusal contact areas detected in the Occlusal Fingerprint Analyser (OFA) software (see also Video S1-S3). B = buccak; D = distal; L = lingual; M = mesial.

Figure 4. The enamel volumetric meshes of specimen ZMB-31435 and ZMB-31435sim.

A, the volumetric mesh of specimen ZMB-31435 LM₂ with highlighted the crests considered in the simulation. B, the volumetric mesh with artificial mesiodistal grooves interrupting the trigonid and entoconid-hypoconulid crests (specimen ZMB-31435sim).

Figure 5. Maximum principal stress distribution observed in ZMB-31435 LM₂ (left), ZMB-31626 LM₂ (middle) and ZMB-83551 LM₂ (right) during maximum intercuspation contact.

Blue areas mark the position were occlusal forces were applied. First row = occlusal view; second row = buccal view; third row = lingual view. B = buccal; D = distal; L = lingual; M = mesial.

Figure 6. Sections of the enamel volumetric meshes along the buccolingual groove (A-A) and mesiodistal groove (B–B).

A, specimen ZMB-31435 LM₂. B, specimen ZMB-31626 LM₂. C, specimen ZMB-83551 LM₂.

Figure 7. Maximum principal stress distribution in ZMB-31435 LM₂ and ZMB-31435sim LM₂ during a representative time-step of phase I.

Blue areas on the occlusal surface mark the position were occlusal forces were applied, and red areas show maximum tensile stress. The plot on the right side shows the differences in tensile stress values between the two specimens based on 14 homologous nodes selected on the occlusal grooves.