Analysis of arch‐like bones: The rodent mandible as a case study

Abstract

Bone strength is determined by the mechanical properties of bone material, and the size and shape of the whole bone, i.e., its architecture. The mandible of vertebrates has been traditionally regarded as a beam oriented in relation to main masticatory loads, i.e., the longer dimension of its cross-section being parallel to the load. Rodents follow this pattern but, in addition, their mandible possesses an intriguing arch-like shape that is apparent when seen in the lateral view. Little attention was given to the structural capacity of this trait. The advantage of an arch is that it can withstand a greater load than a horizontal beam. The objective of this study was to model the rodent mandible like an arch to evaluate its structural strength. The bending moment in an arch-like mandible was 15-25% lower with respect to a beam-like mandible. Further, bending varies with mandible "slenderness" and incisor procumbency, a functionally relevant rodent trait. In the rodent Ctenomys talarum (Caviomorpha; Ctenomyidae), bone stress was substantially reduced when the mandible was modeled as an arch-like structure as compared with a beam-like structure, and safety factors were 15-34% higher. The shape of rodents' mandible might confer a functional advantage to high and repeatedly applied loads resulting from a unique feeding mode: gnawing. J. Morphol. 277:879-887, 2016. © 2016 Wiley Periodicals, Inc.

Mandible strength and geometry in relation to bite force: a study in three caviomorph rodents

“…Condyles and angular processes have a relatively homogeneous distribution of SFs for the three species, with minimum values ranging from 1.8 to 5.5. SF maxima occur at the mandibular corpuses, in agreement with Vassallo's () findings. SF maxima for tuco‐tuco and degu are 2–2.5 times higher than for chinchilla (Table ).…”

“…These authors explained the dorso‐ventral elongation of the mandible shape as an adaptation to the powerful canine biting used by felids to hold and kill their prey, an interpretation that is comparable to the strong incisor biting by rodents. Moreover, rodent mandibles have an arch‐like shape in lateral view, which may enhance its bending strength (Vassallo, ).…”

The biomechanical significance of the elongated rodent incisor root in the mandible during incision

“…The mechanical significance of an arch is its ability to withstand a greater load than an otherwise materially equivalent horizontal beam can support. Vassallo 22 found that as the mandible is exposed to three-point bending, its curved, arched shape reduces the bending moment by up to 25% more than in a beam-like mandible and argued this to be a functional adaptation for the resistance of forces encountered during gnawing. The curvature of the mandibular corpus, associated with the curvature of the underlying incisor, together with the increase in vertical height of the mandibular corpus are more effective ways of providing resistance to this sort of bending than increasing the medio-lateral thickness of the corpus 47 , 48 .…”

“…In rodents, the unerupted portion of the incisor typically extends distally to the end of the tooth row or just beyond 9 , occasionally extending as far as, or into, the condylar process 21 . It has previously been suggested that the deep internal projection of the incisor within the rodent mandibular body is responsible for the arch-like shape of the mandible visible in many rodents and a reduction in the bending moment by up to 25% compared to a beam-like mandible 3 , 22 . In addition, other biological curved structures such as long bones have been proposed to make the location and orientation of strains more predictable 23 , 24 .…”

“…Three main hypotheses will be tested: Reduction in the length of the incisor root will result in an increase in stress across the mandible. Specifically, we expect to see this most clearly in von Mises stresses along the ventral surface of the mandible based on the assumptions that the rodent-like mandible operates in a manner similar to an arch 22 , and that an arch operates in compression. We will test this by comparing the stress magnitudes predicted by the FE models with complete incisors and those with virtually shortened roots.…”