Symphyseal surface strain during in vitro human mandibular wishboning

Abstract

Human symphyseal form, with its distinctive chin, is unlikely to be adapted for countering wishboning loads. Chins are associated with larger than expected strain gradients within and between symphyseal surfaces, which runs counter to the optimality criterion typically invoked in assessing trait performance for signs of adaptation. The implications are twofold: 1) wishboning may not, in fact, be a regular feature of human mastication or 2) wishboning may not pose the same structural risks in human jaws as this load does in other anthropoid primates.

The enduring puzzle of the human chin

“…These authors, in identifying these loads, also noted that wishboning is the most significant of these loading regimes because it is responsible for the largest magnitude strains. Hylander's experimental data from macaques frequently have been used as an analogue for human mastication, though in‐vivo loading regimes have never been observed in humans . However, electromyographic analyses indicate that human and macaque symphyses likely experience very similar strain environments …”

“…Holton et al have shown that as the human symphysis matures, the bone geometry assumes a less biomechanically efficient form for countering wishboning, in much the same way that modern human ontogeny does not organize symphyseal geometry to counter coronal bending moments. Also, Pampush and Daegling, in a series of in‐vitro experiments, measured symphyseal surface strain in modern human mandibles subjected to wishboning loads. We showed that wishboning results in large suboptimal strain concentrations in the symphysis.…”

“…Our current understanding of the forces to which the mandible and its symphysis is subjected during ingestion and mastication is based on macaque analogs and inferences from in‐vitro or simulation‐based studies . We have no in‐vivo data concerning the stress and strain environment of the human symphysis.…”

“…Hylander's experimental data from macaques frequently have been used as an analogue for human mastication, though in-vivo loading regimes have never been observed in humans. 72 However, electromyographic analyses indicate that human and macaque symphyses likely experience very similar strain environments. 73,74 While the human in-vivo masticatory stress environment probably contains all of the loads observed by Hylander -and the resultant stresses are frequently superposedfor heuristic reasons, researchers have considered them in isolation.…”

“…Our current understanding of the forces to which the mandible and its symphysis is subjected during ingestion and mastication is based on macaque analogs 67,70 and inferences from in-vitro or simulation-based studies. 52,65,72 We have no in-vivo data concerning the stress and strain environment of the human symphysis. Therefore, it is possible that our current models aimed at understanding the chin as a masticatory adaptation are based on flawed assumptions of human symphyseal loading.…”