The understanding of the masticatory apparatus including its functional and structural relationship with other components of the cranium increasingly requires an interdisciplinary approach. Recently, "traditional biological sciences" such as anatomy, comparative biology, anthropology and evolution have increasingly meshed with elements from other domains, such as mechanical engineering and material sciences, which has resulted in new and exciting paradigms to be explored. This is particularly true in the field of craniofacial biomechanics yet there are still many unexplored issues and numerous questions that remain unanswered. Numerical modelling in general and Finite Element Analysis (FEA) in particular, represent a numerical experimental procedure to generate such information. Originally derived from the field of structural engineering, FEA has steadily permeated its way into craniofacial biomechanics and has proven itself as a most useful scientific tool.
The present study introduces an engineering-based workframe for applying FEA to craniofacial biomechanical research in a comprehensive manner to cover the entire analytical spectrum, from developing questions to providing their solutions. The study is composed of two major experimental parts addressing both the linear elastic and the non-linear behaviour of some biomaterials encountered in the craniofacial arena. In the first part I analysed mandibular biomechanics using linear elastic models while in the second part I used nonlinear discrete models to determine the optimal elastic properties of the cervical restorative materials.
Modern humans have a number of anatomical features that set us apart from our ancestors. Amongst these perhaps the most striking is the emergence of a protruding chin, otherwise absent in other archaic humans and hominids. While it has been shown that the chin has its embryological origins in the postnatal remodelling of bone in the area around the mandibular symphysis which produces the midline keel in the form of an inverted �T� the functional significance of this novel evolutionary feature is still obscure.
It is accepted that the mandible is optimally designed for resisting masticatory stress, whereby optimal is seen as maximual strength at the lowest biological cost. Here, I tested the currently most accepted theory, namely that the chin provides mechanical resistance to the mandible during mastication. In other words, I tested the hypothesis that a chinned mandible would be stiffer and hence experience lower strains when compared to a non-chinned counterpart under identical loadings. My functional analysis consisted firstly of three simple models which reproduce a simian shelf, a flat and a chinned symphysis, loaded using two unidirectional loadcases (torsion and wishboning) to represent a distortion similar to that which occurs in the mandible during mastication. Secondly, I developed complex geometrical models which incorporated the cortical bone, medullary bone and teeth. The models were then analysed using the same loadcases as those used for the first theoretical models. Additionally, I incorporated the coronal bending and also a coupled loadcase which simulated the complex deformation of the mandible during biting. The aim here was to test the hypothesis that the presence of a chin changed the strain pattern in the mastication-loaded mandible. The results were then interpreted using Frost�s mechanostat theory which relates in a more precise manner the mechanical loading environment to the adaptive response of the bone. My results showed that the calculated strain values for both the chinned and flat mandibles were within the normal bone maintenance levels of the mechanostat during molar biting. In other words, variation in bone strain magnitude across the mandible, which should differ between the chinned and the non-chinned mandibles if the hypothetical mechanical role of the chin is true, is similar in both forms. I concluded that the development of the human chin is thus unrelated to the functional demands placed upon it by mastication.
I suggested a new functional demand associated with pronounced tongue activity during speech. I hypothesise that it is the resistance to stresses induced by strong, repetitive contractions of the tongue and perioral musculature during, phonation that shaped the modern human chin. I tested my hypothesis by loading the symphyseal region with two principal nonmasticatory, muscle systems; firstly, the tongue and secondly the peri-oral muscular curtain, anterior to the symphysis. My results suggested that the flat, non-chinned symphysis when subjected to speech-related genioglossal movements will undergo adaptive changes which would result in an optimised (chinned) shape, such as that found in the modern human symphysis.
These results thus offer a new foundation to an old hypothesis and a solution to the longstanding controversy over the origin of the human chin. I conclude that forces generated by speech rather than those generated by mastication, shaped the chin in anatomically modern humans.
Prompted by an earlier observation I further investigated the apparent cross-over distribution of strains on the mandibular corpora during mastication. In doing so, I tested the hypothesis that this cross-over may be linked with another particular anatomical feature of the mandible that of the postcanine cortical asymmetry, which appears to be stereotypical among anthropoids. The results of my study hence suggest that strain patterns within the human mandible are more complex than previously thought. Not only do strains differ between lingual and buccal aspects of working and non-working sides, but they also differ within these areas (i.e. from alveolus to corpus, to lower border regions). I conclude that postcanine cortical asymmetry may be a retained evolutionary trait rather than the result of masticatory biomechanics.
In the second section of the thesis I introduced a different analysis regime which allows the prediction of fracture initiation and propagation. In this part I analysed the mechanics underlying the failure of the restorations placed in non-carious cervical lesions and suggested changes in the material properties of the restorations used to treat them.
Non-carious cervical lesions (NCCL) include those entities characterised by the cervical loss of hard dental tissue that occurs in the absence of any carious process. To distinguish between lesions that occur due to excessive occlusal load and other non-carious cervical lesion (i.e. erosion and abrasion) the clinical term "abfraction" has been adopted. Although a common clinical issue, failure of restoration placed in these lesions has not been subjected to a rigorous biomechanical analysis.
To determine which of the material�s parameters should be changed and to what extent, I employed a combined numerical approach.
Here I introduced a novel approach in simulating the cracking of restorative materials and tooth tissues which is based on a simpler material formulation and can be used in an advanced nonlinear numerical analysis. The material model I used allows automatic crack insertion and growth and also uniquely accounts for the microdamage which precedes the instalment of macroscopic cracks.
The first step was to balance the factors that may affect failure employing a linear analysis with a stress-based approach to failure. Here, the aim was to investigate the influence of lesion shape and depth as well as the direction of occlusal loading on the mechanical response of the cervical glass-ionomer cements restoration in a lower first premolar. This analysis showed that the direction of loading was the major contributor to the failure of the restoration.
The next step was to apply this fracture model to the restorations of the NCCL in order to verify if the material is able to accurately simulate the location and type of mechanical failure. The data for this problem, i.e. the geometry and the loadcase were derived from the conclusions of linear analysis, that is I chose the "worst case scenario" as the upper boundary of material endurance. My results showed that under the action of para-functional loadings the GIC failed on the cervical margin. I also showed that prior to fracture the restorative material undergoes strain softening, which in turn introduces damage and weakens the materials involved. After successfully testing the proposed model, the final step was to determine which material properties and restorative techniques would be most reliable under given biomechanical conditions. The present work relied on the hypothesis that a more flexible material would partially buffer the local stress concentration and hence reduce the likelihood of mechanical failure of the restoration.
My study, a first of its kind, proposes a radical approach to address the problems of material improvement, namely: numerical-based material optimisation engineering. That is, I aimed to identify the "most favourable" selection of elastic modulus or E value for the restorative material, which will allow it to survive under the unfavourable occlusal loading conditions that may prevail. Two filling techniques were considered; firstly a single bulk material, namely glass-ionomer (GIC) and secondly a layered technique. The latter consisted of a layer of GIC supporting a composite bulk restorative. I chose two thicknesses for the GIC layer, 50 and 150 microns. My results showed that the restorative materials currently used in cervical non-carious lesions are largely unsuitable in terms of resistance to fracture of the restoration mostly because of their relative high stiffness irrespective of the filling technique. The best results are obtained for a bulk filling with a 1GPa elastic modulus material case in which the tensile stresses are about 50% of the failure limit. This approach in determining the mechanical properties of the restorative is novel and unique so far in the dental literature. The direct benefit of this study was the improvement of the restorative material, as it can be engineered to withstand the conditions identified as major cause of failure. This is consonant with the call for new materials better tailored for some specific needs.
| Identifer | oai:union.ndltd.org:ADTP/266543 |
| Date | January 2008 |
| Creators | Ichim, Ionut P, n/a |
| Publisher | University of Otago. School of Dentistry |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Ionut P Ichim |
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