Research article
Occlusal load distribution through the cortical and trabecular bone of the human mid-facial skeleton in natural dentition: A three-dimensional finite element study

https://doi.org/10.1016/j.aanat.2014.09.002Get rights and content

Abstract

Understanding of the occlusal load distribution through the mid-facial skeleton in natural dentition is essential because alterations in magnitude and/or direction of occlusal forces may cause remarkable changes in cortical and trabecular bone structure. Previous analyses by strain gauge technique, photoelastic and, more recently, finite element (FE) methods provided no direct evidence for occlusal load distribution through the cortical and trabecular bone compartments individually. Therefore, we developed an improved three-dimensional FE model of the human skull in order to clarify the distribution of occlusal forces through the cortical and trabecular bone during habitual masticatory activities. Particular focus was placed on the load transfer through the anterior and posterior maxilla. The results were presented in von Mises stress (VMS) and the maximum principal stress, and compared to the reported FE and strain gauge data. Our qualitative stress analysis indicates that occlusal forces distribute through the mid-facial skeleton along five vertical and two horizontal buttresses. We demonstrated that cortical bone has a priority in the transfer of occlusal load in the anterior maxilla, whereas both cortical and trabecular bone in the posterior maxilla are equally involved in performing this task. Observed site dependence of the occlusal load distribution may help clinicians in creating strategies for implantology and orthodontic treatments. Additionally, the magnitude of VMS in our model was significantly lower in comparison to previous FE models composed only of cortical bone. This finding suggests that both cortical and trabecular bone should be modeled whenever stress will be quantitatively analyzed.

Introduction

Distribution of occlusal forces through the mid-facial skeleton in natural dentition had traditionally been explained to occur along specific osseous trajectories known as buttresses (Cryer, 1916, Sicher and Tandler, 1928, Rowe and Williams, 1985). The buttresses were anatomically described as areas in the mid-facial bones that have a form of vertical and horizontal pillars (Fig. 1) composed of a thick cortical bone (Cryer, 1916). Seven vertical buttresses were proposed to transfer most of the occlusal load (Fig. 1A), while three horizontal buttresses (Fig. 1B) were suggested to stabilize the vertical buttresses mechanically by interconnecting them at different levels (Cryer, 1916, Sicher and Tandler, 1928, Rowe and Williams, 1985).

Occlusal force distribution was studied further in dry skulls using strain sensitive transducers, also known as strain gauges (Endo, 1965, Endo, 1966, Endo, 1970). When attached to the facial bone surface, this device registered bone microdeformations in response to artificial tooth loading. During such experiments, Endo, 1965, Endo, 1966, Endo, 1970 detected mainly compressive deformations in the cortical bone along the vertical buttresses and simultaneous tensile deformations in the region of the horizontal buttresses. Endo (1965) also found the highest strain magnitude in the maxillary cortex above the anterior teeth, which decreased gradually as the strain gauge was moved distally along the dental arch. Similar to Endo's findings, Alexandridis et al., 1981, Alexandridis et al., 1985 reported that occlusal forces in the photoelastic skull models distribute predominantly over the anterior maxilla. The skull models used in these studies were formed of a birefringent material, loading of which causes the incident beam of polarized light to split in the direction of stress distribution (Alexandridis et al., 1981, Alexandridis et al., 1985). Although these studies revealed important data related to the mechanical behavior of cortical bone during occlusal loading, neither the contribution of all masticatory muscles nor the role of trabecular bone in the occlusal force distribution were possible to assess by these techniques.

Recent application of finite element (FE) method appeared the most promising for clarifying the pattern of occlusal load distribution through the mid-facial bones (Tanne et al., 1988). However, few published FE studies addressing this problem provided conflicting results that, in some cases, differed significantly from the theory of buttresses and/or strain gauge data. During the simulation of biting, Cattaneo et al. (2003) reported that distribution of von Mises stress (VMS) follows the route of the vertical buttresses, whereas Gross et al. (2001) observed that VMS splits above the loaded tooth and dissipates in two directions different from buttresses. More recently, Prado et al., 2012, Prado et al., 2013 measured VMS along the vertical buttresses during biting, and concluded that stress distributes unevenly through the mid-facial skeleton. Gross et al. (2001) also simulated clenching and described almost uniform distribution of occlusal stress. These studies frequently used oversimplified skull models created only of cortical bone. Bone elastic properties and the magnitude of force applied to the teeth, accurate selection of which is crucial for FE analysis (Strait et al., 2005; Gröning et al., 2012), differed significantly from the experimentally calculated values in healthy dentate individuals. Moreover, these studies did not provide direct evidence for occlusal stress distribution, particularly through the cortical and trabecular bone compartments individually.

In general, there is a need for more detailed investigation of occlusal load distribution through the mid-facial bones in the natural dentition because alterations in magnitude and/or direction of occlusal forces may cause remarkable changes in both cortical and trabecular bone structure (Bresin et al., 1994, Bresin et al., 1999, Mavropoulos et al., 2004, Mavropoulos et al., 2005, Tanaka et al., 2007, Canullo and Götz, 2012, Hasan et al., 2014). Such changes were detected not only in the alveolar bone, but also at the distant sites, e.g. in the zygomatic (Kato et al., 2004, Yoshino et al., 2007) and frontal bones (Dechow et al., 2010).

Therefore, the aim of our study was to clarify the distribution of occlusal forces through the cortical and trabecular bone of the mid-facial skeleton during habitual masticatory activities. Particular focus was on the patterns of load transfer in the anterior and posterior maxilla. In order to overcome simplifications used in previous FE studies, we developed an improved 3D skull model with both cortical and trabecular bone compartments, hollow structures within the mid-facial bones, and jaw-closing muscles. The results presented in equivalent von Misses stress (VMS) and maximum principal stress values were analyzed both qualitatively and quantitatively, and compared with previous finite element studies and patterns of bone strain registered in dry human skulls.

Section snippets

Modeling of the skull

A computer model of the skull was created using CT images of a dry skull of a young adult Caucasian male with the fully dentate maxilla (Fig. 2A). Sexually dimorphic features were moderately expressed so that the skull presented an average anatomical situation. The skull was chosen from the skeletal collection of the Laboratory for Anthropology (Institute of Anatomy, Faculty of Medicine, University of Belgrade), and scanned by Computed Tomography (Siemens Somatom Sensation 16) in 0.75 mm thick

First molar biting

Fig. 3A–C shows the distribution of von Mises stresses within the cortical and trabecular bone during the first molar biting. The highest VMS was registered in the cortical bone of the alveolar process just above the loaded teeth (Fig. 3A and B). High stresses in the anterior maxillary wall, around the piriform aperture, and along the frontal process of the maxilla corresponded to the area of the nasomaxillary buttress. The parts of the zygomaticomaxillary buttress were also visualized by

Discussion

The pattern of occlusal load distribution observed in our study was partially consistent with the classical theory of buttresses. Areas of the nasomaxillary and zygomaticomaxillary buttresses were clearly seen in the cortical bone with the higher stress registered in the former. These sites also exhibited the highest compressive strains in dry human skulls during artificial tooth loading (Endo, 1965). The distribution of compressive and tensile stresses along the two buttresses in our model was

Acknowledgements

The authors acknowledge support from the Ministry of Education and Science of the Republic of Serbia (project numbers: III45005, III41007, ON174028) and from FP7-ICT-2007 project (grant agreement 224297, ARTreat). The authors have no conflict of interest to declare.

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