Mechanical forces, such as mastication, influence morphological characteristics of the cranium. With varying degrees of prognathism found within and between populations, the ability to accommodate masticatory stress may vary, and this will have profound effects on final craniofacial form. The purpose of this research is a two-fold examination of mid-facial prognathism in modern African males. First, an osteometric and morphological examination of specific areas of the cranium involved in the masticatory apparatus was performed, and its relationship with prognathism assessed. Second, finite element analysis (FEA) was used to interpret the distribution of stress during mastication and the contribution of prognathism to this stress distribution. Two diametrically opposed facial forms (prognathic and orthognathic) were modelled to observe variation in displacement, pressure, and Von Mises stress patterns using linear elastic homogenous isotropic material properties.
Boundary conditions simulating muscle contraction of the masseter, medial pterygoid, and temporalis were attributed to the models. A vertical compressive bite-force was applied at the left central incisor and the first molar, respectively. With the use of FEA, differences in the pattern and magnitude of Von Mises stress were noted under simulated mastication. The prognathic model consistently experienced more stress for a molar and incisal bite-force than the orthognathic model. More specifically, the prognathic model accommodated for larger areas of Von Mises stress in the regions of the zygomatic arch, nasal aperture, margins of the orbits, and in the inter-orbital area. As individual muscle forces were modeled, the temporalis and medial pterygoid caused the greatest difference in the stress at the articular eminence between the working and balancing sides. These muscles and their forces should be further investigated to understand their role in temporomandibular joint disorders.
Several cranial dimensions were shown to increase or decrease with prognathism. The relationship between the gnathic index and facial parameters were statistically significant for nine cranial and seven dental dimensions. The orthognathic group showed a larger inter-orbital dimension with a subsequent decrease in stress in that area. The upper facial index, maxillary molar crown area and the dental arcade shape demonstrated statistically significant shape changes associated with the degree of prognathism. Morphological analysis did not show a significant distribution in browridge expression and robusticity as a means to accommodate masticatory stress. Stress distribution patterns were correlated with osteometric data and showed a significant difference in inter-orbital breadth between the two groups.
Mechanical action of mastication may influence prognathic more than orthognatic facial forms. An orthognatic facial form is biomechanically more efficient under masticatory stress. Mechanical loading during mastication greatly influences the morphological patterns of the facial skeleton. Further investigation into patterns of stress is necessary when changes to the masticatory apparatus arises from clinical involvement, trauma, or as a means to avoid or predict failure in the underlying skeletal architectu