The effects of aging and tooth loss to the microstructure of the mandible in South Africans

Abstract

The mandible plays a crucial role in many biological functions (especially mastication, swallowing and speech) and its efficiency to perform these functions depends on its intactness. Aging and tooth loss are biological processes that may compromise the normal functioning of the mandible by changing its morphology. While many studies address the macroscopic mandibular variations, there is a paucity regarding its microstructure. The aim was to investigate microstructural mandibular changes, with reference to the macrostructure, with advancing age and across various tooth loss patterns in South Africans. As the reasons why individuals are differently affected (extent, rate) by senescence or tooth loss are unclear, the influence of other biological factors (sex, ancestry) was also considered. Using micro-focus X-ray Computed Tomography (micro-CT) scans of 333 mandibles, external dimensions, and inner parameters, namely the cortical thickness (CtTh) and cortical density (approximated by histomorphometric parameters, BV/TV) were measured. To assess whether the mandible ages in the same way and rate as the rest of the skeleton, a comparison of mandibular and femoral cortical BV/TV, using a micro-CT subsample of 68 individuals, was included. A comprehensive assessment of the mandibular morphology and cortical microstructure of fully dentate individuals highlighted that the smaller the alveolar height is, the thicker and denser the cortical bone. An inner cortical asymmetry between basal, buccal, and lingual areas was described for CtTh, but was absent for BV/TV. Sexual dimorphism and ancestral variations were confirmed for the external distances and CtTh, but not for BV/TV. With tooth loss, a general decrease in external distances (aggravated by edentulism), CtTh and BV/TV was observed, except at the midline where the lingual CtTh increased with edentulism. Sexual dimorphism and ancestral variations of the external dimensions and CtTh were emphasised in edentulous mandibles. The cortical density decreased with aging, corroborating the general decline in bone mass of the skeleton, as noted in the femur. By contrasting the effects of tooth loss and aging, it was concluded that both external distances and CtTh were mainly influenced by tooth loss and not age, whereas the cortical bone density displayed an age-related decrease independent of tooth loss. To ascertain the applicability of the findings in a dental setting, where Cone-Beam Computed Tomography (CBCT) is commonly used, measurements were performed on 24 mandibles scanned by both CBCT and micro-CT (considered as the reference). The accuracy and repeatability of CBCT was confirmed for large-scale measurements, and CtTh in a lesser manner, whereas results were uncertain for BV/TV, revealing a distinct lack of reliability. In conclusion, this thesis inferred the precise role of aging and tooth loss, but also sex and ancestry, on the variations of the mandibular macro- and microstructure. Not only does knowledge and understanding of these changes have implications in dentistry fields, as cortical thickness and density are essential for many dental procedures; but also in biological anthropology, in which the microstructure of extant human mandibles gives valuable insights into intra- and interspecific variations (e.g., sexual dimorphism), or functional considerations (mastication, diet) of archaeological/fossil specimens.