Edentulation Alters Material Properties of Cortical Bone in the Human Craniofacial Skeleton: Functional Implications for Craniofacial Structure in Primate Evolution

Abstract

Skeletal adaptations to reduced function are an important source of skeletal variation and may be indicative of environmental pressures that lead to evolutionary changes. Humans serve as a model animal to investigate the effects of loss of craniofacial function through edentulation. In the human maxilla, it is known that edentulation leads to significant changes in skeletal structure such as residual ridge resorption and loss of cortical thickness. However, little is known about changes in bone tissue structure and material properties, which are also important for understanding skeletal mechanics but are often ignored. The aims of this study were to determine cortical material properties in edentulous crania and to evaluate differences with dentate crania and thus examine the effects of loss of function on craniofacial structure. Cortical bone samples from fifteen edentulous human skulls were measured for thickness and density. Elastic properties and directions of maximum stiffness were determined by using ultrasonic techniques. These data were compared to those from dentate crania reported in a previous investigation. Cortical bone from all regions of the facial skeleton of edentulous individuals is thinner than in dentate skulls. Elastic and shear moduli, and density are similar or greater in the zygoma and cranial vault of edentulous individuals, while these properties are less in the maxilla. Most cortical bone, especially in edentulous maxillae, has reduced directional orientation. The loss of significant occlusal loads following edentulation may contribute to the change in material properties and the loss of orientation over time during the normal process of bone remodeling. These results suggest that area-specific cortical microstructural changes accompany bone resorption following edentulation. They also suggest that functional forces are important for maintaining bone mass throughout the craniofacial skeleton, even in areas such as the browridges, which have been thought to be little affected by function, because of low in vivo strains found there in several primate studies.

Elastic anisotropy and off-axis ultrasonic velocity distribution in human cortical bone

“…Elastic property variation in the femur (Rho, 1996; Fan et al. 2002; Dechow et al. 2008, 2010) is much less than that in the skull.…”

“…2009; Zapata et al. 2010), three‐dimensional and quantitative relationships with possibly influential microstructural features have received little study (Dechow et al. 2008, 2010).…”

“…2006; Wang et al. 2006, 2010a,b; Dechow et al. 2010), and this information is essential for determining three‐dimensional elastic properties in bone, especially where these directions do not correspond regularly with anatomical axes.…”

Allometry and advancing age significantly structure craniofacial variation in adult female baboons

“…; Dechow et al. ). However, it is not until individuals reach an advanced age that the effects of such remodeling on the CFS are usually deemed a potential source of intraspecific variation.…”

“…Alternatively, as tooth loss significantly alters the material properties of bone (Dechow et al. ), it may be entirely possible that the difference in craniofacial morphology is so drastic as to be easily accounted for when examining morphological variation in aged individuals. It is also unclear whether other factors, such as regional muscle atrophy, may affect the aging cranium in both humans and other mammals.…”