Influence of bone microstructure on the mechanical properties of skull cortical bone – A combined experimental and computational approach

Abstract

The strength and compliance of the dense cortical layers of the human skull have been examined since the beginning of the 20th century with the wide range in the observed mechanical properties attributed to natural biological variance. Since this variance may be explained by the difference in structural arrangement of bone tissue, micro-computed tomography (µCT) was used in conjunction with mechanical testing to study the relationship between the microstructure of human skull cortical coupons and their mechanical response. Ninety-seven bone samples were machined from the cortical tables of the calvaria of ten fresh post mortem human surrogates and tested in dynamic tension until failure. A linear response between stress and strain was observed until close to failure, which occurred at 0.6% strain on average. The effective modulus of elasticity for the coupons was 12.01 ± 3.28GPa. Porosity of the test specimens, determined from µCT, could explain only 51% of the variation of their effective elastic modulus. Finite element (FE) models of the tested specimens built from µCT images indicated that modeling the microstructural arrangement of the bone, in addition to the porosity, led to a marginal improvement of the coefficient of determination to 54%. Modulus for skull cortical bone for an element size of 50µm was estimated to be 19GPa at an average. Unlike the load bearing bones of the body, almost half of the variance in the mechanical properties of cortical bone from the skull may be attributed to differences at the sub-osteon (< 50µm) level. ANOVA tests indicated that effective failure stress and strain varied significantly between the frontal and parietal bones, while the bone phase modulus was different for the superior and inferior aspects of the calvarium. The micro FE models did not indicate any anisotropy attributable to the pores observable under µCT.

Material properties of the skull layers of the primate parietal bone: A single-subject study

“…elastic moduli among parietal, frontal, and sphenoid bones. Complementarily, some authors have previously reported that the human cortical plates are transversely isotropic [4,9,24]. We believe that their results were affected due to the dynamic effect of the strain rate during the tension tests [24], the high average age (68.4 years) of the human subjects [4], and the lack of a pattern orientation when removing the specimens from the skull [9].…”

“…Overall, there were no significant differences in anisotropy (E 2 /E 3 ) between inner and outer parietal cortical plates, suggesting the baboon cranial vault is orthotropic. Complementarily, there were significant differences among the three principal elastic moduli between the inner and the outer cortical tables (E 3 > E 2 > E 1 ) of the parietal bone in baboons suggesting that both cortical tables are orthotropic, and indicating that their osteons are oriented in a relative uniform pattern [9]. Moreover, the principal elastic moduli were larger in the outer cortical plate than in the inner cortical plate (Table 1).…”

“…Recent work has demonstrated that the three-dimensional material properties of cortical bone vary throughout the craniofacial skeleton [8][9][10]. For instance, there is a general correlation of bone anatomical axis and orientations of bone maximum stiffness at some areas, such as the supraorbital torus and zygomatic arch, suggesting a link between bone mechanical properties and structures at the tissue level [11,12].…”

“…Several studies have examined the overall mechanical properties of the cranial vault as a whole by testing the mechanical properties of the vault using: tension test [5,16], compression test [5,7,9], triaxial compression test [7], shear test [7], torsion test [7], three point bending test [16][17][18][19][20][21][22], four point bending test [23],and simple-bending test [24]. However, the results of these studies represent only the overall mechanical properties of all three layers.…”

Bioinspired Collagen Scaffolds in Cranial Bone Regeneration: From Bedside to Bench

“…While cortical bone in the human femur has been reported to have an average elastic modulus of around 20–25 GPa, [65] cortical calvarial bone has been reported to have an elastic modulus of around 12 GPa. [66] When evaluated in children 1–2 years of age, the elastic modulus of the calvaria was found to be between 1.1–1.3 GPa with frontal bone providing greater stiffness than parietal bone. [67] Cancellous bone makes up the remaining 20 percent of the body’s bone mass.…”