Quantitative relationships between microdamage and cancellous bone strength and stiffness

Abstract

Microscopic tissue damage (microdamage) is an aspect of bone quality associated with impaired bone mechanical performance. While it is clear that bone tissue submitted to more severe loading has greater amounts of microdamage (as measured through staining), how microdamage influences future mechanical performance of the bone has not been well studied, yet is necessary for understanding the mechanical consequences of the presence of microdamage. Here we determine how stained microdamage generated by a single compressive overload affects subsequent biomechanical performance of cancellous bone. Human vertebral cancellous bone specimens (n=47) from 23 donors (14 males, 9 females, 64-92years of age) were submitted to a compressive overload, stained for microdamage, then reloaded in compression to determine the relationship between the amount of microdamage caused by the initial load and reductions in mechanical performance during the reload. Damage volume fraction (DV/BV) caused by the initial overload was related to reductions in Young's modulus, yield strength, ultimate strength, and yield strain upon reloading (p<0.05, R(2)=0.18-0.34). The regression models suggest that, on average, relatively small amounts of microdamage are associated with large reductions in reload mechanical properties: a 1.50% DV/BV caused by a compressive overload was associated with an average reduction in Young's modulus of 41.0±3.2% (mean±SE), an average reduction in yield strength of 63.1±4.5% and an average reduction in ultimate strength of 52.7±4.0%. Specimens loaded beyond 1.2% (1.2-4.0% apparent strain) demonstrated a single relationship between reload mechanical properties (Young's modulus, yield strength, and ultimate strength) and bone volume fraction despite a large range in amounts of microdamage. Hence, estimates of future mechanical performance of cancellous bone can be achieved using the bone volume fraction and whether or not a specimen was previously loaded beyond ultimate strain. The empirical relationships provided in this study make it possible to estimate the degree of impaired mechanical performance resulting from an observed amount of stained microdamage.

Keywords: Biomechanics; Bone quality; Cancellous bone; Mechanical testing; Microdamage.

Figure 1

The distribution of donor age across the 10 loading groups is shown with lines connecting specimens from the same donor. Specimens noted with solid symbols provided both mechanical properties and microdamage measurements.

Figure 2

A stress-strain curve illustrating the initial load and reload is shown. Specimens were stained with alizarin complexone, submitted to the initial load and unloaded. Calcein stain was applied and the specimen was then reloaded. The applied strain, residual strain and inelastic strain are also defined. Energy applied and toughness are measured as the area under the curve.

Figure 3

Examples of microdamage measured in the current study are shown including A) microcracks and B) diffuse damage.

Figure 4

The relationships between the amount of microdamage caused by the initial load and the percent reduction in Young’s modulus or Yield Strength upon reloading are shown (p < 0.05). Specimens in which the applied strain in the initial load was less than 1.2% are illustrated with open squares. Regression equations are shown in Table 3.

Figure 5

The relationships between the inelastic strain resulting from the initial load and the resulting amounts of damage volume fraction (DV/BV) and crack density are shown (p < 0.05). Regression equations are in Table 3.

Figure 6

The relationship between mechanical properties and bone volume fraction is illustrated for the initial load as well as for the reload. Specimens in which the applied strain in the initial load was less than 1.2% are illustrated with open squares. Regression equations are shown in Table S1.