Comparison of the Macroscopic Viscoelastic Properties of Cortical Bone to its Nano-Indentation Response

Cortical bone provides a rigid structure allowing for the attachment of skeletal muscles. It makes up approximately 80% of skeletal mass and is a damaging viscoelastic material. Damage is an important aspect to evaluate bone quality since damage accumulation affects cortical bone strength. Fatigue and monotonic strength studies have indicated that damage accumulation mechanisms depend on structural and compositional properties. However, the role of the viscoelastic behavior of cortical bone has been overlooked. Bone is a hierarchical composite material. As such, measurements of multiple length scales are useful in order to evaluate bone viscoelastic properties in different microstructural units. To understand the effect of viscoelasticity of bone at small length scales, cortical bone specimens were evaluated under torsional and tensile loads, by ultrasound and by nanoindentation. Twelve cortical bone specimens were tested using tension, torsion, ultrasonic wave propagation, and indentation test. These tests resulted in the specimens showing stress relaxation and creep effects, which are a key component of viscoelastic behavior. Nanoindentation and torsion mechanical properties were highly correlated, suggesting microscopic properties are reflected at the macroscopic level.