On the Mechanical Function of Marrow in Long Bones

Abstract

Hydraulic strengthening, viscous interaction between marrow and trabecular bone, and viscous resistance by the marrow to shear, appear not to occur in long bones when subjected to non-destructive axially applied compressive loads. These conclusions are based on the following observations. Firstly, volumetric measurements suggest that long bones deform in a way that maintains a near constant volume. Hence the mechanism by which ‘hydraulic strengthening’ could operate seems not to exist. Secondly, with little or no volume change, no significant movement between the marrow and the adjacent trabecular bone is likely to occur. The only mechanism by which the marrow could function mechanically would be by its own viscous resistance to shear deformation. As marrow is near liquid at 37°C (with a measured viscosity of 67 × 10-3 Pa s) this ‘resistance’ is shown to be negligible, even when calculated using values most favourable to the production of large shear forces.

Can the diverse elastic properties of trabecular and cortical bone be attributed to only a few tissue-independent phase properties and their interactions?

“…A prime candidate for a tissue-independent material phase, at the scale of several millimeters where cortical and trabecular bone material can be distinguished, is the marrow saturating the micropores (in the inter-trabecular space as well as in the Haversian and Volkmann canals). The mechanical significance of marrow has been addressed by many authors (Bryant 1983(Bryant , 1988Cowin 1999). All agree that the mechanical properties of marrow are the same for all bone tissues (whether cortical or trabecular), and homogeneous within the micropore space.…”

“…In the physiological state, the marrow in the micropores exhibits the isotropic stiffness of bulk water, c phor;phys = 3Jk H2O ; k H2O ¼ 2:3 GPa, (Bryant 1988); J ijkl =1/3d ij d kl is the volumetric part of the 4 th order unity tensor. Cleared (empty) pores are characterized by c por,cleared =0.…”

Drained and Undrained Poroelastic Properties of Healthy and Pathological Bone: A Poro-Micromechanical Investigation

“…Bryant, (1983Bryant, ( , 1988 argued that the marrow does not play a role during impact loading, given the orders of magnitude of measured intertrabecular pore pressures in non-destructive experiments. On the other hand, Kafka (1983Kafka ( , 1993 underlined the possible significance of ''hydraulic strengthening'' by schematical thought-models while Zhang et al (1998) conducted isotropic poroelastic calculations for the estimation of hydraulic stiffening, reporting a perceivable increase of stiffness under impact conditions.…”

“…On the basis of this model for the transversely isotropic elastic properties of bone, we will compute the tensors of Biot and Skempton coefficients as functions of microporosity, characterizing healthy/osteoporotic, trabecular/cortical bone, as well as of mineral and collagen content, which are species and tissuedependent, and may be affected by bone diseases. As an ultimate model check, we will confront some of our model predictions with corresponding pore pressure experiments of Bryant (1983Bryant ( , 1988.…”

Strain rate dependent poroelastic behavior of bovine vertebral trabecular bone

“…It shows high porosity (more than 85 %) compared to other biological hard tissues such as cortical bone having less than 10 % of porosity (Cowin, 1999). The pore space of trabecular bone is continuous to allow interstitial fluid flow (Hughes et al, 1978) and filled with a highly viscous biological fluid, mainly bone marrow which the viscosity is 67 times of water viscosity at 37·C (Bryant, 1988). Therefore, trabecular bone can be defined as a highly porous structure filled with a highly viscous interstitial fluid.…”