NUMERICAL SIMULATIONS OF INTRAMEDULLARY PRESSURE VARIATION AND ITS INFLUENCES ON INTERSTITIAL FLUID FLOW IN BONE CORTEX

Abstract

Bone fluid flow transports nutrients to, and carries waste from, the bone cells embedded in the bony matrix. In long bones, it is driven by the blood pressure differentials between the medullary cavity and the periosteal surface and it is enhanced by mechanical loading. Loading of bone tissue deforms the bone matrix and changes the volume of the medullary cavity. Both mechanisms alter the interstitial fluid flow in the bone cortex. The former changes the volume of the fluid cavities in the cortex, while the latter modifies the intramedullary pressure (IMP). This study aims to investigate and compare, for the first time, the effects of these two mechanisms combined on the interstitial fluid flow in the bone cortex.

A hydraulic-fluid method is proposed to investigate the enhancement of IMP induced by the external loading. An intact sheep tibia is represented by a hollow cylinder, with the bone marrow being completely constrained in the cavity and assumed to behave as an icompressible liquid. The cortex is supposed to be a purely elastic material, and its permeability is ignored at this stage. The numerical results show that an axial compressive load of 500 N increases the IMP from 4000 Pa to 48900 Pa.

The influence of the enhanced IMP on the interstitial fluid flow is examined in a subsequent poroelastic analysis. At this stage, the cortex is assumed to be a biphasic material that permits fluid perfusion. The poroelastic analyses were conducted for both initial and enhanced IMPs. The results of the simulations demonstrate that the external load induces very high interstitial pressure. The highest pressure could be 25 times higher than the initial marrow pressure, but its magnitude decreases quickly. Furthermore, the influence of the IMP on the interstitial pressure is limited to the inner half of the cortical wall adjacent to the endosteal surface. However, the influence becomes more significant with decreasing load-induced interstitial pressure.

In conclusion, these simulations suggest that the increase in IMP during mechanical loading further enhances interstitial fluid movements in cortical bone, which highlights the importance of mechanical loading for the maintenance of healthy bones.