Pressure and shear stress in trabecular bone marrow during whole bone loading

Area of Science:

• Biomedical Engineering
• Skeletal Biology
• Mechanobiology

Background:

• Skeletal adaptation to mechanical loading is mediated by mechanobiological signaling, primarily involving osteocytes.
• Bone marrow cells also respond to mechanical cues like hydrostatic pressure and shear stress, suggesting a role in skeletal adaptation.
• Trabecular bone's poroelastic nature creates fluid flow and mechanical stress within the marrow during loading.

Purpose of the Study:

• To characterize the in situ mechanical environment of trabecular bone marrow.
• To quantify pressure gradients and shear stress within the bone marrow during mechanical loading.
• To assess if these mechanical cues exceed thresholds for mesenchymal lineage cell mechanostimulation.

Main Methods:

• Measured pore pressure in porcine femurs using miniature pressure transducers during stress-relaxation and cyclic loading.
• Utilized continuum scale poroelastic models to compare measured pressure gradients with theoretical calculations.
• Employed micro-scale computational fluid dynamics (CFD) models based on CT images to calculate micromechanical stress in the marrow.

Main Results:

• Measured pressure gradients within the trabecular bone marrow ranged from 0.013 to 0.46 kPa/mm.
• Volume-averaged shear stress in the marrow during cyclic loading ranged from 1.67 to 24.55 Pa.
• Calculated shear stress levels exceed the known mechanostimulatory threshold for mesenchymal lineage cells.

Conclusions:

• Mechanical loading during daily activities generates significant shear stress in bone marrow.
• This mechanical environment is crucial for bone marrow health and mechanobiology.
• Further research into cell-level responses in healthy and diseased states is warranted to understand marrow mechanobiology fully.