Microstructural strain near osteocyte lacuna in cortical bone in vitro

Area of Science:

• Biomechanics
• Cellular Mechanotransduction
• Bone Physiology

Background:

• Bone remodeling is influenced by mechanical factors, with increased demand promoting bone formation and decreased demand leading to resorption.
• Proposed mechanical signals include stress-generated fluid flow and matrix deformation.
• Current theories suggest bone cells are more responsive to fluid flow than mechanical strain.

Purpose of the Study:

• To investigate the role of cell deformation in bone cell mechanotransduction.
• To compare osteocyte deformation under fluid flow versus substrate strain.
• To determine if cell deformation explains differential cell responses to mechanical stimuli.

Main Methods:

• Bovine cortical bone specimens were subjected to controlled macroscopic strain levels (500–6,000 microstrain).
• MLO-Y4 osteocyte-like cells were exposed to steady fluid flow (16 dynes/cm²) in vitro.
• Digital stereoimaging and strain gauges were used to quantify continuum and cell-level deformations.

Main Results:

• Macroscopic strain of 2,000 microstrain induced local osteocyte strains of 12,000–15,000 microstrain.
• Fluid flow in vitro resulted in significant increases in osteocyte strain, reaching 22,856 microstrain.
• In vivo strains in humans and other species are considerably lower (approx. 1,800–3,000 microstrain).

Conclusions:

• Cell deformation, rather than fluid flow or matrix strain per se, is the primary driver of bone cell biological response.
• Fluid flow generates significantly higher cell deformations than previously considered physiological.
• These findings help explain the heightened sensitivity of bone cells to fluid flow compared to substrate strain.