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Bone Growth and Remodeling: from Concept to Simulations
Kestrel J. Pourchot
Summer 2020
Motivation
figures from M J Schmitz et al.
Actual change in bone densityPredicted change in bone density
Prosthetic implants are being modeled and designed to better be compatible with the human body.
A better understanding of the way our body changes over time in reaction to an implant leads to better implant designs
Better Understanding = Better Implants
Background• Bone cell growth is influenced by the
mechanical environment present.
Bone deposition and absorption as a function of a mechanical signal (adapted from M. J. Schmitz. et al.)
Bone CellBone Cell
Pressure Shear Stress
Mechanical Loading
Bo
ne
den
sity
No change
Density Loss
Density Gain
Two main ways to stimulate bone growth
As loading increases – bone density increases
How do others study these loads?
from D. A. Gaspar et al.Adapted from J.R. Henstock et al.
Bone
Compressor & Release
Valve Membrane
PressurizedChamber
One way is Bio-Reactors!
Porous Substratewith bone cells
Cel
lCell
Cell C
ell
Pore with cells
The problem with Current Bioreactors
Bo
ne
Cel
l
Bo
ne
Cell
Bo
ne
Cell
Bo
ne
Cel
l
Bo
ne
Cel
lB
on
e C
ell
Bo
ne
Cell
Bo
ne
Cell
High Pressure = High Shear
Low Pressure = Low Shear
Pressure and Shear Forces are Linked!
Porous Substratewith bone cells
Looking at single hole
Objective
• Create and simulate a device which can accurately apply fluid shearing to bone cells in a consistent and quantifiable way without affecting the pressure.
Our Design Goals
Create two similar devices that can:
• Provide sufficient nutrient mixing.
• Create a consistent hydrostatic load on cells.
1 Device Maximizes Shearing (same pressure)
1 Device Minimizes Shearing (same pressure)
Maximize Shearing
Minimal Shearing
Maximize Shearing
Minimal Shearing
How Does Our Device Work?
Bone Cell
Bone Cell
Bone Cell
Bone Cell
Bone Cell
Bone Cell
Bone Cell
Shear Stress
Fluid flow
Creation Process
Design Surface MeshImport to Simulation
SoftwareMeshing
Boundary Conditions
Mesh Convergence?
Simulations Results
Design Highlights
✓ Stainless steel internal parts
✓ Fully Autoclavable
✓ Small footprint
✓ Fits 10mm sample
✓ Most parts are pre-made
30cm
15cm
Shearing Mechanism
Initially, fluid was not making contact with surface.
A restriction was added inline to send “jet” of water onto fluid surface.
ModificationsAdded restriction to increase shearing
• With revised design a simplified model is created and exported into the fluid simulation program.
• Only fundamental dimensions are to be retained to minimize simulation meshing time
From Design to Simulations
3D Parts Assembly
High Shear Low Shear
Surface Mesh
Simulation Parameters
Once imported, the simulations where completed in StarCCM+ for both hole placements with a constant velocity.
• 5mm/s for piston motion upwards
• -5mm/s for piston motion downwards
Fluid Meshing
Base size of 0.25 mm
High Shear Low Shear
Maximum size of 200% (0.5mm)
Cross sections of the meshes are shown below
The following meshing parameters are established
Meshing Convergence StudyGoal- Determine if further refining mesh will influences results.
• Distribution of values
• Absolute Values
Base Size- 0.15mm Base Size- 0.25mm
Mesh Convergence – FlowRefined Meshing Original Meshing
0.12m/s0.12m/s
Mesh Convergence – Shear ForcesRefined Meshing Original Meshing
0.05 Pa
0.044 Pa
0.04 Pa
0.45 Pa
Mesh Convergence Study - Conclusions
✓Similar shape and size.
✓Meshing is Functionally the same for our purposes.
Refined Meshing Original Meshing
9.0 Pa
9.0 Pa
1.1m/s
1.1m/s
Final SimulationsHigh Shear Low Shear
Piston UP
Piston DOWN
Piston velocity for all: 0.5cm/s
High Shear Low Shear
Piston UP
Piston DOWN
Velocities
0.12m/s
0.12m/s
0.12m/s
0.12m/s
✓ Flow path is of similar magnitude and direction between configurations.
High Shear Low Shear
Piston UP
Piston DOWN
✓ Similar pressures between all configurations
Pressures
-5 Pa -5 Pa
7 Pa 6 Pa
High Shear Low Shear
Piston UP
Piston DOWN
X Inconsistent shearing on high shear - up
✓ High Shear >> Low Shear
✓ Good maximum shear force values ✓ Goal - ~0.4 Pa
Shear Forces
0.5 Pa
0 Pa
0.25 Pa
0.45Pa
0 Pa
0.05 Pa 0.01 Pa
Piston Upward Motion- Conclusions
Between Configurations:
✓ Similar Pressure
✓ Different Shearing Stresses
High Shear Configuration
X Uniform Shear Distribution
Low Shear Configuration
✓ Minimal Fluid Shearing
High Shear Low Shear
Piston Downward Motion- Conclusions
Between Configurations:
✓ Similar Pressure
✓ Different Shearing Forces
High Shear Configuration
✓ Approximately Uniform Shear Distribution
Low Shear Configuration
✓ Minimal Fluid Shearing
High Shear Low Shear
Limitations
• The results show that the upwards piston motion does not apply consistent hydrostatic shearing.• This can be minimized through a slower upward motion.
High Shear, Upwards MotionNon-uniform Shear Distribution
Future Work
• Device Construction
• Preliminary Testing• Ensure device mechanically functions as intended
• Validation of simulated values
• Testing with bone cells within incubator
• Update to finite element
Acknowledgement
Thank you to the following people and organizations, with out their support, this wouldn’t have been possible:
• Dr. Melanie Coathup and Dr. Luigi Perotti for their extensive guidance and expertise in biomechanics
• Dr. Douglas Hector Fontes for his guidance regarding to StarCCM+
• The Office of Undergraduate Research for their funding through the SURF program
• The Burnette Honors College through the HUT program for the opportunity to conduct this research.
