Bone Growth and Remodeling - WordPress.com

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.