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Design of a mechanical testing device for ESEM. for Bone fracture healing assessment . Participants. Project Sponsor Dr. Stephen Doty, Hospital of Special Surgery Project Advisors Luis Cardoso, Ph.D and Marom Bikson Ph.D from The Biomedical Engineering Department at City College

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design of a mechanical testing device for esem

Design of a mechanical testing device for ESEM

for

Bone fracture healing assessment

participants
Participants
  • Project Sponsor
      • Dr. Stephen Doty, Hospital of Special Surgery
  • Project Advisors
      • Luis Cardoso, Ph.D and Marom Bikson Ph.D from The Biomedical Engineering Department at City College
      • Stewart Russell, Ph.D
  • Students
      • Rasha Aaskar
      • Gaurav Aggarwal
      • Cristina Alexandrescu, Team Leader
      • Francisco Saenz
table of contents
Introduction

Project Goals

Clinical Need

Physiology of Bone Healing

Background

Current Testing Methods for Assessing Healing

Concept Development

Design Specifications

Constraints

Existing Products

Concept Design

Universal External Testing Stage

Concept 1: Piezo Actuator

Concept 2: DC Electric Motor

Advantages & Disadvantages

Conclusion

Table of Contents
project goals
Project Goals
  • Develop a device that is capable to:
    • Perform mechanical testing on fractured bone during the healing process
    • Allow placement inside the ESEM for microscopic analysis
clinical need
Clinical Need
  • Understand the mechanisms of fracture healing
    • Evaluation of the mechanical properties
    • Microscopic assessment of the tissue composition
  • Analyze the effects of different treatments in the fracture repair process
    • Increase in rate of healing
    • Improve the strength of the fracture site
  • Improve patient’s quality of life
physiology of bone healing
Inflammation

Occurs immediately after fracture

Mechanical stability is achieved by presence of hematoma

Callus forms by bridging the fracture site

Takes 2-3 days

Reparation

Callus size increases to unite fracture site and reduce bone motion

Callus begins mineralization and eventually matures into lamellar bone --> bony union occurs

Takes 4-12 weeks

Physiology of Bone Healing
  • Remodeling
    • Characterized by Wolff’s Law
    • Fully restore anatomical configuration of bone
          • Takes 6 months to 1 year in adults
current testing methods for assessing fracture healing
Current Testing Methods for Assessing Fracture Healing
  • Qualitative methods
    • Radiography
    • Densitometry
  • Quantitative methods
    • Mechanical testing
      • Three point bending
      • Four point bending
      • Torsion

These tests measure:

    • Stiffness
    • Ultimate load
    • Work to failure
    • Ultimate displacement

Hiltunen et al

design specifications
Design Specifications
  • Testing Method
    • Four point bending inside the ESEM
  • Components
    • A motor that applies a chosen range of forces
    • Sensors to measure:
      • Displacement
      • Force Applied
  • Materials
    • 440C Stainless Steel
    • UHMWPE
    • Rubber
    • Copper Tubing
  • Design should allow easy visualizations of bone callus for microscopic analysis
  • The Data Acquisition will initially be done via Lab View and NI DAQ Hardware
constraints
Constraints
  • ESEM
    • Minimal alterations to microscope
    • Electromagnetic and environmental conditions
    • Workable space inside the chamber
  • Device Components
    • Satisfy ESEM constraints
    • Must be sturdy and secured inside the chamber
  • Testing Conditions
    • Bone hydration
initial concepts of internal testing system
Initial Concepts of Internal Testing System
  • Modification of existing stage gear system
    • Requires excessive modification of the ESEM
  • Use of the external port of the ESEM
    • Requires the creation of a Vacuum seal
    • Modification of the port assembly of the ESEM

These two concepts might result in damage of the ESEM and are too expensive to be pursued.

existing products
There exist devices that meet the design criteria and overcome the imposed constraints

Prices range from $10,000-30,000

Encompass all testing methods

Customized software applications

Existing Products

Courtesy of www.gatan.com

therefore
Therefore…
  • Existing commercial devices provide an immediate solution to the original design specifications
  • However these systems are too expensive
  • These challenges can be overcome by building an external device as opposed to an internal one. The external testing system will:
    • Be a cheaper alternative to commercial devices
    • Perform the most relevant testing method for fracture healing studies
    • Specifically designed for testing of mouse bones
    • Be portable for usage in multiple microscopes
        • While having a self locking mechanism to maintain deformation
    • Be used as a prototype for preliminary studies to determine clinical relevance
    • Be safe for the ESEM
      • No fragmentation of bone
      • No alterations
      • No EMF
concept designs
Concept Designs
  • Test system:
    • Accommodates motors and linear actuators
    • Minimizes alterations to the stage design.
  • Criteria:
    • Cost
    • Accuracy
    • Size
    • Locking Mechanism
universal external testing stage
Universal External Testing Stage

Interface for bone (consisting of hardened liquid polymer [polyethylene] and metal coupler). Applies four point bending force.

Y

X

Z

Load Cell

Motor / Actuator

Physical stage constructed of Stainless Steel or polyethylene with maximum size of 20 x 8 x 10 cm

LVDT

piezo actuator cont d
Advantages

Self locking when power is removed

Rapid response

High resolution

Not subject to mechanical tear and wear

Eliminates the need for an external LVDT

Disadvantages

Brittle

Repeatability errors due to hysterisis and creep

Higher costs of roughly $500

Piezo Actuator (cont’d)
concept 2 dc electric motor
Concept 2 DC Electric Motor
  • An electrical motor converts electrical energy to mechanical energy using principles of magnetism to propel the armature

http://en.wikipedia.org/wiki/Image:Electric_motor_cycle_1.png

dc electric motor cont d
Advantages

If operated only outside it would not create EMF inside the ESEM

Very Inexpensive

Costs can be less than $100

Disadvantages

Constant power must be applied to maintain load

Special locking clamps would be needed to maintain deformation

Repeatability errors due to hysterisis and creep

Requires external load and displacement sensor

Requires design of gear system for linear displacement

DC Electric Motor (cont’d)
external testing system
Advantages

No EMF inside ESEM

No possible damage to the ESEM

No particle creation inside the ESEM from fracturing

External testing system with the possibility to test inside, with appropriate shielding

Cost effective in manufacturing

Less need for shielding

Disadvantages

Power needs to be removed while imaging in the ESEM for no EMF generation

Possibility of losing deformation during movement

External Testing System
conclusion
Conclusion
  • The risk of modification with an internal system, and the costs of existing devices has lead to the development of an external testing system
  • Our design will provide an alternative solution to the sponsor’s original design specifications while still meeting the requirements of the device