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• 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

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

• 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

•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• 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

• 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

Parameters Value

Workable Area

Length 20 cm

Depth 8 cm

Height 10 cm

Internal Environmental Conditions

Type of Atmosphere Partial ~ 4000 Pa

Temperature 25 Degrees Celsius

Measurement Feedback Scales

Force 0 ~ 30 newtons

Displacement 0 ~ 3 mm

Accuracy

Force 1 micro-newton

Displacement 0.01 mm

Force Lost Due to Components (Gears, shafts, couplers, etc.)

To The Bone 0.01 newtons

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• 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 Courtesy of www.gatan.com

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

• Test system:– Accommodates motors and linear actuators– Minimizes alterations to the stage design.

• Criteria:– Cost– Accuracy– Size– Locking Mechanism

Universal External Testing Stage

Y

Z

X

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

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

Motor / Actuator

Load Cell

LVDT

Concept 1 Piezo Actuator

• Composed of a ceramic material that expands and contracts in response to an applied electrical voltage

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

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

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

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