design of a mechanical testing device for esem for bone fracture healing assessment
TRANSCRIPT
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