| Date post: | 23-Dec-2015 |
| Category: |
Documents |
| Upload: | colleen-mitchell |
| View: | 214 times |
| Download: | 2 times |
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
Structural Analysis of the Femur:
A Collaborative Tool for Surgeons and Engineers
Alice Younge
IStructE YRC15th March 2012
Supervisors: Dr ATM Phillips Prof AA Amis
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
OVERVIEW• Introduction
– The Femur
– Context
– Objectives
• Beam Theory Modelling – Basic Model
– Comparison to finite element Model
– Results of comparison to finite element model
• Applications
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
INTRODUCTION
• Longest and strongest bone in the human body
• Two main functions:
• Support structure• Facilitates movement
Proximal end Hip Joint
Distal end Knee Joint
The Femur:
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
INTRODUCTIONThe Femur:• Consists of two types of bone:
- Cortical
Thin layer on the outside of the bone – Strong, stiff, low porosity shell like structure
- Trabecular
Continuous within the inner surface of the cortical shell – Bony plates and struts, high porosity sponge like structure
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
INTRODUCTION
The joints are surrounded by:
Ligaments -
• Connect bone to bone
• Provide stability for the joint they surround
The Femur:
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
INTRODUCTION
The joints are surrounded by:
Muscles -
• Support and stabilise the joint
• Provide the power for locomotion
The Femur:
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
• From April 2010 to April 2011 the NHS performed over 70,000 hip replacements in England
• We are all expecting to live longer and with a better quality of life
• A replacement hip joint lasts for ~ 15 years
• Additionally increased incidence of patients with musculoskeletal disorders – About 1 in 500 babies born in the UK have Cerebral Palsy
Advances in design of replacement joint and surgical procedure are essential
CONTEXT
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
Ultimate objective: Build joint replacements that outlast the patient
Interim objective: To create a beam theory model which could be used to -
Assess the femur in natural condition and following arthroplasty
Assess various clinical procedures and inform the choice of surgical approach
Predict the effect of changes at one joint, hip or knee, to the other
Inform new and existing designs of artificial hip or knee joint
Assess the effects of muscle damage through disease, injury or surgery
OBJECTIVES
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY Background within biomechanics:
• Popular in early to mid 1900s
• Models crude and calculations carried out by hand
• By 1970s finite element method considered to be superior
• Little development since
• Advancement of computational power and with modern surgical planning time constraints – beam theory modelling is again becoming a viable option
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORYBasic model definition:
1) Medium left fourth generation Sawbones femur scanned
using a computerised tomography (CT) scan at intervals of 0.75 mm.
- Series of x-ray beams scan the bone
- Create detailed images of the structure
2) CT data converted to contour geometry using Mimics.
- Converts 2D image data to 3D model
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY
3) Using Rhino, a CAD package, and the composite femur model a neutral axis was estimated through the head region and down the shaft of the femur. An altered axis was then included to ensure smooth loading through the neck region.
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY4) Using Rhincerous, a CAD package, section
cuts were taken perpendicular to the axis.
- Head region, 2mm intervals 23 sections- Neck region, equally spaced in an arc 30 sections- Shaft region, 5mm intervals 65 sections
5) For each section, cortical and trabecular, line plots in the form of .dxf files were exported.
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY6) The .dxf files were imported in to Oasys GSA as perimeter
sections. Section properties were calculated and assigned for all cortical and trabecular parts.
The section properties included the:
- Area, - Centroid, - Second moments of area,
Custom script written to calculate:
- Torsion constant
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY7) The mid-point of the cortical centroid
values were calculated and nodes were plotted at these points.
- Ensured even distribution of the element section properties between the nodes.
The resulting node path
8) Trabecular sections aligned coincident to cortical sections
- Parallel axis theorem used to calculate new section properties
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY9) Element assigned between
each node with individual section property – cortical and trabecular
Image shows section definition plot of cortical (right) and trabecular (left) parts
10) Muscle origination and insertion nodes (surface nodes)
plotted
- Surface nodes connected to centroid nodes via stiff beam elements to the femur direct transfer of force
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY
Now have the basic beam theory model
Using the basic model one can:
• Rotate the model to any position/stance
• Apply any chosen muscle ligament or joint reaction force
• Alter the position of the muscle origination or insertion site
• Change the material properties of the bone, cortical or trabecular
• Add an additional component such as an artificial joint
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
• Femur position at 12° of adduction & 7° of flexion – representative of single leg stance
• Twenty six muscles & seven ligaments included as cable elements with specific stiffness’s
• Each muscle had a defined force-displacement relationship
Force-displacement curve based on the muscles peak contractile force and stiffness value
Note: Stiffness acted in and tension not in compression
Compare Beam Theory Model to Finite Element Model:
BEAM THEORY
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY
• An equivalent acetabular and condylar structure was defined and connected using stiff beam elements
• Muscles originating on the pelvis connected to the acetabular structure
• Muscles inserting into the tibia were fully constrained
• Force of 835 N applied to L5S1 node
Compare Beam Theory Model to Finite Element Model:
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
Acetabular structure
Muscle origination point
Stiff beam element
Cable element representing muscle
Tibial plateau
BEAM THEORY
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY
Beam theory line plot
Beam theory section plot
Finite element model
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORYProcessing Results:Bending moment output Myy Section definition and properties
Stress calculated using moment, position and section properties Strain calculated using stress and Young’s modulus
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORY
FE shown in black Beam theory shown in red
Time to analyse model:
Beam theory: 1 secondFinite element: 1937 seconds (super computer)
Results – Comparison to Finite Element model:
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
BEAM THEORYFE lateral surface shown in black
FE medial surface shown in blue
Beam theory centroid line shown in red
Reaction forces at condyles
Deflection of femoral head
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
APPLICATIONSBeam theory model main advantages:
Analysis time – very fast
Ease with which parameters can be changed- muscle origination site- loading condition
Beam theory model applications:
Rapidly assess the effects of muscle damage through, disease injury or surgery - changes in muscle attachment etc
Assess femur in natural condition and following arthroplasty. To inform a new design of artificial hip or knee joint
Predict the effect of changes at one joint, hip or knee, to the other
Context
The Femur
Basic Model
Comparison
Objectives
Results
Application
THANK YOU
ANY QUESTIONS Contact: [email protected]
