Investigation of Screw Fixations for Pubic Rami Fractures_AN_KKD
Transcript of Investigation of Screw Fixations for Pubic Rami Fractures_AN_KKD
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Investigation of Screw Fixations for Pubic Rami FracturesPresented by: Shi AnSupervisor: Dr. Kajsa Duke
December 4, 2015
Mechanical EngineeringUniversity of Alberta
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Outline
Introduction
Objectives
Methods
Results
Future work
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Introduction
Pelvic Bone
Ilium
PubisPubic Symphysis
Sacrum
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Introduction
Causes for Pelvic Fracture
Incidents: traffic accidents, falling from
significant height etc..
Osteoporosis
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Introduction
Pelvic Fracture
Superior Pubic Ramus
Dual Rami Fracture
Inferior Pubic Ramus
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Introduction
Treatment of Pubic Rami Fractures
Minimally invasive
Open surgery
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Objectives
Part 1: To evaluate stress in a simple screw block model with different loading
Part 2: To create a cortical bone layer in a pelvis model
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Part 1: Screw block simulationMethodsMethod for simple screw block simulation:
Solid modeling screw and blocks
Different loadings including axial load, shear load and friction
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Part 1: Screw block simulationMethods: ModelingSolid modeling screw and blocks
Screw
Cancellous
Cortical layer
Cortical
Cancellous
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Part 1: Screw block simulationMethods: ModelingMaterial Properties
Cortical Cancellous Stainless Steel
Young's Modulus (MPa)
17,900 1,790 200,000
Poisson's Ratio 0.3 0.3 0.28
Mass Density (kg/) 1,850 460 7,800
Yield Strength (MPa)
110 5.4 172.3
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Part 1: Screw block simulationMethodsComponent Contact
No penetration between interfaces Rigidly bonded for all other components Various frictional factors between interfaces were simulated
Interface (fracture plane)
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Part 1: Screw block simulationMethods: Boundary Conditions & Loadings
Each loading was applied individually
10 N Compression 10 N Tension
10 N Shear Force
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Part 1: Screw block Simulation
Results: Example Von Mises stress plot
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Part 1: Screw block Simulation
Results: Example Normal Stress in X direction
Tension
Compression
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Part 1: Screw block Simulation
Results: Stress
10N Compression
10N Tension
10N Shear Force
No friction No Gap 235 441.7 1,118
Friction factor: 0.46
No Gap 237.1 443.5 837.1
Friction factor: 0.8
No Gap 237.1 443.5 888.1
2-mm Gap 449.5 449.5 12,320
FEA result for Stress
Note: All units in kPa
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Part 1: Screw block Simulation
Results: Displacement
10N Compression
10N Tension
10N Shear Force
No friction No Gap 1.084 2.324 45.78
Friction factor: 0.46
No Gap 1.083 2.319 42.03
Friction factor: 0.8
No Gap 1.083 2.319 42.70
2-mm Gap 2.335 2.335 434.9
Note: All units in mm
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Part 2: Modeling a bone layerMethods Reasons
Current model is a single solidBetter represent the two bone layers
Three methods attempted:Shell featureScalingLofted/boundary cut
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Part 2: Modeling a bone layer
Method 3: Lofted/Boundary cut
Cuts along the specified boundary (required) and follow the path defined (optional)
Boundaries specifiedModeled by Dr. Kajsa Duke
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Part 2: Modeling a bone layer
Method 3: Lofted/Boundary cut
Specify every boundary in multiple planes Surface offset to represent cortical bone
thickness
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Part 2: Modeling a bone layer
Combine
Cancellous core was created using Boolean subtraction, then assembled them together
Cancellous core + Cortical layer = More realistic bone
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Part 2: Modeling a bone layer
Continue Modeling
Main cut, small cut and screw were assembled
Main Cut
Screw
Small cut shell
Small cut core (blue)
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Part 2: Simulation
Component contactMain cut Small cut:
shellSmall cut:
coreScrew
Main cut No penetration
No penetration
Bonded
Small cut: shell
No penetration
Bonded Bonded
Small cut: core
No penetration
Bonded Bonded
Screw Bonded Bonded Bonded
Blue: Small cut coreRed: Screw
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Part 2: SimulationLoading and Boundary Conditions
Fix the Ilium and Pubic Symphysis Apply 500 N force on acetabulum
Pubic Symphysis
Acetabulum
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Part 2: Simulation
Results: Stress Maximum stress found within bolt Stresses concentrated around acetabulum,
fixtures and bolt
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Part 2: Simulation
Results: Displacement Maximum displacement occurs at inferior ~
0.03 mm
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Conclusions
Clinical relevance learned from blockBlock case with Gap was the worstThere should not be a gap between
fractures
Clinical relevance from pelvisGreater displacement in inferior rami
not directly held by screwBut displacement is so small healing is
not a concern
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Challenges & Future work
ChallengesTime consuming method13 more pelvis bones to analyze
Future workApply the method to full pelvisFind a more effective approach of
separating cortical layer and cancellous core
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Questions?