Investigation of Screw Fixations for Pubic Rami Fractures_AN_KKD

Investigation of Screw Fixations for Pubic Rami FracturesPresented by: Shi AnSupervisor: Dr. Kajsa Duke

December 4, 2015

Mechanical EngineeringUniversity of Alberta

Outline

Introduction

Objectives

Methods

Results

Future work

Introduction

Pelvic Bone

Ilium

PubisPubic Symphysis

Sacrum

Introduction

Causes for Pelvic Fracture

Incidents: traffic accidents, falling from

significant height etc..

Osteoporosis

Introduction

Pelvic Fracture

Superior Pubic Ramus

Dual Rami Fracture

Inferior Pubic Ramus

Introduction

Treatment of Pubic Rami Fractures

Minimally invasive

Open surgery

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

Part 1: Screw block simulationMethodsMethod for simple screw block simulation:

Solid modeling screw and blocks

Different loadings including axial load, shear load and friction

Part 1: Screw block simulationMethods: ModelingSolid modeling screw and blocks

Screw

Cancellous

Cortical layer

Cortical

Cancellous

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

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)

Part 1: Screw block simulationMethods: Boundary Conditions & Loadings

Each loading was applied individually

10 N Compression 10 N Tension

10 N Shear Force

Part 1: Screw block Simulation

Results: Example Von Mises stress plot

Part 1: Screw block Simulation

Results: Example Normal Stress in X direction

Tension

Compression

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

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

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

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

Part 2: Modeling a bone layer

Method 3: Lofted/Boundary cut

Specify every boundary in multiple planes Surface offset to represent cortical bone

thickness

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

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)

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

Part 2: SimulationLoading and Boundary Conditions

Fix the Ilium and Pubic Symphysis Apply 500 N force on acetabulum

Pubic Symphysis

Acetabulum

Part 2: Simulation

Results: Stress Maximum stress found within bolt Stresses concentrated around acetabulum,

fixtures and bolt

Part 2: Simulation

Results: Displacement Maximum displacement occurs at inferior ~

0.03 mm

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

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

Questions?