Studying the effects of Implant stiffness on bone implant interface failure and bone remodeling

B.Tech ProjectbyMohit Raut11ME33021

INTRODUCTION

Femur Bone

• Longest, Strongest and the heaviest bone in the

human body

• In most of the daily activities such as running,

jumping etc. , entire weight of the body is

supported by the femur

• It contains two distinct morphological types of

bone namely ,Cortical and Cancellous

FEM Model

FEM-model developed from X-ray scans of femur prosthesis.

Bone and implant are modelled using PLANE183 element, a higher order 2-D, 8-node element with quadratic displacement behaviour

Cortical Bone (purple) has E=7Gpa.

Cancellous Bone (red) has E=1Gpa.

Bone and implant interface contact is defined by CONTA172 and TARGET189 elements in ANSYS.

FAILURE CRITERION

• Finding Bone Implant Interface Failure by Hoffman number (FL).

• Lower the FL, less the probability of failure. FL is calculated for each interface node

(Hoffman, 1967)

BONE REMODELING

Bone has the capability to adapt its structure (external geometry and internal

structure) in response to change in mechanical loading by bone apposition

(formation) and bone resorption (loss)

(Weinans et al 1992)

BONE REMODELING

BONE REMODELING

Finding change of density (bone remodeling) for each element:

BONE REMODELING

Density considerations and Relationship between density and Young’s modulus:

(Zhang et al 2010)

FORCE CALCULATIONS

FLOWCHART OF BONE REMODELLING

RESULTS Maximum Node Interface Stress before remodeling occurs. Stress shielding due to high stiffness visible

0 20 40 60 80 100 1200

2

4

6

8

10

12

14

16

YOUNG'S MODULUS vs MAX INTERFACE STRESS

YOUNG'S MODULUS in GPA

MAX

INTE

RFAC

E ST

RESS

in M

Pal

RESULTS Average change in density for elements after one time step. Bone resorption increases with stiffness.

0 20 40 60 80 100 1200

1

2

3

4

5

6

YOUNG'S MODULUS vs ∆ρ (% of initial value)average(for 1st time)

YOUNG'S MODULUS in GPA

∆P A

VERA

GE(fo

r 1st

tim

e)

RESULTS

Maximum Hoffman number (FL) was calculated for three time steps for implant with 36 GPa(flexible) and 110 GPa (titanium) for each interface node.Probability of failure increases with time due to bone remodelling.

1 2 3 40

2

4

6

8

10

12

14

7.29

9.5

11.2

12.3

4.16

5.46.3

7.1

Hoffman Number vs Loop Number

FlexibleTitanium

Loop Number

Hoffm

an N

umbe

r

CONCLUSIONS

• Bone Resorption increases the chances of the bone-implant interface failure as Hoffman Number (FL) increases after

each time step.

• Bone Resorption increases as stiffness of implant is increased.

• Flexible implants should be used in young patients who have higher bone density than older people

Thank You