MINIMIZING STRESS SHIELDING IN FEMORAL HIP IMPLANTS THROUGH

MATHEMATICAL MODELING AND EXPERIMENTAL VERIFICATION

Justin Fisher, Tyler Grubb, Phuong Huyen, Rohan Yadav

Dr. Abdellah Ait Moussa, Dr. Morshed Khandaker

OVERVIEWObjective: • Reduce stress shielding and interface stress in a hip

replacement by controlling stem stiffness, which is a function of stem geometry and its material properties.

How: • Create a self-regulated software package to optimize the

stem geometry by mathematically modeling and controlling the stem geometry using a fixed number of design parameters, create a solid model of the stem assembly, and conduct the finite element simulation.

• Design and build experimental apparatus to benchmark and confirm the numerical results.

DELIVERABLES

• Numerical analysis method for minimization of stress shielding on a femoral hip implant under static and dynamic loads through geometrical manipulation

• Experimental verification and benchmark of the numerical analysis results.

OSTEOPOROSIS

• Secondary Osteoporosis• Bone fragility due to bone

density reduction• Caused by a variety of

factors

STRESS SHIELDINGFemur Bone Bone with Implant

MATHEMATICAL MODEL

( 𝑥𝑎 )𝑝+( 𝑦𝑏 )

𝑝=1

FINITE ELEMENT ANALYSISANSYS Static Structural in Workbench. Engineering Data- Titanium, PMMA Cement and Cortical Bone.The IGES model imported into Design Modeler.1. Meshing Medium mesh with Tetrahedral elements. Elements – About 90,000-100,000 Grid Independent Test 2. Contact Region Stem- Cement: Rough Bone-Cement: Bonded

FINITE ELEMENT ANALYSIS3. Boundary Conditions• Abductor Muscle force of 1.5 KN at 15°

with vertical.• Joint Reaction force of 2.5 KN at 10°

with vertical. • Fixed Support at Distal End• Simulates average walking conditions.• Fatigue Tool• Goodman Theory• Text file of equivalent alternating

stress.

NUMERICAL ANALYSIS RESULTSCompare stress over the surface of bone.Stress Diff =

NUMERICAL OPTIMIZATION OF STEM GEOMETRYDesign of Experiments Method• Developed by Genichi Taguchi from Japan during late

1940.• Suggested fractional factorial experiments using

orthogonal arrays.• Type of orthogonal array based on the number of

variables and their levels.• Best design parameters will identified from orthogonal

arrays.• About 20 variables for cross section of stem geometry.• L32 orthogonal array was used.

TYPE OF SENSORSensor Electrical

Strain gagePiezoelectric Sensor

Fiber Bragg Sensor

Temperature effect on zero point

Low NA High

Drift Small Large SmallTemperature coefficient of sensitivity

High but compensable

Low High

Linearity High Low NAStatic measurement

Applicable NA Applicable

Dynamics measurement

Applicable Applicable Applicable

MEASUREMENT SYSTEM

The system consists of • Power supply provides ± 15 V, 12V, and 5V• 6 strain gage modules• DAQ device

MEASUREMENT SYSTEM – POWER SUPPLY

MEASUREMENT SYSTEM

MEASUREMENT SYSTEM

LabVIEW Program for DAQ• Measuring the Output voltage of the strain

gage module.• Measuring the Excitation Voltage.• Smoothing the measurement with the Moving

Average Filter.• Computing strain and stress value.• Exporting the data to Excel.

MECHANICAL DESIGN

Designed for Little Tensile TesterMachined

Force Applied PartHip Cup PartBase Plate

MECHANICAL FORCE ANALYSIS

• Sum of moment about L to solve for Fa and Xl such that the give forces for the test condition are met.

BUDGETTotal Budget: $1,000.00

Expenses: Electronic Components: $232.74

Instrumentation Amplifiers (6) Circuit Components Strain Gages (6)

Mechanical Components: $22.98 Crimps and Cable

Total Sent: $255.72Budget Left: $744.28

FUTURE WORK

• Experimentally measure Stress Array in Femur Bone

• Experimentally measure Stress Array in Non-Optimized Implant

• Experimentally measure Stress Array in Optimized Implant

REFERENCES[1] Kurtz S, Ong K, Lau E, Mowat F, Halpern M, Projections of primary and revision hip and knee arthroplasty in the United States from 2005 to 2030, J Bone Joint Surg Am. 2007 Apr;89(4):780-5. [2] Raut, V. V., Siney, P. D., and Wroblewski, B. M., 1995, ‘‘Revision for Aseptic Stem Loosening Using the Cemented Charnley Prosthesis,’’ J. Bone Joint Surg. Br., 77-B, pp. 23–27. [3] Raut, V. V., Siney, P. D., and Wroblewski, B. M., 1995, ‘‘Cemented Revision for Aseptic Acetabular Loosening,’’ J. Bone Joint Surg. Br., 77-B, pp 357-361. [4] Ali Abdulkarim, Prasad Ellanti, Nicola Motterlini, Tom Fahey, and John M. O'Byrne, Cemented versus uncemented fixation in total hip replacement: a systematic review and meta-analysis of randomized controlled trials, Orthop Rev (Pavia). Feb 22, 2013; 5(1): e8 [5] Li C, Granger C, HD. Progressive failure analysis of laminated composite femoral prostheses for total hip arthroplasty. Biomaterials 2002;23:4249–62. [6] Wolfram Mathematica, http://www.wolfram.com/mathematica/ [7] SolidWorks Corporation, http://www.solidworks.com/ [8] ANSYS Corporation, http://www.ansys.com/ [9] Lennon, A.B., McCormack, B.A.O., Prendergast, P.J., 2003. The relationship between cement fatigue damage and implant surface finish in proximal femoral prostheses. Medical Engineering and Physics 25, 833-841. [10] Jeffers, J.R.T., Browne, M., Taylor, M., 2005b. Damage accumulation, fatigue and creep of vacuum mixed bone cement. Biomaterials 26 (27), 5532-5541.