Failure of Orthopedic Implants

7/29/2019 Failure of Orthopedic Implants

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Orthopaedics and total joint

replacements

Stress-shielding

Corrosion

Tribology and wear mechanisms

Case study: Sulzer hip recall

Outline

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Introduction

Arthritis and musculoskeletal disorders are the twomost common causes of debilitating chronic pain.

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Articular Joints

Flexion/extension

rolling, sliding, rotation

Resilience

compression, cyclic

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Cartilage

self-lubrication, hydrodynamiccushion

Bone

shock-absorber,

structural/mechanical support

Muscle

mechanics, shock-absorber

Tendons/ligaments Connect bone-bone, bone-muscle

Functionality Orthopaedic Tissues


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Hip Contact Force5

D.E. Harwitz et al. Journal of Biomechanics 36 (2003) 113-119


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Mechanical Properties ofOrthopaedic Tissues

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Disease Progression: Osteoarthritis

Painful, poorly understood

Arthritic ailments affect 21million Americans with

costs of 1.2% of GNP

OA will inflict 60 millionAmericans by 2020

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Massey JT. Vital Health Statistics


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Total Joint Replacements

The ability of articular cartilage to

repair itself is limited

>1.5 million joint replacements per

year world-wide

200% increase expected each decade

Kurtz et al., The Future Burden of Hip and Knee Revisions 2006. Surgeons. Kurtz, AAOS 2006.


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Early Development of THR

Stainless steel femoral stem with cement

Stem fractures (fatigue problem)

Bending moment/stress tensile field

lateral crack propagation

Solution to fracture problem: engineeringproblem

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Total Joint Replacements

90% success rate at 15 yrs

Wear from metal alloy/UHMWPE linked to

foreign body response and loosening

Need for development of self-lubricating,

wear and fatigue resistant materials

Current: solid/solid bearings

Total joint replacements are highly

successful surgical treatments for

damaged cartilage

700,000 THR and TKR annually

50,000 revisions

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AAOS, 2002

Sir John Charnley


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THR MaterialRequirements

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THR Materials

polyethylene

monomer3-D polyethylene

chain

Surface FinishZirconia, Cobalt Chrome

Osseoinduction Titanium

Low coefficient of frictionUHMWPE


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Bone cement (PMMA) vs. bony in-growth (non-cemented)

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Osseointegration

Load-bearingimmediately

Need to wait forbone in-growth


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Stress-shielding: the reduction in bone density (osteopenia)as a result of removal of normal stress from the bone by animplant.

Osteolysis: active resorption or dissolution of bone tissue aspart of an ongoing disease process.

Look at stress in bone due to axial loading and pure bending

moment (compression) Composite beam theory, neglecting shear

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Bone Loss Stress Shielding


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Uniform attack: generalcorrosion that is evenly

distributed over entire corrosionregion

Rusting of iron, tarnishing of silver Not common in implants

Corrosion

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Wear

Loss of material Abrasion, adhesion, fretting

Delamination (due to fatigue), pitting

Depends on friction, lubrication, contact area, surface finish

and levels of load (stresses, conformity)

Relevance

Over half a million hips and knees are replaced each year

6% need revision within 10 years Polymer component is weak link due to fatigue, fracture,

wear

UHMWPE articulated against Co-Cr

Wear in TJR

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Difference between hip and knee

Multi-axial loading situation (hip) Trade-off

wear, fatigue strength (want both) E, contact area, wear

Conformity and Contact Stress

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Four Modes of

Wear in TJR

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Adhesive Wear

Energy dependent


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Abrasive Wear

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3rd Body Wear

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Fatigue Wear

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Wear

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James,S.P. et al. J Biomed Mater Res (Appl Biomater) 48:374, 1999


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Osteochondral TE Construct

Membrane will preventosseos-tissue formation

throughout.

pHEMA + maleated hyaluronan

pHEMA + hydroxyapatite[38 m]

[200 m]

Chondrocytes

will migrate into

hyaluronan-

loaded scaffold.

MSCs will infiltrate

hydroxyapatite-loaded

scaffold.

Failure of Orthopedic Implants

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