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.