Alloys in Orthopaedics. Consists of various Alloys and biomaterials used in the field of Orthopaedics.

1. Alloys in
Orthopaedics
18/01/16
Chairperson – Dr. Kiran S Patil
Presenter – Dr. Srinath M Gupta
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OutlineOutline
IntroductionIntroduction
Common Orthopaedic implantCommon Orthopaedic implant
materialsmaterials and Clinical applicationsand Clinical applications
General Tissue –General Tissue – Implant responsesImplant responses
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Basic concepts / DefinitionsBasic concepts / Definitions
Recent advancesRecent advances
ConclusionConclusion
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Study of Biomaterials
o Implants are biomaterial devices
o A biomaterial is any substance or combination
of substances (other than a drug), synthetic or
natural in origin, that can be used for any period
of time as a whole or part of a system that
treats, augments or replaces any tissue, organ or
function of the body
o Physical and biological study of materials and
their interactions with the biological
environment.
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o Stress
o Strain
o Young’s modulus of Elasticity
o Ductility
o Brittleness
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Basic concepts and DefinitionsBasic concepts and Definitions
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o STRESSSTRESS: The force applied per unit cross-
sectional area of the body or a test piece
(N/mm²)
o STRAINSTRAIN: The change in length (mm) as a
fraction of the original length (mm)
- relative measure of deformation of the body or
a test piece as a result of loading
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YOUNG’S MODULUS OF ELASTICITYYOUNG’S MODULUS OF ELASTICITY: The stress
per unit strain in the linear elastic portion of the
curve (1N/m² = 1Pascal)
DUCTILITYDUCTILITY: The ability of the material to
undergo a large amount of plastic deformation
before failure e.g metals
BRITTLENESSBRITTLENESS: The material displays elastic
behaviour right up to failure e.g ceramics
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Relative values of
Young's modulus of
elasticity (numbers
correspond to
numbers on illustration
to right)
1.Ceramic (Al2O3)
2.Alloy (Co-Cr-Mo)
3.Stainless steel
4.Titanium
5.Cortical bone
6.Matrix polymers
7.PMMA
8.Polyethylene
9.Cancellous bone
10.Tendon / ligament
11.Cartilage
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STRENGTHSTRENGTH: The degree of resistance to
deformation of a material
- Strong if it has a high tensile strength
FATIGUE FAILUREFATIGUE FAILURE: The failure of a material with
repetitive loading at stress levels below the
ultimate tensile strength
NOTCH SENSITIVITYNOTCH SENSITIVITY: The extent to which
sensitivity of a material to fracture is increased
by cracks or scratches
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ULTIMATE TENSILE STRESSULTIMATE TENSILE STRESS: The maximum
amount of stress the material can withstand
before which fracture is imminent
TOUGHNESSTOUGHNESS: Amount of energy per unit volume
that a material can absorb before failure
ROUGHNESS: Measurement of a surface finish
of a material
HOOKE’S LAWHOOKE’S LAW → Stress α Strain produced
- The material behaves like a spring
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o Bone is anisotropic;
- it’s elastic modulus depends on direction of
loading
- weakest in shear, then tension, then compression
o Bone is also viscoelastic → the stress-strain
characteristics depend on the rate of loading
o Bone density changes with age, disease, use and
disuse
o WOLF’S LAWWOLF’S LAW → Bone remodelling occurs along
the line of stress
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Bone Type Load Type
Elastic
Modulus
(×10 E9 N/m2
)
Ultimate
Stress
(× 10 E6 N/m2
)
Cortical Tension 11.4-19.1 107-146
Compression 15.1-19.7 156-212
Shear 73-82
Cancellous Tension ~0.2-5.0 ~3-20
Compression 0.1-3 1.5-50
Shear 6.6 +/- 1.7
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o Chemically inert
o Non-toxic to the body
o Great strength
o High fatigue resistance
o Low Elastic Modulus
o Absolutely corrosion-proof
o Good wear resistance
o Economical
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IDEAL IMPLANT MATERIALIDEAL IMPLANT MATERIAL
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CLINICAL APPLICATIONS OF
ORTHOPAEDIC IMPLANTS
o Osteosynthesis
o Joint replacements
o Nonconventional modular tumor implants
o Spine implants
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Metal AlloysMetal Alloys:
- stainless steel
- Titanium alloys
- Cobalt chrome alloys
NonmetalsNonmetals:
- Ceramics & Bioactive glasses
(Alumina/Zirconium)
- Polymers (Bone cement, polyethylene)
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CLINICAL APPLICATIONS OF
ORTHOPAEDIC IMPLANTS
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Stainless Steel
ContainsContains:
- Iron (62.97%)
- Chromium (18%)
- Nickel (16%)
- Molybdenum (3%)
- Nitrogen (0.1%)
- Carbon (0.03%)
The form used commonly is 316L (3% molybd,
16% nickel & L = Low carbon content)
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The chromium forms an oxide layer when dipped in
nitric acid to reduce corrosion and the molybdenum
increases this protection when compared to other
steels.
AdvantagesAdvantages:
1. Strong
2. Relatively ductile
3. Biocompatible
4. Relatively cheap
5. Reasonable coorsion
resistance
DisadvantagesDisadvantages:
- Susceptibility to
crevice and stress
corrosion
• Used in plates, screws, IM nails, ext fixators
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22-13-5 stainless steel alloy22-13-5 stainless steel alloy
o Chromium – 20.5 – 23.5%
o Nickel – 11.5 – 13.5%
o Manganese – 4 - 6%
o Nitrogen- 0.2 – 0.4%
o Iron – Remaining%
•Better corossion resistance
•High yield strength
•MRI friendly
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• .
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Titanium alloy
ContainsContains:
- Titanium (89%)
- Aluminium (6%)
- Vanadium (4%)
- Others (1%)
Most commonly orthopaedic titanium alloy is
TITANIUM 64 (Ti-6Al-4v)
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AdvantagesAdvantages:
1. Corrosion resistant
2. Excellent
biocompatibility
3. Ductile
4. Fatigue resistant
5 Low Young’s
modulus
6. MR scan
compatible
• Useful in halos, plates,
IM nails etc.
Disadvantages:Disadvantages:
1. Notch sensitivity
2. poor wear
characteristics
3. Systemic toxicity –
vanadium
4. Relatively
expensive
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• .
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o Contains primarily cobalt (30-60%)
o Chromium (20-30%) added to improve
corrosion resistance
o Minor amounts of carbon, nickel and
molybdenum added
COBALT CHROME ALLOYSCOBALT CHROME ALLOYS
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COBALT CHROME ALLOYSCOBALT CHROME ALLOYS
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AdvantagesAdvantages:
1. Excellent resistance
to corrosion
2. Excellent long-term
biocompatibility
3. Strength (very
strong)
DisadvantagesDisadvantages:
1. Very high Young’s
modulus
-Risk of stress shielding
2.Expensive
3.Nickel sensitivity.
o Used in making arthroplasty implants.
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COMPARISON OF METAL ALLOYSCOMPARISON OF METAL ALLOYS
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o Compounds of metallic elements with
nonmetallic elements. e.g Aluminium bound
ionically or covalently with nonmetallic elements
o Common ceramics include:
- Alumina (aluminium oxide)
- Silica (silicon oxide)
- Zirconia (Zirconium oxide)
- Hydroxyapatite (HA)
- Silicon Nitride (New Alloy)
CERAMICS
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AdvantagesAdvantages:
1. Chemically inert &
insoluble
2. Best
biocompatibility
3. Very strong
4. Osteoconductive
Disadvantages:Disadvantages:
1. Brittleness
2. Very difficult to
process – high melting
point
3. Very expensive
CERAMICS
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o Used for femoral head component of THR
- Not suitable for stem because of its brittleness
o Used as coating for metal implants to increase
biocompatibility e.g HA
CERAMICS
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HA coated implantsHA coated implants
OSSEOINTEGRATION
– Due to presence of Calcium
and phosphorous
– Promotes bone ingrowth
– Increased success rate
– Act as bacteriostatic
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Silicon NitrideSilicon Nitride
o Bacteriostatic – Prevent biofilm formation
o Osseoconductive
o Biocompatible
o Made from elements
present in the human
body
o Used to make variable
Spinal implants
o MRI friendly
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o Consists of many repeating units of a basic
sequence (monomer)
o Used extensively in orthopaedics
o Most commonly used are:
- Polymethylmethacrylate (PMMA, Bone
cement)
- Ultrahigh Molecular Weight Polyethylene
(UHMWPE)
POLYMERSPOLYMERS
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PMMA (BONE CEMENT)PMMA (BONE CEMENT)
o Mainly used to fix prosthesis in place
- can also be used as void fillers
o Available as liquid and powder
o The liquid contains:
→ The monomer N,N-dimethyltoluidine (the
accelerator)
→ Hydroquinone (the inhibitor)
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PMMA (BONE CEMENT)PMMA (BONE CEMENT)
The powder contains:
- PMMA copolymer
- Barium or Zirconium oxide (radio-opacifier)
- Benzoyl peroxide (catalyst)
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The curing process is divided into 4 stages :
1. Mixing
2. Sticking / waiting time (2-3mins)
3. Working Time (5-8mins)
4. Setting time / Hardening (8-10mins)
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USES
used for fixation and load
distribution in conjunction with
orthopeadic implants
Functions by interlocking with
bone
•May be used to fill tumor defects
and minimize local recurrence
Advantages
1)-Reaches ultimate
strength at 24 hours
2)-Strongest in
compression
3)-Young's modulus
between cortical and
cancellous bone
Disadvantages
•poor tensile and shear
strength
•insertion can lead to
dangerous drop in blood
pressure
•failure often caused by
microfracture and
fragmentation
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o A polymer of ethylene with MW of 2-6million
o Used for acetabular cups in THR prostheses
o Metal on polyethylene is gold standard
bearing surface in THR (high success rate)
o Osteolysis produced due to polyethylene wear
debris causes aseptic loosening
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UHMWPEUHMWPE
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UHMWPEUHMWPE
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Disadvantages
1)-Susceptible to abrasion
2)-Wear usually caused by third
body inclusions
3)-Thermoplastic (may be altered by
extreme temperatures)
weaker than bone in tension
Advantages
1)-Tough
2)-Ductile
3)-Resilient
4)-Resistant to wear

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THR IMPLANT BEARING SURFACESTHR IMPLANT BEARING SURFACES
• Metal-on-
polyethylene
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BIODEGRADABLE POLYMERSBIODEGRADABLE POLYMERS
o Ex; Polyglycolic acid, Polylactic acid,
copolymers
o As stiffness of polymer decreases, stiffness of
callus increases
o Hardware removal not necessary (reduces
morbidity and cost)
o Used in phalangeal fractures with good results
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RECENT ADVANCESRECENT ADVANCES
o Aim is to use materials with mechanical
properties that match those of the bone
o Modifications to existing materials to
minimize harmful effects
- Ex; nickel-free metal alloys
o Possibility of use of anti-cytokine in the
prevention of osteolysis around implants
o Antibacterial implant18/01/16 43

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Silicones
o Polymers that are often used for
replacement in non-weight bearing joints
o Disadvantages
• poor strength and wear capability
responsible for frequent synovitis
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Shape-memory polymersShape-memory polymers
(SMPs)(SMPs)
o These are polymeric smart materials that have
the ability to return from a deformed state
(temporary shape) to their original (permanent)
shape induced by an external stimulus (trigger),
such as temperature change.
Ex – Nitinol (Nickel – Titanium alloy)
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o Elemental tantalum metal
o Vapor deposition techniques that create a metallic
strut configuration similar to trabecular bone.
o Crystalline microtexture is conductive to direct bone
apposition.
o Interconnecting pores
•80% porous
•2-3 times greater bone ingrowth compared to
conventional porous coatings
•Double the interface shear strength18/01/16
TRABECULAR METALTRABECULAR METAL
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o All implant materials elicit some response
from the host
o The response occurs at tissue-implant
interface
o Response depend on many factors;
- Type of tissue/organ;
- Mechanical load
- Amount of motion
- Composition of the implant
- Age of patient18/01/16
GENERAL TISSUE-IMPLANTGENERAL TISSUE-IMPLANT
RESPONSESRESPONSES
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There are 4 types of responses (Hench & Wilson,
1993)
1. Toxic response:
- Implant material releases chemicals that
kill cells and cause systemic damage
2. Biologically nearly inert:
- Most common tissue response
- Involves formation of nonadherent fibrous
capsule in an attempt to isolate the implant
- Implant may be surrounded by bone
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TISSUE-IMPLANT RESPONSESTISSUE-IMPLANT RESPONSES
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- Can lead to fibrous encapsulation
- Depend on whether implant has smooth
surface or porous/threaded surface
- Ex; metal alloys, polymers, ceramics
3. Dissolution of implant:
- Resorbable implant are degraded
gradually over time and are replaced by
host tissues
- Implant resorption rate need to match tissue-
repair rates of the body
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TISSUE-IMPLANT RESPONSESTISSUE-IMPLANT RESPONSES
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- Ex; Polylactic and polyglycolic acid polymers
which are metabolized to CO2 & water
4. Bioactive response:
- Implant forms a bond with bone via chemical
reactions at their interface
- Bond involves formation of hydroxyl-
carbonate apatite (HCA) on implant surface
creating what is similar to natural interfaces
between bones and tendons and ligaments
- Ex; hydroxyapatite-coating on implants
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TISSUE-IMPLANT RESPONSESTISSUE-IMPLANT RESPONSES
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o Aseptic Loosening:
- Caused by osteolysis from body’s reaction to
wear debris
o Stress Shielding:
- Implant prevents bone from being properly
loaded
o Corrosion:
- Reaction of the implant with its environment
resulting in its degradation to oxides/hydroxides
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ComplicationsComplications
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Galvanic corrosion-
due to two different metals being used e.g.
stainless steel screws and titanium plate.
Stress corrosion-
The presence of a crack due to stress
Crevice corrosion / fretting occurs
where components have a relative movement
against one another
Pit corrosion-
A local form of crevice corrosion due to
abrasion produces a pit
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o Infection:
- colonization of implant by bacteria and
subsequent systemic inflammatory response
o Metal hypersensitivity
o Manufacturing errors
o VARIOUS FACTORS CONTRIBUTE TO IMPLANT
FAILURE
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ComplicationsComplications
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Adequate knowledge of implant materials is an
essential platform to making best choices for the
patient
Most of the existing implant material falls short
of one or the other criteria to be an IDEAL
IMPLANT.
Advances in biomedical engineering will go a long
way in helping the orthopedic surgeon
The search is on…
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ConclusionConclusion
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