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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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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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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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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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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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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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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
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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
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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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