2. OUTLINE
• INTRODUCTION
• BASIC CONCEPTS/DEFINITIONS
• COMMON ORTHOPAEDIC IMPLANT MATERIALS
& CLINICAL APPLICATIONS
• GENERAL TISSUE-IMPLANT RESPONSES
• COMPLICATIONS ASSOCIATED WITH IMPLANTS
• RECENT ADVANCES
• CONCLUSION
3. INTRODUCTION
• Implants are biomaterial devices
• Essential in the practice of orthopaedics
• 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
4. BASIC CONCEPTS & DEFINITIONS
• STRESS: The force applied per unit cross-
sectional area of the body or a test piece
(N/mm²)
• STRAIN: 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
6. DEFINITIONS
• YOUNG’S MODULUS OF ELASTICITY: The stress
per unit strain in the linear elastic portion of the
curve (1N/m² = 1Pascal)
• DUCTILITY: The ability of the material to
undergo a large amount of plastic deformation
before failure e.g metals
• BRITTLENESS: The material displays elastic
behaviour right up to failure e.g ceramics
7. DEFINITIONS
• STRENGTH: The degree of resistance to
deformation of a material
- Strong if it has a high tensile strength
• FATIGUE FAILURE: The failure of a material with
repetitive loading at stress levels below the
ultimate tensile strength
• NOTCH SENSITIVITY: The extent to which
sensitivity of a material to fracture is increased
by cracks or scratches
8. DEFINITIONS
• ULTIMATE TENSILE STRESS: The maximum
amount of stress the material can withstand
before which fracture is imminent
• TOUGHNESS: Amount of energy per unit volume
that a material can absorb before failure
• ROUGHNESS: Measurement of a surface finish of
a material
• HOOKE’S LAW → Stress α Strain produced
- The material behaves like a spring
9. BONE BIOMECHANICS
• Bone is anisotropic;
- it’s elastic modulus depends on direction of
loading
- weakest in shear, then tension, then
compression
• Bone is also viscoelastic → the stress-strain
characteristics depend on the rate of loading
• Bone density changes with age, disease, use and
disuse
• WOLF’S LAW → Bone remodelling occurs along
the line of stress
10. IDEAL IMPLANT MATERIAL
• Chemically inert
• Non-toxic to the body
• Great strength
• High fatigue resistance
• Low Elastic Modulus
• Absolutely corrosion-proof
• Good wear resistance
• Inexpensive
17. COBALT CHROME ALLOYS
• Contains primarily cobalt (30-60%)
• Chromium (20-30%) added to improve
corrosion resistance
• Minor amounts of carbon, nickel and
molybdenum added
18. COBALT CHROME ALLOYS
• Advantages:
1. Excellent resistance
to corrosion
2. Excellent long-term
biocompatibility
3. Strength (very
strong)
• Disadvantages:
1. Very high Young’s
modulus
- Risk of stress
shielding
2. Expensive
19. YOUNG’S MODULUS AND DENSITY OF
COMMON BIOMATERIALS
MATERIAL YOUNG’S MODULUS (GPa) DENSITY (g/cm³)
Cancellous bone 0.5-1.5 -
UHMWPE 1.2 -
PMMA bone cement 2.2 -
Cortical bone 7-30 2.0
Titanium alloy 110 4.4
Stainless steel 190 8.0
Cobalt chrome 210 8.5
22. CERAMICS
• Compounds of metallic elements e.g
Aluminium bound ionically or covalently with
nonmetallic elements
• Common ceramics include:
- Alumina (aluminium oxide)
- Silica (silicon oxide)
- Zirconia (Zirconium oxide)
- Hydroxyapatite (HA)
23. CERAMICS
• Advantages:
1. Chemically inert &
insoluble
2. Best
biocompatibility
3. Very strong
4. Osteoconductive
• Disadvantages:
1. Brittleness
2. Very difficult to
process – high
melting point
3. Very expensive
24. CERAMICS
• Used for femoral head component of THR
- Not suitable for stem because of its
brittleness
• Used as coating for metal implants to
increase biocompatibility e.g HA
25. POLYMERS
• Consists of many repeating units of a basic
sequence (monomer)
• Used extensively in orthopaedics
• Most commonly used are:
- Polymethylmethacrylate (PMMA, Bone
cement)
- Ultrahigh Molecular Weight Polyethylene
(UHMWPE)
26. PMMA (BONE CEMENT)
• Mainly used to fix prosthesis in place
- can also be used as void fillers
• Available as liquid and powder
• The liquid contains:
→ The monomer N,N-dimethyltoluidine (the
accelerator)
→ Hydroquinone (the inhibitor)
27. PMMA
• The powder contains:
- PMMA copolymer
- Barium or Zirconium oxide (radio-opacifier)
- Benzoyl peroxide (catalyst)
• Clinically relevant stages of cement reaction:
1. Sandy stage
2. Mixture appears stringy
3. Cement is doughy
4. Cement is hard
28. UHMWPE
• A polymer of ethylene with MW of 2-6million
• Used for acetabular cups in THR prostheses
• Metal on polyethylene is gold standard
bearing surface in THR (high success rate)
• Osteolysis produced due to polyethylene
wear debris causes aseptic loosening
32. BIODEGRADABLE POLYMERS
• Ex; Polyglycolic acid, Polylactic acid,
copolymers
• As stiffness of polymer decreases, stiffness of
callus increases
• Hardware removal not necessary (reduces
morbidity and cost)
• Used in phalangeal fractures with good
results
33. GENERAL TISSUE-IMPLANT
RESPONSES
• All implant materials elicit some response from
the host
• The response occurs at tissue-implant interface
• Response depend on many factors;
- Type of tissue/organ;
- Mechanical load
- Amount of motion
- Composition of the implant
- Age of patient
34. TISSUE-IMPLANT RESPONSES
• 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
35. TISSUE-IMPLANT RESPONSES
- 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
36. TISSUE-IMPLANT RESPONSES
- 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
37. COMPLICATIONS
• Aseptic Loosening:
- Caused by osteolysis from body’s reaction to
wear debris
• Stress Shielding:
- Implant prevents bone from being properly
loaded
• Corrosion:
- Reaction of the implant with its environment
resulting in its degradation to oxides/hydroxides
38. COMPLICATIONS
• Infection:
- colonization of implant by bacteria and
subsequent systemic inflammatory response
• Metal hypersensitivity
• Manufacturing errors
• VARIOUS FACTORS CONTRIBUTE TO IMPLANT
FAILURE
39. RECENT ADVANCES
• Aim is to use materials with mechanical
properties that match those of the bone
• Modifications to existing materials to
minimize harmful effects
- Ex; nickel-free metal alloys
• Possibility of use of anti-cytokine in the
prevention of osteolysis around implants
• Antibacterial implant
40. CONCLUSION
• Adequate knowledge of implant materials is
an essential platform to making best choices
for the patient
• No completely satisfying results from use of
existing implant materials
• Advances in biomedical engineering will go a
long way in helping the orthopedic surgeon
• The search is on…
42. REFERENCES
• Manoj Ramachandran et al. Basic Orthopaedic Sciences-The Stanmore
Guide. 1st ed. Hodder Arnold 2007; Ch.17&18, pp 147-163.
• Paul A. Banaszkiewicz et al. Postgraduate orthopaedics-The candidate’s
guide. Cambridge University Press 2009; Ch.24, pp 489-494.
• S. Raymond Golish and William M. Mihalko. Principles of Biomechanics
and Biomaterials in Orthopaedic Surgery. J Bone Joint Surg (Am).
2011;93:207-12.
• Philip H. Long. Medical Devices in orthopedic Applications. Journal of
Toxicologic Pathology 2008;36:85-91.
• Matthew J. Silva and Linda J. Sandell. What’s New in Orthopedic
Research. J Bone Joint Surg (Am). 2002; vol 84-A;8: 1490-96.
