Biomaterials metallic & nonmetallic implants

1. Metallic &
Nonmetallic Implants
In Orthopaedics
Dr Debasis Mukherjee
Dept of Orthopaedics
IPGMER

2. HISTORY
• More than 2000 years ago, Romans, Chinese, and Aztec’s used gold
in dentistry.
• Earliest evidence of fracture treatment with a metallic wire was way
back in 1770.
• In 18th century, came ‘Antisepsis’ and ‘Anaesthesia’, along with
development in metallurgy and plastic and X-RAY.
• 1960 Charnley uses PMMA, ultrahigh-molecular-weight
polyethylend, and stainless steal for total hip replacement.
• Discovery of newer metals and newer alloys.

3. WHY TO KNOW ???
To make an informed selection of the Surgical
implant

4. BIOMATERIALS
• These are natural or synthetic substances ,
capable of being tolerated permanently or
temporarily by the human body.

5. BIOMATERIALS
• Biomaterials used in orthopedics are –
1. Metals and metal alloys
2. Ceramic and ceramometallic materials
3. Tissue adhesives
4. Bone replacement materials
5. Carbon materials and composites

6. Orthopedic Implant
• An orthopedic implant is a medical device
manufactured to replace a missing joint or
bone or to support a damaged bone.
• Implants can be Metallic or Non-metallic.

7. ISSUES
• Biocompatibility
• Strength parameters(tensile,comressive and
torsional strength;stiffness,fatigue
resistance,contourability)
• Resistance to degradation and erosion
• Ease of integration when appropriate
• Minimal adverse effect on immaging

8. Orthopedic Terms
Osteoconductive – The property of a material that
allows for the possible integration of new bone with
the host bone.
Osteoinductive – Characteristic in materials that
promote new bone growth.
Bioresorbable – The ability of a material to be entirely
adsorbed by the body.

9. Orthopedic Terms
• Fatigue
• Endurance limit
• Cyclic Failure- High and Low
• Elongation
• Corrosion and Passivation

10. Basic Biomechanics
• Material Properties
– Elastic-Plastic
– Yield point
– Brittle-Ductile
– Toughness
– Hardness
• Independent of
Shape!
• Structural Properties
– Bending Stiffness
– Torsional Stiffness
– Axial Stiffness
• Depends on Shape
and Material!

11. Basic Biomechanics
Force, Displacement & Stiffness
Force
Displacement
Slope = Stiffness =
Force/Displacement

12. Basic Biomechanics
Stress = Force/Area Strain Change Height (L) /
Original Height(L0)
Force
Area
L

13. Basic Biomechanics
Stress-Strain & Elastic Modulus
Stress =
Force/Area
Strain =
Change in Length/Original Length (L/ L0)
Slope = Elastic
Modulus =
Stress/Strain

14. Basic Biomechanics
Common Materials in Orthopaedics
• Elastic Modulus (GPa) • Stainless Steel 200
• Titanium 100
• Cortical Bone 7-21
• Bone Cement 2.5-3.5
• Cancellous Bone 0.7-4.9
• UHMW-PE 1.4-4.2
Stress
Strain

15. Basic Biomechanics
• Elastic Deformation
• Plastic Deformation
• Energy
Energy
Absorbed
Force
Displacement
PlasticElastic

16. Basic Biomechanics
• Stiffness-Flexibility
• Yield Point
• Failure Point
• Brittle-Ductile
• Toughness-Weakness
Force
Displacement
PlasticElastic
Failure
Yield
Stiffness

17. Stiff
Ductile
Tough
StrongStiff
Brittle
Strong
Ductile
Weak
Brittle
Weak
Strain
Stress

18. Flexible
Ductile
Tough
Strong
Flexible
Brittle
Strong
Flexible
Ductile
Weak
Flexible
Brittle
Weak
Strain
Stress

19. Basic Biomechanics
• Load to Failure
– Continuous application
of force until the
material breaks (failure
point at the ultimate
load).
– Common mode of failure
of bone and reported in
the implant literature.
• Fatigue Failure
– Cyclical sub-threshold
loading may result in
failure due to fatigue.
– Common mode of failure
of orthopaedic implants
and fracture fixation
constructs.

20. Bone Properties
• Density – 2.3g/cm3
• Tensile Strength – 3-20MPa
• Compressive Strength – 15,000 psi
• Shear Strength – 4,000 psi
• Young’s Modulus – 10-40 MPa

21. Metals For Implants
• Must be corrosion resistant
• Mechanical properties must be appropriate
for the desired application
• Areas subjected to cyclic loading must have
good fatigue properties -- implant materials
cannot heal themselves

22. Metals For Implants
• Must be corrosion resistant
• Mechanical properties must be appropriate
for the desired application
• Areas subjected to cyclic loading must have
good fatigue properties -- implant materials
cannot heal themselves

23. Metals Used in Implants
• Three main categories of metals for
orthopedic implants
– stainless steels
– cobalt-chromium alloys
– titanium alloys
– Metallic Foam

24. Stainless Steel
• Generally about 12% chromium,13 to 15.5% nickel
(316L, Fe-Cr-Ni-Mo)
• High elastic modulus, rigid-results in Stress Shielding.
• Low resistance to stress corrosion cracking, pitting and
crevice corrosion, better for temporary use
• Corrosion accelerates fatigue crack growth rate in saline
(and in vivo)
• Intergranular corrosion at chromium poor grain
boundaries -- leads to cracking and failure
• Wear fragments - found in adjacent giant cells
• Cheap

25. Cobalt – Based Alloys
• Co-Cr-Mo
– Used for many years in dental implants; more recently used in artificial
joints
– good corrosion resistance
• Co-Cr-Ni-Mo
– Finer grains
– Typically used for stems of highly loaded implants, such as hip and knee
arthroplasty
• Very high fatigue strengths, high elastic modulus
– High degree of corrosion resistance in salt water when under stress
– Poor frictional properties with itself or any other material

26. Titanium and Titanium Alloys
• Minimal attenuation problem on MRI
• High strength to weight ratio
– Density of 4.5 g/cm3 compared to 7.9 g/cm3 for 316 SS
• Modulus of elasticity for alloys is about 110 GPa
– Not as strong as stainless steel or cobalt based alloys, but
has a higher “specific strength” or strength per density
– Low modulus of elasticity

27. Titanium Alloys
• Co-Ni-Cr-Mo-Ti, Ti6A4V
• Poor shear strength which makes it
undesirable for bone screws or plates
• Tends to seize when in sliding contact with
itself or other metals
• Poor surface wear properties - may be
improved with surface treatments such as
nitriding and oxidizing

28. Best Performance
• Titanium has the best biocompatibility of the
three.
– Metal of choice where tissue or direct bone
contact required (endosseous dental implants or
porous un-cemented orthopedic implants)
– Corrosion resistance due to formation of a solid
oxide layer on surface (TiO2) -- leads to
passivation of the material

29. Metallic Foam
• Types of metallic foams
– Solid metal foam is a generalized term for a material
starting from a liquid-metal foam that was restricted
morphology with closed, round cells.
– Cellular metals:A metallic body in which a gaseous void is
introduced.
– Porous metal: Special type of cellular metal with certain
types of voids, usually round in shape and isolated from
each other.
– Metal Sponges: A morphology of cellular metals with
interconnected voids.

30. Magnesium Foam
Open cellular structure permits ingrowths of new-bone tissue
and transport of the body fluids
– Strength & Modulus can be adjusted through porosity to
match natural bone properties

31. Why Magnesium?
• Bioresorbable
• Biocompatible
– Osteoconductive
– Osteoinductive
• Properties of bone can be easily attained using
varying processing techniques

32. Tantalum
• A newer material, tantalum, is a trabecular metal composed
of a carbon substrate with elemental tantalum deposited
on the surface.
Forms a biological scaffold for new bone formation.
• Modulus of elasticity closer to that of bone than stainless
steel or the cobalt-based alloys.
• Not been used in the manufacture of implants until
more recently. Because of its remarkable resistance to corrosion,
tantalum seems well suited to a biological ingrowth
• Because of its remarkable resistance to corrosion,
tantalum seems well suited to a biological ingrowth
setting, but long-term studies are needed to confirm its
usefulness.

33. Comparisons
Material Density Youngs
Modulus
Tensile
Strength
Estimated
Cost
Ranking
Bone 2.3 10 – 40 3 – 20 Na
Stainless
Steel
7.9 196 290 1
Co
Alloys
8.9 211 345 4
Ti Alloys 4.5 105 200 3
Mg Foam 2.33 10.476 2.843 2

34. Composites
• Manufactured in several ways
– Mechanical bonding between materials (matrix and filler)
– Chemical bonding
– Physical (true mechanical) bonding
• Young’s modulus = 200 GPa
• Benefits
– Extreme variability in properties is possible
• Problems
– Matrix cracking
– Debonding of fiber from matrix
• Examples: concrete, fiberglass, laminates, bone

35. Ceramics
• Materials resulting from ionic bonding of
– A metallic ion and
– A nonmetallic ion (usually oxygen)
• Benefits
– Very hard, strong, and good wear characteristics
– High compressive strength
– Ease of fabrication
• Examples
– Silicates , Metal Oxides - Al2O3, MgO
– Carbides - diamond, graphite, pyrolized carbons
– Ionic salts - NaCl, CsCl, ZnS

36. Ceramics (cont’d)
• Uses
– Surface Replacement
– Joint Replacement
• Problems
– Very brittle & Low tensile strength
• Undergo static fatigue
– Very biocompatible
– Difficult to process
• High melting point
• Expensive

37. Polyethylene
• Ultra high molecular weight (UHMWPE)
• High density
– Molecular weight 2-6 million
• Benefits
– Superior wear characteristics
– Low friction
– Fibers included
• Improve wear properties
• Reduce creep
• Used
– Total joint arthoplasty

38. Bone Cement
• Used to fill gaps between bone and implant
• Example: total hip replacement
– If implant is not exactly the right size, gaps are
filled regardless of bone quality

39. Bone Cement
• Polymethylmethacrylate
• Mixed from powder polymer and liquid
monomer
– In vacuum
• Reduce porosity
• Increase strength
– Catalyst (benzoyl peroxide) may be used
• Benefits
– Stable interface between metal and bone
http://www.totaljoints.info/bone_cement.htm

40. Bone Cement (cont’d)
• Problems
– Inherently weak
• Stronger in compression than tension
• Weakest in shear
– Exothermic reaction
• May lead to bone necrosis
– By handling improperly or less than optimally
• Weaker
– Extra care should be taken to
• Keep debris out of the cement mantle (e.g., blood, fat)
• Make uniform cement mantle of several mm
• Minimize voids in the cement : mixing technique
• Pressurize

41. Biodegradable materials
• These are Poly-Glycolic Acid (PGA), PDS, polylevolactic
acid (PLLA), and racemic poly(D, L)-lactic acid (PDLLA)
• Recently SR-PGA and SR-PLLA
• High glass transition temperature
• Can be covalently linked with HRP, IL-2, BMP-2
• Advantages are gradual load transfer to the healing
tissue, reduced need for hardware removal, and
radiolucency
• Disadvantages like aseptic inflammation and sinus tract
formation,severe synovitis are more prominent with
PGA than PLA

42. Biodegradable materials
• Uses-
Reattachment of
ligaments,tendons,meniscal tears and other
soft tissue structures.
Stabilization of
fractures,osteotomies,bone grafts and fusions.

43. Mechanical Properties of IM
• As Implant materials have to function as
bones, the mechanical properties of interest
are
– Elastic modulus
– Ultimate tensile strength
• They are listed in order of increasing modulus
or strength
(in next 2 slides)

44. Elastic Modulus
in increasing order of strength
1. Cancellous bone
2. Polyethylene
3. PMMA (bone cement)
4. Cortical bone
5. Titanium alloy
6. Stainless steel
7. Cobalt-chromium alloy

45. Ultimate Tensile Strength
in increasing order of strength
1. Cancellous bone
2. Polyethylene
3. PMMA (bone cement)
4. Cortical bone
5. Stainless steel
6. Titanium alloy
7. Cobalt-chromium alloy

46. THANK YOU