METALS AND METALLURGY BASICS OF ORTHOPAEDICS IMPLANT BIOMECHANICS

1. Metals in Orthopaedics
DR. BAIJNATH AGRAHARI
DEPARTMENT OF ORTHOPAEDICS
LMC,PALPA

2. Contents
 History
 Introduction
 Basic contents and definition
 Ideal Metal for Implant
 Properties of implant material
 Commonly Used Metals in Orthopaedic Implants
 Problems Encountered in Orthopaedic Implant

3. • Metals have wide application in Orthopaedics, as
structural and load bearing devices for fracture
fixation and implants for joint replacement.

4. TIMELINE
 Bone pegs -1500
 Brass wire -1775 (wire suture)
 Ivory rod -1890
 Steel plate (Lane) -1905 (Vanadium steel)
 Silver rod -1913
 Steel alloys -1926 (18-8 type SSMo)
 Vitalium (Stellite) -1929 (CC)
 Titanium - 1950s
 Ceramics -1970
 Biodegradable -1980

5. Introduction
A surgical implant may be defined as an object made from a non-
living material that is inserted into a human body, where it is intended
to remain for a significant period of time in order to perform a
specific function.
 The implants for fracture fixation are commonly made of stainless
steel and titanium alloys.
 ALLOYS are materials composed of 2 or more elements, one of which
is a metal.

6. Basic concept and definition
• LOAD: is a force that acts on body.
• STRESS: it is defined as the internal resistance to deformation or the
internal force generated within the substances as a result of application
of external load.
Stress = load/area on which load acts
• 3 types of stress–
1.compressive stress
2.Tensile stress acts perpendicular to a given plane
3.shear stress – acts in the direction parallel to given plane

7. • STRAIN: it is defined as the change in linear dimensions of the body
resulting from the application of a force or a load. (deforming force)
 Tensile strain : is increase in length of a straight edge or a line
drawn on a body.
 Compression strain : is decrease in length of straight edge or a line
drawn on a body.
 Shear strain : is by a change in angular relationship of two lines
drawn on the surface

9. Young’s Modulus of Elasticity
• Its a measure to express the stiffness(ability to resist deformation) or
rigidity under normal stress.
• Its calculated by dividing the (stress) by amount of deflection (strain).
• A high modulus of elasticity indicates that the material is stiff.
• Bone has a lower modulus of elasticity than the metal .

10. 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

12. THE YIELD POINT : or limit of proportionality denotes the end of the
elastic region of the curve.
• It’s a point on the curve at which a marked increase in strain occurs
without significant increase in stress or load
OR
• it’s the stress beyond the elastic limit that results in permanent
bending or deformation

13. ULTIMATE TENSILE STRENGTH(U.T.S)
• The maximum amount of stress the material can with stand
before which fracture is imminent.
• The U.T.S is linearly correlated to the hardness of the metal.
BRITTLENESS:
• A material is brittle if, when subjected to stress, it breaks without
significant plastic deformation.
• Brittle materials absorb relatively little energy prior to fracture,
even those of high strength.
• Breaking is often accompanied by a snapping sound.

14. • ELASTICITY – ability of a material to recover its original shape after
deformation.
• PLASTICITY- ability of a material to be formed to a new shape without
fracture and retain the shape after load removal.

15. DUCTILITY
The ductility of an implant material characterizes its ability to be
deformed under tensile stress and to be stretched into wire without
fracture.
Determines the degree to which the plate, for instance, can be
countered.
Materials of high strength such as titanium alloys or pure titanium
offer less ductility than steel.

16.  STRENGTH : 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.

17.  TOUGHNESS: Amount of energy per unit volume that a material can
absorb before failure , resistance to fracturing.
 ROUGHNESS: Measurement of a surface finish of a material
 HOOKE’S LAW → when a material is loaded in the elastic zone, the
stress is proportional to the strain
Stress α Strain

18. • 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

19. IDEAL METAL FOR IMPLANT
 BIOCOMPATIBLE– NON-TOXIC, NON-CARCINOGENIC, NON-
IMMUNOGENIC
 BIOINERT– NOT ELICIT A RESPONSE
 STRENGTH– COMPRESSIVE, TENSILE, TORSIONAL
 FATIGUE RESISTANCE, CONTOURABILITY
 CORROSION AND DEGRADATION RESISTANCE
 IMAGING COMPATIBLE– MRI, CT SCAN
 ECONOMICAL

20. MAJOR METALS USED
1. Iron based alloys (stainless steel)
2. Cobalt based alloys
3. Titanium based alloys
NEWER METALS
1. Oxinium
2. Trabecular metal
3. Nitinol-nickel titanium alloys

21. STAINLESS STEEL
• PLATES,SCREWS,PINS AND RODS CONTAINS:
- Iron(62.97%)
- Chromium (18%)
- Nickel (16%)
- Molybdenum (3%)
- Nitrogen (0.1%)
-Carbon (0.03%)
• COMMONLY USED TYPES OF STAINLESS STEEL ARE
AISI 316 L, AISI 440 B.

22. STAINLESS STEEL
• Advantages:
1. Relatively ductile
2. Biocompatible
3. Strong
4. Relatively cheap
5. Reasonable corrosion resistance
• Disadvantages :
• -Susceptibility to stress corrosion
• Used in plates, screws, IM nails, external fixators
• 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.

24. TITANIUM ALLOY
• Contains:
- Titanium (89%)
- Aluminium (6%)
- Vanadium (4%)
- Others (1%)
• Most commonly orthopaedic titanium alloy is TITANIUM64 (Ti-6Al-4v)

25. TITANIUM ALLOY
Advantages:
1. Corrosion resistant
2. Excellent biocompatibility
3. Ductile
4. Fatigue resistant
5 Low Young’smodulus
6. MRI scan compatible
Useful in halos, plates, IM
nails etc.
Disadvantages:
1. Notch sensitivity
2. poor wear characteristics
3. Systemic toxicity – vanadium
4. Relatively expensive

26. COBALT-BASED ALLOY
• MAINLY HIP AND KNEE PROSTHESES
• Contains primarily cobalt (30-60%)
• Primarily alloy of cobalt with chromium.
• Chromium (20-30%)
• The chromium forms a strongly adherent oxide film that provides a
passive layer shielding the bulk material from the environment for
corrosion resistance
• Minor amounts of carbon, nickel and molybdenum added

27. COBALT-BASED ALLOY
• 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
3. Nickel sensitivity.
• Used in making arthroplasty implants .

30. NEWER METALS
• Oxinium : oxidized zirconium is a metallic alloy with a ceramic
surface.
• Zirconium: a biocompatible metallic element in the same family as
titanium combines the best of both metal and ceramics.
• excellent fracture toughness like cobalt chrome.
• ceramic surface that offers outstanding wear resistance.

31. TRABECULAR METAL
• Elemental tantalum metal
• Vapor deposition techniques that create a metallic strut configuration
similar to trabecular bone.
• Crystalline microtexture is conductive to direct bone apposition.
• Interconnecting pores
• 80% porous
• 2-3 times greater bone ingrowth compared to conventional porous
coatings
• Double the interface shear strength TRABECULAR METAL

32. Problems Encountered in
Metal Implants

33. INFECTIONS
• EARLY INFECTIONS : Through skin, air or surgical instrumentation
Infection doesn’t subside because revascularisation blocked by
implant
• LATE INFECTIONS : Hematogenous in origin bacteria protected by
glycocalyx present on the coating formed on the surface of the
foreign material .

34. CLINICAL MANIFESTATION OF UNTOWARD
HOST RESPONSE
• INFLAMMATION
• METALLOSIS , OSTEOLYSIS AND LOOSENING
• STERILE ABSCESS
• NEOPLASIA -SCC

35. Fatigue failure
• Fatigue failure is the formation and propagation of cracks due to a
repetitive or cyclic load.
• The failure occurs due to the cyclic nature of the load which causes
microscopic material imperfections (flaws) to grow into a macroscopic
crack (initiation phase).
• Depends upon magnitude of stress and number of cycles.
Three stages –
 crack initiation
 slow, stable crack growth
 rapid fracture.

36. Stress shielding
 Refers to reduction in bone density as a result of removal of typical
stress from the bone by an implant (for instance , femoral component
of hip prosthesis).
 The prosthetic shaft takes off a part of the stress that walking and
other everyday activities put on the upper part of the thigh bone
holding the prosthesis.
 This is because of Wolff’s law , bone in healthy person remodels in
response to the loads it is placed under.

37. Corossion
• Gradual degradation of metals by electrochemical attack ,and is
therefore a concern when placed in electrolytic environment of body.
• Effects- tissue inflammation and necrosis, weakening of implant
TYPES
1. Galvanic corrosion- due to two different metals being used e.g.
stainless steel screws and titanium plate.
2. Crevice corrosion - occurs where metals and alloys depends on oxide
film for corrosion protection.

38.  In narrow gap (crevices)between implants e.g screw head
and plate.
 Can occur in fatigue crack and in defects such as scratch or
fissure.
 Molybdenum tends to limit crevice corrosion.
3. Pit corrosion- A local form of crevice corrosion due to abrasion
produces a pit
4. Stress corrosion- a scratch or crack act as stress raiser.

39. 5. Fretting corrosion -components have a
relative movement against one another
6.Intergranular Corrosion- if impurities
aggregate between grains of relatively
pure alloy ,a localized galvanic corrosion
may exist between the crystals and the
alloy in the grain boundries.

40. THANK YOU