Metallic & Nonmetallic Implants

In Orthopaedics

Dr Debasis Mukherjee

Dept of Orthopaedics

IPGMER

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.

WHY TO KNOW ???

To make an informed selection of the Surgical implant

BIOMATERIALS

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

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

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.

ISSUES

• Biocompatibility

• Strength parameters(tensile,comressive and torsional strength;stiffness,fatigueresistance,contourability)

• Resistance to degradation and erosion

• Ease of integration when appropriate

• Minimal adverse effect on immaging

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.

Orthopedic Terms

• Fatigue

• Endurance limit

• Cyclic Failure- High and Low

• Elongation

• Corrosion and Passivation

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!

Basic BiomechanicsForce, Displacement & Stiffness

Force

Displacement

Slope = Stiffness =

Force/Displacement

Basic Biomechanics

Stress = Force/Area Strain Change Height (L) / Original Height(L0)

Force

AreaL

Basic BiomechanicsStress-Strain & Elastic Modulus

Stress = Force/Area

Strain = Change in Length/Original Length (L/ L0)

Slope = Elastic Modulus = Stress/Strain

Basic BiomechanicsCommon 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

Basic Biomechanics

• Elastic Deformation

• Plastic Deformation

• Energy

Energy Absorbed

Force

Displacement

PlasticElastic

Basic Biomechanics

• Stiffness-Flexibility

• Yield Point

• Failure Point

• Brittle-Ductile

• Toughness-Weakness

Force

Displacement

PlasticElastic

Failure

Yield

Stiffness

StiffDuctile ToughStrongStiff

BrittleStrong

DuctileWeak

BrittleWeak

Strain

Stress

FlexibleDuctile ToughStrong

FlexibleBrittleStrong

FlexibleDuctileWeak

FlexibleBrittleWeak

Strain

Stress

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.

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

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

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

Metals Used in Implants

• Three main categories of metals for orthopedic implants

– stainless steels

– cobalt-chromium alloys

– titanium alloys

– Metallic Foam

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

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

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

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

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

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.

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

Why Magnesium?

• Bioresorbable

• Biocompatible

– Osteoconductive

– Osteoinductive

• Properties of bone can be easily attained using varying processing techniques

Tantalum

• A newer material, tantalum, is a trabecular metal composedof a carbon substrate with elemental tantalum depositedon the surface.

Forms a biological scaffold for new bone formation.• Modulus of elasticity closer to that of bone than stainlesssteel or the cobalt-based alloys.

• Not been used in the manufacture of implants untilmore 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 ingrowthsetting, but long-term studies are needed to confirm itsusefulness.

ComparisonsMaterial 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

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

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

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

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

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

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

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

Biodegradable materials

• These are Poly-Glycolic Acid (PGA), PDS, polylevolacticacid (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

Biodegradable materials

• Uses-

Reattachment of ligaments,tendons,meniscal tears and other soft tissue structures.

Stabilization of fractures,osteotomies,bone grafts and fusions.

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)

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

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

THANK YOU