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Alloys in Orthopaedics
03/05/23
Chairperson – Dr. Kiran S PatilPresenter – Dr. Srinath M Gupta
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11OutlineOutline
IntroductionIntroduction
Common Orthopaedic Common Orthopaedic implant materials implant materials and Clinical and Clinical applicationsapplicationsGeneral Tissue – General Tissue – Implant Implant responsesresponses
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Basic concepts / Basic concepts / DefinitionsDefinitions
Recent advancesRecent advances
ConclusionConclusion
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Study of Biomaterials
o Implants are biomaterial deviceso 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 Brittleness03/05/23
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 steel4.Titanium 5.Cortical bone6.Matrix polymers7.PMMA8.Polyethylene9.Cancellous bone10.Tendon / ligament11.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 failureROUGHNESS: Measurement of a surface finish of a material
HOOKE’S LAW HOOKE’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 LAW WOLF’S LAW → Bone remodelling occurs along the line of stress
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Bone Type Load Type
Elastic Modulus
(×10 E9 N/m2)
Ultimate Stress
(× 10 E6 N/m2)Cortical Tension 11.4-19.1 107-146
Compression 15.1-19.7 156-212Shear 73-82
Cancellous Tension ~0.2-5.0 ~3-20Compression 0.1-3 1.5-50Shear 6.6 +/- 1.7
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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 Economical03/05/23
IDEAL IMPLANT MATERIALIDEAL IMPLANT MATERIAL
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CLINICAL APPLICATIONS OF ORTHOPAEDIC IMPLANTS
o Osteosynthesis
o Joint replacements
o Nonconventional modular tumor implants
o Spine implants
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Metal AlloysMetal Alloys:- stainless steel- Titanium alloys- Cobalt chrome alloys
NonmetalsNonmetals:- Ceramics & Bioactive glasses (Alumina/Zirconium)- Polymers (Bone cement, polyethylene)
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CLINICAL APPLICATIONS OF ORTHOPAEDIC IMPLANTS
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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. Strong2. Relatively ductile3. Biocompatible4. Relatively cheap5. 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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Titanium alloy
ContainsContains:- Titanium (89%)- Aluminium (6%)- Vanadium (4%)- Others (1%)
Most commonly orthopaedic titanium alloy is TITANIUM 64 (Ti-6Al-4v)
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AdvantagesAdvantages:1. Corrosion resistant2. Excellent biocompatibility3. Ductile4. Fatigue resistant5 Low Young’s modulus6. MR scan compatible
• Useful in halos, plates, IM nails etc.
Disadvantages:Disadvantages:1. Notch sensitivity2. poor wear characteristics3. Systemic toxicity – vanadium4. Relatively expensive
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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 corrosion2. Excellent long-term biocompatibility3. Strength (very strong)
DisadvantagesDisadvantages:1. Very high Young’s
modulus-Risk of stress shielding
2. Expensive3. Nickel sensitivity.
o Used in making arthroplasty implants.
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COMPARISON OF METAL ALLOYSCOMPARISON OF METAL ALLOYS
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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 & insoluble2. Best biocompatibility3. Very strong4. Osteoconductive
Disadvantages:Disadvantages:1. Brittleness2. Very difficult to process – high melting point3. 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 formationo Osseoconductiveo Biocompatibleo Made from elements present in the human bodyo Used to make variable Spinal implantso 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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PMMA (BONE CEMENT)PMMA (BONE CEMENT)
The powder contains:- PMMA copolymer- Barium or Zirconium oxide (radio-opacifier)- Benzoyl peroxide (catalyst)
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The curing process is divided into 4 stages :1. Mixing2. Sticking / waiting time (2-3mins)3. Working Time (5-8mins)4. Setting time / Hardening (8-10mins)
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USESused for fixation and load distribution in conjunction with orthopeadic implantsFunctions by interlocking with bone•May be used to fill tumor defects and minimize local recurrence
Advantages1)-Reaches ultimate strength at 24 hours2)-Strongest in compression3)-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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UHMWPEUHMWPE
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Disadvantages1)-Susceptible to abrasion2)-Wear usually caused by third body inclusions3)-Thermoplastic (may be altered by extreme temperatures)weaker than bone in tension
Advantages1)-Tough2)-Ductile3)-Resilient4)-Resistant to wear
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THR IMPLANT BEARING SURFACESTHR IMPLANT BEARING SURFACES
• Metal-on-polyethylene
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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 results03/05/23 41
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RECENT ADVANCESRECENT ADVANCESo 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
o Antibacterial implant03/05/23 43
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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 polymers Shape-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 conventional porous coatings •Double the interface shear strength
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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 patient03/05/23
GENERAL TISSUE-IMPLANT GENERAL 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… 03/05/23
ConclusionConclusion
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