IMPLANT MATERIALS IN ORTHOPAEDICS

BY TELLA A.ONOHD, KANO

18TH JULY, 2013

OUTLINE

• INTRODUCTION• BASIC CONCEPTS/DEFINITIONS• COMMON ORTHOPAEDIC IMPLANT

MATERIALS & CLINICAL APPLICATIONS• GENERAL TISSUE-IMPLANT RESPONSES• COMPLICATIONS ASSOCIATED WITH IMPLANTS• RECENT ADVANCES• CONCLUSION

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

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

STRESS-STRAIN CURVE

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

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

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

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

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

CLINICAL APPLICATIONS OF ORTHOPAEDIC IMPLANTS

• Osteosynthesis

• Joint replacements

• Nonconventional modular tumor implants

• Spine implants

COMMON IMPLANT MATERIALS IN ORTHOPAEDICS

• Metal Alloys:- stainless steel- Titanium alloys- Cobalt chrome alloys

• Nonmetals:- Ceramics & Bioactive glasses- Polymers (Bone cement, polyethylene)

STAINLESS STEEL

• Contains:- Iron (62.97%)- Chromium (18%)- Nickel (16%)- Molybdenum (3%)- Carbon (0.03%)

• The form used commonly is 316L (3% molybd, 16% nickel & L = Low carbon content)

STAINLESS STEEL

• Advantages:1. Strong2. Relatively ductile3. Biocompatible4. Relatively cheap5. Reasonable coorsion resistance

• Used in plates, screws, IM nails, ext fixators

• Disadvantages:- Susceptibility to crevice and stress corrosion

TITANIUM ALLOYS

• Contains:- Titanium (89%)- Aluminium (6%)- Vanadium (4%)- Others (1%)

• Most commonly orthopaedic titanium alloy is TITANIUM 64 (Ti-6Al-4v)

TITANIUM ALLOYS

• Advantages:1. Corrosion resistant2. Excellent biocompatibility3. Ductile4. Fatigue resistant5 Low Young’s modulus6. MR scan compatible

• Useful in halos, plates, IM nails etc.

• Disadvantages:1. Notch sensitivity2. poor wear characteristics3. Systemic toxicity – vanadium4. Relatively expensive

COBALT CHROME ALLOYS

• Contains primarily cobalt (30-60%)

• Chromium (20-30%) added to improve corrosion resistance

• Minor amounts of carbon, nickel and molybdenum added

COBALT CHROME ALLOYS

• Advantages:1. Excellent resistance to corrosion2. Excellent long-term biocompatibility3. Strength (very strong)

• Disadvantages: 1. Very high Young’s modulus - Risk of stress shielding 2. Expensive

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

COMPARISON OF METAL ALLOYSALLOY Young’s modulus

(GPa)Yield strength (MPa)

Ultimate tensile strength (MPa)

Stainless Steel 316L 190 500 750

Titanium 64 110 800 900

Cobalt chrome F562

230 1000 1200

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)

CERAMICS

• Advantages:1. Chemically inert & insoluble2. Best biocompatibility3. Very strong4. Osteoconductive

• Disadvantages:1. Brittleness2. Very difficult to process – high melting point3. Very expensive

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

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)

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)

PMMA

• The powder contains:- PMMA copolymer- Barium or Zirconium oxide (radio-opacifier)- Benzoyl peroxide (catalyst)

• Clinically relevant stages of cement reaction:1. Sandy stage2. Mixture appears stringy3. Cement is doughy4. Cement is hard

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

THR IMPLANT BEARING SURFACES

• Metal-on-polyethylene • Metal-on-metal

BEARING SURFACES

• Ceramic-on-polyethylene

• Ceramic-on-ceramic

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

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

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

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

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

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

COMPLICATIONS

• Infection:- colonization of implant by bacteria and subsequent systemic inflammatory response

• Metal hypersensitivity• Manufacturing errors• VARIOUS FACTORS CONTRIBUTE TO IMPLANT

FAILURE

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

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…

THANK YOU

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.