STAINLESS STEEL AND TITANIUM ALLOYS USED AS BONE FRACTURE

FIXATION

Kaushlendra Sahu, Naman Modi, Tarun Kumar Parashar

Department of Metallurgy National Institute of Technology, Raipur (C.G)

Contents Bio-Materials used as bone fracture fixation

Stainless Steel Titanium Alloys Other Ceramics, Composites and Polymers.

Compositions and properties of these materials

Advantages and Disadvantages of above Materials

Problems in using these materials and their solutions

Titanium alloy v/s Stainless Steel

Important properties for materials

Biocompatibility Density Comparable to bone High mechanical strength Fatigue resistance Low elastic modulus Good wear resistance Stiffness

Chen, Q., Zhu, C., & Thouas, G. A. (2012). Progress and challenges in biomaterials used for bone tissue engineering, Progress in Biomaterials

Different internal fixation by using bio materials.

http://www.mplsortho.com/conditions-treatments/ecategory/47/etopic/4e9a2e8d8583ef20c2f7a72ebfa65420/

Basic introduction Stainless Steel

Stainless Steel is a alloy and its main constituents are Chromium and Nickle Chromium produces a thin layer of oxide on the surface of the steel Stainless Steels are more preferred biomaterials for bone plates because of

its mechanical properties It is cost effective as compared to other metals

Titanium Alloy Titanium alloy mainly consist of aluminum and vanadium ex Ti-6Al-4V. These materials are biological inert It is highly corrosive resistant and have high modules of elasticity Titanium alloy form oxide layer which reduces healing time

Why we use Stainless Steel ?

Stainless steel

Stainless Steel is are made from Nickle, chromium and molybdenum, combined into endless combination for characteristics that are desired for Medical applications.

Basically there are two type of stainless steel used 316 361L

For medical purpose we generally use 316L stainless steel.

Chromium is used for corrosion resistant. Higher the percentage of chromium higher the corrosion resistant.

Copper and nickel discolored bone in which they were embedded. This is the major disadvantage.Because of these stainless steel is used for temporary implantments.

We use 316l stainless for medical purpose Chemical Composition

Chemical Percentage(%) Carbon 0.0 – 0.03 Manganese 0.0 – 2.0 Silicon 0.0 – 0.75 Phosphorous 0.0 – 0.045 Sulphur 0.0 – 0.03 Chromium 16.0 – 18.0 Molybdenum 2.0 – 3.0 Nickle 10.0 – 14.0

Physical properties of stainless steel

Stainless Steel 316L

Density 8000Kg/ Young’s modulus 193GPa Poisson’s ratio 0.30 Thermal expansion 15.9* Tensile strength 485MPa Compressive strength 570MPa

Titanium and its Alloy

Pure titanium (Ti CP) and extra load interstitial Ti-6Al-4V are the two most common titanium based implant biomaterials.

Titanium alloy is mainly made up of 6% aluminum and 4% Vanadium and rest titanium.

These are corrosion resistance and bio inert and less ductility as compared to stainless steel.

They remain essentially unchanged when implanted into human bodies.

Why we use aluminum and vanadium? The addition of alloying elements to titanium enables it to have a

wide range of properties because Aluminum tends to stabilize the α phase and vanadium tend to stabilize β phase, lowering the temperature of transformation from α to β phase.

The α phase promotes good weldability, excellent strength and oxidation resistance.

The addition of controlled amount of vanadium as β stabilizer causes the high strength of β phase to persist below the transformation temperature which results into two phase system.

But also Ti-6Al-4V have some major disadvantages

The elasticity of Ti-6Al-4V has 4 to 6 time less than that of cortical bone and low wear resistance .

Also vanadium can cause potential cytotoxicity and adverse tissue reaction

Aluminum ions from the alloy might cause long term Alzheimer disease.

Mechanical properties

Ti-6Al-4VDensity 4430 kg/Young’s modulus 110 GpaPoisson’s ration 0.342 Thermal Expansion 8*Tensile strength 993 MpaCompressive Strength 1086 Mpa

Phases of Titanium alloy

There are 3 phases of titanium alloy that are used for bone fracture fixation Alpha alloys alpha-beta alloys Beta alloys

Alpha alloy Non heat treatable. Medium strength and good notch toughness Good ductility Pure titanium

Alpha-Beta alloy Heat treatable to some extent medium strength Favorable for Thermal treatment and thermo-processing Ti-6Al-4V

Beta alloy Fully heat treatable High strength Excellent formability Ti-3Al-8V-6Cr-4Zr-4Mo

Problem faced using Titanium alloy

Bulk titanium alloys used in implants present three main problems:

High cost because the amount of processing energy and melting and casting difficulties

Higher elastic modulus compared to bone Although the inert behavior of Ti is a good property, its

bone attachment is difficult because it do not react with the human tissues

Now we are going to discuss the solution of above problems in next page

The processing

A great problem of these new alloys is its fabrication processes because most beta titanium alloys contain considerable amounts of refractory elements with high melting temperatures. This results in heavily weight, difficult melting and solidification processing, low plastic deformability and high materials costs.

Ti is very reactive with oxygen and other atmospheric gases Powder metallurgy (P/M) is an alternative method of fabrication in

which metal powders are utilized by compacting and sintering to form useful products

Production of titanium takes 16 times more energy than the production of steel but after using power metallurgy techniques , the energy reduces upto 50%.

Elastic modulus

the elastic moduli and strength of titanium and its alloys are much higher than those of human bones, which may result in stress shielding and the failure of implants.

B type Ti Alloy are used to reduce the modulus of the implants to the level approaching human bones.

the mechanical properties of porous titanium can be adjusted by pore fraction and morphology, and the stress shielding effect will be reduced

Porous titanium with porosity in a wide range can be prepared with powder metallurgy methods

The inert behavior Titanium has an inert behavior, body tries to encapsulate the titanium

based implants. Titanium does not bound directly to bone resulting in micro moments

and eventually losing of implant. Undesirable moments at the implant tissues interface results in failure cracks of the implant

One method is used to improved implant life time and activeness is to coat the metal with a bio active material.

The bio active material that can promote the formation of adhesion of hydroxyapatite[Ca10(PO4)6(OH)2]. Hardness 5Mohs density 3.1g/cm3 elastic modulus 100GPa Ultimate tensile strength 100MPa.

While using hydroxyapatite for coating of Ti alloy there are 2 problem which arises.

The thermal expansion coefficients of the ceramic and metal are usually different, and as a result, large thermal stresses are generated during processing. These stresses lead to cracks at the interface and compromise coating adhesion.

In addition, chemical reactions between the ceramic and metal can weaken the metal in the vicinity of the interface, reducing the strength of the coated system.

This problem is particularly important when coating Ti alloys, due to their high reactivity with most oxide materials. However, bioactive ceramics coatings on Ti implants further improves the biocompatibility of these implants.

Titanium alloy compatibility.

A biocompatible titanium base alloy suitable for bone implant should meet at least the following requirements Potentially toxic elements, such as vanadium, cooper and tin,

should be avoided completely Elements that may have potential toxicological problems, such as

chromium, nickel and aluminum, should be used only in minimum, acceptable amounts

The alloy should have high corrosion resistance The alloy should have, at least, the following desirable mechanical

properties: low modulus, high strength and good smooth and notched fatigue strength

The alloy should have good workability and ductility.

New generation of titanium alloys

Replacing the aluminum and vanadium with other non toxic components Nb, Fe and Mo are used as a alternative for V. Ta, Hf and Zr are used as a alternative for Al.

By changing the size and shape of the stem to reduce the differences in structural stiffness of the implant and the surrounding bone.

By changing the implant materials from steel to commercially pure titanium or titanium alloy with low modulus.

Metastable Beta Titanium Alloys are used for this purpose having low elastic modulus.

Ti-13Nb-13Zr and Ti-6Al-7Nb are used.

Experiment, observation and Conclusion In a total 34 patients the T-test revealed a significant difference in the

average time taken for adaptation and plating of the two system of plates.

The average time taken for the stainless steel plate was 6.82 minutes and for that of titanium was 3.64 minutes .

The test for comparison of infection rate showed that the 20% of the patient treated with the stainless steel plate and screw head local infection while the success rate of titanium plate was 100%.

In this study of short duration, titanium plates were found to be very idle in the management of fracture. Titanium plates were more biocompatible when compared to stainless steel plate as evidenced by the rate of infection.

Reference

Gross, S. & Abel, E. (2001). A finite element analysis of hollow stemmed hip prostheses as a means of reducing stress shielding of the femur. Journal of Biomechanics, Vol.34, No.8, (2001), pp. 995-1003, ISSN 00219290

Guilemot, F.; Prima, F.; Latta, L. ; Bareille, R. ; Gordin, D. ;Gloriant, T. ; Porté-Durrieu, M. ; Ansel, D. & Baquey, Ch. (2004). Design of new titanium alloys for orthopaedic applications. Medical and Biological Engineering and Computing, Vol.42, No.1, (January 2004), pp. 137-141, ISSN 0140011

Van Noort, R. (1987). Titanium: the implant material for today, Journal of Material Science, Vol.22, (1987), pp. 3801-3811