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Rotary Ni-Ti Files

Physical & Mechanical Properties

Metallurgy of nickeltitanium alloys

Ni-Ti was developed 40 years ago by Buehler et al in the Naval OrdnanceLaboratory (NOL) in Silver Springs, Maryland.

The symbols of the metals were combined with the place of invention, creatingthe acronym NiTiNOL

Using about 55 wt% Ni and 45 wt% Ti and substituting some Ni with less than 2wt% Co, nearly the same number of Ni and Ti atoms are combined, being

reflected in the term equiatomic.

Another type is called 60 NiTiNOL and contains about 5% more nickel. It is used for some hand files but because of different properties (i.e. lower

shape memory effect and increased heat treatability, together with increa sing

hardness) it seems to be less useful than the 55 NiTiNOL.

Phase Transformation of Ni-Ti Alloy

Austenite: (BCC)

High Temperature

Low stress

Martensite: (Mono-Triclinic)

Low Temperature

High stress

R-phase:

Intermediate phase

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This lattice organization can be altered either by temperature or stress. (ShapeMemory Phenomena)

This phenomenon may translate to the ability to remove any deformation withinnickel titanium instruments by heating them above 125C.

This ability of resisting stress without permanent deformation going back to theinitial lattice form is called super elasticity.

The super elasticity is most pronounced at the beginning, when first deformationsof as much as 8% strain can be totally overcome (Spring-Back Phenomena).

After 100 deformations, the tolerance is about 6% and after 100,000deformations, it is about 4%.

Properties of Ni-Ti Files

Ni-Ti alloys overall are softer than stainless steel. Not heat treatable. Low modulus of elasticity (about one fourth to one fifth that of stainless steel). More resilient. Shape memory and super elasticity.

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The Endurance Limit:

It is the level of stress or strain at which a material can be subjected to without failure.At this limit, continued cycling of the material is unrestricted, and would not lead to

failure.

Clinically, the endurance limit, in torsion, is affected by the angle and radius of canal

curvature.

An increased angle or decreased radiussignificantly decreases the number of rotations/

cycles the instrument could withstand to fracture.

Manufacturing Process

Manufacture of files from Ni-Ti alloy promotes work hardening.

The problem is:

1. Microstructural investigations based on X-ray diffraction, scanning electronmicroscopy and microhardness tests revealed a high density of defects in the

alloy that can disturb its phase transformation.

2. A variety of inclusions may become incorporated into the metal resulting inweaknesses at the grain boundaries through which cracks could propagate.

3. The machining often results in an instrument having an irregular surface

characterized by milling grooves, multiple cracks, pits and regions of metal

rollover.

All these sites may act as areas of stress concentration and crack initiation with

eventual flexural failure during clinical use.

How to overcome the above limitations:

1) Electropolishing:

It is a controlled chemical process that minimizes surface defects & consequently may

increase file longevity.

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Electropolishing is hypothesized to improve torsional strength and cyclic fatigue

resistance through levelling the grain boundaries and removal of surface pitting and

machine grooves that can act as centres of fracture propagation.

Herold et al showed that electropolishing did not inhibit the formation of

microfractures.

Cheung et al found that electropolishing did not protect the instrument from low cyclic

fatigue failure.

Bui et alfound that electropolishing did not affect the torsional resistance because it

does not affect the core of the material in which most of the mechanical properties

reside.

Overall,

Electropolished instruments performed significantly better than nonelectropolished

instruments in cyclic fatigue testing and, to a lesser extent, in static torsional loading.

Electropolishing may have beneficial effects in prolonging the fatigue life of rotary Ni-Ti

instruments.

2) Cryogenic treatment:

It involves submersing metal in a super-cooled bath containing liquid nitrogen (-196

degree C/-320 degree F) and then allowing the metal to slowly warm to room

temperature.

This treatment creates a surface layer of titanium nitride which increases cutting

efficiency in that the instrument becomes harder on the surface and thus more effective

in its shaping ability but this increase was not detected clinically.

3) Ion implantation:

This physical method uses an ion accelerator at low energy (100 to 400 keV) to

introduce a known quantity of nitrogen atoms in the Ni-Ti target. Immediate application

is possible on manufactured instruments ready for commercial use. There is an increase

in the resistance to corrosion as a result of implantation.

4)Recent modifications introduced by several manufacturers:

M-Wire (Dentsply Tulsa-Dental Specialties, Tulsa, OK) was developed, and themanufacturer states that a new thermal process is used to produce an alloy that

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provides instruments with greater flexibility and increased resistance to cyclic

fatigue compared to files constructed from the traditional nickel titanium alloy.

A completely different manufacturing process has been developed bySybronEndo (Orange, CA) to create a Twisted File (TF).

It uses twisting of a ground blank in combination with heat treatment to enhance

superelasticity and increase cyclic fatigue resistance.

Liberator files are manufactured using electro-chemical grinding (ECG)techniques. This process employs a liquid chemical that decreases the thermal

impact of the grinding process.

Manufacturers are trying to improve the clinical performance of rotary Ni-Ti files

through developing new design features and new manufacture processes regarding their

physical and mechanical properties.

Design features:

1) Taper:

We basically have two options when instrumenting a root canal.

Firstly,

Same taper but with varying apical tip diameters.

Hand files that all have a consistent taper (.02) but with various tip diameters. A rotary file of constant taper would be the .04 taper (Profile, Liberator) that has

a constant taper (.04) but has varying apical tip diameters.

Secondly,

Varying or graduating tapers.These files have the same apical tip size but their taper varies from .04 to .12.

GT series of files employs a varying taper. Quantec files use a graduated increase in taper. ProTaper that features a progressive taper along its shank.

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The tendency of rotary instruments to lock and/or thread (screw) into the canal is

enhanced when instruments with the same taper, but varying tip diameters, are wedged

into the canal, possibly leading to torsional overloading of the instrument tip.

Analysis of torque & force with different taper rotary instruments showed that

instruments with different tapers offered the advantages of reducing the canal contact

areas and decreasing both torsional and fatigue failure compared to instruments with a

single taper.

The idea is that each successive file is only engaging a minimal aspect of the canal wall.

Therefore, frictional resistance is reduced and requires less torque to properly run the

file.

2) Cross section:

Studies showed that as the instruments cross sectional diameter increases, it becomesless resistant to cyclic fatigue, generate more torque during rotation hence time to

fracture is decreased.

Modified cross sections that offer increased cutting efficiency in addition to reduced

contact areas (no or relieved radial lands) with canal walls reduce torsional loads as well

as the magnitude of internal stresses generated inside the instrument.

Cross sections of rotary systems differ from:

The traditional triangular cs. (RaCe)

Convex triangular cs. (ProTaper) Irregular cs. (K3)

The equilateral triangular cross-section is expected to show a lower torsional resistance

than the convex triangular design because of the high stress concentration at the middle

of each side, the distance between this point and the centroid of the cross-section being

the shortest.

In other words, the resistance of a rotary file to torsional failure is enhanced byincreasing the inner-core diameter of the cross-section.

3) Radial lands:

Radial land is a surface that projects axially from the central axis, between flutes, as far

as the cutting edge.

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It is the combination of a non-cutting tip and radial land that keeps a file centered in the

canal.

Previously, rotary files either had:

Full Radial Lands (Profile, GT) Recessed radial Lands (Quantec)

Concept:

The strength of the file comes from the inner core of the instrument, rather than and

the peripheral area near the cutting blade. This part of the instrument is also called the

radial land.The less blade support (the amount of metal behind the cutting edge) the less resistant

the instrument is to torsional stress.

ProFile

QUANTEC

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K3

3) Cutting Tips:

Aggressive vs. Non-Aggressive Tips

Cutting tips on rotary files make them too aggressive.

Yes, a cutting tip has the ability to enter narrow, somewhat calcified canals.But there are two serious concerns with a cutting tip,1

st, if you accidentally go long past the end of the tooth,

Going long with a non-cutting tip will create a concentric circle at the end of the root.These are easily filled with a non-standardized cone. However, if you go long with a

cutting tip, upon retraction of the file, you generally will create an elliptical tear.This is very difficult to repair and obturate.

2nd, If you place a cutting tip on a non-landed file, you have the distinct possibility of

transportation.This is especially true if you hold the file at length for any period of time. Theoretically, a

cutting tip will not transport if it goes to length and is immediately retracted.

But, how many dentists hold a rotary file at length for less than a second?Some files claim to have modified cutting tips or partially active tips. Fine, but this is

like being a little bit pregnant. Either it is a cutting tip or it is not.

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4) Flute depth:

Instruments with deep cutting flutes & progressively larger variable tapers exhibit

changing cross-sectional diameters along the entire length

These instruments develop high torque levels that make them more prone to metalfatigue and fracture.On the other hand, instruments that have shallow cutting flutes, evenly tapered shafts

and consistently shaped cross-sectional areas are more resistant to torsional overload.

This is because the torsional and bending stresses that develop during use are

distributed uniformly along these instruments entire length.

5) Rake Angle:

The rake angle of a rotary cutting flute is defined as the angle subtended by two

intersecting lines.

The first is a line drawn between the cutting tip and the geometric center of the

instrument.The second is a line tangential to the curve of the cutting face at its tip.

Further studies are needed to evaluate the clinical benefits of a positive versus a

negative rake angle instrument.

Instruments with negative rake angles allow for planning rather than cutting of the

dentinal walls. Thus more pressure is required when enlarging the canal in this manner,

i.e. predisposition to torsional overload as well as to cyclic fatigue (Pro-Taper).

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On the other hand, instruments with positive rake angles, which are marketed to

provide better cutting efficiency, require less energy when enlarging the canal.

However, excessive pressure can cause excessive torsion by forming chips that are too

large to be dislodged.

The K3 is the only third generation file to feature a slightly positive rake angle.

This results in optimum cutting efficiency.

6) Helical Angle:

It is the angle that the cutting edge makes with the long axis of the file.

The first rotary file to take advantage of this factor was the GT.

Constant Helical Angle:

Allow debris to accumulate, particularly in the coronal part of the file. More susceptible to the effect of screwing in forces.

Variable Helical Angle:

By varying the flute angles, debris will be removed in a more efficient manner and the

file will be less likely to screw into the canal.

Varying the helical angle along

the length of the instrument

affects its tendency to screw-in.

If the angle is more closed at the

tip and more open at the junction

between the shank and the

working part, this decreases the

tendency of instrument to screw-

in which is considered a risk

factor that predispose to

torsional failure.

In K3, the helical angle increases from the tip to the handle. This allows for superior

debris removal.

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RaCe file, is unique and utilizes an alternatinghelical design by using spiraled and non

spiraled portions along the working length. This reduces the tendency of the file to get

suckedinto the canal.

Flexmaster Hero

ProTaper RaCe

Non Helical Design:

A unique manufacturing process for Liberator files eliminates the perpendicular micro-

cracks resulting from the manufacturer of helical files.

The absence of the traditional helical flutes in the Liberator files together is claimed

by the manufacturer to reduce dentin contact areas with its subsequent improvement

of torsional resistance.

Liberator

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7) Pitch:

Pitch is the number of spirals or threads per unit length.

Screws historically have had a constant pitch. The result of a constant pitch and constant

helical angle is a pulling down or sucking down into the canal.Diemer and Calas found that increasing the pitch decreases the torsional stresses, and

that short pitched instruments are twice as likely to screw-in as long pitched

instruments which represent a clinical disadvantage.

Long pitch reduces the helical angle which reduces the tendency of a file to screw-in.

The K3 file has a varied pitch that allows debris to effectively

channel coronally. This is important not only during initial

cleaning and shaping but also for retreatment

However,

Instruments with a bigger pitch will have less number of threads.When torque is applied to the shank of the file, the stresses would be dissipated via the

cutting edges (threads).Hence, a decrease in the pitch (more threads) could reduce the resolved shear stress at

the clamped position.

Instruments with a longer pitch and lower helical angle might have resulted in a greaterresolved stress and hence a lower torsional resistance.

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Outcome:

The manufacturing of a superior file is limited to the physical and the mechanicalproperties of the Ni-Ti alloy.

The geometrical designs of the nickel-titanium rotary systems have a directinfluence on their clinical performance.

References:

1) Thompson SA.. International Endodontic Journal 20002) Uhn G, Tavernier B, Jordan L. J Endod 2001;27:516-20.3) Valois, J Endod 2005; 31:8825.4) Herold et al, J Endod 2007; 33:712 4.5) Cheung et al, J Endod 2007; 33: 121712216) Bui et al, J Endod 2008; 34: 190 - 3.7) GT Seriex X Brochure. Tulsa, OK: Dentsply Tulsa Dental Specialties; 20088) TF: The Twisted File Brochure. Orange, CA: SybronEndo; 2008.9) Ullmann CJ, Peters OA. J Endod 2005;31:183 6.10)Diemer F,,Calas P. J Endod 2004;30:716-18.