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april 2012

Design features of rotary root canal instruments

Evgeny Rzhanov (Department of Operative Dentistry and Endodontics, Moscow State University of Medicine and Dentistry, Moscow, Russia)

Tatiana Belyaeva (Department of Operative Dentistry and Endodontics, Moscow State University of Medicine and Dentistry, Moscow, Russia)

Key words
angles of cutting blade, cross section, design features, flute design, rotary nickel-titanium
instruments, self-feeding effect, taper
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Introduction

Design features of any root canal instrument are determined primarily by the aim and the method of its use. Design features are likewise influenced by the material’s properties and its processing methods. On the other hand, all these factors determine the instrument’s operation technique and the range of objectives that could be solved by its use.

There are a vast number of different root canal instruments and systems. From time to time, new instruments or modifications of the existing systems appear. It is not always easy for dental practitioners to be familiar with the whole variety of options.

Nevertheless, understanding the basic features of root canal instruments and their operation principles can help the clinician use these instruments more effectively, and significantly reduce the probability of mistakes.

Preparatory steps completed prior to reviewing the root canal instrument’s design features ought
to clarify some of the basic definitions and terms. Any root canal instrument represents a cutting instrument for the mechanical processing of the root canal wall.

All root canal instruments can be divided into two groups depending on how they are used:

  • hand instruments
  • mechanically driven instruments.

The basic structural elements of the instruments belonging to both groups generally coincide, but the parameters of these elements differ. Because of obvious advantages, the second group of instruments has become the most highly developed. Therefore in the present review, the engine driven nickel-titanium (NiTi) instruments will predominantly be discussed.

In the related literature, rotary instruments for root canal preparation are often termed 'endodontic rotary files'. The term 'file' denotes the cutting instrument that removes the substrate by back-andforth (linear) motions along the working surface. A typical example is the Hedström file. In the case of root canal enlargement with rotary instruments, cutting is carried out by instrument rotation. Therefore it is incorrect to apply the term 'file' to rotary instruments.

With respect to rotary root canal instruments, the term 'reamer' could be applied. It denotes the instrument with rotary cutting motion, which is destined for the enlargement and increase in accuracy of the existing holes. In other words, the term 'engine driven nickel-titanium reamer' is also correct.

It is important to note that in the present paper, a slightly different terminology from those cited in the dental literature is used to identify some components of root canal instruments, because, from the authors' point of view, the proposed terminology more precisely describes the design features of root canal instruments and their functionality.

Components of rotary root canal instruments

Fig 1
Generalised component features of rotary root canal instruments.
Rotary root canal instruments consist of two basic parts, each of which perform a specific function (Fig 1):

  • attachment section
  • working section.

The 'attachment section', or shank, is a part of the instrument meant for attaching to the handpiece, and for the torque transmission from the drive unit to the instrument's working part. In general, all rotary instruments have a shank Type 1, according to the International Organization for Standardization (ISO)1. This shank type has a flat part and a groove on its tip (Fig 1). These elements, together with a retainer located in the head of the handpiece, rigidly bind the instrument to the rotor. According to the ISO standard, this shank type must be 2.35mm in diameter and 13.5 mm in length1. Nevertheless, instrument shanks from different systems can vary in length, from 11 to 15 mm.

As a rule, the shank has identification lines (coloured cross strips or/and notches), which represent a code indicating instrument taper and tip diameter (Figs 1 and 2). However, because NiTi rotary root canal instruments have no ISO standards, a strip col-our on the shank may not relate to the tip diameter, which is coded with such colour according to ISO. The customary colour algorithm is often nominal and is decided by the manufacturer to make the instrument sequences easier to memorise.

The working section (working part) of root canal instrument is meant for root canal preparation. It consists of a few constructive elements, which are functionally divided as:

  • the tip
  • the cutting part
  • the non-cutting part.

The overall length of cutting and non-cutting parts determines the whole length of the working section. This length is usually pointed out on the package. There are three common variants of root canal instruments depending on the working-part length: 21 mm, 25 mm and 31 mm2. NiTi rotary instruments may have a working part of differing length; for example 17 mm, 23 mm or 27 mm.
Fig 2
Shanks of different rotary root canal instruments. From the top downward: K3 (SybronEndo, Orange, USA), RaCe (FKG Dentaire, La-Chaux-de-Fonds, Switzerland), System GT (Tulsa Dental, Tulsa, OK, USA), FlexMaster (VDW, Munich, Germany).
Fig 3
Two types of root canal instrument tips.

The non-cutting part is an element of the working section, which has a smooth cylindrical form and is positioned between the cutting part and the shank (Fig 1). As a rule, the non-cutting part has one or more measuring lines and/or the rubber stop. Both of them allow for the controlling of the 'working length', limiting the instrument insertion into the root canal during the preparation.

The tip is another element of the working part that performs the guiding function (Fig 1). The tip might have a sharp or rounded (bullet-like) configuration, depending on whether it appears:

  • active
  • passive.

An active tip has cutting edges on its surface, which are made for the removal of dentine or obturation material from the root canal (Fig 3). Most instruments with an active tip allow for the removal of obturation material during retreatment. One of the prominent disadvantages of NiTi is the lack of tactile feedback. Therefore, instruments with an active tip require special care when operated because of a significant risk of perforation when deviation from the canal axis occurs, due to insufficient instrument flexibility or the presence of obstacles in the root canal (hard obturation material, separated instrument or ledge).

A passive tip does not have cutting edges and does not possess cutting properties (Fig 3). A passive tip reduces the risk of instrument deviation from the canal axis, and as a consequence the risk of transportation or ledge formation3−5. The majority of NiTi root canal instruments have passive tips.

The cutting part is the prime element of the working section, which has cutting blades that perform the enlargement of the root canal (Fig 1). All the basic parameters of the root canal instrument describe its cutting part and determine the pattern of instrument-substrate interaction, instrument behaviour in the root canal and the operation technique.

All parameters of the cutting part can be conditionally divided into primary and secondary. The first group includes fluting characteristics; and the second, the taper, the length, etc.

The fluting is a specific surface with a certain configuration, which is created on the working part to impart the cutting ability to the instrument. In general, the fluting is formed by grinding out a groove of a specific profile onto the cylindrical or conical nickel-titanium blank — the rod with appropriate diameter (Fig 4). As a result of the grinding process, adjoining flutes form the cutting blade (Figs 5 and 6). The blade is the wedged element of the cutting instrument, which is used for the substrate penetration and the chip separation.

The overwhelming majority of NiTi root canal instruments have a spiral (helical) fluting. The fluting is characterised by the following parameters (Fig 5):

  • helical angle
  • pitch
  • depth of fluting
  • configuration of fluting.
Fig 4
Grinding the cutting part of rotary NiTi root canal
instruments.
Fig 5
Elements of the cutting part of root canal instruments.
Fig 6
External and internal instrument diameters, depth of fluting.
Fig 7
Volume of instrument fluting.
The helical angleis the angle between the instrument axis and the tangent to the cutting edge – b (Fig 5). The physical importance of this parameter is described below.

The pitch is a distance between the edges or the peaks of two nearby cutting blades measured along the instrument axis (Fig 5). The smaller the pitch, the greater the contact area between the instrument and the root canal walls. It increases torsional stress and the drawing of the instrument into the root canal6,7. The pitch may be constant or changing along the instrument.

The depth of fluting is the half of the maximum distance between the external and internal instrument diameters ((De-Di)/2) (Figs 6 and 7). Definitions of the terms 'external and internal diameters' and its physical sense will also be described below.

The pitch and the depth of fluting determine its volume. The fluting volume is a secondary parameter characterising the overall volume of flutes between nearby cutting blades per one pitch (Fig 7). Flutes accumulate a substrate cut off from the root canal walls during the preparation (organic debris, dentine chips or obturation material).

During preparation of the root canal, the fluting volume at a certain level of the instrument may be completely filled with debris. In such cases, further chip separation and the cutting efficiency slump down. The progressive instrument rotation leads to a significant increase of torsional stress and ultimately to instrument breakage8. The greater the fluting volume, the more substrate may be cut off from the root canal walls per unit length of insertion, and the deeper the instrument can be inserted into the root canal without torsion overloading9. Well-timed control over the flute filling can help to maintain the cutting efficiency. This in turn reduces the time needed for root canal preparation. Shortened preparation time decreases the number of instrument rotation cycles inside the root canal, and consequently decreases the risk of instrument cyclic overloading in curved canals. The depth, the volume and the fluting configuration can be evaluated on the instrument cross section (Fig 7).

In turn, fluting configuration determines such important parameters as:

  • external (De) and internal (Di) diameters
  • cutting blade configuration.
Fig 8
Taper of root canal instruments. D0 - instrument diameter at the beginning of cutting part.
D1 – instrument diameter at 1 mm from D0; D2 – instrument diameter at 2 mm from D0, etc.
Fig 9
Elements of the cutting blade.
De is a line segment passing through the instrument axis and connecting two arbitrary points of the circle circumscribed by the cutting edges (Figs 6 and 7). The instrument size is measured according to the ISO standard; for example 10, 15, 20, etc, represents its external diameter at the beginning of the cutting part or D0 (Fig 8). This size is usually pointed out on the packaging.

Di is a line segment passing through the instrument axis and connecting two arbitrary points of the circle, including the deepest flute points (Figs 6 and 7). This circle with the diameter Di determines the size of the instrument central part, the so-called core, which is a very important parameter affecting instrument flexibility (Figs 6 and 9).

The ratio between De and Di is a very essential parameter, influencing instrument resistance to cyclic and torsional stresses10,11. If De tends to Di, the instrument strength and torsional rigidity increase, but its flexibility and resistance to cyclic load decrease12. In addition to the decrease in the ratio (De/Di → 1), the fluting depth and correspondingly the cutting efficiency also decrease.

The ratio between De and Di can be calculated at the cross-sectional view. It is important to take into account the value of this parameter, then evaluate the possibility to use one or another instrument, depending on the anticipated operating conditions. Thus, when working in a curved root canal, if the probability of cyclic overloading is high, then the instrument with a higher De/Di should be chosen. Under the conditions related to high torsional stress (narrow, calcified, but quite straight canals), the instrument with a lower value of De/Di should be chosen. The De/Di ratio may change along the instrument length, depending on fluting changes.

At present, the manufacturers of root canal instruments do not provide detailed information about their products. Only the instrument size (in other words, D0 value) and taper are pointed out in the instructions or on the package, whilst the value of internal or core diameter (Di), or better still De/Di ratio, could be the more important criterion for the practitioner in selecting the right instrument to solve a particular clinical problem.

The configuration of the cutting blades is a combination of surfaces and angles of cutting blades (Fig 9). The basic elements of the cutting blade are13:

  • face surface (or face)
  • flank surface (or flank)
  • cutting edge.
Fig 10
Coordinate planes, according to which the instrument parameters are defined. Instrument fragment is also illustrated schematically by cross sections and cutting edges.
Fig 11
Instrument-substrate interaction pattern
(crosssectional view).
Face Aγ is the surface of the cutting blade contacting with the substrate layer and the chip. Flank Au is the surface of the cutting blade facing the machined surface of dentine. Cutting edge K is the intersection line of the face and flank.

The cutting properties of the instrument are determined by the tool angles. To evaluate the angles of a cutting blade, it is necessary to define the plane in which these angles should be measured. In other words, it is necessary to define a system of coordinates. To define the position in the space of the arbitrary point (here the arbitrary point is on the cutting edge), the coordinate system should consist of three planes, according to which the tool angles are measured14:

  • reference (or base) plane
  • cutting edge plane • orthogonal plane.
Reference plane Pv is the coordinate plane passing through the selected point on the cutting edge transversely to the direction of cutting motion (or the tangent to it).

For the rotary root canal instrument, the cutting motion is the rotation. Therefore the reference plane is built transversely to the tangent of the rotation trajectory of the selected point (Fig 10).

Cutting edge plane Pn is the coordinate plane passing through the selected point on the cutting edge transversely to the reference plane (Fig 10). The vector of cutting motion linear speed V is located in this plane (Fig 11). Orthogonal plane P" is the coordinate plane passing through the selected point on the cutting edge transversely to the intersection of reference plane and cutting edge plane (Fig 10). The orthogonal plane aligns with the instrument cross section (Fig 11). In this plane, the basic angles of cutting blade are measured:

  • clearance angle
  • wedge angle
  • angle of cutting
  • rake angle.
Clearance angle u is the angle between the flank (or its tangent) and the cutting edge plane (Figs 12 and 13). The value of the clearance angle influences the frictional force arising during the cutting process and the degree of instrument penetration into the substrate. The less the clearance angle, the harder the blade penetration into dentine, and the lower the cutting efficiency. Wedge angle β is the angle between the face and the flank (or between tangents to them) (Figs 12 and 13). The wedge angle determines the strength performance of the blade: the greater the wedge angle, the stronger the blade.

Fig 12
Schematic illustration of the tool angles in the case of a negative rake angle.
Fig 13
Schematic illustration of the tool angles in the case of a positive rake angle.
Fig 14
Interaction pattern of the substrate and cutting blade when using an instrument having a negative rake angle.
The angle of cutting (b) is the angle between the cutting edge plane and the face (or its tangent) (Figs 12 and 13). The angle of cutting is defined by the sum of clearance and wedge angles
(b = u + ). The value of the angle of cutting relates linearly to the cutting force and cutting power values. It is calculated on the tool engineering stage, depending on cutting conditions and construction material.

The rake angle (y) is the angle between the face (or its tangent) and the reference plane in the selected point on the cutting edge (Figs 12 and 13). It is assumed that the rake angle may be negative, positive or neutral (equal to zero). The rake angle determines the instrument–substrate interaction pattern.

To determine the rake angle sign, firstly it is necessary to describe the three-dimensional interference of different elements in the instrument–substrate system during the cutting process. In a mathematical sense, vectors can express such interaction patterns.

Any surface in the selected point can be described by the 'normal' to this surface. The normal to the surface in a given point is the vector passing through this point, perpendicular to the tangent plane in the selected point. Thus, the face of the blade can be described by the normal to this surface in the cutting point (vector N in Figs 14 and 15), while the cut dentine surface can be described by the normal to the cutting edge plane in this point (vector n in Figs 14 and 15). The interaction pattern of all elements in the cutting point can be fully described by three vectors: the normal to the face N, the normal to the cutting edge plane n and the linear speed vector V (Figs 14 and 15). The angle 8 between the normal vectors N and n describes the interaction of the face and the cut surface.

The negative rake angle is depicted in Figures 12 and 14. To determine the interaction pattern, measure out a vector N on the direction of the vector n to obtain the projection, which can be expressed as N∙cos8. Here the angle 8 between the normal vectors N and n is more than 90 degrees (turning on the shortest path from the N to n). In that event, cos8 is negative because a cosine of the angle greater than 90 degrees has a minus sign.

According to the reduction formulae, the following is obtained:

N·cosθ = N·cos(90º+γ) = N·(-sinγ) = N·sin(-γ)
It can be seen that the rake angle y is negative. The vertical component of the normal N – vector Ny – is directed away from the substrate (in the coordinate x system y-x in Fig 14 it is directed to the negative zone of ordinate y). Inasmuch as the vertical component of the substrate normal pressure on the blade is directed as the vector Ny, during cutting, the blade would be pushed off from the material. In the case of negative rake angle, cutting is not aggressive; the instrument scratches a surface rather than cuts, torsional load is not too great and is largely determined by the axial force acting on the instrument from the operator.

A positive rake angle is depicted in Figures 13 and 15. In this case, the angle 8 is less than 90 degrees, and its cosine is a positive magnitude. The rake angle is positive because the projection is:

N·cosθ = N·cos(90º-γ) = N·sin(+γ)
Fig 15
Interaction pattern of the substrate and cutting blade when using an instrument having a positive rake angle.
The projection of vector N is positive (directed along the vector n). The vector Ny is directed to the substrate (positive zone of the ordinate y in Fig 15). In the case of a positive rake angle, cutting is very aggressive. The forming chip presses down the blade that causes it to penetrate deeper into the substrate. When operating with such a tool, the torsional load increases and the probability of engagement is higher.

According to available information, all the well-known rotary root canal instruments have a negative or at best a neutral rake angle, despite some different manufacturers' statements8,9,15,16. Some values of the instruments' rake angles are listed in Table 1. This appears to be due to the small size of the instruments, because technologically it is difficult to manufacture a small instrument with a positive rake angle. It is widespread opinion that a positive rake angle is beneficial because it improves the cutting efficiency of the root canal instruments17,18. However due to the fact that a positive rake angle makes an instrument more aggressive and working with it less controllable, the presence of such an angle should be considered as dangerous and undesirable.

Another important parameter of the root canal instrument is the taper. Taper (conicity) is the ratio of the instrument diameters at two different points of the cutting part, to the distance between them. Taper is expressed in fractional form or in percentage. The taper of traditional hand instruments according to the ISO standard is 0.02 mm/mm or 2%2. This implies that for every millimetre of the cutting part, the external instrument diameter (De) increases by

0.02 mm from the tip towards the shank (Fig 8). Moreover, the ascending sequence of internal diameter values (Di) can differ from the progression describing the increase of external diameter (De). In this case, the ratio De/Di will also change along the instrument length.

In the scenario when the instrument taper is constant and the tip size (D0) is known, it is easy to calculate the value of the external diameter in any segment of the cutting part:

Dn = D0 + K∙n where Dn is the required external diameter at n mm from D0 and K is the taper.
K3 ProFile ProTaper RaCe FlexMaster Hero Quantec
Sonntag 8 -19 -39 -50 -40 -55
Johnson 9 -20 -40 -30 -20 -10
Chow et al16 -15 -43
Table 1
The mean values of the rake angles of the most popular root canal instruments.
It is generally considered that a tapered shape enables a decrease of torsional load upon the instrument due to the reduction of contact area between the blades and the root canal walls18. When this area is too great, it is easy to reduce it by taking off the vertical pressure. This action decreases the material volume cut off per each revolution, and thereby prevents the instrument from locking.

However, if when during preparation the shape of a root canal already fits the shape of the rotary instrument, the contact area between the blades and the root canal walls increases and leads to an increase in torsional stresses (taper lock)9. A large taper also increases the probability of root canal transportation due to a decrease in instrument flexibility19. Low flexibility also increases susceptibility to cyclic overloading9. It also makes progression of the instrument deep into the root canal difficult. This leads to the increase in axial force, which the operator should apply to accomplish the cutting. This in turn increases the torsional load on the instrument.
For the same reason, the preparation of narrow root canals with NiTi hand instruments of great taper is a very labour-consuming process, because it necessitates the operator expend considerable efforts for canal enlargement. Significant vertical pressure on the instrument increases the torsional load that could result in instrument breakage.
Torsional rigidity (the ability to withstand torsional loads) is a strong function of the cross-sectional radius. In this context, the tip region of the instrument becomes the weakest zone20. Therefore, when torsional overload occurs, the breakdown is more likely to be at the first quarter of the cutting part, near the thin tip. These conclusions are confirmed by clinical data, showing that rotary root canal instruments more often fracture in the apical third of the root canal21-23, and the removal of fragments from the depth of the root canal presents major difficulties or is even impossible24-26.

Interaction between the rotary root canal instrument and the substrate

As noted above, the design features of the rotary root canal instrument determine the range of its properties, its advantages and disadvantages. It is important to realise which parameters influence one or another instrument's characteristics, and how this influence is accomplished. In addition, certain behaviour patterns of the instrument inside the root canal can be the result of the total influence of different parameters.

One important example of the dependence of instrument behaviour on its constructive features is the 'self-feeding effect'. During root canal preparation, all fluted instruments (if the fluting has the same direction as the rotation motion) are drawn into the canal due to the rotation, even in the absence of vertical feed (vertical pressure from the operator). In general engineering this result is called the self-feeding effect. In the dental literature it is usually called the 'screwing-in effect'. This term is not exactly correct, because the screwing means that during the rotation, the instrument progresses inward in the canal under the axial feed from the operator. In contrast, self-feeding means that the instrument progresses inward in the canal automatically – only due to rotation without any axial feed. To catch a moment of self-feeding effect initiation during preparation is quite difficult and this effect often leads to instrument engagement and breakage inside the root canal.

Fig 16
Interaction pattern between the blades and the root canal wall during full-rotation preparation. V – vector of linear speed of rotational motion of cutting point and elementary area nearby the cutting edge. Fp – force of normal pressure from the cut substrate to elementary area. Ff – frictional force including the cutting resistance force. Fs – second component of the normal pressure force, known as the self-feeding force. – helix angle.
Figure 16 schematically displays the instrument fragment with three blades (plotted with red lines). The torque M applied to the instrument provides its rotational movement. The cutting points belonging to cutting edges move with linear speeds (V), which are in direct relationship to the radius of the instrument. Each point belongs to the elementary area located nearby the cutting edge and subjected to the force of normal pressure from the cut substrate. This force in Figure 16, denoted as Fp, is directed perpendicularly to the elementary area. The normal pressure force Fp could be split into mutually perpendicular components: Ff which is directed against the speed vector V and is named frictional force; and Fs which is directed towards the apical part of the canal and is named self-feeding force. Thus, even in the absence of vertical force from the operator, the instrument is drawn into the canal by force resulting from rotation. The self-feeding force rises with the increase of helix angle under otherwise equal conditions. Thus, the instruments with a large helix angle and a small pitch have an essential drawback – increasing self-feeding force. It is a widespread opinion that varying helical angle and pitch could decrease the self-feeding effect18 – however this is not completely true. According to the rule of vectors, the composition of the total value of self-feeding force depends on the resultant vertical Fs along the whole instrument length, regardless of constant or varying helix angle or pitch.

Summary

Despite the basic design frameworks, all existing rotary NiTi root canal instruments differ from each other by a number of component features and properties. At the same time, unbiased and comprehensive information concerning these differences is absent in the available literature. The information provided by manufacturers is limited and also has advertising targets. It is intended to direct the practitioner's attention to the dissimilarities of the new instrument from the previous one. Therefore information concerning the meaning of those dissimilarities, and its influence on instrument properties, is practically non-existent.

Word combinations such as 'variable taper', 'positive rake' or 'asymmetrical cross section' are the appropriate and fluent phrases to use if the practitioner lacks basic knowledge about the constructive parameters of root canal instruments. In spite of the seeming difficulty of this knowledge, it is crucial to understand what any given instrument is capable of, and how it will behave inside the root canal. It is also important to evaluate how different parameters of the same instrument interact together in determining the instrument's properties. Such information is absolutely necessary for the successful use of rotary root canal instruments in clinical practice.

Knowing the physics of the basic design features, and their correlation with the constructive peculiarities of a given instrument enables the practitioner to choose the right instruments for every case. To understand these differences is essential for determining the operative possibilities of the instruments and correction of their disadvantages by means of improving the technique, or by observing the limitations. Such an approach enables the reduction of possible mistakes that can be made during root canal preparation, and to avoid undesirable complications.

Further laboratory and clinical research studies concerning the comparative evaluation of the basic constructive parameters of the most commonly encountered root canal systems are needed to increase the effectiveness and safety of clinicians' work.

Acknowledgements

The authors thank Prof Edgar Schäfer from the School of Dentistry, University of Münster, and Dr Michael Solomonov from the Department of Endodontics of Hebrew University School of Dental Medicine, for reviewing the manuscript.

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