Design features of rotating (rotary) endodontic instruments.

A. Iandolo (Alfredo Iandolo) Doctor of Dentistry, Professor

The long-term success of endodontic treatment is closely related to adequate cleansing and quality three-dimensional obturation of the complex root canal system. Probably, a significant percentage of failures is due to the presence of residual pulp tissue and insufficient cleaning of the canals.

The endodontic system consists of spaces easily accessible for manual and machine files (main canals) and spaces that are difficult or inaccessible (delta, lateral and auxiliary canals) (Fig. 1, 2).

Rice. 1.

Rice. 2.

Regardless of the technique used, it is impossible to mechanically treat all areas of the root system. For this reason, biochemical purification is necessary. Modern endodontic treatment methods are based on old methods of work: without the help of an operating microscope, with conventional NiTi files, the use of irrigation without activation.

Endodontic treatment can be divided into stages:

1. Opening the pulp chamber is the most difficult phase according to the literature, since an error at this stage may jeopardize further processing. Dissection should be performed under constant magnification and illumination.
2. Forming stage using new modified NiTi tools.
3. Cleansing stage using irrigant activation.
4. Obturation stage.
5. Of course, treatment should end with restoration.

After a thorough analysis of the X-ray and clinical examination data, endodontic treatment can begin.

Opening the pulp chamber

The first step is to isolate the surgical field using a rubber dam. Then, with constant magnification and illumination, we must begin to open the pulp chamber using rotary instruments and ultrasonic tips.

The main function of an operating microscope (Fig. 3) is the ability to distinguish between two points that are very close to each other. The human eye is essentially unable to distinguish between two points separated by a minimum distance of 0.1 mm; it will sum them up as one image. When using an operating microscope, the resolution power increases from 0.1 mm to 0.005 mm, which is 5 microns and allows the human eye to discern more details.

Rice. 3.

Ultrasonic instruments include different types of tips that have different shapes and lengths (Fig. 4). In addition, with the introduction of new and improved ultrasound sources, it has become possible to optimize the use of each type of nozzle with the ability to control the frequency and amplitude of vibration. Ultrasonic handpieces guarantee greater precision thanks to their reduced dimensions, which provide a greater view of the working area than rotary instruments.

Rice. 4.

Only after identifying the orifices (Fig. 5) is it possible to continue treatment.

Rice. 5.

Design features of rotating (rotary) endodontic instruments.

Belyaeva T.S.

postgraduate student at the Department of Therapeutic Dentistry and Endodontics, Moscow State Medical University.
Rzhanov E.A.
Candidate of Medical Sciences, Associate Professor of the Department of Therapeutic Dentistry and Endodontics, Moscow State Medical University.

Summary
Formation of the root canal is one of the most important stages of endodontic treatment, largely determining the possibility of thorough disinfection and hermetically sealed obturation of the root canal system.
Many instruments and their systems have been developed for canal preparation. Today, for a number of reasons, the fastest growing group of instruments are rotating nickel-titanium endodontic instruments. This group of tools undoubtedly has a number of advantages compared to traditional steel tools. It is not without its drawbacks, some of which are due to the characteristics of the alloy, others due to the design features of the tools themselves. However, there is little information available in the available literature regarding design parameters and their influence on the behavior of a rotating endodontic instrument in the canal, and manufacturers' instructions are limited to some empirical recommendations. Meanwhile, understanding the physical meaning and differences in the main design parameters is necessary to assess the capabilities of specific instruments and correct their shortcomings by improving the technique of working with them or limiting the indications for their use. Abstract
Shaping of the root canal is one of the most important stages of endodontic treatment which determines the possibility of thorough disinfection and leak-tight sealing of the root canal system. The numerous systems of instruments have been designed for the preparation of the root canal. Today for a score of reasons rotatable nickel-titanium endodontic instruments have become the most high-developing group of instruments. This group possesses the well known advantages compared with traditional stainless steel devices. There are also some weaknesses, some of which could be explained by alloy characteristics, and others due to deficiency of design and engineering. Nevertheless in available literature full information concerning design parameters and their influence on the instrument behavior in the root canal substantially is not presented. As a rule manufacture's instructions are limited to some empirical recommendations for users. Meanwhile, the detailed understanding of the physics of instrument functioning and difference in the basic design features is essential for determining the operative possibilities of the instruments and correction of it's disadvantages by means of bettering the technique or by use limitations to avoid undesirable complications.

Introduction
The design features of any tool are primarily determined by the purpose and method of its use. Also, the design features are influenced by the properties of the material and technological methods of manufacturing the tool.

On the other hand, all of the above factors determine the technique (methodology) of working with the tool and the range of problems that can be solved with its help. The number of different endodontic instruments and their systems is enormous. New tools or modifications of already known systems appear periodically. At the same time, many of these tools are falling out of use. It is sometimes difficult for practicing doctors to navigate this diversity.

However, knowledge of the basic parameters of endodontic instruments and an understanding of the principles of their operation can help the doctor most effectively use the advantages of certain instruments and dramatically reduce the likelihood of errors.

Before we begin to consider the main design features of endodontic instruments, it seems necessary to clarify some basic definitions, concepts and terms.

So, any endodontic instrument is a cutting instrument designed for mechanical treatment of the root canal

. For convenience, all endodontic instruments can be divided into two large groups according to the method of their use:

• used in manual mode;
• used with a mechanical drive.

The main structural elements of the first and second groups are generally the same; the parameters of these elements differ more. Currently, the second group of instruments, for a number of reasons, is more popular and is developing especially rapidly. In this regard, this work will mainly consider instruments driven by a mechanical drive, or rather endodontic instruments made of nickel-titanium alloy and operating in full rotation mode. The exact full name of this group of instruments sounds like Machine Rotary (Rotating) Nickel-Titanium Endodontic Instruments (from the Latin rotatio

- circular motion, rotation), however, later in the article abbreviated terms will be used, for example, rotational (rotating) endoinstruments.

In the literature, rotary instruments designed for dental root canal treatment are often called endodontic files. The term file

- file) denotes a cutting tool that removes material during a reciprocating movement along the surface being processed. A typical example of this processing technique is the movements produced when working with Hedström files (H-files). When treating a root canal with rotary instruments, dentin is cut as a result of rotation of the instrument. Therefore, it is incorrect to apply the term “file” to rotary instruments.

Correct in relation to rotating endodontic instruments is the term reamer (from the English reamer - reamer), denoting a cutting instrument with a rotational cutting movement, designed to increase the size of an existing hole and increase the accuracy of the shape of this hole. That is, such a concept as “machine nickel-titanium reamer” is terminologically correct.

It should be noted that in this work, in the designation of some components of an endodontic instrument, terminology is used that is slightly different from the generally accepted one or given in the standards. From our point of view, the proposed terminology more accurately and logically characterizes the design features and functional purpose of a particular part of an endodontic instrument.

Structure of a rotary endodontic instrument

The rotating endodontic instrument consists of two main parts, each of which performs its own specific function (Fig. 1):

• fastening part;
• working part.

Fastening part

or
shank
is a part of a tool intended for its installation and fastening in technological equipment (in the tip), through which the torque is transmitted from the drive directly to the working part of the tool.

As a rule, all rotating endodontic instruments have a shank No. 20 according to ISO or type 1 according to GOST [1]. This type of shank has a flat and a groove at the end (Fig. 1). Through these elements of the shank and a special device - a clamp located in the 7th head of the handpiece, the tool is rigidly connected to the rotor of the head. According to the standard, the diameter of such a shank should be 2.35 mm, and its length should not exceed 13.5 mm [2]. However, in different systems of rotating endodontic instruments, the shank can have different lengths - from 11 to 15 mm.

As a rule, identification lines (multi-colored transverse stripes and notches) are applied to the tool shank, which are color and/or relief coding indicating the taper of the tool and the diameter of its tip (Fig. 1, 2). In this case, the color of the strip on the shank of the instrument may not correspond to the size of the tip, which is encoded with this color in the ISO system, but is often conditional and chosen by the manufacturer to make it easier to remember the sequence of using instruments thanks to the color algorithms familiar to the doctor.

The working part
of the endodontic instrument is directly intended for canal preparation, and consists, in turn, of several structural elements, which are functionally divided into:

• top;
• cutting part;
• non-cutting part.

The total length of the cutting and non-cutting parts determines the total length of the working part of the tool. This length is usually indicated on the packaging. According to the ISO standard, four versions of endodontic instruments are available depending on the length of the working part: 21mm, 25mm, 28mm and 31mm [3]. Since the ISO standard only regulates the parameters of hand tools, nickel-titanium rotary tools can have other working lengths, for example 17mm, 23mm or 27mm.

Non-cutting

part is an element of the working part of the tool of a smooth cylindrical shape, located between the cutting part and the shank (Fig. 1). The non-cutting part usually has one or more measuring lines and/or a silicone stop. Both serve to control the so-called “working length” by which the instrument is immersed into the canal during the preparation process.

Top

– this is an element of the working part of the tool that performs a guiding function (Fig. 1). The tip can be sharp or rounded (bullet-shaped), depending on what it is:

• active;
• passive.

The active tip
of the instrument has cutting edges on its surface intended for preparing dentin or removing obturation material from the canal (Fig. 3).
An instrument with an active tip requires special care when working with it, since there is a significant risk of perforation of the canal wall when the instrument deviates from the axis of the canal due to its insufficient flexibility or if there is an obstacle in the canal in the form of hard filling material, a broken instrument, a step, etc. . The passive tip
of the tool does not have cutting edges on its surface and does not have cutting properties (Fig. 3).

Passive top

reduces the risk of instrument deviation from the canal axis and perforation of the root wall. Most rotating nickel titanium endodontic instruments have a passive tip. Some instruments have an active tip and are designed to remove obstructive material from the canal. As a rule, such instruments are used for repeated endodontic treatment.

Cutting part

– this is an element of the working part of the tool with cutting blades, through which mechanical treatment of the root canal is carried out (Fig. 1).

All the most important design parameters of endodontic instruments are the parameters of its cutting part and determine the nature of the interaction of the instrument with the substrate, the behavior of the instrument in the canal, and the method of its use.

Conventionally, we can distinguish the primary and secondary parameters of the cutting part. The primary parameters include such parameters as cutting, the secondary ones include taper, length, etc.

Slicing

- this is a specific surface of a certain configuration, which is created on the working part of the tool to give it cutting properties. Cutting, as a rule, is formed by turning the tool profile from a cylindrical blank - wire of the required diameter (Fig. 4). During the process of turning a profile, adjacent cutting areas form a cutting blade (Fig. 5, 6). A blade is a wedge-shaped element of a cutting tool designed to penetrate the substrate and separate chips.

Most known nickel-titanium tools have a spiral thread. Endodontic instrument cutting is characterized by the following parameters (Fig. 5):

• cutting angle;
• cutting step;
• cutting depth;
• cutting form.

The cutting angle
ω is the angle between the tool axis and the tangent to the cutting edge line -
b
(Fig. 5).

Cutting step

– this is the distance between the edges or vertices of two adjacent cutting blades, measured along the axis of the tool (Fig. 5). The smaller the step, the larger the contact area of ​​the tool surface with the channel walls, which is undesirable, since it increases the torsional load on the tool and promotes screwing it into the channel [5]. The screwing effect will be discussed in detail below. The cutting pitch can be constant or vary along the length of the tool.

Cutting depth

– this is half the maximum value of the difference between the outer and inner diameters of the tool
((D
n -D in )/2) (Fig. 6,7). Definitions of the concepts of outer and inner diameters, as well as their physical meaning, will be discussed below.

The depth and pitch of the cut determine its volume. Cutting volume

– this is a secondary parameter that characterizes the total volume of recesses between adjacent cutting blades per cutting step (Fig. 7). The recesses serve to accumulate the substrate cut from the canal walls during preparation, for example, dentinal chips or filling material. The larger the cutting volume, the greater the amount of substrate that can be cut from the walls of the root canal per unit of time and the deeper the tool is able to move into the canal.

During the process of root canal preparation, a situation may arise when the cutting volume in a certain area of ​​the instrument is completely filled with the substrate. In this case, the possibility of separating new chips is sharply reduced, and cutting efficiency decreases. If the rotational-translational movement of the tool in the channel continues, this leads to a significant increase in the torsional load that is placed on it, and ultimately leads to breakage of the tool in the channel. Timely control over filling the cutting volume will help maintain the cutting efficiency of the tool. This, in turn, helps reduce the time required to process the canal and therefore reduces the likelihood of cyclic overload of the tool when working in a curved canal.

From the above it follows that if the cutting of the tool is shallow, the volume between the blades is filled with sawdust extremely quickly, and the cutting efficiency of such a tool is low. The depth, volume and configuration of the cutting can be judged from the cross section of the tool (Fig. 7).

In turn, the cutting shape determines such important design features of the tool as:

• outer Dн
  and inner
  Dв
  diameters;
• cutting blade shape.

The outer diameter of the tool
Dн is a straight segment passing through the axis of the tool and connecting two arbitrary points of a circle drawn through the cutting edges of the tool (Fig. 6, 7).
ISO tool size, for example 08, 10, 15, 20, etc. – this is its outer diameter at the very beginning of the cutting part or D0
(Fig. 14). This is the size indicated on the packaging.

Inner diameter of tool Dв

– this is a straight line segment passing through the axis of the tool and connecting two arbitrary points of a circle drawn through the deepest cutting points (Fig. 6,7).
In fact, this circle, with a diameter D
in , determines the size of the central part of the tool - the core, a very important parameter of the tool on which its flexibility depends (Fig. 6,8).

The ratio of external or external ( D n

) and internal diameter or core diameter (
D
in ) is a design parameter that significantly affects the tool’s resistance to cyclic and torsional loads.
In the case when Dв
approaches
Dн
, the strength of the tool and resistance to torsional load increases, but at the same time its flexibility and resistance to cyclic loads decreases.
In addition, with an increase in the ratio Dв/Дн
, the cutting depth decreases, and therefore the cutting efficiency decreases.

Ratio Dв/Дн

easy to calculate on the cross section of the tool.
The value of this parameter, along with some others, is important to take into account when assessing the possibility of using a particular tool depending on the expected conditions of its operation in the channel. So, when working in a curved channel, that is, when the probability of cyclic overload of the tool is high, it is necessary to select a tool with a lower value of Dв/Дн
.
In the presence of conditions associated with increased torsion load (narrow, obliterated, but at the same time relatively straight canals), preference should be given to an instrument with a large value of Dв/Дн
.
The ratio Dв/Дн
can vary along the length of the tool depending on the change in the cutting shape.

Currently, manufacturers of endodontic instruments do not provide detailed information about their products; the instructions and packaging indicate only the ISO size of the tool (that is, the D0

) and its taper.
Although the value of the internal diameter ( Dв
), which determines the size of the core, or, better, a parameter such as
Dв/Dн
could serve as an important additional criterion for the doctor when choosing an instrument for solving a specific clinical problem.

Cutting blade shape

– this is a set of surfaces and angles of the cutting blade (Fig. 8). The main elements of the cutting blade are [4]:

• front surface;
• back surface;
• cutting edge.

The front surface Aγ
is the surface of the tool blade that comes into contact with the cut substrate layer and chips during the cutting process.

The back surface
Aα
is the surface of the tool facing the machined surface of the substrate.

The cutting edge
K
is the line of intersection of the front and rear surfaces of the cutting blade.

The characteristics of a tool that determine its cutting properties are the angles of the cutting blade. To determine the angles of the cutting blade, you must first determine the plane in which these angles will be measured, which means you need to specify a coordinate system. To determine the position of any point in space (in this case, an arbitrary point on the cutting edge), the coordinate system must consist of three planes, relative to which the angles of the cutting blade (Fig. 10) and the parameters of the tool movement are determined [6]:

• main plane;
• cutting planes;
• main cutting plane.

The main plane Pv
is a coordinate plane drawn through the point of interest on the cutting edge perpendicular to the direction of its movement during cutting (or tangent to the direction of movement). For a rotary endodontic instrument, the cutting movement will be a rotational movement, therefore, the main plane is constructed perpendicular to the tangent to the rotation path of the considered point of the instrument - line a (Fig. 9).

Cutting plane
Pn
is a coordinate plane drawn through the point of the cutting edge under consideration and perpendicular to the main plane (Fig. 9).
In this plane there is a vector of linear speed of movement during cutting - vector V
(Fig. 10).

The main cutting plane
Pτ
is a coordinate plane drawn through the point of the cutting edge under consideration and perpendicular to the line of intersection of the main plane and the cutting plane (Fig. 9). In this case, the main secant plane will coincide with the cross-sectional plane of the tool (Fig. 10). It is in this plane that the main angles of the cutting blade are determined:

• rear corner;
• point angle;
• cutting angle;
• front corner.

For a detailed examination of the angles of the cutting blade, we will use the diagrams shown in Figures 11 and 12.
The rear angle or clearance angle
α
is the angle between the rear surface of the blade (or the tangent to it) and the cutting plane (Fig. 11, 12). The size of the clearance angle affects the friction force that occurs during the cutting process, as well as the degree of immersion of the tool into the substrate. The smaller the clearance angle, the more difficult it is for the blade to penetrate the dentin, and the cutting efficiency decreases.

The sharpening angle
β
is the angle between the front and rear surfaces of the blade (or the angle between the tangents to them) (Fig. 11, 12). The sharpening angle determines the strength characteristics of the cutting blade: the larger it is, the stronger the blade.

Cutting angle
δ
is the angle between the cutting plane and the front surface of the blade (or tangent to it) (Fig. 11, 12).
This angle should not be terminologically confused with the front angle γ
, which is often found in publications on this topic.
The value of this angle is determined by the sum of the values ​​of the clearance angle and the sharpening angle ( δ = α + β
). The value of the cutting angle is linearly related to the values ​​of cutting force and power. It is set at the tool design stage, depending on the cutting conditions and the material from which the tool is made.

Rake angle
γ
is the angle between the front surface of the blade (or tangent to it) and the main plane at the point of the cutting edge under consideration (Fig. 11, 12). It is generally accepted that the rake angle is either negative (negative), or positive (positive), or neutral, that is, equal to 0. The sign of the rake angle determines the nature of the interaction of the tool with the substrate.

To determine the sign of the rake angle of the blade, it is first necessary to describe the spatial interaction of various elements in the tool-substrate system during the cutting process. From a physical and mathematical point of view, this kind of interaction is usually described in terms of vectors.

Any surface at any specific point can be described by a normal to this surface. The normal to a surface at a given point is a vector passing through that point and perpendicular to the tangent plane at a given point on the surface. Thus, the front surface of the blade can be described by the normal to the front surface of the blade at the cutting point (vector N

in Fig. 13, 14), and the cutting surface can be described by the outer normal to the cutting plane at the same point (vector
n
in Fig. 13, 14).
Thus, with the help of three vectors - the normal to the front surface of the blade, the normal to the cutting plane and the velocity vector (vector V
in Fig. 13, 14) - the nature of the interaction of all elements of the system at a given cutting point can be fully described.
The angle θ
between the normals
N
and
n
describes the nature of the interaction between the front surface of the blade and the cutting surface. Let's look at this issue in more detail and determine how it is related to the sign of the front angle.

Negative rake angle (Fig. 11, 13). To determine the nature of the interaction, we project the vector N

to the direction of vector n, we obtain a projection that can be expressed as
N٠cosθ
.
In this case, the angle θ
between the normals
N
and
n
turns out to be greater than 90º (when rotating along the shortest path from vector
N
to vector
n
), while
cosθ
will be negative, since the cosine of an angle greater than 90º has a minus sign.
Using the reduction formulas, we get that: N cosθ = N cos(90º+γ) = N (-sin γ) = N sin(-γ)
Thus, it is clear that the front angle γ

is negative.
The projection of the vector N
is directed in the negative side of the vertical axis, that is, the vertical component of the normal to the front surface of the blade - the vector
Ny
- is directed from the substrate (in the
y-x
in Fig. 13 it is directed to the negative region of the y ordinate).
Since the vertical component of the force of normal pressure of the substrate on the cutter will be directed in the same way as the vector Ny
, the tool will be pushed out of the material during the cutting process. In the case of a negative (negative) rake angle γ, cutting is not aggressive, the tool rather scrapes the surface, the torsional load is not large and is largely determined by the force acting on the tool from the operator along the direction of the channel.

Positive rake angle (Fig. 12, 14). Angle θ

in this case, less than 90º, its cosine is a positive value - hence the name of the front angle γ, since the projection is equal to:
N٠cosθ = N·cos(90º - γ) = N·sin(+γ)
The projection is positive (directed along the vector n

).
Ny
vector is directed towards the substrate (the positive region of the y ordinate in Fig. 13). In the case of a positive rake angle γ, cutting occurs very aggressively. The forming chips presses the tool blade, which forces it to sink deeper into the substrate. When working with such a tool in a channel, this leads to an increase in torsion load on it and increases the likelihood of jamming.

All known rotary endodontic instruments have a negative rake angle or, at best, a neutral rake angle, despite the claims of some manufacturers [7]. Due to the small size of the tool, it is technologically difficult, or rather practically impossible, to produce it with a positive rake angle. In addition, due to the fact that a positive rake angle makes the instrument more aggressive and the work with it less controlled, the presence of such an angle in an endodontic instrument should be considered dangerous and harmful.

Another very important parameter of an endodontic instrument is its taper.

Taper is the ratio of the difference in diameters of two cross sections of a tool to the distance between them. Taper is expressed as a fraction or percentage. The cutting taper of traditional hand endodontic instruments according to the ISO standard is 0.02mm/mm or 2%. This means that for every millimeter of cutting part length, the outer diameter of the tool ( Dн

) increases in the direction from the top to the shank by 0.02 mm (Fig. 15).
In this case, the increasing sequence of values ​​of the internal diameter ( Dв
) of the tool may differ from the progression characterizing the increase in the external diameter of the tool (
Dн
).
In this case, as the tool moves, the ratio Dв/Дн
, as noted above.

In the case when the taper of the tool is constant and its size is known, that is, the value of D0

, it is easy to calculate the value of its outer diameter at any section of the cutting part:
Dn=D0+K·n
, where
Dn
is the desired outer diameter at a distance of n millimeters from
D0
, and
K
is the taper.

It is believed that the conical shape makes it possible to reduce the torsional load on the instrument by reducing the contact area of ​​the blades with the canal walls. When this area is large, it can be easily reduced by simply removing the vertical force from the tool. This results in a rapid reduction in the volume of material cut per revolution, thereby preventing tool jamming.

However, when, as a result of preparation, the shape of the root canal already largely begins to correspond to the shape of the rotating endoinstrument, this sharply increases the contact area of ​​the instrument surface with the canal walls and leads to a significant increase in torsion load. It should also be noted that a large taper of the tool increases the likelihood of channel transportation, as it leads to a decrease in the flexibility of the tool, which in turn increases its susceptibility to cyclic loads.

In addition, the large taper of the instrument itself makes it difficult to advance deep into the root canal, which leads to an increase in the axial force that the doctor must apply to carry out cutting. This in turn again leads to an increase in torsional load on the tool. For the same reason, the preparation of narrow root canals with hand-held nickel-titanium instruments of large taper is a labor-intensive process, since it forces the operator to exert significant effort to widen the canal. With significant vertical forces, the feeling of torsional load applied to the tool is lost, which can lead to its breakage. Since the torsional stiffness of a tool (that is, its ability to withstand torsional loads) is highly dependent on the cross-sectional radius, the tool tip area is the weakest point in this regard. Therefore, as a result of torsional overload when using instruments of increased taper, the area most susceptible to fracture is located in the apical quarter of the instrument, the extraction of which from the depths of the canal is the most difficult, and often simply impossible.

Interaction of rotary endodontic instrument and substrate

As noted above, the design parameters of a rotating endool determine the range of its basic properties, its advantages and disadvantages. At the same time, it is important to understand what specific instrument parameters affect certain of its characteristics, as well as how exactly this influence is carried out. In addition, a certain behavior of the tool in the channel may be a consequence of the combined influence of a number of parameters. An important example of the dependence of the behavior of a tool in a channel on its design features is the well-known effect of screwing.

The screw-in phenomenon is that the tool is pulled into the canal as a result of rotation, even in the absence of vertical force on the part of the operator. It is very difficult to detect the moment when the screwing effect occurs during the preparation process, and this often ends in jamming and then breaking of the instrument in the canal.

Fig. 16 shows a fragment of a tool with three blades, shown conventionally (red lines). Torque M

, applied to the tool, ensures its rotational movement, while the cutting points belonging to the cutting edges move with linear velocities
V
, which are directly dependent on the distance from this point to the axis of rotation.
Each such point belongs to an elementary area located near the cutting edge, which is pressed by the normal pressure force from the substrate being cut. This force in Fig. 16 is designated as FD
and is directed perpendicular to the elementary area.
The normal pressure force FD
can be decomposed into mutually perpendicular components:
FT
, which is a force directed strictly against the velocity vector
V
, and is called the friction force;
and component FB
, which is directed towards the apical part of the canal and is called the twisting force. Thus, even in the absence of vertical force on the part of the operator, there is a force that draws the tool into the channel, which is due to rotation. The screwing force increases with increasing cutting angle ω due to a decrease in the friction force, all other things being equal. Thus, tools with a large cutting angle and, accordingly, a small cutting pitch have a significant drawback - this design increases the screwing force. This requires special care when using them in practice.

Conclusion

Today, all manufactured rotary nickel-titanium endodontic instruments and their systems, despite the general principle of structure, differ from each other in a number of design parameters and properties. At the same time, objective and comprehensive information regarding these differences is lacking in the available literature. The information provided by manufacturing companies is quite limited, is of an advertising nature and is aimed at drawing the doctor’s attention to some differences between the next instrument and the previous one. Moreover, there is often a complete lack of information about the meaning of these differences and their impact on the properties of the tool. Phrases such as “variable taper,” “positive cutting angle,” or “asymmetrical cross-sectional design” do not in themselves convey meaning unless the clinician has a basic knowledge of the design parameters of endodontic instruments. Despite the apparent complexity of this knowledge, without it it is almost impossible to understand what a particular instrument is and to assume the nature of its behavior in the channel. It is also impossible to assess how certain parameters of the same instrument interact with each other, determining its properties. This type of information is absolutely necessary for the successful use of rotary endodontic instruments in practice. Knowing the physical meaning of the main design parameters and correlating it with the design features of a particular instrument gives the doctor the opportunity to select the most suitable instrument with the properties necessary for each specific situation. Understanding these differences is necessary to assess the possibilities of correcting the shortcomings of specific instruments by improving the technique of working with them or limiting the indications for their use. This approach will reduce the number of possible errors that occur during root canal preparation and reduce the number of complications. In addition, further laboratory and clinical studies are needed regarding the comparative assessment of the main design parameters of the most common systems of rotary nickel-titanium endodontic instruments, aimed at increasing the efficiency and safety of working with them.

Used Books

1. GOST 26634-91. Rotating dental instruments. Shanks. – M.: IPK Standards Publishing House, 1991. – 7 p.
2. GOST 50350.1-92. Dental rotary instruments. Digital designation system. Part 1. General characteristics. – M.: IPK Standards Publishing House, 1993. – 17 p.
3. GOST 50351.1-92. Dental instruments for treatment and treatment of the tooth root canal. Part 1. Root files, drills, pulp extractors, rasps, channel fillers, probes and cotton needles. – M.: IPK Standards Publishing House, 1993. – 27 p.
4. Cutting tools: textbook. Benefit / E.E. Feldshtein, M.A. Kornievich, M.I. Mikhailov. – Minsk: New knowledge, 2007. – 400 pp.: ill.
5. Rzhanov E.A., Bolyachin A.V. Endodontic nickel-titanium instruments. Part I. Properties of nickel-titanium alloy. Design features of instruments. Clinical Endodontics, 2007; Vol.I., No. 3–4: 3–7.
6. Cutting theory: textbook/ P.I. Yashcheritsyn, E.E. Feldshtein, M.A. Kornievich. – 2nd ed., rev. and additional – Minsk: New knowledge, 2006. – 512 p.: ill.
7. Chow DY, Stover SE, Bahcall JK, Jaunberzins A., Toth JM An in vitro comparison of the rake angles between K3 and ProFile endodontic file systems. Journal of Endodontics, 2005; 31(3): 180-182.

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2021-09-01BASIC COURSE ON DENTAL IMPLANTATION WITH DR. FRIEDMAN FOR STUDENTS AND RESIDENTS (Annual course)

2022-02-05International Implantology Congress

The main properties of NiTi are shape memory and superelasticity (or pseudoelasticity), although the first characteristic is not used in endodontics. Superelasticity, or pseudoelasticity, is particularly useful because it gives the alloy the ability to flex and conform to the shape of the channel, allowing the channel to be formed in rotation while maintaining a centered position even in the presence of accentuated curvature. Thus, negative effects (perforations, steps) on the original channel path are minimized. Superelastic or pseudoelastic behavior depends on changes in crystalline organization. Although the use of NiTi offers several advantages, the use of these rotary instruments in endodontics may increase the risk of fracture compared with the use of steel instruments.

Fracture of a rotating instrument most often depends on bending resistance. There are many NiTi instruments available in dentistry today, in this study we used a new set of rotary instruments - ProTaper Next, as their use in endodontic treatment is very effective (Fig. 6).

Rice. 6.

ProTaper Next is the fifth generation of instruments, created using modern M-Wire technology, with a rectangular cross-section and an asymmetrical center of rotation. This tool, rotating in the channel, has a larger cutting surface than a tool with the same caliber, square cross-section and symmetrical center of rotation.

The rectangular cross-section and asymmetrical center reduce blade-to-wall contact, providing more debris clearance and increased flexibility. In addition, the new alloy improves resistance to tool cyclic fatigue, allowing you to work more safely even in highly curved canals (Fig. 7-10).

Rice. 7.

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Rice. 10.

As shown in the literature, files are not able to contact all endodontic spaces, for this reason active cleaning is necessary to maximize the cleanliness of a complex endodontic system.

What instruments are used to treat root canals?

For routine root canal treatment, the dentist must have basic endodontic equipment at his disposal. As is known, the success of endodontic treatment directly depends on the thoroughness of root canal cleaning and the reliability of obturation, which is practically impossible without instrumentation, therefore a wide variety of instruments for intracanal treatment have been created.

Criteria for systematization of endodontic instruments:

• length and cross-sectional size;
• mode of application;
• method of actuation (manual, machine);
• the shape of the working part and the top of the tool;
• the composition of the alloy from which it is made (as a rule, nickel-titanium alloy is used for instruments intended for machine treatment of the root canal).

Classification of endodontic instruments by purpose:

– for the passage of a root canal;

– for preparing a tooth cavity;

– to expand the canal mouth;

– to expand the root canal;

– for filling the root canal.

When preparing a dental cavity, fissure burs and spherical burs are most often used. Endodontic files are used for the root canal enlargement procedure.

What is included in the endodontic instrument set:

• Tweezers;
• Diagnostic probe;
• Magnifying dental mirror;
• An excavator with a small working surface (if you need to remove the roof of the tooth cavity);
• Periodontal probe for diagnosing root canal orifices;
• ironer;
• Ruler;
• Syringes for irrigating root canals and endodontic needles, which have a side hole and a rounded tip;
• Holder for small endodontic instruments;
• Cotton rolls, sterile balls.

In addition to these instruments, there are others required for endodontic therapy:

3D cleansing

The most common irrigant used for cleansing is sodium hypochlorite. Several authors have described various methods for increasing the effectiveness of sodium hypochlorite, including using more quantity and preheating.

Heated sodium hypochlorite has a greater ability to dissolve pulp tissue and clean the canal. The rate at which a chemical reaction occurs increases with increasing temperature, pressure, activation and concentration. Since the pressure within the root canal system cannot be increased, it is possible to speed up the cleansing by increasing concentration, heat and activation.

Activation is easily achieved with sound or ultrasonic sources (Fig. 11, 12). The concentration of solutions available on the market today to prevent possible irritant reactions does not exceed 6%.

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So let's move on to heating. Usually the solution is preheated to a temperature of 50°. Preheated solutions are of limited benefit as they quickly stabilize at room temperature.

New technique for heating sodium hypochlorite: working protocol

Sodium hypochlorite has a boiling point of 96°-120°. We use a heating plugger (System-B or similar). The temperature is set to 150°. The plugger used will be 30/04 so that the working length can be easily achieved without excessive preparation.

The root canal is filled with sodium hypochlorite through an endodontic needle. The plugger is inserted to a level of no more than -3 mm from the working length, and then activated. Each activation cycle lasts 5 seconds with further intervals of 5 seconds. When activated, the plugger makes short up and down movements of a few millimeters to agitate the irrigant.

The most important aspect is that there is no contact with the canal walls during activation of the plugger. After each cycle, the irrigant is replaced with a fresh solution to have a larger amount of hypochlorite with active chlorine. The activation cycle is repeated 5 times. During each activation, vapors are absorbed by the cannula.

The main indicator is the heating of the outer surface of the root in the coronal, middle, apical thirds and at the level of the apical foramen. When the irrigant was activated, the temperature on the outer surface of the root was measured with an infrared thermometer (resolution 0.1°). Using the values ​​set in the operating protocol, no external heating above 42.5° was detected. In this way, temperatures close to 47°, which are dangerous for the periodontal ligament, can be avoided. After chemical-mechanical cleansing (Fig. 13-15), we proceed to three-dimensional obturation using thermoplastic gutta-percha.

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Machine method of root canal treatment

The result of endodontic treatment in general largely depends on the effectiveness of mechanical treatment of the root canals. At this stage, the canals are completely cleaned of microbes and dentin sawdust, they are expanded and brought to the shape necessary for fixing the filling material.

There are 2 methods of mechanical treatment of canals: machine and manual. Machine preparation is carried out by rotating nickel-titanium (Ni-Ti) files with special endodontic tips and micromotors that drive them.

The most common Ni-Ti tools are profiles (Pro-File), propapers (Pro Taper), MTU system (Mtwo).

The profiles are made from a super-flexible alloy (nickel-titanium), as a result of which they have high ductility, which allows even curved channels to be processed. Greater Taper (GT-rotating files) are the latest generation of nickel-titanium endodontic instruments. They are maximally adapted for root canal preparation using the Crown Down technique.

Advantages of machine root canal treatment:

• The presence of a special top on machine files, which avoids damage to the root wall;
• The doctor’s labor costs are reduced and the time for canal preparation is reduced;
• Instruments made of a flexible alloy (nickel-titanium) allow them to prepare even curved canals (up to 90 degrees);
• The cutting edges of the instruments have a special shape, which makes it possible to effectively remove dentin sawdust and nerve remains from the canal, and ensure its complete rinsing;
• The canal is given the desired shape for three-dimensional filling with filling material.

Disadvantages of machine root canal treatment:

• special skills are required to operate machine tools;
• tactile quality control of root processing deteriorates;
• high cost of equipment and treatment;
• Initial passage and processing of canals should be performed using hand tools. Therefore, today the most correct method is considered to be a combined preparation method using manual and machine files.

Files

There are many different file types on the market.

As their name indicates (files), this tool is used for filing the walls of the canal. The instrument is inserted to the apex and, when some clamping of the instrument is felt, it is withdrawn, while the walls of the canal are scraped. In this case, the tool is either turned very little or not at all. During the preparation process, the instrument is removed, then reinserted and pressed with the working edge in another part of the perimeter of the root configuration, and in this way most of the walls are prepared from the mouth to the apical third. If necessary, the tool can also be used as an example.

K-type files

These tools are made from high quality steel wire and are sharpened into a square or triangle cross section. Then the workpiece is twisted in a spiral from 0.88 to 1.97 curls (cutting edges) per millimeter.

Recently, K-file microboring technology has been used and the result is an ultra-sharp tool with increased flexibility.

Made from triangular wire, K-files are ultra-sharp and efficient. Thanks to their increased flexibility, they fit into the channel without deviation from their course.

K-flex files

The cross-section of the blanks is diamond-shaped and, after twisting, they form sharp (less than 600) cutting edges and a blunt, non-cutting tip.

The cutting efficiency of K-flex files is higher than other types of files. This is achieved due to increased flexibility and increased ability to remove sawdust, as well as the relatively obtuse angle of the grooves, which act as a reservoir for sawdust.

The main disadvantage is the rapid loss of cutting efficiency.

Flexofile is the same K-file made of triangular wire made of high-quality steel with increased flexibility.

Flex-R file

Many root tools have a sharp tip. Removing the sharp cutting tip of the tool prevents the undesirable effect of creating ledges and perforations. The Flex-R file is deprived of the ability to form ledges, since its tip is deprived of cutting capabilities. This allows the tip to slide along the channel without penetrating into its walls. It is made of triangular wire, is very flexible and can fit even into highly tortuous canals up to the apex. This was a step forward in endodontic instrument design.

Handstrom file

It is made from a round wire billet, has rising cutting edges, and seems to consist of cones that decrease in size. Although the design of the tool may be very flexible, it is very fragile due to sudden changes in diameter, and is prone to breakage. Cutting efficiency, unlike other files, only when moving towards you. These files can be helpful in removing instrument debris and silver pins from the canal. Two files are inserted on the sides of the fragment and can be successfully removed. Effective in removing gutta-percha from canals.

S-file

Proposed by the Swedes and has an S-shape in cross-section. Manufactured by stretching. Tougher than a handstrom file. According to the manufacturers, it can perform both the function of a file with increased cutting efficiency and the function of a reamer. The tip of the instrument is 90* to form an effective canal shape in the area of ​​physiological constriction.