Vibration method for cutting teeth

(54) Description Device for vibration processing of teeth (56) Publications cited: - \ (57) Abstract:

The invention relates to a device for vibration processing of teeth and comprises a fast rotating cutting tool and a tooth excitation device. The tooth excitation device comprises an acoustic transducer (23) for generating vibrations of predetermined frequency, a vibration transmitting means (24) having a protruding front portion (24b) for actuating the tooth. The acoustic transducer (2J) has a vibration frequency higher than the tooth's own frequency. The vibration processing device meets either the condition that the cutting speed of the cutting tool is less than the maximum vibrational speed of the tooth, seen in the feeding direction, or the condition that the cutting depth of the cutting tool in the tooth is less than the amplitude of the vibration to which the tooth is subjected.

DB 603415

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The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code

449 296

The invention relates to a device for vibration processing of teeth, in particular an ultrasonic tooth excitation device for use with a fast-rotating cutting tool of known type, and a device for processing teeth with such a tool and such device.

Various cutting tools, handpieces and angular pieces have been developed for use in machining a tooth to relieve discomfort, dread and pain during machining. In addition, a number of theoretical and technological developments have been made regarding stunning. The material in the cutting tools has been changed from high-speed steel to cemented carbide tools, whereby the tool life has been improved while the cutting edge sharpening has been improved for a longer time. The cutting accuracy of the cutting edge is uniform, so that irregularities in the sharpness are avoided, which reduces the variation in the force required for a machining operation. Similarly, the manufacturing accuracy for various parts, such as handpieces and angular pieces, has been improved. This provides an improvement in the rotational accuracy of rotating parts and reduces the deflection of the axis of rotation. Improvements in accuracy in the manufacture and assembly of ball bearings enable rapid rotation with a speed of the order of 300,000 - 500,000 rpm. The development of cutting tools using diamonds has made it possible to reduce the required cutting force as a result of the high speed. An analysis of the unit's design, including detailed examination of work ability, stability and the like, helps to reduce discomfort, which results in a considerable improvement in function and efficiency.

Despite the measures taken, the discomfort, dread and pain in teeth processing remain. Since this is a mechanical machining of teeth with a cutting tool, any measure to reduce the required mechanical force, to suppress the dynamic behavior of the teeth during machining, and to reduce dynamic directional factors on the nerves, provides an opportunity to relieve the pain. To relieve the pain, one can therefore 1) fix the tooth, 2) reduce the required force and

3) use stunning. For fixing teeth, it is known to use bridges that utilize nearby teeth. However, due to the difficult preparations required, this method was used to a small extent in practice. Step 2) can be achieved by using an improved diamond sharpening tool.

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This results in some of the desired effect with reduced cutting force, reducing the dynamic appearance of the tooth and alleviating the pain. However, a patient is still subjected to discomfort, fear and pain. Therefore, if necessary, step 3) is utilized by injecting anesthetic or using nitrous oxide to suppress the pain. In recent times, however, the use of common anesthesia has often been banned due to the risk of drug shock. If use of anesthetic is allowed, longer recovery time is required. It would therefore appear that it is desirable to provide teeth processing without discomfort, fear and pain while avoiding the use of anesthesia.

Japanese Patent Specification 296,443, published October 1961, shows that a significant reduction in pain, as compared to normal high-speed rotation, can be achieved by vibrating teeth in such a way that a pulsed waveform is obtained. According to the Japanese patent specification, a cutting tool is subjected to an ultrasonic vibration in the direction of rotation with a frequency f and an amplitude a, the tool being put into high-speed rotation with a cutting speed V selected less than 2iraf. However, if the cutting tool is to meet these requirements, the size of its bracket must be increased, which presents difficulties in carrying out a careful machining by hand. This limits its use to the preparation of front teeth and dental technician work. The use of ultrasound technology in the dental field is described in U.S. Pat.

Vibration machining or vibration cutting using pulsed shear force in waveform is previously known in the field of mechanical precision machining. As shown in Fig. 1, which shows the principle of operation in two-dimensional metal machining, a workpiece 100 moves with the cutting speed v. A cutting tool 101 is vibrated in the same direction as the cutting direction shown by arrow 105, with the frequency f and the amplitude a. If the cutting speed v is chosen in such a way that v &lt;2Traf, a pulsating cutting force of waveform is obtained as shown at 106 and 107 in the right part of Fig. 1. The cutting edge of the tool 101 then initiates a vibration at an initial position 0 and out of 449 296 chip 102 during a cutting time tc relating to an arc EFA. At point A, where the vibration velocity is equal, the chip 102 begins to move away from the release surface of the tool, and after time t? relative to the starting point o, the tool begins to produce another chip 103 and a new chip 104 during a cutting time tc relating to an arc BGD. From this it can be seen that the cutting force is effective only during the time tc, while it is inactive otherwise. In this way you get a pulsating cutting force with waveform that occurs in sequence. As the cutting speed v is increased to a value where v = 2iraf the pulsating waveform disappears and is replaced by a conventional waveform which includes a high speed cutting force t with waveform P + p sin iot. The pulsating cutting force of waveform can be expressed mathematically as follows:

°0

P (t) = - i £ p + ^ P Σ - sin n it cos n mt ^{1 77} n = ¹

The dynamic process of a horizontal displacement x of a workpiece subjected to an elastic oscillation when the pulsating cutting force of waveform is applied is analyzed for a turning process shown in Fig. 2. An equation of motion for the horizontal displacement x of a workpiece 110 is written as follows:

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dt where M is the mass of the workpiece, C is the damping coefficient and K is the spring constant theft. This means that

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tc T

1 tc

P. + - P. Σ - sin n -sr π cos n ut (1)

The dynamic displacement x of the workpiece can therefore be expressed as follows:

tc T

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x

449 296 there

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About the working octagon 110 horizontal angular frequency ω_η is less than the angular frequency ω of a cutting tool 111, i.e. ω_η (= 2fffn) &lt;&lt; ω (= 2irf) you get tea

T

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..............(2)

If an applied oscillation of the tool 111 at the frequency f and the amplitude a is suppressed to allow the workpiece 110 to rotate at high speed to provide normal high-speed cutting conditions, the waveform of the cutting force will be matched by + pt its mt. Therefore, the equation of motion for the horizontal displacement x of the workpiece becomes as follows:

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+ pt sin wt

...............(3)

About the horizontal angular frequency of the workpiece ω_η in the above equation is less than frequency ω, ie (= 2irfn) &lt;&lt; ω (= 2irf), the angular force of the cutting force is obtained

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.................... (Ό

It should be noted that neither Equation (2) nor Equation (4) include time as a variable. This means that the workpiece does not oscillate with time but only moves from the starting point 0, which represents the center of rotation before the start of the operation, a distance corresponding to the static displacement as expressed in any of equations (2) and (4). If you use a regular low speed cutting operation p, the equation (4) is reduced to χ _ t₊ and it can be seen that the workpiece thereby extends to a large extent over time, which deteriorates the machining accuracy. It should be noted that the requirement for high-speed operation is due to the desire to make the workpiece static as given in equation (4). It is an object of the invention to provide this fact as the basis for teeth processing. The waveform of the pulsating cutting force is used to perform a machining operation, since the workpiece

449 296 then a short distance is moved to the position represented by equation (2), where the displacement is reduced relative to the displacement according to equation (4) by a factor tc, 1. 1&gt;.

T 10 'to obtain a static effect.

A closer study of a corresponding model of a machining operation with a lathe, as shown in Fig. 2, reveals that a spring k and a damping member C correspond to the area of ​​the root membrane between the jaw bone and the teeth, the workpiece 110 corresponding to a tooth. Therefore, in carrying out a high-speed cutting operation through the high-speed rotation of the cutting tool, the displacement of the tooth x, due to the relation according to equation (4), will exhibit slight oscillation, so that the pain is relieved accordingly. This corresponds to a high-speed dental machining using a diamond tool, which can now achieve as high a speed as 300,000 - 550,000 rpm. However, a study shows that the high-speed power indicated by Equation (4) cannot be achieved in machining a tooth if this technique is used without the use of anesthesia unless the tool speed is increased in magnitude. High speed rotation within this speed range gives a high cutting tone, and the heat generated by the friction reduces the cutting tool's service life. In addition, the sharpness can be changed, giving variations in the size of the cutting force P. This results in an uneven movement of the tooth, which in turn causes pain. Therefore, close adjustments of the rotational accuracy of the rotating parts of the handpiece or angular piece or frequent replacements of the cutting tool are required, and in some circumstances, anesthesia must be used as pain occurs. However, as can be seen from equation (2), the displacement of the tooth x during a vibration operation is as follows:

tc. p

Even assuming that the machining force or cutting force Pt is the same under a normal high speed machining with the same conditions, i.e., the area of ​​the tooth being machined, feed rate and tool design, which when using vibration machining (although the size of P in practice) becomes the apparent processing power or cutting force that affects the displacement x of the tooth ητ- P_t{j ^ -ΡΡ)} as shown in Equation (5), which gives

449 296 a considerable reduction in cutting force compared to the usual method. It has been found that vibration processing of a tooth, where a pulsating cutting force with waveform is applied to the tooth, which is subjected to an elastic oscillation, reduces the perceived pain compared to a normal high speed processing. On the basis of this discovery, Japanese Patent Specification 296,443 disclosed above discloses a method for teeth processing, which method involves subjecting a cutting tool to a rotating ultrasonic vibration of frequency f and amplitude a while rotating the tool at a cutting speed V less than 2vaf . However, a device for vibrating teeth to perform this procedure requires a larger size bracket which prevents the use of this method in practice.

The object of the invention is to provide a device for processing a tooth substantially without causing pain to the patient.

Another object of the invention is to provide a tooth excitation device useful in conjunction with a known high speed rotary cutting tool.

According to the invention there is provided a device for vibration processing of teeth and comprising a cutting tool which is arranged to rotate at high speed and has a tip for machining a tooth. The device is characterized according to the invention in that a tooth excitation device is arranged to be brought into contact with a tooth for excitation thereof, which tooth excitation device consists of a cylindrical housing with an opening at the front end, an acoustic converter in the housing for generating vibrational energy at a predetermined frequency, a vibration transfer means having a rear portion located in the housing, the end of which is connected to the transducer; and a front portion projecting through the opening in the housing, the free end of which is arranged to be brought into contact with the tooth, support means for attaching a vibrator of the acoustic transducer and the transfer means to the housing at a balance point, and electrical means for supplying energy to the tooth. the acoustic transducer, which has a vibration frequency higher than the tooth's own frequency, co-cites the vibration transmitting means, so that the vibration amplitude at its free end is at most 30 µm, and either the vibration processing device satisfies either the condition that s &lt;2iraf, where s is the feed rate of the cutting tool, a is the amplitude measured at the free end of vibration transfer.

449 296 means, f is the vibration frequency of the acoustic converter and 2iraf is the greatest vibrational velocity of the tool, seen in the feed direction, or condition that the cutting depth of the cutting tool is so adapted to the amplitude a that t &lt;a. of an ultrasound • • c.csl I. Lalor nom produces or v ibration; energy in the ultra.1 Judaism area.

In accordance with the invention, the vibrational energy supplied to the tooth by the ultrasonic transducer through the vibration transmitter means that its apparent spring constant is increased to a high value so that the tooth is held in a manner corresponding to it being fixed in a fixed position. A machining is done under these conditions, so that the cutting tool provides a pulsating cutting force with waveform, and dental treatment can be performed without exposing the patient to significant pain. In carrying out machining in accordance with the invention, it is desirable that high pressure water is sprayed onto the tooth and cutting tool in a manner similar to a conventional machining.

Before describing the invention in detail, a summary of the theoretical background to the invention is made below. As a result of basic research on the technique for the relief of pain in the treatment of teeth, the inventor has successfully provided a model for a pain transmission system in the processing of a tooth with a cutting tool, which tooth is connected to the jawbone by means of the root canal and is made up of cement, dentine, dental pulp and enamel. Fig. 3 shows such a model. A tooth of mass M is attached to a jaw bone 1 by means of a spring 2 having the spring constant k and a damping member 3, the spring and damping member corresponding to the root membrane, and the mass of the tooth is surrounded by cement 4 and enamel 11. Between the enamel 11 and the dentin 5 there is tomeska threads 9 and odontoblasts 8, which are supported by dentine fluid 10 in dentine ducts and which are supported by a group of nerve fibers in the dental pulp by means of a spring 6 having the spring constant k and a damping member 7 with the damping constant c. Static or dynamic displacement of the tomes threads 9 and odontoblasts 8 produces a tension in the spring 6 in the nerve fibers of the dental pulp. The magnitude of the voltage can be measured with a receiver 1J in the nervous system, which receiver corresponds to a wire strain sensor. The voltage can be amplified by means of an amplifier 15 with frequency reproduction and then recorded on a paper strip 17 in a recording device 16. It is believed that the height of the resulting waveform is proportional to the degree of pain in the dental pulp. Further, a displacement of the spring k can be measured by means of a wire strain sensor corresponding to a size up

449 296 of the nerves in the rhombus, so that in this way it is possible to determine the voltage in the spring 2. The magnitude of this voltage can be amplified by an amplifier 14 with frequency reproduction and recorded on the paper 17 · It is considered that the height of the waveform obtained is proportional to the degree of pain in the nerve group of the rhythm. It will be apparent that this creative model enables recognition of the pattern of the recorded waveform, assuming that the degree of pain is proportional to the height of the waveform. In this way, the pain can be relieved by attempting to reduce the height of the peaks in the recorded waveform. When a tooth is processed with a cutting tool 19, the movement of a recording pen in the recording device will be non-existent, so the recorded waveform is linear, meaning that the pain is completely eliminated, provided that no tension is produced in the spring 2 in the root or spring 6 in the tooth pulp. . This corresponds to dental work when the pulp and thus the spring 6 have been removed and the tooth is attached to the jawbone, so that no changes in the spring 2 can occur.

In general, a tooth that is processed has different inflammations of the root membrane, which means that the spring is tensioned as a result of extension or compression compared to the spring position for a balanced and stress-free, healthy tooth. As a result, the waveform recorded on the paper undergoes a periodic oscillation and the patient is exposed to pain. An intense pain arises as a result of an examination that imposes very little external force. Fixing the tooth using a bridge that is mounted between nearby teeth can involve contact with the tooth in question, causing intense pain. It is therefore clear that a reduction in the magnitude of the cutting force is the first measure to be considered for reducing the pain. Since the cutting edge of the tool comes into direct contact with the tooth in question to impose a force on it in accordance with Newton's law, the use of the pulsating cutting force with waveform in accordance with this cutting theory and technology cannot reduce the movement of the tooth or spring 2 to zero even if the displacement is greatly reduced as shown in equation (2). In a case where the spring 2 in the root canal is healthy but the spring 6 in the dental pulp has different inflammations that give either extension or shortening, so that the spring is tensioned, the waveform recorded on the paper undergoes a peroidal oscillation and the patient suffers from stabbing pain. When a treatment is started at the enamel on the surface of the tooth, intense pain will immediately

449 296 is felt in the boundary region against the dentin if the tooth itself or the spring 2 is fixed, such as when the tooth is fixed. This can be explained as a result of an elastic displacement or oscillation of the spring 6, which is effected by dynamic displacement of the tomesian fibers 9, which represents a group of small masses located in the dentine fluid 10, which displacement is effected by the fluid flow and the application of the cutting force. , and by a dynamic displacement of the odontoblasts 8, which displacement is effected by the cutting force. In order to reduce the arising pain, it will appear from FIG. 3 that the elastic movement or oscillation of the spring 6 must be minimized. Since the very purpose is to minimize the elastic displacement or oscillation of the odontoblasts 8 and the tomes fibers 9, it will appear that the use of the pulsating shear force of waveform in accordance with equation (2) is the best way to carry out the processing. However, some static movement of the spring 6 is impossible to avoid. Injection of anesthetic or the use of nitrous oxide must be resorted to if the pain cannot be cured even though the best way to reduce the cutting force is used. Referring to Fig. 3, the use of anesthetic means corresponds to breaking the connection with the amplifiers 14, 15 in the transmission circuit of Fig. 3, which prevents any signal from being fed to the recording device 16. The use of nitrous oxide corresponds to interruption of the connection to the recording device 16, which in turn prevents any pain signal from being recorded.

In order to determine a specific technique that further reduces the cutting force, an experiment has been carried out. Block-shaped samples of ceramic materials, including magnesium oxide (Mohr hardness 6), mullite (Mohr hardness 7)_} zirconia (Mohr hardness 8) and alumina (Mohr hardness 9) were glued using epoxy resin at the free end of an amplitude amplifying horn, which is subjected to an ultrasonic vibration with a frequency of 20 kHz and an amplitude of 8 µm. A 1 mm diameter diamond tool is pressed against the sample at a constant load of 0.4 N. By using a combination of an electric motor and an air turbine, the tool is operated at a speed varying between 10,000 and 300,000 rpm. When the direction of the applied load coincides with the direction of vibration, the depth in the obtained depression is measured. At alumina and a rotation speed of 300,000 rpm, h = 500 µm was measured.

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The depth h shows an increase in proportion to an increase in the speed of the tool and the applied load. As the speed is increased by a factor of about four, the depth h of the depression increases by a factor of about two, which appears to be characteristic of hard and brittle materials. If the ultrasonic vibration of the ceramic sample is suppressed and the diamond tool is allowed to rotate in the usual way, a shallow depression of h = 200 µm is obtained. The ultrasonic vibration of the ceramic sample provides an increase in the depth of the obtained depression by as much as 2.5 times what can be achieved with ordinary machining without the use of ultrasonic vibration. The fact that the working depth increases at the same applied load means that the resistance to machining decreases. If one tries to achieve such a result only by increasing the speed, then as high speed as

900 000 rpm, which cannot be achieved with air turbine technology, which gives an upper limit of the order of 550 000 rpm. The ultrasonic vibration of the ceramic specimen drastically improves the sharpness of the diamond tool at the now useful speeds. If the same sharpness is to be maintained, the speed can be lowered from 300,000 to 10,000 rpm. Since the tool life is proportional to the cutting speed, this means that the tool life can be increased. Therefore, it is found that ultrasonic vibration of a hard and brittle material improves the sharpness of the cutting tool or increases the life of the tool.

A homogeneous quality ivory specimen is subjected to an ultrasonic vibration with a frequency of 28 kHz and an amplitude of 8 µm. A diamond tool with a diameter of 1 mm and rotating at a speed of 30,000 rpm is pressed against the specimen with a load of 0.9 N for a specified time. When compared to results obtained with ordinary high-speed rotation, the depth of the formed depression (H) is increased by a factor of approx.

2.5 to 5, but this depends to some extent on the direction of the fibers in the ivory piece. Thus, unexpected results are obtained in ivory as well as in ceramic materials. Similarly, various teeth of patients have been glued with the aid of araldite to the free end of a horn subjected to a longitudinal ultrasonic vibration with a frequency of 28 kHz and an amplitude of 8 µm. A diamond tool with a diameter of 1 mm and rotating at a speed of 300,000 rpm is pressed against each tooth with a constant load of 0.9 N for a predetermined time. When compared to conventional cutting technology without the use of ultrasonic vibration, it has been found that an increase in the depth of the obtained effect is obtained.

449 296 ft. Depth in a similar manner to that of ceramic materials and ivory. The use of ultrasonic vibration has thus increased the depth by a factor of about 2-6 compared to previously known high speed rotation. This means that the resistance to the cutting is reduced by a factor of 2 - 6. This may be due to a decrease in the mechanical strength of the tooth itself, for example the tensile strength, which is caused by stresses and stresses due to the ultrasonic vibration, which affects the cutting conditions of a hard and brittle material during cutting or machining.

For application of the invention to a tooth, a method of ultrasonic vibration is described below to process the tooth which is connected to the jawbone through the root membrane. The tooth's own frequency fn is calculated taking into account the mass M of the tooth and the spring constant k of the spring 2 in the root membrane. Measurements have found that it has a value between 300 and 2000 Hz. Thus, the tooth is excited for oscillation at a higher frequency than fn. An actuator 21 is caused to pivot in the high frequency direction shown by an arrow 22 in the ultrasonic region and is lightly pressed against the tooth as shown by arrow A, so that the tooth can be subjected to an ultrasonic vibration with the actuator 21 frequency f and with sinusoidal waveform with an amplitude near the amplitude of the actuator a. For example, the tooth's ultrasonic vibration may have f = 60 kHz and a = 4 µm. A diamond tool 19 which rotates at high speed is brought into engagement with the tooth in the direction shown by an arrow 20 and a substantially constant feed rate S, but some variation in the feed rate is inevitable as a result of manual operation. By selecting the feed rate S and the maximum vibration rate 2 iraf, seen in the feed direction, such that S &lt;2iraf and by selecting the cutting depth t for the cutting tool in relation to the amplitude, that t &lt;a, one can provide a machining mechanism. where the rotating tool is alternately brought into contact with the tooth and alternately moved away from it, providing an efficient pulsating cutting force with waveform. Thus, when analyzing the feed direction of the cutting tool in the form of a vector with respect to the direction of tooth excitation, the first difference applies when the feed direction is parallel to the direction of tooth excitation, while the second difference applies when the feed direction is perpendicular to the direction of tooth excitation. In this way, the displacement can be reduced to tj as shown in equation (2), and the cutting force can be reliably

449 296 tc is reduced by a factor -ψ-, for example, a 1/3 - 1/20, compared to the cutting force P which arises with ordinary high speed rotation without using the invention.

It is generally known that when a wire strain sensor is adhered to a test piece of elastic material subjected to a high frequency vibration such as within the ultrasonic range and a change in the voltage due to a change in the resistance, the wire strain sensor is amplified by a recording amplifier. on a paper for determining the frequency and / or amplitude of the sample vibration, accurate indication or recording can be prevented and the indication or recording result may be lower than the actual amplitude of the vibration if the amplifier, indicator or recording device has poor frequency reproduction. This can be explained if one considers that an alternating current meter of 50 Hz cannot be used to give accurate determination of the absolute value of a high frequency current. When using an electromagnetic oscillograph, a recording pen, if using a galvanometer with an intrinsic frequency as low as 500 Hz to record an ultrasonic current of 20 kHz on a paper, will not perform any displacement proportional to the current flow without remaining at a point corresponding to the starting position. A similar phenomenon also occurs in the pain transfer system shown in Fig. 3. If a detector corresponding to a wire strain sensor is used to detect vibrations of a tooth vibrating at a frequency of 60 kHz and amplitude 4 µm, in combination with an amplifier corresponding to a sensory nerve and a recording device corresponding to a sensorium, both of which have poor frequency reproduction, you get little movement of the recording pen depending on the vibration of the tooth and the pen is essentially stationary and gives no curve on the recording paper. This means that if the tooth is moved statically at 8 pm, you will experience a pain, but if the tooth is subjected to an ultrasonic vibration at the frequency of 60 kHz and the amplitude 4 pm, the resulting movement cannot be sensed, which means that no pain occurs. This effect apparently corresponds to the anesthetic effect obtained when injecting anesthetic or using nitrous oxide, and the effect is referred to below as dynamic anesthesia.

As previously mentioned, the intrinsic frequency of a tooth is in the range between about 300 and 2000 Hz. For example, assuming that the frequency of the ultrasonic vibration applied according to the invention is 60 kHz, the tooth will “vibrate

449 296 at this frequency, which means that the apparent apparent frequency of the tooth is increased during processing. There is an expression ω_η = / k / M where ω_η is the tooth's own frequency when twisting, k is the spring constant of the root and M is the mass of the tooth. If the intrinsic frequency of the tooth to be treated is 600 Hz, it follows that the spring constant has apparently increased by a factor of 60,000/600 τ 100 or about 10,000. In other words, the apparent spring constant of the tooth is increased to a value approximately 10,000 times greater. than the spring constant when vibrational energy from an ultrasonic oscillator is not applied. In other words, the wave-shaped vibration applied to a tooth corresponds to the fixation of the tooth in accordance with the known bridging method to enable the tooth to be processed while being held still. A rounded tip on an actuator which oscillates longitudinally at the frequency of 60 kHz and the amplitude 4 µm and gives an output of 20 W is brought to light contact against a tooth to be processed to impart an ultrasonic vibration to it. A diamond tool with a diameter of 1 mm is rotated at a speed of 300,000 rpm and is fed with reduced pressure on the tooth subjected to ultrasonic vibration, mainly in the cutting direction. This provides a pulsating cutting force with waveform, which reduces the cutting force which gives rise to dynamic displacement and thereby pain, and in addition, the cutting force is significantly reduced by using the dynamic properties of the tooth vibration system. In this way, the shape of the ultrasonic oscillation in the tooth, which is produced by a small output power of the order of 20 W, is not affected, but the usual ultrasonic vibration is maintained while increasing the apparent spring constant to produce a dynamic stunning effect. In this way, the pains that would otherwise arise in the treatment of the tooth will be drastically reduced or completely eliminated. It is necessary that dynamic displacement of the tooth due to the cutting force be kept in the same order of magnitude as the amplitude of the ultrasonic vibration, which in this example is 4 µm. For ordinary machining, amplitude values ​​of 4 - 10 µm are satisfactory. If the cutting force becomes too great. For example, the ultrasonic vibration of the tooth can be impaired so that the amplitude decreases. If the amplitude is reduced to 0, the power of the invention disappears. In this case, it is necessary to restore the ultrasonic vibration of the tooth by increasing the output power or amplitude.

Fig. 1 is a schematic illustration of the principle of cutting vibration processing applied to a two-dimensional

449 Fig. 296 shows a cutting operation in a metal workpiece; Fig. 2 shows schematically a corresponding model in a cutting vibration processing by means of a lathe; Fig. 3 schematically shows a corresponding model of cutting vibration processing of a tooth in accordance with the invention, and Figures 4, 5 and 6 are schematic, partially cut side views of a tooth excitation device according to various embodiments of the invention.

As can be seen from the above description, the device for vibration processing of teeth according to the invention can be constructed using a dental processing tool of known kind, which rotates at high speed, and a tooth excitation device according to the invention. The machining tool or cutting tool is well known and therefore does not require a detailed description. Therefore, only the tooth excitation device will be described in detail. In the various embodiments which will be described below, the corresponding details are denoted by the same reference numerals.

Fig. 4 shows the use of a longitudinal ultrasonic vibrator 23 and an actuator 24 arranged to perform a longitudinal ultrasonic vibration and having a length adapted to resonate at the frequency of the vibrator 23. The vibrator 23 may be an electrostrictive or magnetostrictive vibrator and may have a vibration frequency greater than 20 kHz, i.e. in the ultrasonic range. The use of such a high frequency makes it possible to design the tooth excitation device according to the invention with lower weight and smaller size. The actuator 24 may be formed as a metal bar or may in some cases be a bar of non-metallic material subjected to elastic deformation. The actuator 24 includes a rear portion 24a connected to the vibrator 23 as well as a front portion 24b with a free end 24c. Since the free end 24c is to be brought into contact with a tooth 25 to transmit vibration to it without damaging the tooth, the free end is rounded so that the tooth surface cannot be damaged. In some cases, the free end may have a curved surface for surface contact with the tooth so that the contact is stabilized. Since the free end of the actuator is not subject to abrasion, as opposed to a cutting tool, there is no reason to replace it, and therefore it can be made integrally with the actuator 24a. Different diameters can. required depending on the application. Therefore, the front portion 24b can be made separate from the actuator 24a and coupled thereto with a threaded joint or a conical coupling. The excitation 449 296 device further comprises a pair of connecting means 26 which can be connected to the output of an ultrasonic oscillator. Next, the free end of actuator 24b performs an ultrasonic vibration in a direction indicated by arrow 27. A holder or housing 28 is provided at a vibration node of actuator 24a. The vibrator 23 and the rear portion 24a of the actuator 24 &lt;'k' disposed inside, the casing 28, and the front portion 24b project forward through an opening 28a in the casing 28. When the housing is held in the hand, the free end 24c of the actuator 24 can be easily pressed against the tooth 25, as indicated by the arrow 29, to provide a vibration of the tooth, which is then subjected to a machining or cutting operation by a cutting tool 31, which may be formed. of a diamond grinding wheel rotating at high speed as indicated by arrow 30.

If the front of the tooth 25 is covered by a metal crown, it may be necessary to bring the actuator 24 into contact with the back surface of the tooth, which is not covered by the crown, since the presence of a metal crown may impair the transmission of the vibration. Fig. 5 shows a device which can be used in such a case. This includes a longitudinally acting ultrasonic vibrator 23, a longitudinally vibrating horn 32 having a length that resonates at the frequency of vibrator 23, and an actuator 33 designed for bending resonance and attached to the free end of the horn 32 at an antinode thereof. At the free end of the actuator 33, which corresponds to an anti-node, is a projection 33a, which is rounded in the same way as the free end of the actuator 24 according to Fig. 4. The projection 33a performs an ultrasonic vibration in the direction indicated by the arrow 27. The actuator is pressed against a tooth as shown by the arrow 29 to provide a vibration in the tooth 25 while being subjected to machining by a high speed rotary cutting tool. As shown, the horn 32 comprises a rear portion 32a, the end of which is connected to the vibrator 23, and a front portion 32b, at which end the actuator 33 is attached.

Fig. 6 shows a tooth excitation device which includes a longitudinal vibrator and a curved actuator 34 for increasing the amplitude. The actuator 34 has a tip 34a arranged to vibrate in the direction shown by arrow 27 · When the tip is easily pressed against the tooth 25 as indicated by arrow 29 to provide vibration in the tooth, it can be machined with a high speed rotary cutting tool.

449 296

If only high-speed rotation is utilized, as is the case with a conventional air turbine, any reduction in tool sharpness will cause discomfort and pain. This determines the tool's service life, which means that the tool often needs to be replaced. However, using the present invention, the tool life is increased by a factor of 3 - 10. When the waveform pulsed shear force is used, the heat generated during processing also undergoes a pulsating process, and a frequency response occurs in the heat transfer system in a similar manner to the frequency response in a dynamic system, which drastically reduces the pain due to the heat generated. By means of the invention, teeth which previously had to be treated under anesthesia can be processed without the use of anesthesia and without causing pain, discomfort or fear. This is especially important as the use of anesthesia must be avoided in view of the risk of drug shock. Another advantage is a significant reduction in the sound produced during processing, which has a psychological effect.

The effects of the invention can be described with particular results:

1) A cavity can be most appropriately designed in accordance with the invention in a patient suffering from morbidly increased tooth irritability and feeling pain when the tooth is touched.

2) The invention can be applied to a tooth with caries of the third degree with pain sensations upon touching, wherein the upper part of the pulp can be processed without pain.

3) Treatment can be done without anesthesia in teeth with acute purulent inflammation of the root membrane, chronic sore inflammation of the dental pulp, necrosis of the dental pulp, partially acute permanent inflammation of the dental pulp, etc.

As can be seen from the model shown in Fig. 3, the springs and 6, in addition to the deliberately applied cutting force, can be subjected to extension or shortening for various reasons, for example by inflammation in the surrounding area, which gives the springs an extension. When stresses are created in this way and cause stinging pain, the actuator of the invention can be applied to the tooth in question, whereupon an ultrasonic vibration is applied with an amplitude corresponding to the size of the stresses so that the pain is relieved.

The invention has been described above in connection with the use of ultrasonic vibrations at a frequency of at least 20 kHz, which is in the ultrasonic range, but the invention can also be applied with corresponding results using high frequency

449 296 which is at least three times the tooth's own frequency.