J Med Dent Sci 2006; 53: 93–101
Original Article
Influence of Number of Dental Autoclave Treatment Cycles on Rotational
Performance of Commercially Available Air-Turbine Handpieces
Masahiro Nagai and Kazuo Takakuda
Department of Biodesign, Division of Biosystems, Institute of Biomaterials and Bioengineering, Tokyo Medical
and Dental University
The influence of number of autoclave treatment
cycles (N ) on rotational speed and total indicated
run-out of commercially available air-turbine
handpieces from five manufacturers was investigated at N =0, 50, 100, 150, 200, 250 and 300
cycles, and the significance in the test results was
assessed by Dunnett’’s multiple comparison test.
Some air-turbine handpieces showed the significant differences in rotational speed at N =300
cycles, however, the decreases of the rotational
speeds were only 1 to 3.5 percent. Some air-turbine
handpieces showed the significant differences in
total indicated run-out, however, the respective
values were smaller than that at N =0 cycle.
Accordingly, it can be considered that the ball
bearing in the air-turbine handpieces is not affected significantly by autoclave. To further evaluate
rotational performance, this study focused on the
rotational vibration of the ball bearing components of the air-turbine, as measured by Fast
Fourier Transform (FFT) analysis; the power spectra of frequency of the ball’’s revolution, frequency
of the cage’’s rotation and frequency of the ball’’s
rotation were comparatively investigated at N =0,
150 and 300 cycles, and the influence of autoclave
was evaluated qualitatively. No abnormalities in the
ball bearings were recognized.
Key words:
Autoclave sterilization, Rotational
speed, Total indicated run-out, Air-turbine handpieces.
Corresponding Author: Masahiro Nagai
2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan
Received December 6, 2005; Accepted March 17, 2006
Introduction
Social awareness of infectious disease such as
hepatitis B, hepatitis C and acquired immune deficiency syndrome (AIDS) is increasing. In dental fields, there
has long been concern that patients may contract
such infectious diseases via contact with poorly sterilized instrument or appliances, particularly because
patients commonly bleed during dental treatment.
Furthermore, dentists, dental hygienists and assistants are also at risk of contracting such diseases via
contact with patient blood and saliva. Therefore, sufficient measures to protect against infection must be
taken1,2.
In light of this social background, manufacturers of
air-turbine handpieces have actively improved their
products to facilitate autoclave sterilization. Dentists
also understand the necessity of sterilizing air-turbine
handpieces after each patient. However, there is
some concern among dentists that if air-turbine handpieces are sterilized frequently under severe environmental conditions, such as those present in an autoclave, the rotational performance is adversely affected.
In order to alleviate such misgivings, we performed a
recent literature search regarding the durability of airturbine handpieces with sterilization, but were able to
find only three reports3-5. Two reports3,4 did not focus on
the rotational speed or eccentricity of the test mandrel;
the testing criteria focused on the number of cycles until
use of the air-turbine handpiece was impossible.
Accordingly, no correlations between changes in rotational performance and sterilization treatment could be
determined. The third report5 focused on the rotational
speed, the stall torque, the bearing resistance, the
94
M. NAGAI and K. TAKAKUDA
noise and the light output of fiber-optic of the air-turbine
handpieces during the course of daily use in general
dental practice. However, the report5 did not focus on
the eccentricity of a rotational tool.
In the present study, we investigated the influence of
the number of autoclave cycles on the rotational performance (rotational speed and eccentricity) of the most
commonly used commercially available air-turbine
handpieces in Japan. Furthermore, we diagnosed the
rotational conditions of ball bearing in the air-turbine
handpieces by using the Fast Fourier Transform (FFT)
analyzer.
MATERIALS AND METHODS
Air-turbine handpieces
The air-turbine handpieces used in this study were
commercially available products. They were made by
four Japanese manufacturers and one German manufacturer, as shown in Table 1, with two products coming
from each manufacturer. The air-turbine handpieces
made by Osada and Yoshida used steel ball bearings,
while the air-turbine handpieces made by Morita,
Nakanishi and Kavo used ceramic ball bearings. In
addition, the air-turbine handpieces made by Kavo were
equipped with an automatic air pressure regulator.
Furthermore, all the air-turbine handpieces included a
device in the head that prevented oral fluids and other
foreign substances from entering.
J Med Dent Sci
Autoclave sterilization conditions
The sterilization conditions for small medical auto6
claves are specified in JIS T7324-2005 . In addition, the
sterilization conditions for air-turbine handpieces, as
specified in “Dental handpiece - high-speed air-turbine
7
handpieces” (JIS T5906 ), call for a sterilization temperature of 132±2°C and a sterilization pressure of 0.2
MPa for 5 min. Rotational performance must not deteriorate after autoclaving, even if sterilization treatment is
repeated for 250 cycles. In this study, the autoclave
used was the Super Clave HF260 (Hillson Dec Co.) and
the sterilization temperature and pressure were set at
132±2°C and 0.2 MPa, respectively, as specified in JIS
T59067. However, sterilization was performed for 15 min
and the number of cycles was 300. These conditions
were severer than the JIS regulations7.
Testing procedure
For the repeated sterilization test, one cycle (N=1
cycle) was as follows. First, the air-turbine handpieces
were lubricated with turbine spray using nozzles provided by the manufacturers for 2 s, were immediately
rotated without applied load for 30 s after inserting a
test mandrel (Í1.6 × 19 mm), and were sterilized for
15 min in the autoclave after removing the test mandrel.
When setting the air-turbine handpieces into the autoclave chamber, all air-turbine handpieces were tilted at
about 20°, with the heads placed upward, in stainless
steel container. The container was then set in the autoclave chamber. In addition, the positions of the air-turbine handpieces in the container were changed after
Table 1. Commercially available air-turbine handpieces used in this test.
INFULUENCE OF DENTAL AUTOCLAVE ON AIR-TURBINE
every cycle in order to minimize the influence of air-turbine handpiece position. After each cycle, the container in which the air-turbine handpieces were placed was
immediately removed from the autoclave chamber
and was allowed to cool to room temperature.
Lubrication was repeated as described above, and this
was repeated for 300 cycles. None of the air-turbine
handpieces were subjected to load during testing.
This is because the present study was conducted in
order to investigate the influence of autoclave on the
ball bearing parts of the air-turbine handpiece.
Evaluation method
Rotational performance of the air-turbine handpieces was evaluated based on rotational speed,
eccentricity of test mandrel in rotation and power
spectral analysis of the ball bearings supporting the
body of the rotating air turbine, without applied load.
Rotational speed and eccentricity were measured
every 50 cycles, and power spectral analysis was performed every 150 cycles using sound level meter (NA40, Rion Co.) and FFT analyzer (SA-74, Rion Co.).
Rotational performance tests were carried out as follows: 1) Air-turbine handpieces were rotated without
applied load; however, at cycle N=0, when the rotational
speed of each air-turbine handpiece reached the
manufacturer’s recommended rotational speed (R* in
Table 2) by adjusting the compressor air valve, the supply air pressure, po, was measured using a digital pressure gauge connected at the coupling joint of the airturbine handpiece. This value (po) was used when supplying the air pressure used to rotate the respective
air-turbine handpieces in the rotational performance
Table 2. Each manufacturer’s recommended rotational speed, R *
and the supply air pressure, po at N=0 cycle.
95
tests. In Table 2 the measured values of R* and po for
each air-turbine handpiece are shown.
2) When arriving at the predetermined number of
autoclave cycles, the air pressure of each air-turbine
handpiece was adjusted to po via the compressor air
valve (Table 2).
3) After the rotational speed of each air-turbine
handpiece became stable, as the rotational performance tests, the rotational speed, RN, and the total indicated run-out, δN, were measured simultaneously
according to the testing methods depicted in Fig. 1. The
tests were performed continually from 30 to 50 s and
was performed three times. Power spectrum was
measured based on the sound generated from the airturbine head.
Test mandrels (φ1.6×19 mm) having the dimensions
specified in the International Standard (ISO8) and JIS7
documents were prepared. The test mandrels were
magnetized using a permanent magnet and were
inserted into the spring-type chuck of the air-turbine
handpieces. The necessity of such magnetization
depends on the type rotational speed sensor used. For
measurement of rotational speed and total indicated
run-out, as shown in Fig. 1, after fixing the air-turbine
handpiece with a fastening device in order to maintain
the test mandrel in a horizontal position, the rotational
speed sensor was set below the test mandrel. The
laser beam of the laser displacement meter was
focused on the test mandrel 6 mm from the air-turbine
head, and both tests were carried out after the air-turbine handpiece was activated. The laser displacement meter measures the distance between specific
points on the test mandrel. And it was able to follow the
movement of the test mandrel in direction of the laser
beam axis, because the laser displacement meter
shown in Fig. 1 had a maximum response frequency of
10 kHz. The “peak-to-peak” output mode was thus
selected for the laser displacement meter. This output
mode measures the difference between the maximum
and minimum values over the measured distance,
and thus it was possible to measure the total indicated
run-out change of the test mandrel. The data shown in
Fig. 1 were transferred to a personal computer via the
sensor interface and were converted to rotational
speed and a total indicated run-out‐time diagrams,
respectively. For power spectral analysis, the air-turbine
handpiece was hung in balance from the ceiling. The
rotational axis of the test mandrel coincided with the
direction of gravity. A microphone of the sound level
meter to record sound levels was set at a position 0.45
m away from the air-turbine head. The power spectra of
96
M. NAGAI and K. TAKAKUDA
J Med Dent Sci
Fig. 1. Measuring methods of rotational speed and total indicated run-out. [Rotational speed meter: JADAS-701, Ogura jewel Ind., Laser sensor: LC-2440, Keyence Co., Laser displacement meter: LC-2400, Keyence Co. and Sensor interface: PCD-320A, Kyowa E. I. Co.]
predicted frequencies described later and a major
spectrum with comparatively large power value were
observed scientifically in the screen of 10-kHz and 20kHz display modes of the FFT analyzer, and then the
two images were recorded immediately. Furthermore,
those images were compared with those at N=0
cycle.
FFT analysis
In the case of ball bearings in the dental air-turbine
handpieces, the outer ring is fixed and the inner ring is
rotated at a predetermined rotational speed. The rotational speed of the air-turbine handpiece, fr, the frequency of the ball’s revolution, fa, the frequency of the
cage’s rotation, fb, and the frequency of the ball’s rotation, fc, were calculated from the following equations:
(1)
(2)
(3)
where, d is the ball diameter, D is the pitch circle of
the ball and α is the contact angle of the ball. If the ball
bearing has a dent or a scratch on the outer ring, and
when number of balls is z, the frequency, fd, that these
balls pass over the dent or scratch is given by the following equation:
(4)
In addition, if the ball bearing has a dent or a
scratch on the inner ring, because the relative rotational
speed of the inner ring for the frequency of the ball’s
revolution is (fr ‐ fa), where, the number of balls is z,
the frequency, fe, that the balls pass over the dimple or
scratch is given by the following equation:
(5)
The frequencies, fa, fb, fc, fd and fe of each air-turbine
handpiece in five manufacturers were predicted by
using Eqs.(1) to (5), where, fr converted the R* values
shown in Table 2 to Hz, d=1 mm, D=4.76 mm, α=0°
and z=7. Here, the numerical values for the ball bearing
were provided by the five manufacturers.
Statistical analysis
The significances of the rotational speed (RN) and
the total indicated run-out (δN) at N=50, 100, 150, 200,
250 and 300 cycles were assessed by Dunnett’s multiple comparison test; free software:R9 (significance
level: α =0.05) in comparison with RN and δN at N=0
cycles for control group, in order to examine influence of
autoclave sterilization of an air-turbine handpiece.
INFULUENCE OF DENTAL AUTOCLAVE ON AIR-TURBINE
RESULTS
Rotational speed and total indicated run-out
In the time-series diagrams, there were almost no
changes in rotational speed and total indicated run-out
during the course of measurement in all air-turbine
handpieces. Table 3 shows the means of three test values of the rotational speed (RN) measured at the predetermined number of sterilization treatment cycles
(N=0, 50, 100, 150, 200, 250, 300 cycles) with all airturbine handpieces used in this study. The numerical
values in parenthesis under each RN value indicate
standard deviation. The value of RN for each air-turbine
handpiece fluctuated as sterilization cycles increased.
However, the standard deviations for all RN values were
extremely small. This may be because establishing
equal air pressure at cycle 0 was difficult due to fluctuations in the digital air pressure gauge reading. When
the air pressure was fixed, rotational speed was stable,
even when measurement was repeated 3 times.
Consequently, small standard deviations were
obtained.
Table 4 shows the mean total indicated run-out val-
97
ues (δN) calculated by the same procedure as for rotational speed. The numerical values in parenthesis
under each δN value indicate standard deviation. The
values for δN were around 2 µm and were extremely
small. Incidentally, as specified in ISO8, eccentricity
should not exceed a total indicated run-out of 0.03 mm.
However, the air-turbine handpieces used in this
study gave significantly smaller values than the permissible values specified in ISO. However, the values
fluctuated somewhat as sterilization cycles increased,
regardless of the type of air-turbine handpiece.
Furthermore, the standard deviations of all δN values
were very large, as shown in Table 4. This demonstrates that the δN data, as measured 3 times for each
test, varied widely. Such scattering may have been due
to problems in the dynamic measurement system,
including the resolution (0.2 µm) of the laser displacement meter, laser sensor installation, fixing of the airturbine handpiece and stiffness of the table that
instruments were placed on.
The mean values shown in Tables 3 and 4 were plotted as RN‐N and δN‐N diagrams in Fig. 2. The symbols for each product in Fig. 2 correspond to those
shown in Table 1. On the whole, the rotational speed of
Table 3. Mean values of rotational speed measured at predetermined number of sterilization treatment cycles.
98
M. NAGAI and K. TAKAKUDA
J Med Dent Sci
Table 4. Mean values of total indicated run-out measured at predetermined
number of sterilization treatment cycles.
each air-turbine handpiece seems not to be affected by
increasing N values, except those of Nakanishi. The
details are determined by Dunnett’s multiple comparison test to be described next. However, both the rotational speeds of N1 and N2 showed a tendency to gradually decrease by increasing N values.
The results assessed by the Dunnett’s multiple
comparison test were shown in Tables 5 and 6. In case
of rotational speed, as shown in Table 5, some of
results of O1-1 and Y2 decreased significantly. In O1-1
which is similar to O1-2, although O1-2 showed no significant difference in all cycles of sterilization, the significant difference was observed at N=150, 200 and
300 cycles. Similarly, Y1 showed no significant difference at all cycles of sterilization. However, the significant difference was observed at N=100, 200, 250 and
300 cycles in Y2. Although the Y1 and the Y2 were of
different types, only the ball bearing part was same.
Meanwhile, in case of RN values in N1 and N2, the significant differences were observed at all cycles of
sterilization.
As shown in Table 6, in case of Y2 and K1-1, the significant differences were observed in δN values at
N=200 and 250 cycles, and in δN values at N=150, 200,
Fig. 2. RN and δN - N diagrams in each air-turbine handpiece, where
the symbols for each product correspond to those shown in Table 1.
INFULUENCE OF DENTAL AUTOCLAVE ON AIR-TURBINE
99
Table 5. The results of Dunnett’s multiple comparison test (α=0.05) of RN.
Table 6. The results of Dunnett’s multiple comparison test (α=0.05) of δN.
Fig. 3. Representative power spectra of the air-turbine handpiece,
which were measured after predetermined number of sterilization
treatment cycles, where the respective arrows indicate fr=6.2 kHz, fa=
fb=2.4 kHz, fc=14.0 kHz and fd=17.1 kHz.
250 and 300 cycles, respectively. However, as shown in
Table 4, the δN values in Y2 or K1-1 were smaller than
those N=0 cycle. N1 and N2 showed no significant differences in δN values, although both the rotational
speeds showed a tendency to gradually decrease and
differed significantly. This is noticeable result and suggests that it is not a problem of ball bearing.
FFT analysis
Figure 3 shows representative power spectra of airturbine handpiece. The frequency domain of noise in
the measurement room was equal to or less than 2
kHz. The power spectral diagrams were recorded
respectively when the air-turbine handpiece: M1-1
was tested at N=0, 150 and 300 cycles. The arrows in
Fig. 3 indicate each predicted frequency calculated by
Eqs.(1) to (4): fr=6.2 kHz, fa= fb=2.4 kHz, fc=14.0 kHz
and fd=17.1 kHz. As shown in Fig. 3, M1-1 after sterilized did not exhibit characteristic difference in the
power spectra in comparison with those at N=0 cycle,
because similar power spectra were observed at different stages of the sterilization treatment and none of
the power spectra varied in both sides on frequencyaxis when the air-turbine handpiece after sterilized was
tested. Additionally, an abnormal power spectrum was
not observed besides the predicted frequencies. Such
similarities were observed for all air-turbine handpieces tested.
DISCUSSION
Rotational speed and total indicated run-out
Monaghan and others5 discussed the influence of
100
M. NAGAI and K. TAKAKUDA
autoclave on ball bearing by means of the bearing
resistance. A wear condition of the ball bearing might
be investigated from such bearing resistance.
However, their method is not commonly used. The evaluation of the ball bearing is considered of value in the
case of air-turbine handpiece that the dentist used in
dental practice. In case of this study, it was selected to
evaluate by means of the total indicated run-out and
FFT analysis, because we employed the air-turbine
handpieces without tooth cutting in this test.
The significance regarding rotational speed and
total indicated run-out is discussed as follows. In O1-1,
the RN values at N=150, 200 and 300 cycles, as
shown in Table 3, decreased 1 to 3 percent in comparison with that at N=0 cycle, although the significant differences were observed at those N values as shown in
Table 5. However, O1-1 showed no significant differences in total indicated run-out as shown in Table 6.
Similar to O1-1, the RN values in Y2 decreased 1 to 2
percent in comparison with that at N=0 cycle. And the
Y2 showed the significant differences in the δN values.
However, the δN values, as shown in Table 4, were
smaller than that at N=0 cycle. Incidentally, the δN values of K1-1 that the significant differences were
shown in total indicated run-out were small than that at
N=0 cycle. If the ball bearing was damaged by autoclave, the total indicated run-out of test mandrel would
increase suddenly even if the damage of the ball
bearing was a little. Therefore, such the result may have
depended on the quality of the equipment. In regard to
the measurement of a total indicated run-out, the ball
bearing rotates at super-high-speed and sufficient
measurement technology has not been provided at present. The development of new measuring equipment is
expected in future.
In case of N1 and N2, the significant differences were
observed in all RN values, furthermore, both the RN values gradually decreased as shown in Fig. 2.
Especially, both the RN values decreased instantly to
3.5 and 2.2 percent at N=50 cycles, respectively. A
hypothesis that the ball bearing was affected by autoclave at the smaller N was rejected, because a cage of
the ball bearing is made by heat-resistant resin.
Accordingly, it was speculated that such the phenomenon depended on other problems aside from ball
bearing itself. On the contrary, Monaghan and others5
reported that the rotational speed of several air-turbine
handpieces increased at early use stage. They suggested that initial conditions or variation of quality of ball
bearing might become a problem as the reason for this.
However, the dentist will try enough lubrication and
J Med Dent Sci
free-running of the air-turbine handpiece before used in
clinical practice for the first time, and then will sterilize
the air-turbine handpieces. Accordingly, their consideration is hard to be understood. Hence, it is considered
that such phenomena would depend on account of others. And it is hard to make a clear consideration for the
early decrease in this paper. The aim of this paper is to
evaluate the affect of an autoclave on the rotational performance of ball bearing. So, the ratio of RN at N=300
cycles to that at N=50 cycles in N1 and N2 was calculated, respectively. As the result, the decreases were 5
percent together. From such the decrease rates, it may
be evaluated that the durability of the air-turbine
handpieces
were
deteriorated
inconsiderably.
However, it was suggested that the ball bearing did not
become dynamically the serious situation at N=300
cycles, because N1 and N2 showed no significant differences in the δN values in all cycles of sterilization. If
the ball bearing was influenced by autoclave, the δN values must have shown a tendency to increase. In the
actual clinical case, small decrease of rotational
speed has no influence on the tooth cutting technique
for a dentist.
FFT analysis
Among the components of the ball bearing, the
cage is made of heat-resistant resin. The ball bearing
may thus be gradually influenced by the heat of sterilization, even if heat-resistant resins are used. This may
result in reduced rotational performance. However,
from the former results of total indicated run-out, it was
guessed that the ball bearing would not be influenced
by autoclave. Furthermore, as another test for evaluating the ball bearing, a qualitative diagnosis of ball bearing was performed while observing scientifically major
power spectra displaying on the screen of FFT analyzer, on the basis of each predicted frequency of the components in the ball bearing.
Each arrow in Fig. 3 indicates the frequencies for fr, fa
(=fb), fc and fd as a reference. Thus, each predicted frequency, as shown for each result after a predetermined
number of sterilization treatment cycles (Fig. 3),
agreed with almost each peak value from the spectra.
However, the spectra for fc and fd were masked by noise
because the amplitudes of both frequencies were
extremely small. This suggests that an abnormal
sound was generated in the frequencies of fc and fd
because the ball bearing showed no signs of damage
after N=300 cycles. Consequently, this suggests that
the ball bearing did not receive any type of damage by
autoclave. Comprehensively, it was demonstrated that
INFULUENCE OF DENTAL AUTOCLAVE ON AIR-TURBINE
all air-turbine handpieces rotated normally after
repeated sterilization and were not affected by the autoclave.
From the above-mentioned results, we may speculate that the rotational performance of air-turbine
handpieces would not be influenced by sterilization
treatment until N=900 cycles, because the sterilization
period was 3 times longer in this study than that recommended by the JIS. One air-turbine handpiece is
sterilized 2-3 times per day, and five days per week at a
typical Japanese dental clinic. This means that the airturbine handpieces were subjected to the equivalent of
more than two years of sterilization treatment in the present study.
101
frequency of the ball’s rotation were comparatively
investigated at N=0, 150 and 300 cycles, and the
influence of autoclave was determined. No abnormalities in the ball bearings were recognized.
ACKNOWLEDGEMENTS
On the occasion of implementation of this research,
authors wish to thank Morita Mfg. Corp., Nakanishi Inc.,
Osada Electric Co., Yoshida Dental Mfg. Co., and
Shirokusu Dental Supply Works for supplying those
respective air-turbine handpieces shown in Table 1, and
Mr. Masayuki Kobayashi for his assistance in sterilization treatment cyclic test for air-turbine handpiece.
CONCLUSION
REFERENCES
The influence of number of autoclave treatment
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from five manufacturers was investigated at N=0, 50,
100, 150, 200, 250 and 300 cycles, and the significance
in the test results was assessed by Dunnett’s multiple
comparison test. Some air-turbine handpieces
showed the significant differences in rotational speed at
N=300 cycles, however, the decreases of the rotational speeds were only 1 to 3.5 percent. Therefore, it is not
concluded that those ball bearings received a considerable damage by autoclave. Some air-turbine handpieces showed the significant differences in total indicated run-out, however, the respective values were
smaller than that at N=0 cycle. Therefore, it was suggested to be influenced variation of data. Accordingly, it
can be considered that the ball bearing in the air-turbine handpieces is not affected significantly by autoclave.
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study focused on the rotational vibration of the ball
bearing components of the air-turbine, as measured by
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