Advance Online Publication

ODEP Vol. 1, No. 1

Original Article

Development of Chairside Dry Heat Sterilizer Using Halogen Lamps
as a Thermal Radiation Source
SAIGUSA Keisuke, TAKAHASHI Yukihiro＊,
MAEDA Munehiro and IGARASHI Masaru
Department of Endodontics, The Nippon Dental University, School of Life Dentistry at Tokyo＊

Department of Microbiology, The Nippon Dental University, School of Life Dentistry at Tokyo

Abstract
Purpose: A chairside dry heat sterilizer using halogen lamps as a heat radiation source was developed, andits potential for clinical application was investigated.
Methods: The temperature in the sterilization chamber was measured in two groups: one group in which
the central temperature was set and maintained at 330℃, and another group in which the central temperature continuously increased. Then, changes in the physical properties of a stainless-steel K-file irradiated withthe lamp were examined by observing the surface by scanning electron microscope and by bending and torsion tests. Finally, sterilization tests were conducted using spore-forming bacteria and indigenous oral bacteria.
Results: The temperature in the light condensing area stabilized after about 20 s, with little effect on thephysical properties of the K-file. Sterilization of spore-forming bacteria took several tens of seconds, whilethat of indigenous oral bacteria took 3 5 s on a paper point and 7 10 s on a cotton ball.
Conclusion: It is suggested that the chairside dry heat sterilization using a thermal radiation source is useful for dental materials.
Key words: simple dry heat sterilizer, halogen lamp, stainless-steel K-file, strength test, spore-forming bacteria

Corresponding author: Dr. SAIGUSA, Department of Endodontics, The Nippon Dental University, School of Life Dentistry atTokyo, 1 9 20, Fujimi, Chiyoda-ku, Tokyo 102 8159, Japan
TEL: ＋81 3 3261 5698, FAX: ＋81 3 5216 3718, E-mail: k-saigusa̲d2117007@tky.ndu.ac.jp
Received for Publication: January 6, 2021/Accepted for Publication: January 15, 2021
DOI: 10.11471/odep.2021 002

Advance Online Publication

ODEP Vol. 1, No. 1

to the sterilized instrument and be transported directly
into the root canal, resulting in blockage of theIntroduction

canal11,12）, harm to the human body due to heavy metal
Aseptic treatment in dentistry is one of the most

poisoning, and burns due to scattering of hot material.

important principles to improve treatment outcomes

Meanwhile, other sterilizers also have problems, suchand prevent the spread of infection via treatment

as unbalancing the table of the dental unit due to the

instruments. In particular, endodontic treatment

excessive weight of the entire sterilizer, media spillage

requires thorough removal of bacterially infected toothwhen the sterilizer is tipped over, and salt coagulation

structures and antiseptic treatment of healthy dentin.

due to humidity in hot salt sterilizers. Furthermore, all

In addition, root canal treatment possesses the risk ofsterilizers take time to heat the media, so advance

contamination by blood and saliva during the proce-

preparation is necessary. Finally, in recent years, oppor-dure, and also by necrotic tissue that has been infected

tunities for home-visit dental care, in addition to dental

by bacteria, making it necessary to re-disinfect thecare in the clinic, have been increasing, and thus there

treatment instruments during the procedure.

is a demand for easy, safe, and portable sterilizers that

In recent years, from the perspective of nosocomial

can be brought into a typical residence.

infection, dental care workers have needed to be espe-

In this study, we developed a simple dry heat steril-cially careful about their own infection potential to pre-izer that uses halogen lamps as a thermal radiation

vent bacterial infections caused by methicillin-resistantsource to convert electrical energy into thermal energy

,

, and

and directly apply it to materials to be sterilized in the

1,2）. Viral infections include infec-

form of electromagnetic waves. Then, we investigatedtions caused by viruses of the herpesvirus family, hepa-

the characteristics of the sterilizer to determine its

titis B and C viruses, and human immunodeficiency

potential for clinical application.

virus, and these microorganisms have also been
detected in dental pulp and periodontal tissues 3 5）.

Materials and methods

Recently, severe acute respiratory syndrome coronavirus 2（SARS-CoV-2）has caused a worldwide pandemic
and the infection is still spreading, so patients must betreated with caution during dental care6,7）.

1 ．Experimental equipment
An overview of the prototype sterilizer and peripheral apparatus is shown in Fig. 1. The prototype steril-

Small sharp instruments such as reamers and files,

izer was composed of two metal plates, each 154 mm inwhich are frequently used in endodontics, are classified

width by 70 mm in height, and two heat sinks, eachas critical instruments and are sterilized by gas or

100 mm in width by 70 mm in height. These were com-high-pressure steam sterilization before use8）. However,bined on a metal chassis measuring 180 mm wide byonce they come into contact with fingers, saliva, or

120 mm deep by 55 mm tall. A hole 1 cm in diameter

infected dentin, bacterial contamination of the instru-

was made in the center of the upper metal cover, andments is inevitable, and because these instruments arehalogen lamps were installed in the heat sinks.

used repeatedly during the procedure, frequent sterilization during treatment is necessary.

A halogen lamp spot heater（HSH-30, Fintech, Hyogo,
Japan）with a focal length of 40 mm, a light condensing

Many endodontic dry heat sterilizers that can bearea diameter of 10 mm, and power of 75 watts wasused chairside have been described in the literature,used for the experiment. Irradiation was carried out inincluding 1）devices using molten tin-based alloys, 2）both directions on opposite sides of the deviceʼs coaxial

devices using salt as a medium, 3）bead sterilizers using

axis, and the material to be sterilized was placed in thesmall glass beads or metal balls, 4）electric furnace-typeirradiation field in the middle of the heater. The voltage

dry heat sterilizers with heaters wrapped aroundwas supplied by a 100 V household power supply, whichquartz tubes, and 5）sterilizers that heat brass blocks

was converted to 24 V for use, and the current was set

with slits 9 18）. However, sterilizers that use molten

at 3.125 A.

metal or small balls, such as glass beads, introduce the

In Fig. 2, K-type thermocouples（Φ 0.3 mm）were

possibility that the metal or balls will become attachedplaced alternately facing each other at 5-mm intervals

100 mm

154 mmE

70 mm

Heat sink

55 mm

D

A

※

C

B

180 mm

120 mmLong side

Short sideabFig. 1（a）Overview of prototype sterilizer and peripheral apparatus：（A）Prototype sterilizer,（B）Control apparatus,（C）Power
supply unit,（D）Irradiation switch,（E）Temperature control apparatus（b）Schematic diagram of prototype sterilizer（※：Base of the halogen lamp）

40 mm

B

A

A

B

C

10 mm

C

a

b

Fig. 2（a）Prototype sterilizer（Fig. 1, A）, top view：（A）Halogen lamps,（B）K-type thermocouples,（C）Heatsinks（b）Cross-sectional schematic diagram of the prototype sterilizer：the entire circle is the light condensing area, and the circumference is the outer edge. The black points represent the tips of theK-type thermocouples（depths of 10, 15, 20, 25, 30, and 35 mm from the top）.

up to 35 mm from the side of the apparatus to measure

Ⅰ: When the control switch is turned on, the lamp

the temperature of the sterilization area. The light con-cycles on and off to reach and maintain the set tem-densing area（10 mm）at the focus of the halogen lamp

perature automatically.

was set at a depth of 15 25 mm from the hole, with 15

Ⅱ: When the control switch is turned off, continuousand 25 mm at the outer edge of the light condensing

irradiation is performed and the temperature increases

area and 20 mm at the center of the light condensing

until the switch is turned off.

area.

2 ．Experimental details

The irradiation time could be set from 0.1 to 99.9 s at

The following items were investigated to determine

0.1-s intervals, and a countdown system was adopted.

the effects of this device on endodontic instruments andThere was also a temperature control switch that could

materials.

be set from 100℃ to 500℃ in 1℃ increments. Two settings for halogen lamp irradiation were established:

Advance Online Publication

ODEP Vol. 1, No. 1

Load cell

Load cell
45°

3 mm

Fig. 3 Bending test machine
The first 3 mm of the file tip
is fixed with a chuck, and thebending torque is measured
when the file is bent to 45 .

Fig. 4 Torsion test machine
The first 3 mm of the file tip is fixed with achuck, the file is twisted until it breaks, and thetorsion torque and rotation angle at the time of
failure are measured.

the top, then the K-file was allowed to cool by natural
air cooling to room temperature, and this process was

1 ）Measurement of temperature in the light condensing area
The K-type thermocouples placed inside the sterilizer

repeated 10 times. We observed the surface changes by
SEM for each group（n＝1）.
3 ）Strength testing of files after heat irradiation

were used to record the temperature change over time

The changes in physical properties of the files after

at 100-ms intervals when the temperature was con-irradiation with halogen lamps were tested according totrolled at 330℃ with setting Ⅰ and when the tempera-

the international standard ISO 3630-119）for small dental

ture was continuously increased with setting Ⅱ. The

instruments and judged according to the standard val-

irradiation time was set to 90 s for both settings.

ues of ISO 3630-5.20）Two types of #15 40 stainless-steel

The measurement data were recorded on a PC

hand K-file were used for the test: K-file and Senseus（Inspiron 15 7000 Gaming, DELL, Round Rock, TX,

K-FlexoFile（Dentsply Sirona, Charlotte, NC, USA）.

USA）using a data logger（NR-1000, KEYENCE, Osaka,

Since there was a risk of bias with a single type of sam-Japan）. Temperature measurements were performedple, we made a comprehensive judgment by comparing

at room temperature（25℃）and humidity（55％）.

two K-files with their respective untreated groups.

2 ）Surface observation of K-files after heat irradiation

Non-irradiated files were used as a control group, andthe experimental group consisted of files that were

The surface of #40 stainless-steel hand K-files（MANI,

irradiated 10 times with halogen lamps for 10 s at set-

Tochigi, Japan）
was observed under a scanning electronting Ⅱ（n＝10 for each group）. All instruments weremicroscope（SEM, S-4000, Hitachi, Tokyo, Japan）after

allowed to stand at room temperature（25℃）for 30 min

halogen lamp irradiation to detect any change in theafter each irradiation before the next irradiation. Before

surface properties.

each test, the handle of the file was removed with wire

Five experimental groups were established. The first

cutters at the point where it was attached to thegroup was subjected to natural air cooling to room tem-

instrument shaft.

perature after 5 s of irradiation, and this cycle was（1）Bending test（ISO 3630-1）

repeated 10 times at setting Ⅱ. The second and third

The bending test machine and test method aregroups were subjected to the same treatment, but atshown in Fig. 3. A section of the K-file up to 3 mm from

irradiation times of 10 s and 15 s. In addition, two con-the tip was fixed vertically in the chuck section of thetrol groups were set. An untreated group was used as abending test machine and rotated at a speed of 100

negative control and a flame-treated group was used as

mm/min to induce bending. The maximum bending

a positive control. In the flame-treated group, the blade

torque was measured and the average value of 10 files

was set at the position of the oxidizing flame of the gas

was used as the maximum bending torque（mN・m）.

burner（about 1,500℃）and heated for 10 s by grasping

－Sterility Test－
Halogenlamp
K－fileHalogenlamp

K－filePaper point Cotton ball

Paper point

Cotton ball

3. Irradiate contaminatedmaterials with halogen lamps

1. Prepare solution ofeach bacteria species 2. Contaminate samples
with each solution

Fig. 5

4. After irradiation, placesamples in sterile Tryptic
soy broth

5. Incubate cultures at 37℃
for up to 2 wk and visually
determine the results

Sterilization test procedure（2）Torsion test（ISO 3630-1）

method, the spores were collected by centrifugation at

The torsion test machine and test method used are

3,000× for 15 min, washed three times with sterile

shown in Fig. 4. Each sample was set in the chuck of

distilled water, and then heat-treated at 75℃ for 30 min

the motor, and a section of the file up to 3 mm from theto kill the vegetative form bacteria. Other bacteria

tip was fixed in the chuck on the opposite side. The

were inoculated in brain heart infusion（BHI: Becton,

torque measuring device on the top of the machine was

Dickinson and Company, Sparks, MD, USA）broth andconnected to the torsion testing machine by a wire, andincubated at 37℃ for 24 h.

the motor was operated at 100 mm/min. The average

For experimental samples, we used a #40 stain-

value of 10 files was used as the maximum torsional

less-steel hand K-file, #40 sterile paper points（Absor-

torque（mN・m）and fracture angle（ ）.

bent Paper Points, Zipperer, Germany）, and cotton balls

4 ）Heat sterilization tests for K-files, paper points,
and cotton ballswith a diameter of 3 mm（Micro Cotton Ball #3, Iwatsuki, Tokyo, Japan）. The K-file was cut with wire nip-

Various microorganisms were applied to different

pers just below the handle, and the cotton balls weredental materials to evaluate the sterilization effective-

removed from their container, placed in sterilizationness.

bags, and sterilized by high-pressure steam in an auto-

ATCC9372 was used because

it is regarded as an indicator organism for dry heat21）

sterilization in ISO 11138-4 .

clave for the experiments.
The procedure of the sterilization test is shown in

ATCC23726,

NCTC11324,

Fig. 5. The K-file was immersed up to 16 mm from the（formerly known as）

tip of the blade in a bacterial solution adjusted to 1.0×

MT814823） were

108 colony forming units（CFU）/m for each species.

used as representative of indigenous oral bacteria. In

Contaminating bacteria on the paper points and cotton

addition,balls were prepared at approximately 2.0×103 CFU/m .

T14V22）, andW3110 was also included, for a

total of six bacteria species used in the experiment.
andThe paper points were immersed in 2.0×106 CFU/m

were purchasedof bacterial solution up to 10 mm from the tip（aboutfrom the American Type Culture Collection（ATCC）,

1μ ）, and the cotton balls were contaminated by drop-andping 10μ of 2.0×105 CFU/m bacterial solution. The

was purchased from the National

Collection of Type Cultures（NCTC）.

W3110 wascotton balls were then placed in a sterile Petri dish for

obtained from the Institute of Medical Science, Univer-

30 min at 37℃ to dry them before use in the experi-sity of Tokyo.

ments.
was inoculated in 50 m of

For the K-files, the irradiation time was set to 0［Con-Shaefferʼs medium24） as a spore-forming medium, andtrol（＋）］, 3, 5, 7, and 10 s with setting Ⅱ for all bacte-cultured at 37℃ with shaking for 24 h. The sporesria. For the paper and cotton samples, setting Ⅰ waswere then removed from the shaking incubator andused, and the irradiation time was set to 0［Control

stored at room temperature for 1 wk. After confirming（＋）］, 3, 5, 7, 10, 15, 20, 30, and 40 s for

sufficient spore formation by Wirtzʼs spore staining

and 0, 3, 5, 7, and 10 s for other bacteria（n＝20 for eachFirst, onlyAdvance Online Publication
500

500

450

450

300

25 mm

250

15 mm

200

10 mm

150

30 mm

100

0

15 mm

300
250

10 mm

200

30 mm

150

35 mm

50

10 20 30 40 50 60 70 80 90 100 110 120130140 150160 170180190 200
Time
（s）

10mm

25 mm

350

100

35 mm

50
0

20 mm

400

20 mm

350

Temperature（℃）

Temperature（℃）

400

ODEP Vol. 1, No. 1

15mm

20mm

25mm

0

0

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
Time（s）

30mm

35mm

Temperature measurement during irradiation
at setting Ⅰ
Irradiation was started 10 s after the start of tem-

10mm

15mm

25mm

30mm

35mm

Temperature measurement during irradiation
at setting Ⅱ
Irradiation was started 10 s after the start of tem-

Fig. 6

Fig. 7

perature measurement.

perature measurement.

group）. The test samples were irradiated with halogen

20mm

until the end of irradiation.

lamps and incubated in sterile Tryptic soy broth（Bec-

At 15 mm and 25 mm, which were the outer edges ofton, Dickinson and Company）at 37℃ for 24 h, 48 h, 72

the condensing area, the temperature rose to approxi-

h, 1 wk, and 2 wk, the results were visually checked,

mately 260℃ and remained at around 240 250℃ until

and those with increased turbidity or precipitation

the end of irradiation. The temperature dropped

were judged as positive for culture. For each group, thesharply outside the condensing area, reaching only 120

number of samples that were culture negative was

150℃ at 10 mm and 30 mm and 70 90℃ at 35 mm.

counted, divided by 20（the total number of samples）,

In setting Ⅱ, continuously increasing irradiation wasand multiplied by 100 to calculate the culture negative

performed for 90 s, and the results were measured overachievement rate.

time（Fig. 7）. In the light condensing area at a depth of

The paper points were cut 10 mm from the contaminated tip before being fed into the sterilizer.

15 25 mm, the temperature rose rapidly as soon as
irradiation started, reaching 275℃ at the center and3 ．Statistical analysis of strength

215 226℃ at the outer edge 5 s after the start of irradi-

Studentʼs -test was performed on the data obtained

ation. After another 10 s, the temperature reachedfrom the strength tests. The statistical significance

368℃ at the center and 280 289℃ at the outer edge,

level was set at less than 5％, and IBM SPSS ver. 25

and after 15 s, 408℃ at the center and 307 317℃ at thewas used as the statistical software.

outer edge. The highest temperatures after 90 s of irradiation were 464℃ at the center and 367℃ at the outer

Resultsedge.
2 ．Surface observation of K-files after heat irradi-

1 ．Temperature measurement of sterilization
area

ation
The surface of each file was observed by SEM（Fig.

The temperature of the sterilization area was mea-

8）. The flame-treated group showed a loss of continuity

sured for 90 s when the temperature was controlled todue to the collapse of the file edge, with the metal onaround 330℃ with setting Ⅰ（Fig. 6）. When this tem-the file surface peeling off. In the group irradiated byperature was reached, the control apparatus could not

the halogen lamp, there was no major change in thereact quickly enough, and the center temperature rose

surface properties of the files irradiated for 5, 10, andto 349℃. However, 20 s after the start of control, the15 s compared with the untreated group, either bytemperature stabilized at around 330℃. The amplitudevisual inspection without magnification or in the SEM

during each on/off switching cycle was about 10℃,

images.

which maintained the temperature at around 325 335℃

Flame treatment group［Control（+）］

Untreated group［Control（－）］

5 s irradiation repeated 10 times group10 s irradiation repeated 10 times group15 s irradiation repeated 10 times group

Fig. 8

Surface observation by SEM of K-file irradiated by halogen lamp

3 ．Strength testing of K-files after heat irradiation

K-FlexoFile. The bending torque values in both groups

The results of the strength tests are shown in Fig. 9.

were within the ISO standard values.

1）Bending test2）Torsion test

The torques required to bend the files up to 45 are

The maximum torque and fracture angle until frac-shown in Fig. 9A and B. There was a significant differ-ture when torsion was applied to the file are shown inence between the control and irradiated groups for the

Fig. 9C F. For the #25 MANI K-file, results for the irra-#15 MANI K-files（p＜0.05）, and there was no signifi-

diated group were significantly lower than those for thecant difference between the groups for the Senseus

untreated group, and for the #35 K-file, results for theBending torque（mN・m）

25
20
15
10
＊

5
0

#15

#20

#25

#30

Torsional torque（mN・m）

Control

#40

A
25
20
15

＊

10

＊

5
0

#15

#20

#25

#30

#35

1,500
1,000
500
#15

#20

#25

Control

#30

10
5
0

#15

#20

#35

#40

#25

#30

#35

#40

#35

#40

#35

#40

Heat

B
25
20
15
10
5
0

#15

#20

#25
ControlAngular deflection（ ° ）

2,000

0

15

*p<0.05

Heat

C

2,500

20

Control#40

ODEP Vol. 1, No. 125*p<0.05

Heat

ControlAngular deflection（ ° ）#35Torsional torque（mN・m）

Bending torque（mN・m）

Advance Online Publication

#30
Heat

D

2,500
2,000
1,500
1,000
500
0

#15

Heat

#20

#25

Control

E

#30
Heat

F

Fig. 9 Bending test and torsion test results（p＜0.05, n＝20）（A）MANI K-file bending test results；（B）Senseus K-FlexoFile bending test results；（C）（E）MANI,K-file torsion test results；（D）（F）Senseus,
K-FlexoFile torsion test results

untreated group were significantly lower than those forafter 40 s forthe irradiated group（p＜0.05）. The torque failure values, and, after 10 s for,, and after 7 s for other bacteria.

for both groups were within the ISO standard values.
There was no significant difference between the

Discussion

groups for Senseus K-FlexoFile, but only one of the 10#40 K-files had a fracture angle below the ISO standard

Most conventional simple dry heat sterilizers provideof 360 . Therefore, 10 additional tests were conducted

sterilization heat through a heating medium, such asin accordance with the standard, and all of them wereglass pellets or molten metal, and it takes several min-confirmed to meet the standard.

utes for the heating medium to rise to the sterilization

4 ．Sterilization tests

temperature. In contrast, the prototype of the simple

The results of the sterilization tests are shown in

dry heat sterilizer used in this study heats the materi-

Tables 1 3. The culture results for K-files were all neg-als to be sterilized by directly radiated heat, which has

ative after 10 s of irradiation forand after

the advantage of eliminating the need to turn on the

7 s for other bacteria. Results for the paper points werepower supply and heat the medium beforehand because

negative after 30 s forthere is no heating medium. Therefore, it is possible toand, after 5 s for, and after 3 s for other bacteria.

The culture results for cotton balls were all negative

rapidly heat an object by turning on the power andpressing the irradiation switch.

Table 1

Sterilization test results of K-file（％）

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

Species and Irradiation

0s

0

0

0

0

0

0s

0

0

0

0

0

3s

0
70

0
0

0
0

0
0

0
0

3s
5s

60
65

60
65

60
65

60
65

60
65

80

35

30

30

30

7s

100

100

100

100

100

10 s

100

100

100

100

100

10 s

100

100

100

100

100

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

0s
3s

0

0

0

0

0

0s

0

0

0

0

0

90

80

80

80

80

3s

80

80

80

80

80

5s
7s

24 h

48 h

72 h

1 wk

2 wk

5s

95

95

95

95

95

5s

95

95

95

95

95

7s

100

100

100

100

100

7s

100

100

100

100

100

10 s

100

100

100

100

100

10 s

100

100

100

100

100

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

0s

100

0

0

0

0

0s

0

0

0

0

0

3s

100

0

0

0

0

3s

0

0

0

0

0

5s

100

90

65

65

65

5s

85

85

85

85

85

7s
10 s

100

100

100

100

100

7s

100

100

100

100

100

100

100

100

100

100

10 s

100

100

100

100

100

48 h

72 h

1 wk

2 wk

The percentages of negative achievement rate are indicated（n＝20）.

Table 2

Sterilization test results of paper point（％）

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

Species and Irradiation

24 h

0s
3s

0

0

0

0

0

0

0

0

0

0

0

0

0

0s
3s

0

0

100

100

100

100

100

5s
7s
10 s

15
25

0
25

0
25

0
25

0
25

100
100

100
100

100
100

100
100

100
100

50

50

50

50

50

5s
7s
10 s

100

100

100

100

100

15 s

60

60

55

55

55

20 s

75

65

65

65

65

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

30 s

100

100

100

100

100

0

0

0

0

0

40 s

100

100

100

100

100

0s
3s

100

100

100

100

100

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

5s

100

100

100

100

100

7s

100

100

100

100

100

0s

0

0

0

0

0

10 s

100

100

100

100

100

3s

100

95

65

65

65

5s

100

100

100

100

100

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

7s

100

100

100

100

100

0s

0

0

0

0

0

10 s

100

100

100

100

100

3s

100

100

100

100

100

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

5s

100

100

100

100

100

7s

100

100

100

100

100

0s

100

0

0

0

0

10 s

100

100

100

100

100

3s

100

35

35

35

35

5s

100

100

100

100

100

7s

100

100

100

100

100

10 s

100

100

100

100

100

The percentages of negative achievement rate are indicated（n＝20）.

Advance Online Publication

ODEP Vol. 1, No. 1

Sterilization test results of cotton ball（％）

Table 3Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

Species and Irradiation

0s

0

0

0

0

0

0s

0

0

0

0

0

3s

0
0

0
0

0
0

0
0

0
0

3s

10
80

10
80

10
80

10
80

0

0

0

0

0

5s
7s

10
95
100

100

100

100

100

10 s

0

0

0

0

0

10 s

100

100

100

100

100

15 s

0

0

0

0

0

20 s

0

0

0

0

0

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

30 s

45

45

45

45

45

0

0

0

0

0

40 s

100

100

100

100

100

0s
3s

60

25

25

25

25

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

0s
3s

0

0

0

0

15

0

0

0

5s

100

45

30

25

25

7s

100

80

65

55

55

10 s

100

100

100

100

100

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

0s

100

0

0

0

0

3s

100

0

0

0

0

5s

100

60

45

45

45

7s

100

95

95

95

95

10 s

100

100

100

100

100

5s
7s

24 h

48 h

72 h

1 wk

2 wk

5s

95

95

95

95

95

7s

100

100

100

100

100

0

10 s

100

100

100

100

100

0

Species and Irradiation

24 h

48 h

72 h

1 wk

2 wk

0s

0

0

0

0

0

3s

10

0

0

0

0

5s

70

50

50

50

50

7s
10 s

75

70

70

70

70

100

100

100

100

100

The percentages of negative achievement rate are indicated（n＝20）.

In our prototype sterilizer, the time from pressing
the

on

9 18）
tional sterilizers（200 280℃）, is that the prototype

switch to stabilizing the temperature in thesterilizer reflects more of the radiant heat projected

light condensing area was about 20 s. It was reported

onto the sterilized materials compared with the conven-that it takes 9 min for the temperature to stabilize attional type that uses a heating medium, and not all of

230℃ for glass beads, 5 min for salt, and 6 min for mol-the heat energy incident on the materials can be used.

13）.

This is also the reason why the core temperature of N.I.

Therefore, the setup time was greatly reduced, and the

dry heat sterilizers using radiation heat has beensterilizer could be used immediately. This feature isincreased to around 360℃ in the past16）.

ten metal, which are conventional heating media

useful not only in daily clinical practice, but also in

There is no clear ISO standard for simple dry heat

home-visit dental care if the device can be madesterilizers, and each sterilizer is designed with its own

smaller.

optimal temperature. The temperature difference

In addition, it is possible to set the temperature at

between the center of the light condensing area and thethe center of the light condensing area with a control

outer edge of the light condensing area was 80℃.

switch and select setting Ⅰ, which controls the tem-

Although the temperature at the outer edge was notperature by cycling the lamp, or setting Ⅱ, which per-lower than the temperature of a conventional sterilizer,

forms continuously increasing irradiation. The paper

the temperature of the sterilized material may be lower

points and cotton balls, which have poor thermal con-than the apparent temperature for paper and cotton

ductivity, were irradiated with setting Ⅰ, while K-files,

products in a sterilizer with low thermal energy effi-

which are stainless steel and have good thermal con-ciency. Currently, almost no sterilizers available on theductivity, were irradiated with setting Ⅱ.

market can heat K-files up to 330℃, and N.I. sterilizers

The reason why the temperature was 330℃ in setting Ⅰ, which is slightly higher than that of conven-have only been reported to heat cotton plugs and paper
points16）.

In this study, we observed the surface of the heatedof treatment, just before the working length was deter-

files and performed bending and torsion tests according

mined and provisionally sealed, has been reported to be

to ISO 3630-1 to observe the changes in physical prop-

approximately 1.0 3.0×103 CFU27）.

erties（Figs. 7 and 8）. Compared with the untreated

For the paper points, negative cultures weregroup, images of the flame-treated group showed metal

obtained after 3 5 s of irradiation for non-spore-forming

peeling off the surface of the file and the blade disinte-

bacteria, but after 30 s for spore-forming

grating and blunting due to heat, while the halogen

Because the central temperature of the condensing

lamp-irradiated group showed no major changes.

area rises to 330℃, this area seems to be able to kill.

It has long been believed that sensitization（a chro-bacteria in a short time. However, the temperaturemium deficiency phenomenon caused by grain precipi-

decreases away from the center, and the temperatures

tation due to chromium carbonization on the surface ofof the ends of the samples（tip of the paper point andthe file）, which is the cause of intergranular corrosion

edge of the cut）located in the outer edge reach onlyin 18-8 stainless steel, occurs when steel is heated for a

240 250℃. Therefore, the spores that survive there

long time at 500 800℃. In this experiment, the tem-

could germinate.

perature was about 410℃ even after the longest irradi-

Cotton balls were culture negative after 7 10 s ofation of 15 s, so it is thought that the files were undam-

irradiation for non-spore-forming bacteria, but took 40 s

aged because the temperature was not high enough tofor spore-forming

25）. The longer timerequired for a negative culture compared to the papercause grain precipitation .

In the bending tests, torque was significantly lower inpoints for all species was thought to be due to the fact

the MANI #15 K-file, and in the torsion tests, torquethat the depth from the surface to the center of the cot-

was significantly lower in the MANI #25 K-file withton ball required more time for heat conduction, allow-

respect to the untreated group. However, in both tests,

ing interior bacteria to survive.

the torque did not decrease significantly compared to

In the K-files, the length of contamination was 16

the untreated group, and the ISO standard values20）mm, some of which was not placed within the condens-

were met, so the K-files would still be acceptable foring area, and despite the large number of contaminat-

clinical use.

ing bacteria, the culture was negative after 10 s for

There was no significant difference in the fracture

spore-forming

and after 7 s for other bac-angle among all groups, but one Senseus K-FlexoFile

teria species. This is probably due to the good thermal#40 fractured at an angle under the ISO standard

conductivity of the metal, unlike paper or cotton.

value. The additional 10 pieces tested in accordance

Because it is rare to come into contact with highlywith the standard all exceeded the standard value. The

thermostable bacteria in general dental practice, theresults of these strength tests are in agreement withresults confirmed that clinical sterilization could be26）

those of Asakura , who reported that the mechanicalachieved by irradiating the K-files for 7 s with setting

properties of K-files were not affected until they wereⅡ, and the paper points for 5 s and small cotton balls

heated at 400℃ for 1 h in an electric furnace.

for 10 s with setting Ⅰ at the temperature settings

In the sterilization test, we used

used in this study.

（Gram-positive bacteria）, which is an indicator bacte-

The results show that it was able to quickly andrium for dry heat sterilization and has the ability tosafely heat the materials to be sterilized and achieve

form spores;assterilization in the light condensing area. This proto-Gram-positive bacteria indigenous to the oral cavity;

type sterilizer had the necessary control boards andandas Gram-negative bacteria. In addi-

equipment in order to achieve sufficient performance.

tion,

was used as a general Gram-negative bac-

However, since some equipment could be omitted interium. The amount of bacteria applied to the paper

actual clinical applications, it is possible to reduce the

,

, and

3

points and cotton balls was set at 2.0×10 CFU/m ,size of the sterilizer, and since there is no heating

under the assumption that these materials would be

medium compared to the conventional type, it is possi-

used before provisional sealing at the end of treatment,

ble to reduce the weight. Moreover, the lack of a heat-because the number of glove bacteria in the second half

ing medium prevented the material from being bent

Advance Online Publication

ODEP Vol. 1, No. 1

when it was inserted into the sterilization area andence was observed between the conditions with andeliminated contamination of the material by a heating

without irradiation.

medium. The results also suggest that this sterilizer is

5. The time required for 100% of the spore-forming

effective for infected root canal treatment, where abacteria to be sterilized was 10 s for the K file, 30 s forlarge amount of necrotic pulp or infected dentin is han-the paper point, and 40 s for the cotton ball. For otherdled, or where contamination is suspected due to small

bacterial species, it took 7 s for the K file, 3 to 5 s for

instruments falling outside the clean area or being

the paper point, and 7 to 10 s for the cotton ball.

exposed to saliva. Moreover, although they do not come

These results suggest that this sterilizer is effective

into direct contact with the sterilized materials, theas used in this experiment, which was conducted underinternal walls of the equipment can be sterilized by

the assumption of clinical procedures.

autoclaving if they are assembled, with the exception of
the halogen lamps, which require wiring, so maintenance is considered possible.

The authors declare no conflicts of interest associatedwith this manuscript.

However, as with conventional sterilizers, it is necessary to insert long instruments, such as tweezers or
K-file handles, that are not heat resistant to direct light
exposure. In addition, sterilization of materials exposed
to a large amount of microorganism contamination will
require a conventional autoclave. The bacterial countset for this study corresponds to that observed during
clinical procedures, and it is therefore desirable to sterilize any suspected contaminated items after cleaning
them with gauze.
In addition, the temperature tended to drop rapidly
away from the center of the light condensing area.
Thus, this instrument is effective for sterilizing small
areas contaminated during use, and conventional methods are required to achieve complete sterilization of theentire instrument. Once used, instruments should only
be used for the same patient, and autoclaves or dryheat sterilizers should be used to sterilize instruments
between patients.

Conclusion
1. In settingⅠin which the central temperature of thecondensing area was maintained, the temperature wasstable at a central of around 330℃ and an outer edge ofaround 250℃ after irradiation for 20 s.
2. In settingⅡwith continuous irradiation, the center
temperature reached 368℃ and the outer edge temperature increased to around 280℃ after irradiation for
10 s.
3. In SEM observation of the irradiated K-file, there
was no change compared to unirradiated file.
4. In the bending test and the torsion test of theK-file, there was a tendency that no significant differ-

References
1）Klevens RM, Gorwitz RJ, Collins AS. Methicillin-resistant Staphylococcus aureus: a primer for dentists. J Am
Dent Assoc 2008; 139: 1328 1337.
2）Volgenant CMC, de Soet JJ. Cross-transmission in thedental office: Does this make you ill? Curr Oral HealthRep 2018; 5: 221 228.
3）Glick M, Trope M, Bagasra O, Pliskin ME. Human immunodeficiency virus infection of fibroblasts of dental pulp
in seropositive patients. Oral Surg Oral Med Oral
Pathol 1991; 71: 733 736.
4）Li H, Chen V, Chen Y, Baumgartner JC, Machida CA.
Herpesviruses in endodontic pathoses: association of
Epstein-Barr virus with irreversible pulpitis and apicalperiodontitis. J Endod 2009; 35: 23 29.
5）Sabeti M, Slots J. Herpesviral-bacterial coinfection inperiapical pathosis. J Endod 2004; 30: 69 72.
6）Meng L, Hua F, Bian Z. Coronavirus disease 2019
（COVID-19）: Emerging and future challenges for dental
and oral medicine. J Dent Res 2020; 99: 481 487.
7）Amato A, Caggiano M, Amato M, Moccia G, Capunzo M,
De Caro F. Infection control in dental practice during
the COVID-19 pandemic. Int J Environ Res Public
Health 2020; 17: 4769.
8）Centers for Disease Control and Prevention. Summaryof infection prevention practices in dental settings:
Basic expectations for safe care. Atlanta: Centers for
Disease Control and Prevention, US Dept of Health andHuman Services; October 2016. https://www.cdc.gov/
oralhealth/infectioncontrol/pdf/safe-care2.pdf.（cited
2019. 3. 1）
9）Stewart GG, Williams NB. A preliminary report on theefficacy of molten metal for sterilization of root canal
instruments and materials. Oral Surg Oral Med Oral
Pathol 1950; 3: 256 261.

10）Findlay J. A report on the efficacy of molten metal andball bearings as media for sterilisation. Br Dent J 1955;
98: 318 323.
11）Grossman LI. Hot salt sterilizer. Br Dent J 1956; 100:283.
12）Oliet S, Sorin S, Brown H. A temperature analysis ofthermostatically controlled root canal sterilizers using
molten metal, glass beads, or salt. Oral Surg Oral Med
Oral Pathol 1958; 11: 37 42.
13）Yasuoka M, Ideguchi Y, Hiramine K, Osada T. A studyof hot salt sterilizer for root canal instruments. Jpn J
Conserv Dent 1975; 18: 282 291.（in Japanese）
14）Spring PN. Bacteriologic evaluation of the glass beadsterilizer for endodontics. Oral Surg Oral Med Oral
Pathol 1959; 12: 353 357.
15）Kitajima K, Eguchi M, Kitano Y, Ezura A, Igarashi M,
Kawasaki K. A study on the efficacy for the sterilization of a newly-designed electric glass bead sterilizer
with digital thermometer. Jpn J Conserv Dent 1993; 36:
241 251.（in Japanese）
16）Tanaka I, Kojima K, Suzuki K, Matsuda J, Nakamura H.
A study of a special electric drying sterilizer. ―An
improved drying apparatus of N.I. system―. Jpn J Conserv Dent 1976; 19: 323 331.（in Japanese）
17）Tamazawa K, Horiuchi H. A new endodontic sterilizer
using a heated metal slit. Jpn J Conserv Dent 1980; 23:
209 212.（in Japanese）
18）Sawada K, Yazaki M, Kanda M, Suda H, Sunada I. A
study on the efficacy for the sterilization of SL sterilizer. Jpn J Conserv Dent 1981; 24: 246 251.（in Japanese）
19）International Organization for Standardization. ISO

3630-1: 2019（E）Dentistry―Root-canal instruments―
part 1: General requirements and test methods, 2019.
20）International Organization for Standardization. ISO
3630-5: 2020（E）Dentistry―Endodontic instruments―
part 5: Shaping and cleaning instruments, 2020.

21）International Organization for Standardization. ISO
11138-4: 2017（E）Sterilization of health care products―
Biological indicators―Part 4: Biological indicators fordry heat sterilization processes, 2017.
22）Cisar JO, Vatter AE, Clark WB, Curl SH, Hurst-Calderone S, Sandberg AL. Mutants of
T14V lacking type 1, type 2, or both types of fimbriae. Infect Immun 1988; 56: 2984 2989.
23）Okahashi N, Sasakawa C, Yoshikawa M, Hamada S,
Koga T. Cloning of a surface protein antigen gene fromserotype. Mol Microbiol 1989; 3:
221 228.
24）Bott KF, Davidoff-Abelson R. Altered sporulation andrespiratory patterns in mutants of
induced by acridine orange. J Bacteriol 1966; 92: 229240.
25）Umemura F. Non-destructive evaluation method for
degree of sensitization of stainless steel. J Jpn Weld Soc
1987; 56: 411 416.
26）Asakura T. The effect of heating temperature for the
cutting efficiency of the K-file for an enlargement of aroot canal. J Fukuoka Dent Coll 2002; 29: 169 180.
（inJapanese）
27）Niazi SA, Vincer L, Mannocci F. Glove contamination
during endodontic treatment is one of the sources of
nosocomial endodontic
infections. J Endod 2016; 42: 1202 1211.