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APOS Trends in Orthodontics
Original Research

Effectiveness of low binding frictional
materials: Evaluation of the binding frictional
resistance of improved superelastic nickel-titanium
alloy wires with different bracket combinations
Yoshio Shima1,2, Akihiro Koyama2, Motohiro Uo3, Takashi Ono2
Private Practice, Fukui, Departments of 2Orthodontic Science, 3Advanced Biomaterials, Graduate School of Medical and Dental Sciences, Tokyo Medical and
Dental University, Tokyo, Japan.
1

ABSTRACTIntroduction: is study aimed to evaluate the binding frictional resistance of improved superelastic nickeltitanium alloy wires (ISW) with different bracket combinations and to verify the effectiveness of low binding
frictional materials by applying them in orthodontic treatment.
*Corresponding author:
Dr. Yoshio Shima,
Shima Orthodontic Office,1-21-22 Chuo, Fukui-Shi,
Fukui, Japan.
shima.ortho.office@gmail.com
Received : 28 March 19
Accepted : 24 April 19
Published : 28 September 19
DOI
10.25259/APOS_25_2019

Materials and Methods: Straight stainless steel wire (SSW; 0.016 × 0.022-inch) and straight ISW (0.016 × 0.022inch) were set to each displaced bracket, and the tensile resistance load was measured. The maximum tensile
resistance load was statistically compared using the Tukey test. For exemplification, we treated a typical extraction
case of Angle Class I crowding malocclusion with lip protrusion using lower binding frictional materials, whichwere selected based on tensile test results.
Results: The SSW and metal bracket combination had the largest maximum tensile resistance load, and the ISW
and metal slot-equipped plastic bracket combination had the smallest load (P < 0.01). In a patient treated using
lower binding frictional materials, the active treatment period was 9 months. Satisfactory patient results wereobtained without using reinforced anchorage.
Conclusions: Binding frictional resistance varies, depending on the archwire and bracket combination. In a
multibracket appliance, selecting materials with as low a binding frictional resistance as possible may make amore effective treatment.
Keywords: Binding friction, Materials, Improved superelastic nickel-titanium alloy wire, Extraction case

Quick Response Code:INTRODUCTION
Superelastic nickel-titanium (Ni-Ti) alloy has superior mechanical properties such as
superelasticity, shape memory effect, high spring back property, and low stiffness,[1-4] and it has
been widely used in clinical orthodontic treatment for >30 years. The load delivered from thesuperelastic Ni-Ti alloy wire is influenced by oral temperature changes: When the temperaturechanges during reverse transformation due to unloading, the load has a higher value than theinitial load. The tendency of the load to increase depends on its stress hysteresis.[5,6] Furthermore,
the appliance was improved so that it would deliver a more stable orthodontic force under changing

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Shima, et al.: Effectiveness of low binding frictional materials

oral temperatures, and an improved superelastic Ni-Ti
alloy wire (ISW) with lower stress hysteresis was developed(L and H Titan; Tomy International, Tokyo, Japan).[7] Dueto the characteristics of the ISW, three-dimensional tooth
movement was possible during the early stage of treatment byincorporating bends that were formed using a heat-bending
machine (SOARER-X; Tomy International).[8-12]
However, the orthodontic force delivered from the archwire
in the oral environment during orthodontic treatment isdifferent from the bending properties; the orthodontic force
is always changing due to friction with the bracket slot.[13-15]
Friction is classified as “classical friction,” “binding friction,”and “notching friction.” Of these, binding friction has a great
influence during various tooth movement patterns such asleveling and space closing.[16-18] The mechanical properties ofthe orthodontic wire and the bracket slot type greatly affectbinding friction.[19-22] Due to its lower stress hysteresis andstiffness, the binding friction of the ISW is lower than that ofother Ni-Ti wires.[23]

a

b

c

Figure 1: Magnified photographs of the materials used in the
study. (a) IPASS II (Ortho-Dentaurum, Tokyo, Japan). (b) Clear
bracket SL+ (Dentsply-Sirona K. K. Tokyo, Japan). (c) Metal bracket(Dentsply-Sirona).

MATERIALS AND METHODS

which was similar to the setting of the conventional threepoint bending test [Figure 2c].[3-7] To exclude the influenceof various ligature forces, a 0.016 × 0.022 – inch straight
SSW (3M Unitek, Tokyo, Japan) and a 0.016 × 0.022 – inchstraight ISW were set to the bracket without ligation. All
tensile tests were conducted at a speed of 1.0 mm/minutefrom a state where the deflection was applied, and thetensile resistance load was measured [Figure 2d]. The test
was performed on four samples for each combination in achamber maintained at 37.0°C. A statistical comparison of
the average values of the maximum tensile resistance load
between three brackets was evaluated using the Tukey test
(P < 0.01).

Ethical statement

Treatment case

The study was performed in accordance with the Declarationof Helsinki, and the treatment plan was approved by the
director of Shima Orthodontic Office.

The patient was a woman, aged 22 years and 1 month, who
presented with a chief complaint about her lateral profile
protrusion and irregularity of maxillary anterior teeth. The
lateral facial photograph shows protrusion of the upper andlower lips [Figure 3]. Pre-treatment dental casts revealed thatthe first molar relationship was Angle Class I on both sides[Figure 4].

In this paper, we evaluated the binding frictional resistance of
ISWs with different bracket combinations. We selected lower
binding frictional materials, based on tensile test results, andexemplified the use of the materials in a typical extraction
case.

Tensile test for the evaluation of binding friction
We conducted tensile tests to evaluate the binding
frictional resistance between the different wire andbracket combinations. The testing materials were as
follows: Metal slot-equipped plastic brackets [Figure 1a;
IPASS II, Ortho-Dentaurum, Tokyo, Japan], different
types of metal slot-equipped plastic brackets [Figure 1b;
Clear bracket SL+, Dentsply-Sirona K. K., Tokyo, Japan],and metal brackets [Figure 1c; Metal bracket, DentsplySirona K. K.]. All brackets were the standard type with a
0.018 × 0.025 – inch slot for the mandibular anterior teeth.
We used a universal testing machine (EZ-SX; Shimadzu
Corp., Kyoto, Japan) to measure the tensile resistance load
[Figure 2a].
Each bracket was fixed on an acrylic plate in a position
that would give a displacement of 0.5 mm in the verticaldirection using a full-size 0.018 × 0.025 – inch stainless steel
wire (SSW) (3M Unitek, Tokyo, Japan) with a 0.5 mm offset
bend [Figure 2b]. The interbracket distance was 7.0 mm,

Lateral cephalometric analysis demonstrated a normal
position between the maxilla and mandible and Frankfortmandibular plane angle showed a high angle. The maxillary
and mandibular anterior teeth were inclined labially tothe cranial base [Figure 5 and Table 1].[24] Based on thisinformation, the patient was diagnosed with an Angle
Class I malocclusion with lip protrusion, high mandibular
plane angle, maxillary anterior tooth crowding, and labial
inclination of the maxillary and mandibular anterior teeth.
To resolve the discrepancy and to improve the labial
inclination of the anterior teeth, we decided to extract the
maxillary and mandibular bilateral first premolars. The
anchorage value was maximum. However, binding friction
was expected to be reduced as much as possible by basing
the selection of materials on the tensile test results instead ofusing a reinforced anchorage device.

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Shima, et al.: Effectiveness of low binding frictional materials

b

a

c

d

Figure 2: Materials and tensile test apparatus. (a) Tensile test apparatus. (b) The 0.018 × 0.025 – inch bended stainless steel wire for fixing
brackets. (c) Brackets bonded on an acrylic plate. (d) Schematic drawing of the tensile test apparatus.

Figure 3: Pre-treatment facial and intraoral photographs.

RESULTS
Tensile test for the evaluation of binding friction
With the SSW, the average value of the maximum tensile
resistance load in combination with IPASS II bracket was
smallest at 1.04 N (106.0 gf), compared with the Clear
bracket SL+ value (1.53 N [156.0 gf]), and the metal bracket
APOS Trends in Orthodontics • Volume 9 • Issue 3 • July-September 2019

had the largest value at 2.21 N (225.3 gf) [Figure 6]. There
were significant differences between them. However, with the
ISW, the IPASS II bracket had a significantly smaller value
at 0.26 N (26.5 gf), compared with the Clear bracket SL+(0.47 N [47.9 gf]) and the Metal bracket (0.51 N [52.0 gf]).
Among all wire and bracket combinations, the SSW andmetal bracket combination had the largest maximum| 158

Shima, et al.: Effectiveness of low binding frictional materials

Treatment case
The intraoral photographs of the treatment progressat all patients’ visit are shown in Figures 7a-7i. After
extracting the maxillary and mandibular first premolars,
0.018 × 0.025 – inch pre-adjusted IPASS II brackets wereapplied to the maxillary arch, and a 0.016 – inch Ni-Tiround wire (Sentalloy Blue; Tomy International) was applied
as the initial archwire [Figure 7a]. In the following month,
the same brackets were also applied to the mandibular archand treatment was started with 0.016 × 0.022 – inch ISW[Figure 7b]. 2 months after beginning active treatment, a
0.016 × 0.022 – inch ISW was installed to the maxillary archand space closure was continued [Figure 7c]. 3 months after
initiating the active treatment, the use of the intermaxillaryelastics (3/16 M; Tomy International) was initiated to
improve the intermaxillary relationship [Figure 7d]. At
the same time, the maxillary canine distalization wascompleted, and maxillary anterior retraction was initiatedby sliding mechanics. Mandibular molar mesialization andlower anterior retraction were administered simultaneously.
The maxillary and mandibular arches were treated using
the same ISW wire until the end of the active treatment[Figures 7e-7i].

Figure 4: Pre-treatment dental casts.

Figure 5: Pre-treatment lateral and posteroanterior cephalometricand panoramic radiographs.

Figure 6: Results of the tensile test. **P < 0.01. ISW, improved
superelastic nickel-titanium alloy wire; SSW, stainless steel wire.

tensile resistance load, and the ISW and IPASS II bracket
combination had the smallest load: The difference wasapproximately eight-fold. The SSW had approximately
3–4 times the value of the ISW with the same bracket.

A rigid occlusal relationship was thereafter obtained, and thebrackets were removed [Figure 8]. The total active treatment
period was 9 months. After removing the devices, lingual
fixed retainers were bonded to the maxillary and mandibular
anterior teeth to stabilize the occlusion.
The patient’s lateral profile improved with retractionof the upper and lower lips [Figure 8]. Post-treatment
oral photographs and dental casts showed that the ideal
individual teeth arrangement and adequate occlusal
relationship were obtained [Figure 9]. The post-treatment
panoramic radiograph shows sufficient parallelism of the
tooth root [Figure 10]. Post-treatment cephalometric analysis
and comparison of the pre-treatment and post-treatment
cephalometric radiographs revealed that the maxillary andmandibular anterior teeth inclinations improved [Table 1].
The labiolingual position of the maxillary anterior teeth
also improved. Only intermaxillary elastics were used as thereinforced anchorage, although the amount of the maxillary
first molar mesialization was 1.0 mm [Figure 11]. The
patient was satisfied with the occlusion and facial esthetics.
Postretention stability of the occlusion was observed after
2 years [Figures 12 and 13].

DISCUSSION
The efficiency of orthodontic tooth movement in a
multibracket appliance is greatly affected by different typesof friction generated between the archwire and the bracket
slot.[13-20] With regard to friction classification, binding

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Shima, et al.: Effectiveness of low binding frictional materials

a

b

c

d

e

f

g

h

i

Figure 7: Intraoral changes in treatment progress. (a) At the start of active treatment. (b) 1 month later. (c) 2 months
later. (d) 3 months later. (e) 4 months later. (f) 5 months later. (g) 6 months later. (h) 7 months later. (i) 8 months later.
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Shima, et al.: Effectiveness of low binding frictional materials

Figure 8: Post-treatment facial and intraoral photographs.

Figure 9: Post-treatment dental casts.

friction is generated by tipping the archwire in the bracket
slot, which greatly affects tooth movement.[16] Movement
occurs immediately after the wire bending exceeds
the critical contact angle, and classical friction gives way
to elastic binding at this point [Figure 14].[16,17] The criticalcontact angle depends on the materials and design of thewire and the bracket. Various studies[13-22,25-29] on binding
friction have been reported. It is affected by the mechanicalproperties of the orthodontic archwire. Stiffer archwires
exacerbate binding and then increase the frictional
resistance between the archwire and the bracket slot.[20-22]
In the comparison between different wire materials, Ni-Ti
produced the least amount of friction.[19] As the angulation(and hence the binding) of the wire increases, there is a

Figure 10: Post-treatment lateral and posteroanterior cephalometricand panoramic radiographs.

greater increase in frictional force with stainless steel thanwith Ni-Ti. This phenomenon is attributed to the excellent
springback properties of Ni-Ti. Furthermore, based on one
study,[23] which was conducted using Ni-Ti orthodontic
wire products, ISW had a lower binding frictional
resistance due to its lower stress hysteresis and stiffness,compared with conventional Ni-Ti wire (Sentalloy Yellow;
Tomy International). Based on the findings in several
reports,[19-23] the Ni-Ti round wire and ISW have lower
binding frictional resistance. In particular, ISW has one

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Shima, et al.: Effectiveness of low binding frictional materialsof the least binding frictional resistances among various
orthodontic wires.
The frictional resistance is lower with the metal bracket thanwith the plastic bracket or the ceramic bracket.[13,19] For thisreason, metal slots are designed to be incorporated in many
plastic brackets, and the opening of the slot shape is often
devised in the ceramic bracket. Binding frictional resistanceof the IPASS II bracket is smaller than that of metal brackets
and other metal slot-equipped plastic brackets. The tensile

resistance load is the frictional resistance that is generated
between the deflected wire and the bracket slot. Since thewire is identical, it may be that the difference reflects thebinding frictional resistance, based on the slot type.
Binding friction in the vertical direction appears from thepoint where the wire tips and over the critical contact angle forbinding in the bracket slot.[16,17] The mesiodistal opening of ageneral bracket slot is a right angle and the opening of the IPASSII bracket is an obtuse angle, although the critical contact angle
for binding is larger and the binding friction may be smaller.
In the patient’s treatment, alignment was completed inapproximately 2 months [Figure 7]. Alignment of the anterior

Table 1: Changes in cephalometric variables.

Figure 11: Superimposed cephalometric tracings: Pre-treatment(black line) and post-treatment (red line).

SNA (°)
SNB (°)ANB (°)FMA (°)
U−1–SN (°)
U−1–NP (mm)
IMPA (°)
Interincisal
angle (°)

Pre‑treatment

Post‑treatment

82.3±3.578.9±3.5
3.4±1.828.8±5.2
104.5±5.6
11.7±2.7
96.3±5.8
124.1±7.6

82.8
77.7
5.1
37.8
113.1
16.9
93.0
109.0

82.6
76.8
5.8
38.0
99.1
11.8
84.4
131.2

FMA: Frankfort‑mandibular plane angle

Figure 12: Facial and intraoral photographs 2 years after treatment.

APOS Trends in Orthodontics • Volume 9 • Issue 3 • July-September 2019Japanese
norm

| 162

Shima, et al.: Effectiveness of low binding frictional materials

frictional resistance. This reduction could contribute as one
factor for more efficient orthodontic treatment in a shorttreatment period. However, the resistance to friction bybinding examined in this report is a factor that should beconsidered clinically, and more detailed examination of thefrictional behavior will be necessary with regard to the oral
environment (e.g., under the wet condition).

CONCLUSIONS••

Figure 13: Superimposed cephalometric tracings: Post-treatment(red line) and 2 years after treatment (green line).

•

The ISW had low binding frictional resistance, which
varied depending on different combinations of brackets.
A successful treatment was possible using ISW with alower binding frictional bracket in a typical extractioncase.
An effective treatment can be achieved by reducing thebinding frictional resistance.

Declaration of patient consent
The authors certify that they have obtained all appropriate patientconsent forms. In the form, the patient has given his consent for
his images and other clinical information to be reported in thejournal. The patients understand that their names and initials
will not be published and due efforts will be made to conceal
their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Nil.
Figure 14: Friction classifications and the interrelationship between
each classification.

teeth proceeded, along with canine distalization. Flare out of
the maxillary anterior teeth was not observed. The anterior
retraction was thereafter applied by sliding mechanics
using the ISW, and space closure was nearly completed in
4 months. The low binding friction of the ISW combinedwith the IPASS II bracket seemed to work effectively during
space closure.
The anchorage value of the maxillary molar was diagnosed
as maximum. As described in the treatment objectives, weapplied anchorage control using a light orthodontic force
without reinforced anchorage devices. As a result, a possible
reason for minimum (i.e., 1.0 mm) anchorage loss of themaxillary molar could be the low binding friction and the lightorthodontic force. It is noteworthy that the superelasticityof the ISW is not only effective in the alignment, the ISW
combined with the IPASS II bracket is but also effective, evenin the space closure, due to the low frictional properties.
Based on the findings of this report, the combined useof the ISW and IPASS II bracket may reduce the binding

Conflicts of interest
There are no conflicts of interest.

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How to cite this article: Shima Y, Koyama A, Uo M, Ono T. Effectivenessof low binding frictional materials: Evaluation of the binding frictional
resistance of improved superelastic nickel-titanium alloy wires withdifferent bracket combinations. APOS Trends Orthod 2019;9(3):156-64.