Download Interlot variations of transition temperature range and force delivery

ORIGINAL ARTICLE
Interlot variations of transition temperature range
and force delivery in copper-nickel-titanium
orthodontic wires
e C. Pompei-Reynoldsa and Georgios Kanavakisb
Rene
New York, NY, and Boston, Mass
Introduction: The manufacturing process for copper-nickel-titanium archwires is technique sensitive. The
primary aim of this investigation was to examine the interlot consistency of the mechanical properties of
copper-nickel-titanium wires from 2 manufacturers. Methods: Wires of 2 sizes (0.016 and 0.016 3 0.022 in)
and 3 advertised austenite finish temperatures (27 C, 35 C, and 40 C) from 2 manufacturers were tested for
transition temperature ranges and force delivery using differential scanning calorimetry and the 3-point bend
test, respectively. Variations of these properties were analyzed for statistical significance by calculating the
F statistic for equality of variances for transition temperature and force delivery in each group of wires. All
statistical analyses were performed at the 0.05 level of significance. Results: Statistically significant interlot variations in austenite finish were found for the 0.016 in/27 C (P 5 0.041) and 0.016 3 0.022 in/35 C (P 5 0.048)
wire categories, and in austenite start for the 0.016 3 0.022 in/35 C wire category (P 5 0.01). In addition, significant variations in force delivery were found between the 2 manufacturers for the 0.016 in/27 C (P 5 0.002),
0.016 in/35.0 C (P 5 0.049), and 0.016 3 0.022 in/35 C (P 5 0.031) wires. Conclusions: Orthodontic wires of
the same material, dimension, and manufacturer but from different production lots do not always have similar
mechanical properties. Clinicians should be aware that copper-nickel-titanium wires might not always deliver
the expected force, even when they come from the same manufacturer, because of interlot variations in the
performance of the material. (Am J Orthod Dentofacial Orthop 2014;146:215-26)
T
he introduction of copper-nickel-titanium (CuNiTi) archwires to the orthodontic specialty is
relatively recent. Although small variations in
the ratio of nickel to titanium can have meaningful
effects on the mechanical properties of orthodontic
archwires, the substitution of copper for some nickel
can maintain the shape-memory properties that make
nickel-titanium (NiTi) wires so popular, and make the
wire more stable and less sensitive to exact proportions
in the alloy.1 The clinically relevant claimed benefits of
CuNiTi over NiTi wires include more constant force
generation over longer activation spans, greater resistance to permanent deformation, more stable
a
Private practice, New York, NY.
Assistant professor, Department of Orthodontics and Dentofacial Orthopedics,
Tufts University School of Dental Medicine, Boston, Mass.
All authors have completed and submitted the ICMJE Form for Disclosure of
Potential Conflicts of Interest, and none were reported.
Address correspondence to: Georgios Kanavakis, 1 Kneeland St, Department of
Orthodontics 11th floor, Boston, MA 02111; e-mail, Georgios.Kanavakis@
tufts.edu.
Submitted, September 2013; revised and accepted, May 2014.
0889-5406/$36.00
Copyright Ó 2014 by the American Association of Orthodontists.
http://dx.doi.org/10.1016/j.ajodo.2014.05.017
b
superelasticity characteristics when cyclically loaded,
better spring-back, and less hysteresis.2 Additionally,
there have been claims that the CuNiTi manufacturing
process allows for more consistent transition temperatures, thus providing controlled force delivery individualized for each patient.2
NiTi wires can exist in 1 of 2 different physical states or
phases of molecular arrangement: martensite and
austenite. Martensite is the pliable, low-temperature
state, and austenite is the stiffer, high-temperature state.
The transformation temperature range consists of the
austenite start (As) temperature, when the alloy first begins
the transformation from martensite to austenite, and the
austenite finish (Af) temperature, when the transformation is complete and the alloy becomes uniformly
austenite. It is well known that the superelastic properties
and thus the clinical performance of NiTi archwires
directly depend on the transition temperatures of the alloy
and the alloy's potential to undergo molecular changes after mechanical (deflection) or thermal (temperature) stimuli.3 Therefore, if the addition of copper into the NiTi alloy
allows for more consistency in the transition temperature
of the produced wires, that should also be directly related
to better consistency in clinical performance.
215
Pompei-Reynolds and Kanavakis
216
According to the International Standards Organization, an accepted method of determining transition temperatures of superelastic alloys is thermal analysis via
differential scanning calorimetry (DSC).4 In this test, a
sample of metal is placed in a controlled chamber and
put through a cycle of cooling and heating. As superelastic wires transform through their various phases with
temperature changes, enthalpy is measured and
graphed. Peaks on the resultant curves represent the
temperatures at which the phase changes began and
ended, thus allowing determination of the transformation temperature ranges.5
Wire properties are extremely sensitive to the alloy
ratio; small amounts of dissolved interstitial elements
act as impurities and disrupt the NiTi crystal matrix and
therefore its transformation behavior. Additionally, the
manufacturer-specified parameters for the amount of
cold work and the duration and temperature of the heat
treatment and annealing processes greatly affect the
archwire's final transition temperature range and therefore its force delivery.6 Thus, it has been established
that wires of similar types from different manufacturers
do not necessarily possess similar properties because
these manufacturing conditions are not consistent.3
Furthermore, it is unclear to what extent all of these
manufacturing materials and conditions are tightly
controlled from production lot to production lot of wires
from the same manufacturer. Previous studies have
investigated the mechanical and thermal properties of
NiTi wires. However, the authors of these studies assumed
manufacturing consistency within companies, since
nearly all previous wire studies that compared manufacturers used only 1 wire sample from each manufacturer to
make comparisons.6-8 Bradley et al,6 examining transition temperature via DSC, first conducted a pilot study
and established that “excellent reproducibility was
achieved between nominally identical five segment samples of the same NiTi alloy,” leading the authors to
conclude that 1 sample from each lot was sufficient to
compare the wires from different manufacturers. Interestingly, potential differences between wires made from
the same manufacturer—but in different lots—have not
been explored. Therefore, the purpose of this descriptive
pilot study was to test the potential variability in mechanical and thermal properties among CuNiTi wires with the
same advertised characteristics (ie, dimensions, Af) from
the same company to determine whether future indepth studies of interlot variations are warranted. To
our knowledge, no peer-reviewed study detailing the
consistency of the NiTi manufacturing process has been
published in the orthodontic literature to date.
Transformation temperature range and force delivery
are 2 clinically relevant and intimately linked properties
August 2014 Vol 146 Issue 2
Table I. Number of wire types from different lots
tested via DSC to determine austenite start and finish
temperatures and via 3-point bend test to determine
force delivery
Wire specimens
tested, each
from a different
lot (n)
Manufacturer
Ormco
RMO
Size
0.016 in
0.016 3 0.022 in
0.016 in
0.016 3 0.022 in
0.016 3 0.022 in
0.016 in
0.016 3 0.022 in
0.016 in
0.016 3 0.022 in
0.016 3 0.022 in
ThreeMaterial
DSC point bend
CuNiTi 27 C
4
4
5
5
CuNiTi 27 C
CuNiTi 35 C
9
9
CuNiTi 35 C
4
4
CuNiTi 40 C
2
2
3
3
CuNiTi 27 C
CuNiTi 27 C
3
3
CuNiTi 35 C
3
3
3
3
CuNiTi 35 C
CuNiTi 40 C
3
3
Totals
39
39
Fig 1. TA Instruments DSC machine.
of NiTi wires: the force delivered by a wire depends on
whether a deflected wire is in the austenitic or martensitic configuration or a mixture thereof. The aim of this
in-vitro investigation was to evaluate the interlot consistency in the mechanical properties of CuNiTi orthodontic
archwires, by attempting to detect differences in As, Af,
and force delivery between different manufacturers.
When this study was conducted, CuNiTi archwires were
commercially available from only 2 manufacturers:
Rocky Mountain Orthodontics (RMO, Denver, Colo)
American Journal of Orthodontics and Dentofacial Orthopedics
Pompei-Reynolds and Kanavakis
217
and Ormco (Glendora, Calif). Therefore, all the CuNiTi
archwires tested were provided by those companies.
MATERIAL AND METHODS
The experimental sample consisted of CuNiTi wires of
2 dimensions (0.016 and 0.016 3 0.022 in), with variable advertised Afs (27 C, 35 C, and 40 C) and from
different production lots. The wires were grouped based
on the combination of dimension and Af, so each group
consisted of wires with the same dimension and the
same Af, but from different production lots. Production
lots were determined by the information available on the
package of each wire. The exact numbers of each type of
archwire used in this investigation are shown in Table I.
In total, 39 wire specimens were tested; 24 were Ormco
wires, and 15 were RMO wires.
The difference in numbers between the 2 companies
was because no more than a few lots were readily available from RMO at the time of the study. The examiner
(R.C.P.-R.) tested as many wires per lot as were easily
obtainable commercially. It was considered reasonable
to begin with the premise that enough precision should
exist in the materials manufacturing process from one
lot to another so that if even 2 lots of the same wire
type from the same manufacturer showed significant
differences in mechanical properties, important clinical
implications could be made, or at least this would elicit
the potential need for future studies. Wires from the
same manufacturer and the same production lot were
assumed to be identical at a molecular level, and therefore only 1 specimen from each lot was used to test the
mechanical properties of the lot.
To evaluate the consistency of transition temperature
ranges, each wire specimen was tested via thermal analysis with DSC (TA Instruments-Waters, New Castle, Del)
as shown in Figure 1. In preparation for DSC analysis, the
wires were sectioned in approximately 4-mm segments
weighing approximately 3.5 mg, taken from the posterior area of each sample arch form.
Each wire segment was weighed to the nearest
0.01 mg, placed in an aluminum crucible, and sealed.
An empty aluminum crucible served as the reference
during the DSC measurements. The temperatures of
the crucibles were scanned with nitrogen gas coolant
from 60 C to 60 C and back to 60 C to protect the
stability in the cell. Heating was achieved with electric
heat.
Each wire specimen and reference chamber were
heated and then cooled at 10 C per minute. The DSC
plots were qualitatively and quantitatively analyzed by
the DSC manufacturer's software, Universal Analysis
2000 (TA Instruments-Waters). Onset and end-set
Fig 2. Tinius Olsen HIKS 3-point bend machine.
temperatures of the martensitic and austenitic phases
were derived from the graphs produced by the software
according to standard transition temperature determination methods.
To measure the force produced by a wire as it returns to its original shape after it has been deflected,
in other words, the force experienced by teeth as they
are moved by a wire, a 3-point bending test was conducted by an examiner (R.C.P.-R.) using the Tinius Olsen HIKS testing machine (Tinius Olsen, Horsham, Pa).
The test was performed in accordance with ISO 15841
standard.4 The machine has 2 stainless steel fulcrums
10 mm apart between their respective centers to serve
as supporting points for the wire specimen, shown in
Figure 2. The entire apparatus was housed in a closed
chamber maintained at 37 C to simulate average oral
temperature.7
All wires were sectioned into 20-mm-long specimens, rested across the 2 supporting points, and left in
the temperature-stabilized testing chamber for 3 minutes, allowing the wire to reach the chamber temperature. Then each specimen was deflected from 0 to
3.1 mm and back to 0 mm with a stainless steel pointed
bar measuring 0.1 mm at its head, traveling at a speed of
10.0 mm per minute. A computer attached to the testing
machine recorded 1000 data points for the force placed
American Journal of Orthodontics and Dentofacial Orthopedics
August 2014 Vol 146 Issue 2
Pompei-Reynolds and Kanavakis
218
Table II. Af and As values for Ormco and RMO CuNiTi wires grouped by dimension and reported Af temperature
Wire (no. of
specimens
tested)
RMO 0.016
in/27 C (3)
Ormco 0.016
in/27 C (4)
Average
Af to
Af
As
Af
As
reported
difference
experimental experimental As As P value
Af
experimental experimental
from
Af Af P value P value
SD (F statistic) reported
mean
range
mean
range
reported SD (F statistic) (t statistic)
16.31
0.43
0.43
27.00
28.52
0.71
1.52 0.39
5.69
3.57
1.56
27.00
24.98
6.10
2.02
2.69
RMO 0.016
in/35 C (3)
Ormco 0.016
in/35 C (9)
18.25
2.78
1.44
35.00
31.34
2.35
3.66
1.18
14.75
12.94
4.49
35.00
34.11
7.17
0.89
2.35
RMO 0.016 3
0.022
in/27 C (3)
Ormco 0.016 3
0.022
in/27 C (5)
12.53
3.47
1.74
27.00
26.36
3.31
0.64
1.66
4.76
12.98
5.14
27.00
25.05
6.81
1.95
3.22
RMO 0.016 3
0.022
in/35 C (3)
Ormco 0.016 3
0.022
in/35 C (4)
15.40
0.82
0.42
35.00
30.00
0.71
5.00
0.36
12.39
12.39
5.74
35.00
32.17
5.18
2.83
2.29
RMO 0.016 3
0.022
in/40 C (3)
Ormco 0.016 3
0.022
in/40 C (2)
21.11
2.22
1.12
40.00
33.26
1.12
6.74
0.57
22.29
3.46
2.45
40.00
34.54
0.32
5.46
0.23
0.14
0.194
0.211
0.010y
0.319
0.041*
0.627
0.432
0.110
0.443
0.160
0.048*
0.136
0.540
0.063
*P \0.05; yP \0.010.
on and delivered by the wire through the entire loading
and unloading process. Rectangular wires were
measured under vertical loads that were applied on their
long sides, mimicking the forces that teeth would experience when being leveled occlusogingivally. To eliminate variability from cyclic loading, measurements
were taken only on specimens that had never been tested
or deflected before. To compare forces upon unloading,
values recorded at 2 mm of deflection were used for all
statistical analyses.
STATISTICAL ANALYSIS
Fig 3. Sample DSC graph showing how the intersection
of tangents to peak slopes are used to determine various
transition temperature points.
August 2014 Vol 146 Issue 2
To determine the statistical significance of the interlot variances between manufacturers in As and Af temperatures for each type of wire sampled, the F test of
equality of variances was conducted using the statistical
American Journal of Orthodontics and Dentofacial Orthopedics
Pompei-Reynolds and Kanavakis
219
Fig 4. Interlot DSC graphs for each type of wire tested, overlayed.
software R (version 2.11.1; R Development Core Team,
GNU Corporation, Auckland, New Zealand). The F test
was also used to compare the variances of the force exerted by each wire on unloading, at 2 mm of deflection
during the 3-point bend test.
One-sample t tests were performed to compare the
mean Af temperatures to those reported by the manufacturer. All statistical analyses were performed at the
P #0.05 level of significance.
RESULTS
The average As and Af temperatures for each type of
wire tested, along with descriptive statistics detailing the
spread of data in each group, are summarized in Table II.
Also, the graphs produced by the DSC testing machine
are presented in Figures 3 and 4.
DSC testing of 0.016-in archwires with an advertised
Af of 27 C showed a statistically significant difference in
American Journal of Orthodontics and Dentofacial Orthopedics
August 2014 Vol 146 Issue 2
Pompei-Reynolds and Kanavakis
220
Fig 4. (continued).
interlot variation in Af between the 2 manufacturers
(P 5 0.041). The average Af for the RMO 0.016 in/
27 C archwires was 28.52 C (SD, 0.39 C); for the Ormco
wires, the average Af was 24.98 C (SD, 2.69 C). No
significant differences were detected regarding As
(P 5 0.14).
DSC testing of the 0.016-in archwires with an advertised Af of 35 C showed no significant differences
between the 2 manufacturers in interlot variations of
Af (P 5 0.432) or As (P 5 0.194).
In DSC testing of the 0.016 3 0.022-in archwires
with an advertised Af of 27 C, similarly, the 2 manufacturers did not differ significantly in interlot variations of
Af or As for these archwires (Af, P 5 0.443; As,
P 5 0.211).
DSC testing of the 0.016 3 0.022-in archwires with
an advertised Af of 35 C showed a statistically significant
difference in interlot variations of both Af (P 5 0.048)
and As (P 5 0.010). It appeared that the variations in
Af and As were much greater in wires of this type made
by Ormco, as clearly shown by the significantly higher
standard deviations (Table II).
August 2014 Vol 146 Issue 2
The DSC testing of the 0.016 3 0.022-in archwires
with an advertised Af of 40 C showed no significant difference in interlot variations of Af and As between the 2
manufacturers (Af, P 5 0.540; As, P 5 0.319).
When the mean Af values of the archwires from both
manufacturers were compared with the advertised Af
values, no significant differences were apparent
(0.627 $ P $0.063), although most values were lower
than the advertised ones.
Furthermore, it appeared that for wires with a dimension of 0.016 3 0.022 in and an advertised Af of 40 C,
the actual Af values measured with the DSC test were
not only lower than 40 C but even lower than 35 C;
this places them in a totally different category of archwires (Table II).
The results of the 3-point bend test showed statistically significant differences between manufacturers in
interlot force delivery variations on unloading, after
2 mm of wire deflection (Table III). In both types of
0.016-in archwires, Ormco wires appeared to have
significantly greater interlot variations in force delivery
(27 C, P 5 0.02; 35 C, P 5 0.049). Similarly, when
American Journal of Orthodontics and Dentofacial Orthopedics
Pompei-Reynolds and Kanavakis
221
Table III. Unloading force exerted by wires at 2 mm
of unloading
Wire (no. of
specimens tested)
RMO 0.016 in/27 C (3)
Ormco 0.016 in/27 C (4)
Average
load at
2 mm (g)
94.38
112.91
Load at
2 mm,
range
2.27
87.62
Load at
2 mm,
SD
1.20
40.93
RMO 0.016 in/35 C (3)
Ormco 0.016 in/35 C (9)
87.16
95.44
8.67
87.17
4.52
28.42
RMO 0.016 3 0.022
in/27 C (3)
Ormco 0.016 3 0.022
in/27 C (5)
191.35
13.73
7.30
283.18
117.30
42.15
RMO 0.016 3 0.022
in/35 C (3)
Ormco 0.016 3 0.022
in/35 C (4)
160.50
13.23
7.07
214.18
131.81
56.19
RMO 0.016 3 0.022
in/40 C (3)
Ormco 0.016 3 0.022
in/40 C (2)
106.99
19.42
10.04
95.65
44.95
31.78
P
value
0.002y
0.049*
0.057
0.031*
*P \0.05; yP \0.010.
0.016 3 0.022-in archwires with an advertised Af of
35 C were tested, Ormco wires also had significantly
greater interlot variations in force delivery (P 5 0.031).
However, other types of 0.016 3 0.022-in archwires
did not appear to be statistically different between manufacturers in interlot force delivery variation (27 C,
P 5 0.057; 40 C, P 5 0.174).
DISCUSSION
One unique aspect of CuNiTi wires that has been
touted in some of its marketing is that each wire has
been engineered to have a specific Af temperature,
implying a significant correlation between a tightly
controlled Af and a specific force delivery. This allows
the clinician to choose a CuNiTi wire of 1 dimension
but with 3 varying degrees of force: the 27 C wire
would be austentic both at room temperature (average,
22 C) and at most oral temperatures (average, 36 C)9
for maximum force delivery. The 35 C wire should be
martensitic at room temperature but partially to totally
austenitic at oral temperatures. The 40 C wire is expected to be martensitic at both room and average
oral temperatures, theoretically allowing for the lightest delivery of forces. The validity of these claims depends on the accuracy and precision of the transition
temperature range for each wire; in this study, we attempted to confirm this while also correlating the
experimental transition temperatures to the force delivery, which is ultimately the property of concern to
clinicians.
The novelty of this study lies in its questioning of the
easy assumption that all wires of a similar type from the
same manufacturer have similar clinical behavior. However, our results demonstrate that this is not necessarily
the case. Data from the 2 manufacturers show different
degrees of variations in forces among production lots.
When interlot force variations are large, the doctor
cannot be assured of the same clinical outcomes among
patients.
Previous investigations have also tested the accuracy
of transition temperature values reported by wire
manufacturing companies. Kusy and Whitley8 used
DSC testing to evaluate the mechanical properties of
different types of archwires and found that actual Af
values are mostly a few degrees different from those reported by the manufacturers. Thus, they concluded that
companies tend to be relatively accurate in reporting the
Af values of their archwires. Similarly, Biermann et al10
examined the transition temperatures of Ormco CuNiTi
wires via DSC and found that for wires advertised to
have Afs of 27 C and 35 C, the experimental Af was
within 2 C of the claimed Af for every wire tested. For
the wires advertised to have an Af of 40 C, the experimentally determined Af was on average 4 C less at
36 C. This finding agrees with our results, which also
showed a significant difference between the actual and
reported Af values of CuNiTi archwires that are advertised to have an Af of 40 C.
Bierman et al10 speculated that this 4 C difference
could be related to an inherent variation between production lots of wire made by the same manufacturer.
Our study confirmed that interlot variations in transition
temperatures and force delivery are not uncommon in
CuNiTi archwires. As a result, commercially available
archwires do not necessarily perform as expected. This
is particularly true for wires with a higher advertised Af.
If there is variation among the transition temperatures of wires made by the same manufacturer because
of the technique-sensitive nature of the manufacturing
process, it stands to reason that a NiTi wire and its variants made by different manufactures would also exhibit
differences in mechanical properties. This question was
examined by Nakano et al,7 who conducted 3-point
bend tests on 42 categories of NiTi wires, including CuNiTi, made by 9 manufacturers. Their results showed
that NiTi wires (including thermal CuNiTi and superelastic NiTi) of different brands but of the same diameter
varied in unloading force at 1.5 mm of deflection by
as much as 337 g. For orthodontic purposes, forces
much lighter than this, in the range of 35 to 100 g, are
American Journal of Orthodontics and Dentofacial Orthopedics
August 2014 Vol 146 Issue 2
Pompei-Reynolds and Kanavakis
222
Table IV. Trend among As, Af, and force delivery on unloading at 2 mm of deflection
Wire and lot
RMO 0.016 in/27 C, 1218227
RMO 0.016 in/27 C, 1218629
RMO 0.016 in/27 C, 1218716
As-low ( C)
15.88*
16.31
16.74y
Af-low ( C)
28.26*
28.34
28.97y
As-Af range ( C)
12.38
12.03
12.23
Load at 2 mm (g)
93.47*
95.74y
93.92
RMO 0.016 in/35 C, 1221161
RMO 0.016 in/35 C, 1216937
RMO 0.016 in/35 C, 1216934
17.09*
17.8
19.87y
30.22*
32.57y
31.22
13.13
14.77
11.35
90.76y
88.63
82.09*
Ormco 0.016 in/27 C, 03C51C
Ormco 0.016 in/27 C, 05F24F
Ormco 0.016 in/27 C, 040928478
Ormco 0.016 in/27 C, 05B54B
4.26*
4.85
5.81
7.83y
27.5y
21.4*
24.51
26.52
23.24
16.55
18.7
18.69
114.98
181.79y
163.66
94.17*
Ormco 0.016 in/35 C, 04l316l
Ormco 0.016 in/35 C, 040932770
Ormco 0.016 in/35 C, 04l144l
Ormco 0.016 in/35 C, 05A451A
Ormco 0.016 in/35 C, 06A218A
Ormco 0.016 in/35 C, 04J505J
Ormco 0.016 in/35 C, 02l134l
Ormco 0.016 in/35 C, 02K591K
Ormco 0.016 in/35 C, 02J25J
7.18*
8.52
12.2
15.6
15.74
17.44
17.65
18.28
20.12y
35.57
29.19*
35.66
32.22
32.37
34.91
35.67
36.36y
35.05
28.39
20.67
23.46
16.62
16.63
17.47
18.02
18.08
14.93
128.24
141.13y
104.66
111.95
87.45
87.37
73.00
71.21
53.96*
RMO 0.016 3 0.022 in/27 C, 1218238
RMO 0.016 3 0.022 in/27 C, 1213370
RMO 0.016 3 0.022 in/27 C, 1216921
10.84*
12.44
14.31y
24.78*
26.2
28.09y
13.94
13.76
13.78
188.48
199.65y
185.92*
RMO 0.016 3 0.022 in/35 C, 1218663
RMO 0.016 3 0.022 in/35 C, 1216929
RMO 0.016 3 0.022 in/35 C, 1216930
15.03*
15.32
15.85y
29.97
29.66*
30.37y
14.94
14.34
14.52
163.38
165.68y
152.45*
RM0 0.016 3 0.022 in/40 C, 1216945
RMO 0.016 3 0.022 in/40 C, 1216941
RMO 0.016 3 0.022 in/40 C, 1213371
20.07*
20.98
22.29y
32.64*
33.39
33.76y
12.57
12.41
11.47
115.23y
109.92
95.81*
Ormco 0.016 3 0.022 in/27 C, 050943902 S1
Ormco 0.016 3 0.022 in/27 C, 06C327C
Ormco 0.016 3 0.022 in/27 C, 091260639
Ormco 0.016 3 0.022 in/27 C, 04J417J
Ormco 0.016 3 0.022 in/27 C, 02A411A*
3.65*
4.65
5.01
8.48
9.33y
21.95
21.8*
24.92
27.97
28.61y
25.6
17.15
19.91
19.49
19.28
343.80y
289.00
286.60
226.50*
270.00
Ormco 0.016 3 0.022 in/35 C, 05B79
Ormco 0.016 3 0.022 in/35 C, 050934403
Ormco 0.016 3 0.022 in/35 C, 06A490A
Ormco 0.016 3 0.022 in/35 C, 03A382A
2.12*
12.88
13.12
14.51y
30.32*
31.6
31.25
35.5y
28.2
18.72
18.13
20.99
291.80y
191.62
213.30
159.99*
Ormco 0.016 3 0.022 in/40 C, 06A317A
Ormco 0.016 3 0.022 in/40 C, 011228333
20.56*
24.02y
34.38*
34.7y
13.82
10.68
118.12y
73.17*
*Denotes the lowest value in the column category among the lots of the same wire type and manufacturer tested; ydenotes the highest value in the
column category among the lots of the same wire type and manufacturer tested.
This table shows evidence that there may be a correlation between low As and Af and higher forces; and conversely, a correlation between high As
and Af and lower forces.
considered optimal for tipping, rotating, extruding, and
root uprighting, the actions most often intended to be
the result of initial leveling and aligning with NiTi
wires.11
August 2014 Vol 146 Issue 2
If the force a superelastic orthodontic wire exerts on
a tooth is largely dependent on the temperature at
which it begins to transform from martensite to
austenite (As) and the temperature at which that
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Pompei-Reynolds and Kanavakis
223
Fig 5. Interlot load vs deflection curves for each type of wire tested, overlayed.
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Pompei-Reynolds and Kanavakis
224
Fig 5. (continued).
transformation is complete (Af), it follows that precise
engineering of each wire's transition temperature
range—both As and Af—is key for the wire to behave
clinically as expected. The data in Table IV agree with
this because they demonstrate a possible relationship
between both As and Af—not just Af as advertised—
and force delivery. Visual observations of these data
suggest a potential inverse relationship between As,
Af, and force delivery.
Several factors influence the final transition temperatures of a wire: the initial proportion of metals in the
alloy ingot, the annealing conditions, the amount of
cold work done, and the amount of time and temperature at which the wire is heat treated.6 Small variations
in these factors can yield huge effects on phase transformation temperatures. If the manufacturing process is
indeed as sensitive as indicated, then the question is
raised as to whether an orthodontist can reasonably
August 2014 Vol 146 Issue 2
expect manufacturers to be consistent in the force delivery properties of their archwires.
The results of this study demonstrate notable differences among production lots of the same wire type
from the same manufacturer. In addition, significant
differences were found between manufacturers in
certain wire types. It appeared that certain archwires
made by Ormco had greater interlot variabilities in transition temperature and force delivery values. This was
true for 0.016-in (Af, 27 C), 0.016-in (Af, 35 C), and
0.016 3 0.022-in (Af, 35 C) archwires. A cursory visual
inspection of the DSC graphs (Fig 4) and the load vs
deflection graphs (Fig 5) also highlights the differences
among the same wire types from various lots from the 2
manufacturers. On the DSC graphs, the slopes of
enthalpy and peak heights for each wire of the same
type and maker but from different lots are notably
not parallel for the Ormco wires. This indicates
American Journal of Orthodontics and Dentofacial Orthopedics
Pompei-Reynolds and Kanavakis
variations in martensite start, martensite finish, As, and Af
from lot to lot. Regarding RMO wires, the slopes of
enthalpy and peak heights from the DSC tests demonstrated that most wires in every category had near
parallel slopes of similar heights, indicating greater consistency in the transition temperature ranges for each
wire in every lot. Similar observations can be made
when looking at the overlaid graphs of load vs deflection.
A possible explanation for the differences between
the 2 manufacturers is related to the length of time
each company has had its product commercially available. Since Ormco CuNiTi wires have been available on
the market for several years, it is possible that Ormco
has been changing its parameters for these wires over
time, and this might account for the changes seen in
transition temperatures and force delivery. Since the
dates of wire manufacturing were not available on
the wire packages, it is possible that the wires we tested
were made several years ago, when Ormco might have
had a different manufacturing protocol. RMO has just
recently released its CuNiTi wire and presumably determined one formulation that has stayed consistent on
the few batches of wires released to date.
Another possible reasoning behind these differences
could be associated with how different factors are being
considered by companies during the manufacturing process. For example, it appears that not only Af but also As
could impact the overall performance of a wire and
affect force delivery values.
There are several limitations of this study. Because of
the availability of wires at the time of data collection,
the sample sizes for each type of wire were small and
not equal for RMO and Ormco. In addition, there was
no method of randomization in the selection of wires to
be tested, so it is not clear whether the wire samples
were equally representative of all the lots produced by
each company. Nevertheless, it was considered reasonable
to assume that a manufacturing process should be precise
enough to have low tolerances in material properties from
one production lot to another. Although a small sample
size might have reduced the ability to determine the statistical significance in the variations, it is clinically significant that the ranges of force delivery from lot to lot were
as high as 131.8 g of force for what should have been the
same wire. Since only one sample from every lot was
tested there is the possibility of intralot variation, which
could not be detected but is reasonably expected to be
significantly less than interlot variation. Therefore, our results must be reinforced by future studies including larger
samples of wires. However, it is important to identify significant variations of mechanical and thermal properties
in the same manufacturer. Due to these variations, results
from previous studies should be viewed critically because
225
they are based on measurements of one wire from each
manufacturer, assuming excellent quality control by the
companies.6-8,10
It should also be considered that the 3-point bend
test is an in-vitro test that poorly mirrors in-vivo conditions. Clinically, when a wire is engaged in the brackets,
the incisogingival and buccolingual displacements of
every tooth as well as the mesiodistal distances between
brackets all affect the amount of force delivered by a
wire to a tooth. Additionally, as the wire undergoes cyclic
loading with masticatory function and temperature
variations throughout the day in the moist oral environment, force delivery is again affected. Therefore, it is
important to remember that the numbers given here
and in similar studies for force delivery are valid only
in that they allow comparison of one wire's behavior
relative to another's in one isolated environment under
similar conditions and do not actually reflect the forces
experienced by teeth in a given clinical situation.
CONCLUSIONS
Wires of the same materials, dimensions, and
manufacturer but from different production lots do
not always have similar mechanical properties. Differences in interlot variations exist between manufacturers
of CuNiTi archwires. Improvements should be made in
the manufacturing process of archwires to provide clinicians with CuNiTi archwires that have consistent mechanical properties.
ACKNOWLEDGMENTS
The authors would like to thank Matthew Finkelman,
PhD, for his help with the statistical analysis.
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