Download Article Reference - Archive ouverte UNIGE

Article
Light-curing time reduction with a new high-power halogen lamp
STAUDT, Christine Bettina, et al.
Abstract
Orthodontic brackets are routinely bonded with light-cured adhesives. Conventional halogen
lights used in bonding have the disadvantage of a long curing time, and the available
alternatives (laser and plasma lights) are expensive. Our aim was to investigate the minimum
time necessary to bond brackets with a new, relatively low-priced, high-power halogen light.
Reference
STAUDT, Christine Bettina, et al. Light-curing time reduction with a new high-power halogen
lamp. American Journal of Orthodontics and Dentofacial Orthopedics, 2005, vol. 128, no.
6, p. 749-54
DOI : 10.1016/j.ajodo.2004.08.020
PMID : 16360916
Available at:
http://archive-ouverte.unige.ch/unige:85013
Disclaimer: layout of this document may differ from the published version.
ORIGINAL ARTICLE
Light-curing time reduction with a new
high-power halogen lamp
Christine Bettina Staudt,a Anestis Mavropoulos,a Serge Bouillaguet,b Stavros Kiliaridis,c and Ivo Krejcid
Geneva, Switzerland
Introduction: Orthodontic brackets are routinely bonded with light-cured adhesives. Conventional halogen
lights used in bonding have the disadvantage of a long curing time, and the available alternatives (laser and
plasma lights) are expensive. Our aim was to investigate the minimum time necessary to bond brackets with
a new, relatively low-priced, high-power halogen light. Methods: Five groups of 15 deciduous bovine
incisors were bonded with stainless steel brackets (Mini Diamond Twin, Ormco, Orange, Calif) by using
different lamps and curing times. Three of the groups were bonded by using a high-power halogen light
(Swiss Master Light, Electro Medical Systems, Nyon, Switzerland) for 2, 3, and 6 seconds, respectively. The
fourth group, bonded with a fast halogen light (Optilux 501, Sybron Dental Specialties, Danbury, Conn) for 40
seconds, served as the positive control group. The fifth group, the comparison group, was bonded with a
plasma light (Remecure, Remedent, Deurle, Belgium) for 4 seconds. After storage for 24 hours in the dark at
37°C in water, shear bond strength was measured with a universal testing machine. Results: A curing time
of 2 seconds with the high-power halogen light negatively affected the bond strength and the probability of
bond survival. The adhesive remnant index scores were not significantly different among the groups. Most
failures (⬎ 60%) occurred at the bracket base/adhesive interface. Conclusions: The high-power halogen
light seems to be a cost-effective solution to reducing curing time. The recommended curing times to bond
stainless steel brackets are 6 seconds and, with caution, even 3 seconds. (Am J Orthod Dentofacial Orthop
2005;128:749-54)
O
rthodontic brackets are routinely bonded by
using chemical or light-curing adhesive systems. With light-curing materials, the clinician
has sufficient time to position the brackets accurately
and initiate polymerization when ready. The disadvantage, however, of conventional halogen lights is the
long curing time required for each bracket (up to 40
seconds); this wastes valuable clinical time.
The curing efficiency of conventional halogen
lights is limited, because 98% of their radiation does
not contribute to polymerization but is lost as heat.1 In
addition, only part of the halogen light spectrum is
useful because the absorption spectrum of camphorquinone is comparatively narrow.2 To reduce curing time,
light sources with higher power densities have been
introduced.
Argon lasers can achieve bond strengths comparaFrom the University of Geneva, Geneva, Switzerland.
a
Research associate, Department of Orthodontics.
b
Lecturer, Department of Cariology and Endodontology.
c
Professor and chair, Department of Orthodontics.
d
Professor and chair, Department of Cariology and Endodontology.
Reprint requests to: Dr Christine Bettina Staudt, Faculté de Médecine, Section
de Médecine Dentaire, Division d’Orthodontie, 19, rue Barthélemy-Menn,
CH-1205 Geneva, Switzerland; e-mail, [email protected].
Submitted, June 2004; revised and accepted, August 2004.
0889-5406/$30.00
Copyright © 2005 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2004.08.020
ble to the 40 seconds of light-curing with conventional
devices in only 10 seconds3-6 or even 5 seconds.7
Unfortunately, argon lasers are larger and more expensive than the conventional devices.
Xenon plasma lights are less expensive than argon
laser lights, but they are 3 times more expensive than
halogen lights.8 Conflicting information exists concerning curing time for xenon plasma lights. Some authors
recommend short exposure times of 2 or 3 seconds.9-11
Others suggest longer exposure times from 6 to 9
seconds2,12-14 to avoid compromising the mechanical
properties or eluting of monomers.15
Light-emitting diode (LED) units vary widely in
their power density. The LEDs evaluated until now for
bonding orthodontic brackets have relatively low
power densities with irradiation times of 20 to 40
seconds.8,16-18 These irradiation times are similar to
halogen lights. Only 1 study describes a curing time of
10 seconds as sufficient for a specific LED,17 but this
low curing time does not agree with findings that 20
seconds are necessary with this lamp.18 LED units are
relatively inexpensive. No information is available on
the new generation of high intensity LEDs.
The light-curing devices investigated in other studies have either a long curing time (conventional halogen lights, low power density LEDs) or a comparatively high price (laser, plasma-lights).
749
750 Staudt et al
Table I.
American Journal of Orthodontics and Dentofacial Orthopedics
December 2005
Technical characteristics of 3 curing lights
Curing device
Swiss Master Light
Optilux 501
Remecure
Type
High-power halogen
Fast halogen
Plasma
The aim of this study was to evaluate a relatively
low-cost, high-power halogen light (Swiss Master
Light, no. M 00042, Electro Medical Systems SA,
Nyon, Switzerland) to estimate the minimum exposure
time necessary to achieve bond strength equivalent to
the bond strength obtained with a fast halogen light.
These results were also compared with a plasma light,
which until now has been the most advanced device
available for curing time and price. Our null hypothesis
was that the shear bond strength obtained with the new
high-power halogen light for short irradiation periods
would be equivalent to a fast halogen light with a
relatively long curing time.
MATERIAL AND METHODS
Seventy-five deciduous bovine incisors (extracted
about age 18 months) were divided into 5 groups of 15
teeth. They were stored in 0.2% thymol for 2 to 3
months. The root of each tooth was cut off, and the
crown was polished for 15 seconds with a handpiecebrush by using water and a fluoride-free pumice
(Bimsstein Pulver, Prochimie, Avenches, Switzerland).
Each tooth was then rinsed thoroughly under tap water
for 20 seconds.
The teeth were dried with oil-free air for 5 seconds
and etched on the buccal side for 30 seconds with 35%
phosphoric acid gel (Transbond XT etching gel, no.
712-039, 3M Unitek, Monrovia, Calif). Each tooth was
then rinsed under tap water for 30 seconds and dried
again with oil-free air for 5 seconds, followed by
application of the primer (Transbond XT Light Cure
Adhesive Primer, no. 712-034, 3M Unitek). Composite
(Transbond XT Light Cure Paste, no. 712-036, 3M
Unitek) was applied to the bracket-base (stainless steel
brackets: Mini Diamond Twin, for the maxillary right
central incisor, no. 351-0130, Ormco, Orange Calif).
The bracket was then firmly placed onto the flattest area
of the crown with respect to the occlusal/gingival side.
Excess composite was carefully removed. Light-curing
was performed with the following devices and curing
times:
Three groups were bonded with a high-power
halogen light (Swiss Master Light) in the fast-cure
mode. In the first 2 groups, curing times of 2 and 3
seconds were used. The light tip was positioned on the
Power density
Wavelength
Tip diameter
3000 mW/cm2
1000 mW/cm2
1600 mW/cm2
390-530 nm
400-520 nm
435-490 nm
11 mm
8 mm
5.5 mm
middle of the bracket during each exposure. For the
third group, the total curing time was 6 seconds.
Because the manufacturer suggested not more than 4
seconds of curing time at once, the time of 6 seconds
was divided into 3 seconds on the mesial side and 3
seconds on the distal side of the bracket.
The fourth group was bonded with a fast halogen
light (Optilux 501 with turbo light guide, serial no.
53109080, Sybron Dental Specialities, Danbury, Conn)
for 20 seconds on the mesial side and 20 seconds on the
distal side of the bracket (40 seconds total). This group
served as a positive control.
The fifth group, the comparison group, was bonded
with a xenon plasma arc curing light (Remecure, model
CL-15E, serial no. CL15E-A0100, Remedent, Deurle,
Belgium) for 2 seconds on the mesial side and 2
seconds on the distal side of the bracket. The total
curing time of 4 seconds was according to the light’s
operation instructions. The technical characteristics
of these curing lights are described in Table I.
We made sure that the polymerization of the composite was not affected by the position of the lightguiding tip with respect to the bracket. The large tip of
the high-power halogen lamp during the 1-shot light
exposure covered the bracket completely. Throughout
the mesial and distal light exposures, the tip overlapped
the middle of the bracket (about 1 mm) to resemble the
coverage of the high-power halogen lamp. Therefore,
even with the smallest light-guiding tip diameter, the 2
radii of light it produced covered all areas around the
bracket. Furthermore, the tip was held at an angle of
90° to the bracket to direct the light at the same angle
on each occasion.
The shear bond strength was measured with a
universal testing machine (Model 1114, serial no.
H-1377, Instron Corp, Canton, Mass) (Fig 1). A rectangular rod transmitted the shear force on the occlusal
edge of the bracket base until the bracket failed. The
following procedure of standardization was developed
to ensure that the bracket base would be parallel to the
force-exerting rod during bond strength testing.9,19
Each bonded specimen was embedded in a rectangular acrylic block (Technovit 4071, Lot 021584,
Ch.-B. 011102, Heraeus Kulzer, Wehrheim/Taunus,
Germany). For this purpose, a metallic cubic blade (2 ⫻
Staudt et al 751
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 128, Number 6
tooth; 3, all adhesive left on the tooth with a distinct
impression of the bracket mesh.
Statistical analyses
Fig 1. Assembly for shear bond strength testing: acrylic
block with bonded tooth secured onto lower jaw of
testing machine. Force-exerting rod moved along axis
parallel to tooth-bracket interface, and its extremity was
parallel to occlusal border of bracket base.
2 mm) was fixed into the vertical slot of the bracket to
ensure that the tooth-bracket interface was parallel to
and projecting slightly above the acrylic surface.10
When this blade was laid over the mold, the bracket
was situated in the middle of the acrylic block, and the
edges of the bracket base were parallel to the edges of
the block. After storage for 24 hours in the dark at
37°C2,12,20 in water,21 the acrylic block with the bonded
tooth was secured onto the lower jaw of the testing
machine. The shear bond force was applied by the rod.
The samples were stressed in an occlusogingival direction with a crosshead speed of 0.5 mm/minute.22 Due to
our special embedding procedure, the direction of the
shear force exerted by the rod was parallel to the
tooth-bracket interface, and the edge of the rod was
parallel to the occlusal edge of the bracket base (Fig 1).
The force necessary to debond the brackets was recorded in Newtons and then converted into megapascals (MPa) by dividing the Newtons by the surface area
12.41 mm2 of the bracket base.9,19
The adhesive remnant index (ARI) was used to
measure according to the following 4-point scale introduced by Årtun and Bergland23: 0, no adhesive left on
the tooth; 1, less than half of the adhesive left on the
tooth; 2, more than half of the adhesive left on the
For the statistical evaluation, SPSS software (version 11.5, SPSS, Chicago, Ill) was used. The level of
significance was set at P ⬍ .05.
Because the distribution of the shear bond strength
values was normal, parametric tests were applied. The
mean, standard deviation, and 95% confidence interval
of the mean were estimated for each group.
Comparisons of shear bond strength between lights
and curing times were performed by analysis of variance (ANOVA). If a statistically significant difference
was found, the post-hoc Tukey test was used to identify
which groups were different. Additionally, the groups
were classified into Tukey homogeneous subsets. The
Weibull survival analysis was also applied to compare
the performance of each sample by calculating survival
probability at given values of shear stress.10,13
The nonparametric Kruskal-Wallis test was used to
test for significant differences between groups regarding the ARI.
RESULTS
Descriptive statistics for shear bond strength of the
different groups are shown in Table II.
ANOVA indicated statistically significant differences between groups (P ⬍ .001). Shear bond strength
was significantly lower for the 2-second high-power
halogen light than the 6-second high-power halogen
light (P ⫽ .001) , the fast halogen (P ⫽ .002), and the
xenon plasma arc light (P ⫽ .007) (Table II).
The Tukey homogeneous subsets test showed a
weaker subset, which included the 2- and 3-second
high-power halogen groups (P ⫽ .06). The stronger
subset included the 3- and 6-second high-power halogen light, the fast halogen, and the plasma light (P ⫽
.565).
The bracket survival probability at given levels
of shear stress, estimated with the Weibull survival
distribution, is shown in Figure 2. The curve of the
2-second high-power halogen lamp differs significantly: the survival probability declines more rapidly
at a lower stress level; eg, the vertical dotted lines in
Figure 2 correspond to 6-8 MPa that is suggested as
the minimum acceptable bracket bond strength for
clinical use.24 In the range of 6-8 MPa, the probability of survival would be approximately 80% for
the 2-second high-power halogen light, whereas it
would be above 90% for the other groups. A shear
stress of approximately 10 MPa would cause 50%
bond failures in the case of the 2-second high-power
752 Staudt et al
Table II.
American Journal of Orthodontics and Dentofacial Orthopedics
December 2005
Descriptive statistics for shear bond strength (MPa)
95%
Curing device*
Swiss Master Light
Optilux 501
Remecure
Total
Time
(seconds)
2
3
6
40
4
†
N
Mean
SD
Lower
Upper
Minimum
Maximum
15
15
15
15
15
75
10.4b
16.6
20.1a
19.2a
18.5a
17.0
4.1
5.4
6.6
6.9
7.8
7.0
8.1
13.6
16.4
15.4
14.1
15.3
12.6
19.6
23.7
23.0
22.8
18.6
4.8
8.6
9.5
7.2
8.5
4.8
22.6
25.0
32.7
29.5
33.2
33.2
*Swiss Master Light, high-power halogen light; Optilux 501, fast halogen light; Remecure, xenon plasma arc light.
†a, b
significant difference (P ⬍ .05) between groups with different symbols.
Fig 2. Bracket survival probability plotted against shear stress [MPa] (Weibull survival analysis) for
high-power halogen (Swiss Master Light, SML), fast halogen (Optilux 501), and xenon plasma arc
light (Remecure) with different curing times (seconds). Vertical dotted lines represent stress range
of 6-8 MPa. Horizontal dotted line corresponds to failure probability of 50%.
halogen lamp, whereas, to reach the same bond
failure rate, a stress of over 15 MPa would be
required for the other groups.
No statistically significant differences were found
between the groups regarding the ARI (P ⫽ .72) (Table
III). More than half of the adhesive was left on the tooth
in most cases (⬎60%) in every group after debonding.
DISCUSSION
These results support our hypothesis that the highpower halogen light minimizes curing time dramatically
without compromising the bond strength of orthodontic
brackets. Exposure to the high-power halogen light for 6
seconds and even for 3 seconds led to shear bond strengths
equivalent to those achieved after 40 seconds of exposure
to the fast halogen light. An exposure time of only 2
seconds is, however, not recommended because it led to
significantly inferior shear bond strength values. An exposure time of 3 seconds should be used with caution,
because the 3-second group was not significantly different
from the 2-second group. We, therefore, recommend a
curing time of 6 seconds for bonding stainless steel
brackets. For ceramic brackets, the curing time might be
less because the light polymerizes the composite through
transillumination of the bracket as well.
Curing time is reportedly not influenced by the
bandwidth of the emission spectrum of different lights
as long as it corresponds approximately to the absorption spectrum of camphorquinone. It is shown that the
double bond conversion rate of resin composites is not
influenced by the emission spectrum, if the irradiated
light energy is the same.25 Therefore, the parameter that
allowed relevant reduction in curing time in our study
seems to be the increase in power density (3000
mW/cm2 for the high-power halogen light compared
with 1000 mW/cm2 for the fast halogen light). However, such a high power density might not be necessary
to reduce curing time in the order of magnitude we
Staudt et al 753
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 128, Number 6
Table III.
Frequency distribution of ARI
ARI scores†
Curing device*
Time (seconds)
0
1
2
3
Swiss Master Light
2
3
6
40
4
0 (0.0%)
0 (0.0%)
0 (0.0%)
0 (0.0%)
0 (0.0%)
1 (6.7%)
0 (0.0%)
1 (6.7%)
2 (13.3%)
2 (13.3%)
9 (70.3%)
13 (87.1%)
10 (60.7%)
11 (73.3%)
9 (70.3%)
5 (33.3%)
2 (13.3%)
4 (26.6%)
2 (13.3%)
4 (26.6%)
Optilux 501
Remecure
*Swiss Master, high-power halogen light; Optilux 501, fast halogen light; Remecure, xenon plasma arc light.
†
0, no adhesive left on tooth; 1, less than half of adhesive left on tooth; 2, more than half of adhesive left on tooth; 3, all adhesive left on tooth.
demonstrated. The xenon plasma arc light had a power
density of only 1600 mW/cm2. Exposure to the xenon
plasma arc light for 4 seconds was sufficient to obtain
bond strength equivalent to that achieved after 3 or 6
seconds of exposure to the high-power halogen light
and after 40 seconds to the fast halogen light. This
observation leads to the assumption that, from a certain
power density on, further increase in power density
ceases to reduce curing time considerably. Such a
threshold was previously detected, suggesting that there
might be no benefit to polymerization of resin composite when power density is increased above approximately 1000 mW/cm2.26
The innovations of the high-power halogen light are
its high power density and the unique size of its
light-guiding tip (diameter, 11 mm). The light-guiding
tip covers the entire bracket, and, in our study, a 1-shot
application of 3 seconds was shown to be acceptable.
However, the power increase of the high-power halogen light, made possible by an innovative watercooling system, is accompanied by a bulky (270 ⫻ 205
⫻ 256 mm) and heavy (7 kg) device. Nevertheless, the
high-power halogen lamp is considerably less expensive than the laser or the plasma lamp. Only the LED
lamps are less expensive, but no information is available on the new generation of high-intensity LEDs.
The bond strength to bovine enamel is weaker than
to human enamel.27 Nevertheless, the use of bovine
enamel from deciduous teeth (as in this study) instead
of permanent bovine enamel resembles more closely
the bond strength values of human enamel.27 Even
when human enamel is used in vitro and in vivo, there
is a difference in bond strength: in-vivo values are
significantly lower.28 Thus, a direct extrapolation of
in-vitro observations to the clinical situation is difficult,
and clinical studies are necessary. Furthermore, clinical
studies could determine the minimum bracket bond
strength necessary to withstand the forces due to
occlusal loading. Recent information on such a threshold is lacking in the literature, and the only indication
for the minimum necessary bond strength is given by
Reynolds,24 who suggested 6-8 MPa. If we rely on this
finding, the mean shear bond strength of every group in
our study would be sufficient (Table II, Fig 2). Even the
minimum in every group would be within Reynolds’
suggested limits, with the exception of the 2-second
light-curing with the high-power halogen light. However, the bond strength values measured in this study
are common in analogous studies.2,9,12,20 One must also
consider that in-vivo bond strength values are lower.28
The site of bond failure was unaffected by light
source and curing time. Most failures at debonding
were located at the bracket-adhesive interface; this
agrees with other shear-bond testing studies.6,8,11,13,19
The relatively weak mechanical retention between the
composite and the bracket base reduces the risk of
enamel fractures.
CONCLUSIONS
The results of this study support the use of the
high-power halogen light. It is a cost-effective solution
to reduce light-curing time. The recommended curing
time for bonding stainless steel brackets is 6 seconds or,
with caution, even 3 seconds. Our results are based on
in-vitro laboratory conditions, and the clinical relevance of our findings should be confirmed in vivo.
We thank Dr M. Cattani for his help with the shear
bond strength testing.
REFERENCES
1. Althoff O, Hartung M. Advances in light curing. Am J Dent
2000;13(spec no):77D-81D.
2. Oesterle LJ, Newman SM, Shellhart WC. Rapid curing of
bonding composite with a xenon plasma arc light. Am J Orthod
Dentofacial Orthop 2001;119:610-6.
3. Blankenau RJ, Kelsey WP, Powell GL, Shearer GO, Barkmeier
WW, Cavel XT. Degree for composite resin polymerization with
visible light and argon laser. Am J Dent 1991;4:40-2.
4. Kurchak M, Desantos B, Powers J, Turner D. Argon laser for
light-curing adhesives. J Clin Orthod 1997;31:371-4.
754 Staudt et al
5. Weinberger SJ, Foley TF, McConnell RJ, Wright GZ. Bond
strengths of two ceramic brackets using argon laser, light, and
chemically cured resin systems. Angle Orthod 1997;67:173-8.
6. Talbot TQ, Blankenau RJ, Zobitz ME, Weaver AL, Lohse CM,
Rebellato J. Effect of argon laser irradiation on shear bond
strength of orthodontic brackets: an in vitro study. Am J Orthod
Dentofacial Orthop 2000;118:274-9.
7. Lalani N, Foley TF, Voth R, Banting D, Mamandras A. Polymerization with the argon laser: curing time and shear bond
strength. Angle Orthod 2000;70:28-33.
8. Dunn WJ, Taloumis LJ. Polymerization of orthodontic resin
cement with light-emitting diode curing units. Am J Orthod
Dentofacial Orthop 2002;122:236-41.
9. Sfondrini MF, Cacciafesta V, Pistorio A, Sfondrini G. Effects of
conventional and high-intensity light-curing on enamel shear
bond strength of composite resin and resin-modified glassionomer. Am J Orthod Dentofacial Orthop 2001;119:30-5.
10. Pettemerides AP, Ireland AJ, Sherriff M. An ex vivo investigation into the use of a plasma arc light when using a visible
light-cured composite and a resin-modified glass poly(alkenoate)
cement in orthodontic bonding. J Orthod 2001;28:237-44.
11. Klocke A, Korbmacher HM, Luck LG, Ghosh J, Kahl-Nieke B.
Plasma arc curing of ceramic brackets: an evaluation of shear
bond strength and debonding characteristics. Am J Orthod
Dentofacial Orthop 2003;124:309-15.
12. Ishikawa H, Komori A, Kojima I, Ando F. Orthodontic bracket
bonding with a plasma-arc light and resin-reinforced glass
ionomer cement. Am J Orthod Dentofacial Orthop 2001;120:5863.
13. Klocke A, Korbmacher HM, Huck LG, Kahl-Nieke B. Plasma
arc curing lights for orthodontic bonding. Am J Orthod Dentofacial Orthop 2002;122:643-8.
14. Dietschi D, Marret N, Krecji I. Comparative efficiency of plasma
and halogen light sources on composite micro-hardness in
different curing conditions. Dent Mater 2003;19:493-500.
15. Munksgaard EC, Peutzfeldt A, Asmussen E. Elution of TEGDMA and BisGMA from a resin and a resin composite cured
with halogen or plasma light. Eur J Oral Sci 2000;108:341-5.
American Journal of Orthodontics and Dentofacial Orthopedics
December 2005
16. Bishara SE, Ajlouni R, Oonsombat C. Evaluation of a new curing
light on the shear bond strength of orthodontic brackets. Angle
Orthod 2003;73:431-5.
17. Swanson T, Dunn WJ, Childers DE, Taloumis LJ. Shear bond
strength of orthodontic brackets bonded with light-emitting diode
curing units at various polymerization times. Am J Orthod
Dentofacial Orthop 2004;125:337-41.
18. Usumez S, Buyukyilmaz T, Karaman AI. Effect of light-emitting
diode on bond strength of orthodontic brackets. Angle Orthod
2004;74:259-63.
19. Sfondrini MF, Cacciafesta V, Klersy C. Halogen versus highintensity light-curing of uncoated and pre-coated brackets: a
shear bond strength study. J Orthod 2002;29:45-50.
20. Oesterle LJ, Newman SM, Shellhart WC. Comparative bond
strength of brackets cured using a pulsed xenon curing light with
2 different light-guide sizes. Am J Orthod Dentofacial Orthop
2002;122:242-50.
21. Fox NA, McCabe JF, Buckley JG. A critique of bond strength
testing in orthodontics. Br J Orthod 1994;21:33-43.
22. Eliades T, Brantley WA. The inappropriateness of conventional
orthodontic bond strength assessment protocols. Eur J Orthod
2000;22:13-23.
23. Årtun J, Bergland S. Clinical trials with crystal growth conditioning as an alternative to acid-etch enamel pretreatment. Am J
Orthod 1984;85:333-40.
24. Reynolds IR. A review of direct orthodontic bonding. Br J
Orthod 1975;2:171-8.
25. Yoon TH, Lee YK, Lim BS, Kim CW. Degree of polymerization
of resin composites by different light sources. J Oral Rehabil
2002;29:1165-73.
26. Musanje L, Darvell BW. Polymerization of resin composite
restorative materials: exposure reciprocity. Dent Mater 2003;19:
531-41.
27. Oesterle LJ, Shellhart WC, Bellanger GK. The use of bovine
enamel in bonding studies. Am J Orthod Dentofacial Orthop
1998;113:514-9.
28. Murray SD, Hobson RS. Comparison of in vivo and in vitro
shear bond strength. Am J Orthod Dentofacial Orthop 2003;123:
2-9.