Download effect of light intensity on the cure characteristics of photo

EFFECT OF LIGHT
INTENSITY ON THE
CURE
KASSIM B. A.
CHARACTERISTICS OF
JUNE 15, 2011.
PHOTO-POLYMERISED
AUTHORS
KASSIM B. A
Tutorial Fellow, Department of
Conservative and Prosthetic Dentistry,
University of Nairobi.
KISUMBI B. K.
Senior lecturer, Department of
Conservative and Prosthetic Dentistry,
University of Nairobi.
Introduction
The demand for aesthetic restorative
materials has led to a widespread use
of photo-activated direct resin
composites in many parts of the world
(Bala et al 2005)
Modern dental composite restorations
are wholly dependent on the use of
Visible Light Curing devices.
The output of these devices is among
many interrelated factors that
influence the polymerisation and
quality of light activated resin
composites.
The factors include
material characteristics,
the tooth structure
the light source and manner of use and the
duration of exposure of the material to the
light. (Ralph Rawls 2003, Rahiotis et al 2004,
Gritsch et al 2008).
The main factors that are important in
the light source
intensity output
the band width
curing mode
filter quality (where applicable)
light tip characteristics – including
diameter, distance from the
restoration and direction of the light
Naturally, the desire among restorative
dentists is to produce an optimally
polymerised (cured) composite
restoration within a short time so as to
save chair time
Control the cost of the restoration.
This desire has fuelled the search for light
curing machines with ideal output
characteristics
The focus in this search has been to
increase the light intensity output,
narrow the band width
Remove unnecessary heat and glare.
The outcome has been the arrival of
light curing devices with intensities as
high as 3000mWcm-2.
(Rahiotis et al 2004, Deb et al 2003)
however, there was a drawback
heat generation and temperature
increase within the unit and in the
restoration.
The heat causes
deterioration of the unit
injurious to the pulp. (Zach, Cohen
1965)
Concerns have also been raised about
the effect of the high intensity on the
cure properties of the material.
(Knezevic et al 2002, Feilzer et al
In order to shine more light on these
factors which influence the
polymerisation and quality of light
activated resin composites,
this study set out to determine the
light intensity emitted by light curing
units (LCUs) in dental clinics in Nairobi
and study its effect on the cure
characteristics (depth of cure and
MATERIALS AND METHODS
Study design: This was a crosssectional analytical study.
Study area: The study was set in
private and public dental clinics in
Nairobi, Kenya.
A total of 83 LCUs were studied.
Data collection
Light intensity of LCUs in dental
clinics was measured using a digital
dental radiometer (CURE RITE,
CAULK-DENTSPLY, USA)
Depth of cure and surface microhardness were measured on
cylindrical resin composite specimens
made using custom-made split brass
moulds and polymerised with LCUs in
the dental clinics.
The DOC specimen: 4mm in diameter
and 6mm in depth.
Micro-hardness specimen: 8mm
diameter and 3mm thickness.
Depth of cure evaluation:
The specimen was immersed in a
capsule containing acetone solvent
and shaken in a mixing device.
The remaining length of the specimen
was measured using an electronic
digital Vanier caliper (Shengya
Machine & Tools Co., Ltd. China) and
the DOC taken as half of this length.
Surface micro-hardness evaluation
Three surface indentations were made with
a diamond indenter using a load of 200g
and a dwell time of 15-seconds.
A conversion table was used to convert
measurements from indentations into
hardness numbers.
The average of the three measurements
was taken as the surface micro-hardness of
the specimen.
Click to edit Master text styles
Second level
● Third level
● Fourth level
Fifth level
●
Split brass mould
for making microhardness specimen
Micro-hardness
and DOC
Composite
specimens
Click to edit Master text styles
Second level
● Third level
● Fourth level
● Fifth level
DATA ANALYSIS
The data was analysed using SPSS
version 12.
One way analysis of variance (ANOVA)
between groups with Post Hoc test was
used to compare the mean DOC and
surface micro-hardness among
different light intensity groups.
RESULTS
A total of 83 LCUs were examined.
The light intensity, and DOC were
determined for all the 83 units
Surface micro-hardness evaluation was done
only for 58 of the LCUs.
Table 5: Light intensity frequencies and associated mean DOC
and surface micro-hardness
Light
intensity
(mW/cm2)
Depth of cure (mm)
Surface microhardness
n
mean
SD
n
mean
SD
0 - 300
35
1.3399
0.26842 24
46.6042 19.17284
301 - 600
17
1.7106
0.12442 11
60.4727 5.89476
601 - 900
14
1.9654
0.17397 9
64.3444 6.82113
901 - 1200 12
2.1671
0.13168 11
64.1091 4.56584
1201 - 1500 2
2.3200
0.04243 1
71.6000 -
1501 - 1800 3
1.9833
0.29771 2
63.8500 6.43467
One way ANOVA showed that there is a
significant difference in the mean DOC
for the different intensity groups
(p=0.000).
The mean VHN for the different light
intensity groups also significantly
differed (p=0.002).
Relationship of light intensity with
depth of cure and surface microhardness
The relationship was investigated in
their continuous form and found to be
curvilinear.
2.50
Depth of cure(mm)
2.00
1.50
1.00
0.50
0.00
500.00
1000.00
1500.00
light intensity mW/cm2
2000.00
100.00
surface hardness (VHN)
80.00
60.00
40.00
20.00
0.00
0.00
500.00
1000.00
light intensity mW/cm2
1500.00
Discussion
Discussion
2.50
Depth of cure(mm)
2.00
1.50
1.00
0.50
0.00
500.00
1000.00
1500.00
light intensity mW/cm2
2000.00
100.00
surface hardness (VHN)
80.00
60.00
40.00
20.00
0.00
0.00
500.00
1000.00
light intensity mW/cm2
1500.00
Pilo et al(1999) reported on the
relationship between light intensity
and surface hardness.
Previous studies(Lohbauer et al 2005,
Peutzfieldt et al 2000) report that LCUs
with very high intensities give low DOC
and surface hardness.
The rapid polymerisation associated
with the high intensity is thought to
result in shortened lifespan of the free
radicals(Feng et al 2009)thus causing
saturated curing at the resin
composite surface,
trapping of unpolymerised monomer
CONCLUSION
Light intensity output of LCUs has a
significant influence on the cure
characteristics of dental composites
with both DOC and surface microhardness increasing with increase in
light intensity up to 1200mWcm-2.
RECOMMENDATION
light curing units with light intensity of
400-1200mWcm-2 should be used to
achieve acceptable cure
characteristics of dental composites.
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