J. Dent. 1992;
20: 183-l
88
183
Curing light performance and polymerization of composite restorative materials R. L. Sakaguchi,
W. H. Douglas and M. C. R. B. Peters*
Biomaterials Research Center, University The Netherlands
of Minnesota
School of Dentistry and *TRIKON, University of Nijmegen,
ABSTRACT The majority of modern composite restorative materials require light activation for polymerization. Variables affecting light energy absorption by the composite have been examined for their effect on the polymerization contraction. Since the polymerization contraction is closely associated in a complex way to the degree of cure of the restoration, this parameter served as an empirical indicator for the extent of polymerization. Variables included the composite shade, distance between the light source and composite sample, and light intensity. Three resin composites are evaluated. Post-gel polymerization contraction was evaluated using a strain gauge method. Curing light intensity diminished rapidly for distances greater than 2 mm between the tip of the light
guide and material surface. A linear relationship was demonstrated between polymerization contraction and light intensity. The polymerization contraction of a microfilled composite and posterior composite, using a constant curing time and light intensity. decreased linearly with increasing sample thickness. Less than optimal light output of the curing light source can be compensated by increasing application time within reasonable limits. KEY WORDS: J. Dent. 1992; 1991)
Composites, Polymerization, 20:
183-I
88 (Received
Light curing
1 August
1991;
reviewed
3 October 1991;
accepted 25 October
Correspondence should be addressed to: Dr Fi. L. Sakaguchi, Biomaterials Research Centre, Department of Oral Science, School of Dentistry, 16-2 12 Malcolm Moos Health SciencesTower, Minneapolis, MN 55455, USA.
INTRODUCTION Composite restoratives demonstrate beneficial features such as aesthetics and the ability to bond to enamel and dentine through bonding agents. However, these materials are more technique sensitive than amalgam alloys because of the steps involved in placement. When all of the procedures and materials function ideally, the composite restoration becomes integrated with the remaining tooth structure so that the tooth and restoration functions as a unit (Morin et al.. 1988). However, because of the equipment and procedures involved, these ideal physical properties are often not attained. Almost all direct filling composite restoratives polymerize through light activation. Recommended curing times provided by composite manufacturers are based on optimal or near optimal performance of the curing light. Less than optimal light source output may provide lower @ 1992 Butterworth-Heinemann 0300-5712/92/030183-06
Ltd.
degrees of cure which result in diminished physical properties. Many factors affect the performance of curing light sources. Long-term use of the light source ages the bulb, reflector, filter and light guide. These are factors which can be checked visually. Performance of the power supply also affects light output but is not easily evaluated. Other factors such as the distance and orientation of the light source (Kelsey et al., 1987). and shade of the composite (Kanca, 1986; Sakaguchi et al., 1991) affect polymerization. Two physical phenomena invariably accompany the polymerization of resins of dental interest. These are the evolution of heat and the volumetric shrinkage of the cured material compared with the prepolymer. The bulk manifestations of these two phenomena are closely associated with the polymerization reaction in a complex way. Thus the exotherm appears as a pulse of heat after the
184
J. Dent.
1992;
20:
No. 3
Table 1. Study design
Materials
Shades
P-50
Silux Plus
Herculite
XR
U XL G Y U G DG L U DG L DY
Polym exotherm
Test variables Comp Comp sample shade thickness X X X X X X X X X X X X
X
X
X
Neutral density
initial gel formation. In a similar way the shrinkage of the cured resin can be usefully divided in two parts. These are the pre-gel shrinkage in which volumetric change can be compensated for by continued flow of the material and the post-gel shrinkage in which the shrinkage of the polymer is accompanied by the development of a modulus of elasticity. The post-gel shrinkage is easily measured by the use of strain gauges (Sakaguchi et al., 1990, 1991). and in the present study it is related to the parameters of light absorption by the photocurable polymers. The objective of this study is an evaluation of the effect of light intensity and application technique on the polymerization contraction of resin composite using a strain gauge method for real-time measurement of composite polymerization.
METHODS Curing
Light intensity IO 40 60
(%) 100
X
x
x
x
x
X
x
x
x
x
X
x
x
x
x
1
0.4
0.2
0
(ND) values:
Everett, WA, USA) was placed on the surface of the tip of the light guide of a Visilux 2 light source. Temperature was recorded by the digital thermometer for the duration of a 120 s curing cycle.
Polymerization exotherm restorative materials
of composite
A chromel-alumel thermocouple (Fluke Model 52) was embedded in a 2 mm thick composite sample to record polymerization exotherm. The universal shade of three composites (Herculite XR, Kerr Manufacturing Co., Romulus, MI, USA P-50,3M Co., St Paul, MN, USA and Silux Plus, 3M Co.) was evaluated. A glass slide was placed between the curing light source and composite sample to serve as a platform for the composite sample. A Visilux 2 light source was used for polymerization. The curing light was applied for 60 s.
AND MATERIALS
light source intensity
Three new Visilux 2 (3M Co., St Paul, MN, USA) curing light sources were evaluated for power output as a function of distance between the light source and detector. The light guide of the curing source was mounted on a graduated stage which permitted accurate measurements of the distance between the end of the light guide and the detector at 0.25 mm increments. The detector of a Metrologic photometer (Model 45-230, Metrologic Instruments, Inc., Bellmawr, NJ, USA) was mounted coaxially with the light guide. The detector consisted of a photocell and 468 nm narrow bandwidth filter. Power output (mW cm-*) was measured at each 0.25 mm separation increment from contact between the light guide and detector (0 separation) to 15 mm separation.
Temperature elevation curing light source
generated
by the
A chromel-alumel thermocouple (Fluke Model 52 Digital Thermometer, John Fluke Manufacturing Co., Inc.,
Effect of composite shade and sample thickness on polymerization contraction Three composite restorative materials (P-50, Silux Plus and Herculite XR) were evaluated for polymerization contraction as a function of material shade and sample thickness. The effect of composite shade was evaluated using 1 mm thick samples of the composite material placed in a strain gauge evaluation apparatus (Sakaguchi et al., 1991). A strain gauge was placed on the sample on the side opposite the curing light source. The strain gauge measured post-gel polymerization contraction of the composite. The samples were cured for 60 s with a Visilux 2 light source. The materials are listed in Table I. Samples were prepared with thicknesses ranging from 0.9 mm to 3.0 mm using the universal shade of P-50, Silux Plus and Herculite XR. The samples were placed in the test apparatus described above with the strain gauge opposite the light guide. The samples were cured for 60 s. After completion of the strain measurements, the sample was removed and its thickness was measured with a digital micrometer (Scherr-Tumico Digital Caliper, S-T
Sakaguchi
et a/.: Light-activated
composite
185
polymerization
45 40 35 z e
0
I
I
I
I
I
1
I
L
2
4
6
8
10
12
14
16
Distance
between
light
source
and
Fig. 1. Effect on power output as distance light guide and light detector is increased.
detector
between
30
f 3 zk
25
g
15
C
10
20
0
20
end of
80
60
40
(mm)
100
120
Time (sl
Fig, 2. Temperature at tip of curing light source light guide during a 60 s curing cycle.
Industries, Inc., St James, MN, USA). Polymerization contraction was plotted as a function of final sample thickness and linear regression analysis was performed.
Effect of light intensity contraction
on polymerization
One millimetre thick samples of universal shade P-50 and Herculite XR were prepared and placed in the test apparatus. Kodak Wratten (Kodak Co., Rochester, NY, USA) neutral density filters were placed in the path between the light guide and the composite sample to attenuate the light. Kodak neutral density filters are designed to attenuate light intensity without shifting the colour spectrum. The filters and their respective degree of attenuation are listed in Table 1. Samples were cured for 60 s with a Visilux 2 curing light. Polymerization contraction was measured by the strain gauge for three attenuated curing light intensities.
RESULTS Curing
Y
c
XL
U
DC
L
DY
Shade
Fig. 3. Polymerization contraction for various ahdes of P-50 (m), Silux Plus (tZl) and Herculite XR (B) after 60 s curing cycle.
Polymerization exotherm restorative materials
of composite
The polymerization exotherms for the three materials are listed in Table ZZ.The exotherm for P-50 (ll.S”C f 1.0) was significantly different from the exotherm for Herculite XR (16.5”C f 2.1) and Silux Plus (16.4”C + 3.1) at P = 0.05 (Student-Newman-Keuls multiple range test).
light source intensity
A 5 s warm-up time was required for the light sources to reach maximum intensity as measured by the power meter. The mean power output of the Visilux 2 light sources is plotted as a function of distance between the light source and detector in Fig. 1. Power output decreased rapidly at distances greater than 2 mm. At 2 mm, power output was 93 per cent of maximum, at 4 mm 75 per cent of maximum, and at 6 mm 59 per cent of maximum.
Effect of composite shade and sample thickness on polymerization contraction The effect of material shade is illustrated in Fig. 3. Silux Plus dark grey (DG) exhibited significantly less polymerization contraction than all other shades of all other materials (P < 0.05). Silux Plus light was significantly lower than P-50 yellow, grey and universal, but not Table II. Temperature restorative
Temperature elevation curing light source
generated
materials
by the
The light sources exhibited a mean temperature of 42.2”C f 0.59 at the tip of the light guide after 60 s of operation. The mean temperature elevation was 14.4”C f 0.33. A temperature plateau was reached at 50 s of light application (Fig. 2).
Material Herculite XR P-50 Silux Plus
elevations for three composite during 60 s light activation Temperature Mean 16.5 11.8 16.4
elevation (“C) s.d. 2.1 1 .o 3.1
P-50 is significantly different from Herculite XR and Silux Plus at P = 0.05 (Student-Newman-Keuls multiple range test).
J. Dent 1992; 20: No. 3
186
0.25 o,o
0 0.5
0
1
1.5
2
Sample
3
2.5
3
3.5
10
4
thickness (mm)
20
30
40
50
60
70
80
90
100
% Optimal light transmission Fig. 5.
0.25 -
Effect of light transmission on polymerization contraction of three composite resins. W, Herculite XR; 0, P-50; A, Silux Plus.
Herculite XR
with an unfiltered light source is plotted as a function of percentage light transmission in Fig. 5 for each of the three materials. The values for the dependent variable were calculated by dividing the polymerization contraction at each light intensity by the contraction exhibited when a non-attenuated light source was used. The proportion for polymerization contraction with a non-attenuated light source was therefore 1.0. Regression coefficients using a linear regression analysis were calculated: Herculite XR (0.93) P-50 (0.96) Silux Plus (0.95). contraction
b g
0.05
t
q,, 1
0.5
0
Sample
,
,
,
,
1.5
2
2.5
3
thickness (mm1 P-50
DISCUSSION
I
P
0
I
0.5
I
1
I
I
I
1.5
2
2.5
Sample
thickness
I
I
3
3.5
(mm]
Fig. 4. Regression analysis for polymerization contraction versus sample thickness for Silux Plus, Herculite XR and P-50.
significantly different from other shades in the Silux Plus family. No other significant differences were found. Scatter plots of contraction shrinkage vs. sample thickness for the three test materials are shown in Fig. 4. Using an analysis of variance test for linearity of regression, Silux Plus and P-50 were statistically significant at P = 0.0001 (Silux Plus F = 153.8. P-50 F = 58.7). The data from Herculite XR did not appear to lit any pattern.
Effect of light intensity contraction
on polymerization
The ratio of observed polymerization contraction with an attenuated light source to maximal polymerization
The method utilized in this study for determination of polymerization contraction measures post-gel contraction in the linear dimension (Sakaguchiet al., 1991). In the pregel phase, before the material has developed a modulus of elasticity, flow compensates for polymerization contraction (Davidson and deGee, 1984; Feilzer et al., 1990). After a sufficient modulus has been achieved which is able to deform the strain gauge, the gauge monitors the deformation and hence the shrinkage in real time. The gauge is also able to record the expansion of the composite due to radiant heat from the curing light source and the exotherm from the polymerization reaction. Since this method measures only the post-gel contraction in the linear dimension, the absolute magnitude of the measured contraction is lower than the consensus of literature values, although it is thought that the strain gauge method may provide a better indicator of the clinically significant component of contraction. In this study post-gel contraction is used as a general indicator of the extent of polymerization of the composite. Although contraction is not a reliable measure of differences in degree of cure between different materials because of variation in filler loading, resin content and resin composition, it is a good indicator for differences in polymerization within the same material group. Most modern dental adhesive materials and resin composites require light energy for activation of the polymerization process. The energy output, proximity and
Sakaguchi
duration of application of the light determine the degree of cure of the composite restoration, other things being equal. It is nearly impossible to assess the performance of a curing light source visually. As the curing light ages, the light output diminishes because of deterioration of the bulb, reflector, power supply and light guide. Compensations must be made as the curing light source deteriorates to ensure optimal cure of the restoration and optimal physical properties. An attempt was made to simulate typical light-activated polymerization procedures as performed in a clinical setting. For Class II restorations it is impossible to place the light guide directly on top of the composite restoration. The light can be shadowed by tooth structure or opaque matrix bands. Aged light sources do not provide optimal light energy output. Also, placement of composite increments greater than 2 mm in thickness may diminish light transmission. All of these factors tend to compromise the degree of cure of the restoration which could result in diminished physical properties and decreased longevity of the restoration. For the curing lights tested. maximum power output was achieved after an initial 5 s warm-up time. The temperature at the tip of the light guide increased rapidly for the first 20 s then reached a plateau at approximately 50 s (Fig. 2). Minimal elevation in temperature was noted beyond 50 s. Extended curing times beyond 50 s do not cause increased temperature elevations above 14.4”C at the tip of the light guide but do increase the total heat energy applied to the tooth. The temperature elevation for P-50 (11.8”C) was significantly lower than that of Herculite XR (16.5”C) and Silux Plus (16.4”C) at P = 0.05 using the StudentNewman-Keuls multirange test (Table II). The difference in temperature elevation for P-50 was not surprising considering the lower resin content of P-50 relative to the other materials. The temperature elevations listed in Table II include the thermal contribution from the curing light source due to the radiant heat generated. If the temperature rise measured at the tip of the light curing source is subtracted from the temperature elevations during light activation of the composite, a relative indicator of the polymerization exotherm can be calculated. However, the composite can be expected to provide some thermal diffusivity which is comparable to dentine and lining cements (Lloyd et al., 1986: Smail et al., 1988). The thermal energy contributions by the curing light source and polymerization exotherm of composites can be potentially damaging to the pulp of the tooth (Lloyd et al.. 1986; Smail et al., 1988). Zach and Cohen (1965) found that. in monkeys, a temperature elevation of 5.5 “C in the pulp resulted in loss of vitality in 1.5 per cent of teeth. Light output diminished by 7 per cent when the light guide tip and detector were separated by 2 mm (Fig. 1). At a 4 mm separation, light output diminished by 25 per cent. When the initial increment of composite is placed at the gingival aspect of a proximal box in a Class II restoration,
et al.: Light-activated
composite
polymerization
187
obstruction of the light guide by marginal ridges or adjacent cusps may cause the light guide to be separated from the composite by 4 mm or more. In this situation. increased curing time is necessary to compensate for the increased distance. The polymerization contractions of the universal shade of Silux Plus, Herculite XR and P-50 were not significantly different at the 95 per cent confidence level. The lightest and darkest shades for each of the materials tended to exhibit lower polymerization contraction than the shades within the middle range (universal, yellow, grey), although the differences were not significantly different at P = 0.05 (Fig. 3). Light transmission through the darker shades is diminished because of the opacity. The white pigments in the very light shades also tend to make the material more opaque. Also. the white pigments diffuse the light which limits the light penetration through the composite. Therefore, very dark and very light shades require additional light application time. The relationship of polymerization contraction to sample thickness (Fig. 4) was found to be linear for Silux Plus (r2 = 0.737) and P-50 (r2 = 0.701). A simple relationship for Herculite XR was not found. The polymerization contraction of Silux Plus decreased by 40 per cent for each 1 mm increase in thickness of the sample beyond 1.8 mm. For P-50. a 25 per cent decrease in contraction was observed per 1 mm increase in thickness greater than 1.2 mm. This suggests an optimal increment thickness of < 2 mm for Silux Plus and < 1.5 mm for P-50. Thicker increments may require longer than 60 s curing times. A linear relationship was noted between light intensity and polymerization contraction for each of the three materials tested (Fig. 5). Regression analysis for the materials (Herculite XR r2 = 0.93, P-50 r2 = 0.96, Silux Plus r2 = 0.95) demonstrated good agreement at P = 0.05. To a first approximation only. the light energy supplied is proportional to the product of light intensity and time. Therefore, in clinical situations where the light intensity is less than optimal, a proportional increase in curing time can be applied to compensate for the diminished output of the light or attenuation due to properties of the composite used. Excessive curing times will enhance the physical properties of the material up to the optimal limit. However. extending the curing light application time increases the period of heat application. Of course, if the intensity of specific wavelengths (470 nm) in the light source is inadequate to activate the polymerization reaction. no compensatory mechanisms are available to produce a satisfactory composite restoration.
CONCLUSIONS 1. Visilux 2 light sources produce a temperature elevation of 14.2”C at the tip of the light guide during the time of light application. A 5 s warm-up time is necessary before maximum light intensity is achieved. When the sample surface is 4 mm from the surface of light guide, 25 per cent of the light intensity is lost.
188
J. Dent. 1992;
20: No. 3
2. The temperature elevation of P-50 is significantly lower than that of Silux Plus and Herculite XR during 60 s light activation. 3. There is a linear relationship between polymerization contraction and light intensity in the dental resin composites investigated. 4. Less than optimal light intensity can be compensated by increased curing times within reasonable limits.
Acknowledgements The authors thank Mr T. Martin from 3M Dental Products and MS M. Bunczak and Dr C. Sasik for their contributions to this study.
References Davidson C. L. and deGee A. J. (1984) Relaxation of polymerization contraction stresses by flow in dental composites. J. Dent. Res. 63, 146-148. Feilzer A. J., deGee A. J. and Davidson C. L. (1990) Relaxation of polymerization contraction shear stress. .I. Dent. Res. 69, 36-39.
Kanca J. (1986) The effect of thickness and shade on the polymerization of light-activated posterior composite resins. Quintessence Int. 17, 809-8 11. Kelsey W. P., Shearer G. O., Cave1 W. T. et al. (1987) The effects of wand positioning on the polymerization of composite resin. J. Am. Dent Assoc. 117, 213-215. Lloyd C. H., Joshi A. and McGlynn E. (1986) Temperature rises produced by light sources and composites during curing. Dent. Mater. 2, 170-174. Morin D. L., Douglas W. H., Cross M. et al. (1988) Biophysical stress analysis of restored teeth: experimental strain measurement. Dent Mater. 4,41-48. Sakaguchi R. L., Salsik C. T., Bunczak M. A. et al. (1991) Post-gel polymerization contraction of composite resins. (submitted). Sakaguchi R. L., Bunczak M. A. and Douglas W. H. (1990) Strain gauge measurement of post-gel polymerization contraction of composite resins. _I. Dent. Res. 69, 309 (abstr. 1601). Smail S. R. J., Patterson C. J. W., McLundie A. C. er al. (1988) In vitro temperature rises during visible-light curing of a lining material and a posterior composite. J. Oral Rehahil. 15, 361-366. Zach L. and Cohen G. (1965) Pulp response to externally applied heat. Oral Surg Oral Med. Oral Pathol. 19, 515-530.
Book Review A Textbook of Orthodontics. T. D. Foster. Pp. 347. 1990. Oxford, Blackwell Scientific Publications. Hardback, f27.50. Functional Orthodontic Appliances. K. G. Isaacson, R. T. Reed and C. D. Stephens. Pp. 131. 1990. Oxford, Blackwell Scientific Publications. Hardback, f 39.50. The first four chapters of Foster’s A Textbook of Orthodontics deal in a conventional fashion with mechanisms of growth, their rates and racial variations with occlusal development. Malocclusion classifications are discussed proceeding to the aetiology of dento-alveolar and skeletal discrepancies. Lateral skull cephalometric films receive some detail with special reference to growth points for longitudinal measurements. The next chapter covers occlusal developments, soft tissue, dental factors, arch and tooth size discrepancies. Space maintenance is discussed as well as localized factors. Foster then moves on to the biology of tooth movement, treatment planning and the pros and cons of extractions. Chapters 12, 13 and 14 deal with principles of the various orthodontic appliances, and are well illustrated, albeit some of the photographs are quite antiquated showing bands on the anterior teeth. However, on page 233, fig. 12.2 shows an upper removable appliance with a latex elastic to retrocline and align the upper incisors: this is to be deplored and should not have been included. Functional appliances are then discussed with illustrations of Andresen, two Frankel appliances and an oral screen. The remaining chapters deal with treatment of the various malocclusions. The author relates growth and timing of treatment and effect upon growth extraoral and functional appliance forces. Lastly, orthodontics and preventive
dentistry are covered. There is no separate chapter on interceptive orthodontics: the topic is covered fragmentally throughout, and there is a complete absence of any on orthognathic surgery as an integrated treatment plan with orthodontic appliance therapy. This book can be only recommended for undergraduates. It is easy enough to read but lacks sufficient depth for postgraduate students. By contrast, Functional Orthodontic Appliances is more specialized in scope. The first four chapters are devoted to growth and its modification by their use. Much stress is laid on velocity growth spurts and timing of treatment. With few exceptions, the authors’ feel most cases should start at 10 + years of age to obviate protracted treatment times and ensure maximum patient cooperation. Their mode of action is dealt with simply but in sufficient detail. Included are pre-, during- and post-treatment cephalometric lateral skull tracings with clear black and white photographs. There follows a fairly detailed description of the more commonly used functional appliances with clear indications and contraindications for their use. These include Andresen, Harvold activator (the three authors’ preferred appliance), Bionator, Clark Twin Block and a brief mention of Bimmler and Herbst appliances. Combination treatments of functional followed by fixed or removable appliances are discussed with emphasis upon maintenance of Class II or maxillary extraoral force lest relapse occur. Sadly, no mention was made of the (Stockli) Teusher appliance which is gaining rapid popularity within the UK. An excellent summary of published papers and laboratory appliance construction appendices complete a praiseworthy book which should be a ‘must’ for all engaged in postgraduate orthodontics and can be wholeheartedly recommended. B. S. Feldman
20: 183-l
88
183
Curing light performance and polymerization of composite restorative materials R. L. Sakaguchi,
W. H. Douglas and M. C. R. B. Peters*
Biomaterials Research Center, University The Netherlands
of Minnesota
School of Dentistry and *TRIKON, University of Nijmegen,
ABSTRACT The majority of modern composite restorative materials require light activation for polymerization. Variables affecting light energy absorption by the composite have been examined for their effect on the polymerization contraction. Since the polymerization contraction is closely associated in a complex way to the degree of cure of the restoration, this parameter served as an empirical indicator for the extent of polymerization. Variables included the composite shade, distance between the light source and composite sample, and light intensity. Three resin composites are evaluated. Post-gel polymerization contraction was evaluated using a strain gauge method. Curing light intensity diminished rapidly for distances greater than 2 mm between the tip of the light
guide and material surface. A linear relationship was demonstrated between polymerization contraction and light intensity. The polymerization contraction of a microfilled composite and posterior composite, using a constant curing time and light intensity. decreased linearly with increasing sample thickness. Less than optimal light output of the curing light source can be compensated by increasing application time within reasonable limits. KEY WORDS: J. Dent. 1992; 1991)
Composites, Polymerization, 20:
183-I
88 (Received
Light curing
1 August
1991;
reviewed
3 October 1991;
accepted 25 October
Correspondence should be addressed to: Dr Fi. L. Sakaguchi, Biomaterials Research Centre, Department of Oral Science, School of Dentistry, 16-2 12 Malcolm Moos Health SciencesTower, Minneapolis, MN 55455, USA.
INTRODUCTION Composite restoratives demonstrate beneficial features such as aesthetics and the ability to bond to enamel and dentine through bonding agents. However, these materials are more technique sensitive than amalgam alloys because of the steps involved in placement. When all of the procedures and materials function ideally, the composite restoration becomes integrated with the remaining tooth structure so that the tooth and restoration functions as a unit (Morin et al.. 1988). However, because of the equipment and procedures involved, these ideal physical properties are often not attained. Almost all direct filling composite restoratives polymerize through light activation. Recommended curing times provided by composite manufacturers are based on optimal or near optimal performance of the curing light. Less than optimal light source output may provide lower @ 1992 Butterworth-Heinemann 0300-5712/92/030183-06
Ltd.
degrees of cure which result in diminished physical properties. Many factors affect the performance of curing light sources. Long-term use of the light source ages the bulb, reflector, filter and light guide. These are factors which can be checked visually. Performance of the power supply also affects light output but is not easily evaluated. Other factors such as the distance and orientation of the light source (Kelsey et al., 1987). and shade of the composite (Kanca, 1986; Sakaguchi et al., 1991) affect polymerization. Two physical phenomena invariably accompany the polymerization of resins of dental interest. These are the evolution of heat and the volumetric shrinkage of the cured material compared with the prepolymer. The bulk manifestations of these two phenomena are closely associated with the polymerization reaction in a complex way. Thus the exotherm appears as a pulse of heat after the
184
J. Dent.
1992;
20:
No. 3
Table 1. Study design
Materials
Shades
P-50
Silux Plus
Herculite
XR
U XL G Y U G DG L U DG L DY
Polym exotherm
Test variables Comp Comp sample shade thickness X X X X X X X X X X X X
X
X
X
Neutral density
initial gel formation. In a similar way the shrinkage of the cured resin can be usefully divided in two parts. These are the pre-gel shrinkage in which volumetric change can be compensated for by continued flow of the material and the post-gel shrinkage in which the shrinkage of the polymer is accompanied by the development of a modulus of elasticity. The post-gel shrinkage is easily measured by the use of strain gauges (Sakaguchi et al., 1990, 1991). and in the present study it is related to the parameters of light absorption by the photocurable polymers. The objective of this study is an evaluation of the effect of light intensity and application technique on the polymerization contraction of resin composite using a strain gauge method for real-time measurement of composite polymerization.
METHODS Curing
Light intensity IO 40 60
(%) 100
X
x
x
x
x
X
x
x
x
x
X
x
x
x
x
1
0.4
0.2
0
(ND) values:
Everett, WA, USA) was placed on the surface of the tip of the light guide of a Visilux 2 light source. Temperature was recorded by the digital thermometer for the duration of a 120 s curing cycle.
Polymerization exotherm restorative materials
of composite
A chromel-alumel thermocouple (Fluke Model 52) was embedded in a 2 mm thick composite sample to record polymerization exotherm. The universal shade of three composites (Herculite XR, Kerr Manufacturing Co., Romulus, MI, USA P-50,3M Co., St Paul, MN, USA and Silux Plus, 3M Co.) was evaluated. A glass slide was placed between the curing light source and composite sample to serve as a platform for the composite sample. A Visilux 2 light source was used for polymerization. The curing light was applied for 60 s.
AND MATERIALS
light source intensity
Three new Visilux 2 (3M Co., St Paul, MN, USA) curing light sources were evaluated for power output as a function of distance between the light source and detector. The light guide of the curing source was mounted on a graduated stage which permitted accurate measurements of the distance between the end of the light guide and the detector at 0.25 mm increments. The detector of a Metrologic photometer (Model 45-230, Metrologic Instruments, Inc., Bellmawr, NJ, USA) was mounted coaxially with the light guide. The detector consisted of a photocell and 468 nm narrow bandwidth filter. Power output (mW cm-*) was measured at each 0.25 mm separation increment from contact between the light guide and detector (0 separation) to 15 mm separation.
Temperature elevation curing light source
generated
by the
A chromel-alumel thermocouple (Fluke Model 52 Digital Thermometer, John Fluke Manufacturing Co., Inc.,
Effect of composite shade and sample thickness on polymerization contraction Three composite restorative materials (P-50, Silux Plus and Herculite XR) were evaluated for polymerization contraction as a function of material shade and sample thickness. The effect of composite shade was evaluated using 1 mm thick samples of the composite material placed in a strain gauge evaluation apparatus (Sakaguchi et al., 1991). A strain gauge was placed on the sample on the side opposite the curing light source. The strain gauge measured post-gel polymerization contraction of the composite. The samples were cured for 60 s with a Visilux 2 light source. The materials are listed in Table I. Samples were prepared with thicknesses ranging from 0.9 mm to 3.0 mm using the universal shade of P-50, Silux Plus and Herculite XR. The samples were placed in the test apparatus described above with the strain gauge opposite the light guide. The samples were cured for 60 s. After completion of the strain measurements, the sample was removed and its thickness was measured with a digital micrometer (Scherr-Tumico Digital Caliper, S-T
Sakaguchi
et a/.: Light-activated
composite
185
polymerization
45 40 35 z e
0
I
I
I
I
I
1
I
L
2
4
6
8
10
12
14
16
Distance
between
light
source
and
Fig. 1. Effect on power output as distance light guide and light detector is increased.
detector
between
30
f 3 zk
25
g
15
C
10
20
0
20
end of
80
60
40
(mm)
100
120
Time (sl
Fig, 2. Temperature at tip of curing light source light guide during a 60 s curing cycle.
Industries, Inc., St James, MN, USA). Polymerization contraction was plotted as a function of final sample thickness and linear regression analysis was performed.
Effect of light intensity contraction
on polymerization
One millimetre thick samples of universal shade P-50 and Herculite XR were prepared and placed in the test apparatus. Kodak Wratten (Kodak Co., Rochester, NY, USA) neutral density filters were placed in the path between the light guide and the composite sample to attenuate the light. Kodak neutral density filters are designed to attenuate light intensity without shifting the colour spectrum. The filters and their respective degree of attenuation are listed in Table 1. Samples were cured for 60 s with a Visilux 2 curing light. Polymerization contraction was measured by the strain gauge for three attenuated curing light intensities.
RESULTS Curing
Y
c
XL
U
DC
L
DY
Shade
Fig. 3. Polymerization contraction for various ahdes of P-50 (m), Silux Plus (tZl) and Herculite XR (B) after 60 s curing cycle.
Polymerization exotherm restorative materials
of composite
The polymerization exotherms for the three materials are listed in Table ZZ.The exotherm for P-50 (ll.S”C f 1.0) was significantly different from the exotherm for Herculite XR (16.5”C f 2.1) and Silux Plus (16.4”C + 3.1) at P = 0.05 (Student-Newman-Keuls multiple range test).
light source intensity
A 5 s warm-up time was required for the light sources to reach maximum intensity as measured by the power meter. The mean power output of the Visilux 2 light sources is plotted as a function of distance between the light source and detector in Fig. 1. Power output decreased rapidly at distances greater than 2 mm. At 2 mm, power output was 93 per cent of maximum, at 4 mm 75 per cent of maximum, and at 6 mm 59 per cent of maximum.
Effect of composite shade and sample thickness on polymerization contraction The effect of material shade is illustrated in Fig. 3. Silux Plus dark grey (DG) exhibited significantly less polymerization contraction than all other shades of all other materials (P < 0.05). Silux Plus light was significantly lower than P-50 yellow, grey and universal, but not Table II. Temperature restorative
Temperature elevation curing light source
generated
materials
by the
The light sources exhibited a mean temperature of 42.2”C f 0.59 at the tip of the light guide after 60 s of operation. The mean temperature elevation was 14.4”C f 0.33. A temperature plateau was reached at 50 s of light application (Fig. 2).
Material Herculite XR P-50 Silux Plus
elevations for three composite during 60 s light activation Temperature Mean 16.5 11.8 16.4
elevation (“C) s.d. 2.1 1 .o 3.1
P-50 is significantly different from Herculite XR and Silux Plus at P = 0.05 (Student-Newman-Keuls multiple range test).
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186
0.25 o,o
0 0.5
0
1
1.5
2
Sample
3
2.5
3
3.5
10
4
thickness (mm)
20
30
40
50
60
70
80
90
100
% Optimal light transmission Fig. 5.
0.25 -
Effect of light transmission on polymerization contraction of three composite resins. W, Herculite XR; 0, P-50; A, Silux Plus.
Herculite XR
with an unfiltered light source is plotted as a function of percentage light transmission in Fig. 5 for each of the three materials. The values for the dependent variable were calculated by dividing the polymerization contraction at each light intensity by the contraction exhibited when a non-attenuated light source was used. The proportion for polymerization contraction with a non-attenuated light source was therefore 1.0. Regression coefficients using a linear regression analysis were calculated: Herculite XR (0.93) P-50 (0.96) Silux Plus (0.95). contraction
b g
0.05
t
q,, 1
0.5
0
Sample
,
,
,
,
1.5
2
2.5
3
thickness (mm1 P-50
DISCUSSION
I
P
0
I
0.5
I
1
I
I
I
1.5
2
2.5
Sample
thickness
I
I
3
3.5
(mm]
Fig. 4. Regression analysis for polymerization contraction versus sample thickness for Silux Plus, Herculite XR and P-50.
significantly different from other shades in the Silux Plus family. No other significant differences were found. Scatter plots of contraction shrinkage vs. sample thickness for the three test materials are shown in Fig. 4. Using an analysis of variance test for linearity of regression, Silux Plus and P-50 were statistically significant at P = 0.0001 (Silux Plus F = 153.8. P-50 F = 58.7). The data from Herculite XR did not appear to lit any pattern.
Effect of light intensity contraction
on polymerization
The ratio of observed polymerization contraction with an attenuated light source to maximal polymerization
The method utilized in this study for determination of polymerization contraction measures post-gel contraction in the linear dimension (Sakaguchiet al., 1991). In the pregel phase, before the material has developed a modulus of elasticity, flow compensates for polymerization contraction (Davidson and deGee, 1984; Feilzer et al., 1990). After a sufficient modulus has been achieved which is able to deform the strain gauge, the gauge monitors the deformation and hence the shrinkage in real time. The gauge is also able to record the expansion of the composite due to radiant heat from the curing light source and the exotherm from the polymerization reaction. Since this method measures only the post-gel contraction in the linear dimension, the absolute magnitude of the measured contraction is lower than the consensus of literature values, although it is thought that the strain gauge method may provide a better indicator of the clinically significant component of contraction. In this study post-gel contraction is used as a general indicator of the extent of polymerization of the composite. Although contraction is not a reliable measure of differences in degree of cure between different materials because of variation in filler loading, resin content and resin composition, it is a good indicator for differences in polymerization within the same material group. Most modern dental adhesive materials and resin composites require light energy for activation of the polymerization process. The energy output, proximity and
Sakaguchi
duration of application of the light determine the degree of cure of the composite restoration, other things being equal. It is nearly impossible to assess the performance of a curing light source visually. As the curing light ages, the light output diminishes because of deterioration of the bulb, reflector, power supply and light guide. Compensations must be made as the curing light source deteriorates to ensure optimal cure of the restoration and optimal physical properties. An attempt was made to simulate typical light-activated polymerization procedures as performed in a clinical setting. For Class II restorations it is impossible to place the light guide directly on top of the composite restoration. The light can be shadowed by tooth structure or opaque matrix bands. Aged light sources do not provide optimal light energy output. Also, placement of composite increments greater than 2 mm in thickness may diminish light transmission. All of these factors tend to compromise the degree of cure of the restoration which could result in diminished physical properties and decreased longevity of the restoration. For the curing lights tested. maximum power output was achieved after an initial 5 s warm-up time. The temperature at the tip of the light guide increased rapidly for the first 20 s then reached a plateau at approximately 50 s (Fig. 2). Minimal elevation in temperature was noted beyond 50 s. Extended curing times beyond 50 s do not cause increased temperature elevations above 14.4”C at the tip of the light guide but do increase the total heat energy applied to the tooth. The temperature elevation for P-50 (11.8”C) was significantly lower than that of Herculite XR (16.5”C) and Silux Plus (16.4”C) at P = 0.05 using the StudentNewman-Keuls multirange test (Table II). The difference in temperature elevation for P-50 was not surprising considering the lower resin content of P-50 relative to the other materials. The temperature elevations listed in Table II include the thermal contribution from the curing light source due to the radiant heat generated. If the temperature rise measured at the tip of the light curing source is subtracted from the temperature elevations during light activation of the composite, a relative indicator of the polymerization exotherm can be calculated. However, the composite can be expected to provide some thermal diffusivity which is comparable to dentine and lining cements (Lloyd et al., 1986: Smail et al., 1988). The thermal energy contributions by the curing light source and polymerization exotherm of composites can be potentially damaging to the pulp of the tooth (Lloyd et al.. 1986; Smail et al., 1988). Zach and Cohen (1965) found that. in monkeys, a temperature elevation of 5.5 “C in the pulp resulted in loss of vitality in 1.5 per cent of teeth. Light output diminished by 7 per cent when the light guide tip and detector were separated by 2 mm (Fig. 1). At a 4 mm separation, light output diminished by 25 per cent. When the initial increment of composite is placed at the gingival aspect of a proximal box in a Class II restoration,
et al.: Light-activated
composite
polymerization
187
obstruction of the light guide by marginal ridges or adjacent cusps may cause the light guide to be separated from the composite by 4 mm or more. In this situation. increased curing time is necessary to compensate for the increased distance. The polymerization contractions of the universal shade of Silux Plus, Herculite XR and P-50 were not significantly different at the 95 per cent confidence level. The lightest and darkest shades for each of the materials tended to exhibit lower polymerization contraction than the shades within the middle range (universal, yellow, grey), although the differences were not significantly different at P = 0.05 (Fig. 3). Light transmission through the darker shades is diminished because of the opacity. The white pigments in the very light shades also tend to make the material more opaque. Also. the white pigments diffuse the light which limits the light penetration through the composite. Therefore, very dark and very light shades require additional light application time. The relationship of polymerization contraction to sample thickness (Fig. 4) was found to be linear for Silux Plus (r2 = 0.737) and P-50 (r2 = 0.701). A simple relationship for Herculite XR was not found. The polymerization contraction of Silux Plus decreased by 40 per cent for each 1 mm increase in thickness of the sample beyond 1.8 mm. For P-50. a 25 per cent decrease in contraction was observed per 1 mm increase in thickness greater than 1.2 mm. This suggests an optimal increment thickness of < 2 mm for Silux Plus and < 1.5 mm for P-50. Thicker increments may require longer than 60 s curing times. A linear relationship was noted between light intensity and polymerization contraction for each of the three materials tested (Fig. 5). Regression analysis for the materials (Herculite XR r2 = 0.93, P-50 r2 = 0.96, Silux Plus r2 = 0.95) demonstrated good agreement at P = 0.05. To a first approximation only. the light energy supplied is proportional to the product of light intensity and time. Therefore, in clinical situations where the light intensity is less than optimal, a proportional increase in curing time can be applied to compensate for the diminished output of the light or attenuation due to properties of the composite used. Excessive curing times will enhance the physical properties of the material up to the optimal limit. However. extending the curing light application time increases the period of heat application. Of course, if the intensity of specific wavelengths (470 nm) in the light source is inadequate to activate the polymerization reaction. no compensatory mechanisms are available to produce a satisfactory composite restoration.
CONCLUSIONS 1. Visilux 2 light sources produce a temperature elevation of 14.2”C at the tip of the light guide during the time of light application. A 5 s warm-up time is necessary before maximum light intensity is achieved. When the sample surface is 4 mm from the surface of light guide, 25 per cent of the light intensity is lost.
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2. The temperature elevation of P-50 is significantly lower than that of Silux Plus and Herculite XR during 60 s light activation. 3. There is a linear relationship between polymerization contraction and light intensity in the dental resin composites investigated. 4. Less than optimal light intensity can be compensated by increased curing times within reasonable limits.
Acknowledgements The authors thank Mr T. Martin from 3M Dental Products and MS M. Bunczak and Dr C. Sasik for their contributions to this study.
References Davidson C. L. and deGee A. J. (1984) Relaxation of polymerization contraction stresses by flow in dental composites. J. Dent. Res. 63, 146-148. Feilzer A. J., deGee A. J. and Davidson C. L. (1990) Relaxation of polymerization contraction shear stress. .I. Dent. Res. 69, 36-39.
Kanca J. (1986) The effect of thickness and shade on the polymerization of light-activated posterior composite resins. Quintessence Int. 17, 809-8 11. Kelsey W. P., Shearer G. O., Cave1 W. T. et al. (1987) The effects of wand positioning on the polymerization of composite resin. J. Am. Dent Assoc. 117, 213-215. Lloyd C. H., Joshi A. and McGlynn E. (1986) Temperature rises produced by light sources and composites during curing. Dent. Mater. 2, 170-174. Morin D. L., Douglas W. H., Cross M. et al. (1988) Biophysical stress analysis of restored teeth: experimental strain measurement. Dent Mater. 4,41-48. Sakaguchi R. L., Salsik C. T., Bunczak M. A. et al. (1991) Post-gel polymerization contraction of composite resins. (submitted). Sakaguchi R. L., Bunczak M. A. and Douglas W. H. (1990) Strain gauge measurement of post-gel polymerization contraction of composite resins. _I. Dent. Res. 69, 309 (abstr. 1601). Smail S. R. J., Patterson C. J. W., McLundie A. C. er al. (1988) In vitro temperature rises during visible-light curing of a lining material and a posterior composite. J. Oral Rehahil. 15, 361-366. Zach L. and Cohen G. (1965) Pulp response to externally applied heat. Oral Surg Oral Med. Oral Pathol. 19, 515-530.
Book Review A Textbook of Orthodontics. T. D. Foster. Pp. 347. 1990. Oxford, Blackwell Scientific Publications. Hardback, f27.50. Functional Orthodontic Appliances. K. G. Isaacson, R. T. Reed and C. D. Stephens. Pp. 131. 1990. Oxford, Blackwell Scientific Publications. Hardback, f 39.50. The first four chapters of Foster’s A Textbook of Orthodontics deal in a conventional fashion with mechanisms of growth, their rates and racial variations with occlusal development. Malocclusion classifications are discussed proceeding to the aetiology of dento-alveolar and skeletal discrepancies. Lateral skull cephalometric films receive some detail with special reference to growth points for longitudinal measurements. The next chapter covers occlusal developments, soft tissue, dental factors, arch and tooth size discrepancies. Space maintenance is discussed as well as localized factors. Foster then moves on to the biology of tooth movement, treatment planning and the pros and cons of extractions. Chapters 12, 13 and 14 deal with principles of the various orthodontic appliances, and are well illustrated, albeit some of the photographs are quite antiquated showing bands on the anterior teeth. However, on page 233, fig. 12.2 shows an upper removable appliance with a latex elastic to retrocline and align the upper incisors: this is to be deplored and should not have been included. Functional appliances are then discussed with illustrations of Andresen, two Frankel appliances and an oral screen. The remaining chapters deal with treatment of the various malocclusions. The author relates growth and timing of treatment and effect upon growth extraoral and functional appliance forces. Lastly, orthodontics and preventive
dentistry are covered. There is no separate chapter on interceptive orthodontics: the topic is covered fragmentally throughout, and there is a complete absence of any on orthognathic surgery as an integrated treatment plan with orthodontic appliance therapy. This book can be only recommended for undergraduates. It is easy enough to read but lacks sufficient depth for postgraduate students. By contrast, Functional Orthodontic Appliances is more specialized in scope. The first four chapters are devoted to growth and its modification by their use. Much stress is laid on velocity growth spurts and timing of treatment. With few exceptions, the authors’ feel most cases should start at 10 + years of age to obviate protracted treatment times and ensure maximum patient cooperation. Their mode of action is dealt with simply but in sufficient detail. Included are pre-, during- and post-treatment cephalometric lateral skull tracings with clear black and white photographs. There follows a fairly detailed description of the more commonly used functional appliances with clear indications and contraindications for their use. These include Andresen, Harvold activator (the three authors’ preferred appliance), Bionator, Clark Twin Block and a brief mention of Bimmler and Herbst appliances. Combination treatments of functional followed by fixed or removable appliances are discussed with emphasis upon maintenance of Class II or maxillary extraoral force lest relapse occur. Sadly, no mention was made of the (Stockli) Teusher appliance which is gaining rapid popularity within the UK. An excellent summary of published papers and laboratory appliance construction appendices complete a praiseworthy book which should be a ‘must’ for all engaged in postgraduate orthodontics and can be wholeheartedly recommended. B. S. Feldman
