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Factors contributing to the temperature rise during polymerization of resinmodified glass-ionomer cements
PI1
SO142-9612
(96)
Biomoteriols 17 (1996)23054312 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0142-9612/96/$X5.00
00065-B
Factors contributing to the temperature rise during polymerization of resinmodified glass-ionomer cements Widchaya Kanchanavasita, Biomaterials
Department,
Gavin J, Pearson and H.M. Anstice
Eastman Dental Institute, 256 Gray’s Inn Road, London WClX 8LD, UK
Part of the setting reaction of a resin-modified glass-ionomer cement (RMGIC) is a photoinitiated polymerization. As a result of the polymerization exotherm, the temperature of the cement may rise during setting. This study investigated the temperature rise for two liner/base- and two restorativetype RMGICs. The effects of factors such as specimen thickness, exposure time and environment temperature were investigated. The thermal diffusivity of the cements was also evaluated. Temperatures were measured using a thermocouple embedded in the centre of 6-mm diameter specimen discs of 1, 2 or 3 mm thickness. The exposure times used to cure the specimens varied from 15 to 60 s. The tests were carried out at either 25 or 37°C. The temperature rises attributable to the polymerization reaction ranged from 11 to 26°C for the liner/base cements and from 8 to 17°C for the restorative cements. Increasing the specimen thickness reduced the temperature rise only when inadequate exposure times were used. Raising the environmental temperature resulted in a smaller temperature rise. The thermal diffusivities were determined from cylindrical specimens. These ranged from 1.9 x 10e3 to 2.5 x 10m3cm*.Y’, the lining cements showing lower values than the restorative materials. 0 1996 Elsevier Science Limited. Keywords:
Dental
materials,
resin-modified
glass-ionomer
cements,
polymerization,
thermal
diffusivity
Received 3 November 1995;accepted 5 April 1996
and differential thermal analyser (DTA)SP1O. Crisp et Ill.11monitored the temperature rises of some dental including conventional cements, glass-ionomer materials. They found that zinc phosphate cements showed the greatest temperature rise on setting (22°C) whereas glass-ionomer cements gave the smallest (4°C). Modifications of the DTA have been made to monitor the exotherm of light-curing composite resins2*‘2. Bourke et a1.13 applied the use of a DTA to investigate the setting reactions and temperature rises of two liner/base RMGICs. In the present study, the temperature rises of liner/base RMGICs were compared with those of restorative-type RMGICs. The effects on the temperature rise produced by varying the times for which the materials were exposed to light activation, the thickness of the cement layers and the environmental conditions were also examined.
resin-modified glass-ionomer cements Recently, (RMGICs) have been developed for use as either liner/ base or restorative materials. These cements set by two mechanisms: the acid-base reaction of the conventional glass-ionomer component and a polymerization reaction of the monomeric constituents contained in the formulation’. Such polymerization reactions are known to be exothermic in nature, producing a rise in temperature in the cement during setting. Heating from the light-curing source used in photopolymerization may also contribute to the total temperature rise observed’. The liner/base cements are normally used in deeper parts of the cavities before placement of other restorative materials, such as amalgam or composite resin. In contrast, restorative cements may be placed either incrementally or in bulk to fill up the whole cavity. Generally, a large temperature rise may have a detrimental effect on the tooth tissue, particularly the pu1p3. Most of the previous work monitoring the changes in temperature of the setting restorative materials has been carried out on resin materials. The instrumentation used varied fiorn simple devices using thermocouples4-6 to more sophisticated commercial equipment such as a differential scanning calorimeter (DSC)7*8 Correspondence
MATERIALS AND METHODS Four hand-mixed RMGICs were examined in this study (Table 2). Vitremer (VM) contains a radiopaque, fluoroaluminosilicate glass powder and a lightsensitive aqueous solution of a modified poly(alkenoic) acid with some pendant methacrylate end groups, some methacrylate monomers, usually 2-hydroxyethyl
to Dr ‘G.J.Pearson.
2305
Biomaterials
1996, Vol. 17 No. 24
Polymerization
2306 Table 1 Resin-modified
of resin-modified
glass-ionomer
cements:
W. Kanchanavasita
et a/.
GlCs used in this study
Product
Manufacturer
Batch no.
Mixing ratio (g/g)
Thermal diffusivitya (x10-3cm’s-‘)
Restorative Vitremer (VM)
3M Co., St. Paul, MN, USA
Powder Liquid Powder Liquid
2.511 .O
2.50 (0.12)
3.011 .o
2.47 (0.09)
1.411.0 1.411 .o
2.18 (0.14) 1.91 (0.30)
Fuji II LC (FC)
Lining Vitrebond (VB) Fuji Lining LC (FL)
inAverages of five determinations. (P > 0.05)
GC Dental, Tokyo, Japan
3M Co., St. Paul, MN, USA GC Dental, Tokyo, Japan
The figures in parentheses
19940727 Powder 230341 Liquid 250341
are 1 standard deviation. The values connected by a straight line were not significantly
(HEMA), and various polymerization initiators14. In Fuji II LC (FC) the glass is a strontiumcontaining fluoroaluminosilicate glass while the liquid consists of an aqueous solution of a copolymer of acrylic-maleic acid, HEMA, and an initiator and activator15. Both VM and FC are recommended for use as aesthetic filling materials in non-load-bearing areas in the adult dentition, such as cervical lesions and Class III lesions, and occlusal restorations of deciduous dentition. Vitrebond (VB) and Fuji Lining LC (FL) are low-viscosity lining materials used primarily for pulp protection under other restorative materials. The formulation contains similar components but in the lining materials the powder/liquid ratio is lower. Before mixing each material, the powder and liquid were weighed out to the manufacturers’ recommended mixing ratio using a digital balance (R-20, Oerting Ltd., Kent, UK). The experiment was divided into four phases: methacrylate
Measurement of temperature increase during initial polymerization Measurement of temperature increase attributable to the polymerizing light source Measurement of temperature increase during material addition Measurement of thermal diffusivity Tests were carried out at either 25 or 37°C in a thermostatically controlled cabinet (Compenstat, UK). A minimum of five recordings was taken for each test and each combination of material and condition. All the results were subjected to the Kruskal-Wallis test. If differences were found, further analyses were carried out using the Mann-Whitney U test.
Measurement of temperature increase during initial polymerization (Ti) The materials under investigation were mixed at room temperature in an environmental chamber according to the manufacturers’ instructions and then inserted into a polytetrafluoroethylene (PTFE) cylindrical mould with an internal diameter of 6mm and a thickness of either 1, 2 or 3 mm. Single continuous exposures for specific times from 15 to 60s were applied to the surfaces of the specimens using a polymerizing light source (Luxor, ICI Dental, Macclesfield, UK). The increase in temperature with time during polymerizaBiomaterials 1996, Vol. 17 No. 24
19941206 19941020 150981 030981
different
tion was monitored using a thermocouple which had been placed centrally in the mould before insertion of the materials. The signals from the thermocouple were fed to a digital thermometer including a cold junction and also to a chart recorder (Linseis LS 52-4, Germany). The temperature change recorded was the result of the combined effects of the heat from the light source and the heat produced by the polymerization reaction. From the temperature-time plot, the maximum temperature rise (T’i), which is the difference between the maximum temperature and the temperature of the ambient, and the time to reach the maximum temperature rise (ti) were determined.
Measurement of temperature increase attributable to the polymerizing light source (T,) The second experiment was carried out to distinguish the temperature rise caused by the heating effect of the light from that attributable to the polymerization reaction. The specimens subjected to an initial 60s exposure during polymerization were monitored until their temperatures returned to that of the ambient and maintained at that temperature for a further 15 min. The specimens were then re-exposed for another 60 s using the same light source. The temperature changes were monitored and recorded. To obtain the temperature rise due to polymerization (T,) alone, the temperature rise during the second exposure (T,) occurring at the same time as the maximum temperature during the first exposure was subtracted from the maximum temperature rise during the first exposure (T, = Ti - T,) (Figure I).
Measurement of temperature increase during material addition (T,) To measure the temperature rise during second layer PTFE mould was curing, a second 2-mm-thick positioned over the 2-mm-thick mould containing the previously cured specimen in which the thermocouple was buried. The freshly mixed cement of the same type was then placed in the upper mould and cured for 60 s. The temperature rise with respect to time was measured by the thermocouple embedded in the cured cement in the lower mould as described earlier. This phase of the experiment was carried out only for the restorative materials, i.e. Vitremer and Fuji II LC, being evaluated.
Polymerization
Tempsmtun
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et al.
2307
were obtained. Each sample was then plunged into another bath maintained at approximately 5°C (To). The change in temperature with respect to time was recorded until the specimen had re-equilibrated. A plot of ln(T - T,)/(T, - TI) versus time t, where T is the temperature within the specimen at any time t, was produced and the slope (S) was obtained horn the linear region of the plot. The thermal diffusivity was calculated using the equation D = -S/Y, where Y = (5.7832/R’) + (?/L’) for a cylindrical specimen of radius R and length L17.
he
RFSULTS Figure 1 A schematic plot of temperature rise versus time recorded during initial polymerization and during second exposure using a 60-s exposure time. The difference between maximum temperature rise during initial polymerization (Ti) and temperature rise during second exposure
which occurred at the same time as Ti (TJ is the temperature rise resulting frorn the heat of polymerization (T,,).
Measurement of thermal diffbsivity The thermal diffusivity (0) for each cement was determined using the method described initially by Braden’” and modified by Watts and Smith17. The cylindrical-shaped s;pecimens (6 mm in diameter and 6 mm in length) with a thermocouple embedded in the centre were polymerized and then stored in distilled water at 37°C for 24 h. The specimens were first immersed in a warm water bath (TI = SO’C) and the temperatures were monitored until constant readings Table 2 Maximum temperature Material
rise during initial polymerization,
Environmental tempe,rature (“C)
Specimen thickness (mm)
Fuji II LC
Vitrebond
Fuji Lining LC
The maximum temperature rises (Ti) and the times to reach the maximum temperature (tJ during initial polymerization for different exposure times, specimen thicknesses and experimental temperatures are shown in Tables 2 and 3 respectively. The average maximum temperature rises produced by the RMGICs when subjected to an initial exposure of 60 s ranged from 11 to 26°C for the liner/base cements and from 8 to 17°C for their restorative counterparts (Table 2), the ranking between materials being in ascending order, VM < VB < FC < FL for all specimen thicknesses and test temperatures (P < 0.01). Increasing exposure times horn 15 to 60s did not always result in a greater increase in temperature, particularly for thin specimens where the temperature 7, (“C)
Ti (“C)” after exposure time (s) 15
Vitremer
Measurement of temperature increase during initial polymerization
20
30
40
60
25
1 2 3
12.1 (0.3) 12.3 (0.8) 8.4 (1.2)
12.3 (0.1) 13.3 (0.4) 8.1 (0.6)
13.7 (0.7) 12.4 (0.8) 11.1 (0.6)
13.2 (0.6) 12.9 (0.7) 10.9 (0.7)
37
1 2 3
10.3 (0.7) 3.8 (0.2) 2.4 (0.3)
10.1 (0.5) 9.0 (0.8) 8.1 (0.9)
10.6 (0.5) 8.7 (1.0) 8.5 (0.5)
11.1 (0.5) 8.9 (0.5) 7.9 (0.2)
25
1 2 3
16.3 (1.0) 16.7 (1.2) 12.3 (1.1)
16.6 (1.0) 16.6 (1.1) 14.7 (1.3)
16.2 (1.1) 16.4 (0.4) 16.5 (1.0)
16.3 (0.9) 16.9 (0.2) 16.8 (1.3)
37
1 2 3
12.2 (1.0) 10.0 (0.5) 11.4 (0.3)
13.3 (0.2) 12.7 (0.9) 13.5 (0.5)
14.0 (0.7) 12.1 (0.8) 13.9 (0.3)
14.8 (0.8) 13.5 (1.7) 12.9 (0.2)
25
1 2 3
13.9 (2.2) 14.0 (0.5) 11.6 (1.2)
14.7 (0.5) 14.6 (0.6) 11.7 (0.9)
15.2 (1.3) 15.5 (0.2) 13.6 (0.5)
14.1 (0.3) 15.3 (0.6) 13.0 (0.7)
15.7 (1.2) 15.2 (0.4) 14.1 (0.9)
37
1 2 3
10.1 (0.4) 11 .o (0.3) 8.4 (0.6)
11.2 (0.8) 12.4 (0.4) 9.1 (0.6)
12.6 (0.8) 12.5 (0.9) 10.9 (0.3)
12.3 (0.6) 11.9 (0.8) 11.2 (0.7)
13.0 (0.8) 13.0 (1.1) 11.9 (0.8)
25
1 2 3
18.3 (2.0) 16.4 (3.0) 13.3 (1.2)
20.4 (0.9) 17.9 (1.0) 18.6 (1.3)
23.9 (1.6) 23.5 (0.6) 21.9 (1.8)
25.9 (0.3) 24.0 (0.6) 23.4 (0.3)
25.6 (1.3) 24.9 (0.5) 24.9 (1 .O)
37
1 2 3
20.4 (1 .O) 13.3 (0.5) 13.2 (1.1)
21.5 (0.3) 14.3 (0.2) 14.1 (0.8)
22.4 (1.1) 22.5 (0.5) 23.7 (1 .O)
23.5 (1 .O) 23.0 (0.4) 25.2 (1.4)
24.8 (0.3) 23.3 (0.2) 23.2 (1.4)
‘Averages of five determinal.ions. The figures in parentheses are 1 standard deviation. Biomaterials 1996. Vol. 17 No. 24
2308
Polymerization
Table 3 Time to reach maximum temperature Material
Test temperature
(“C)
of resin-modified
rise during initial polymerization,
Specimen thickness (mm)
Fuji II LC
Vitrebond
Fuji Lining LC
cements:
W. Kanchanavasita
et
a/.
ti (s)
ti (s)’ after exposure time (s) 15
Vitremer
glass-ionomer
25
20
30
40
60
20 (0) 20 (0)
21 (1)
23 (1) 29 (3) 36 (4)
23 (1) 35 (2) 42 (4)
22 (3)
27 (1) 26 (0)
37
16 (1) 20 (0) 26 (0)
17 (1) 23 (0) 30 (0)
16 (1) 25 (2) 39 (2)b
16 (1) 26 (2) 39 (2)b
25
20 (0) 20 (0) 21 (2)
21 (1) 20 (1) 29 (2)
21 (1) 21 (2) 35 (4)
22 (1) 21 (2) 33 (2)
37
‘6 (1) 20 (0) 20 (0)
17 (1) 23 (0) 29 (1)
16 (1) 22 (1) 30 (o)b
‘9 (1) 22 (1) 29 (2)
25
15 (0) 15 (0) 15 (0)
16 (1) 19 (0) 19 (0)
17 (2) 20 (1) 26 (3)
19 (0) 19 (0) 29 (2)
16 (2) 19 (0) 26 (4)
37
15 (0) 15 (0) 15 (0)
16 (1) ‘9 (0) 21 (2)
21 (2) 20 (1) 26 (0)b
16 (1) 19 (0) 24 (2)
21 (2) 19 (0) 24 (2)
25
15 (0) 15 (0) 15 (0)
19 (0) 20 (0) 23 (0)
23 (0) 25 (1) 29 (3)
24 (2) 25 (1) 32 (3)
23 (0) 26 (3) 32 (3)
37
15 (0) 15 (0) 15 (0)
20 (1) 20 (0) 20 (1)
22 (1) 24 (1) 27 (4)
24 (2) 23 (1) 30 (3)b
24 (3) 23 (2) 26 (2)
aAverages of five determinations. The figures in parentheses are 1 standard deviation. bThese-values were the greatest in each-material group at 37°C.
rise nearly reached its peak when using short exposure time. For l-mm-thick specimens, the maximum temperature rises obtained at different exposure times from 15 to 60s were not statistically different (P < 0.05). However, for 2- and 3-mm-thick specimens, the maximum temperature rises were obtained only by using the exposure times more than 20s. Increasing specimen thickness also resulted in a reduction in the rise in cement temperature for exposure times less than 30 s. The times required for the temperature to reach its peak ranged from 18s for l-mm-thick specimens for Vitrebond to 42 s for 3-mm-thick specimens for Vitremer (Table 3). Thicker specimens generally took longer to reach their peak temperatures than thinner specimens of the same material. The temperature of the test environment had an effect on Ti. Increasing the environmental temperature from 25 to 37°C resulted in a significant decrease in 7’i. However, this only affected ti slightly.
Measurement of temperature increase attributable to the polymerizing light source Figures ~a and ~b illustrate the temperature rises due to the polymerization reaction (T,) together with those attributable to the polymerizing light source (T,). TP ranged from 8 to 22°C for the liner/base and from 5 to 15°C for the restorative cements. The ranking for TP in ascending order is VM < VB < FC < FL for all specimen thicknesses and test temperatures (P < 0.01). The contribution of T, to the overall temperature rise Biomaterials 1996, Vol. 17 No. 24
was relatively small and in no case being greater than 6°C.
Measurement of temperature increase during material addition The temperature rises measured during curing of the second layer for 60s (7’,) are shown in Table 4. These were 6.7 and 3.9”C for Vitremer and 7.5 and 4.8% for Fuji II LC at 25 and 37”C, respectively. These values were always less than those obtained during the initial polymerization. The times required to reach the maximum temperatures obtained from the initial polymerization and from curing the material addition for the same conditions were similar (Table 4).
Measurement of thermal diffusivity The thermal diffusivities of the RMGICs determined in this study ranged from 1.9 x 10e3 to 2.5 x 10m3cm2 s-l (Table I). The values for the lining cements were significantly lower than those for the restorative cements (P < 0.01).
DISCUSSION The results from this study showed marked temperature rises (up to 22°C) produced by four RMGICs during polymerization with a light source, even when the temperature rise attributable to the light source was excluded. The heat from the polymerization of these materials therefore played a significant part in
Polymerization
of resin-modified
glass-ionomer
cements:
W. Kanchanavasita
2309
et al. 30
30
25
25 ,-Vitremer
I
]
f-&Fuji II LC
i
20
o^ 20 e
4
‘C
$
15
1 I-
10
5
0 lmm
2mm 0 Tp l37’C)
2mm
lmm
3mm n
0 Tp l25.W
0 Ts (25W
Ts 137’C)
j
1 Vitrebond
3mm
[
Fuji Lining LC
-
] ‘7
-
25
20
-
20
2c
15
-
15
1 f
10
-
10
25
G “L
-5
5
-0
0
0 Tp (37°C)
b
3mm
3mm
2mm
lmm
0 Tp (25W
E TS l25”CI
H Ts 137’C)
Figure 2 Temperature rise produced by polymerization reaction (Tp) and by the heat from the light source (7,) for: a, two restorative RGMICs, Fuji II LC (FC) and Vitremer (VM), during 60-s exposures; b, two lining RMGICs, Fuji Lining LC (FL) and Vitrebond (VB), during 60-s exposures.
Table 4 Maximum temperature rises and times to reach the maximum during material addition (Ta and tJa Material
2!j”Cb
temperature
rise during
initial
polymerization
(Ti and ti) and
37”Cb
7, (“C)
Ta (“Cl
ti
(S)
kl
(s)
T
(“C)
Ta
(“‘7
ti
(S)
kl
(9
Vitremer
lZ.9 (0.7)
6.7 (1.1)
35 (2)
31 (2)
8.9 (0.5)
3.9 (0.5)
26 (2)
23 (0)
Fuji II LC
16.9 (0.2)
7.5 (0.1)
21 (2)
26 (0)
13.5 (1.7)
4.8 (0.3)
22 (1)
23 (4)
‘Averages of five determinations. bEnvironmental temperature
The figures in parentheses
are 1 standard deviation
Biomaterials
1996, Vol. 17 No. 24
2310
Polymerization
of resin-modified
temperature rises which were observed. This finding contrasted with the results from previous studies on composite resins which showed that most of the temperature rises observed were from the light source2’18. Bourke et aLI measured the temperature rise in a Vitrebond specimen (diameter 5.5mm and thickness 2.1 mm) with a DTA operating at 37°C using a different light source and 30 s curing time. They found that the maximum temperature rise was 15.3 f 2.1”C (after taking account of the effect of temperature rise from the light source which was 5°C). The time to reach the maximum temperature was 45 s. These values were higher than those found in this study (T, = 9.4 f 2.1%; ti = 20s) even though the light source they used produced a smaller energy output than the Luxor unit used here4*18. The difference is probably attributable to the method used to measure the temperature rise, the specimen container, and the site of the thermocouple in the material. However, these two studies confirm the substantial temperature rise as a result of heat of reaction produced by the RMGICs used as lining materials. Another RMGIC, Fuji Lining LC, which, according to this study, produced a larger temperature rise than Vitrebond, is also marketed as a lining material. The manufacturer does not advise the use of any sub-lining before placing the cement in deep cavities. Zach and Cohen3 have suggested that a rise in temperature of approximately 5°C may cause pulp tissue damage. Therefore, the marked temperature rise during the polymerization process may potentially cause damage to the pulp. Moreover, the combined effect of the cement exotherm and the heat from the light source should be considered since clinically the tooth is subjected to both sources of heat simultaneously. The use of a light source which produces greater heat than the Luxor unit used in this study5 may further increase the temperature rise. The effect of these temperature rises may be reduced or modified by the surrounding tooth tissue and dentine as these are good thermal insulators and will limit the rise in temperature in the base of the activity. The exotherm produced by RMGIC during polymerization is substantially greater than that for composite probably as a result of the use of lowresinsI molecular-weight monomers such as HEMA. Lowmolecular-weight monomers are known to produce higher exotherm, as well as higher shrinkage, when compared with high-molecular-weight monomers. This contributes to the temperature rise observed from these cements. The concentration of the monomers contained in the cement formulation also has an effect on the amount of exotherm. The higher temperature rise produced by the liner/base RMGICs compared with the restorative materials may be attributable to the presence of a higher proportion of monomers as a result of the lower powder/liquid ratio used. Polymerization of unsaturated monomers like HEMA involves the reaction of the double bonds contained in the molecules. At the termination of the reaction, some of the monomeric component may remain unconverted. These are then incorporated within the set material. High degree of polymerization or conversion is desirable in order to reduce water sorption and the
Biomaterials
1996, Vol. 17 No. 24
!Yass-ionomer
cements: W. Kanchanavasita
et al.
solubility and increase mechanical properties. However, high conversion also results in increased exotherm which is not desirable in the oral environment. In contrast, a low degree of conversion in which the amount of residual monomer is high produces low exotherm. The unreacted monomeric component, which may be toxic, can diffuse out of the cement into the oral environment. Temperature rises were reduced as the specimen thickness increased. This may be due to the greater distance between the thermocouple and the light source as the specimen thickness increased, reducing the contribution of the heat from the light source. It may also be a result of the attenuation of light in the thicker specimen causing less effective curing of material in the depth of the specimen. In this study, unlike earlier investigations2*‘, varying the specimen thickness did not significantly reduce the temperature rise. A reason for this is that in this study the temperature rise was measured at the centre of, rather than beneath, the specimen. The time to reach maximum temperature rise (ti) did not coincide with exposure times greater than 20s (Table 3), ti tending to occur before the light was switched off, especially in thin specimens. This may indicate that the polymerization reaction went to completion (optimum conversion occurred) more quickly in thinner specimens. This explains why increasing the exposure time does not cause a significant increase in peak temperature in a thin specimen (Table 2). Longer exposure times result in a slower decline in the temperature of the specimen. This is due to the heating effect from the light source which is still applied to the specimen after the maximum temperature rise is attainedlg. The point at which the temperature is at a maximum represents the point at which the conversion of the monomer has been maximized under these sets of conditions. From this the optimum exposure times were determined from Table 3 for each material. These are given in Table 5 together with the exposure times recommended by the manufacturers. It can be seen that both values are in agreement except for Fuji II LC where the recommended exposure time of 20s appears to be too short, especially for curing thicker specimens. It may be that using an incremental build-up technique instead of a bulk placement and a short exposure time may reduce the maximum temperature rise. However, the temperature rise measured when Table 5 Comparison between exposure times recommended by the manufacturers from Table 3
and the optimum
exposures
determined
Material
Exposure times recommended by the manufacturers (s)
Exposure times determined from Table 3 at 37°C (s)’
Vitremer Vitrebond Fuji II LC Fuji Lining LC
40 30 20 30
39 26 30 30
eAverages of five determinations. deviation.
(2) (0) (0) (3)
The figures in parentheses
are 1 standard
Polymerization
of resin-modified
glass-ionomer
cements: W. Kanchanavasita
curing an overlay of 2-mm-thick Vitremer or Fuji II LC (T,) at 37”C, although much lower than Tie was still in the range of 5°C (Table 4, the level of which was often cited to cause pulpal damage3. When the cement is used to build up a cavity in layers, the tooth is subjected to both the heat from curing the first layer and the subsequent second layer. The thermal diffusivities of the RMGICs determined in this study are the average values obtained between 5 and 6O”C, the range normally occurring in the oral environment (Tabjle 1). Since thermal diffusivity indicates the rate of thermal diffusion or transfer through a material;:‘, a material with higher thermal diffusivity will permit more rapid transfer of heat according to the thermal through it. Therefore, diffusivity values, the rate of transmission of heat in VM and FC should be higher than that in VB and FL when subjected to identical exposures. However, this is not the case in this study where the temperature rises produced by VM, FL and FC during second exposure were slightly higher than those produced by VB. This may be attributable to the relatively small difference in the thermal diffusivities among these four cements and possible inhomogeneity in the specimens. A material with lower thermal diffusivity is more effective as an insulator for temperature changes. Therefore, Vitrebond (VB) and Fuji Lining LC (FL) could be regarded as better insulators than Vitremer (VM) and Fuji II LC (FC). This may be attributable to the lower powder to liquid (P/L) ratio (1.4:l.o by mass) used for mixing VB and FL (P/L ratios for VM and FC are 2.5:l.O and 3.0:1.0, respectively) rather than the difference in composition. The effect of P/L ratio on thermal diffusivity in conventional GICs has been determined in many studies21-23, increasing the P/L ratio resulting in an increase in thermal diffusivities of the cements. The data available for the thermal diffusivity of some RMGICs have been found in one studyz4. From that study the thermal diffusivity for Vitrebond (1.8 x 1Clm3 cm’s_‘) agrees well with the value obtained from this study (2.2 x 10m3cm’s_l). The difference may be from the geometry of the specimens and the temperature over which the specimens were cooled. From this study, the thermal diffusivity values of RMGICs (1.9 x 10-32.5 x 10e3 cm2 s-‘) are comparable to those of dentine (1.8 x 10-3-2.6 x 10e3 cm2s-1)16,25*26, suggesting that RMGICs will provide a similar degree of insulation to dentine. The efficiency of a lining material is given by its thickness divided by the square root of its thermal diffusivity16. Furthermore, the time for the temperature of the external stimuli to reach a given value within a specimen is proportional to the square of its thickness’“*“. Therefore, the differences in the thermal protection efficiencies between materials are more dependent on the variation in the thickness than in the thermal diffusivity. However, clinically where the thickness of the lining material must be kept to a minimum in order to provide enough thickness for the overlying material, or in the case where a thin layer of material can be applied to any cavities such as in Class V, the value of thermal diffusivity will be a significant factor in determining which material to be used.
2311
et a/.
CONCLUSIONS 1. Polymerization
of RMGICs resulted in marked rises in temperature, the liner/base cements showing higher temperature rises than the restorative cements. Increasing specimen thickness reduced the temperature rise for exposure time shorter than or equal to 30s. Thus, inadequate exposure time or thick layer of material resulted indicating in temperature rise, reduced incomplete curing. The time required to reach the maximum temperature rise was shorter in thinner specimens than in thicker ones for the same exposure time. Increasing the environmental temperature from 25 to 37°C resulted in a decrease in the temperature rise. 2. Using a thermocouple as a means to measure temperature rises during polymerization can determine the depth of cure and exposure time where curing becomes inadequate. 3. When supplementary layers of the cements are polymerized, the temperature rise measured underneath was still in the range of 5”C, the level of which has been cited as sufficient to cause pulp injury. 4. The thermal diffusivities of the liner/base RMGICs are lower than those for the restorative cements, indicating that they are better insulators. However, both cements have similar values of thermal diffnsivity to that of dentine and can be used as dentine substitutes. From this study, no correlation was found between the thermal diffusivity value and the temperature rise on polymerization.
REFERENCES 1
Wilson AD. Resin-modified glass-ionomer cements. Int/ Prosthodont
2
3
1990; 3: 425429.
Lloyd CH, Joshi A, McGlynn E. Temperature rises produced by light sources and composites during curing.Dent Mater 1986;2:170-174. Zach L, Cohen G. Pulp responses to externally applied heat. Oral Surg Oral Med Oral Path01 1965; 9: 515530.
4
Hansen EK, Asmussen E. Correlation between depth of cure and temperature rise of a light-activated resin.
5
Masutani S, Setcos JC, Schnell RJ, Philips RW. Temperature rise during polymerization of visible light-activated composite resins. Dent Mater 1988; 4:
Stand J Dent Res 1993; 101:176179.
174-178. 6
Smail SRJ, Patterson CJW, McLundie AC, Strang R. In vitro temperature rises during visible-light curing of lining material and a posterior composite. J Oral Rehabill966; 15:361-366.
7
Antonucci J, Toth EE. Extent of polymerisation of dental resins by differential scanning calorimetry. J Dent Res
a
McCabe JF, Wilson HJ. The use of differential scanning calorimetry for the evaluation of dental materials. J
9
Adamson M, Lloyd CH, Watts DC. The temperature rise beneath a light-cured cement lining during light curing.
1963; 62:121-125.
Oral Rehabil1995:
7: 103-110.
JDent 19aa;16:182-187. 10
Lloyd CH. A differential thermal analysis (DTA) for the Biomaterials 1996,Vol. 17 No. 24
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11
12
13
14 15 16 17
18
19
Polymerization
of resin-modified
heats of reaction and temperature rises produced during the setting of tooth coloured restorative materials. I Oral Rehabil1984; 11: 111-121. Crisp S, Jennings MA, Wilson AD. A study of temperature changes occurring in setting dental cements. I Oral Rehabill995; 5: 139-144. McCabe JF. Cure performance of light-activated composites by differential thermal analysis (DTA). Dent Mater 1985; 1: 231-234. Bourke AM, Walls AW, McCabe JF. Light-activated glass polyalkenoate (ionomer) cements: the setting reaction. JDent 1992; 20: 115-120. Vitremer Tri-cure Glass Ionomer System Technical Product Profile. 3M Dental Products, 1993. Saito S. Clinical application of GC Fuji II LC-theory and practice. 1995. Braden M. Heat conduction in teeth and the effect of lining materials. JDent Res 1964; 43: 315-322. Watts DC, Smith R. Thermal diffusivity in finite cylindrical specimens of dental cements. JDent Res 1981; 60: 1972-1976. McAndrew R, Lloyd CH, Watts DC. The effect of a cement lining upon the temperature rise during the curing of composite by visible light. I Dent 1987; 15: 218-221. Kanchanavasita W, Pearson GJ, Anstice HM. Tempera-
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1996, Vol. 17 No. 24
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24
25 26
27
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W. Kanchanavasifa
et a/.
ture rise in ion-leachable cements during setting reaction. Biomateriak 1995; 16: 1261-1265. Watts DC. Dental restorative materials. In: Cahn RW, Haasen P, Kramer EJ, eds. Materials Science and Technology: a Comprehensive Treatment. Weinheim, Germany: VCH Verlagsgesellschaft, 1992: 209-258. Inoue T, Saitoh M, Nishiyama M. Thermal properties of glass ionomer cement. J IVihon LJniv Sch Dent 1993; 35: 252-257. Tay WM, Braden M. Thermal diffusivity of glassionomer cements. JDent Res 1987; 66: 1040-1043. Watts DC, Smith R. Thermal diffusion in some polyelectrolyte dental cements: the effect of powder/liquid ratio. J Oral Rehabill984; 11: 285-288. Brantley WA, Kerby RR. Thermal diffusivity of glass ionomer cement systems. J Oral Rehabil 1993; 20: 6168. Brown WS, Dewey WA, Jacobs MR. Thermal properties of teeth. JDent Res 1970; 49: 752-755. Fukase Y, Saitoh M, Kaketani M, Nishiyama M. Thermal coefficients of paste-paste type pulp capping cements and dentin by xenon flash method. Dent Mater J 1992; 11: 189-196. Baffa 0, Mielnik J, Panzeri H, Vinha D. Thermal diffusivity of dental cements. Aust Dent J 1986; 31: 268-272.
SO142-9612
(96)
Biomoteriols 17 (1996)23054312 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0142-9612/96/$X5.00
00065-B
Factors contributing to the temperature rise during polymerization of resinmodified glass-ionomer cements Widchaya Kanchanavasita, Biomaterials
Department,
Gavin J, Pearson and H.M. Anstice
Eastman Dental Institute, 256 Gray’s Inn Road, London WClX 8LD, UK
Part of the setting reaction of a resin-modified glass-ionomer cement (RMGIC) is a photoinitiated polymerization. As a result of the polymerization exotherm, the temperature of the cement may rise during setting. This study investigated the temperature rise for two liner/base- and two restorativetype RMGICs. The effects of factors such as specimen thickness, exposure time and environment temperature were investigated. The thermal diffusivity of the cements was also evaluated. Temperatures were measured using a thermocouple embedded in the centre of 6-mm diameter specimen discs of 1, 2 or 3 mm thickness. The exposure times used to cure the specimens varied from 15 to 60 s. The tests were carried out at either 25 or 37°C. The temperature rises attributable to the polymerization reaction ranged from 11 to 26°C for the liner/base cements and from 8 to 17°C for the restorative cements. Increasing the specimen thickness reduced the temperature rise only when inadequate exposure times were used. Raising the environmental temperature resulted in a smaller temperature rise. The thermal diffusivities were determined from cylindrical specimens. These ranged from 1.9 x 10e3 to 2.5 x 10m3cm*.Y’, the lining cements showing lower values than the restorative materials. 0 1996 Elsevier Science Limited. Keywords:
Dental
materials,
resin-modified
glass-ionomer
cements,
polymerization,
thermal
diffusivity
Received 3 November 1995;accepted 5 April 1996
and differential thermal analyser (DTA)SP1O. Crisp et Ill.11monitored the temperature rises of some dental including conventional cements, glass-ionomer materials. They found that zinc phosphate cements showed the greatest temperature rise on setting (22°C) whereas glass-ionomer cements gave the smallest (4°C). Modifications of the DTA have been made to monitor the exotherm of light-curing composite resins2*‘2. Bourke et a1.13 applied the use of a DTA to investigate the setting reactions and temperature rises of two liner/base RMGICs. In the present study, the temperature rises of liner/base RMGICs were compared with those of restorative-type RMGICs. The effects on the temperature rise produced by varying the times for which the materials were exposed to light activation, the thickness of the cement layers and the environmental conditions were also examined.
resin-modified glass-ionomer cements Recently, (RMGICs) have been developed for use as either liner/ base or restorative materials. These cements set by two mechanisms: the acid-base reaction of the conventional glass-ionomer component and a polymerization reaction of the monomeric constituents contained in the formulation’. Such polymerization reactions are known to be exothermic in nature, producing a rise in temperature in the cement during setting. Heating from the light-curing source used in photopolymerization may also contribute to the total temperature rise observed’. The liner/base cements are normally used in deeper parts of the cavities before placement of other restorative materials, such as amalgam or composite resin. In contrast, restorative cements may be placed either incrementally or in bulk to fill up the whole cavity. Generally, a large temperature rise may have a detrimental effect on the tooth tissue, particularly the pu1p3. Most of the previous work monitoring the changes in temperature of the setting restorative materials has been carried out on resin materials. The instrumentation used varied fiorn simple devices using thermocouples4-6 to more sophisticated commercial equipment such as a differential scanning calorimeter (DSC)7*8 Correspondence
MATERIALS AND METHODS Four hand-mixed RMGICs were examined in this study (Table 2). Vitremer (VM) contains a radiopaque, fluoroaluminosilicate glass powder and a lightsensitive aqueous solution of a modified poly(alkenoic) acid with some pendant methacrylate end groups, some methacrylate monomers, usually 2-hydroxyethyl
to Dr ‘G.J.Pearson.
2305
Biomaterials
1996, Vol. 17 No. 24
Polymerization
2306 Table 1 Resin-modified
of resin-modified
glass-ionomer
cements:
W. Kanchanavasita
et a/.
GlCs used in this study
Product
Manufacturer
Batch no.
Mixing ratio (g/g)
Thermal diffusivitya (x10-3cm’s-‘)
Restorative Vitremer (VM)
3M Co., St. Paul, MN, USA
Powder Liquid Powder Liquid
2.511 .O
2.50 (0.12)
3.011 .o
2.47 (0.09)
1.411.0 1.411 .o
2.18 (0.14) 1.91 (0.30)
Fuji II LC (FC)
Lining Vitrebond (VB) Fuji Lining LC (FL)
inAverages of five determinations. (P > 0.05)
GC Dental, Tokyo, Japan
3M Co., St. Paul, MN, USA GC Dental, Tokyo, Japan
The figures in parentheses
19940727 Powder 230341 Liquid 250341
are 1 standard deviation. The values connected by a straight line were not significantly
(HEMA), and various polymerization initiators14. In Fuji II LC (FC) the glass is a strontiumcontaining fluoroaluminosilicate glass while the liquid consists of an aqueous solution of a copolymer of acrylic-maleic acid, HEMA, and an initiator and activator15. Both VM and FC are recommended for use as aesthetic filling materials in non-load-bearing areas in the adult dentition, such as cervical lesions and Class III lesions, and occlusal restorations of deciduous dentition. Vitrebond (VB) and Fuji Lining LC (FL) are low-viscosity lining materials used primarily for pulp protection under other restorative materials. The formulation contains similar components but in the lining materials the powder/liquid ratio is lower. Before mixing each material, the powder and liquid were weighed out to the manufacturers’ recommended mixing ratio using a digital balance (R-20, Oerting Ltd., Kent, UK). The experiment was divided into four phases: methacrylate
Measurement of temperature increase during initial polymerization Measurement of temperature increase attributable to the polymerizing light source Measurement of temperature increase during material addition Measurement of thermal diffusivity Tests were carried out at either 25 or 37°C in a thermostatically controlled cabinet (Compenstat, UK). A minimum of five recordings was taken for each test and each combination of material and condition. All the results were subjected to the Kruskal-Wallis test. If differences were found, further analyses were carried out using the Mann-Whitney U test.
Measurement of temperature increase during initial polymerization (Ti) The materials under investigation were mixed at room temperature in an environmental chamber according to the manufacturers’ instructions and then inserted into a polytetrafluoroethylene (PTFE) cylindrical mould with an internal diameter of 6mm and a thickness of either 1, 2 or 3 mm. Single continuous exposures for specific times from 15 to 60s were applied to the surfaces of the specimens using a polymerizing light source (Luxor, ICI Dental, Macclesfield, UK). The increase in temperature with time during polymerizaBiomaterials 1996, Vol. 17 No. 24
19941206 19941020 150981 030981
different
tion was monitored using a thermocouple which had been placed centrally in the mould before insertion of the materials. The signals from the thermocouple were fed to a digital thermometer including a cold junction and also to a chart recorder (Linseis LS 52-4, Germany). The temperature change recorded was the result of the combined effects of the heat from the light source and the heat produced by the polymerization reaction. From the temperature-time plot, the maximum temperature rise (T’i), which is the difference between the maximum temperature and the temperature of the ambient, and the time to reach the maximum temperature rise (ti) were determined.
Measurement of temperature increase attributable to the polymerizing light source (T,) The second experiment was carried out to distinguish the temperature rise caused by the heating effect of the light from that attributable to the polymerization reaction. The specimens subjected to an initial 60s exposure during polymerization were monitored until their temperatures returned to that of the ambient and maintained at that temperature for a further 15 min. The specimens were then re-exposed for another 60 s using the same light source. The temperature changes were monitored and recorded. To obtain the temperature rise due to polymerization (T,) alone, the temperature rise during the second exposure (T,) occurring at the same time as the maximum temperature during the first exposure was subtracted from the maximum temperature rise during the first exposure (T, = Ti - T,) (Figure I).
Measurement of temperature increase during material addition (T,) To measure the temperature rise during second layer PTFE mould was curing, a second 2-mm-thick positioned over the 2-mm-thick mould containing the previously cured specimen in which the thermocouple was buried. The freshly mixed cement of the same type was then placed in the upper mould and cured for 60 s. The temperature rise with respect to time was measured by the thermocouple embedded in the cured cement in the lower mould as described earlier. This phase of the experiment was carried out only for the restorative materials, i.e. Vitremer and Fuji II LC, being evaluated.
Polymerization
Tempsmtun
of resin-modified
glass-ionomer
cements:
W. Kanchanavasita
et al.
2307
were obtained. Each sample was then plunged into another bath maintained at approximately 5°C (To). The change in temperature with respect to time was recorded until the specimen had re-equilibrated. A plot of ln(T - T,)/(T, - TI) versus time t, where T is the temperature within the specimen at any time t, was produced and the slope (S) was obtained horn the linear region of the plot. The thermal diffusivity was calculated using the equation D = -S/Y, where Y = (5.7832/R’) + (?/L’) for a cylindrical specimen of radius R and length L17.
he
RFSULTS Figure 1 A schematic plot of temperature rise versus time recorded during initial polymerization and during second exposure using a 60-s exposure time. The difference between maximum temperature rise during initial polymerization (Ti) and temperature rise during second exposure
which occurred at the same time as Ti (TJ is the temperature rise resulting frorn the heat of polymerization (T,,).
Measurement of thermal diffbsivity The thermal diffusivity (0) for each cement was determined using the method described initially by Braden’” and modified by Watts and Smith17. The cylindrical-shaped s;pecimens (6 mm in diameter and 6 mm in length) with a thermocouple embedded in the centre were polymerized and then stored in distilled water at 37°C for 24 h. The specimens were first immersed in a warm water bath (TI = SO’C) and the temperatures were monitored until constant readings Table 2 Maximum temperature Material
rise during initial polymerization,
Environmental tempe,rature (“C)
Specimen thickness (mm)
Fuji II LC
Vitrebond
Fuji Lining LC
The maximum temperature rises (Ti) and the times to reach the maximum temperature (tJ during initial polymerization for different exposure times, specimen thicknesses and experimental temperatures are shown in Tables 2 and 3 respectively. The average maximum temperature rises produced by the RMGICs when subjected to an initial exposure of 60 s ranged from 11 to 26°C for the liner/base cements and from 8 to 17°C for their restorative counterparts (Table 2), the ranking between materials being in ascending order, VM < VB < FC < FL for all specimen thicknesses and test temperatures (P < 0.01). Increasing exposure times horn 15 to 60s did not always result in a greater increase in temperature, particularly for thin specimens where the temperature 7, (“C)
Ti (“C)” after exposure time (s) 15
Vitremer
Measurement of temperature increase during initial polymerization
20
30
40
60
25
1 2 3
12.1 (0.3) 12.3 (0.8) 8.4 (1.2)
12.3 (0.1) 13.3 (0.4) 8.1 (0.6)
13.7 (0.7) 12.4 (0.8) 11.1 (0.6)
13.2 (0.6) 12.9 (0.7) 10.9 (0.7)
37
1 2 3
10.3 (0.7) 3.8 (0.2) 2.4 (0.3)
10.1 (0.5) 9.0 (0.8) 8.1 (0.9)
10.6 (0.5) 8.7 (1.0) 8.5 (0.5)
11.1 (0.5) 8.9 (0.5) 7.9 (0.2)
25
1 2 3
16.3 (1.0) 16.7 (1.2) 12.3 (1.1)
16.6 (1.0) 16.6 (1.1) 14.7 (1.3)
16.2 (1.1) 16.4 (0.4) 16.5 (1.0)
16.3 (0.9) 16.9 (0.2) 16.8 (1.3)
37
1 2 3
12.2 (1.0) 10.0 (0.5) 11.4 (0.3)
13.3 (0.2) 12.7 (0.9) 13.5 (0.5)
14.0 (0.7) 12.1 (0.8) 13.9 (0.3)
14.8 (0.8) 13.5 (1.7) 12.9 (0.2)
25
1 2 3
13.9 (2.2) 14.0 (0.5) 11.6 (1.2)
14.7 (0.5) 14.6 (0.6) 11.7 (0.9)
15.2 (1.3) 15.5 (0.2) 13.6 (0.5)
14.1 (0.3) 15.3 (0.6) 13.0 (0.7)
15.7 (1.2) 15.2 (0.4) 14.1 (0.9)
37
1 2 3
10.1 (0.4) 11 .o (0.3) 8.4 (0.6)
11.2 (0.8) 12.4 (0.4) 9.1 (0.6)
12.6 (0.8) 12.5 (0.9) 10.9 (0.3)
12.3 (0.6) 11.9 (0.8) 11.2 (0.7)
13.0 (0.8) 13.0 (1.1) 11.9 (0.8)
25
1 2 3
18.3 (2.0) 16.4 (3.0) 13.3 (1.2)
20.4 (0.9) 17.9 (1.0) 18.6 (1.3)
23.9 (1.6) 23.5 (0.6) 21.9 (1.8)
25.9 (0.3) 24.0 (0.6) 23.4 (0.3)
25.6 (1.3) 24.9 (0.5) 24.9 (1 .O)
37
1 2 3
20.4 (1 .O) 13.3 (0.5) 13.2 (1.1)
21.5 (0.3) 14.3 (0.2) 14.1 (0.8)
22.4 (1.1) 22.5 (0.5) 23.7 (1 .O)
23.5 (1 .O) 23.0 (0.4) 25.2 (1.4)
24.8 (0.3) 23.3 (0.2) 23.2 (1.4)
‘Averages of five determinal.ions. The figures in parentheses are 1 standard deviation. Biomaterials 1996. Vol. 17 No. 24
2308
Polymerization
Table 3 Time to reach maximum temperature Material
Test temperature
(“C)
of resin-modified
rise during initial polymerization,
Specimen thickness (mm)
Fuji II LC
Vitrebond
Fuji Lining LC
cements:
W. Kanchanavasita
et
a/.
ti (s)
ti (s)’ after exposure time (s) 15
Vitremer
glass-ionomer
25
20
30
40
60
20 (0) 20 (0)
21 (1)
23 (1) 29 (3) 36 (4)
23 (1) 35 (2) 42 (4)
22 (3)
27 (1) 26 (0)
37
16 (1) 20 (0) 26 (0)
17 (1) 23 (0) 30 (0)
16 (1) 25 (2) 39 (2)b
16 (1) 26 (2) 39 (2)b
25
20 (0) 20 (0) 21 (2)
21 (1) 20 (1) 29 (2)
21 (1) 21 (2) 35 (4)
22 (1) 21 (2) 33 (2)
37
‘6 (1) 20 (0) 20 (0)
17 (1) 23 (0) 29 (1)
16 (1) 22 (1) 30 (o)b
‘9 (1) 22 (1) 29 (2)
25
15 (0) 15 (0) 15 (0)
16 (1) 19 (0) 19 (0)
17 (2) 20 (1) 26 (3)
19 (0) 19 (0) 29 (2)
16 (2) 19 (0) 26 (4)
37
15 (0) 15 (0) 15 (0)
16 (1) ‘9 (0) 21 (2)
21 (2) 20 (1) 26 (0)b
16 (1) 19 (0) 24 (2)
21 (2) 19 (0) 24 (2)
25
15 (0) 15 (0) 15 (0)
19 (0) 20 (0) 23 (0)
23 (0) 25 (1) 29 (3)
24 (2) 25 (1) 32 (3)
23 (0) 26 (3) 32 (3)
37
15 (0) 15 (0) 15 (0)
20 (1) 20 (0) 20 (1)
22 (1) 24 (1) 27 (4)
24 (2) 23 (1) 30 (3)b
24 (3) 23 (2) 26 (2)
aAverages of five determinations. The figures in parentheses are 1 standard deviation. bThese-values were the greatest in each-material group at 37°C.
rise nearly reached its peak when using short exposure time. For l-mm-thick specimens, the maximum temperature rises obtained at different exposure times from 15 to 60s were not statistically different (P < 0.05). However, for 2- and 3-mm-thick specimens, the maximum temperature rises were obtained only by using the exposure times more than 20s. Increasing specimen thickness also resulted in a reduction in the rise in cement temperature for exposure times less than 30 s. The times required for the temperature to reach its peak ranged from 18s for l-mm-thick specimens for Vitrebond to 42 s for 3-mm-thick specimens for Vitremer (Table 3). Thicker specimens generally took longer to reach their peak temperatures than thinner specimens of the same material. The temperature of the test environment had an effect on Ti. Increasing the environmental temperature from 25 to 37°C resulted in a significant decrease in 7’i. However, this only affected ti slightly.
Measurement of temperature increase attributable to the polymerizing light source Figures ~a and ~b illustrate the temperature rises due to the polymerization reaction (T,) together with those attributable to the polymerizing light source (T,). TP ranged from 8 to 22°C for the liner/base and from 5 to 15°C for the restorative cements. The ranking for TP in ascending order is VM < VB < FC < FL for all specimen thicknesses and test temperatures (P < 0.01). The contribution of T, to the overall temperature rise Biomaterials 1996, Vol. 17 No. 24
was relatively small and in no case being greater than 6°C.
Measurement of temperature increase during material addition The temperature rises measured during curing of the second layer for 60s (7’,) are shown in Table 4. These were 6.7 and 3.9”C for Vitremer and 7.5 and 4.8% for Fuji II LC at 25 and 37”C, respectively. These values were always less than those obtained during the initial polymerization. The times required to reach the maximum temperatures obtained from the initial polymerization and from curing the material addition for the same conditions were similar (Table 4).
Measurement of thermal diffusivity The thermal diffusivities of the RMGICs determined in this study ranged from 1.9 x 10e3 to 2.5 x 10m3cm2 s-l (Table I). The values for the lining cements were significantly lower than those for the restorative cements (P < 0.01).
DISCUSSION The results from this study showed marked temperature rises (up to 22°C) produced by four RMGICs during polymerization with a light source, even when the temperature rise attributable to the light source was excluded. The heat from the polymerization of these materials therefore played a significant part in
Polymerization
of resin-modified
glass-ionomer
cements:
W. Kanchanavasita
2309
et al. 30
30
25
25 ,-Vitremer
I
]
f-&Fuji II LC
i
20
o^ 20 e
4
‘C
$
15
1 I-
10
5
0 lmm
2mm 0 Tp l37’C)
2mm
lmm
3mm n
0 Tp l25.W
0 Ts (25W
Ts 137’C)
j
1 Vitrebond
3mm
[
Fuji Lining LC
-
] ‘7
-
25
20
-
20
2c
15
-
15
1 f
10
-
10
25
G “L
-5
5
-0
0
0 Tp (37°C)
b
3mm
3mm
2mm
lmm
0 Tp (25W
E TS l25”CI
H Ts 137’C)
Figure 2 Temperature rise produced by polymerization reaction (Tp) and by the heat from the light source (7,) for: a, two restorative RGMICs, Fuji II LC (FC) and Vitremer (VM), during 60-s exposures; b, two lining RMGICs, Fuji Lining LC (FL) and Vitrebond (VB), during 60-s exposures.
Table 4 Maximum temperature rises and times to reach the maximum during material addition (Ta and tJa Material
2!j”Cb
temperature
rise during
initial
polymerization
(Ti and ti) and
37”Cb
7, (“C)
Ta (“Cl
ti
(S)
kl
(s)
T
(“C)
Ta
(“‘7
ti
(S)
kl
(9
Vitremer
lZ.9 (0.7)
6.7 (1.1)
35 (2)
31 (2)
8.9 (0.5)
3.9 (0.5)
26 (2)
23 (0)
Fuji II LC
16.9 (0.2)
7.5 (0.1)
21 (2)
26 (0)
13.5 (1.7)
4.8 (0.3)
22 (1)
23 (4)
‘Averages of five determinations. bEnvironmental temperature
The figures in parentheses
are 1 standard deviation
Biomaterials
1996, Vol. 17 No. 24
2310
Polymerization
of resin-modified
temperature rises which were observed. This finding contrasted with the results from previous studies on composite resins which showed that most of the temperature rises observed were from the light source2’18. Bourke et aLI measured the temperature rise in a Vitrebond specimen (diameter 5.5mm and thickness 2.1 mm) with a DTA operating at 37°C using a different light source and 30 s curing time. They found that the maximum temperature rise was 15.3 f 2.1”C (after taking account of the effect of temperature rise from the light source which was 5°C). The time to reach the maximum temperature was 45 s. These values were higher than those found in this study (T, = 9.4 f 2.1%; ti = 20s) even though the light source they used produced a smaller energy output than the Luxor unit used here4*18. The difference is probably attributable to the method used to measure the temperature rise, the specimen container, and the site of the thermocouple in the material. However, these two studies confirm the substantial temperature rise as a result of heat of reaction produced by the RMGICs used as lining materials. Another RMGIC, Fuji Lining LC, which, according to this study, produced a larger temperature rise than Vitrebond, is also marketed as a lining material. The manufacturer does not advise the use of any sub-lining before placing the cement in deep cavities. Zach and Cohen3 have suggested that a rise in temperature of approximately 5°C may cause pulp tissue damage. Therefore, the marked temperature rise during the polymerization process may potentially cause damage to the pulp. Moreover, the combined effect of the cement exotherm and the heat from the light source should be considered since clinically the tooth is subjected to both sources of heat simultaneously. The use of a light source which produces greater heat than the Luxor unit used in this study5 may further increase the temperature rise. The effect of these temperature rises may be reduced or modified by the surrounding tooth tissue and dentine as these are good thermal insulators and will limit the rise in temperature in the base of the activity. The exotherm produced by RMGIC during polymerization is substantially greater than that for composite probably as a result of the use of lowresinsI molecular-weight monomers such as HEMA. Lowmolecular-weight monomers are known to produce higher exotherm, as well as higher shrinkage, when compared with high-molecular-weight monomers. This contributes to the temperature rise observed from these cements. The concentration of the monomers contained in the cement formulation also has an effect on the amount of exotherm. The higher temperature rise produced by the liner/base RMGICs compared with the restorative materials may be attributable to the presence of a higher proportion of monomers as a result of the lower powder/liquid ratio used. Polymerization of unsaturated monomers like HEMA involves the reaction of the double bonds contained in the molecules. At the termination of the reaction, some of the monomeric component may remain unconverted. These are then incorporated within the set material. High degree of polymerization or conversion is desirable in order to reduce water sorption and the
Biomaterials
1996, Vol. 17 No. 24
!Yass-ionomer
cements: W. Kanchanavasita
et al.
solubility and increase mechanical properties. However, high conversion also results in increased exotherm which is not desirable in the oral environment. In contrast, a low degree of conversion in which the amount of residual monomer is high produces low exotherm. The unreacted monomeric component, which may be toxic, can diffuse out of the cement into the oral environment. Temperature rises were reduced as the specimen thickness increased. This may be due to the greater distance between the thermocouple and the light source as the specimen thickness increased, reducing the contribution of the heat from the light source. It may also be a result of the attenuation of light in the thicker specimen causing less effective curing of material in the depth of the specimen. In this study, unlike earlier investigations2*‘, varying the specimen thickness did not significantly reduce the temperature rise. A reason for this is that in this study the temperature rise was measured at the centre of, rather than beneath, the specimen. The time to reach maximum temperature rise (ti) did not coincide with exposure times greater than 20s (Table 3), ti tending to occur before the light was switched off, especially in thin specimens. This may indicate that the polymerization reaction went to completion (optimum conversion occurred) more quickly in thinner specimens. This explains why increasing the exposure time does not cause a significant increase in peak temperature in a thin specimen (Table 2). Longer exposure times result in a slower decline in the temperature of the specimen. This is due to the heating effect from the light source which is still applied to the specimen after the maximum temperature rise is attainedlg. The point at which the temperature is at a maximum represents the point at which the conversion of the monomer has been maximized under these sets of conditions. From this the optimum exposure times were determined from Table 3 for each material. These are given in Table 5 together with the exposure times recommended by the manufacturers. It can be seen that both values are in agreement except for Fuji II LC where the recommended exposure time of 20s appears to be too short, especially for curing thicker specimens. It may be that using an incremental build-up technique instead of a bulk placement and a short exposure time may reduce the maximum temperature rise. However, the temperature rise measured when Table 5 Comparison between exposure times recommended by the manufacturers from Table 3
and the optimum
exposures
determined
Material
Exposure times recommended by the manufacturers (s)
Exposure times determined from Table 3 at 37°C (s)’
Vitremer Vitrebond Fuji II LC Fuji Lining LC
40 30 20 30
39 26 30 30
eAverages of five determinations. deviation.
(2) (0) (0) (3)
The figures in parentheses
are 1 standard
Polymerization
of resin-modified
glass-ionomer
cements: W. Kanchanavasita
curing an overlay of 2-mm-thick Vitremer or Fuji II LC (T,) at 37”C, although much lower than Tie was still in the range of 5°C (Table 4, the level of which was often cited to cause pulpal damage3. When the cement is used to build up a cavity in layers, the tooth is subjected to both the heat from curing the first layer and the subsequent second layer. The thermal diffusivities of the RMGICs determined in this study are the average values obtained between 5 and 6O”C, the range normally occurring in the oral environment (Tabjle 1). Since thermal diffusivity indicates the rate of thermal diffusion or transfer through a material;:‘, a material with higher thermal diffusivity will permit more rapid transfer of heat according to the thermal through it. Therefore, diffusivity values, the rate of transmission of heat in VM and FC should be higher than that in VB and FL when subjected to identical exposures. However, this is not the case in this study where the temperature rises produced by VM, FL and FC during second exposure were slightly higher than those produced by VB. This may be attributable to the relatively small difference in the thermal diffusivities among these four cements and possible inhomogeneity in the specimens. A material with lower thermal diffusivity is more effective as an insulator for temperature changes. Therefore, Vitrebond (VB) and Fuji Lining LC (FL) could be regarded as better insulators than Vitremer (VM) and Fuji II LC (FC). This may be attributable to the lower powder to liquid (P/L) ratio (1.4:l.o by mass) used for mixing VB and FL (P/L ratios for VM and FC are 2.5:l.O and 3.0:1.0, respectively) rather than the difference in composition. The effect of P/L ratio on thermal diffusivity in conventional GICs has been determined in many studies21-23, increasing the P/L ratio resulting in an increase in thermal diffusivities of the cements. The data available for the thermal diffusivity of some RMGICs have been found in one studyz4. From that study the thermal diffusivity for Vitrebond (1.8 x 1Clm3 cm’s_‘) agrees well with the value obtained from this study (2.2 x 10m3cm’s_l). The difference may be from the geometry of the specimens and the temperature over which the specimens were cooled. From this study, the thermal diffusivity values of RMGICs (1.9 x 10-32.5 x 10e3 cm2 s-‘) are comparable to those of dentine (1.8 x 10-3-2.6 x 10e3 cm2s-1)16,25*26, suggesting that RMGICs will provide a similar degree of insulation to dentine. The efficiency of a lining material is given by its thickness divided by the square root of its thermal diffusivity16. Furthermore, the time for the temperature of the external stimuli to reach a given value within a specimen is proportional to the square of its thickness’“*“. Therefore, the differences in the thermal protection efficiencies between materials are more dependent on the variation in the thickness than in the thermal diffusivity. However, clinically where the thickness of the lining material must be kept to a minimum in order to provide enough thickness for the overlying material, or in the case where a thin layer of material can be applied to any cavities such as in Class V, the value of thermal diffusivity will be a significant factor in determining which material to be used.
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et a/.
CONCLUSIONS 1. Polymerization
of RMGICs resulted in marked rises in temperature, the liner/base cements showing higher temperature rises than the restorative cements. Increasing specimen thickness reduced the temperature rise for exposure time shorter than or equal to 30s. Thus, inadequate exposure time or thick layer of material resulted indicating in temperature rise, reduced incomplete curing. The time required to reach the maximum temperature rise was shorter in thinner specimens than in thicker ones for the same exposure time. Increasing the environmental temperature from 25 to 37°C resulted in a decrease in the temperature rise. 2. Using a thermocouple as a means to measure temperature rises during polymerization can determine the depth of cure and exposure time where curing becomes inadequate. 3. When supplementary layers of the cements are polymerized, the temperature rise measured underneath was still in the range of 5”C, the level of which has been cited as sufficient to cause pulp injury. 4. The thermal diffusivities of the liner/base RMGICs are lower than those for the restorative cements, indicating that they are better insulators. However, both cements have similar values of thermal diffnsivity to that of dentine and can be used as dentine substitutes. From this study, no correlation was found between the thermal diffusivity value and the temperature rise on polymerization.
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