Thermal Properties of Dental Materials - JST

Dental Materials Journal 23 (3): 399-405, 2004

Thermal Properties of Dental Materials-Cavity Liner and Pulp Capping Agent-

Masahiro SAITOH1, Shigeyuki MASUTANI2, Taishi KOJIMA1, Masataka SAIGOH1, Hideharu HIROSE1,3 and Minoru NISHIYAMA1,31Department of Dental Materials, Nihon University School of Dentistry, 1-8-13, Kanda Surugadai, Chiyoda-ku, Tokyo 101-8310, Japan2Division of Clinical Research, Dental Research Center, Nihon University School of Dentistry, 1-8-13, Kanda Surugadai, Chiyoda-ku, Tokyo 101-8310, Japan3Division of Biomaterials Science, Dental Research Center, Nihon University School of Dentistry, 1-8-13, Kanda Surugadai, Chiyoda-ku, Tokyo 101-8310, JapanCorresponding author, E-mail:[email protected]

Received April 20, 2004/Accepted July 14, 2004

We studied the thermal properties of cavity liners that included calcium phosphate as inorganic filler, in contrast to the con-

ventional pulp capping agents. Therefore, thermal diffusivity, specific heat capacity, and thermal conductivity were meas-

ured. In addition, thermal conductivity results were compared with those of restorative materials and human dentin to

examine thermal insulation effects. The thermal conductivity of cavity liners ranged from 0.23 to 0.28Wm-1K-1, and that

of pulp capping agents ranged from 0.44 to 0.48Wm-1K-1. Test results indicated that the thermal conductivity of cavity

liner was lower than those of human dentin, pulp capping agent, cast alloy, and composite resin for restoration, hence sug-

gesting that cavity liner has a good thermal insulation effect.

Key words: Thermal conductivity, Cavity liner, Calcium phosphate

INTRODUCTION

Cavity liner (hereafter referred to as liner), which reportedly includes highly biocompatible calcium

phosphate, is now widely used in clinical applications. Liner is applicable even to deep cavities because it can also serve as pulp capping agent1).

Thermal insulation effect is one of liner's key

properties, but the effect varies significantly depend-ing on the composition. Thermal insulation effect of materials with relatively high thermal conductivity could be small even when thickened. For example, in cast restorations with crowns or bridges, the thermal conductivity values of 20K and 12%Au-Ag-Pd are much higher than that of human tooth2,3). Therefore thermal insulation effect is required in liner, as it is required in luting agent, when a cavity is so deep and so close to the dental pulp. Also, as one of the clinical techniques in composite resin restoration, liner is thinned while the restorative material is thickened to preclude the color of the liner from af-fecting that of the restorative material. In addition, some direct composite resin products show higher thermal conductivity than that of human dentin4). All this serves to highlight the importance of a good cognizance of liner's thermal conductivity, in particu-lar its behavior toward thermal stimuli caused by hot or cold ingesta.

Accordingly, we studied the thermal properties of liners that included calcium phosphate as inorganic filler, in contrast to the conventional pulp capping

agents. For this purpose, thermal diffusivity, spe-

cific heat capacity, and thermal conductivity were

measured. In addition, the thermal conductivity re-

sults were compared with those of restorative materi-

als and human dentin to examine thermal insulation

effects.

MATERIALS AND METHODS

Materials

Table 1 lists the three liners and two pulp capping

agents widely used in clinical practice. The three lin-

ers used in this study were Cavios(R) (Neo Dental

Chemical Products Co. Ltd., Tokyo, Japan), Ultra-

blend (Ultradent Products Inc., South Jordan, Utah,

USA), and Cavalite (Kerr, Romulus, Michigan,

USA), and the two pulp capping agents used were

namely Life (Kerr, Romulus, Michigan, USA) and

Dycal (L.D. Caulk Co., Midford, Delaware, USA).

Methods

1) Qualitative analysis

a) Measurement of inorganic filler content

To measure the inorganic filler content, thermo-

gravimetry (TG) was carried out by a differential

thermal analyzer (Thermo Flex 8100 Series, Rigaku).

Before heating, the weight of each sample was 50

mg. At a heating rate of 10•Ž/minute, the samples

were heated up to 800•Ž. Following which, three

measurements were carried out on each sample to ob-

tain the mean value of the post-heating weight. The

400 CAVITY LINER AND PULP CAPPING AGENT

Table 1 Materials used in this study

Table 2 Inorganic filler content of cavity liners and pulp capping agents

weight difference between the pre-heating weight of

each sample (i.e., 50mg) and its post-heating weight

was then used as the inorganic filler weight. As for

inorganic filler content, it was calculated as a per-

centage of inorganic filler weight to sample weight

(50mg).

b) Compositional analysis

Compositional analysis was conducted as follows

using Fourier transform infrared spectrophotometer

(FTIR-4300, Shimadzu). Each sample was put into

benzene for dissolution. The supernatant solution

was applied to KBr plates to let the solvent evapo-

rate, and the measurement was performed under the

condition of fifty times' accumulation of the double-

beam method. The composition was identified based

on the obtained IR spectrum of various monomers as

well as the IR spectrum of organic solvent.

For the insoluble components in benzene, their

deposits were removed by a centrifuge, then dried

and triturated. Following which, elemental analysis

was conducted with a x-ray dif f ractometer (RAD-

IIA, Rigaku) and a wavelength dispersive x-ray

microanalyzer (JXA 8600, JEOL).

The measurement condition was set as CuKƒ¿,

40kV-20mA for the former and 15kV for the latter,

with three prism channels - namely TAP, PET, and

LiF - for a spectrometer. The compositions were

identified based on the obtained x-ray diffractogram

and x-ray spectrum.

2) Specimens preparation

The specimens were prepared as follows in accordance

with the procedure recorded in previous report).

Each material was injected into a Teflon mold 1.0

mm thick with a pore 10mm in diameter. It was

then held between a cellulose acetate film and a glass

plate using a C-type clamp, and cured following the

manufacturer's instruction (Table 1). Light-cure

type materials were polymerized using a visible light

curing unit (VL-II, GC).

Five specimens were provided for each sample.

They were made in a lab with a constant room tem-

perature of 23•}1•Ž and relative humidity at 50•}5%.

After which, they were immersed in distilled water

in an incubator at 37•Ž for a week.

3) Measurement of density

Bulk density was measured by a specific gravity

measuring instrument (ED-120T, Mirage Boeki).

4) Measurement of thermal properties

In accordance with the method given in previous re-

ports2,4-6), thermal dif f usivity and specific heat capac-

ity were measured by a thermal measuring

instrument (TC-2000L, Shinku Riko). Then, thermal

conductivity was calculated using the following equa-

tion.

K=ƒ¿•EC•EƒÏ

k: Thermal conductivity (Wm-1K-1)ƒ¿

: Thermal diffusivity (cm2s-1)

C: Specific heat capacity (Jg-1K-1)ƒÏ

: Density (gcm-3)

5) Statistical analysis

Bartlett test followed by Tukey-Kramer test (ƒ¿=

0.05) was applied to the results to compare amongst

the thermal properties' measurements.

RESULTS

Composition1) Inorganic filler content

Table 2 shows the inorganic filler content results. The inorganic filler content was 55.6wt% in CV, 36.1wt% in UB, and 54.7wt% in CL. As for the

SAITOH et al. 401

Table 3 Characteristic absorption of main components of cavity liners

Table 4 Characteristic absorption of main components of pulp capping agents

pulp capping agents, it was 59.0wt% in DY and 56.6wt% in LF.2) Compositional analysisTables 3, 4, and 5 show the analysis results. The compositions of the samples could be divided into ei-ther benzene-soluble or benzene-insoluble components. Based on the peaks assigned to the IR spectra, it was determined that the soluble components in CV and UB were UDMA, and the mixture in CL composed of Bis-GMA and Bis-MPEPP. While in DY and LF, the soluble components were determined to be N-ethyl-p-toluenesulfonic acid and salicylic ester respectively.

On the other hand, x-ray diffraction and wave-length dispersive x-ray microanalysis identified the insoluble components in the liners as follows: Ca3(PO4)2 and BaKxSO4 in CV, and BaSO4 and Ca5(PO4)3(OH) in UB and CL. As for the pulp cap-

ping agents, they were ZnO, Ca5(PO4)3(OH), and Ca(OH)2 in DY, and ZnO, BaSO4, Ca(OH)2 and TiO2 in LF.

DensityFig. 1 shows the measurement results. The density of the liners ranged from 1.49 to 1.80gcm-3, and that of the pulp capping agents ranged from 1.83 to 1.84gcm-3.

Comparison among the liners showed statistical differences except for CV-CL in the liners group. No statistical differences were recognized between the

pulp capping agents, LF and DY.Comparing the liners group and the pulp capping

agents group, liners showed a significantly lower density value than those of the pulp capping agents.

Thermal properties1) Thermal diffusivity

Fig. 2 shows the measurement results. The thermal

diffusivity of the liners was 0.12-0.15•~10-2m2s-1,

while that of the pulp capping agents was 0.18-0.19

•~10-2cm2s-1.

Comparison among the products showed statisti-

cal differences except for CV-CL in the liners group,

while LF showed significantly higher diffusivity than

DY.

Comparing the liners group and the pulp capping

agents group, the thermal dif f usivity of the former

was significantly lower than that of the latter.

2) Specific heat capacity

Fig. 3 shows the measurement results. The specific

heat capacity of the liners ranged from 1.01 to 1.27

Jg-1K-1, and that of the pulp capping agents ranged

from 1.28 to 1.47Jg-1K-1.

Comparison among the products showed statisti-

cal differences except for CV-CL in the liners group.

In the pulp capping agents group, DY showed a

higher capacity than LF.

Comparing the liners group and the pulp capping

agents group, liners showed a significantly lower

specific heat capacity value than the pulp capping

agents.

3) Thermal conductivity

Fig. 4 shows the measurement results. The liners'

ranged from 0.23 to 0.28Wm-1K-1, while the pulp

capping agents' ranged from 0.44 to 0.48Wm-1K-1.

Comparison among the liner products showed

statistical differences except for CV-CL. In the pulp

capping agents group, there were no statistical dif-

ferences between LF and DY.

When comparing between the two groups, the

liners had significantly lower conductivity than the

pulp capping agents.


Table 5 Main diffraction angle (2ƒÆ) of cavity liners and pulp capping agents

a: Ca3(PO4)2b: BaKxSO4c: BaSO4d: Ca5(PO4)3(OH)e: ZnOf: Ca(OH)2

g: TiO2u: unknown

DISCUSSION

CompositionFor the liners, the benzene-soluble components were either UDMA or Bis-GMA and the benzene-insoluble ones were calcium phosphate and barium sulfate. Therefore, difference in the composition of inorganic components, which significantly affects thermal con-ductivity, was found to be small. However, the inor-

ganic filler content differed significantly from UB to CV and CL (Table 2). For the pulp capping agents, the benzene-soluble components differed from each other. As for the benzene-insoluble components (i . e.,

the inorganic components which impact thermal con-ductivity), although there were some similarities

(Table 5, ZnO and Ca(OH)2), there was a confirmed difference between the products. A slight difference was also found in the inorganic filler content be-tween the products (Table 2).

Density and thermal propertiesIn the liners group, UB had relatively low density

(Fig, 1) and thermal diffusivity (Fig. 2), as it had less inorganic filler content such as calcium phos-

phate or barium sulfate. On the other hand, CV and CL exhibited high values due to their larger amount

SAITOH et al. 403

Density (gcm)-3

n=5, Identical letters indicate that values are not significantly different (p>0.05)

Fig. 1 Density of cavity liners and pulp capping agents.

Specific heat capacity (Jg-1K-1)


Fig. 3 Specific heat capacity of cavity liners and pulp capping agents.

Thermal diffusivity (x10-2cm2s-1)


Fig. 2 Thermal diffusivity of cavity liners and pulp capping agents.

Thermal conductivity (Wm-1K-1)


Fig. 4 Thermal conductivity of cavity liners and pulp capping agents.


of inorganic filler content. Considering the fact that the inorganic filler components were almost similar among the liners, their amount linearly increased the density and thermal diffusivity values. Specific heat capacity (Fig. 3) was higher in UB which had less in-organic filler content. This must be because the in-organic filler - with high values of density and thermal diffusivity - shows low specific heat capac-ity4,5). In addition, UB - which had the least amount of inorganic filler content among the liners - exhibited a lower specific heat capacity than the

pulp capping agents which had more inorganic filler content. This must be due to the composition differ-ence between the liners and the pulp capping agents. As for thermal conductivity (Fig. 4), products with more inorganic filler content exhibited a higher value as in the cases of density and thermal diffusivity. This is because the thermal conductivity value is de-rived from the multiplication of density, thermal diffusivity, and specific heat capacity. In other words, reduction in density and thermal diffusivity had a greater impact than an increase in specific heat capacity.

In the pulp capping agents group, LF and DY showed similar density values. However, LF - which had less inorganic filler content - exhibited a higher

thermal diffusivity value. The difference in inorganic filler content among the pulp capping agents was not sufficiently huge to impact density. However, LF ex-hibited a higher thermal diffusivity because it con-tained titanium oxide which has higher thermal diffusivity than calcium phosphate. As for specific heat capacity, DY - which had a large amount of inorganic filler content - exhibited a high value. This could stem from its components composition. DY also showed high thermal conductivity, probably due to its higher specific heat capacity value.

By comparing the two groups, the liners' ther-mal conductivity was lower than the pulp capping agents' by 37-52%. Therefore, the heat transference of liners should be lower than that of pulp capping agents.

As discussed above, the thermal conductivity of liners and pulp capping agents seemed to depend on the main components' composition and the inorganic filler content. According to the thermal conductivity data search we conducted for each component4,6-8), it was found that Ca5(PO4)3(OH) was 1.3Wm-1K-1, ZnO was 54Wm-1K-1, and TiO2 was 8.4Wm-1K-1 in the benzene-insoluble group of the pulp capping agents. In the benzene-soluble group of the liners, resin-cured material that was fabricated with Bis-

Fig. 5 Thermal conductivity of cavity liner and dental materials.

SAITOH et al. 405

GMA and TEGDMA at the ratio of 6:4 (mole ratio) was 0.21Wm-1K-1, and that of Bis-MPEPP cured material was 0.19Wm-1K-1. From the fact that the thermal conductivity of benzene-insoluble components (inorganic filler) is 6-100 times higher than that of benzene-soluble components (matrix), it is obvious that compositional difference affected the thermal conductivity of the products, be it liners or pulp cap-ping agents.

Thermal insulation effect of linersFig. 5 shows the thermal insulation effects of differ-ent materials (including human dentin and dental materials) to illustrate the relative positioning of each material in terms of thermal insulation effect.

The thermal conductivity of liners (0.23-0.28Wm-1K-1) was extremely low: 50-59% lower than that of human dentin (0.56Wm-1K-1)2), less than 1% that of 20K (44.9Wm-1K-1) and 12%Au-Ag-Pd (38.5Wm-1K-1)1), and 13-70% lower than that of composite resin (0.32-0.77Wm-1K-1)4). Compared with dental cements5-12), it was lower than zinc phos-

phate cement for luting (1.13-1.15Wm-1K-1) by 75-80%, and glass ionomer cement for luting (0.50-0.54Wm-1K-1, GIC for luting) by 44-57%. Compared to resin luting (0.22-0.75Wm-1K-1), the values were al-most the same or 69% lower. When compared with

glass ionomer cement for lining (0.54-0.65Wm-1K-1, GIC for lining), the liners' were 48-65% lower; and when compared with light-activated glass ionomer ce-ment (0.37-0.45Wm-1K-1, light-activated GIC), the liners' were 24-49% lower.

Based on the above, it is obvious that liners have significant thermal insulation effect. It is possible to use liners with thinner lamina. Moreover, the liner with thinner lamina is more suitable to be used in composite restorations without affecting the color of the restorative materials. And coupled with its good thermal insulation effect and biocompatibility1), liners are applicable to deep cavities in cast restorations with crowns and bridges instead of pulp capping agents.

CONCLUSION

To explore the thermal properties of liner, measure-

ments on its thermal diffusivity, specific heat capac-

ity, thermal conductivity, as well as density were

conducted. Facts obtained through this study are

presented as follows.

1. For liner, its density ranged from 1.49 to 1.80

gcm-3; the thermal diffusivity was 0.12-0.15•~10-2

cm2s-1; the specific heat capacity was 1.01-1.27

Jg-1K-1; and the thermal conductivity was 0.23-0.28

Wm-1K-1. For the pulp capping agent, its density

ranged from 1.83 to 1.84gcm-3; the thermal

diffusivity was 0.18-0.19•~10-2cm2s-1; the specific

heat capacity was 1.28-1.47Jg-1K-1; and the thermal

conductivity was 0.44-0.48Wm-1K-1.

2. The thermal conductivity of liner was lower than

those of human dentin, pulp capping agent, cast

alloy, and composite resin for restoration - suggest-

ing that liner has a good thermal insulation effect.

REFERENCES

1) Seino E, Nakazawa Y, Hirai Y, Ishikawa T, Toda T,

Hata G, Baba T. A study on light-cured lining materi-

als - Part 1: Clinical evaluation of •gCavios(R)•h. Japan

J Conserv Dent 1998; 41: 933-939.

2) Saitoh M, Yamanaka N, Kaneko K, Horie Y, Hayashi

J, Mori T, Nishiyama M. Thermal properties of dental

materials. 7. Thermal properties of dental metals. J J

Dent Mater 1994; 13: 340-345.

3) Anusavice KJ. Phillips' science of dental materials,

10th ed, WB Saunders Co., 1996, pp. 134-135.

4) Hirano S, Saitoh M, Nishiyama M, Hirasawa T. Ther-

mal properties of light-cured composite resins. J J

Dent Mater 1993; 12: 759-764.

5) Saitoh M. Study on thermal properties of dental ce-

ment - Xenon flash method. J J Dent Mater 1991; 10:

328-343.

6) Shirasaki S. Property data of ZnO. Ceramics Japan

1983; 18: 965-969.

7) Kanazawa T, Umegaki T, Monma H, Yamashita K.

Materials chemistry of apatites. Gypsum & Lime 1987;

210: 261-273.

8) Shadanhoujin Nihonkagakukai. Kagakubinran Kisohen

II, 4th ed, Maruzen Co., Tokyo, 1993, pp. II-69.

9) Inoue T, Saitoh M, Miyazaki K, Anabuki M,

Nishiyama M. A study on thermal properties of dental

materials - thermal properties of resin cements. J J

Dent Mater 1992; 11: 922-927.

10) Inoue T, Saitoh M, Kaketani, M, Ishikawa Y, Sunaji

M, Nishiyama M. A study on thermal properties of

dental materials. 6. Thermal properties of light-cured

type glass ionomer cements. J J Dent Mater 1994; 13:

163-169.

11) Saitoh M, Ohki Y, Usui N, Yui S, Nakajima Y,

Nishiyama M, Moro H, Igarashi T. Thermal properties

of dental materials. 8. Thermal properties of resin-

modified glass ionomer cement. J J Dent Mater 1998;

17: 225-230.

12) Moro H, Saitoh M, Kakehashi Y, Igarashi T,

Nishiyama M. A study on thermal properties of all ce-

ramics - adhesive resin cements. Nihon Univ Dent J

1999; 73: 82-87.

Thermal Properties of Dental Materials - JST

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