Pulpal Temperature Rise: Evaluation after Light Activation of Newer Pulp-Capping Materials and Resin Composite.

BACKGROUND: To evaluate temperature changes in pulp chamber during light activation of newer pulp- capping materials and composite resin using light-emitting diode.
MATERIALS AND METHODS: A standardized Class I cavity was prepared in 80 extracted, intact, noncarious mandibular first molars, keeping remaining dentin thickness of 0.5 mm. The teeth were divided into four groups of 20 teeth each. Following this, apical third of the mesial root of each tooth was cut and a K type thermocouple attached to digital thermometer was inserted into pulp chamber from the sectioned mesial root. Whole assembly with teeth was suspended in water bath with constant temperature at 37°C. The previously divided teeth in four groups, were lined with Calcimol LC (Group A), Activa (Group B), TheraCal LC (Group C), and Ionoseal (Group D), followed by 3 increments of Filtek Z350 × T universal restorative. The temperature rise following light activation of pulp-capping material, bonding agent, and composite was noted.
RESULTS: The temperature rise in the pulp chamber after light activation of Activa was highest among all pulp-capping materials, followed by teeth lined with Calcimol LC, Ionoseal, and least in teeth with TheraCal LC.
CONCLUSIONS: Temperature rise in the pulp chamber after light activation of newer pulp-capping materials and composite was below critical threshold for irreversible pulpal damage. Among all the pulp-capping materials, TheraCal LC showed lowest temperature rise in pulp chamber. Copyright:

Introduction

The main goal of the restorative dentistry is to restore and maintain tooth health in order to protect as well as reestablishes the function of the pulp. Pulp plays a vital role in the formation and nutrition of dentin as well as in the innervation and defense of the teeth. Many factors, including physical, chemical, biological, and thermal factors, can damage the dental pulp during restorative procedures.[12] This holds, particularly true for deep cavities, where the amount of remaining dentin thickness (RDT), types of pulp-capping material, and the heat generation during operative procedures stand out as factors that affect the health of the pulp.[345] In such cases, pulp-capping materials can be used to preserve the dentin–pulp complex against increasing temperature, which also shows antibacterial activity, by blocking bacterial transition in the pulp chamber.[6]

The materials available for this purpose are glass ionomer cement, resins, adhesive systems, and calcium hydroxide (Ca[OH]2) base cement. Ca(OH)2 is the most widely used pulp-capping material in restorative dentistry, due to its ability to induce new dentin formation, antibacterial effect, and alkaline pH.[789] Despite its popularity, however, the physical properties of conventional Ca(OH)2 such as water solubility, bond strength to dental hard tissues, and compressive strength, are relatively poor.[9] Due to these disadvantages, newer light-cure bioactive pulp-capping materials such as Calcimol LC, Activa, TheraCal LC, and Ionoseal were developed to treat deep cavities. However, the heat generated during light activation of these materials could lead to an irreversible pulpal damage. This increase in temperature could be originated from both the exothermic polymerization of the material and the energy absorbed from the light curing unit (LCU).[10]

Thus, the purpose of this study was to compare the temperature rise in the pulp chamber during light activation of four newer light cure pulp-capping materials and composite resin using a light-emitting diode (LED)-LCU.

Materials and Methods

Eighty extracted, intact, noncarious mandibular first molars were cleaned using a periodontal scaler and pumice slurry and were stored in 0.5% chloramine T solution until use (Max 1 month). Class I cavity of width 2 mm and length 5 mm was prepared with a number 2 round and a number 245 carbide burs (SS White, Lakewood, NJ, USA) in all the teeth. The depth of the cavity was prepared in such a way that the RDT was 0.5 mm, which was checked by taking an intraoral periapical radiograph (IOPA) of each tooth with a radiographic grid. The average depth of the cavity preparation in all the teeth was determined and teeth with cavity depth with more than ±0.5 mm difference were discarded to maintain standardization of the cavity preparation. After this, all prepared 80 teeth were randomly divided into four groups of 20 teeth each [Table 1].

Table 1

Materials used in the study

Groups (n=20)	Materials used	Manufacturer	Product description
Group A	Calcimol LC	Voco GmbH, Cuxhaven, Germany	Light curing calcium hydroxide
Group B	Activa Bioactive-base/liner	Pulpdent Corporation, Watertown, MA, USA	Light curing bioactive ionic resin with reactive glass filler
Group C	Theracal LC	Bisco Inc, Schaumburg, IL, USA	Light curing resin-modified calcium silicate
Group D	Ionoseal	Voco, GmbH, Cuxhaven, Germany	Light curing glass ionomer composite cement

After cavity preparation, each tooth from the respective group was taken and the apical 3 mm of the mesial root of each tooth was sectioned, perpendicular to the long axis of the tooth, with a water-cooled diamond disk. The mesiobuccal root canal space was prepared through the cut mesial root surface up to No. 100 K file (Mani Inc., Tochigi, Japan) and the pulp chamber was irrigated with 5.25% sodium hypochlorite (Vishal Dentocare Pvt Ltd, India), followed by flushing out with normal saline and drying with paper points.

An electrocardiogram (ECG) gel (to facilitate the transfer of heat from the walls of the pulp chamber to the thermocouple) was injected into the pulp chamber through the prepared apical portion of the root. ECG gel is a water-based gel and the pulp's main component is also water, so ECG gel was used to mimic heat transfer of pulpal tissue. The K-type thermocouple TC (CIC, Patel heater and control Pvt. Ltd., Vadodara, India), connected to a digital thermometer was passed through the sectioned apex [Figure 1] and placed into the pulp chamber, touching the chamber's roof. The position of the thermocouple was confirmed on IOPA [Figure 2] and then root end was sealed with a cyanoacrylate adhesive (Fevikwik Adhesive, Pidilite Industries Ltd., Mumbai, India) to stabilize thermocouple.

Figure 1

K-type thermocouple and its insertion through sectioned mesial root

Figure 2

Radiograph conforming position of the thermocouple

The whole assembly of tooth with thermocouple was submerged into water of the water bath machine up to the cementoenamel junction of the tooth, simulating level of attachment of periodontal ligament, with the help of custom made-acrylic platform [Figure 3]. Waterbath machine keeps water at a constant temperature of 37°C (i.e., to simulate human body temperature). The schematic representation of the experiment is shown in Figure 3.

Figure 3

Schematic representation of the experiment

A marking of 1 mm was made from the floor of the cavity using UNC-15 Probe and a pencil. The pulp-capping material (belonging to the respective group) was then placed to the depth of the cavity till 1 mm marking and was light activated with the LED LCU (Elipar™, 3M ESPE, Germany) for 20 s. The highest temperature rise in the pulp chamber during 20 s of light activation was noted. After this, an interval time of 30 s was kept and sixth-generation bonding agent (Adper single bond universal, 3M ESPE, Germany) was applied and light activated for 20 s. Again, the highest temperature rise was measured during the light activation of bonding agent. Following this, composite (Filtek Z350, 3M ESPE, Germany) was added incrementally in three layers with 2 mm increment each, with the oblique layering technique and the temperature rise was noted after the light activation of each increment for 40 s. Between each increments, an interval time of 30 s was kept to stabilize the temperature in the pulp chamber. The LED-LCU was placed at the occlusal surface of the tooth during each light activation. The same procedure was repeated for all the samples.

The results of rise in temperature were tabulated and statistically analyzed by IBM SPSS Statistics version 20.0 statistical package (SPSS, Chicago, IL, USA). One-way ANOVA test was used to compare temperature rise between all the groups and post hoc Tukey test was used for pairwise comparison between groups.

Results

Results of one-way ANOVA test [Table 2] showed that there was a statistical high significant difference found between groups in temperature rise following pulp-capping material application, bonding agent and first increment of composite (P < 0.001), but there was no significant difference found between groups following second increment (P = 0.203 > 0.05) and third increment (P = 0.917 > 0.05) placement. The overall highest temperature rise and the lowest temperature rise were calculated for Activa and the TheraCal LC, respectively.

Table 2

One-way ANOVA and post hoc Tukey test

	Liner	Number of samples	Mean	SD	Statistics/mean squares	df2 (Welch)/F (ANOVA)	P
Temperature difference following liner application	Group A (Calcimol LC)	20	2.605A	0.83507	6.673	11.826	<0.001
	Group B (Activa Liner)	20	2.62A	0.508455
	Group C (TheraCal LC)	20	1.465B	0.762458
	Group D (Ionoseal)	20	1.825B	0.848451
	Total	80	2.1287	0.892329
Temperature difference following bonding agent	Group A (Calcimol LC)	20	1.115A	0.537318	10.435	41.292	<0.001
	Group B (Activa Liner)	20	1.63B	0.357035
	Group C (TheraCal LC)	20	0.965A	0.494469
	Group D (Ionoseal)	20	1.03A	0.628365
	Total	80	1.185	0.568832
Temperature difference following first increment	Group A (Calcimol LC)	20	0.48A	0.362157	3.569	22.083	<0.001
	Group B (Activa Liner)	20	1.435B	0.434408
	Group C (TheraCal LC)	20	0.76A	0.334664
	Group D (Ionoseal)	20	0.625A	0.463255
	Total	80	0.825	0.539456
Temperature difference following second increment	Group A (Calcimol LC)	20	0.37A	0.453176	1.606	40.743	0.203
	Group B (Activa Liner)	20	0.43A	0.18666
	Group C (TheraCal LC)	20	0.415A	0.311659
	Group D (Ionoseal)	20	0.29A	0.22219
	Total	80	0.3762	0.309857
Temperature difference following third increment	Group A (Calcimol LC)	20	0.28A	0.276444	0.024	0.169	0.917
	Group B (Activa Liner)	20	0.24A	0.358946
	Group C (TheraCal LC)	20	0.195A	0.40714
	Group D (Ionoseal)	20	0.245A	0.453611
	Total	80	0.24	0.373692

Superscripted capital letters refer to statistical grouping in the row. Different letters indicate statistical differences between groups (P<0.05). SD: Standard deviation

Post hoc Turkey test Table 2 shows that, following pulp-capping material application, there is no significant difference between Ionoseal and TheraCal group; and between Calcimol and Activa group, while all the other groups show statistically significant difference. Following bonding agent application and first composite increment, the temperature rise for Activa group was highest and having significant difference with all the other pulp-capping materials. For second and third increment, none of the groups shows significant difference.

Discussion

During the light-activated polymerization process of resin composites and pulp-capping materials, temperature increases as a result of the exothermic reaction process and energy absorption during irradiation.[1011] According to the findings of a study[12] on rhesus monkeys, a 5.6°C temperature rise in the pulp chamber caused irreversible pulp damage in 15% of the monkeys, while 11°C and 16.6°C increases caused irreversible pulp damage, respectively, in 60% and 100% of the monkeys. In our study, light activation of all the pulp-capping materials and composite have shown increase in pulpal temperature but not beyond the critical value of 5.6°C.

Previously, different techniques were used to evaluate pulpal temperature increase, such as calorimeter, thermocouple, differential thermal analysis, and infrared cameras. In the current study, the thermocouple technique was selected to measure temperature changes during polymerization of cavity liners due to its reliable and precise results.[131415]

In our study, to replicate pulp tissue, ECG gel was placed in the pulp chamber and each tooth was placed in a water bath at 37°. ECG gel is a water-based gel and the pulp's main component is also water, so it mimics the heat transfer of the pulp tissue.

The RDT plays an important role in the thermal insulation of the pulp since it contributes to the thermal diffusivity due to its low thermal conductivity coefficient. Thicker the dentin, that is, more the RDT, less is the pulpal temperature rise.[16] For this reason, in our study, the critical dentin thickness for deep cavities (i.e. 0.5-mm RDT) was selected to simulate the deep cavity preparation in clinical situations.

In this study we tested, a light cure Ca(OH)2 liner (Calcimol LC), Bioactive Resin (Activa), light cure calcium silicate liner (TheraCal LC), and resin-modified glass ionomer liner (Ionoseal). Results showed that TheraCal LC had the lowest mean temperature increase out of all the other light cure pulp-capping materials. TheraCal LC contains resin-modified calcium silicate filler which has a low specific heat capacity.[17] The specific heat capacity is directly proportional with the thermal conductivity of the materials. Thus, low specific heat capacity leads to lower thermal conductivity and higher insulation properties. In industries, calcium silicate has been used over the years as an insulation material.[1819] Thus, TheraCal has shown lower temperature rise due to higher insulation characteristic of calcium silicate. A study by Savas et al.[10] has shown similar results, whereby TheraCal has shown lowest temperature rise in pulp chamber compared to other light curing Ca(OH)2-based pulp-capping materials.

Low molecular-weight monomers are known to produce higher exotherm, as well as higher shrinkage, when compared with high molecular-weight monomers.[20] Calcimol LC is a light curing Ca(OH)2-based liner which contains urethane dimethacrylate, triethylene glycol dimethacrylate, and dimethylaminoethyl-methacrylate in its resin component. These are low molecular weight monomers. The high temperature rise in the pulp chamber following the polymerization of Calcimol LC could be attributed to these components in its composition. Ionoseal, on the other hand, has bisphenol A glycidyl dimethacrylate (Bis-GMA) as its major resin component. Bis-GMA is a high molecular weight monomer and consequently it could be this component in this material that is responsible for its comparatively lower temperature rise in the pulp chamber following the light curing of Ionoseal.

Activa was found to have the highest temperature rise in the pulp chamber among all the pulp-capping materials. Although the manufacturer's claim, this material is bioactive with its patented composition, according to the results obtained from the present study suggested that it might have high thermal conductivity. This consequently could prove detrimental to the pulp if not used judiciously.

The temperature rise in the pulp chamber following the light activation of bonding agent was found to be higher than that of the subsequent increments of the composite. This was in accordance with the results found by Millen et al.[21] The reason for this could be the high thermal conductivity of bonding agents, as only a thin film of the bonding agent needs to be applied and hence this provides minimal insulation.

The temperature rise in the pulp chamber following the polymerization of the first increment of the composite was found to be higher than that of the following two increments. This suggests the insulating property of composite. The temperature rise was higher with the first increment as the thickness of the composite was not sufficient enough to induce thermal insulation. However, with the subsequent two increments, the composite acted as a buffer due to the increasing thickness of the composite. Similar results have been seen in a study conducted by Mahant et al.[22]

However, it must be emphasized that the results of this study cannot be directly extrapolated to the clinical situation. Heat dissipation in the tooth can occur through pulpal blood circulation and heat may also be absorbed by the gingival connective tissues.[23] The present study was a laboratory investigation and experimental set up did not account for blood circulation which occurs in the natural, vital pulp chamber. The experimental values obtained in our study may therefore be higher than those occurring in vivo. Different results might be achieved with intraoral conditions.

Conclusions

Within the limitations of this study, the following conclusions can be drawn:

The temperature rises in the pulp chamber following the polymerization of all the pulp-capping materials were below the critical threshold for irreversible pulpal damage, and hence they can be used in clinical practice without concerns regarding their thermal conductivity

The temperature rises in the pulp chamber following the polymerization of pulp-capping materials and resin composite was noted to be highest in the teeth lined with Activa Liner, followed by Calcimol LC, Ionoseal, and least in TheraCal LC.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.