Evaluation of mineral trioxide aggregate (MTA) versus calcium hydroxide cement (Dycal®) in the formation of a dentine bridge: a randomised controlled trial

Abstract

Aim: To assess the effectiveness of mineral trioxide aggregate (MTA) used as an indirect pulp-capping material in human molar and premolar teeth. Methodology: We conducted a clinical evaluation of 60 teeth, which underwent an indirect pulp-capping procedure with either MTA or calcium hydroxide cement (Dycal®). Calcium hydroxide was compared with MTA and the thickness of the newly formed dentine was measured at regular time intervals. The follow-up was at 3 and 6 months, and dentine formation was monitored by radiological measurements on digitised images using Mesurim Pro® software. Results: At 3 months, the clinical success rates of MTA and calcium hydroxide were 93% and 73%, respectively (P = 0.02). At 6 months, the success rate was 89.6% with MTA, and remained steady at 73% with calcium hydroxide (P = 0.63). The mean initial residual dentine thickness was 0.23 mm, and increased by 0.121 mm with MTA and by 0.136 mm with calcium hydroxide at 3 months. At 6 months, there was an increase of 0.235 mm with MTA and of 0.221 mm with calcium hydroxide. Conclusions: A higher success rate was observed in the MTA group relative to the Dycal® group after 3 months, which was statistically significant. After 6 months, no statistically significant difference was found in the dentine thickness between the two groups. Additional histological investigations are needed to support these findings.

INTRODUCTION

The consequences of pulp exposure from caries, trauma or unexpected tooth preparation procedures can be severe, with pain and infection. Pulp capping, in which a medicament is placed directly over the exposed pulp (direct pulp cap), or a cavity liner or sealer is placed over residual caries (indirect pulp cap), is an attempt to maintain pulp vitality and avoid more extensive treatments¹.

There are key procedures in the management of vital teeth with deep carious lesions2., 3., 4., which can be performed with high predictable long-term success rates⁵.

Calcium hydroxide is the gold standard for pulp capping, following the initial publication by Zander⁶ in 1939. It allows for the formation of a reparative dentine bridge through cellular differentiation, extracellular matrix secretion and subsequent mineralisation7., 8.. From a clinical point of view, it enables successful maintenance of pulp vitality², protects the pulp against thermoelectric stimuli and has an antimicrobial action. Calcium hydroxide is used as a reference for other capping agents, such as glass ionomer cement and adhesives9., 10., 11., 12..

However, in long-term clinical studies of pulp capping with calcium hydroxide-based materials, failure rates increase with the follow-up time³. Known disadvantages for this material include gradual degradation and tunnel defects in the newly formed dentine. In addition, an increased frequency of inflammatory cells and localised areas of pulp necrosis have been reported over time13., 14., 15., 16..

Mineral trioxide aggregate (MTA) is a pulp-sealing agent, essentially composed of a mixture of tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite and calcium sulphate dehydrate – which are the main components of Portland cement – with an addition of bismuth oxide in a 4 : 1 ratio for radio-opacity properties17., 18..

This bioactive silicate cement has been shown to be an effective pulp-capping material in canine models and in nonhuman primates¹⁹. The material appears to be successful because of its small particle size, sealing ability, alkaline pH when set and slow release of calcium ions. Investigators have reported that MTA induces pulp cell proliferation, cytokine release and subsequent hard tissue formation with the synthesis of a mineralised dentine interface similar to that of biological hydroxyapatite¹⁹.

Most of the investigations conducted on MTA have involved the evaluation of the clinical and radiographic outcomes of pulp-capping procedures in either human primary20., 21. or permanent²⁰ teeth. Other human studies have reported histological observations of the pulp-capping procedure15., 16., 22., 23., 24.; their results confirm those reported in animal models13., 25., 26..

Therefore, the capping ability of MTA is comparable with that of calcium hydroxide, but few clinical studies have evaluated both compounds simultaneously. For this reason, we conducted a prospective evaluation with both MTA and calcium hydroxide. The aim of this study was to assess the effectiveness of the pulp-capping materials by measuring the thickness of the newly formed dentine.

MATERIALS AND METHODS

The study design was a single-blind clinical trial realised in a sample of 60 paired permanent teeth (30 in each group) according to their type and site of caries.

Selection criteria

The teeth were selected from patients, aged 16–34 years, attending the faculty clinic of the Department of Dentistry, University Cheikh Anta Diop, Dakar, Senegal. The descriptive characteristics of the sample are given in Table 1. The mean age of the patients was 23.37 ± 4.92 years for the calcium hydroxide group and 24.30 ± 5.30 years for the MTA group. Thirty teeth were considered for each material (Table 1).

Table 1.

Distribution of teeth according to the age and gender of the subjects and the type of pulp-capping material

Capping material	Gender	n	Age (years)				t-test*	t-test*†
			Mean	SD	Min.	Max.	P value	P value
MTA	Female	12	22.75	5.74	16	34	0.58	0.48
	Male	18	23.78	4.43	16	32
	Total	30	23.37	4.92	16	34
Ca(OH)2	Female	14	23.43	3.857	16	30	0.39
	Male	16	25.06	6.34	16	34
	Total	30	24.30	5.30	16	34

MTA, mineral trioxide aggregate; SD, standard deviation.

^*t-test between males and females in each pulp-capping material group.

^†t-test between males and females in the two pulp-capping material groups.

The sample size was determined according to the literature review and in order to yield statistically significant results for the measurement of the thickness of newly formed dentine in each group. No changes occurred in the outcomes after the trial had commenced.

All the selected teeth presented an active deep carious lesion on either the occlusal or proximal surface. Reversible pulp inflammation was present in all cases, as demonstrated by the transient painful response to pulp testing.

Teeth with periodontal lesions, internal or external root resorptions, and patients with systemic medical conditions, were excluded from the study.

Patients were informed about the procedure and provided written informed consent after the study had been approved by the ethics commission of our institution.

All procedures were performed by one of the investigators in the study (FL) who is a qualified endodontist at our institution.

Pulp vitality was tested by submitting the tooth to thermal and electrical testing:

• A cold stimulus was given by the use of ethyl chloride

• Electrical testing was carried out using an electric pulp tester (Electric Pulp Tester Averon® PT 2.0, VEGA-PRO, Ekaterinburg, Russia).

These tests were carried out at baseline, before pulp capping, and at the 3- and 6-month post-operative follow-up visits.

Pulp-capping procedure

MTA(ProRoot; Dentsply/Tulsa Dental, Tulsa, OK, USA) and calcium hydroxide material (Dycal® Ivory, Dentsply Caulk, Dentsply, L.D. Caulk, Milford, DE, USA) were used as pulp-capping agents.

The operative procedure was performed as follows:

• After local peri-apical or intraligamentary anaesthesia of the tooth, rubber dam isolation was provided and carious lesions were removed using a three-step procedure: (i) high-speed carious enamel removal with a round diamond bur; (ii) dentine mechanical curettage with a low-speed powered tungsten carbide bur (H1 314 014 or H1 314 012, Dentsply, Maillefer, Tulsa, OK, USA); (iii) manual final dentine curettage using a spoon excavator (no 49, 61 or 73, Dentsply, Maillefer) making it possible to see the pulp by transparency

• MTA powder was mixed with sterile water in a 3 : 1 ratio, placed on the operative site with plastic amalgam carrier-like instruments (MTA Gun) and applied by light pressure with moist cotton pellets. Hard-setting calcium hydroxide paste (Dycal®) was mixed according to the manufacturer’s instructions and applied to the sites with ball-ended instruments. Glass ionomer cement (GC Fuji IX, GC EUROPE, Leuven, Belgium) was placed over both materials as a filling material during the 6-month evaluation period of the study. The final restoration was placed over the glass ionomer after 6 months, with either amalgam or composite following the study period

• A simple randomisation was used with a single sequence of random assignment, without any restriction.The first tooth was randomly assigned to MTA and the 30 following cases were alternatively assigned to either calcium hydroxide or MTA. Each tooth was secondarily paired with a control case with the other pulp-capping material. The two paired cases differed only by the pulp-capping material (MTA or calcium hydroxide) and were paired for the type of tooth (premolar, molar), age range (as shown in Table 1) and gender of the patient. Table 2 displays the tooth distribution and sites of the initial carious lesions. Occlusal lesions represented 45% of the sample, and proximal lesions made up 55%; upper premolars and first lower molars accounted for 58% of the total treated teeth.

Table 2.

Distribution of teeth according to the site of caries

Tooth type	Site of caries						Total
	Occlusal		Mesio-occlusal		Disto-occlusal		n	(%)
	n	(%)	n	(%)	n	(%)
Upper premolar	2	(3.33)	2	(3.33)	14	(23.33)	18	(30.00)
Lower premolar	0	(0)	0	(0)	2	(3.33)	2	(3.33)
1st upper molar	2	(3.33)	5	(8.33)	2	(3.33)	9	(15.00)
1st lower molar	14	(23.33)	1	(1.66)	2	(3.33)	17	(28.33)
2nd or 3rd upper molar	5	(8.33)	0	(0)	0	(0)	5	(8.33)
2nd or 3rd lower molar	4	(6.66)	2	(3.33)	3	(5.00)	9	(15.00)
Total	27	(45.0)	10	(16.7)	23	(38.3)	(60)	(100)

Clinical and radiographic follow-ups were carried out at baseline, 3 and 6 months. The treatment was considered to be clinically successful when the pulp remained vital with a normal response to thermal and electrical tests without signs of spontaneous pain. The treatment was considered to be radiographically successful when the dentine bridge was present over the lesion and no furcation radiolucency, periodontal ligament space widening, internal or external root resorptions were noted.

The study and recruitment of the patients were carried out from 21 May 2007 to 31 December 2008; the study was terminated 6 months later (June 2009) following the last follow-up of the final patient.

Radiographic assessment of the dentine thickness

All measurements of dentine thickness were performed with Mesurim Pro® Software (©J-F. Madre, Academy of Amiens, Amiens, France). This software is intended to collect data on digitised images (e.g. counting of elements on an image, measurements of surface light or length). A 1-mm Fixott–Everett grid (Fixott–Everett X-Ray Grid Large Ea, Miltex Instrument Co, Inc., York, PA, USA) was used. The Fixott–Everett grid (Figure 1) is a metallic incorporated device placed in contact with the X-ray film during exposure, and results in a grid of known size being imaged. It was used for radiological scaling for standardised measurements on the digitised images. All the radiographs were subsequently scanned and transferred to the computer for digital analysis.

Figure 1.

Fixot–Everett grid and intrabuccal radiographic film.

Measurements on the digitised radiograph were performed at baseline (after the indirect pulp-capping procedure) and at 3 and 6 months. On each digitised radiograph, the scale for the measurements was determined using the space between two lines of the grid, which was assigned a value of 1 mm; the dentine thickness on each film was measured with Mesurim Pro® software accordingly to this scale calculation. The range of measurements was 10⁻³ mm.

The measurements on the digitised images were performed by one investigator in this study (HMB) who was blind to the clinical procedure and the nature of the pulp-capping material.

Statistical analysis

Statistical analysis was performed using SPSS software (version 11.0 for Windows, SPSS Inc., Chicago, IL, USA). Cohen’s kappa statistic test for qualitative measurements was used to assess the reliability of electrical pulp testing (K = 0.624, substantial agreement according to Landis and Koch²⁷); it was also used for the intra-class correlation coefficient (ICC) for the reliability of the radiographic measurements employing Mesurim Pro® Software (quantitative) with two-way random single measures (consistency based on absolute agreement) [ICC = 0.722; confidence interval (CI), 0.575; 0.824; P < 0.001].

Losses to follow-up were analysed as intention to treat, i.e. regarded as failures. Means and proportions for personal characteristics and clinical parameters were calculated for both groups. The significance of any difference in the means was tested using Student’s t-test, and the significance of any difference in proportions was tested using Pearson’s chi-squared test. The relationship between the independent variables and the pulp-capping outcomes, considered as a dependent variable, was assessed using multivariate logistic regression analysis on subject-based data. The variables which were statistically insignificant in univariate analysis were not considered for further analysis. Statistical significance was defined as P < 0.05.

RESULTS

Failure and success rates

The failures were defined as negative pulp vitality tests on examination. As illustrated in the flow diagram of the trial (Figure 2), at 3 months, there were four failures in the calcium hydroxide group and one in the MTA group. One additional failure at 6 months was found in the MTA group.

Figure 2.

Flow diagram of trial.

At 3 months, the success rate was 93.1% for MTA and 73.3% for calcium hydroxide, whereas, at 6 months, it was 89.6% for MTA and remained unchanged for calcium hydroxide (Figure 3). At 3 months, the rate of failure was four times greater with calcium hydroxide than with MTA, which was statistically significant (P = 0.02). After 6 months, with one additional failure in the MTA group, the difference between the two groups was not significant (P = 0.63).

Figure 3.

Success rates at 3 and 6 months according to the pulp-capping material.

Newly formed dentine thickness

The average thicknesses of newly formed dentine at 3 and 6 months are shown in Table 3. Using Mesurim Pro® Software, at 3 months, the measurements were 0.121 ± 0.050 mm in the MTA group and 0.136 ± 0.060 mm in the calcium hydroxide group (P = 0.380). At 6 months, the averages were 0.235 ± 0.110 mm in the MTA group and 0.221 ± 0.059 mm in the calcium hydroxide group (P = 0.594). In each group, the thickness of the dentine bridge at 6 months was approximately two-fold higher than that at 3 months, with a statistically significant difference (P < 0. 0001).

Table 3.

Average thicknesses of tertiary dentine at 3 and 6 months according to the pulp-capping material

Pulp-capping material	Follow-up duration		t-test*	t-test†
	3 months	6 months	P value	P value
MTA
n	28	27	0.000	<0.0001
Mean (mm)	0.121	0.235
SD (mm)	0.059	0.110
Ca(OH)2
n	22	22	0.000
Mean (mm)	0.136	0.221
SD (mm)	0.060	0.059

MTA, mineral trioxide aggregate; SD, standard deviation.

^*t-test between male and females in each pulp-capping material group.

^†t-test between male and females in two pulp-capping material groups.

Statistical analysis

Logistic regression analysis was performed to obtain a predictive model for the pulp-capping outcome. Univariate analysis of the personal characteristics and clinical parameters showed that only the disto-occlusal site of caries and the type of capping material may affect the pulp-capping outcome (Table 4). Multivariate logistic regression analysis showed that only MTA and the disto-occlusal site of caries (independent variables) were predictive factors for the pulp-capping outcome as dependent variable. The odds ratio (OR) from the logistic regression showed the effects of the selected independent variables (Table 5). The disto-occlusal site appeared to be an unfavourable factor and showed (B = −2.123) the strongest evidence as an explanatory variable (P = 0.003). The final predictive model showed that the success of the pulp-capping procedure can be predicted up to 43.8% of times when MTA is used, and that failure can be predicted up to 90.9% of times when the carious lesion is on the disto-occlusal site of the tooth.

Table 4.

Univariate analysis of the effects of personal characteristics and clinical parameters on the pulp-capping outcome

Variable	Score	df	Pvalue
Age	0.179	1	0.672
Gender	0.302	1	0.582
Tooth localisation	0.075	1	0.785
Tooth type
Upper premolar	4.156	1	0.041
Lower premolar	0.752	1	0.386
1st upper molar	1.310	1	0.252
1st lower molar	0.091	1	0.762
2nd or 3rd upper molar	1.983	1	0.159
2nd or 3rd lower molar	0.107	1	0.744
Site of caries
Occlusal	3.526	1	0.060
Mesio-occlusal	1.705	1	0.192
Disto-occlusal	8.539	1	0.003
Pulp-capping material
Ca(OH)2	5.455	1	0.020
MTA	5.455	1	0.020

MTA, mineral trioxide aggregate.

Table 5.

Multivariate logistic regression analysis with predictive factors as independent variables and pulp-capping outcome as dependent variable

Variable	B	SE	Significance	Exp(B) or OR	95% CI
					Lower	Upper
Disto-occlusal caries	−2.123	0.724	0.003	0.120	0.029	0.495
MTA	1.870	0.754	0.013	6.489	1.481	28.439

CI, confidence interval; MTA, mineral trioxide aggregate; OR, odds ratio; SE, standard error.

DISCUSSION

This study was designed as a prospective, randomised, paired clinical study. The patients were young, reflecting the youth of the Senegalese population, and the recruitment of patients at our institution. With regard to the operating protocol, we placed a glass ionomer cement over the capping material, which was used as restoration material during the time of the study; thus, electrical pulp testing for vitality could be performed accurately during the follow-up period. Following the pulp-capping procedure, bacterial leakage through the final restoration material is considered to be more detrimental to the outcome than bacterial contamination at the time of treatment²⁸. This finding underlines the need for a good seal in the final restoration material after the completion of the pulp-capping procedure. In this study, failure occurred in one case in the MTA group, because of the loss of the restoration material.

As MTA and calcium hydroxide can be distinguished by the operator on performing the pulp-capping procedure, a double-blind clinical trial could not be performed here.

The success rates were comparable for MTA and calcium hydroxide at 6 months, but they differed at 3 months. Thus, the critical period for the success of the capping procedure seems to be within the first 3 months. When looking at these success rates, and with regard to the potential toxic effects of the capping materials, we considered, as reported by Pashley ²⁹, that there was no difference between direct and indirect pulp capping for the restoration of deep cavities, because of the fast increase in dentine permeability near the pulp. In deep cavities with a residual thickness of dentine of less than 0.5 mm, the number and size of open tubuli are such that communication with the pulp is comparable with that of a true pulp exposure ³⁰.

The 93% success rate at 3 months with MTA is in accordance with the results obtained by Bogen et al.¹⁹ in a 9-year follow-up study of direct pulp capping among 40 patients aged between 7 and 45 years; they reported successful pulp capping in 49 of 53 teeth (97.96%) on the basis of radiographic criteria, subjective symptoms and cold testing of pulp vitality. In another clinical and radiographic, 24-month, follow-up study of direct pulp capping on temporary molars, Tuna and Olmez³¹ recorded good results for both MTA and calcium hydroxide (up to 100%).

The good clinical success rates are related to the thickness of the newly formed dentine. In a reference study using calcium hydroxide, Stanley et al.³² showed that the thickness of the dentine bridge did not exceed 250 μm after 66 days, and reached up to 0.5 mm after 200 days. In our study, average thicknesses of the dentine bridge were two-fold lower than those estimated by Stanley et al.³². This difference could be related to direct or indirect capping and the assessment of the newly formed dentine thickness. In addition, our measurements were made radiographically at baseline, 3 and 6 months, whereas Stanley et al.³² measured histological cuts of dentine formation. The thickness of the newly formed dentine, using MTA as a pulp-capping material, was not documented. In this study, we found a slower formation of the dentine bridge from baseline to 3 months in the MTA group than in the calcium hydroxide group. Between 3 and 6 months, this difference was not maintained, and no difference was found at 6 months. This could be explained by the fact that MTA serves as a reservoir for calcium hydroxide and the calcium release from MTA materials decreases slightly over time¹.

The clinical and radiographic data reported here may be related to the cellular and biomechanical mechanisms of reparative dentine formation. Calcium hydroxide promotes the dentine repair of pulp wounds, and the presence of superficial pulp tissue necrosis is crucial and serves as a stimulus for the initiation of the hard tissue repair process². Calcium hydroxide has the ability to dissolve the dentine, and thus gradually release growth factors³³.

MTA does not contain calcium hydroxide but, after hardening, calcium oxide is formed that can react with tissue fluids to give calcium hydroxide¹⁸; this can induce the secretion of fibronectin by the pulp cells adjacent to the necrotic layer under the capping material³⁴. MTA is able to stimulate reparative dentine formation by the stereotypic defensive mechanism of early pulp wound healing²⁶.

The liberation of dentine factors by MTA has been demonstrated, but at different concentrations to those released by calcium hydroxide³⁵. These differences could account for the different kinetics of dentine formation in our study, and may lead to dentine bridges of different quality; the quality of the newly formed dentine is a significant factor for the success of the capping procedure. Histological studies have shown a greater frequency of inflammatory cells and zones of pulp necrosis when calcium hydroxide is used for capping13., 14., 15., 16., 36.. In vivo studies have shown that MTA induces the formation of a high-quality thicker dentine bridge15., 16., 37..

In this study, logistic regression analysis identified two variables predictive of the capping outcome. In the final predictive model, MTA was significantly predictive of the success of pulp capping and a disto-occlusal site of caries significantly increased the risk of failure. The difficult visual access and control of dentine curettage in some areas of the teeth and the better quality of the dentine bridge and sealing ability of MTA support these findings13., 15., 16., 37..

Other predictive models for pulp capping have emphasised age as a dependent variable for success of the procedure. This was not apparent in the logistic regression analysis performed in this study, as the sample population was homogeneous for age and mainly involved young adults; these cases are easily managed and inflammatory involvement is minimal, as suggested by bleeding that is easy to stop ².

Further clinical studies need to be performed on a larger sample in order to check whether the disto-occlusal site of caries is a predictive, and not operator-dependent, factor. As no detrimental effect was demonstrated with MTA, its use may appear to be of long-term benefit. Further generalisability of these results will require a larger sample and longer follow-up duration.

CONCLUSION

Based on the results of this short-term clinical and radiographic study, a higher success rate was observed in the MTA group relative to the Dycal® group after 3 months, which was statistically significant. After 6 months, however, no statistically significant difference was found in the dentine thickness between the two groups. Additional histological investigations are needed to support these findings.

Acknowledgements

The authors thank Dr Papa Ibrahima Ngom, Associate Professor in Orthodontics at University Cheikh Anta Diop, Dakar, Senegal, who performed statistical analysis, and Roland Arsan for the gracious provision of products that enabled this study to be conducted.

Conflicts of interest

This study was not financed by any company or manufacturer and has no commercial aim.