Effects of pH and Time on Nickel Ion Release from Pediatric Stainless-Steel Crowns: An In-Vitro Comparative Study.

Background: Frequent use of stainless-steel crowns in pediatric dentistry has led to concerns that heavy metals in the crowns could be released into the mouth and potentially trigger allergic reactions. Of these constituents, nickel is known to be a common cause of hypersensitivity reactions. Aim: To evaluate and compare nickel ion release from pediatric stainless-steel crowns of 3M ESPE and DNTO Kids Crown at pH levels of 4.3, 5.5, and 6.3 for days 1, 7, 15, and 30.
Methods: In this in-vitro study, nickel ion release (in PPM) from stainless steel crowns of 3M ESPE (n = 60) and DNTO Kids Crown (n = 60) in artificial saliva of pH 4.3, 5.5, and 6.3 on days 1, 7, 15, and 30 was analyzed using inductively coupled plasma-atomic emission spectrometry at Indian Institute of Technology-Bombay. Statistical analysis was performed using two-way and three-way analysis of variance (ANOVA) followed by least significant difference post hoc test and Spearman's rank order correlation. P < 0.05 was considered statistically significant.
Results: In both groups (3M ESPE and DNTO Kids Crown), a significantly higher amount of nickel ion release was observed at pH 4.3. Among different time intervals, significantly maximum nickel ion release was observed on day 7. Nickel ion release from DNTO Kids Crowns was significantly higher than 3M ESPE at all the pH levels and time intervals. Conclusions: The pH of artificial saliva and nickel ion release is inversely related. The manufacturing process may affect the biodegradability of stainless-steel crowns. The maximum average nickel ion release from stainless steel crowns is below the recommended dietary intake but sufficient to cause allergic reactions. Copyright:

INTRODUCTION

Prefabricated stainless steel crowns (SSCs) are the second most common dental device containing stainless steel used in children.[1] SSCs are extremely durable, relatively inexpensive, subject to minimal technique sensitivity during placement, and offer the advantage of full coronal coverage.[2]

The frequent use of preformed crowns for primary tooth restoration in clinical pediatric dentistry has led to concerns that heavy metals in the crowns could be released into the mouth and accumulate in the body.[3] The oral environment is dynamic, where factors such as pH variation, temperature, salivary conditions, mechanical load, microbiological and enzymatic activity, and bacterial acidic production affect the corrosion rates and thus the oral release of metals.[3456]

The leached metals from SSCs can trigger an allergic reaction. Of these constituents, nickel is known to be a very common cause of contact allergy and hypersensitivity reactions.[178] Although the older generation SSCs with almost 72% nickel have been replaced by stainless steel alloys, in which nickel concentration is limited to 8%–13%, it should be noted that the release rate of metallic ions is not proportional to their concentration but depends mainly on the alloy's corrosion resistance during clinical service.[9]

A limited number of studies are available that have investigated the release of nickel ions from stainless steel crowns of different manufacturers at different pH levels and time intervals. Thus, the purpose of this study was to do a comparative evaluation of nickel ion release from pediatric stainless-steel crowns of 3M ESPE and DNTO Kids Crown at pH levels of 4.3, 5.5, and 6.3 on days 1, 7, 15, and 30.

MATERIALS AND METHODS

In the present in-vitro experimental study, 120 stainless steel crowns for the left upper second primary molar (FDI tooth notation: 65) were analyzed, out of which 60 were of 3M ESPE (St. Paul, Minnesota, USA/E7 size) and 60 of DNTO Kids Crown (Peng Lim, Taiwan/J7 size). The study was conducted for a period of 30 days. Ethical clearance was taken from the institutional ethics committee on 11/06/2021.

Artificial saliva

Artificial saliva was prepared using the formula recommended by Gjerdet and Hero (1987).[10] The pH values of the artificial saliva samples (pH 4.3, 5.5, and 6.3) were adjusted using 74% sodium hydroxide (Thermo Fisher Scientific India Pvt. Ltd., Powai, Mumbai, India) and 95% hydrochloric acid (Thermo Fisher Scientific India Pvt. Ltd., Powai, Mumbai, India).

Study procedure

Prior to the experiment, stainless steel crowns and polyethylene tubes were cleaned in a solution of 50% v/v ethanol to remove oily dirt, rinsed with distilled water, and then dried for 15 min. Zinc polycarboxylate cement (HY-Bond Polycarboxylate Cement, SHOFU Dental Corporation, San Marcos, California, USA) was used to fill the inner surface of crowns to prevent contact with artificial saliva solutions. The polyethylene tubes were used for the immersion of 120 stainless steel crowns in artificial saliva. Polyethylene tubes were divided into three groups as per the pH of artificial saliva (pH 4.3, 5.5, and 6.3).

In each group of polyethylene tubes with 10 mL of artificial saliva solution of a particular pH, 20 stainless steel crowns of 3M ESPE and 20 stainless steel crowns of DNTO Kids Crown were soaked (one crown in each tube). After days 1, 7, and 15, crowns were taken out from the solution and placed in another polyethylene tubes containing 10 mL of artificial saliva of the same pH for the subsequent time interval. These polyethylene tubes were stored at 37°C in an incubator to simulate the oral conditions. After removal of crowns from artificial saliva samples, digestion of solutions was done using 1.5 mL of 65% nitric acid (Thermo Fisher Scientific India Pvt. Ltd., Powai, Mumbai, India) and 0.5 mL of 70% perchloric acid (Thermo Fisher Scientific India Pvt. Ltd., Powai, Mumbai, India). The digestion process prevented the growth of fungi during storage. Digested samples were stored in a refrigerator till the analysis of nickel concentration.

Blinding: The experimental solution containing polyethylene tubes was coded by another person unrelated to the experiments so that the researcher and analyzer of nickel concentration in solution were blinded about the type of stainless-steel crown, pH, and time interval of solution.

Analysis of Nickel concentration

Nickel concentration in digested artificial saliva samples was evaluated at Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology-Bombay (IIT-B), Mumbai, Maharashtra, India by using inductively coupled plasma-atomic emission spectrometry (ICP-AES) (ARCOS, Spectro, Kelve, Germany).

Statistical analysis

Along with computation of descriptive statistics, data analysis was performed using two-way and three-way analysis of variance (ANOVA) followed by least significant difference (LSD) post hoc test, and Spearman's rank-order correlation. P < 0.05 was accepted as statistically significant. All analyses were performed using version 23.0 of the Statistical Package for Social Sciences (IBM Corporation, Armonk, New York, USA).

RESULTS

Table 1 shows nickel ion release (on days 1, 7, 15, 30, and cumulative) at artificial saliva of pH levels of 4.3, 5.5, and 6.3 in stainless steel crowns of 3M ESPE and DNTO Kids Crown.

Table 1

Nickel ion release (days 1, 7, 15, 30, and cumulative) at artificial saliva of pH 4.3, 5.5, and 6.3 in stainless steel crowns of 3M ESPE and DNTO Kids Crown

Group	Time Interval	Nickel ion release (ppm) at different pH levels
		pH 4.3		pH 5.5		pH 6.3
		Mean±SD	Min-Max	Mean±SD	Min-Max	Mean±SD	Min-Max
3M ESPE	Day 1	0.053±0.001	0.051-0.055	0.030±0.001	0.028-0.032	0.018±0.001	0.017-0.020
	Day 7	0.105±0.001	0.102-0.107	0.058±0.001	0.057-0.060	0.038±0.001	0.037-0.040
	Day 15	0.088±0.001	0.086-0.091	0.046±0.001	0.043-0.048	0.032±0.001	0.029-0.034
	Day 30	0.079±0.001	0.077-0.082	0.037±0.001	0.034-0.040	0.025±0.001	0.023-0.028
	Cumulative	0.325±0.002	0.320-0.329	0.170±0.003	0.166-0.175	0.114±0.002	0.109-0.118
DNTO Kids Crown	Day 1	0.068±0.001	0.066-0.069	0.039±0.001	0.036-0.041	0.026±0.001	0.024-0.028
	Day 7	0.126±0.001	0.124-0.128	0.075±0.001	0.074-0.078	0.052±0.001	0.051-0.054
	Day 15	0.099±0.001	0.098-0.101	0.055±0.001	0.053-0.056	0.040±0.001	0.038-0.041
	Day 30	0.090±0.001	0.088-0.091	0.046±0.001	0.044-0.047	0.033±0.001	0.031-0.034
	Cumulative	0.382±0.003	0.377-0.387	0.214±0.003	0.208-0.220	0.150±0.003	0.146-0.155

For comparison of nickel ion release on days 1, 7, 15, and 30 at artificial saliva of pH 4.3, 5.5, and 6.3 in stainless steel crowns of 3M ESPE and DNTO Kids Crown, three-way mixed factorial ANOVA was used. The interaction of groups, pH of artificial saliva, and time intervals on nickel ion release was statistically significant {F (6, 342) = 12.232, P < 0.001}. When LSD post hoc test was applied for pairwise comparison, it showed the following observations:

In both groups, for all three pH levels, significantly maximum nickel ion release was observed on day 7 followed by day 15 and day 30. The least nickel ion release was observed on day 1.

In both groups, for all the time intervals (days 01, 07, 15, and 30), significantly maximum nickel ion release was observed at pH 4.3 followed by pH 5.5, and the least nickel ion release was observed at pH 6.3.

For all the time intervals (days 1, 7, 15, and 30), at all three pH levels, a significantly higher amount of nickel release was observed with DNTO Kids Crown than with 3M ESPE stainless steel crowns.

For comparison of cumulative nickel ion release at artificial saliva of pH 4.3, 5.5, and 6.3 in stainless steel crowns of 3M ESPE and DNTO Kids Crown, 2-way ANOVA was used. The interaction of groups and pH of artificial saliva on cumulative nickel ion release was statistically significant {F (2, 114) = 151.969, P < 0.001}. When LSD post hoc test was applied for pairwise comparison, it showed the following observations:

In both groups, the maximum cumulative nickel ion release was observed at pH 4.3 followed by pH 5.5, and the least cumulative nickel ion release was observed at pH 6.3.

For all three pH levels, a significantly higher amount of cumulative nickel release was observed with DNTO Kids Crown than with 3M ESPE stainless steel crowns.

Table 2 shows the correlation between the pH of artificial saliva (pH 4.3, 5.5, and 6.3) and nickel ion release (days 1, 7, 15, 30, and cumulative) in stainless steel crowns of 3M ESPE and DNTO Kids Crown. In both groups, a statistically significant and very strong negative relationship was observed between pH of artificial saliva and nickel ion release (days 1, 7, 15, 30, and cumulative).

Table 2

Correlation between pH of artificial saliva (pH 4.3, 5.5, and 6.3) and nickel ion release (days 1, 7, 15, 30, and cumulative) in stainless steel crowns of 3M ESPE and DNTO Kids Crown

Correlation Between	3M ESPE		DNTO Kids Crown
	Correlation Coefficient {Spearman’s rho (ρ)}	P	Correlation Coefficient {Spearman’s rho (ρ)}	P
pH and nickel ion release on day 1	−0.951	0.000 (<0.001),	−0.947	0.000 (<0.001),
	Very strong negative relationship	Significant	Very strong negative relationship	Significant
pH and nickel ion release on day 7	−0.947	0.000 (<0.001),	−0.947	0.000 (<0.001),
	Very strong negative relationship	Significant	Very strong negative relationship	Significant
pH and nickel ion release on day 15	−0.947	0.000 (<0.001),	−0.949	0.000 (<0.001),
	Very strong negative relationship	Significant	Very strong negative relationship	Significant
pH and nickel ion release on day 30	−0.949	0.000 (<0.001),	−0.948	0.000 (<0.001),
	Very strong negative relationship	Significant	Very strong negative relationship	Significant
pH and cumulative nickel ion release	−0.944	0.000 (<0.001),	−0.944	0.000 (<0.001),
	Very strong negative relationship	Significant	Very strong negative relationship	Significant

DISCUSSION

In the present study, pH 4.3 was investigated to simulate a condition that occurs when people consume acidic fruits or drinks (e.g., lemon juice and cola).[11] According to research, pH 5.5 is the critical pH and pH 6.3 is the normal pH of saliva in human subjects.[12] In the present study, the concentration of nickel ions in artificial saliva samples was analyzed using ICP-AES. The ICP-AES has proven to be a highly successful technique for multi-element analysis by atomic emission spectroscopy.[10] ICP has the advantage of extracting each ion simultaneously and detecting the metals without the interference of other ions.[3]

In the present investigation, a higher amount of nickel ion release in artificial saliva of pH 4.3, 5.5, and 6.3 was observed with DNTO Kids Crown than with 3M ESPE stainless steel crowns. The corrosion of stainless-steel appliances and subsequent metal ion release in the oral environment is governed by two main factors. The first is the manufacturing process, which includes the type of alloy, characteristics of the metals used, and surface finishing. The second is environmental factors, such as mechanical stress, diet, time of the day, salivary flow rate, and health and psychosomatic condition of the individual.[13] The earlier studies have reported that surface defects due to the manufacturing process are sites that are susceptible to corrosion and leaching of nickel in stainless steel orthodontic wires.[7] In the present study, environmental factors were the same but the manufacturing process could be a possible answer for high nickel ion release from DNTO Kids crowns.

In the present study, it was observed that in stainless steel crowns of 3M ESPE and DNTO Kids Crown in pH 4.3, 5.5, and 6.3, nickel ion release first increased till day 7 and then decreased on day 15 and day 30. These results were consistent with several earlier studies conducted on stainless steel crowns, space maintainers, and orthodontic appliances.[814] The possible explanation of kinetics of ion release could be that nickel present on the surface of the crowns may quickly corrode during the first 7 days of the experiment; then, the rate of release drops off as surface nickel depleted. Second, after 7 days, the degradation of stainless steel slows down due to the formation of corrosion product on the crown surface.[15] On the contrary, studies conducted by Menek et al.,[2] Arab-Nozari et al.,[5] Ramazani et al.,[15] and Kulkarni et al.[16] concluded that nickel ion release from stainless steel crowns decreased from day 1 till the end of the study period.

In the present study, maximum nickel ion release was observed at pH 4.3 followed by pH 5.5, and the least nickel ion release was observed at pH 6.3 in 3M ESPE and DNTO Kids crown. A strong negative correlation between the pH of artificial saliva and nickel ion release was seen. The stainless-steel oxide film required for corrosion endurance is less stable in acidic conditions.[11] Similar results were observed in earlier studies.[21117] In an investigation, Milheiro et al.[6] stated that pH had a drastic effect on nickel release. The abrupt drop of pH after each meal and the microbiological activity in plaque contributes to periods of low pH. This reflects the importance of maintaining perfect oral hygiene in preventing the corrosion process.

As reported in the studies, the average dietary intake of nickel is 300–500 μg per day.[41011] In the present study, the measured quantities of released metal ions are minor from a systemic toxicologic viewpoint. Results obtained in our study are in accordance with earlier studies.[1617] However, these levels are sufficient to cause an allergic reaction because of the high haptenic potential of nickel, especially because stainless steel crowns remain in the oral cavity for a long time.[1016] The amount required to create a contact hypersensitivity reaction depends on the individual; a “safety threshold” of nickel release without biological repercussions has not been determined.[68]

The results of the present in-vitro tests are limited, and extrapolations to the clinical situation are difficult because the methodology used is unable to precisely reproduce the highly complex and dynamic oral environment. Thus, further studies are needed to simulate this additional variable in the analysis of ion release from stainless steel crowns as well as to assess the absorption of released metal ions and their long-term effects on oral tissues.

CONCLUSIONS

Based on the findings of the present study, the pH of artificial saliva is inversely related with nickel ion release from SSCs. The nickel ion release first increases till day 7 and then decreases with time. The manufacturing process may affect the biodegradability of SSCs. The maximum average nickel ion release from stainless steel crowns of 3M ESPE and DNTO Kids Crown is below the recommended dietary intake of nickel but sufficient to cause allergic reactions.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.