Effect of autoclave sterilization on nickel ion release from stainless steel crowns and space maintainer bands

Abstract

This study aimed to investigate the effect of autoclave sterilization on the leaching of nickel ions from stainless steel crowns and space maintainer bands used in pediatric dentistry. Sixty space maintainer bands and 60 stainless steel crowns were divided into sterilized (Amsco Century V‐120 Prevac steam sterilizer) and non‐sterilized groups. These samples were immersed in artificial saliva, placed in individual petri dishes, and then incubated at 37°C. The samples were analyzed for 42 days using a gas chromatography‐mass spectrometry (GC‐MS) machine to quantify the release of nickel ions. Nickel release was higher in the sterilized samples than in the non‐sterilized samples for both space maintainer bands and stainless steel crowns. This trend persisted throughout most days of the experiment, with a peak release observed on day 7. Autoclave sterilization had a significant impact on ion release, raising potential concerns regarding the material's biocompatibility. Dentists should be mindful of the increased nickel release observed from sterilized crowns and bands, as this information could influence treatment decisions, especially in children who are predisposed to allergies.

INTRODUCTION

Prefabricated stainless steel crowns and space maintainer bands have become essential dental devices in pediatric dentistry owing to the high prevalence of dental caries among children [1, 2].

Stainless steel crowns are preformed crowns that are used for a wealth of purposes: restoration of deciduous molars that have undergone pulpal therapy; as multi‐surface restorations for caries; as protection in patients at high risk of developing caries; as protection of deciduous teeth with developmental defects such as amelogenesis imperfecta, dentinogenesis imperfecta, or enamel hypoplasia; and as protection of teeth showing extensive wear [3]. These crowns protect primary molars by reducing the risk of post‐operative symptoms [4]. Their prefabricated design simplifies use and reduces chair time and cost [5].

In contrast, space maintainer bands are metal bands placed on an abutment tooth as part of a space‐maintaining appliance. They prevent space loss and tooth drift and maintain proper alignment [6]. Space maintainer bands are indicated following the early loss of a deciduous tooth to preserve space for the eruption of permanent teeth. Their appropriate use can prevent the need for extensive orthodontic treatment [6].

The popularity of stainless steel crowns and space maintainers is due to their ease of use and affordability. However, concerns have been raised about the nickel (Ni) content in these alloys, which has been linked to allergic reactions [7, 8]. The release of nickel into the oral cavity after cementation has prompted inquiries into the biocompatibility of stainless steel crowns and space maintainers [3, 9]. Factors such as temperature, pH, moisture, and oral conditions may influence the rate of nickel ions leaching from these materials [10, 11].

Nickel is added to enhance durability and to reduce costs, but its release may exceed safe daily intake limits, leading to health risks such as allergies, dermatitis, gingivitis, and tooth enamel damage. These potential effects highlight the need for monitoring and safer material alternatives [12].

The impact of the heat and sterilization procedures commonly used in dental clinics on the leaching of ions from stainless steel crowns and space maintainer bands has not been studied thoroughly. This knowledge gap necessitates further investigation, especially considering that fitting the stainless steel crowns and space maintainer bands often involves trying multiple sizes until an optimal fit is achieved [13]. This trial‐and‐error process results in unused crowns and bands that may be contaminated with blood, saliva, or infectious microorganisms [14]. Therefore, sterilization of these materials before insertion into another patient's mouth is crucial to prevent cross‐contamination [15].

Numerous disinfection and sterilization techniques are available in dental clinics. However, autoclaves are widely regarded as the most effective for eliminating harmful pathogens from contaminated crowns [15]. Therefore, this study aimed to evaluate the in vitro effect of autoclave sterilization on the release of nickel ions from pediatric stainless steel crowns and space maintainer bands when immersed in artificial saliva. The null hypothesis was that autoclave sterilization has no statistically significant effect on the release of nickel ions from pediatric stainless steel crowns and space maintainer bands when immersed in artificial saliva.

MATERIAL AND METHODS

This study was approved by the College of Dentistry Research Center at King Saud University (FR0653). The in vitro experiments were conducted at the Dental University Hospital, Riyadh, Saudi Arabia.

A total of 60 space maintainer bands (size 36, 3M Unitek) and 60 stainless steel crowns (size lower E5, 3M ESPE) were purchased. The chemical compositions analyzed, based on manufacturer specifications, were as follows: iron 5%–74%, chromium 17%–19%, and nickel 9%–13%.

Following the method of a previous study [16], the space maintainer bands and stainless steel crowns were allocated to one of two interventions, autoclave sterilization or no sterilization. The sterilization process was carried out using an Amsco Century V‐120 Prevac steam sterilizer (STERIS) for 4 min and a temperature of 121°C (250°F), followed by a drying period of 15 min.

Following the intervention, each specimen was immersed in 20 mL of artificial saliva (consisting of 0.8 g NaCl, 2.4 g KCl, 1.5 g NaH₂PO₄, 0.1 g Na₂S, and 2 g CO(NH₂)₂), and the pH was adjusted to 6.75 ± 0.05. Each dish was sealed securely to prevent evaporation of the artificial saliva solution. Each crown or band was placed in an individual petri dish on a plastic stand to ensure no contact with any metallic material during the test, and incubated at 37°C using a general incubator (model JSGI‐150T, JS Research). The required saliva samples (2 mL) were drawn from each solution and analyzed separately on days 0, 7, 14, 21, and 42 to measure the concentration of released Ni ions in artificial saliva using a gas chromatography‐mass spectrometer (GC‐MS, Thermo Fisher Scientific).

To maintain the original composition of the artificial saliva and to prevent saturation with released ions, fresh artificial saliva samples were used after each nickel (Ni) ion concentration measurement. This procedure was performed on days 7, 14, and 21; the saliva in the samples was completely replaced with new artificial saliva. All dishes were gently agitated after immersing the stainless steel crowns or space maintainer bands to ensure thorough coverage and to achieve a uniform distribution of the solution. Seven days after the initial immersion of the stainless steel crowns and space maintainer bands, samples of the artificial saliva were analyzed, and readings were recorded (second reading). This procedure was repeated for the third, fourth, and fifth readings on days 14, 21, and 42.

Statistical analysis

Data analysis was performed using an independent sample t‐test. Statistical significance was set at a critical value of p = 0.05, with p < 0.05 considered statistically significant. All analyses were performed using spss statistics for windows, version 29.0.2.0 (IBM).

RESULTS

The maximum release of Ni was observed in sterilized stainless steel crowns on day 7, 537,811.3 ppm after which it decreased to 44,036.2 ppm on day 14 and 177.8 ppm on day 21. Significantly higher amounts of ions were released in the sterilized group than in the non‐sterilized group on days 7, 14, and 12 (Table 1). On day 42, no nickel release was observed irrespective of the sterilization status.

TABLE 1.

The release of nickel ions from stainless steel crowns and space maintainer bands as a function of time for sterilized and non‐sterilized specimens.

		Stainless steel crowns (N = 30)		Space maintainer bands (N = 30)
Time point	Procedure	Mean (ppm) ± SD	p‐value	Mean (ppm) ± SD	p‐value
Day 0	Non‐sterilized	0.0		0.0
	Sterilized	0.0		0.0
Day 7	Non‐sterilized	16,685.5 ± 2792.1	<0.001*	1214.2 ± 239.6	<0.001*
	Sterilized	537,811.3 ± 27,740.5		233,463.7 ± 11,142.4
Day 14	Non‐sterilized	4985.7 ± 617.7	<0.001*	769.4 ± 41.4	<0.001*
	Sterilized	44,036.2 ± 5768.9		12,881.9 ± 1095.7
Day 21	Non‐sterilized	177.8 ± 43.1	<0.001*	36.4 ± 5.9	<0.001*
	Sterilized	243.9 ± 80.7		2239.6 ± 156.1
Day 42	Non‐sterilized	0.0		0.0
	Sterilized	0.0		0.0

The highest release of Ni ions was observed in the sterilized space maintainer bands on day 7 (233,463.7 ppm) after which it decreased to 12,881.9 ppm on day 14 and to 2239.6 ppm on day 21. Significantly higher levels of ions were released in the sterilized space maintainer bands than in the non‐sterilized bands between days 7 and 21.

The samples of the stainless steel crowns and space maintainer bands exhibited a higher Ni release from the sterilized samples than from the non‐sterilized samples (Figure 1).

FIGURE 1.

Mean Ni ion concentrations released from stainless‐steel crowns (SSC) and space maintainer bands (SM) before and after sterilization.

DISCUSSION

The use of stainless steel crowns and space maintainer bands in pediatric dentistry has revolutionized the treatment of primary dentition. However, these nickel‐containing appliances are expected to corrode because the alloys are exposed to varying pH levels and temperatures in the oral cavity [10].

In the present study, a higher release of Ni ions from sterilized stainless steel crowns was observed on days 7 and 14 than from non‐sterilized crowns. This phenomenon can be attributed to the combined effects of heat sterilization and corrosion on the sterilized stainless steel crowns. Sterilization processes can alter the surface chemistry of stainless steel [17] because they often involve high temperatures and/or harsh chemicals. These conditions can lead to the formation of a new surface layer on stainless steel, which affects its susceptibility to corrosion and ion leaching [18]. Additionally, high temperatures can alter the microstructure of stainless steels, creating new grain boundaries and other metal defects that serve as pathways for ion leaching [19].

The amount of Ni ions leached from stainless steel crowns after sterilization depends on several factors, such as the type of sterilization process used, sterilization temperature and duration, and the composition of the stainless steel [17]. Similar findings regarding ion levels over time were reported by Amanna et al. [9], Tiwari et al. [10], and Kulkarni et al. [20] who studied the release rates of ions and observed a decrease in Ni ion release from stainless steel crowns as the study period progressed. This is consistent with the findings of the present study.

The decrease in nickel ion release observed on day 21 could be due to the material reaching a saturation point. After an initial period of higher ion release, the material may have released the majority of the available nickel ions, leading to a decrease in further release over time. This phenomenon has been observed in studies in which nickel release from stainless steel crowns decreased significantly after the first week, with levels dropping to zero after 2 weeks [9, 21].

The pH level plays a crucial role, as highlighted by Menek et al. [22] who evaluated Ni release from stainless steel crowns immersed in artificial saliva for 4 weeks across various pH values. They noted a consistent increase in the Ni release over time under highly acidic conditions (pH = 2.5). Conversely, in more neutral environments (pH = 6.25), the Ni release steadily decreased over time. The highest amount of Ni was released on the first day and decreased by the end of the study period. Overall, they observed that lower pH levels were correlated with increased Ni release. This can be explained by the formation of a passivation layer. When a stainless steel crown is initially placed in the mouth, it comes into contact with various corrosive agents in saliva, food, and drinks [10]. These agents can dissolve the surface of stainless steel, resulting in the release of Ni ions into the surrounding environment [23]. However, over time, a passivation layer develops on the surface of the stainless steel crown. This passivation layer is a thin oxidized film that acts as a protective barrier, shielding the underlying stainless steel from further corrosion [21]. As this passivation layer becomes thicker, it inhibits the release of Ni ions from the stainless steel crown. This phenomenon likely explains why the initial ion leaching activity was the highest and then decreased over time in the study samples.

In addition, anodic inhibitors are substances that reduce the corrosion rates of metals such as titanium by altering the electrochemical behavior at the metal surface. They typically work by promoting the formation of a more stable and protective oxide layer. In the case of titanium, anodic inhibitors help to strengthen the passive oxide layer, which is crucial for the material's corrosion resistance [24]. These inhibitors play crucial roles in maintaining the stability of dental alloys. However, if a patient consumes a diet rich in acidic foods and beverages, the anodic inhibitors present in the saliva may become depleted. This depletion can accelerate the corrosion rate of metal alloys used in dental applications. Dietary habits and oral hygiene practices can significantly influence the longevity and performance of dental restorations.

Furthermore, several other factors can influence the ion leaching activity in stainless steel crowns over time, including the composition of the stainless steel, manufacturing process, patient's diet, and oral hygiene habits [25]. The ion leaching activity of stainless steel crowns is a complex process that is influenced by multiple variables, including sterilization methods. These factors collectively determine the extent and rate of Ni ion release from stainless steel crowns in the oral environment.

The European Food Safety Authority (EFSA) has established a Tolerable Daily Intake (TDI) for nickel of 13 µg/kg body weight per day [26]. However, in individuals with nickel allergies, even small amounts of nickel can trigger allergic reactions. This occurs because their immune systems are sensitized to nickel, recognizing it as a harmful substance and causing hypersensitive responses upon exposure. Research shows that even minimal concentrations of nickel, as low as 0.5 ppm, can provoke reactions in sensitized individuals. These findings highlight that, for people with nickel allergies, even trace amounts of nickel can be problematic. Therefore, it is essential for these individuals to avoid exposure to nickel‐containing items to prevent allergic reactions [27].

It was found that 23.7% of pediatric patients in the USA tested positive for nickel allergy across various age groups [28], while the prevalence ranged from 8% to 10% in the European population [29]. Nickel allergy is a significant concern in pediatric populations, with a notable increase in prevalence linked to early and frequent nickel exposures. Limiting such exposures during childhood may help to reduce the risk of developing nickel allergy [30].

As a result, it is advisable not to reuse sterilized bands or crowns to minimize the nickel exposure. Furthermore, exploring alternative materials and limiting unnecessary nickel exposure can help to reduce the associated risks. Material choices are customized based on individual allergy risks, and may be the most effective strategy to address this issue while preserving the functional benefits of benefits of stainless steel crowns and space maintainers.

Healthcare providers should focus on identifying at risk patients through allergy testing and history taking before selecting materials. Additionally, manufacturers could explore modifications to reduce nickel ion release from existing materials, potentially minimizing the risk of sensitization.

Based on the results obtained, the null hypothesis is rejected in favor of the alternative hypothesis, which stated that autoclave sterilization significantly affects the release of Ni ions.

Overall, the findings of the present study suggest that sterilization procedures may affect the biocompatibility of stainless steel crowns and space maintainers, and careful consideration is needed when using these materials in pediatric dentistry, especially for patients with a potential nickel allergy.

Even though other materials and brands may have slight differences in composition, they could still exhibit similar characteristics. If the composition of other brands is identical to the one studied, it is likely that the results would be consistent, so testing additional brands of stainless steel crowns and space maintainers could offer greater insights and improve the generalizability of the findings.

The experiment was conducted in vitro, with samples immersed in artificial saliva and incubated at a controlled temperature. While this provides useful data on ion leaching, it may not fully replicate the complex biological environment inside the human mouth, where factors such as pH, temperature fluctuations, and mechanical stress can influence ion release.

Further studies could be conducted with long‐term clinical studies involving pediatric patients to evaluate the actual impact of sterilization on nickel ion release in a real‐world setting, to investigate the effects of various sterilization techniques (e.g., autoclave, dry heat, chemical sterilization, ultraviolet light) on the leaching of nickel and other metals from stainless steel crowns and space maintainers, and to extend the research to analyze the leaching of other metals such as chromium.

AUTHOR CONTRIBUTIONS

Conceptualization: N. Alessa, H. Alhamdan, S. Alrobaish; Formal analysis: N. Alessa, H. Alhamdan, S. Alrobaish, N. AlQahtani, N. Alarifi; Investigation: N. Alessa, N. AlQahtani, N. Alarifi, R. Alessa; Methodology: N. Alessa, H. Alhamdan, S. Alrobaish, N. AlQahtani, N. Alarifi, R. Alessa; Writing — original draft: H. Alhamdan, S. Alrobaish, N. AlQahtani, N. Alarifi; Writing — review & editing: N. Alessa, M. Almutairi, L. Alhowaish, M. Al‐Homaidhi, R. Alessa.

CONFLICT OF INTEREST STATEMENT

There are no conflicts of interest to declare.

Alessa N, Alhamdan H, Alrobaish S, AlQahtani N, Alarifi N, Almutairi M, et al. Effect of autoclave sterilization on nickel ion release from stainless steel crowns and space maintainer bands. Eur J Oral Sci. 2025;133:e70013. 10.1111/eos.70013