International Journal of Clinical Pediatric Dentistry
Volume 16 | Issue S-2 | Year 2023

Evaluation of Streptococcus mutans Colonization and Oral Hygiene Status in Primary Molars Restored with Two Different Crowns: A Randomized Clinical Trial

JA Amilia Elizabeth1https://orcid.org/0000-0002-8229-2654, Hemalatha Ramkumar2, Shankar Paulindraraj3https://orcid.org/0000-0003-4524-6875, Senthil Dakshinamurthy4https://orcid.org/0000-0003-0262-2703, Trophimus G Jayakaran5https://orcid.org/0000-0002-8285-7424, Rajkumar Manoharan6https://orcid.org/0000-0003-1537-1579

1Department of Pediatric and Preventive Dentistry, Meenakshi Ammal Dental College and Hospital, Maduravoyal, Chennai, Tamil Nadu, India

2–6Department of Pediatric and Preventive Dentistry, SRM Dental College and Hospital, Ramapuram, Chennai, Tamil Nadu, India

Corresponding Author: JA Amilia Elizabeth, Department of Pediatric and Preventive Dentistry, Meenakshi Ammal Dental College and Hospital, Maduravoyal, Chennai, Tamil Nadu, India, Phone: +91 9566125909, e-mail: amilia2810@gmail.com


Background: Full-coverage restorations are commonly employed choice of treatment in treating multisurface carious lesions in primary teeth.

Aim: To assess the amount of Streptococcus mutans (S. mutans) colonization and oral hygiene status in deciduous molars restored with preformed zirconia and stainless steel crowns (SSC).

Materials and methods: A total of 21 children aged between 4 and 7 years with bilateral carious primary molars were randomly divided into two groups of zirconia and SSC. Plaque collection was done using sterile swabs at baseline and 1-month intervals. S. mutans was cultured on mitis salivarius-bacitracin agar (MSBA). The microorganisms were then counted and expressed as colony-forming units. The plaque and gingival indices were recorded during the follow-up visits. All data were tabulated and subjected to statistical analysis, with the level of significance set at 5%.

Results: A total of 21 children with 42 teeth were randomized into group I—SSC and group II—zirconia crowns using the split-mouth technique, respectively. The zirconia group showed a statistically significant reduction in the adhesion of S. mutans (p < 0.001). On comparing the plaque and gingival indices between the groups, plaque index (PI) and gingival index (GI) scores were significantly low in the zirconia group as compared with the SSC, with a mean difference of 0.08 at 3 months in group II (p < 0.001).

Conclusion: Streptococcus mutans (S. mutans) adhesion to zirconia crowns was significantly less when compared with SSC, with better gingival health and oral hygiene.

How to cite this article: Elizabeth JAA, Ramkumar H, Paulindraraj S, et al. Evaluation of Streptococcus mutans Colonization and Oral Hygiene Status in Primary Molars Restored with Two Different Crowns: A Randomized Clinical Trial. Int J Clin Pediatr Dent 2023;16(S-2):S183–S189.

Source of support: Nil

Conflict of interest: None

Keywords: Colony-forming units, Microbial count, Stainless steel crowns, Streptococcus mutans, Zirconia crowns


Dental caries is an oral disease most ubiquitous among children, characterized by a series of demineralization and remineralization cycles. If left untreated, it can progress to cavitated lesions.1-3 Usually, the familiar method of managing multi-surface carious lesions in primary teeth includes full-coverage restoration with stainless steel crowns (SSC).4 Humphrey (1950) introduced the prefabricated SSC into the field of pediatric dentistry.5 The advantages of SSC over other commercially available crowns are longevity, less technique sensitivity, low failure rate, and cost-effectiveness.6 In spite of these benefits, one of the known disadvantages of SSCs is esthetics due to their metallic form, which is not well received by parents and patients.7 Due to this drawback, the demand for esthetically pleasing full-coverage restorations increased. Therefore, the clinicians searched for a crown that would combine the characteristics of SSCs and still be esthetically acceptable. As an outcome of this research, “EZ-Pedo, Loomis, California, United States of America” in the year 2008 launched the first prefabricated pediatric zirconia crown, which exhibited excellent esthetics and enhanced clinical properties.8 These crowns are made up of monolithic zirconia reinforced with alumina and yttria for high flexural strength and hardness to withstand massive masticatory loads.9 In addition, these crowns have less volume and provide enormous strength.10

When a primary tooth is restored with a full-coverage restoration, it establishes a fresh environment conducive to microbial adhesion.11 Streptococcus mutans (S. mutans) is considered to be the main colonizer and primary etiological factor in caries progression as it stimulates the production of glucans from the food substrates.12 In the literature, there are studies that reported S. mutans could be isolated from plaque samples.13,14 Therefore, the long-term success of the restored tooth is directly influenced by the adhesion of S. mutans to the preformed crowns.

In literature, there are studies that compared the periodontal health of children after restoring primary molars with SSC and zirconia crowns and have concluded that zirconia crowns have better gingival health when compared to SSC.15,16 Till date, no research has been conducted to compare the colonization of S. mutans on the surfaces of prefabricated zirconia and SSCs. Hence, this clinical trial was conducted to assess the adhesion of S. mutans on prefabricated SSCs and zirconia crowns under clinical conditions.


Study Design and Setting

To assess the level of S. mutans colonization and oral hygiene status in primary molars of 4–7-year-old children restored with prefabricated zirconia and SSC, a split-mouth, randomized control clinical study was designed with an allocation ratio of 1:1. The study was carried out among the pediatric patients visiting the Department of Pediatric and Preventive Dentistry, SRM Dental College and Hospital, Ramapuram. Prior to the commencement of the procedure, the parents of the participants were provided with detailed information about the study, and informed consent was obtained from them.

Ethical Clearance and Trial Registration

The institutional review board granted ethical clearance approval and assigned the approval number (SRMDC/IRB/2019/MDS No. 801A). Additionally, the trial registration was done under Clinical Trials Registry—India (CTRI)—CTRI/2020/09/027729.

Sample Size Calculation

The sample size derived from the study by AlShaibah et al. in the year 201211 was calculated using G*Power software, with 95% power. Considering a 20% attrition rate and maintaining α-error at 5% and β-error at 20%, the final sample size was determined as 21 individuals, with 42 teeth in each group.

Source of Sample

The source of sample comprised of children aged 4–7 years who visited the Department of Pediatric and Preventive Dentistry, SRM Dental College and Hospital, Ramapuram.

Eligibility Criteria

Participants were recruited based on the following criteria:

Inclusion Criteria

  • Participants aged between 4 and 7 years.

  • Pulp therapy treated primary molars on the contralateral side, requiring a full coverage restoration (split-mouth technique).

  • No history of any antibiotic consumption 2 weeks before the procedure.

  • No history of fluoride application 2 weeks before the procedure.

Exclusion Criteria

  • Special children with physical disabilities or emotional disturbances.

  • Children diagnosed with systemic diseases.

  • Grossly destructed teeth with nonrestorable crown structure.

  • Primary teeth with preshedding or pathological tooth mobility.

Recruitment of Trial Participants

A total of 100 children were initially assessed for eligibility, out of which 79 declined to participate in the study. The study procedure, along with its details, was thoroughly explained to the remaining 21 children and their parents, following which written informed consent was obtained. For the split-mouth study design, the oral cavities of the 21 study subjects were randomly assigned into two groups, each consisting of 21 teeth, to receive the respective interventions. The randomization was performed using a table of random numbers generated via the website www.randomizer.org. The allocation of intervention to the two groups was determined through a lottery method. In the present study, allocation concealment was ensured by employing sequentially numbered opaque sealed envelopes.


A single-trained pediatric dentist placed both the SSC and the zirconia crowns to reduce operator variability. The principal investigator or the participant could not be blinded due to the nature of the intervention being provided. The microbiologist, as an outcome assessor, was blinded by coding the data sheets.


The procedure of the study was designed as a split-mouth, randomized controlled trial, where the patient, after pulp therapy, was subjected to SSC [Korean Innovative Dental System crown, Shinhung Co. Ltd., South Korea] on one side and zirconia crown [NuSmile Zirconia Reinforced (ZR) Primary Crowns, Houston, Texas, United States of America] on the contralateral side as their full coverage restorations. The duration between these crown placements in an individual varied from 7 to 10 days. Plaque samples were collected from the gingival crevices, buccal mucosa, and occlusal surfaces of the teeth to be treated using sterile cotton swabs initially and at the end of the 1-month study period from the respective crown surfaces. After collecting the samples, they were vortexed for about 15 seconds and then diluted using the Ringer’s solution. A 0.1 mL of the diluted sample was extracted and inoculated onto a plate of mitis salivarius-bacitracin agar (MSBA), a selective growth media for cultivating S. mutans. The inoculated plates were then incubated for 2 days at room temperature in 5–10% of carbon dioxide. Using a digital colony counter, the colony-forming units (CFU/mL) of S. mutans were determined (Figs 1 and 2).

Fig. 1: Preoperative clinical photo and colony-forming unit of S. mutans

Fig. 2: A 1-month postoperative clinical photo and colony-forming unit of S. mutans

To assess the gingival health, gingival index (GI) and plaque index (PI) were recorded according to Silness and Loe,17 before the commencement of the procedure, at 1 and 3-month follow-ups for each individual.

Statistical Analysis

The statistical analysis was done using the Statistical Package for the Social Sciences (SPSS) software (SPSS software for Windows; version 22.0 Armonk, New York, United States of America). Intragroup comparisons of S. mutans were done using paired t-test, while the Intergroup comparison of S. mutans was done using an unpaired t-test. Intragroup comparison of GI and PI was assessed by repeated measures analysis of variance, and intergroup assessment of GI and PI was done by unpaired test. The significance level was set at 5%.


In this study, 21 subjects were enrolled, comprising 10 males and 11 females, with mean ages of 5.86 ± 1.014, respectively. The study population demonstrated a balanced representation of age and gender (Table 1 and Flowchart 1). Figure 3 illustrates the graphical distribution based on gender.

Table 1: Gender and age distribution of patients
Total (N) Male Female Mean age
21 10 (47.6%) 11 (52.4%) 5.86

Flowchart 1: Consolidated Standards of Reporting Trials (CONSORT) flowchart indicating the enrollment, allocation, and follow-up activities of the present study

Fig. 3: Graphical representation of gender-based distribution at baseline

The adhesion of S. mutans showed a significant decrease from baseline to 1 month in both stainless steel and zirconia groups. Statistically significant differences were seen between the baseline and 1-month interval values in both groups (p ≤ 0.01) (Table 2).

Table 2: Intragroup comparison of S. mutans colonization within stainless steel and zirconia crowns
Group Time period Mean Standard deviation Mean difference t-value p-value Lower bound* Upper bound*
Stainless steel Baseline 91.24 77.84 75.24 5.68 0.01 47.6 102.87
1 month 16 24.09
Zirconia crowns Baseline 92.52 83.21 88.57 5.16 0.01 52.79 124.35
1 month 3.95 9.68

*, 95% Confidence Interval of the mean difference

Table 3 represents the intergroup comparison between the groups based on colony-forming units at 1-month intervals. The baseline values were similar between both groups. The zirconia crowns showed significantly less colonization of S. mutans when compared with SSCs (p ≤ 0.04). Figure 4 shows the graphical representation of S. mutans colony-forming units at both baseline and 1-month intervals.

Table 3: Intergroup comparison of S. mutans colonization among the groups at 1-month interval
Group Baseline Mean ± standard deviation Mean difference p-value 1-month mean ± standard deviation Mean difference p-value
Group I—stainless steel 91.24 ± 77.84 –1.28 0.95 16 ± 24.09 12.05 0.04
Group II—zirconia 92.52 ± 83.21 3.95 ± 9.68

Fig. 4: Graph showing colony-forming units of stainless steel and zirconia crowns at baseline and 1-month interv

On comparing the PI and GI scores at baseline, 1-month, and 3-month intervals within SSCs and zirconia crowns, statistically improved gingival health was observed from baseline to 1-month and 3-month intervals in both the groups (Table 4).

Table 4: Intragroup comparison of plaque and GI scores within SSCs and zirconia crown
Group Index Time interval Mean Standard deviation F-value p-value
Stainless steel Plaque Baseline 1.58 0.55 58.76 0.01
1 month 1.03 0.52
3 months 0.87 0.52
Gingival Baseline 0.66 0.39 53.11 0.01
1 month 0.40 0.30
3 months 0.17 0.18
Zirconia Plaque Baseline 1.58 0.55 131.09 0.01
1 month 0.78 0.41
3 months 0.30 0.37
Gingival Baseline 0.66 0.39 52.69 0.01
1 month 0.38 0.31
3 months 0.10 0.14

The mean PI scores at 1-month and 3-month intervals in SSCs were 1.03 ± 0.52 and 0.87 ± 0.52, and in zirconia crowns were 0.78 ± 0.41 and 0.30 ± 0.37, respectively. The Zirconia group showed statistically significantly lower PI scores at 3-month intervals when compared with the stainless steel group (p ≤ 0.01). The mean GI scores at 1-month and 3-month intervals in SSCs were 0.40 ± 0.30 and 0.17 ± 0.18, and zirconia crowns were 0.38 ± 0.31 and 0.10 ± 0.14, respectively, and the values were statistically insignificant (Table 5). Figure 5 shows the graphical representation of PI and GI scores at baseline, 1-month, and 3-month intervals.

Table 5: Intergroup comparison of plaque and GI scores among stainless steel and zirconia crowns
Index Time interval Group Mean Standard deviation Mean difference t-value p-value Lower bound* Upper bound*
Plaque 1 month SSC 1.03 0.52 0.25 1.74 0.90 –0.04 –0.55
Zirconia 0.78 0.41
3 months SSC 0.87 0.52 0.56 4.06 0.01 0.28 0.85
Zirconia 0.30 0.37
Gingival 1 month SSC 0.40 0.30 0.02 0.25 0.80 –0.17 0.21
Zirconia 0.38 0.31
3 months SSC 0.17 0.18 0.08 1.35 0.18 –0.03 0.16
Zirconia 0.10 0.14

*, 95% Confidence Interval of the mean difference

Fig. 5: Graph showing PI and GI scores of stainless-steel crowns and zirconia crowns at various points


The oral cavity is a complex environment that collects a variety of products, including saliva, food, oral biofilms, and their metabolites.17 Oral biofilms are bacterial communities that adhere to teeth, dental materials, and oral soft and hard tissues, which are reasoned to be the primary cause of dental caries and gingival inflammation in children.18 Among the various oral microorganisms, S. mutans is commonly associated with dental caries. It orchestrates the creation of cariogenic biofilms by directing the building of a matrix rich in insoluble exopolysaccharides, primarily α 1,3-glucans.19 Thus, the colonization of S. mutans over these full-coverage restorations was assessed in this present study.

To standardize the clinical situation, patients were carefully selected based on the eligibility criteria. Children included in the present study were devoid of systemic diseases as they might act as a confounder for bacterial colonization and can have a deteriorating effect on gingival health. In the present study, children with no previous history of any antibiotic intake for at least 14 days prior to crown preparation and crown placement were included because regular usage of broad-spectrum antibiotics disrupts the normal organization of bacterial flora, and the participants did not undergo any professional fluoride application up to 2 weeks before the placement of the crowns, as studies have shown fluoride to have no effect on the microbiota of plaque, although higher fluoride levels may eliminate vulnerable species and alter the plaque ecology.20,21

This study adopted a split-mouth design since both SSC and zirconia crowns would be exposed to a similar oral environment, making it easier to compare the oral hygiene practices of the participants. In this study, tooth preparation was done using the conventional technique for SSC,22 and for zirconia crowns, the operator followed the instructions of the manufacturer. The crown preparation and crown placement were done by a single trained pediatric dentist, and therefore, the probability of operator variability was significantly reduced.

In this study, the S. mutans was obtained by isolating it with a sterile cotton swab from the gingival crevices and occlusal and buccal surfaces of the teeth to be treated as they harbor more microorganisms. Subsequently, the swabs were transferred into a tube containing 1 mL of sterile isotonic saline (0.15 M sodium chloride solution), in accordance with Motisuki et al.,23 who claimed there was a statistically insignificant difference between pooled plaque technique and collected swab samples for the detection of the S. mutans organism. Therefore, swab collection was deemed as the optimal method for the collection and detection of S. mutans, as it integrates both saliva and plaque samples, enabling a unified plating procedure.

For the growth of S. mutans, a selective medium of MSBA was selected. This media allows the growth of S. mutans with maximum inhibition of the other streptococcal flora.24 According to Scheuerman et al., rough crown surfaces exhibit a faster colonization rate compared to smooth surfaces. This is attributed to the increased surface area available for bacterial attachment, and the initial accumulation tends to occur predominantly in the bottom areas of roughness, where there is protection from shear forces.25 This correlates to the fact that various surface properties of the crown determine microbial adhesion. On the contrary, Myers et al. exemplified in their study that plaque easily accumulates and deposits over the SSCs, irrespective of the polishing procedure used. Therefore, the author suggested that SSCs should be meticulously placed to avoid any gingival irritation to minimize the accumulation of bacterial plaque.26 Similarly, in the present study, SSC exhibited a significantly increased amount of colonization of S. mutans when compared with zirconia. This could be due to the roughness created on the crown surface by cutting, crimping, and trimming of the SSC, which acts as an important feature for biofilm formation, whereas the preformed zirconia crown was not trimmed/adjusted, and this protects the surface finish, preventing biofilm coating.

Prabhu et al. quoted that plaque accumulation on SSC-restored teeth was comparatively lesser than on the control teeth with no full coverage restoration.27 This finding aligns with the results of the current study, wherein the baseline scores of PI and GI were immensely high before the placement of both the full coverage restorations and drastically reduced after their placement. The results of the present study shows the scores of plaque and gingival indices were relatively higher in SSC groups, indicating poorer gingival health and increased plaque accumulation when compared to zirconia crowns at 1 and 3-month follow-ups.

These findings are consistent with Taran and Kaya15 and Abdulhadi et al.,16 who agreed that zirconia crowns had better gingival health and less plaque build-up performance than SSCs. These results are due to the fact that when the tooth comes in contact with saliva in the oral cavity, the enamel gets coated with saliva, and its vulnerability to attract bacterial colonizers increases.28 Similarly, Walia et al. observed a significant reduction in plaque formation on the prefabricated zirconia crowns for its well-glazed and polished surface.4 Consistent with these findings, the present study revealed lower biofilm formation and lesser plaque accumulation over zirconia surfaces compared to SSCs.


The recently introduced pediatric zirconia crowns are esthetic full-coverage restorations that offer esthetics with better gingival health compared to SSC.

Zirconia crowns possess a highly polished surface that minimizes surface roughness, thereby decreasing the adhesion of S. mutans when compared with SSC.


The strength of the present study includes the following:

The limitations of our study includes:


Within the limitations of this randomized clinical trial, the following conclusions could be drawn:


Amilia Elizabeth https://orcid.org/0000-0002-8229-2654

Shankar Paulindraraj https://orcid.org/0000-0003-4524-6875

Senthil Dakshinamurthy https://orcid.org/0000-0003-0262-2703

Trophimus G Jayakaran https://orcid.org/0000-0002-8285-7424

Rajkumar Manoharan https://orcid.org/0000-0003-1537-1579


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