Reasons for Crown Failures in Primary Teeth: Systematic Review and Meta-Analysis

Introduction

Dental caries is a highly prevalent chronic disease affecting millions of children worldwide, leading to pain, infection, and difficulties in eating and speaking [1-3]. Untreated caries of the primary teeth may affect over 621 million children globally [1]. Early intervention with fillings is essential to manage caries, but in cases where the tooth structure is significantly compromised, pediatric dental crowns become necessary to preserve the tooth’s functionality and prevent premature loss [4].

Several studies have documented the usefulness and efficacy of materials such as stainless steel, zirconia, composite resin, and polycarbonate materials for fabricating pediatric dental crowns [5-8]. Despite notable advancements in dental materials and restorative techniques, various biological and technical factors persist in influencing the longevity and acceptability of pediatric dental crowns [8]. Biological factors such as the oral microbiome, the presence of cariogenic bacteria, and the immune response play significant roles in the success or failure of pediatric dental crowns [8]. Additionally, technical factors including crown preparation, cementation methods, and occlusal adjustment can impact the longevity and performance of these restorations [8]. Understanding these factors is crucial for clinicians to make informed decisions regarding treatment planning and material selection.

Chisini et al [8], in their systematic review, highlighted that composite resin crowns exhibited the lowest annual failure rate (1.7% to 12.9%), while stainless steel crowns had the highest success rate (96.1%). This report [8], published in 2018, identified secondary caries as the main reason for the failure of pediatric crowns and recommended an anticariogenic, health-promoting approach. Furthermore, several systematic reviews (published between 2021 and 2023) have also reported on the efficacy of individual restorative materials such as stainless steel, zirconia, and prefabricated crowns [9-11]. Since the publication of these systematic reviews, several randomized controlled trials (RCTs) have been published, warranting an updated systematic review to include these additional articles.

In the context of overall public health, ensuring the longevity and success of pediatric dental crowns not only addresses the immediate dental needs of children but also contributes to their overall health and well-being [12,13]. Improving the success rates of these restorations can reduce the burden of dental diseases and associated health care costs, enhancing the quality of life for affected children. On the premise of this background, this systematic review and meta-analysis aims to provide a comprehensive qualitative assessment of crown-retention rates and the impact of technical and biological factors on crown success and acceptability for primary teeth.

Methods

Overview

The protocol for this systematic review and meta-analysis [14] was registered with PROSPERO (CRD42023442266) and conducted in accordance with the MOOSE (Meta-Analysis of Observational Studies in Epidemiology) and PRISMA-P (Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols) guidelines [15,16].

Eligibility Criteria

RCTs and clinical studies (prospective and retrospective) evaluating crowns fabricated using stainless steel, zirconia, composite resin, compomer, and resin-modified glass ionomer cement (RMGIC) were included. The predefined inclusion criteria for this systematic review encompassed studies with an English abstract that evaluated crown restorations in pediatric patients aged 1-10 years, reporting crown-retention data, reasons for retentive loss, and technical and biological complications. Qualitative interviews, quasi-experimental studies, single-case studies, and series of single-case studies were excluded. Additionally, articles based on conference abstracts and dissertations were excluded.

Biological factors in this analysis included secondary caries and periodontal pathology (including the periodontal index, which assesses the severity of periodontal disease). Clinical parameters included the gingival index (evaluating gingival inflammation) and other periodontal health indicators. Technical factors included anatomic form, marginal adaptation, color match, surface texture, and wear on the opposing tooth. Success rates, if reported, were also included for the purposes of this systematic review and meta-analysis.

Data Sources and Search Strategy

Using a database-appropriate search strategy, electronic databases, including Cochrane, Embase, and PubMed (MEDLINE), were systematically searched to identify RCTs and clinical studies (prospective and retrospective) based on the eligibility criteria described above. The PICOS (Population, Intervention, Comparison, Outcomes, and Study design) format for this systematic review and meta-analysis was as follows:

• Population: children with primary tooth decay
• Intervention: crown restorations
• Comparators: materials such as stainless steel, zirconia, composite resin, compomer, and RMGIC
• Outcomes: crown retention, as well as technical and biological factors
• Study design: RCTs and clinical studies (prospective and retrospective)

Multimedia Appendix 1 presents the search syntax used for searching the electronic databases. The search strategy was considered adequate to reduce the risk of selection and detection bias.

Selection of Studies

The search results were imported into Zotero, and duplicates were removed to create a virtual library. Study selection was conducted in 2 stages. In the first stage, 2 independent reviewers (SL and KJ) screened the titles and abstracts of identified studies to determine their eligibility. In the second stage, the same 2 reviewers independently assessed the full-text articles of potentially relevant studies to confirm whether they met the eligibility criteria. Quality assessment of included RCTs and prospective and retrospective clinical studies was also independently performed by SL and KJ. Any disagreements at any stage, including study selection, quality assessment, and data extraction, were resolved through discussion. If consensus could not be reached, a third expert (DG) was consulted, with DG making the final decision.

Data Extraction

Data extraction was independently conducted by 2 reviewers (SL and KJ) using a standardized data extraction form. Extracted data included author details, year of publication, number of children, age, number of teeth, tooth type, crown material, dropout, follow-up duration, crown retention, and retention complications including biological and technical complications. Discrepancies in data extraction were resolved through discussion between the 2 reviewers. If consensus could not be reached, a third expert (DG) was consulted to make the final decision.

Risk-of-Bias Assessments

The Risk of Bias 2 tool developed by the Cochrane Collaboration was used to assess the quality of included RCTs [17]. Two independent reviewers evaluated various bias domains in RCTs, including randomization, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, and other sources of bias. After assessing each included RCT for risk-of-bias domains specified by the Risk of Bias 2 tool, an overall risk-of-bias judgment (low, some concerns, or high) was assigned to each trial [17]. A checklist proposed by Moga et al [18] was adapted for assessing the risk of bias in the included prospective and retrospective clinical studies. Only studies with a moderate or low risk of bias were included in this analysis. Furthermore, a funnel plot was generated to assess for any publication bias.

Meta-Analytic Approach

For quantitative analysis (meta-analysis), retention was defined as the number of crowns that were in situ, regardless of technical and biological complications. Total exposure time refers to the cumulative duration that all crowns remain in the mouth across all included studies. It is calculated by summing the duration each crown is in place for each study and then aggregating these times across all studies. Failure rates resulting from retentive complications were calculated by dividing the number of failures by the total exposure time. Exposure time for each included study was calculated by summing the exposure time for all restorations. A Poisson regression model was used to analyze the calculated rates. Proportions of crowns retained at 3 years and 5 years were estimated with an assumption of constant event rates. The Pearson goodness-of-fit test was performed to assess heterogeneity across studies and to determine whether a fixed-effects or random-effects model should be used. Since no statistically significant heterogeneity was detected in any of the individual groups and rates (P>.05), a fixed-effects Poisson regression model was used to estimate the parameters. A P value <.05 was considered significant. All analyses were performed using R statistical software (version 4.1.2; R Core Team).

Results

Selection of Studies and Risk-of-Bias Assessments

A total of 13 RCTs and 5 clinical studies (prospective and retrospective) were included in this systematic review [19-31]. Figure 1 presents the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flow diagram illustrating the study selection process. Tables 1 and 2 provide the risk-of-bias assessments for the included RCTs and clinical studies, respectively.

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Qualitative Analysis of the Included Studies

Table 3 summarizes the 13 included RCTs, presenting data on crown retention and retentive complications [19-31]. These RCTs were conducted between 2014 and 2022 and recruited 454 children who received a total of 1172 crowns [19-31]. The follow-up duration ranged from 6 to 36 (median 12, IQR 9) months, with a cumulative dropout rate of 10.6% (124/1172 crowns). Table 4 outlines the 5 included clinical studies, reporting data on crown retention and retentive complications [32-36]. These clinical studies were conducted between 2008 and 2020 and included data from 810 children, with a follow-up duration ranging from 12 to 24 (median 20.8, IQR 5) months [32-36].

Figure 2 provides a descriptive analysis of retention rates for crowns fabricated with different materials, as reported in the included RCTs [19-31]. Retention rates varied from 77.8% to 100%, from 80.6% to 100%, and from 92.3% to 100% for compomer, composite resin, and stainless steel crowns, respectively. Additionally, retention rates for strip crowns and zirconia crowns ranged from 78% to 100% and from 86.4% to 100%, respectively. Table 5 illustrates various retentive complications reported in the RCTs for different materials. Regarding clinical studies, debonding and loss of crown were reported for zirconia crowns, while crown fractures were reported for composite resin crowns (Table 4).

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Biological Complications Reported in the Included Studies

Table 6 depicts the distribution of biological complications reported in the included RCTs [18-30]. In the prospective and retrospective clinical studies, 8.3% (6/67) of children receiving compomer crowns and 8.8% (11/121) receiving composite resin crowns reported secondary caries [31-35]. Additionally, 3.3% (10/289) and 21.8% (80/368) of children with stainless steel crowns and zirconia crowns, respectively, in these studies exhibited gingival inflammation [32-36].

Technical Complications Reported in the Included Studies

Table 7 outlines the distribution of technical complications reported in the included RCTs [18-30]. In the prospective and retrospective clinical studies, 29% (19/67) of compomer crowns exhibited marginal adaptation complications, while stainless steel crowns did not demonstrate marginal adaptation issues [32-36]. No marginal discoloration or plaque retention complications were observed for compomer crowns. Among the 399 zirconia crowns evaluated, 4.8% (19/399) demonstrated shade mismatches, and 1% (4/399) exhibited marginal integrity complications [32-36].

Quantitative Analysis (Meta-Analysis) of the Included Studies

Table 8 presents the estimated 3-year and 5-year retention rates of crowns fabricated using various materials, as reported in the included RCTs [19-31].

The Pearson goodness-of-fit test was performed to assess heterogeneity across studies and to determine whether a fixed-effects or random-effects model should be used. Since no statistically significant heterogeneity was detected in any of the individual groups and rates (P>.05), a fixed-effects Poisson regression model was used to estimate the parameters.

Meta-analysis of pooled 5-year retention rates for different crown materials revealed the following retention rates: 88.90% for compomer crowns (Figure 3), 92.18% for composite resin crowns (Figure 4), 90.30% for RMGIC crowns (Figure 5), and 97.88% for stainless steel crowns (Figure 6). Additionally, strip crowns exhibited a retention rate of 83.48% (Figure 7), while zirconia crowns had a retention rate of 97.09% (Figure 8). Figure 9 presents the retention rates, irrespective of the material, and found a retention rate of 92.20% (95% CI 92.14%‐92.26%).

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Discussion

This systematic review examined data from RCTs and prospective and retrospective clinical studies to evaluate the performance of dental crowns for primary teeth. Key findings include significant biases in RCTs affecting the assessment of crown efficacy, variability in age distribution impacting retention rates, and varying complications across different crown materials. Despite these challenges, stainless steel crowns consistently demonstrated superior retention rates, while complications such as secondary caries and gingival inflammation varied among materials.

In our analysis, stainless steel crowns exhibited the highest retention rates, corroborating findings from previous studies [8,10], which also highlighted the superior performance of stainless steel crowns in terms of retention and durability. The study by Chisini et al [8] indicated an annual failure rate between 0% and 29.9%, with stainless steel crowns showing the highest success rate of 96.1%. In this study, the estimated 3-year and 5-year retention rates ranged from 75.7% to 100% and from 62.8% to 100%, respectively. Zirconia and composite resin crowns also demonstrated high retention rates, although they were associated with different types of complications. These results align with another study by Alzanbaqi et al [9], which noted improved gingival and periodontal health, excellent retention, and high fracture resistance with zirconia crowns for primary teeth. Regarding complications, strip and composite resin crowns showed higher instances of chipping and partial material loss, likely due to the inherent brittleness of these materials and their susceptibility to fracture under masticatory forces [37-39]. Crowns fabricated with composite resins and newer materials like compomer exhibited a higher incidence of fractures, possibly due to inadequate bonding strength and less favorable mechanical properties compared with stainless steel and zirconia [39,40]. Composite resin crowns also exhibited higher rates of gingival inflammation, likely due to plaque accumulation and potential marginal leakage. Conversely, zirconia crowns showed minimal gingival inflammation, attributed to their smooth surface and biocompatibility [40-42]. Secondary caries was a significant issue across all materials but was particularly prevalent in crowns with poor marginal adaptation, emphasizing the need for precise placement and proper maintenance to mitigate caries risk [42,43]. While the findings demonstrate the efficacy of stainless steel crowns, material choice should consider the specific needs and conditions of pediatric patients. Clinicians must weigh the benefits of high retention rates against the potential for biological complications. The included RCTs displayed a notable prevalence of allocation concealment, performance bias, and detection bias (Table 1) [19-31]. These biases, inherent to the nature of interventions that often preclude optimal double blinding, necessitate careful consideration in assessing the risk of bias for studies involving crowns for primary teeth. These findings align with a previous systematic review by Chisini et al [8], which also identified a high risk of bias. The assessment of publication bias in our meta-analysis revealed noteworthy findings (Figure 10). The Egger test for zirconia crowns suggested potential publication bias. In contrast, for stainless steel, composite resin, and strip crowns, the limited number of studies precluded robust evaluation of small study effects. Thus, caution is warranted in interpreting these outcomes due to the small sample size. The significant results indicate asymmetry in the funnel plot, potentially influencing effect size estimates.

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The age distribution of the study population is often not reported, posing challenges in determining optimal retention rates for a smooth transition from primary to permanent teeth, especially in younger age groups. The age of children in the RCTs and prospective and retrospective clinical studies ranged from 1 to 10 years and from 3 to 10 years, respectively [19-31]. The reported crown-retention rates for different age groups suggest potential nuances in the transition from primary to permanent teeth, underscoring the need for future studies to provide detailed age-distribution data. Another major limitation of this review is the varying sample sizes across studies, which could influence the generalizability of the results. However, the application of Poisson regression allowed for the adjustment of these differences, providing a more accurate comparison of retention rates [44]. Despite these limitations, this review provides a comprehensive analysis of crown retention and associated complications in primary teeth, offering valuable insights for evidence-based clinical decision-making.

Future studies should aim to include larger sample sizes and longer follow-up periods to validate the findings of this review. Additionally, research should focus on the development and evaluation of new materials and techniques that could enhance crown retention and reduce complications. Investigating advanced fabrication methods such as 3D printing and computer-aided design and computer-aided manufacturing technology holds promise for improving the fit and longevity of dental crowns.

In conclusion, while stainless steel crowns remain a gold standard for primary teeth restorations due to their high retention rates and durability, the choice of material should be tailored to individual patient needs, considering both mechanical properties and potential biological complications. Ongoing research and clinical advancements will continue to refine these treatment modalities, ultimately improving outcomes for pediatric dental patients.