Marginal and internal adaptation of esthetic crowns for primary molars: an in-vitro study

Abstract

Purpose

To assess the marginal and internal adaptation of the prefabricated esthetic crowns for primary molars.

Methods

Eighty human primary mandibular second molars were distributed into four groups: stainless steel crowns (SS), preveneered stainless steel crowns (VSS), prefabricated zirconia crowns (PZ), and BioFlx crowns (BF). After cementation, ten samples per group were subjected to 5000 thermal cycles of 5 ± 0.5 °C to 55 ± 0.5 °C to simulate six months of clinical service, followed by marginal- and internal adaptation assessment (µm) using the cross-sectioning method of all samples. Differences between the groups’ internal and marginal adaptation were tested using analysis of variance (ANOVA), followed by Tukey’s post hoc test, while differences within each group before and after thermocycling were tested using Student’s t-test at P = 0.05 using SPSS version 25 software.

Results

ANOVA tests revealed a significant difference in marginal and internal adaptation among the test groups. Before thermocycling, SS crowns demonstrated the highest marginal (33.5 ± 3.87 μm) and internal (37.12 ± 3.11 μm) adaptation values, whereas PZ showed the lesser values (37.63 ± 2.56 μm and 42.31 ± 1.45 μm respectively). Both BF and VSS presented intermediate performance. Following thermocycling, all crowns exhibited increased gap values, with SS maintaining the most favorable marginal (36.07 ± 1.79 μm) and internal (38.75 ± 1.93 μm) adaptation, while PZ and BF showed the greater deterioration of marginal (44.79 ± 1.75 μm and 45.78 ± 1.99 μm) and internal adaptation (45.37 ± 1.75 μm and 46.22 ± 1.69 μm) respectively. VSS remained comparable to SS in both marginal and internal adaptation.

Conclusions

Stainless steel crowns demonstrated the most favorable and stable marginal and internal adaptation, while the greatest discrepancies after aging were detected in prefabricated zirconia and BioFlx crowns.

Introduction

Prefabricated full coverage crowns are commonly used in Pediatric Dentistry with a high rate of success to restore caries-affected primary molars as well as non-caries affected developmental enamel defects as hypoplasia and hypomineralization conditions with a higher clinical long-term success for restoring multi-surface cavitated defects in primary molars compared to intra-coronal restorations [1, 2].

Stainless steel crowns (SSCs) are considered the “gold standard” restoration for primary teeth, offering enhanced durability, reduced recurrent caries, fewer pulp-related complications, and preservation of arch length integrity [3]. They also provide a high success rate when used to restore root canal-treated permanent molars in children [4]. However, their metallic appearance contributes to reduced parental and patients’ acceptance [5].

Increasing esthetic expectations have driven the introduction of tooth colored prefabricated crown systems for primary molars [6]. Pre-veneered stainless-steel crowns were developed as an aesthetic alternative to traditional SSCs, however; several limitations persist [7]. These crowns often require increased tooth reduction due to their added bulk at the buccal surface. In addition, the esthetic facings of these crowns are prone to chipping or complete detachment over time within a few years of placement, particularly in patients with deep overbite [8]. Such failures not only compromise the appearance but may also affect the longevity and functionality of the restoration.

Prefabricated zirconia crowns have become a widely adopted esthetic alternative, offering enhanced appearance and biocompatibility [9]. However, their rigid structure necessitates extensive tooth preparation to achieve passive crown seating, and concerns persist regarding time efficiency and cost-effectiveness [10]. Their highly glazed surface reduces the risk of gingival inflammation [11]. Their prefabricated design may compromise the marginal adaptation, necessitating additional circumferential axial reduction (1.0–1.5 mm) and careful cementation [12–14].

The emergence of BioFlx crowns provided an advancement in pediatric dental restorations via offering a blend between an esthetically appealing full coverage crown with an acceptable degree of flexibility to enhance their marginal adaptability and withstand the applied masticatory load. Tooth preparation to receive BioFlx crowns resembles that for SSCs, widely familiar to dental professionals [15]. The crowns are constructed from hybrid polymer resin and are devoid of metal or bisphenol A-glycidyl methacrylate, thus; enhancing their suitability for use in young children [16, 17] In a 12-month comparative clinical study, BioFlx crowns presented with a high rate of clinical and parental satisfaction as compared to zirconia or stainless-steel crowns. Thus, they are considered an economical, esthetic, full-coverage alternative for primary teeth [18]. The BioFlx crowns showed minimal tooth reduction, good color stability, and less wear of the opposing natural teeth compared to zirconia [19]. Evidence regarding the internal and marginal adaptation of BioFlx crowns is still limited, leaving uncertainty about their comparative clinical performance.

Marginal and internal adaptations are critical factors that determine the success of the restoration. The marginal misfit was reported to be related to cement dissolution, microleakage, plaque accumulation, secondary caries, and periodontal disease. Poor marginal adaptation of the crown will result in a thick cement film, exposing the luting material to the oral environment and promoting its dissolution over time [20]. The internal misfit was frequently associated with poor mechanical retention and reduced fracture strength [21]. The adaptation may vary depending on the restorative material or production method of the restoration [22]. The assessment of marginal and internal adaption of esthetic crowns for primary teeth could provide an insight on the clinical concern of its effect on crown retention, long-term pulpal health, gingival condition, and the eventual impact on the success of the restorative therapy for children.

Only few comparative studies have assessed the marginal and internal adaptation of different types of prefabricated full-coverage crowns for primary molars, particularly the adaptation stability of BioFlx crowns following thermocycling aging. Thereby, this study aimed to compare the marginal and internal adaptation of prefabricated zirconia crowns, preveneered stainless-steel crowns, and BioFlx crowns for primary molars compared to SSCs, both before and after thermocycling. The null hypothesis of this study stated that no significant differences would exist in marginal or internal adaptation among the tested crown systems for primary molars, before and after thermocycling challenge.

Materials and methods

Sample size calculation

For sample size calculation to determine the presence of statistical significance between the study groups, using a study power of 80% at alpha significance level of 5% (0.05), with about 54 μm mean value of zirconia crowns marginal discrepancy compared to 47 ± 5 μm of stainless steel crowns [23], a minimum of eight teeth per group was used. The ClinCalc tool (ClinCalc LLC, Chicago, Illinois, USA) was used for sample size calculation.

Study design

The tested prefabricated crowns for primary teeth are as follows: stainless-steel crowns (SS), preveneered stainless steel crowns (VSS), prefabricated zirconia crowns (PZ), and BioFlx crowns (BF). All tested crowns were manufactured by NuSmile^® Ltd. (Houston, Texas, USA).

A total of 80 primary mandibular second molars were included in this study. The teeth included in this study were obtained following exfoliation or extracted by clinicians due to prolonged retention beyond their expected exfoliation time, where no teeth were extracted exclusively for research purposes. Teeth with sufficient root length (about one-third) to ensure stable retention in acrylic resin blocks, and suitable for accommodating size four crowns with mesiodistal dimension of 8.7–9.2 mm as determined using a graduated periodontal probe, were selected for inclusion. All teeth were thoroughly cleaned and carefully inspected to ensure absence of dental caries, restorations or any developmental defect. The collected teeth were stored in a 0.2% Thymol solution at room temperature until usage within 90 days to preserve structural integrity of specimens [24]. Each tooth was vertically placed to the level of the cemento-enamel junction in a plastic mold (two cm height x 1.5 cm diameter) filled with auto-cured acrylic resin (Acrostone Dental Factory, Industrial Zone, Salam City, Cairo, A.R.E-WHW Plastic, England) and left for curing overnight. The specimens were numbered and randomly allocated to one of the test groups (n = 20 crowns per group) using a computer-generated random number function in Microsoft Excel (RAND). The teeth were prepared by the same pediatric dentist using a high-speed 169 L taper carbide bur under water cooling.

Each specimen in the SS, VSS and BF groups received 0.8–1.0 mm of proximal surface reduction, creating a feather-edge finish line with slight occlusal convergence and 1.0 to 1.5 mm occlusal clearance (Fig. 1). The active cutting head of 169 L bur was used as a reference guide. The preparation was subsequently verified with a periodontal probe and confirmed by the passive seating of the corresponding crown. A greater depth of reduction, from 1.5 to 2 mm, at the buccal surface was added to samples receiving VSS crowns with removal of the buccal cervical bulge as a design requirement of VSS crowns rather a routine preparation depth for all crown types in primary teeth. The SS crowns and the lingual aspect of the VSS received a uniform crimping of the cervical margin using a standardized crimping plier, to enhance marginal adaptation simulating clinical practice steps. For the PZ group, a 1.0–1.5 mm circumferential axial surface reduction, with rounded lines and point angles was conducted to facilitate passive fit of the crowns (Fig. 2).

Fig. 1.

Digital image depicting specimen preparation specifications to accommodate stainless steel (SS), veneered stainless steel (VSS), and biologic functional (BF) crowns. The illustration depicts the required occlusal reduction and proximal wall reduction details

Fig. 2.

Digital image depicting specimen preparation specifications to accommodate prefabricated zirconia (PZ) crowns. The illustration depicts the required occlusal reduction and circumferential axial walls reduction details

Each crown was cemented to its corresponding prepared tooth using glass-ionomer cement (Medicem, Promedica, Cuxhaven, Niedersachsen, Germany), following the manufacturer’s instructions and applied to the fitting surfaces of the crowns. The crowns were accurately placed on their corresponding prepared teeth and allowed to set under a static finger load applied for five minutes, followed by removal of the excess cement. All specimens were stored in distilled water for 24 h at 37 degrees Celsius (°C).

Thermocycling

Ten specimens per group were subjected to thermocycling challenge using an automatic thermocycling device (Thermocycler 1100/1200, SD Mechatronik, Feldkirchen- Westerham, Bavaria, Germany). The samples were placed in a basket that oscillates between hot and cold tanks at five °C ± 0.5 °C and 55 °C ± 0.5 °C, respectively, for 5000 cycles to mimic six months of clinical aging [25]. The device dwell time was adjusted to 30 s per cycle via the device control panel to ensure sufficient heat transfer to the tested specimen.

Marginal adaptation assessment

For the assessment of marginal adaptation, each specimen was photographed using Digital microscope with a built-in camera (Nikon SMZ745T stereo microscope, Japan) connected with an IBM compatible personal computer at a fixed magnification of ×25. A digital image analysis system (Omnimet image analysis software, Buehler, USA) was used to evaluate the marginal discrepancy width. Shots of the margins were taken and for each surface of the examined specimen, four equidistant landmarks along the cervical circumference (mesial, buccal, distal, and lingual) were assessed and the average values per surface were calculated.

Internal adaptation assessment

The internal discrepancies of the specimens were measured using the cross-section technique. For specimen stabilization during sectioning, each specimen was inserted (from its coronal side) into a plastic cylindrical mold poured with auto-polymerizing acrylic resin (Acrostone Dental Factory, Industrial Zone, Salam City, Cairo, A.R.E-WHW Plastic, England) and left for curing overnight. The specimens were carefully separated from their plastic molds, and each specimen was mounted into a fixture compartment. Then, the embedded crowns were vertically sectioned in bucco-lingual, mesio-distal, mesio-buccal to disto-lingual and disto-buccal to mesio-lingual directions (Fig. 3), to provide a representation of the crown internal adaptation across different orientations. Specimens’ sectioning was carried using a Buehler IsoMetâ 4000 linear precision saw (Buehler Ltd., Lake Bluff, IL, USA). A digital photograph of each section interface was obtained under a stereomicroscope (Nikon SMZ745T stereo microscope, Japan) at a magnification of ×25. The discrepancy width measurements were conducted using a digital image analysis system (Omnimet image analysis software, Buehler, USA).

Fig. 3.

Digital image illustrating the directions of sectioning in; bucco-lingual, mesio-distal, from mesio-buccal to disto-lingual and from disto-buccal to mesio-lingual direction. Specimen aspects: Buccal (B), Lingual (L), Mesial (M), and Distal (D)

From each obtained section, four assessment points were evaluated; Marginal discrepancy (MD): reflecting the average measurement of the horizontal distance at the cervical 1/3 of the axial wall from the prepared tooth surface to the internal surface of the crown, middle-axial (MA): denoting the average of horizontal measurement at the middle 1/3 of the axial wall, axial-occlusal (AO): the distance at the point of intersection between a line parallel to the axial wall and the occlusal plane, and occlusal discrepancy (OD): indicating the average of vertical discrepancy detected toward the occlusal surface (Fig. 4). For each surface, the four measurements were taken, and the average values were calculated.

Fig. 4.

Digital light microscope image depicting specimen’s section for internal adaptation assessment; MD: marginal discrepancy, MA: middle-axial, AO: axial-occlusal, and OD: occlusal discrepancy. Arrows pointing measurement points (magnification ×25)

All measurements were carried by a single calibrated examiner who was blinded to the sample group allocation.

Statistical analysis

The mean (± SD) marginal and internal discrepancy values were statistically analyzed using the Statistical Package for Social Sciences (SPSS) software version 25 (SPSS Inc., Chicago IL, USA) at significance level of 0.05. The Shapiro–Wilk test was applied to assess the normality of data distribution. Intra-examiner reliability was evaluated using the Intraclass Correlation Coefficient (ICC), with values greater than 0.90 considered indicative of excellent agreement.

One-way ANOVA test was used to test for statistical significance difference of marginal and internal discrepancy at different assessment points of all surfaces, followed by Tukey’s post hoc test. Student t-test was used to compare the discrepancy mean values before and after thermocycling within each test group.

Results

Reliability of measurements was statistically verified using ICC, which demonstrated excellent agreement for both marginal and internal assessments (ICC > 0.90). Specifically, ICC values for marginal adaptation were 0.92 (95% CI: 0.88–0.95), and for internal adaptation 0.95 (95% CI: 0.82–0.95). The Shapiro–Wilk test confirmed that the collected data were normally distributed for all groups and measurement points (P < 0.05).

Analysis of the marginal adaptation revealed statistically significant differences among the test groups before and after thermocycling (P < 0.05), as in Table 1. Before thermocycling, SS crowns showed the smallest marginal discrepancies, followed by BF, VSS and PZ groups (F = 4.712, P = 0.006, partial η² = 0.28, reflecting a large effect size). Within-group, comparisons across measured surfaces showed no significant variation, except for the buccal surface of the VSS group, which exhibited greater discrepancies than other surfaces (F = 15.111, P < 0.001, partial η² = 0.507, reflecting a large effect size).

Table 1.

Comparison of marginal adaptation between different surfaces of the test groups before and after thermocycling

		Group	Marginal adaptation (µm) Mean +/- SD*					95% CI
			Buccal	Mesial	Lingual	Distal	Average	L-U Bound
Thermocycling	Before N = 10	SS**	34.88 ± 2.64A, a	31.56 ± 8.20A, a	34.94 ± 7.44A, a	32.37 ± 8.35A, a	33.50 ± 3.87A	31.04–35.96
		VSS**	37.51 ± 2.86B, a	33.89 ± 4.23A, b	34.89 ± 4.59A, b	33.89 ± 5.73A, b	35.05 ± 2.62A, B	33.38–36.72
		PZ**	39.88 ± 3.47B, a	36.91 ± 4.13A, a	37.91 ± 3.91A, a	35.81 ± 3.93A, a	37.63 ± 2.56B	36.00-39.25
		BF**	37.21 ± 4.58A, B,a	35.49 ± 4.11A, a	36.70 ± 3.91A, a	34.76 ± 4.61A, a	36.21 ± 1.68A, B	35.14–37.27
	P-value†		<0.05
	After N = 10	SS	36.86 ± 3.04A, a	33.86 ± 3.25A, a	37.58 ± 3.01A, a	35.99 ± 3.25A, a	36.07 ± 1.79A	34.94–37.20
		VSS	38.55 ± 3.05A, a	35.98 ± 2.75A, a	36.76 ± 3.91A, a	36.59 ± 2.35A, a	36.97 ± 1.15A	36.48–37.95
		PZ	44.54 ± 3.59B, a	43.19 ± 3.64B, a	45.79 ± 1.76B, a	45.64 ± 2.34B, a	44.79 ± 1.75B	43.67–45.90
		BF	45.13 ± 4.09B, a	44.13 ± 2.82B, a	47.69 ± 4.33B, a	46.17 ± 2.95B, a	45.78 ± 1.99B	44.51–47.04
	P-value†		<0.05

*µm  micrometer, SD  standard deviation, **SS  stainless-steel crown, VSS  veneered stainless-steel crown, BF  BioFlx crown, PZ  prefabricated zirconia crown, CI  confidence interval, L-U bounds  lower and upper bounds,n  Number of samples and gp  Group

**Different upper-case superscript letters within the same column indicate a significant difference between tested groups at P < 0.05. Different lowercase superscript letters within the same row indicate a significant difference within the same group at P < 0.05

†The comparison of the marginal adaptation mean values (± SD) between the different test groups was conducted using the analysis of variance test followed by Tukey’s post hoc test

Thermocycling significantly increased marginal discrepancy values in SS and VSS groups, and a highly significant increase was identified in PZ and BF groups (F = 105.01, P < 0.001, partial η² = 0.90, reflecting an extremely large effect size).

The internal adaptation values differed significantly among the tested groups before and after thermocycling (P < 0.05) as in Table 2. Surface-specific comparisons within groups revealed no significant differences for SS crowns, while the buccal surface exhibited significantly higher internal discrepancy in VSS crowns (F = 6.537, P < 0.001, partial η² = 0.308, reflecting medium-to-large effect size). The effect of thermocycling was not statistically significant on internal discrepancy across the SS and VSS crown surfaces. In contrast, PZ and BF crowns presented a highly significant increase in discrepancy values compared to the baseline records (F = 1.21, P < 0.001, partial η² = 0.425, reflecting a large effect size).

Table 2.

Comparison of internal adaptation between different surfaces of the test groups before and after thermocycling

		Group	Internal adaptation (mm) Mean +/- SD*					95% CI
			Buccal	Mesial	Lingual	Distal	Average	L-U Bound
Thermocycling	Before N = 10	SS**	38.25 ± 4.57A, a	36.29 ± 5.66A, a	37.58 ± 5.07A, a	36.34 ± 5.54A, a	37.12 ± 3.11A	35.58–38.39
		VSS**	40.09 ± 1.94A, C,a	38.01 ± 1.45A, B,b	37.80 ± 2.55A, b	38.52 ± 3.73A, C,b	38.61 ± 1.51B	37.64–39.57
		PZ**	43.68 ± 2.80B, a	41.80 ± 2.64B, a	42.43 ± 3.08A, a	41.32 ± 1.96B, Ca	42.31 ± 1.45C	41.22–43.39
		BF**	43.03 ± 3.25B, C,a	42.01 ± 4.56B, a	41.22 ± 2.24A, a	39.80 ± 1.90A, C,a	41.46 ± 1.19C	40.52–42.40
	P-value†			<0.05
	After N = 10	SS	38.47 ± 1.52A, a	37.19 ± 3.03A, a	38.97 ± 1.70A, a	38.18 ± 3.20A, a	38.75 ± 1.93A	37.59–39.69
		VSS	40.58 ± 2.32A, a	38.72 ± 2.35A, b	38.89 ± 3.34A, b	39.34 ± 2.54A, b	39.38 ± 1.21A	38.60-40.15
		PZ	45.04 ± 4.89B, a	44.13 ± 3.30B, a	45.99 ± 2.91B, a	46.29 ± 2.09B, a	45.37 ± 1.75B	44.24–46.48
		BF	46.78 ± 2.58B, a	45.28 ± 3.16B, a	46.02 ± 3.33B, a	46.83 ± 2.86B, a	46.22 ± 1.69B	45.15–47.03
	P-value†		<0.05

*µm  micrometer, SD standard deviation, **SS stainless-steel crown, VSS veneered stainless-steel crown, BF BioFlx crown, PZ  prefabricated zirconia crown, CI confidence interval, L-U bounds = lower and upper bounds, N = Number of samples and gp  Group

**Different upper-case superscript letters within the same column indicate a significant difference between tested groups at P < 0.05. Different lowercase superscript letters within the same row indicate a significant difference within the same group at P < 0.05

†The comparison of the marginal adaptation mean values (± SD) between the different test groups was conducted using the analysis of variance test followed by Tukey’s post hoc test

Discussion

Despite the widespread clinical utilization of esthetic crowns for primary teeth, there is a notable paucity of research comparing their marginal and internal adaptation. This study compared the marginal and internal adaptation of different prefabricated esthetic crowns for primary molars with conventional stainless-steel crowns. The results demonstrated that SS crowns exhibited the smallest, and most stable discrepancies, both initially and after thermocycling, confirming their superior adaptability. In contrast, BF and PZ crowns showed significantly larger marginal and internal discrepancies, which increased significantly following thermal aging. The VSS crowns performed intermediately, with larger buccal discrepancies attributed to the esthetic facing. Therefore, these findings led to the rejection of the null hypothesis, confirming significant differences in crown adaptation among the tested crown systems.

A conventional glass ionomer cement (GIC) was used in this study as it is considered a reliable luting agent for different types of full coverage crowns in primary teeth, with good rate of retention and adequate marginal integrity [9, 26, 27].

Marginal adaptation assessment using digital microscope is a non-destructive technique that allows precise evaluation of the vertical distance at the cervical margin of crowns. This technique is considered cost-effective, less time-consuming, and fewer expected errors due to its simple steps [23]. Internal adaptation was assessed using the cross-sectional method, a frequently used technique for the evaluating the internal gap between the crown’s internal surface and the prepared tooth surface [28]. Despite its destructive nature and its limitations in comprehensive gap assessment due to the omission of recommended measurement points, this method remains a commonly used valid standard, as it allows direct assessment of the actual interface [28].

Our findings align with previous studies reporting superior adaptability of conventional SS crowns in primary dentition [23]. Crimping and enhancing the marginal adaptation are critical procedures that reduce discrepancies and improve clinical longevity [29, 30]. Veneered stainless steel crowns exhibited significantly larger gaps on the buccal surface, due to the bulk of the esthetic facing and limitations in crimping capacity inherent to the veneered layer. Similar to Abo-Elsoud et al., the present findings confirmed that marginal and internal discrepancies of the BF and PZ crowns were greater than SS and VSS crowns, particularly after thermocycling [23]. Qualitative assessment of the internal discrepancies revealed that higher mean values were frequently located occlusally, while lower measurements were detected along the axial surfaces. Anatomical occlusal reduction has been reported to enhance crown fit compared to semi- or non-anatomic reduction [31]. Increased occlusal discrepancies may invite stress concentration, which can adversely affect the mechanical performance of crown restorations and the supporting tooth structure [32].

Tooth preparation for prefabricated zirconia crowns is more extensive, involving all tooth surfaces and removal of coronal bulges. Unlike SS crowns, zirconia crowns lack crimping ability, which may compromise protection of the cement line, and increase the risk of microleakage, and discrepancies under thermal and masticatory stresses [33]. The reduction of BF crown adaptation after thermocycling could be attributed to its polymeric structure’s susceptibility to thermal changes, water sorption, matrix plasticization, and potential cement degradation at the interface [23, 34]. Since BF crowns are made of fiber reinforced polymer material, applying crimping pressure at the cervical margin could risk the crown’s integrity with microcracks. According to the manufacturer’s recommendations, adjusting the BF crown adaptation can be mainly achieved via selective tooth preparation rather than mechanical bending of the crown margin [35].

While discrepancies were statistically significant among crown types, all values remained within clinically acceptable range; from 100 to 150 μm for the marginal gap and from 200 to 300 μm for the internal gap [36, 37]. This suggests that despite the limited adaptation compared to SS crowns, esthetic crowns may still perform satisfactorily in clinical practice [6]. However, dental practitioners should recognize that larger discrepancies, particularly in BF and PZ crowns, may increase the risk of microleakage, recurrent caries, sensitivity, plaque accumulation, gingival inflammation, and restoration failure under long-term thermal and masticatory stresses [29, 33]. Accordingly, crown selection should balance esthetic demands with functional longevity, and meticulous tooth preparation is integral to optimizing crown adaptation.

Strengths of this study include the comparison of marginal and internal adaptation of different prefabricated crowns for primary molars, and subjecting specimens to 5000 thermocycles to simulate six months of oral thermal stresses, thus providing an insight into the clinical relevance of crowns’ performance over time. In addition, the use of human primary molars could enhance the clinical applicability and the anatomical relevance of the findings. The cross-sectioning method provided precise visualization of the crown-tooth interface, enabling accurate quantification of the internal gap across multiple areas, thereby enhancing reproducibility of the measurements.

The limitations of this study could arise from the in-vitro design, which does not fully replicate the complex biological conditions of the oral environment, or the long-term effects of masticatory forces on crown adaptation. The cross-sectioning technique, though accurate, is a destructive method that cannot be used in a clinical setting for assessment of crown adaptation in patients. Although the sample size was statistically adequate, a larger sample would better account for natural morphological variations between teeth. The study evaluated performance under thermal aging only; where the addition of mechanical fatigue cycling would provide a more comprehensive understanding of long-term adaptation. Furthermore, the use of single luting cement may not reflect the potential differences that could arise with alternative cement types. Accordingly, further research should focus on refining non-invasive techniques to assess crown adaptation under long-term thermomechanical loading to better simulate the clinical conditions.

This study confirms significant differences in marginal and internal adaptation among prefabricated crown systems for primary molars. The SS crowns remain the most reliable option due to their superior adaptability and stability. Although BF and PZ crowns demonstrated large discrepancies, these remained within clinically acceptable ranges, suggesting they can be used when esthetics are prioritized, provided clinicians account for their limitations. Careful tooth preparation and awareness of material-specific properties are essential to minimize risks of microleakage and restoration failure.

Conclusions

Under the limitations of this study, the following conclusions could be drawn:

1. Stainless steel crowns exhibited superior marginal and internal adaptation and maintained stability after thermocycling.

2. Prefabricated zirconia and BioFlx crowns showed greater discrepancies, particularly following thermal aging, while veneered stainless steel crowns performed intermediately.

3. Thermocycling increased discrepancies across all crown types, with changes most pronounced in zirconia and BioFlx crowns.

Authors’ contributions

The authors confirm contribution to the paper as follows: study conception and design: A.A.; data collection: M.F.; analysis and interpretation of results: A.A. and M.F.; draft manuscript: A.A. All authors reviewed the results and approved the final version of the manuscript.

Funding

The authors received no financial support for conducting this study.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval

This study was conducted in accordance with the ethical principles outlined in the Declaration of Helsinki. The study followed the institutional and national guidelines governing the use of biological specimens of human teeth in scientific research. Ethical approval was obtained from the Biomedical Research Ethics Committee, Umm Al-Qura University (Institutional Review Board Approval No.: HAPO-02-K-012-2023-10-1773).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Clinical trial registration details

Not applicable. The study is not a clinical trial, but an in-vitro study. Accordingly, no trial registration, no registration number, no data of registration.