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

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