The internal fit and fracture resistance of stainless-steel crowns and porcelain fused to metal crowns in intact and severely damaged permanent molars: an in vitro study

Severe caries in mixed dentition frequently affects mandibular first permanent molars, often necessitating full-coverage restorations. Stainless steel crowns (SSCs) and porcelain-fused-to-metal (PFM) crowns are among the most commonly used options. This study aimed to compare the internal fit and fracture resistance of SSCs and PFMs in both intact and restored mandibular first molars. Forty extracted mandibular first molars were randomly divided into four groups: (1) intact teeth–SSCs, (2) intact teeth–PFMs, (3) restored teeth–SSCs, and (4) restored teeth–PFMs. In restored samples, access cavities and mesiodistal boxes were prepared and restored with composite resin. Following tooth preparation, PFM crowns were fabricated, and appropriate SSCs were selected. The internal fit was assessed using the silicone replica technique under a digital microscope. Crowns were then cemented with glass ionomer cement, and fracture resistance was measured using a universal testing machine. Data were analyzed using two-way ANOVA, independent t -test, and Chi-square test (α = 0.05). SSCs demonstrated significantly greater marginal gap (intact: 313.20 ± 44.46 μm; restored: 304.90 ± 62.03 μm), axial gap, and total internal gap compared with PFMs (marginal gap: intact: 183.00 ± 41.29 μm; restored: 222.10 ± 38.25 μm; p < 0.001). Tooth condition had no significant effect on SSC adaptation; however, PFM marginal and total internal gap values increased significantly following restoration ( p = 0.048). PFMs demonstrated significantly higher fracture resistance than SSCs under both conditions ( p < 0.001). Restored SSCs exhibited significantly higher fracture resistance than intact SSCs ( p = 0.029), while no significant difference was observed between intact and restored PFMs. SSCs showed a significantly higher proportion of repairable failures (40–80%) compared with PFMs, in which all fractures were non-repairable ( p < 0.001). Under the conditions of this in vitro study, PFMs demonstrated superior internal adaptation and overall fracture resistance compared with SSCs; however, SSCs exhibited more favorable and repairable fracture patterns. Long-term clinical studies are warranted to validate these findings.