Effect of cavity depth on degree of conversion and microhardness of low-shrinkage resin composites

Background: The study aimed to investigate the effect of different cavity depths on the degree of conversion (DC) and surface microhardness (MH) of low-shrinkage resin composites. Methods: Three low-shrinkage resin composites (Clearfil Majestry Posterior/CMP, Beautifil II LS/BL, Charisma Diamond/CD) and one conventional nanohybrid resin composite (Filtek Z550/FZ) as control were evaluated. For each composite material, a total of 12 disc-shaped samples were prepared using plastic molds with two different depths (2 mm and 4 mm) (n = 6). The same sample preparation protocol was applied for the DC and MH tests, and a total of 96 samples were prepared, 48 for each test. Resin composites filled into molds with the incremental technique were cured with LED light unit only from the top surface. DC was determined by Fourier transform infrared (FTIR) spectroscopy; MH was determined using the Vickers hardness tester. Data were statistically analyzed using Robust and two-way variance analyses (p < 0.05). Results: For all resin composites, there was no significantly difference in DC and MH between the 2 mm and 4 mm cavity depths. BL showed the highest DC values at both cavity depths, while CD showed the lowest (p < 0.001). At both cavity depths, the highest MH values on bottom-top surfaces ​​were obtained in CMP, while the lowest ​​were in BL. The top surface MH values ​​were higher in all resin composites than the bottom surfaces. Conclusions: Increasing cavity depth may not negatively affect the DC and MH of low-shrinkage resin composites. Moreover, resin composite material formulations may have a higher potential effect on these parameters. Clinical trial number: Not applicable. © The Author(s) 2025.