To polish or not to polish? An evaluation of the accuracy in measuring depth of cure calculated by microhardness ratio.

Aim: This study aimed to evaluate the influence of surface polishing on the reliability of microhardness ratio-based assessment of depth of cure in resin-based composites.

Materials and methods: A total of 60 composite discs (n = 10 per group) were prepared using six different resin-based composite materials. The tested materials included two conventional sculptable composites (Tetric Prime and Neo Spectra ST HV), one conventional flowable composite (Tetric EvoFlow), one bulk-fill sculptable composite (Tetric PowerFill), and two bulk-fill flowable composites (Tetric PowerFlow and SDR Plus), all prepared according to the respective manufacturers' instructions. Prior to microhardness measurements, the bottom-to-top surface microhardness ratios were assessed using three distinct approaches: 1)unpolished bottom/unpolished top, 2) unpolished bottom/polished top, 3) polished bottom/polished top. Vickers Microhardness Numbers (VHN) were determined under a load of approximately 50 g applied for 15 s. The threshold value for acceptable depth of cure was set at 80%.

Results: Statistical analysis revealed significant differences in the depth of cure measurements of resin-based materials when evaluated using the unpolished bottom/polished top, polished bottom/top, and unpolished bottom/top approaches (p < 0.001). Among these, the lowest degree of conversion was consistently observed in the configuration where the bottom surface remained unpolished while the top surface was polished. Notably, SDR was the only material that surpassed the 80% threshold across all three evaluation methods.

Conclusion: The unpolished bottom/polished top configuration resulted in the lowest conversion rates, primarily due to the increase in microhardness values caused by polishing the top surface. Whether or not surface finishing is applied has a substantial impact on the mechanical performance, and should therefore be thoughtfully considered when interpreting the effectiveness of polymerization.