In-Vitro Evaluation of effect of Pre-Heating on Degree of Conversion and Surface Micro-Hardness of Bulk-Fill Composite at Different Depths

Abstract

Objective: This in-vitro study objective is to evaluate whether preheating improves degree of conversion and surface micro-hardness of the Filtek bulk-fill posterior composite.

Study Design: An in-vitro experimental laboratory study
Place and Duration of Study: This study was conducted at the Conservative Dental Sciences Department, College of Dentistry, Qassim University. KSA from 20 May 2026 to 30 June 2026.

Methods: This study was conducted to evaluate the effect of preheating temperature on the degree of conversion and Vickers microhardness of FiltekTM Bulk Fill Composite. Sixty specimens were prepared in 5mm diameter, 4mm depth molds, and assigned to 23°C, 37°C, 0r 54°C groups (n=20 each). Each group was divided for ATR-FTIR degree of conversion and Vicker’s micro-hardness testing. Specimens were stored in distilled water at 37°C for 24 hours before analysis. The composite was cured for 30 seconds using LED light at 1200mW/cm2.

Results: Preheating significantly increased degree of conversion and micro-hardness with 54°C producing the highest values and 23°C the lowest. In all groups, the top surface showed higher values than the bottom surface, and the bottom to top hardness ratios remained clinically acceptable. Degree of conversion (DC) strongly and positively correlates
with micro-hardness.

Conclusion: Preheating the tested bulk fill composite improved polymerization efficiency and surface hardness, particularly at 54°C, and may enhance performance in deeper restorations. The Data also showed a clear depth effect; top surface values exceeded bottom surface values across temperatures, suggesting that preheating improved but did not eliminate light attenuation through the 4mm increment. Because the correlation between conversion and hardness was strong, micro-hardness may serve as a practical indirect marker of polymerization in this material.

Clinical Relevance: Within the limits of this single material in-vitro study, preheating appears to be useful strategy for improving bulk fill composite polymerization especially when deeper restorations are planned with clinically relevant chairside implications for practice.