Effect of sintering temperature on mechanical properties and ion release of fluorohydroxyapatite (FHA)-filled dental resin composites

Objective: Smart ion-releasing restorative materials may increase the success and survival rate of composite dental restorations. This study aimed to evaluate the impact of sintering temperature on the mechanical properties and ion-releasing behavior of fluorohydroxyapatite (FHA)-filled dental resin composites. Methods: The FHA was synthesized via a sol-gel method and sintered at three temperatures (250, 500, and 1000ºC). FHA fillers were characterized using X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy, then coated with citric acid and incorporated (40% w/w) into a photo-curable resin matrix (Bis-GMA, TEGDMA, HEMA; weight ratio 2:1:1). Flexural strength (FS) and diametral tensile strength (DTS) were measured, as well as the depth of cure, which was determined by the Vickers microhardness test. The release of calcium and fluoride ions was monitored for a month in diluted citric acid (pH=4.3), and the effect of acidic storage on mechanical properties was evaluated via DTS testing. The filler distribution was observed via scanning electron microscopy. The influence of sintering temperature on the measured variables was statistically analyzed using one-way and two-way ANOVA (α=0.05). Results: The XRD results revealed increased crystallinity with higher sintering temperatures. Correspondingly, mechanical properties improved with increasing sintering temperature (P 0.05), whereas the depth of cure, as well as ion release (Ca+2 and F-), decreased (P 0.05). Conclusions: Sintering at elevated temperatures yielded a greater amount of crystalline apatite structure, and enhanced mechanical properties of FHA-filled dental resin composites, but reduced ion release. Based on these findings, FHA fillers sintered at approximately 500°C appear optimal for creating smart dental resin composite.