The effect of hydrochloric acid on resin composite hardness

Objectives: Patients with persistent acid reflux, regurgitation and vomiting often have severe erosion of the surfaces of teeth due to exposure to stomach acid (HCl). Consequently, any permanent restorative material used to repair these teeth needs to be resistant to damage by HCl. We have studied a range of commercially available composites to establish the effect HCl exposure has on hardness.

Methods: Six commercially available composites (table 1) were selected to represent the major monomer systems available. Disc specimens (diameter=12mm, thickness=3mm, n=20) were made for each material in custom-made PMMA moulds. Specimens were polymerised for 40s on top and bottom faces (Coltolux LED, 500mW/cm²) and then left to condition for 28-days in PBS at 37°C. Next Martens baseline hardness was measured (Zwick Z 2.5) for all specimens (5 indents per disc) and specimens were divided into 5 groups (n=5), relating to either differing concentrations of HCl (pH1–2.5) or PBS (pH7.4). After storage for another 28-days at 37°C hardness was measured on the opposite face and the %change calculated.

Results: There were significant hardness differences between all composites at baseline (table 1). Four of the composites exhibited a significant decrease in hardness after exposure to pH1 HCl, of which two also exhibited a significant decrease in hardness for pH1.5. No significant differences were found after exposure to HCl at pH above 1.5 or exposure to PBS. The composites with the best performance used either a silorane system or a mix of Bis GMA, UDMA and BisEMA. Composites containing mainly UDMA appeared to soften more at pH1 and 1.5.

Conclusion: While we have not taken filler type or concentration into account, these results suggest that composites in which the main monomer is UDMA may be more susceptible to HCl erosion than those containing Bis-GMA.

ECHCPMS-epoxycyclohexylcyclopolymethylsiloxane,

ECHCEPMS-epoxycyclohexylethylphenylmethylsilane