Ability of restorative materials to buffer acids from bacterial origin

Objectives: Composites seem to be more susceptible to secondary caries (SC) compared to other restorative materials. One of the reasons for this could be their inability to buffer acids produced by cariogenic bacteria. However, this material property has not been investigated before. The aim of our study was to test the ability of different restorative materials and hydroxyapatite (HA) to neutralize lactic acid and to increase the pH.
Methods: Disks of composite (Z100, 3M ESPE, Germany), amalgam (Cavex, Netherlands), glass-ionomer (GIC, Ketac Fil, 3M ESPE) and giomer (Beautifil II, Shofu, Japan) were prepared; same size HA disks were obtained from Himed (USA). Solutions of lactic acid in distilled water with a final pH of 4, 5.5 and 7 were prepared. In addition, Streptococcus mutans cultures were grown in Brain Heart Infusion broth diluted with physiological saline (1:10) at 37°C. After 24h, the bacterial suspension was centrifuged and the supernatant containing bacteria-produced acid (pH=5.02) was collected. Material specimens were exposed to 500 µl of the prepared solutions and supernatant in 48 well-plates (n=3), and the pH of the solutions was measured daily for 3 days. Solutions without specimens served as control.
Results: Already after 24h, all materials except for composite increased the pH of solutions from 4 and 5.5 up to 6.3 (GIC and giomer) or 7.5 (HA and amalgam); the pH in the composite group remained the same as that of the control during all 3 days. A similar effect was observed with the supernatants which contained S. mutans-produced acid: while amalgam increased the pH from 5 to 5.9 (24h) and to 6.3 (72h), the pH in composite group remained unchanged after 72h.
Conclusions: Composite, unlike other restorative materials, lacks the ability to buffer low pH caused by bacteria-produced acids, which may be one of the reasons for its higher susceptibility to SC.