The pH and Concentration of Citric Acid – Impact upon Dentine’s Topographical, Morphological, and Mechanical Properties

Abstract

Dentine erosion is a condition, increasing worldwide, that negatively impacts an individual’s quality of life, which is linked to acidic fruits and beverages. However, the effects of dietary acids on dentine erosion, particularly during normal consumption times, remains poorly understood. This in vitro study investigates the effect of citric acid on selected dentine properties, following short exposure times. Polished dentine samples were subjected to simulated dietary erosion using 1% and 6% citric acid (CA), in buffered (pH3.8) or unbuffered states (pH2.55 and pH2.06). Bovine dentine was initially measured with contact profilometry and microhardness testing, to establish their suitability to detect early erosion. Bovine and human dentine were subsequently measured with atomic force microscopy (AFM) in a simulated intra-oral environment, where samples were eroded at 20s cycles, over 2 minutes. Changes to the morphology, roughness (Sa), modulus of elasticity (MoE), and stiffness were monitored using AFM. Profilometry and microhardness were unable to detect early CA erosion changes. In comparison, AFM analysis showed dentine erosion after only 20s. For human dentine, 6% unbuffered CA had the greatest impact on morphological, Sa, MoE and stiffness measurements, whereas 1% buffered CA had the least erosive changes. The 1% unbuffered group was significantly more eroded than 6% buffered samples, for all AFM measurements. Bovine dentine was comparatively more susceptible to 6% buffered CA. Peritubular and extrafibrillar mineral were lost at the same rate during erosion, however, the intrafibrillar mineral remained intact for all samples, even after 2-minute exposures. Early nanoscale surface changes to peritubular and intertubular mineral could be measured using AFM. CA pH was more critical for erosive changes on human dentine compared with concentration. Intrafibrillar mineral was afforded the greatest protection from erosion compared with other sites. The different bovine erosion trends means that it cannot be used to replace human specimens for clinically relevant outcomes.