FRACTURING DENTIN EXPOSED TO BIOFILM
Oral biofilms produce organic acids, which degrade the dental resin-based composite restorations, and promote demineralization of enamel and dentin, thus compromising marginal integrity and leading to secondary caries increasing potential for tooth failure.
We were intrigued by the synergy of mastication loads and acidic oral environments on fracture and fatigue resistance of human dentin.
A series of experiments were conducted to study the fatigue and fatigue crack growth response of dentin exposed to acidic environment, mimicking the oral environment conditions.
For the first time, we revealed that exposing dentin to lactic acids caused a significant reduction on the fatigue strength after only 5.5 hours of exposure (see left Figures below with S/N curve).
In addition, results revealed that cracks required lower stress to initiate and propagation happened at faster rates (see right Figure below with Fatigue Crack Growth Response) facilitating failures.
In other words, acids not only erode dentin but also reduces the mechanical strength only after few hours of acid exposure. Such reduction could lead to premature tooth failure.
We revealed the mechanisms responsible for the reduction on the strength and mechanical integrity of dentin after exposed to acidis. The toughening mechanisms activated during crack propagation (i.e. extrinsic toughening) and the loss of stiffness on a subsurface level promoted by removal of minerals (demineralization) were the dominant factors.
SEM images presented below show the crack path of dentin after 6 hours of acid exposure revealing changes of the toughening mechanisms.
Finally, we conducted additional experiments evaluating the fatigue strength of human dentin when exposed to a simultaneous challenge of cyclic loading and oral bacteria with different cariogenic protocols.
Results revealed that dentin exposed to the biofilm model with 2.0% sucrose supplements pulsed twice per day caused a significant reduction in the fatigue strength.
As a reference and according to Surwit et al. a high-sucrose diet consists of 43% of the total daily energy intake, whereas a low-sucrose diet contains only 4%.
Despite being indicative of a very low cariogenic diet, the in-vitro biofilm model with only 2.0%v/v sucrose concentration was sufficient to reduce the fatigue resistance of dentin by 60%.
Details and more results of the work can be found in different publications: