Synergistic effects of bacteria, enzymes, and cyclic mechanical stresses on the bond strength of composite restorations

Abstract

Predicting how tooth and dental material bonds perform in the mouth requires a deep understanding of degrading factors. Yet, this understanding is incomplete, leading to significant uncertainties in designing and evaluating new dental adhesives. The durability of dental bonding interfaces in the oral microenvironment is compromised by bacterial acids, salivary enzymes, and masticatory fatigue. These factors degrade the bond between dental resins and tooth surfaces, making the strength of these bonds difficult to predict. Traditionally studied separately, a combined kinetic analysis of these interactions could enhance our understanding and improvement of dental adhesive durability. To address this issue, we developed and validated an original model to evaluate the bond strength of dental restorations using realistic environments that consider the different mechanical, chemical, and biological degradative challenges working simultaneously: bacteria, salivary esterases, and cyclic loading. We herein describe a comprehensive investigation on dissociating the factors that degrade the bond strength of dental restorations. Our results showed that cariogenic bacteria are the number one factor contributing to the degradation of the bonded interface, followed by cyclic loading and salivary esterases. When tested in combinatorial mode, negative and positive synergies towards the degradation of the interface were observed. Masticatory loads (i.e., cycling loading) enhanced the lactic acid bacterial production and the area occupied by the biofilm at the bonding interface, resulting in more damage at the interface and a reduction of 73 % in bond strength compared to no-degraded samples. Salivary enzymes also produced bond degradation caused by changes in the chemical composition of the resin/adhesive. However, the degradation rates are slowed compared to the bacteria and cyclic loading. These results demonstrate that our synergetic model could guide the design of new dental adhesives for biological applications without laborious trial-and-error experimentation.

Keywords: Bacteria; Biofilms; Bonding strength; Composite restoration; Degradation; Dental adhesives; Dental caries; Dental composites; Dental materials; Dental restoration failure; Saliva esterases.

Figure 1.. Integrated modeling of the multiple degradation sources affecting adhesive-composite-dentin bonded interfaces.

(a) Schematic diagram showing the preparation of twin-interface samples. (b) A three-factor experimental design was used to evaluate the contribution of each degradative source, including bacteria, saliva (i.e., pseudocholinesterase (PCE) and cholesterol esterase (CE) enzymatic activities), and cyclic loading and their combinations/interactions on the reduction of the bond strength. (c) Bacteria supplementation during experiments. After bacterial adhesion, samples were incubated in phosphate-buffered saline (PBS) at 37°C. BHI media and sucrose supplementation were conducted 5 times/day by temporarily submerging the colonized samples for 1 minute.

Figure 2.. Bacteria activity at the bonded interfaces.

(a) Fracture surface CLSM images of selected samples subjected to different challenges, including phosphate-buffered saline (PBS), saliva, bacteria, cyclic loading, and their interactions. Samples were stained with Syto-9 (green) and propidium iodide (red) to indicate live and dead bacteria, respectively. (b) Quantification of the surface area occupied/covered by bacteria. N= 3 samples for each group. (c) Coefficient estimates from GLM statistical analysis for bacteria surface coverage response. (d) L-lactate quantity along the bonded interface for all tested groups. Data is expressed as millimolar (mM) of L-lactate. N = 6 samples for each group. (e) Coefficient estimates from GLM for L-lactate concentration responses. (f) Metabolic activity of S. mutans cells at the bonded interface for all evaluated conditions. N = 4 samples for each group. (g) Coefficient estimates from GLM for metabolic activity responses at the bonded interface. (h) Number of viable bacteria (CFU) present at the bonded interface. N= 4 samples for each group. (i) Coefficient estimates from GLM for the number of viable cells at the bonded interface. All GLM coefficients were classified as detrimental (red bars) or beneficial (green bars) to the degradation of the bonded interface. Significance of the factors and interactions were assessed by multifactor ANOVA (<0.05).

Figure 3.. Bond strength responses and bonded interface conditions after challenged by multiple degradative sources.

(a) Comparison of the bond strength for interfaces subjected to different challenges, including phosphate-buffered saline (PBS), saliva, bacteria, cyclic loading, and their interactions. The duration of the challenges was 6-days (or ~1 M cycles for cyclically loaded cases). N= 6 samples for each group. (b) Coefficient estimates from GLM for residual bond strength responses. Coefficients were classified as detrimental (red bars) or beneficial (green bars) to the degradation of the bonded interface. Significance of the factors and interactions were assessed by multifactor ANOVA (<0.05). (c) Representative micrographs of the bonded interfaces after different degradative challenges, including bacterial and bacteria/saliva under cyclic loading conditions and phosphate-buffered saline (PBS) and bacteria under static conditions. “C” indicates the composite material, “A” represents the adhesive interface, and “D” corresponds to dentin. The yellow and orange arrows on the micrographs correspond to the adhesive and composite material damage, respectively.

Figure 4.. Chemical changes of the biomaterials at the bonded interfaces after challenged by multiple degradative sources, including phosphate-buffered saline (PBS), bacteria, saliva, cyclic loading, and their interactions.

Hyperspectral images representing changes in the intensity of peaks associated with the (a) ester (C=O), (b) Si-O-Si, (c) ν₁,ν₃ PO₄, and (d) Amide I bonds. Degraded samples were compared with “As prepared” (no-degraded) samples. Color scale bars on the hyperspectral image correspond to the relative O-PTIR intensity, with dark blue as the lowest and red as the highest peak intensity.