Degradation in the dentin-composite interface subjected to multi-species biofilm challenges

Abstract

Oral biofilms can degrade the components in dental resin-based composite restorations, thus compromising marginal integrity and leading to secondary caries. This study investigates the mechanical integrity of the dentin-composite interface challenged with multi-species oral biofilms. While most studies used single-species biofilms, the present study used a more realistic, diverse biofilm model produced directly from plaques collected from donors with a history of early childhood caries. Dentin-composite disks were made using bovine incisor roots filled with Z100(TM) or Filtek(TM) LS (3M ESPE). The disks were incubated for 72 h in paired CDC biofilm reactors, using a previously published protocol. One reactor was pulsed with sucrose, and the other was not. A sterile saliva-only control group was run with sucrose pulsing. The disks were fractured under diametral compression to evaluate their interfacial bond strength. The surface deformation of the disks was mapped using digital image correlation to ascertain the fracture origin. Fracture surfaces were examined using scanning electron microscopy/energy-dispersive X-ray spectroscopy to assess demineralization and interfacial degradation. Dentin demineralization was greater under sucrose-pulsed biofilms, as the pH dropped <5.5 during pulsing, with LS and Z100 specimens suffering similar degrees of surface mineral loss. Biofilm growth with sucrose pulsing also caused preferential degradation of the composite-dentin interface, depending on the composite/adhesive system used. Specifically, Z100 specimens showed greater bond strength reduction and more frequent cohesive failure in the adhesive layer. This was attributed to the inferior dentin coverage by Z100 adhesive, which possibly led to a higher level of chemical and enzymatic degradation. The results suggested that factors other than dentin demineralization were also responsible for interfacial degradation. A clinically relevant in vitro biofilm model was therefore developed, which would effectively allow assessment of the degradation of the dentin-composite interface subjected to multi-species biofilm challenge.

Keywords: Bond strength; Dental restoration; Interfacial Degradation; Multi-species biofilm; Resin composite.

Figure 1

Schematic illustrations of dentin-composite disks of LS (A) and Z100 (B). SEM images of LS (C) and Z100 (D) dentin-composite disks after demineralization and deproteinization, showing the adhesive layers and resin tags. SEM images of one layer (E) and two layers (F) of Z100 adhesive applied on etched dentin surfaces showing incomplete coverage.

Figure 2

Real-time pH recordings from the bioreactors during the 72-hr biofilm challenge (subject #778). (-○-) Biofilm challenge without sucrose pulsing, (-□-) Biofilm challenge with sucrose pulsing, (---) Control with sucrose pulsing but no biofilms.

Figure 3

SEM, EDS mapping and elemental analysis of dentin-composite disks exposed to biofilm challenges. (A) LS after biofilm challenge with sucrose pulsing. (B) Z100 after biofilm challenge with sucrose pulsing. (C) LS after biofilm challenge without sucrose pulsing. (D) Z100 after biofilm challenge without sucrose pulsing. Regions marked as 1 are exposed dentin and 2 are dentin covered with nail varnish during the biofilm challenge. (E) SEM image of fractured surface of an LS disk specimen after biofilm challenge with sucrose pulsing, showing a dark band of decalcified dentin at the surface. (F) Higher-magnification SEM image of the demineralized dentin showing exposed collagen fibril networks and resin tags.

Figure 4

Typical load-displacement curves and acoustic emission (AE) signals (bars) from the diametral compression tests of LS and Z100 disks after being subjected to biofilm challenges with or without sucrose pulsing. (A) LS after biofilm challenge without sucrose pulsing. (B) Z100 after biofilm challenge without sucrose pulsing. (C) LS after biofilm challenge with sucrose pulsing. (D) Z100 after biofilm challenge with sucrose pulsing. Inserts are images from digital image correlation showing surface strains on the disks during loading. The arrows indicate load drops due to debonding.

Figure 5

(A) Debonding loads averaged over the 12 subjects of dentin-composite disks with different bioreactor conditions. Ctrl: Control with no biofilms; BNS: biofilm without sucrose pulsing; BWS: biofilm with sucrose pulsing. The top of each bar indicates the mean of the values for all 12 subjects, and the error bars are standard deviations. (B) SEM image of the fracture surface of a Z100 disk specimen showing broken resin tags and adhesive. Insert is a lower-magnification SEM image of the same fracture surface: the top layer is the second adhesive layer (L2) debonded from the composite, while another thin layer of adhesive (L1) on dentin can be found below L2. The steps between the two layers indicate that part of the top layer was delaminated from the bottom layer.