Degradation of resin-bonded human dentin after 3 years of storage.

PURPOSE: To test, in vitro, the null hypothesis that there was no difference in the ultrastructure of adhesive-bonded, acid-etched dentin aged under accelerated conditions in mineral oil or artificial saliva. METHODS: Beams of human dentin bonded with three total-etch adhesives were retrieved from the two storage media after 3 years and prepared for transmission electron microscopy. RESULTS: Hybrid layers from specimens aged in mineral oil exhibited structural integrity of the collagen network. Conversely, abnormal hybrid layers were seen in specimens aged in artificial saliva, with progressive disintegration of the fibrillar network to the extent that it was beyond detection by collagen staining. Self-destruction of collagen matrices can occur in resin-infiltrated dentin, and to a lesser degree, in mineralized dentin in the absence of bacterial or salivary enzymes.

Reactivation of inactivated endogenous proteolytic activities in phosphoric acid-etched dentine by etch-and-rinse adhesives.

Auto-degradation of collagen matrices occurs in resin-infiltrated dentine by the slow action of host-derived matrix metalloproteinases. As phosphoric acid-etching inactivates these endogenous enzymes, it is puzzling how hybrid layers created by simplified etch-and-rinse adhesives can degrade in vivo. This study tested the null hypothesis that there are no differences in the relative proteolytic activities of mineralised dentine, acid-etched dentine, and etch-and-rinse adhesive-treated acid-etched dentine. Powdered dentine prepared from extracted human teeth was treated with 17% EDTA, 10% phosphoric acid, or with five simplified etch-and-rinse adhesives that were applied to 10% phosphoric acid-etched dentine. The gelatinolytic activity of the dentine powder was assayed using fluorescein-labelled gelatine. TEM examination of the air-dried, treated dentine powder was performed to confirm the presence of remnant mineralised dentine after acid-etching. 17% EDTA significantly reduced the relative proteolytic activity (73.2%) of the untreated mineralised dentine powder (control), while 10% phosphoric acid-etched dentine exhibited the highest reduction (98.1%). Treating the acid-etched dentine powder with any of the five simplified etch-and-rinse adhesives resulted in the reactivation of the proteolytic activity, with a significant negative linear correlation (P<0.05) between the increases in fluorescence and the corresponding pH values of the adhesives. It is concluded that simplified etch-and-rinse adhesives can reactivate endogenous enzymatic activities in dentine that are previously inactivated by phosphoric acid-etching. The amount of enzyme reactivated may even exceed the original quantity present in untreated mineralised dentine. This provides an explanation for the degradation of hybrid layers after acid-etched dentine matrices are infiltrated with these adhesives.

Effect of resin hydrophilicity on water-vapour permeability of dental adhesive films.

This study examined the water-vapour permeability of thin polymerized resin films fabricated from five co-monomer blends of increasing degrees of hydrophilicity, as measured by their Hoy's solubility parameters. Neat resin films were prepared from five experimental light-curable resins (n = 10). Each film was mounted in a Fisher permeability cup with 8 g of water placed inside the cup. The experiments were conducted in a modified twin-outlet desiccator connected to a vacuum pump in one outlet to permit a continuous airflow to encourage water evaporation. Weight losses by water evaporation were measured at 3, 6, 9, 24, 30, and 48 h by using an analytical balance. Additional resin films were examined by using transmission electron microscopy (TEM) after immersion in ammoniacal silver nitrate. A significant correlation was observed between the cumulative water loss at 48 h and the Hoy's total cohesive energy density (delta(t)). Transmission electron microscopy revealed silver-filled channels along film peripheries and silver grains of decreasing dimensions toward the film centres in co-monomer blends 3, 4, and 5 of increasing hydrophilicity. Hydrophilic dentin adhesives polymerized in thin films are prone to water loss by evaporation. This probably accounts for the water droplets seen on the surface of vital-bonded dentin after the application of simplified dentin adhesives.

Solvent and water retention in dental adhesive blends after evaporation.

This study examined the extent of organic solvent and water retention in comonomer blends with different hydrophilicity (Hoy's solubility parameter for hydrogen bonding, delta(h)) after solvent evaporation, and the extent of tracer penetration in polymerised films prepared from these resins. For each comonomer blend, adhesive/solvent mixtures were prepared by addition of (1) 50 wt% acetone, (2) 50 wt% ethanol, (3) 30 wt% acetone and 20 wt% water and (4) 30 wt% ethanol and 20 wt% water. The mixtures were placed in glass wells and evaporated for 30-60s for acetone-based resins, and 60-120 s for ethanol-based resins. The weight of the comonomer mixtures was measured before and after solvent evaporation. Resin films were prepared for transmission electron microscopy (TEM) after immersion in ammonical silver nitrate. The percentages of solvent and water retained in the comonomer mixtures, and between the acetone and ethanol groups were measured gravimetrically and were statistically compared. In comonomer-organic solvent mixtures, the percentage of solvent retained in acetone and ethanol-based mixtures increased significantly with hydrophilicity of the comonomer blends (P < 0.05). In resin-organic solvent-water mixtures, significantly more solvent and water were retained in the ethanol-based mixtures (P < 0.0001), when compared to acetone-based mixtures after 60s of air-drying. TEM revealed residual water being trapped as droplets in resin films containing acetone and water. Water-filled channels were seen along the film periphery of all groups and throughout the entire resin films containing ethanol and water. The addition of water to comonomer-ethanol mixtures results in increased retention of both ethanol and water because both solvents can hydrogen bond to the monomers.

Submicron hiati in acid-etched dentin are artifacts of desiccation.

OBJECTIVES: The submicron hiatus represents a potential space between the base of the collagen network and the mineralized dentin when dentin is acid-etched for bonding. These spaces were observed in SEM studies after acid-etched dentin specimens were critical point dried or dehydrated in hexamethyldisilasane. However, they have never been identified in TEM studies of dentin hybrid layers. This study critically examined the cause of submicron hiati formation using a silver staining technique to measure nanoleakage. METHODS: Two multi-step, total-etch adhesives (One-Step, Bisco; Single Bond, 3M) and two single-step, self-etching adhesives (Prompt L-Pop, ESPE; One-Up Bond F, Tokuyama) were examined. Flat dentin surfaces were bonded with these adhesives and a lining composite. In each adhesive group, 0.8mm thick slabs from the same bonded tooth were coated with nail varnish applied 1mm from the bonded interfaces. The varnish was either left to dry completely for 10min before immersing in 50wt% silver nitrate (AgNO(3)) for 24h (group D), or painted on blotted tooth slabs that were immediately dropped into the AgNO(3) solution (group M). After developing, undemineralized, unstained, epoxy resin-embedded sections were prepared for transmission electron microscopy (TEM) to identify the amount and distribution of silver uptake. RESULTS: Nanoleakage patterns were observed in all adhesive-bonded teeth, regardless of brand. Fine reticular silver deposits were also found in the underlying undemineralized dentin. In group D, submicron hiati were seen as tunnels of heavy silver deposits beneath hybrid layers. Specifically, a hiatus occurred between the undemineralized intertubular dentin and a cohesively fractured layer of the same matrix that was attached to the base of the hybrid layer. Hiati were completely absent in group M, regardless of the brand of adhesive. SIGNIFICANCE: Submicron hiati are artifacts created by desiccation during specimen processing, and should be referred to as such in future studies of bonded dentin interfaces.

Effects of one versus two applications of an unfilled, all-in-one adhesive on dentine bonding.

OBJECTIVES: This study examined the null hypothesis that there is no difference between the effect of a one versus two-layer applications of Prompt L-Pop (3M ESPE, Seefeld, Germany) to sound, abraded human coronal dentine. METHODS: In group I, the mixed adhesive was applied for 15s, and light-cured for 10s. In group II, after light-curing the first layer, the adhesive was re-applied and light-cured. Specimens bonded with a hybrid composite were sectioned into beams for microtensile bond strength evaluation. Additional teeth from each group were bonded similarly using a lining composite for transmission electron microscopy examination of the resin-dentine interfaces, and nanoleakage evaluation using ammoniacal silver nitrate. RESULTS: A significant difference (p<0.001) was detected between microtensile bond strengths in the two groups. Stained, demineralized sections revealed 3-5 microm thick hybrid layers in both groups. An electron-lucent layer between 7 and 20 microm thick was present between the adhesive and the overlying composite. This layer was absent from the interfaces after removal with ethanol before composite placement. The use of a single application in group I resulted in the direct contact of the electron-lucent layer with the dentine surface and tubular orifices. In unstained, undemineralized sections used to evaluate nanoleakage, silver deposits were found mostly in the hybrid layer in group II, but throughout the entire adhesive layer in group I. CONCLUSION: Bonding of this unfilled all-in-one adhesive to dentine may be improved by application of a second adhesive layer after light-curing the first layer. This ensures that the exposed dentine surface and dentinal tubules are coated with adhesive that is adequately polymerized.

The effects of dentin permeability on restorative dentistry.

The permeability properties of dentin determine its sensitivity and the degree of pulpal response to restorative procedure materials and microleakage. Most pulpal reactions are due to bacteria or bacterial products that permeate across dentin. These reactions can be prevented if dentin is sealed with resins as soon as it is exposed. In the future, restorative dentists may employ topical application of biologic growth factors to permeate across dentin to modify the formation of reactionary or reparative dentin, thereby lowering dentin permeability and protecting the pulp.