Abstract
Objective
To determine if the results of resin–dentin microtensile bond strength (μTBS) is correlated with the outcome parameters of clinical studies on non-retentive Class V restorations.
Methods
Resin–dentin μTBS data were obtained from one test center; the in vitro tests were all performed by the same operator. The μTBS testing was performed 8 h after bonding and after 6 months of storing the specimens in water. Pre-test failures (PTFs) of specimens were included in the analysis, attributing them a value of 1 MPa. Prospective clinical studies on cervical restorations (Class V) with an observation period of at least 18 months were searched in the literature. The clinical outcome variables were retention loss, marginal discoloration and marginal integrity. Furthermore, an index was formulated to be better able to compare the laboratory and clinical results. Estimates of adhesive effects in a linear mixed model were used to summarize the clinical performance of each adhesive between 12 and 36 months. Spearman correlations between these clinical performances and the μTBS values were calculated subsequently.
Results
Thirty-six clinical studies with 15 adhesive/restorative systems for which μTBS data were also available were included in the statistical analysis. In general 3-step and 2-step etch-and-rinse systems showed higher bond strength values than the 2-step/3-step self-etching systems, which, however, produced higher values than the 1-step self-etching and the resin modified glass ionomer systems. Prolonged water storage of specimens resulted in a significant decrease of the mean bond strength values in 5 adhesive systems (Wilcoxon, p < 0.05). There was a significant correlation between μTBS values both after 8 h and 6 months of storage and marginal discoloration ( r = 0.54 and r = 0.67, respectively). However, the same correlation was not found between μTBS values and the retention rate, clinical index or marginal integrity.
Significance
As μTBS data of adhesive systems, especially after water storage for 6 months, showed a good correlation with marginal discoloration in short-term clinical Class V restorations, longitudinal clinical trials should explore whether early marginal staining is predictive for future retention loss in non-carious cervical restorations.
1
Introduction
Dental adhesive systems are evaluated in the laboratory using various methods: bond strength tests, penetration of tracers along the restorative material/tooth substance interface, morphological characteristics of the interface, and marginal adaptation of restorations . Bond strength tests are the most widely used laboratory method for assessing the ability of a dental bonding system to establish a bond between the restorative material and the biologic substrate, i.e. dentin and/or enamel. There are a variety of different approaches to test for the bonding capabilities. To date, there is no international consensus on which is the most appropriate approach and what should be the adequate parameters to evaluate the bond strength. The ISO Technical Specification on “Testing the adhesion to tooth structure” (No. 11405, first edition 1994, second edition 2003) describes the methodology for shear and tensile bond strength tests, which are also the most popular tests .
Application of the μTBS test has substantially increased over the last 3–5 years. Many studies publish tests after only 24 h of water storage. However, it has been proven that prolonged water storage is a challenge for the interface between restorative material and tooth substance and may degrade this bond over time . The different dental adhesive system formulations and the components of the resin–dentin bond react differently to this challenge.
The clinical significance of bond strength tests has been discussed in many papers ; however, many researchers doubt the importance of these laboratory tests. The failure to correlate bond strength results on a product level with clinical outcomes, even with early adhesive systems that showed low bond strength, supports this skepticism .
Bond strength may be correlated to the ability of a restorative material to be held in place when mechanical retention is weak or missing. This is notably the case with cervical non-retentive lesions (Class V), also called non-carious cervical lesions (NCCL) that are restored with resin-based composites (RBC), mostly without further cavity preparation or roughening of dentin. The American Dental Association (ADA) previously defined an adhesive system to be adequate and acceptable for clinical use (“full acceptance”) if the retention rate of restorations placed in non-carious lesions is higher than 90% after an observation period of 1.5 years . Many of the newer adhesive systems, especially the one-step self-etching systems, would not have received ADA acceptance. The ADA acceptance program was abandoned by the end of 2008 .
The question is whether the high frequency of clinical failures of a specific product is predictable by bond strength tests. If there were an acceptable correlation between bond strength test results and the clinical performance of an adhesive system in cervical restorations, one would be able to improve the adhesive prior to the time-consuming and expensive clinical trial phase. Both dentists and patients would benefit from such an approach.
The objective of the present study was to determine whether results from μTBS 8 h post-bonding or 6 months of water storage of the specimens of a variety of different adhesive/restorative systems correlate with the clinical outcome of non-retentive cervical restorations, notably with retention loss, marginal discoloration and marginal integrity. The null hypothesis was that there was no significant correlation between the 8 h or 6-month μTBS on dentin and the outcome of the clinical trials.
2
Materials and methods
2.1
Microtensile bond strength
The μTBS testing procedure for all adhesive/restorative materials was performed by the same operator (TC) in the same laboratory with the same methodology. Sixty-four caries- and defect-free extracted human molar teeth, obtained according to institutional review board requirements, were stored in 0.5% chloramine T at 4 °C and used within 6 months after extraction. The teeth were cleaned and mounted in dental stone (Die-Keen ® Green, Heraeus Kulzer, Inc., Armunk, NY). Occlusal enamel was partially removed using a 600-grit wheel model trimmer (3/4HP Wet Model Trimmer, Whip Mix Corporation, Louisville, KY, USA). The remaining occlusal enamel was removed with a water-cooled carbide bur (#55, Brasseler, Savannah, Georgia) in an electric handpiece rotating at 200,000 rpm (KaVo Electromatic and Intramatic 25LHA: KaVo America Corporation, Lake Zurich, IL) mounted in a CNC Specimen Former (University of Iowa, Iowa City, IA) to expose superficial to middle dentin. After rinsing for 5 s with 35% phosphoric acid to confirm that all central enamel had been removed, an additional 0.1 mm of occlusal dentin was removed to expose unaltered dentin substrate for bonding.
The teeth were randomly distributed across 16 adhesive/restorative systems (four teeth per adhesive). Manufacturer’s instructions were followed for adhesive application as closely as possible with the following exceptions: (1) drying distance, pressure, and angulations were modified to ensure that excess moisture/solvent was completely removed from the flat occlusal surface, and (2) based on pilot study results, priming and priming/adhesive application times and the number of coats were increased for Syntac, Futurabond NR, and Xeno III to reduce the frequency of pre-test failures. See Table 1 for adhesive system composition, batch number and mode of application.
| Materials | Components | Batch no. | Manu-facturer | Procedure |
|---|---|---|---|---|
| Three-step etch-and-rinse systems | ||||
| Adper Scotchbond Multi-purpose | Etchant: 35% phosphoric acid | 6GK | 3M ESPE | Etch 15 s, Rinse 15 s, Dry 5 s from 0.5 cm, Prime 30 s A , Dry 5 s from 0.5 cm, Adhesive 2 coats A , LC 10 s |
| Primer: HEMA, polyalkenoic acid copolymer, water | 6BB | |||
| Adhesive: bis-GMA, HEMA, photoinitiator | 6PK | |||
| All-Bond 2 | Etchant: 32% phosphoric acid with benzalkonium chloride | 0600001909 | Bisco | Etch 15 s, Rinse 15 s, Dry 2 s from 0.5 cm, Prime 5 coats A , Dry 5 s from 0.5 cm, Adhesive 2 coats A , LC 20 s |
| Primer A: NTG-GMA, acetone, ethanol, water | 0600001616 | |||
| Primer B: BPDM, acetone, ethanol, photoinitiator | 0600001614 | |||
| Adhesive: bis-GMA, HEMA, camphorquinone, amine activator | 0600001539 | |||
| OptiBond FL | Etchant: 37.5% phosphoric acid | 444288 | Kerr | Etch 15 s, Rinse 20 s, Dry 5 s from 0.5 cm, Prime 30 s with light scrubbing B , Dry 5 s from 0.5 cm, Adhesive 2 coats A , LC 30 s |
| Primer: HEMA, GPDM, PAMM, ethyl alcohol, camphorquinone, water | 423881 | |||
| Adhesive: bis-GMA, HEMA, barium aluminum borosilicate glass, fumed silica, disodiumhexafluorosilicate, glycerol dimethacrylate, camphorquinone | 440996 | |||
| PermaQuick | Etchant: 35% phosphoric acid | B0FTQ | Ultradent | Etch 15 s, Rinse 15 s, Dry with Kimwipes, Prime 30 s A , Dry 12 s from 1 cm, LC 20 s, Adhesive 2 coats A , Dry 5 s from 2 cm, LC 20 s |
| Primer: Canadian balsam, HEMA, methacrylic acid, camphorquinone, phosphate monomer in ethanol | B1Z74 | |||
| Adhesive: bis-GMA, TEGDMA, HEMA, diluent monomer, tertiary amine, camphorquinone, proprietary glass silicate filler | B1YFR | |||
| Two-step etch-and-rinse systems | ||||
| One-Step | Etchant: 32% phosphoric acid with benzalkonium chloride | 0500011271 | Bisco | Etch 15 s, Rinse 15 s, Dry with Kimwipes, Adhesive 2 coats A with agitation, Dry 10 s from 2 cm, LC 10 s |
| Adhesive: bis-GMA, HEMA, BPDM, acetone, photoinitiator | 0500020767 | |||
| Prime&Bond NT Dual Cure | Etchant: 34% phosphoric acid, water, silicon dioxide, surfactants, blue colorant | 051282 | Dentsply | Etch 15 s, Rinse 10 s, Dry with Kimwipes, Adhesive 2 coats A 30 s, Dry 5 s from 2 cm, LC 10 s |
| Adhesive: di- and trimethacrylate resins, dipentaerythritol penta acrylate monophosphate, nanofillers, amorphous silicon dioxide, photoinitiators, stabilizers, cetylamine hydrofluoride, acetone | 051211 | |||
| Two-step self-etch systems | ||||
| Clearfil SE Bond | Primer: 10-MDP, HEMA, hydrophilic dimethacrylate, dl-camphorquinone, N,N-diethanol-p-toluidine, water | 00599B | Kuraray | Prime 20 s A , Dry from 0.5 cm, Adhesive 2 coats A , Dry from 2 cm, LC 10 s |
| Adhesive: 10-MDP, bis-GMA, HEMA, hydrophobic dimethacrylate, dl-camphorquinone, N,N-diethanol-p-toluidine, silanated, colloidal silica | 00844A | |||
| Syntac | Primer: polyethylene glycol dimethacrylate, maleic acid, ketone | G04550 | Ivoclar-Vivadent | Prime 30 s A , Dry from 0.5 cm, Adhesive 2 coats A 10 s, Dry from 0.5 cm, Heliobond 2 coats A , Dry 5 s from 2 cm, LC 20 s |
| Adhesive: polyethylene glycol dimethacrylate, glutaraldehyde | G08116 | |||
| Heliobond: bis-GMA, triethylene glycoldimethacrylate, photoinitiator | G04743 | |||
| One-step self-etch systems | ||||
| Adper Prompt L-Pop | Compartment #1: methacrylate phosphates, photoinitiator, stabilizer | 240465 | 3M ESPE | Mix Unidose 5 s, Adhesive 15 s B did not replenish, Dry from 1 cm until no fluid movement then from 0.5–1 cm, Second coat of adhesive, Dry 1 cm until no fluid movement, LC 10 s |
| Compartment #2: water, complexed fluorides, stabilizer | ||||
| Futurabond NR | Adhesive: bis-GMA, hydroxyethylmethacrylate, BHT, ethanol, organic acids, fluoride, initiator | 610064 | VOCO | Mix 5 s A , Adhesive 1 coat 20 s A , Dry 5 s from 2 cm, LC 10 s |
| iBond | Adhesive: acetone, 4-META, glutaraldehyde, initiator | 010074 | Heraeus-Kulzer | Adhesive 3 coats 30 s A , Dry from 5 cm until no fluid movement then additional drying from 1 cm until dry, LC 20 s |
| Xeno III | Liquid A: HEMA, purified water, ethanol, BHT, highly dispersed silicon dioxide | 0508001872 | Dentsply | Mix 5 s, Adhesive 45 s A , Dry 5 s from 2 cm until no fluid movement, LC 10 s |
| Liquid B: phosphoric acid modified polymethacrylate resin, mono fluorophosphazene modified methacrylate resin, UDMA | 0509000753 | |||
| Glass-ionomer systems | ||||
| Fuji BOND LC | Cavity conditioner: 20% polyalkenoic acid, 3% aluminum chloride | 0503021 | GC | Condition 10 s B , Rinse 10 s, Dry with Kimwipes, Mix powder/liquid 10 s B , Adhesive 2 coats, LC 20 s, Apply vaseline after RBC placement then left in humidity 1 wk |
| Powder: fluoroaluminosilicate glass | 0503031 | |||
| Liquid: polyalkenoic acid, HEMA, dimethacrylate, camphorquinone, water | 0503021 | |||
| Fuji II LC | Cavity conditioner: 20% polyacrylic acid, 3% aluminum chloride | 0503021 | GC | Condition 10 s B , Rinse 10 s, Dry with Kimwipes, Mix capsule 10 s with 4,000 RPM, LC 20 s |
| Capsule: fluoroaluminum silicate glass, polyacrylic acid, HEMA | 0510241 | |||
| Ketac-Fil Plus | Powder: calcium aluminofluorosilicate glass | 240436 | 3M ESPE | Condition 10 s B , Rinse 15 s, Dry with Kimwipes Mix capsule 10 s with 4,200 RPM, Apply Vaseline, then leave in humidity 1 wk |
| Liquid: polyethylene polycarbonic acid, tartaric acid, water | 131595 | |||
| Vitremer | Primer: Vitrebond copolymer, HEMA, ethanol, photoinitiators | 6BF | 3M ESPE | Prime 30 s from 2 cm B , Dry until no fluid movement, LC 20 s, Mix powder/liquid 45 s, LC 40 s |
| Powder: fluoroaluminosilicate glass, microencapsulated potassium persulfate, ascorbic acid, pigments | 6EE | |||
| Liquid: polycarboxylic acid modified with pendant methacrylate groups, Vitrebond copolymer, water, HEMA, photoinitiators | 6ET | |||
Light curing was performed with an Optilux 500 curing unit (Demetron/Kerr, Danbury, CT, USA) using a radiant emittance of no less than 550 mW/cm 2 as measured with a radiometer in the wavelength range of 400–500 nm (Curing Radiometer model 100, Demetron Research Corp., Danbury, CT, USA). Immediately after the application of the adhesive, the resin-based composite Z100 (shade A1, 3 M ESPE, St Paul, MN, USA) was used to incrementally build a composite “crown” that was 4–5 mm in height with peripheral borders maintained entirely in dentin. The initial increment was limited to approximately 0.5 mm thickness. The composite was heated up to 54 °C (Calset Model 201 120 V, AdDent Inc., Danbury, CT, USA) to increase flow and adaptability . Each increment was light cured for 40 s from a distance of 1 mm using the same light curing unit and radiant emittance. After completion of the buildup, all restored teeth were left on the bench for 15 min before storing them in 100% humidity. After 1 h of storage, the restored teeth (except for Fuji BOND LC and Ketac-Fil Plus, which were stored for 1 week) were sectioned perpendicular to the adhesive-tooth interface with three water-cooled low-speed diamond saw blades (IsoMet 1000, Buehler Ltd., Lake Bluff, IL, USA) separated by 2 mm spacers using a cutting speed of 125 rpm and the application of a 150-g weight. Two sections were accomplished at right angles to yield four rectangular 2 mm × 2 mm sticks that were sectioned free from the dental stone mounting using a 1 in. diamond disk mounted in a dental lab motor (Kavo EWL Typ 950, Kavo Dental Corporation, Lake Zurich, IL, USA). Using a CNC Specimen Former, the sticks ( n = 256) were trimmed with a 0.8-μm, ultrafine cylindrical diamond bur (#012, Brasseler, Savannah, Georgia, USA) into dumbbell-shaped tensile test specimens with a round cross-sectional area of 0.5 mm 2 , a gage length of 1 mm, and a radius of curvature or ‘neck’ of 0.6 mm. They were examined under a stereomicroscope at 50× magnification (Stemi 2000, Zeiss, NY, USA) to verify proper fabrication. The diameter of each specimen was measured with a digital caliper (Digimatic caliper, Mitutoyo Corporation, Kawasaki, Japan). Equal numbers of the specimens from each tooth were randomly allocated to two groups: (1) 8 h post-bonding (which corresponds to immediate testing), and (2) 6-month storage at 37 °C in artificial saliva , which contained 0.1% sodium azide to inhibit microbial growth .
Microtensile testing was performed at a crosshead speed of 1 mm/min with a calibrated Zwick material testing machine (Zwick Materials Testing Machine Z2.5/TN1S, Zwick/Roell, Ulm, Germany) and testXpert software after the defined storage times. The specimen was gripped centrally with respect to the test axis with a non-gluing passive gripping device (Dircks Device, The University of Iowa, Iowa City, IA, USA). No specimens fractured outside of the test region (gage area) and the mean μTBS per tooth was used as the statistical unit.
2.2
Collection of clinical trial data and adhesive-related parameters in non-carious Class V restorations
Prospective clinical studies on Class V restorations were searched in MEDLINE (search period 12/2008) and IADR abstracts (1994–2008). The search words were “Class V” or “cervical” or “abfraction lesion” and “clinical”. The inclusion criteria were as follows:
- 1.
Prospective clinical trial involving at least 1 adhesive/restorative (ARS) system in Class V cavities.
- 2.
Minimum duration of 18 months.
- 3.
The study had to report the following outcome variables: retention, marginal discoloration, marginal integrity, secondary caries.
2.3
Statistical analysis
To test whether the μTBS results after 8 h of water storage differ from those after 6 months, the non-parametric Wilcoxon test was applied ( p < 0.05). The following clinical outcomes were retrieved from the studies: R = 100 − (% of retention loss), MD = 100 − (% of marginal discoloration) and MI = 100 − (% of detectable margins). Since most experiments had 0% of secondary caries, this outcome variable was not considered. Following a comparative study of marginal adaptation and the outcome of Class V clinical studies , the clinical performance was summarized by combining three clinical outcomes into one clinical index: CI = (4 R + 2MD + 1MI)/7.
A first look at the data roughly revealed that linear deterioration occurred over time. However, a simple regression model Y = 100− βt + error (equivalent to (100 − Y )/ t = β + error, where Y = dependent variable (retention, marginal discoloration), β = coefficient based on study characteristics (type of adhesive, dentin preparation, random effect) and t = time) was not adequate, as the error was not normally distributed (data not shown). Thus, we applied a square root transformation and opted for the model
100 − Y t = β + error
for which normality could be assumed. Since this model implies Y = 100−( β + error) 2 t , the slope −( β + error) 2 characterizing deterioration was, logically, forced to be negative and the quantity − β 2 was interpreted as median slope of deterioration.
Using a linear mixed model, we allowed the above coefficient beta to depend on the fixed effects of preparation, beveling, type of isolation and the adhesive and the random effect of the experiment to account for the fact that measurements within the same experiment were correlated. We did not include a study effect, since it would have been too much confounded with the adhesive effect. Estimates of the adhesive effects in this model were used to summarize the clinical performance of each adhesive between 12 and 36 months. They were inverted and centered in such a manner that a positive value corresponds to a performance above average, and a negative value to a performance below average (a zero value represents average performance). Spearman correlations between these clinical performances and the μTBS values were calculated.
2
Materials and methods
2.1
Microtensile bond strength
The μTBS testing procedure for all adhesive/restorative materials was performed by the same operator (TC) in the same laboratory with the same methodology. Sixty-four caries- and defect-free extracted human molar teeth, obtained according to institutional review board requirements, were stored in 0.5% chloramine T at 4 °C and used within 6 months after extraction. The teeth were cleaned and mounted in dental stone (Die-Keen ® Green, Heraeus Kulzer, Inc., Armunk, NY). Occlusal enamel was partially removed using a 600-grit wheel model trimmer (3/4HP Wet Model Trimmer, Whip Mix Corporation, Louisville, KY, USA). The remaining occlusal enamel was removed with a water-cooled carbide bur (#55, Brasseler, Savannah, Georgia) in an electric handpiece rotating at 200,000 rpm (KaVo Electromatic and Intramatic 25LHA: KaVo America Corporation, Lake Zurich, IL) mounted in a CNC Specimen Former (University of Iowa, Iowa City, IA) to expose superficial to middle dentin. After rinsing for 5 s with 35% phosphoric acid to confirm that all central enamel had been removed, an additional 0.1 mm of occlusal dentin was removed to expose unaltered dentin substrate for bonding.
The teeth were randomly distributed across 16 adhesive/restorative systems (four teeth per adhesive). Manufacturer’s instructions were followed for adhesive application as closely as possible with the following exceptions: (1) drying distance, pressure, and angulations were modified to ensure that excess moisture/solvent was completely removed from the flat occlusal surface, and (2) based on pilot study results, priming and priming/adhesive application times and the number of coats were increased for Syntac, Futurabond NR, and Xeno III to reduce the frequency of pre-test failures. See Table 1 for adhesive system composition, batch number and mode of application.
