Abstract
Objective
An ideal dental adhesive should provide retentive strength, marginal seal, be relatively simple to achieve and demonstrate clinical durability. Future improvements in adhesive bonding to tooth structure require in vitro test methods that provide reliable data for materials development and/or evaluation of experimental variables. The objective of this project was to identify a test method that is relatively easy to perform, repeatable and ultimately useful for predicting clinical outcomes.
Methods
The Academy of Dental Materials initiated a project to develop and distribute guidance documents on laboratory test methods that are useful for the evaluation of dental adhesives and cements, composite resins and ceramics.
Results
The dental adhesive sub-group has identified the micro-tensile bond strength test, especially after subjecting the specimens to a durability challenge, as currently the best practical surrogate measure of dental composite restoration retention.
Conclusion
The following μTBS guidance is meant to aid the researcher in conducting the μTBS test. The authors, while recognizing the limitations of a static, strength-based test method, welcome comments and suggestions for improvements of this guidance document in future revisions.
1
Introduction
Future improvements in adhesive bonding to tooth structure require in vitro test methods that provide reliable data for materials development and/or evaluation of experimental variables. Multiple test methods are necessary for understanding and improving the clinical performance of dental materials; however, a chosen test method should be relatively easy to perform and must be repeatable within and between laboratories, and ultimately useful for predicting clinical outcomes . Ideally, adhesion to tooth structure should provide retentive strength, marginal seal, be relatively simple to achieve and have clinical durability. The micro-tensile bond strength test (μTBS), especially after subjecting the specimens to a durability challenge, is currently recommended by the authors as the best surrogate measure of dental composite restoration retention . The following guidelines are meant to aid the researcher in conducting the μTBS test. The authors welcome comments and suggestions for improvements of guidelines in future revisions.
1.1
Terminology
- a)
Passive gripping device ( Fig. 1 A): specimen is placed in a testing device without the aid of glue or mechanical gripping; device should self-align the specimen parallel to the tensile load.
Fig. 1 (A) Passive gripping device; (B) active gripping device. - b)
Active gripping device ( Fig. 1 B): mechanical fastening of specimen to gripping device, such as with the use of glue or “clamps”.
- c)
Stick specimen ( Fig. 2 A): cross-sectionally square micro-specimen, also referred to as a “non-trimmed specimen” or sometimes as a “beam”.
Fig. 2 (A) Stick specimen; (B) dumbbell specimen; (C) hourglass specimen. - d)
Dumbbell specimen ( Fig. 2 B): a smoothly notched specimen with a defined radius of curvature from the end or shoulder of the specimen to a straight gauge length that defines the testing region and includes the adhesive joint. This specimen is one kind of a “trimmed specimen”. Although trimming by hand has been done, it is highly preferable to machine the dumbbells using mechanically or computer-controlled high-speed hand piece movement to produce exacting geometries with identical fabrication histories ( Fig. 3 ).
Fig. 3 Schematic illustrating the preparation of a ‘trimmed’ dumbbell specimen. Dumbbell micro-specimens are preferentially prepared using mechanically or computer-controlled high-speed hand piece movement, e.g. MicroSpecimen Former (University of Iowa). - e)
Hourglass specimen ( Fig. 2 C): a notched specimen with a defined radius of curvature from the end or shoulder of the specimen to the adhesive joint without a defined straight gauge length. This specimen is the other kind of a “trimmed” specimen.
- f)
Pre – testing failures (“ ptf ”): a specimen failure that occurred before tensile testing and cannot be attributed to any human manipulation error (see below); it possesses an unknown bond strength, which is an observation commonly referring to weak adhesive performance and therefore cannot be ignored; it has also been referred to as left-censored data.
- g)
Manipulation error : a specimen failure that occurred at any moment during specimen preparation or testing and should be attributed to a human manipulation error and should be clearly differed from the abovementioned pre-testing failures. Occurrence and number needs to be explicitly noted, but are excluded from the dataset.
- h)
Tooth dependency : when testing multiple specimens from the same tooth. Tooth dependency must be accounted for if each specimen is considered as an experimental unit, e.g. random effects for ANOVA, frailty effects for Weibull . Seek expert statistical support.
2
Guidelines/specific recommendations for adhesive–dentin μTBS testing (recommended approach highlighted in bold when multiple options are given)
- 1.
Selection of the teeth:
- a)
Ethics: Project must comply with local Medical Ethics Committee requirements. Local Medical Ethics Committees may grant an exemption for formal review when using teeth that have been discarded as medical waste material and recuperated for secondary use without patient identifiers.
- b)
Substrate: Human permanent molars (or premolars) (deciduous teeth if required by research question).
- c)
Age of teeth: should be described if available, but is essential if age of substrate is the variable to be investigated.
Note: “It is preferable to use third permanent molars from 16- to 40-year-old individuals if possible” (according to ISO/DTS 11405 Dentistry — Testing of adhesion to tooth structure) .
- d)
Number of teeth: Ideally based upon power calculation performed in planning stages of study; if the specimen is used as the statistical unit, an absolute minimum of three teeth per experimental group with appropriate consideration of tooth dependency/clustering; if tooth is the statistical unit at least 5 and by preference 8–10 teeth per experimental group.
- e)
Post-extraction time: preferable less than 1 month.
Note: less than 6 months according to ISO/DTS 11405 .
- f)
Condition of teeth: non-restored, not root-canal treated, caries free, intact in area of bonding interest and ultimate test specimen geometry.
- g)
Additional substrate conditions: determined by research questions and research design, e.g. caries-affected, sclerotic, erosive dentin, etc.
- a)
- 2.
Storage of extracted teeth:
- a)
Upon extraction: Store teeth in 0.5% Chloramine-T immediately after extraction.
- b)
Upon receipt in the laboratory: Remove adherent tissue, clean teeth and place in fresh 0.5% Chloramine-T and store at 4–7 °C.
- c)
Tooth hydration: Do not allow the tooth to dehydrate from point of extraction throughout the entire experimental process (continuously immerse in aqueous media without antimicrobial agent or cover with wet tissue paper in between the processing steps).
- d)
Aqueous storage media type: Laboratory grade water is not necessary, but if using deionized water be aware that tooth surface demineralization has been documented, therefore a phosphate-buffered salt solution of neutral pH , e.g. Dulbecco’s Phosphate-Buffered Saline (DPBS), Hank’s balanced salts solution (HBSS) is recommended .
- a)
- 3.
Tooth mounting and preparation:
- a)
Mounting: mount the complete tooth (root still present; do not use dentin disks) by preference in dental stone (or acrylic resin) for subsequent surface preparation and specimen fabrication, taking care to carefully align the surface of interest ( Fig. 4 ).
Fig. 4 Schematic illustrating the successive steps of tooth mounting and bonding. - b)
Create retention, for instance, by preparing two shallow grooves at two opposite root sites for securing tooth in stone throughout specimen fabrication steps.
- c)
Tooth region of interest should be above level of stone to avoid subsequent contamination during application of adhesive and dental composite.
- d)
Exposure of dentin at the occlusal surface is recommended unless research design dictates otherwise.
- e)
Continue specimen preparation ‘immediately’ after dentin exposure with smear layer preparation, adhesive treatment, etc., to avoid abundant water uptake by the exposed dentin (if needed, short coverage with wet tissue paper is recommended rather than complete immersion in water).
- a)
- 4.
Tooth dependency:
- a)
Use as much as possible a research design balanced by tooth dependency, e.g. 24 h, 3 mo., 6 mo., 12 mo. aqueous storage with random assignment of individual specimens from the same tooth to each of the four storage groups ( Fig. 5 A). As the micro-specimens are cut prior to the long-term storage, any pre-testing failures that may have occurred should be equally divided over the different experimental groups; in case of an odd ptf number, one more ptf is assigned to the longest storage group.
Fig. 5 Research designs: (A) balanced tooth dependency design with only one adhesive treatment; individual specimens from the same tooth are randomly assigned to each of two, three, or four experimental aging groups. (B) Split-tooth design; each tooth halve serves as a separate experimental group involving a different adhesive application protocol; up to four different adhesive treatments can be applied per tooth, when the tooth surface is separated in 4 quarters. - b)
Split-tooth design, e.g. two bonding procedures, with maximum 4 different conditions per tooth: The areas should be separated by a thin and shallow groove (cut using a thin 150-μm diamond blade), in which a barrier (i.e. razor blade, cover slip) can be positioned to separate both tooth halves ( Fig. 5 B).
- c)
Tooth dependency cannot be ignored in data reporting and statistical analyses — see data handling and statistical reporting .
- a)
- 5.
Cavity and surface preparation:
- a)
Cavity preparation: The standard preparation is a flat surface for preparation simplicity and control of variables; however, cavities can also be prepared depending upon the needs of the research design, e.g. study of C-factor, bond to different cavity walls .
- i)
Bonding surfaces tested must be perpendicular to ultimate loading direction.
- ii)
To simulate bonding to the cavity floor, burs and instruments should be aligned perpendicular to the bonding surface, which may require special equipment (e.g. MicroSpecimen Former, University of Iowa). Monitor bur wear and change regularly, e.g. every 6–12 teeth.
- i)
- b)
Smear layer creation:
- i)
Most appropriate if created immediately after dentin exposure (see above) as well as shortly before bonding.
- ii)
Burs: Clinically relevant smear layer is best produced by carbide and diamond dental burs that are chosen according to research design ( Table 1 ).
Table 1Burs to prepare a bur-cut clinically relevant smear layer.Bur (μm) Notes 8-fluted carbide NA Simulates DIRECT bonding Medium grit diamond 107 Simulates DIRECT bonding Fine grit diamond 46 Simulates INDIRECT bonding - iii)
Abrasive papers: Clinically simulated smear layer produced with abrasive papers requires careful attention to align flat surfaces perpendicular to load direction and random surface scratches using a circular motion (rather than in one direction); wet-sanding is preferred ( Table 2 ). Grit sizes are not directly comparable between bur and silicon carbide abrasive paper preparation methods, as different surfaces and smear layers are created.
Table 2Silicon carbide papers to prepare a clinically relevant smear layer.Paper (μm) Notes FEPA P320 46.2 ± 1.5 Simulates DIRECT bonding J360 48 ANSI240 51 FEPA P600 25.8 ± 1 Simulates INDIRECT bonding a J600 29 ANSI360 28 FEPA P = Federation of European Producers of Abrasives, sanding paper; J = Japanese Industrial Standard; ANSI = American National Standards Institute. - iv)
Other preparation instruments (e.g. laser, sono abrasion, etc.) may be necessary based upon specific research designs, which also need to be employed in an as uniform as possible way.
- i)
- c)
Occlusal depth: ‘superficial’, by removing last remnant of enamel, to ‘mid-coronal’ dentin. Check with light microscope and/or 30–40% phosphoric acid for 3–5 s to confirm enamel removal followed by additional 0.1-mm surface removal.
- d)
Surface region: to reduce regional and tooth variability, a fixed number of specimens per tooth from a central region of the teeth should be obtained . For example, four sticks of 2-mm width per tooth for ‘trimmed’ (dumbbell, hourglass) micro-specimen fabrication (to reach an interface diameter of 0.8–1 mm; 0.8 mm cylindrical dumbbell has a cross-sectional area of 0.5 mm 2 ) ( Fig. 6 a); 8–16 sticks of 1-mm width per tooth for the ‘non-trimmed’ technique ( Fig. 6 b).
Fig. 6 Producing a fixed number of micro-specimens with a favorable regional distribution. (A) 4 sticks per tooth for a TRIMMED micro-specimen fabrication; (B) 8–16 sticks per tooth for a NON-TRIMMED micro-specimen fabrication.
- a)
- 6.
Adhesive application:
- a)
A detailed description of the application of the adhesive should be provided, as “ Applied by manufacturer ’ s directions ” is inadequate to reproduce employed methodology; this should include:
- i)
Conditions in which restorative procedure was completed:
- ii)
Absolute field control/rubber dam simulation (room conditions): 23 ± 2 °C; 50 ± 5%RH.
- iii)
Relative field control simulation (mouth conditions): 35 ± 2 °C; 90 ± 5%RH.
- ii)
- iv)
Tips and applicators used.
- v)
Dispensing/mixing.
- vi)
Time of application.
- vii)
Method of application (left untouched, actively rubbed, repeated fresh application, etc.).
- viii)
Air-drying method, pressure, time, etc.
- ix)
Photopolymerization: see “Photopolymerization” below.
- x)
Etc.
- i)
- a)
- 7.
Dental composite:
- a)
Research design permitting, use same dental composite, with sufficiently high mechanical properties, for all groups and for all studies. An agreed upon industrial-supplied standard dental composite would be a welcome and useful addition to the field. An exception is when one wants to investigate the contribution of the dental composite to the bond strength; then different dental composites should be combined with same adhesive.
- b)
Provide detailed description of dental composite application (layering, bulk, instrument kind, etc.).
- c)
Do not exceed recommended increment thickness for specific material, e.g. 2 mm (unless required by research design, e.g. 4-mm bulk-filling).
- d)
Apply dental composite without voids or defects. One approach is to apply dental composite directly from compule, then carefully remove “wiped” component of dental composite from tooth and compule tip before photopolymerization and next dental composite application.
- e)
Build up to reach at least a 4-mm thickness (will serve as support and connection to jig).
- f)
Final layer can be covered with glass plate using light finger pressure before curing.
- a)
- 8.
Photopolymerization:
- a)
Use a curing light that has a radiant exitance from 500 to 2000 mW/cm 2 and deliver sufficient energy to cure materials (adhesive, composite) and report radiant exposure (energy delivered) (J/cm 2 ) to the material surface for each light delivery step . As research design permits, recommend delivering 150% required incident radiant exposure for adhesive and each dental composite increment to help reduce effect of polymerization as a systematic experimental error.
- b)
Use photopolymerization unit that matches photo-initiator absorption requirements and report spectral irradiance.
- c)
Use photopolymerization unit with homogenous irradiance and spectral emission from the photopolymerization tip .
- d)
If possible, use photopolymerization tip that effectively covers entire restoration to avoid necessity of overlapping light curing and report light tip size.
- e)
Photopolymerization unit is best held in a fixture from a fixed distance perpendicular to the material surface to apply the required radiant exposure. The light tip is kept in the same orientation for all specimens; alternatively, keep tip end parallel with material surface at 1-mm distance for first 5 s and then let tip end rest on the material surface (plastic protection around tip end needed).
- a)
- 9.
Macro-specimen storage/aging (bonded tooth):
Aqueous media aging is better suited to micro-specimens for accelerated aging , but can also be performed in the macro-specimen form. Mechanical fatigue and thermo-mechanical aging are currently best performed in the macro-specimen form.
a) Storage in water or by preference in artificial saliva (see micro-specimen storage/aging below); alternative accelerated aging solutions (as used in literature: propionic acid, acetic acid, ethanol, NaOCl, etc.) could be used, but cannot be considered as standard and therefore should be described in sufficient detail to replicate in future studies.
b) Thermo-cycling: Minimum 10,000 cycles (and by preference more) in aqueous media between 5 °C and 55 °C, starting after 1–7 day storage in aqueous media at 37 °C. The exposure to each bath should be at least 20 s, and the transfer time between baths should be as short as possible (e.g. 5–10 s). (ISO/DTS 11405 Test type 2 of 500 cycles is inadequate to produce a significant effect.)
c) Mechanical or thermo-mechanical fatigue: Chewing simulator with bonded teeth in single or multiple chambers: use a round 6-mm diameter steatite agonist, apply (for example) 100,000 cycles at 50 N at a frequency of 0.5 Hz with (optionally) 2500 simultaneous thermal cycles between 5 °C and 55 °C for 30 s at each temperature .
- 10.
MACRO- to MICRO-specimen stick formation by sectioning ( Figs. 5 and 6 ):
- a)
Diamond saw sectioning: Specify speed, pressure/load, etc.; monitor wear and regularly change blades.
- b)
Gauge saw sectioning is recommended, that is spacers of desired width between sets of saw blades for single pass sectioning.
- c)
Reinforce/support sectioned sticks/slabs with dental stone (or alginate or other more rigid impression material) between first and second saw pass.
- d)
1 × 1 mm maximum size for ‘non-trimmed’ μTBS testing; 2 × 2 mm specimen dimensions for ‘trimmed’ μTBS testing.
- e)
Micro-specimens that de-bond during this sectioning process should be gathered, and recorded as ‘pre-testing failures’.
- f)
Diamond wire sectioning has been reported to reduce sectioning defects , but has not been commonly used to date.
- g)
Remaining dentin thickness should be measured for each micro-specimen and recorded for possible influence on resultant bond strength . A minimum of 1 mm remaining dentin thickness should be present for testing.
- a)
- 11.
Additional specimen processing to create ‘trimmed’ (dumbbell, hourglass) micro-specimens:
- a)
Do not allow the tooth to dehydrate during the sectioning process.
- b)
Dumbbell recommended over other geometries due to presence of straight gauge region producing uniformly stressed testing area of interest. Fractures that occur outside of this uniformly stressed gauge region are not recommended for direct calculation of mean μTBS (ASTM C1273-15 Standard Test Method for Tensile Strength of Monolithic Advanced Ceramics at Ambient Temperatures).
- c)
Circular neck and gauge region formation preferred over sharp edged to avoid stress concentration at corners.
- d)
Requires larger sticks for trimming, e.g. 2 × 2 mm (see above).
- e)
Requires specialized mechanical or computer-driven equipment for consistent and uniform size and geometry, as well as consistent specimen loading (e.g. MicroSpecimen Former, University of Iowa); preparation by hand is NOT acceptable.
- f)
Micro-specimens that de-bond during this trimming process should be gathered, and recorded as ‘pre-testing failures’.
- a)
- 12.
Micro-specimen storage/aging (non-trimmed and trimmed sticks):
- a)
Aqueous storage (see above for alternative aging solutions)
- i)
Short-term (<1 month) — aqueous storage media is replaced weekly to avoid contamination.
- ii)
Medium-term (1–6 months) — aqueous storage media containing 0.5% Chloramine-T is replaced biweekly to avoid contamination.
- iii)
Long-term (6 months or longer) — aqueous storage media containing 0.5% Chloramine-T is replaced biweekly to avoid contamination (ISO/DTS 11405 Test Type 3) .
- i)
- b)
Thermo-cycling
- i)
Special containers to hold the micro-specimens are needed to maintain them securely and allow efficient water exchange. Although micro-specimens may show degradative effects more readily than macro-specimens, the thermo-cycling regimen described above is recommended .
- i)
- c)
Mechanical fatigue or thermo-mechanical fatigue
- i)
Four-point bending cyclic loading of individual μTBS stick specimens until failure or 10 6 cycles (specify load and fatigue frequency) .
- ii)
Cyclic tensile fatigue loading of micro-specimens fixed by their bottom and top into a material tester that enables cyclic loading .
- iii)
Micro-rotary fatigue of individual μTBS dumbbells: specimen rotated at 4 Hz around their main axis, while being loaded at a fixed distance from the adhesive interface to induce a sinusoidal cycling tensile and compression stress at the outer surface of the adhesive–tooth interface. The specimen is continuously sprayed with water at 37 °C throughout testing. Specimens stressed until failure or until 100,000 cycles are reached to report a median micro-rotary fatigue resistance .
- iv)
Chewing simulator with adhesive–dentin specimens in multiple chambers: using a round 6 mm diameter steatite agonist to apply (for example) 100,000 cycles at 20 N at a frequency of 0.5 Hz with (optionally) 2500 simultaneous thermal cycles between 5 °C and 55 °C for 30 s at each temperature .
- i)
- a)
- 13.
Specimen gripping device, AKA “jig”
- a)
Active gripping jigs ( Fig. 7 ):
- a)
