Shear bond strength of orthodontic brackets bonded with primer-incorporated orthodontic adhesives and unpolymerized 3-dimensional printing materials on 3-dimensional-printed crowns

Introduction

The use of 3-dimensional (3D) printing techniques in fabricating crowns has increased the demand for bracket bonding onto these surfaces. The objective was to evaluate the shear bond strength (SBS) of orthodontic brackets bonded onto 3D-printed crowns using primer-incorporated orthodontic adhesives and 3D printing materials as orthodontic adhesives.

Methods

A total of 160 crowns were printed with two 3D printing materials, DentaTOOTH (Asiga, Sydney, Australia) (group A) and NextDent C&B Micro Filled Hybrid (3D Systems, Soesterberg, Netherlands) (group N). Each group was randomly divided into 4 adhesive subgroups (n = 20): Transbond XT (for groups A [ATX] and N [NTX]; 3M Unitek, Monrovia, Calif), Ortho Connect (for groups A [AOC] and N [NOC]; GC Corporation., Tokyo, Japan), Orthomite LC (for groups A [AOM] and N [NOM]; Sun Medical, Co Ltd, Moriyama, Shiga, Japan), and unpolymerized liquid state of 3D printing resin (for groups A [AA] and N [NN]). SBS was measured with a universal testing machine at a crosshead speed of 0.5 mm/min. The adhesive remnant index and the mode of failure were analyzed under the microscope. Statistical analysis was performed at a significance level of α = 0.05.

Results

When used as adhesives (AA and NN), 3D printing materials showed no statistically significant difference in SBS compared with Transbond XT (ATX and NTX, respectively). In group N, NN showed a significantly higher SBS than primer-incorporated orthodontic adhesives (NOC and NOM; P <0.001). Adhesive failures were only observed in primer-incorporated orthodontic adhesives (AOC, NOC, AOM, and NOM).

Conclusions

Primer-incorporated orthodontic adhesives, as well as unpolymerized 3D printing materials employed as orthodontic adhesives on 3D-printed crowns, exhibited comparable bonding strength to Transbond XT without surface modification. Despite variations in adhesive-related factors, all measurements stayed within clinically acceptable ranges, highlighting the potential of these materials for orthodontic bonding on 3D-printed crowns, simplifying clinical procedures without compromising bond strength.

Highlights

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Unpolymerized 3D printing materials can bond brackets on 3D-printed crowns without surface prep.
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Primer-incorporated orthodontic adhesives have clinically acceptable SBS on 3D-printed crowns.
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Bonding strengths and failure modes differed depending on the adhesives used on 3D-printed crowns.

Interdisciplinary treatment plans cover a wide range of dental issues, from minor tooth movement to more complicated issues. In interdisciplinary patients involving orthodontic and restorative dentistry, it is not unusual for patients to wear provisional crowns during their orthodontics and before placement of definitive restorations. These provisional crowns can be 3-dimensional (3D) printed and may require orthodontic brackets bonded directly to them. Along with advancements in 3D printing, also known as additive manufacturing, there has been an overall increase in the use of 3D-printed materials for provisional restorations. Consequently, there is a growing demand for bonding brackets onto these materials.

Among 3D printing systems, printing 3D objects by photopolymerization is commonly used in dentistry. This is the process of exposing liquid materials to ultraviolet light for polymerization. DentaTOOTH (Asiga, Sydney, Australia) is a light polymerizing 3D printing material used in the fabrication of denture resin teeth, inlay, onlay, veneer, crowns, and bridge provisional restorations. NextDent C&B Micro Filled Hybrid (MFH; 3D Systems, Soesterberg, Netherlands) is also an ultraviolet light polymerizing 3D printing provisional material characterized by MFH filler particles that improve mechanical properties compared with other 3D printing materials.

Orthodontic adhesives are primarily designed to obtain a durable bond between the bracket and the enamel. Therefore, it is essential to evaluate the bond strength of orthodontic adhesives on surfaces of restorative materials. Conventional bonding requires multiple steps, including etching, priming, and adhesive resin applications. The introduction of a primer-incorporated orthodontic adhesive has simplified the bonding process by eliminating the surface treatment and priming steps. One-step primer-incorporated orthodontic adhesives save time and reduce the risk of contamination because fewer steps are involved in bonding.

Unlike conventional bracket bonding to enamel surfaces, additional surface treatments, such as sandblasting, additional primer application, or surface silanization, are required on gold, ceramic, and zirconia surfaces because of the higher incidence of bonding failure. As such, conventional bonding strategies may not be effective in bonding brackets to 3D-printed materials. Despite the studies on bonding on tooth surfaces, there is still a lack of evidence on bracket bonding on 3D-printed restorations. A bonding strategy involving the application of unpolymerized liquid-state 3D printing material onto 3D-printed dentures, aided by a dental light-curing unit (LCU), has been reported as effective. Thus, the evaluation of the use of unpolymerized 3D printing material as an orthodontic adhesive was included in this study.

This study aimed to determine the shear bond strength (SBS) of ceramic brackets bonded with primer-incorporated orthodontic adhesives and unpolymerized liquid state of 3D printing resin on 3D-printed crowns. The null hypothesis posits that there are no significant differences in SBS, adhesive remnant index (ARI), and mode of failure among the adhesives used within the groups of 3D-printed crowns.

Material and methods

A total of 160 crowns were printed with 2 different 3D printing materials, DentaTOOTH (group A) and NextDent C&B MFH (group N) ( Fig 1 ; Table I ). Each group was randomly divided into 4 adhesive subgroups (n = 20): One conventional orthodontic adhesive, Transbond XT (for groups A [ATX] and N [NTX]; 3M Unitek, Monrovia, Calif); 2 primer-incorporated orthodontic adhesives, Ortho Connect (for groups A [AOC] and N [NOC]; GC Corporation., Tokyo, Japan) and Orthomite LC (for groups A [AOM] and N [NOM]; Sun Medical, Co Ltd, Moriyama, Shiga, Japan); and unpolymerized 3D printing materials, DentaTOOTH (for group A [AA]) and NextDent C&B MFH (for group N [NN]).

Table I

Composition of 3D printing materials and orthodontic adhesives

Product	Composition	Manufacturer
3D printing crown and bridge material
NextDent C&B MFH	Methacrylate oligomer, methacrylate monomer, inorganic filler, phosphine oxides	3D Systems, Soesterberg, Netherlands
Asiga DentaTOOTH	Bismethacrylate, tetrahydrofurfuryl, methacrylate, diphenylphosphine oxide	Asiga, Sydney, Australia
Orthodontic adhesive
Transbond XT (conventional)	Primer: TEGDMA, Bis-GMA, DMAEMA, CQ, hydroquinone Adhesive: TEGDMA, Bis-GMA, DMAEMA silane-treated quartz, Bis-EMA, silane-treated silica, TEGDMA, CQ	3M Unitek, Monrovia, Calif
Ortho Connect (Primer-incorporated)	Bis-EMA, UDMA, phosphoric acid ester monomer, photoinitiator, stabilizer	GC Corporation, Tokyo, Japan
Orthomite LC (Primer-incorporated)	HEMA, filler, methacrylate monomers, UDMA, 2-propenoic acid, 2-methyl-, 1,3-phenylene bis(oxy-2,1-ethanediyl) ester	Sun Medical, Co Ltd, Moriyama, Japan

TEGDMA , triethylene glycol dimethacrylate; Bis-GMA , bisphenol A glycerolate dimethacrylate; DMAEMA , dimethylaminoethyl methacrylate; CQ , camphorquinone; Bis-EMA , bisphenol A ethoxylate dimethacrylate; UDMA , urethane dimethacrylate; HEMA , 2-hydroxyethyl methacrylate.

A maxillary right central incisor was designed using software (Exocad DentalCAD; Exocad GmbH, Darmstadt, Germany; Fig 2 , A ).

In group A, DentaTOOTH crowns were printed with an Asiga MAX printer (Asiga) (n = 80). The design file was exported to the Asiga MAX slicing software (version 1.1.7; Asiga Composer) and printed at a resolution of 1920 × 1080 pixels and a build volume of 119 × 67 × 75 mm with 5 base layers of 50 μm thickness each. The printed specimens without any defects were selected by visual inspection and rinsed in 99.8% isopropanol (Sigma Aldrich, St Louis, Mo) for 5 minutes. For the final rinse, the specimens were placed in an ultrasonically activated container with fresh isopropanol for 5 minutes according to the manufacturer’s instructions. Specimens were postpolymerized for 3 minutes using Flash Cure (Asiga) with 90 seconds of exposure on both labial and lingual sides.

In group N, NextDent C&B MFH was printed with a DLP type printer, NextDent 5100 (3D Systems) (n = 80). The design file was exported to the slicing software 3D Sprint (3D Systems) and printed at a resolution of 1920 × 1080 pixels and a build volume of 125 × 70 × 196 mm with 5 base layers of 50 μm thickness each. Printed specimens without defects were selected by visual inspection and rinsed in 99.8% isopropanol for 5 minutes. For the final rinse, the specimens were placed in an ultrasonically activated container with fresh isopropanol for 5 minutes according to the manufacturer’s instructions. Specimens were postpolymerized with LC-3DPrint Box (3D Systems) for 30 minutes.

During the above process, all of the tests were done by 1 operator (Y.C.). The specimens were placed centrally to minimize light disturbance during the postpolymerization. The specimens were immediately stored in distilled water at 37°C for 24 hours.

After the storage, the printed crown surface was gently dried with air before the bracket bonding. Maxillary right central incisor ceramic brackets (Perfect Clear 2; Osstem Orthodontics Inc, Uiwang-si, Gyeonggi-do, South Korea) were used for all tests.

In ATX and NTX, a thin coat of Transbond XT primer was applied on the buccal surface of the crown. The primer was dried with gentle air and light-cured with a monowave light-emitting diode dental LCU (Elipar DeepCure-S; 3M Oral Care, St. Paul, Minn) for 20 seconds (1470 mW/cm ² ). Transbond XT adhesive resin was applied to the bracket base, and the bracket was positioned and pressed using hand pressure onto the crown for 5 seconds. Excess material was removed using a dental explorer, and the adhesive was light-cured with Elipar DeepCure-S for a total of 20 seconds, 10 seconds each, on the mesial and distal sides.

In groups AOC, NOC, AOM, and NOM, primer-incorporated orthodontic adhesives were applied on the bracket base and cured as in ATX and NTX.

In group AA, unpolymerized liquid state DentaTOOTH was used as an orthodontic adhesive. It was applied on the bracket base and bonded onto the buccal surface of the crown. Excess material was removed using a dental explorer and light-cured with a polywave LCU (Bluephase N G4; Ivoclar Vivadent, Schaan, Liechtenstein), with high mode (1200 mW/cm ² ) for 20 seconds, 10 seconds each on mesial and distal sides.

In group NN, unpolymerized liquid state NextDent C&B MFH was used as an orthodontic adhesive. It was applied on the bracket base and bonded onto the buccal surface of the crown as in group AA.

Each adhesive system was used according to the manufacturer’s instructions. After bonding, specimens were immersed in distilled water and stored at 37°C for 24 hours before the SBS measurement.

The SBS test was performed using a universal testing machine (Instron 8848; Instron, Canton, Mass) with a 500 N load cell and a crosshead speed of 0.5 mm/min (ISO 11405). The force was applied from the incisal edge parallel to the bracket-crown interface ( Fig 2 , B ). The maximum force (N) required to debond the ceramic bracket from the surface was recorded. The SBS values (MPa) were calculated by dividing the force by the surface area of the bracket base.

The surfaces of debonded specimens were evaluated under a stereomicroscope (Optinity KS-208; Korea Lab Tech, Seongnam-si, Gyeonggi-do, South Korea) at 20× magnification. The ARI and mode of failure were assessed.

The ARI was scored as follows : 0 (no adhesive remains on the crown surface), 1 (less than half the adhesive remains on the crown surface), 2 (more than half the adhesive remains on the crown surface), and 3 (all the adhesive remains on the crown surface).

The mode of failure was categorized into 3 groups: cohesive failure for those occurring within the adhesive layer; adhesive failure for the failures involving debonding at the interface of the crown and the adhesive; and mixed failure was a combination of these 2 types.

The sample size was determined after a pilot test with 10 specimens from the subgroup ATX. With the calculation of the margin of error obtained from the test, the sample size was determined to be 11. Therefore, 20 specimens per subgroup were tested.

The normality and homogeneity of variances were analyzed using the Shapiro-Wilk test and Bartlett’s test, respectively, using SPSS software (version 23; IBM, Armonk, NY). When both normality and homogeneity of variances were satisfied, a 1-way analysis of variance with Tukey’s correction was used. When homogeneity was not satisfied, Welch’s analysis of variance with Dunnett’s T3 post-hoc test was used at a significance level of α = 0.05. The χ ² test was used to evaluate the statistical significance of ARI scores and mode of failure analysis within the groups.

Results

The results of the SBS were presented as mean ± standard deviation ( Table II ). In group A, there was no significant difference between the subgroups ( P >0.05; Fig 3 , A ). In group N, the difference between subgroup NTX and NN was not statistically significant (11.99 ± 4.51 MPa and 15.29 ± 2.34 MPa, respectively; P >0.05). Subgroup NN showed a statistically significant higher bond strength than NOC (10.93 ± 3.56 MPa; P < 0.001) and NOM (11.13 ± 2.61 MPa; P <0.001) ( Fig 3 , B ). Among the samples, 4 outliers and 3 specimens with excessive resin remaining on the crown surface because of incomplete resin removal when evaluated under the stereoscopic microscope (20×) were excluded from the analysis.

Table II

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