Contrary to an old belief, four corner curing does not provide more bond strength or polymerization in bonding of orthodontic metal brackets

Sampling

Seventy-five extracted human upper premolars were collected and divided randomly into three groups each 25 teeth. The inclusion criteria of selected teeth were intact buccal enamel, no history of the application of chemical adhesives, no visible cracks, and no restorations and caries extended to the buccal surface. We washed the teeth with water to remove contaminations and then stored them in a bottle containing distilled water at 4°C. The time between extraction and the tests was no longer than 3 months.

Specimen preparation

The buccal surfaces of all teeth were cleaned with oil-free, non-fluoride pumice, and then rinsed with water and dried with moisture-free Airstream for 20 s. Then, the enamel was etched with a conventional 35% phosphoric acid (Ultra Etch, Ultradent Products Inc, South Jordan, USA) for 20 s. Then, the etched surface was rinsed for 20 s until the etchant was completely removed and then dried with oil and moisture-free air source to achieve a frosted appearance. A thin uniform coat of Transbond XT primer (3 M Unitek, Monrovia, CA, USA) was applied to the etched enamel surfaces and Transbond XT (3 M Unitek, Monrovia, CA, USA) adhesive was applied to the bracket bases. The brackets used in this study were stainless steel mesh base premolar with a 0.022-in slot (Master Series Brackets, American Orthodontics, WI, USA). Immediately, the brackets were placed on the tooth surface in their standard position and then pressed firmly. Excess resin material was carefully removed from around the bracket with a dental probe without disturbing it before polymerization.

Before light curing, the teeth were randomly divided into three groups each containing 25 teeth based on the mode of curing [Figure 1]:

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Figure 1:

(a-c) Three groups based on the mode of irradiation; Group A: One-sided, Group B: Two-sides, and Group C: Four corners curing.

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Group A: 20 s curing from the occlusal aspect of the bracket.

Group B: 10 s from the occlusal and 10 s from the cervical aspect.

Group C: 5 s from four corners of the bracket.

The brackets were light-cured with a conventional QTH curing light (Litex 680A, Dentamerica Inc., California, USA) for 20 s. Irradiation was performed from the bracket edges at an angle of 45° to the surface. All bonding procedures were performed by an orthodontist. For keeping blindness, the remaining process of the study was done by another orthodontist who was not aware of the types of curing. For this, the first clinician placed the samples in three different boxes with the codes unrelated to the type of bonding and the study proceeded with these codes. After finishing the tests and statistical analyzes, the codes were translated to group names by the first clinician.

After bonding, the teeth were embedded in self-curing acrylic resin using a standard plastic cylinder which was prepared in the size to be fitted in the universal testing machine (Hounsfield Test Equipment: H5K Model, England). We used a jig to position the buccal surface of the premolars perpendicular to the base of the cylindrical specimen holder ring with the help of the slot of the bracket. Therefore, the buccal surface of the teeth will be parallel to the arm of the universal testing machine. Then, the embedded teeth were stored in distilled water at 37°C for 24 h.

Tastings

FTIR spectroscopy

Immediately after debonding, the remnant composite was collected and composite particles were ground into a fine powder. Then, the powder was stored in a dark box until the onset of the FTIR test. Ten milligrams of the fine ground powder were mixed with 100 mg of KBr powder. This mixture was placed into a pelleting device and then pressed for 1 min to obtain a pellet. Then, the pellet was placed into a holder attached to the spectrophotometer.

The polymerization ratio was monitored by the FTIR spectroscopic analysis in transmission mode and it can be convertible to absorbance mode, using the device’s software. The FTIR spectrometer (Bruker Tensor 27 IR, Bruker, Germany) ran under the following conditions: Spectral range: 7500–370 cm⁻¹, wavenumber range accuracy: 0.01 cm⁻¹, resolution: Better than 1 cm^–1, and scan speed: 20 kHz (12.7 mm/s opd, from 32 scans). The percentage of unreacted carbon-carbon double bonds (C=C) was determined from the ratio of absorbance intensities of aliphatic C=C. The transmission ratio of the aliphatic C=C peak at 1638 cm⁻¹ can be determined with this test.

Statistical analysis

We use a one-sample Kolmogorov–Smirnov test for checking the normality. The data were not normal. Hence, we used Kruskal–Wallis test for assessing the significance between the groups. All statistical analysis was done with SPSS (version 24.0, Chicago, IL) and P < 0.05 was considered significant.

SBS

The overall average of SBS in the present study was 93.81 N. The area of the bracket base was calculated at 10.31 mm². Hence, the mean pressure for debonding was 9.09 MPa which is comparable with the results of Endo et al.^[13] and Bishara et al.^[3] who reported 11.94 MPa and 7.0–12.2 MPa, respectively, for permanent teeth. They also used a universal testing machine for debonding. In the present study, SBS of all groups was well beyond the acceptable range of bond strength for successful clinical performance which is 6–8 MPa.^[14] This shows that all three directions of light curing can be acceptable. Hobson et al. emphasized that, at this range of bond strength, < 14% of bonds would fail in the worst scenario.^[15]

The theory behind the suggestion of four corner curing is that metal brackets are not translucent and the light cannot pass through them, so some authors propose curing from different directions to achieve adequate bond strength by ensuring that photons of light cure reach all layers of composite and completely polymerize it.^[16,17] However, our study showed that this theory is not true. A possible explanation is that the diameter of light tube (8 mm) is completely larger compared to the width of the bracket (3.5 mm); therefore, with curing from one aspect, other aspects receive a degree of curing automatically and changing the direction of the tube may not be necessary.

Polymerization ratio

By polymerization of the Bis-GMA, double carbon-carbon bonds of monomers change into single bonds.^[18] A single carbon-carbon bond absorbs fewer photons in wavelengths of 1638 cm^–1 compared to double carbon-carbon bonds; therefore, the amount of transmitted or absorbed photons can reflect the degree of polymerization.^[19,20] In the present study, we found that one-sided curing results in a higher polymerization ratio compared to other groups.

The question is why SBS did not increase significantly despite higher polymerization in one-sided curing group? Can it be related to other types of polymerization like circular or crosslinking reaction rather than linear propagation that increase the transmission but do not necessarily increase the bond strength?^[20] It seems that for bonding of metal brackets with continuous unidirectional 20 s curing results in more polymerization compared to interrupt multidirectional curing.

Some researchers investigated the effect of transillumination curing from the lingual aspect of the tooth on bond strength.^[4] Their theory was that since the composite polymerizes toward the source of energy so the resin tags will penetrate deeply into enamel rods and this, in turn, will result in a stronger bond.^[21] The other theory was that because the tooth structure is partially transparent light photons that can reach the adhesive through the tooth. However, a lot of studies showed that transillumination on itself cannot increase the bond strength of orthodontic brackets unless there was increased curing time and light intensity.^[4,22] However, this increases chair time and the risk of pulpal damage and reduces the efficiency of the procedure.^[23,24]

On the other hand, a higher polymerization ratio which was seen in one-sided curing has other potential advantages rather than SBS. It has been demonstrated that higher DC decreases the age-dependent discoloration of composite.^[25] Furthermore, the release of uncured Bis-GMA may be hazardous for the patient^[9] and higher DC can prevent these side effects.^[26] A high degree of monomer convergence is vital to enhance color stability and biocompatibility.^[27]

Limitations and suggestions

We suggest repeating the study with attachments with smaller or larger bonding bases like lingual buttons and buccal tubes. Needless to say that we cannot overgeneralize the results of the present study to porcelain brackets because of their transparency. Furthermore, the question, why the increase in the degree of conversion does not guarantee higher bond strength, remained unanswered and more investigations are needed to answer that. Furthermore, we suggest repeating the study by adding the aging process. We suggest adding thermocycling of the samples before debonding and use of LED units in future studies.