Comparison of the shear bond strength of reconditioned metal brackets used in indirect bonding: an in vitro pilot study

Orthodontic brackets have been developed as attachments to facilitate the movement of teeth to desired positions. In early appliances, the bracket was welded to a stainless steel band which was cemented to the teeth. This method required a tooth separation process and produced a proximal band space after appliance removal.¹

In compensation, Newman² developed a method that directly bonded orthodontic brackets to the teeth. However, due to the diversity of crown shapes and limited visualisation of the teeth, this method often incorrectly positioned the brackets.³

In 1972, Silverman⁴ introduced indirect bonding to overcome the placement disadvantages of direct bonding. Indirect bonding is a technique by which brackets are directly bonded in desired positions to teeth on a dental cast and then, indirectly attached to the teeth in the mouth using a transfer tray. The advantage of indirect bonding is that it reduces bracket positioning errors and chair time, thereby providing more comfort for patients. However, indirect bonding has the disadvantage of requiring additional laboratory procedures and costs.⁵ In addition, since the bracket must be bonded twice with a bonding agent, the strength of the bond could be reduced.⁶

To overcome bonding strength degradation, surface treatment of the brackets to be bonded is required which involves removing the previous bonding agent without damaging the base mesh structure of the bracket.⁷

Past surface treatment methods have included mechanical abrasion using a greenstone,⁸ a carbide bur,⁹ sandblasting,¹⁰ a chemical method,¹¹ and a thermal method.¹² While it is crucial to know which chairside surface treatment method is best, it is further important to determine the appropriate conditions for possible sandblasting because bond strength can vary depending upon the size of Al₂O₃ particles and sandblasting time. A previous study¹³ recently evaluated the change in bonding strength according to the Al₂O₃ particle size, but no study has yet evaluated the change in related bonding strength. If the sandblasting time is too short, it can leave the bonding material in the bracket base; however, if the sandblasting time is too long, damage can occur to the mesh of the bracket base.¹⁴

The present study aimed to compare the differences in shear bond strength (SBS) related to the surface treatment methods of debonded brackets before indirect rebonding and to evaluate changes in SBS according to sandblasting time.

The study was approved by the Institutional Review Board (PNUDH-2021-013) of the Pusan National University Dental Hospital, South Korea.

Indirect bonding procedure

All bonding procedures were performed by a single operator with more than 8 years of clinical experience. All materials were used according to the manufacturer’s instructions. The method for bracket bonding by the indirect technique was:¹⁵

Six teeth were embedded in a white stone block and orientated such that the crown axes were perpendicular to the base surface of the block. A total of 24 teeth were used in each group.

A dental cast of the embedded teeth was made from an alginate impression (Figure 1).

A separating medium (Dentsply, PA, USA) was applied twice to the cast teeth.

The brackets were bonded to the centre of the buccal clinical crown using an orthodontic bonding agent (Transbond XT, 3M ESPE, CA, USA). Excess resin was removed using a sharp instrument and the bonding material subsequently polymerised using a photopolymer (VALO, Ultradent, South Jordan, USA) at 1000 mW/cm² for 30 sec. The undercut-like bracket hooks were blocked out with transparent silicone (Emiluma, Ortho Kinetics, Vista, CA, USA).

A transfer tray was made using a 0.020-inch vacuum-formed stent (Raintree Essix, LA, USA) (Figure 2) and then removed from the dental cast.

Surface treatment methods were applied to the base of the debonded bracket after ensuring that no dental stone was incorporated.

The teeth were polished using pumice (Whipmix Co., Louisville, USA) and a rubber cup for 10 sec and washed and dried with compressed air. The teeth were then etched for 30 sec using 37% phosphoric acid which was rinsed and the teeth redried for 30 sec. A bonding primer (Transbond XT primer, 3M ESPE, CA, USA) was applied to the teeth and an orthodontic bonding agent (Transbond XT, 3M ESPE, CA, USA) was applied to the bracket base. The transfer tray was brought close to the teeth and firmly seated. The brackets were polymerised using a photopolymer (VALO, Ultradent, South Jordan, USA) for 30 sec.

The transfer tray was removed from the teeth. The specimens were stored at 37°C and 100% relative humidity for 24 hr before the SBS test.

Figure 1.

Figure 2.

Shear bond strength

To evaluate the bonding strength, SBS tests were performed using a universal testing machine (Instron, Canton, MA, USA). The crosshead speed was set at 1 mm/min. After placing the machine’s steel rod as close as possible to the bracket, the maximum load (N) was measured when the bracket detached from the tooth (Figure 3). The measured values were divided into a bracket base area (11.97 mm²) to convert them into MPa units.

Figure 3.

Figure 4 and Table III present the results of the SBS study. The SBS (11.2 ± 2.2 MPa) of Group VII was not statistically different from the control group (9.1 ± 3.0 MPa). SBS (16.7 ± 1.8 MPa) of Group VI was not statistically different from Group II (16.8 ± 2.1 MPa) but higher than in the control group (P < 0.05). There was no statistical difference between Groups III (21.0 ± 2.8 MPa), IV (21.4 ± 3.0 MPa), and V (20.2 ± 2.9 MPa), but was higher than that of the other groups (P < 0.05).

Figure 4.

The ARI values are presented in Table IV and showed that all groups had low remnant scores. There was a significant difference in the ARI distribution between each group (P = 0.000).

The microscopic photographs and SEM images of the bracket base according to the surface treatment method are shown in Figures 5 and 6, respectively. In Group I, the mesh structure was obscured because many bonding agents remained in the base of the bracket. In Group II, the bonding agent was partially removed and remained in the mesh structure. In Group III, most of the bonding agent was removed, but some was still found in the base mesh. In Group IV, the bonding agent was completely removed to fully reveal the mesh structure. In Group V, the bonding agent was completely removed, but some of the mesh pattern was damaged. Group VI had significant damage to the mesh pattern and much bonding agent remained.

Figure 5.

Figure 6.

For the commonly-used straight wire appliance, it is essential to bond brackets in their ideal position on the teeth so that appliance prescription is correctly expressed. Andrews¹⁸ found that measuring from the incisal edge, as had been done with standard edgewise brackets, did not allow consistent expression of the three-dimensional bracket characteristics and recommended anatomical landmarks to locate the brackets. This allowed orthodontists to accurately position the brackets so that fewer arch wire adjustments were required, bracket repositioning minimised, and treatment progress was efficient.

Various indirect bonding methods have been developed to improve the accuracy of bracket positioning.¹⁵ Regardless of the modification of indirect bonding methods, the basic indirect bonding procedure is to first directly bond the brackets to a dental cast, and secondly, to rebond to a prepared tooth using a transfer tray. As a result, indirect bonding varies from direct bonding in that there is a possible interface between the polymerised bonding agent and the new bonding agent, which may reduce bond strength.⁶ While there are various surface treatment methods to improve the bonding strength, the three methods employed in the present study (round bur, sandblasting, and adhesion booster) are the most commonly used at chairside.

Specimens that underwent surface treatment by sandblasting for more than 6 sec recorded a higher SBS than the other methods. During sandblasting, Al₂O₃ with a particle size of 50 μ m was able to completely remove the previously polymerised bonding agent on the bracket base, resulting in a high SBS value. Especially for metal brackets, attachment is achieved by mechanical retention between the bracket base and bonding agent and so, it is important to maintain the integrity of the bracket base. Surface treatment with a round bur resulted in a low SBS because bonding agent smaller than the bur dimension remained in the base mesh. In addition, the bur damaged the mesh structure of the bracket base (Figures 5 and 6).

There were no statistically significant differences between the adhesion booster and control groups. Adhesion boosters did not significantly contribute to the attachment of polymerised bonding agents to fresh bonding agents, which was consistent with previous studies.¹⁹ The plastic conditioner may be used to enhance the SBS of acrylic appliances or plastic brackets.¹⁹ As the methyl methacrylate (MMA) resin cannot react chemically with Bis-GMA or TEGDMA, this product cannot produce a positive effect on the SBS of composite resin.¹⁹

The bonding strength may change depending on sandblasting time and so, related experiments were conducted. Sandblasting for 3 sec significantly increased the SBS compared to the control group. Sandblasting for 6 sec statistically increased SBS compared to sandblasting for 3 sec. This means that the polymerised bonding agent was not removed after 3 sec of sandblasting, and at least 6 sec of sandblasting is required for complete removal. However, the SBS did not increase proportionally when sandblasting was performed for more than 6 sec. When comparing sandblasting for 9 sec and 12 sec, a lower SBS was recorded in sandblasting for 12 sec, although there were no statistical differences. This means that excessive sandblasting can damage the mesh structure of the bracket base and reduce SBS (Figure 6).

The average SBS for each group ranged from 11.2 MPa to 21.4 MPa. Previous studies have not specified the minimum SBS required for clinical purposes although one study²⁰ stated that 13.0 to 21.0 MPa was required, while another study²¹ recorded 6.0 to 8.0 MPa. Most of the groups assessed in the current study matched the previous results, but it should be appreciated that laboratory and clinical conditions can be different. For direct bonding, earlier studies^22,23 showed that the average SBS was 19.0 to 25.6 MPa, which was similar to the current group which received sandblasting for more than 6 sec.

The ARI was used to evaluate the types of bonding failure. An ARI score of 4 and 5 indicated that the bonding failure occurred at the enamel–resin interface, and ARI 1 and 2 indicated that the bonding failure occurred at the bracket base-resin interface. Ideally, failure at the enamel–resin interface is desirable because removing the remaining bonding agent from the teeth is facilitated. It is not desirable to have a very strong SBS because the enamel layer may be damaged during debonding. Because an acid etching technique was used, it is known that most bonding failures occur at the resin–bracket interface.²⁴ The distribution of ARI in each group was statistically different. The lower the SBS value, the more likely the bonding failure occurred in the resin–bracket interface. This means that bonding failure frequently occurs at the interface between the polymerised bonding agent and the fresh bonding agent, which is consistent with the results of previous studies.²²

The present study had limitations as the sample size was small. Secondly, technique sensitivity during indirect bonding may have affected the results, even though an orthodontist with more than 8 years of clinical experience conducted the experiment. In particular, brackets were placed with sufficient pressure on the dental cast so that the resin thickness was consistent. Nevertheless, resin thickness could affect the results of the study. Thirdly, indirect bonding would artificially assign a resin base of varying thicknesses as needed. Despite these limitations, the current study will help clinicians determine the surface treatment method which improves bonding strength during indirect bonding. In addition, the results may be used to determine the optimal bracket recycling method for debonded brackets.