Testing the tensile fracture strength tie-wings of different orthodontic ceramic bracket using an Instron machine

Abstract

Introduction

Newly released products should address the shortcomings of the older ones. Frequent breakages have always been a major drawback when using ceramic brackets. This study assessed the difference in tensile fracture strength at maximum load of tie-wings of different orthodontic ceramic brackets recently available for clinical use.

Materials and Methods

In this in-vitro study, four ceramic brackets were examined. Two monocrystalline brackets (CLEAR™, Adanta®, Germany; Inspire ICE™, Ormco®, USA), one polycrystalline bracket (Symtri Clear™, Ormco®, USA), and one clear hybrid bracket (DISCREET™, Adanta®, Germany). A steel ligature wire was placed around the tie-wing and on the Instron machine to apply tension. The mean maximum load (MxL) and fracture strength (FS) was evaluated and recorded. The significance level was set at p ≤ 0.05.

Results

Statistical difference in fracture strength of the tie-wing fracture was noted among all four groups. Inspire ICE™ showed the highest maximum load and fracture strength with (202.78 N and 107.3 MPa), followed by Symtri Clear™ (111.99 N and 59.25 MPa). In contrast, CLEAR™ and DISCREET™ showed lower MxL values (79.63 N and 47.01 N). The monocrystalline Inspire ICE™ brackets showed the greatest tie-wing fracture resistance and the hybrid clear ceramic bracket DISCREET™ exposed the least resistance to fracture.

Conclusion

Brand (manufacturing specifications) as well the bracket crystalline structure seems to have a direct effect on its tie-wing strength.

1. Introduction

Concerning esthetic appearance, the innovation of manufacturing ceramic brackets exhibited a breakthrough in orthodontics. To date esthetic brackets are considered the only esthetic route that permits conventional orthodontic mechanics practice.

Currently, most of the available orthodontic ceramic brackets mainly comprise aluminum oxide (alumina) in its structure. They belong to one of the two classes of ceramics brackets: monocrystalline (MC) or polycrystalline (PCR). Both of which differ in the manufacturing process. These differences are evident in bracket performance throughout the treatment. For monocrystalline brackets, the particles are processed to melt and crystallize into single crystal upon controlled cooling. After that, milling takes place using diamond tools followed by further heating to counteract the impurities formation (Bishara and Fehr, 1997). In contrast, PCR brackets are manufactured by a ceramic injection molding technique, where particles are mixed with a binder and pressed into mold through pressurization. Subsequently, the binder burns out through the process of sintering. To eliminate surface imperfections and reduce stresses, the brackets are heat treated afterwards (Bishara and Fehr, 1997).

DISCREET™ orthodontic bracket was recently introduced to the market. These brackets are made of clear hybrid material that includes ceramic (α-ceramic compound copolymers) described by the material safety and data sheet (https://www.adenta.com/discreettrade-ultra-low-friction-bracket1.html). DISCREET™ is a one-piece-bracket manufactured by Adenta® (Gilching, Germany) with a unique laser aided sinter line technology. The producing company claims that this material displays high material stability, less brittleness when compared with PCR ceramic, sapphire or MC ceramics.

When comparing mechanical properties, brittleness is an inherent characteristic of both monocrystalline and polycrystalline ceramic brackets (Karamouzos et al., 1997). However, when subjected to stress PCR brackets appear to have greater strength and fracture resistance than MC brackets. This can be explained by the presence of grain boundaries in PCR ceramics which slows down the crack propagation. As opposed to MC ceramic brackets which would fracture at once (Bishara and Fehr, 1997). Unfortunately, there is no data available in literature regarding the physical performance of the α-ceramic hybrid copolymers used in brackets.

Two properties play a major role in the ceramic’s tensile strength: the surface condition and material thickness (Holt et al., 1991, Johnson et al., 2005). The thickness of the bracket varies widely among the manufacturers. In 1990, Flores et al. (Flores et al., 1990) compared the response of the scratched PCR versus the MC brackets. The effect of the surface damage (scratching) was significant and unfavorable in regard to the single-crystal brackets, while the PCR brackets were not significantly affected.

A common place of failure is the tie-wing either at the junction of the tie-wing and base or the tie-wing complex itself (Johnson et al., 2005). This is mainly attributed to the geomorphology of the bracket, the location of the tie-wings, and mechanical stresses frequently concentrated over the tie-wings and bracket slots, leading to their fracture (Ghosh et al., 1995). Frequently stresses are created unintentionally, for instance, while cutting the ligature wires, engaging a heavy arch wire, or even while forceful ligation. The failure of the tie-wings may not only increase the duration, compromise the outcome of treatment, and adds to the expenses, but also risk the health of the patient with possible aspiration (Waly, 2013).

Some commercially available ceramic brackets are offered as semi-wings, which are believed to possess more toughness than true wings. The true twin configuration has four tie-wings that are independently projected from the bracket base, while the semi-wing brackets have a ceramic connector that joins the mesial and distal tie-wings together (Johnson et al., 2005). In this study, none of the brackets included were of the semi-wing type to provide better standardization while assessment.

Since the production of new esthetic bracket is an ongoing process, it is valuable to commence a comparative study that would verify the manufacturers claims. To our knowledge, two of the tested brackets (DISCREET™, CLEAR™) were not included in any of the previous studies. The main objective of the study is to evaluate the tensile maximum load and fracture strength (FS) of the tie-wings of four ceramic brackets from two manufacturers. In addition, to compare between two well-known ceramic bracket types (mono- and polycrystalline) in addition to a newly launched bracket type (α-ceramic compound copolymers).

2. Materials and methods

2.1. Sample selection

Table 1 presents the bracket systems chosen to be tested in this study (two MC, one PCR, and one clear hybrid ceramic bracket). Inspire ICE™ brackets were chosen as they were available in the market for many years to be compared with the other three brackets which were more recently released.

Table 1.

Bracket sample selection.

Brand	Code	Manufacturer	Bracket slot/ prescription	Composition/ Crystalline Structure
Inspire ICE™	ICE	Ormco®, Orange, Ca, USA	0.022-inch MBT	Monocrystalline alumina
Symtri Clear™	SC	Ormco®, Orange, Ca, USA	0.022-inch MBT	Polycrystalline alumina
CLEAR™	CR	Adanta®,Gilching, Germany	0.022-inch MBT	Monocrystalline alumina
DISCREET™	DT	Adanta®,Gilching, Germany	0.022-inch MBT	Hybrid clear α-ceramic compound copolymers

MBT, system developed by McLaughlin, Bennett, and Trevisi.

The level of significance was set at α = 0.05, effect size of 20 %, and power of 85 % each group has to have at least 18 brackets.

2.2. Specimens preparation

The methodology followed was derived from a previously published study by Johnson et al. (Johnson et al., 2005) with slight modifications. The upper right central incisor brackets were chosen with a slot size of 0.022-inch and MBT (MBT, system developed by McLaughlin, Bennett, and Trevisi) prescription. Seventy-two metal slabs were prepared with specific dimensions (15 mm x 40 mm x 2 mm) to fit the undercut of a customized holder on the lower jaw of the Instron machine (Instron 5965 Corp., Norwood, MA, USA) shown in Fig. 1.

Fig. 1.

Demonstration of the mechanical testing setup using the Instron Machine.

These metal slabs were used to hold the brackets in place while testing. Only one bracket was placed on each slab to avoid possible effects on the other brackets during testing. The metal surface was roughened using acrylic bur to ensure better bonding. A strong epoxy system offering a very high tensile strength on full cure, JB Weld (JB Weld, Sulphur Springs, Tex, USA), was used to secure bracket and prevent the fracture of the tie-wing from happening before the debonding of the bracket from the metal slab. The 2-parts of the epoxy system were mixed in equal amounts as recommended by the manufacturer. A fine layer of glue was placed on each slab. Each bracket was seated over the glue using bracket holder and allowed to set. The initial set was achieved after 4–6 h; however, all the samples were not tested before 24 h to allow the full cure.

2.3. Testing apparatus, procedure, and data recording

The disto-incisal wing was selected for testing. A small gauge steel ligature tie wire (GAC International, Bohemia, NY, USA) was looped under and tied to the upper jaw of the Instron machine. Once used the ligature wire was disposed, and a new ligature wire was inserted for each sample. A 0.012-inch ligature wire was chosen to enable a secure engagement of the tie-wing while testing. Instron Universal testing machine (Instron Corp., Canton, MA, USA) was utilized to apply a vertical force perpendicular to the long axis of the bracket specifically on the disto-incisal wings of each bracket. A customized holder with a clamp to hold and secure the sample in place was used during the tensile test. The samples were loaded and tested until failure, at a crosshead speed of 10 mm/min. Merlin software (v 5.43, Instron Corp.) was used to record the maximum load (MxL) at failure in Newtons (N) and fracture strength (FS) at maximum load in megapascal (MPa) (Waly, 2013, Kim, 2020). To calculate fracture strength (MPa), the force (N) was divided by the area of contact between the ligature wire and tie-wing (Johnson et al., 2005).

2.4. Statistical analysis

IBM SPSS Version 25.0 (IBM Corp., Armonk, NY, USA) was used to enter and analyze the data. Intra-examiner reliability analysis was performed to overcome possible observer bias. Intraclass correlation coefficient (ICC) estimates at 95 % confidence interval were calculated using SPSS. ICC estimate was 0.84 indicating good reliability.

The P-value was set at the level of 0.05, and any P-value less than that was considered significant (p ≤ 0.05). Two variables were tested Fracture strength at maximum load (MPa) and the maximum load at failure (N). One-way ANOVA test followed by Tukey’s post hoc test was performed to assess the difference among the groups.

3. Results

The means and standard deviations of tensile fracture strength at maximum load (MPa) and the maximum loads at failure (N) values were recorded and presented in (Fig. 2 a,b). Inspire ICE™ showed the highest MxL and FS with (202.78 N ± 2.47 and 107.3 MPa ± 1.3), followed by Symtri Clear™ (111.99 N ± 3.89 and 59.25 MPa ± 2.1). In contrast, CLEAR™ and DISCREET™ showed a lower MxL and FS values as (79.63 N ± 2.84 and 42.13 MPa ± 1.5) and (47.01 N ± 1.87 and 24.87 MPa ± 0.99) respectively.

Fig. 2.

Comparison of the means. (a) the mean values of fracture strength (maximum load in megapascal [MPa]) of the brackets; (b) the mean values of mean maximum load at failure (newtons [N]) of the brackets.

One-way ANOVA statistics showed a significant statistical difference in the values of FS and MxL at failure (p < 0.05). Upon applying the Tukey’s post hoc test, statistically significant differences were indicated in FS and MxL between all brackets (p < 0.05) (Table 2, Table 3).

Table 2.

Comparison of the mean value (±standard deviation) of fracture strength at maximum load [MPa] between the brackets.

	Mean	Standard Deviation	Standard Error	95 % Confidence Interval for Mean		Minimum	Maximum	P value
				Lower Bound	Upper Bound
Inspire ICE™	202.7802a	2.47098	0.58242	201.5515	204.009	198.96	206.76	0.000
Symtri Clear™	111.9885a	3.89453	0.91795	110.0518	113.925	106.90	120.85
CLEAR™	79.6336 a	2.84036	0.66948	78.2211	81.0461	75.74	84.90
DISCREET™	47.0131a	1.87401	0.44171	46.0812	47.9450	44.39	50.36

Vertically, similar letter groups show a statistically significant difference (p < 0.05).

Table 3.

Comparison of the mean value (±standard deviation) of the maximum load at failure [N] between the brackets.

	Mean	Standard Deviation	Standard Error	95 % Confidence Interval for Mean		Minimum	Maximum	p value
				Lower Bound	Upper Bound
Inspire ICE™	107.2911a	1.30740	0.30816	106.6410	107.941	105.27	109.40	0.000
Symtri Clear™	59.2532 a	2.06060	0.48569	58.2285	60.2779	56.56	63.94
CLEAR™	42.1342 a	1.50284	0.35422	41.3868	42.8815	40.07	44.92
DISCREET™	24.8747 a	0.99154	0.23371	24.3816	25.3678	23.48	26.64

Vertically, similar letter groups show a statistically significant difference (p < 0.05).

4. Discussion

In the field of orthodontics, it is clearly perceived that esthetic alternatives are taking over and replacing the unesthetic ones. Ceramic brackets remain the most appropriate replacement to metal brackets if maintaining conventional mechanics is a concern. Regardless of their esthetic advantage, the use ceramic brackets have been associated with certain limitations, mainly the high fracture rates due to their low fracture toughness, especially the tie-wings (Scott, 1988, Birnie, 1990). When proposing ceramic brackets as a treatment modality, it should essentially retain its integrity throughout the treatment. This would reduce the cost on the patient and maintain the treatment efficiency and overall treatment time on the orthodontist (Khanapure et al., 2016). Throughout the past 18 years (2005–2022), only six studies were available in literature discussing the performance of the tie-wings, comparing the two classes of ceramic brackets (i.e., the MC and PCR). However, this is considered scant and not conclusive. Therefore, the present study aimed to compare the clinically relevant imperfection of the two well-known ceramic bracket types (mono- and polycrystalline) in addition to a new bracket type (clear hybrid ceramic brackets), which is the strength of their tie-wing.

The results of this study should reflect the effect of the manufacturing specifications of the bracket (brand) and the structural configuration of brackets on the fracture strength of the tie-wing. Perhaps, this can be obtained as these values are taken on new “unused” brackets with no interventions.

As mentioned previously the methodology described by Johnson et al. (Johnson et al., 2005) was followed with slight modifications. A 0.012-inch steel ligature wire was passed under the disto-incisal wing and secured to the upper jaw of the Instron machine. While it was recommended in Johnsons’et al.’s (Johnson et al., 2005) study not to use ligature wires smaller than 0.014-inch as they were prone to failure before the bracket tie-wing fracture happens, a more recent study in 2022 has shown that with the use of 0.012″ no ligature breakages was noticed during testing (Johnson et al., 2005, Kate James et al., 2022). Therefore, a smaller gauge of the ligature wire was chosen over the 0.014-inch to facilitate better engagement of the tie-wing in addition to its availability in the market.

Half of the previous papers have reported their data in [N] and the other half in [MPa]. The maximum load at failure (MxL-[N]), represents how much force can the bracket handle before failing which relates to the amount of force applied on the bracket clinically by the clinician. However, the tensile fracture strength (FS-[MPa]) indicates how the geometry and the design of the bracket can affect its strength (the force divided by cross-sectional area of the tie-wing). Manufacturers can increase the thickness of components to balance the inadequacy in mechanical properties of the material used. Owing to the importance of both measurements and due to the limited data available for comparison regarding this subject, both MxL and FS were recorded in this study and reported. Nevertheless, knowing the FS can be taken into consideration while bracket selection and MxL can affect the clinical decision during treatment.

The outcomes of this study supports that the manufacturing specifications and the bracket structural configuration has a direct effect on its tie-wing strength. As general comparison between the two manufacturers (Ormco® VS Adenta®) it was clear that Ormco® brackets has shown the highest fracture strength in both types (mono- and polycrystalline) ranging from (107.29–59.25 MPa), while Adenta® brackets fell on the lower limit ranging between (42.13–24.87 MPa). The difference between the two brands was statistically significant as per the post-hoc analysis (p < 0.05).

When comparing the brackets according to their structural configuration, it appears that there is significant difference among all bracket types. In this study the monocrystalline brackets of both brands showed a higher mean of FS at maximum load compared to PC and hybrid clear ceramic brackets. This comes in agreement with what was concluded by Johnsen’ et al. (Johnson et al., 2005). Recent studies were not conclusive about this matter. Some attributed that inconclusiveness to the small sample size and others to the intervention variables (sliding before testing, saliva immersion and fluoride effect) made on the brackets prior to the fracture test (Waly, 2013, Kate James et al., 2022). It is documented in literature that MC ceramics shows higher fracture resistance at first due to reduced imperfections during manufacturing, however, once the crack is present, its propagation in MC ceramic brackets is quite easy when compared with PCR ceramic brackets due to the presence of grain boundaries that resist the crack propagation on the latter one (Elekdag-Türk and Yilmaz, 2019, Flores et al., 1990). Therefore, it is expected to see lower fracture resistance of MC brackets on the tie-wing once they are subjected to scratched during clinical use (Waly, 2013, Flores et al., 1990).

In terms of the new material launched by Adenta® company (Clear Hybrid material) represented by DISCREET™, it was noted that it has the lowest fracture resistance when compared to MC and PCR ceramic brackets contrasting the manufacturer claims. This can be explained by the structural blend of the hybrid material.

The results of this study can be relatively compared to three of the six published papers as similar methodology was followed (testing new as-received brackets with no intervention) (Johnson et al., 2005, Kim, 2020, Kate James et al., 2022). From the selected sample, Inspire ICE™ and Symetri clear™ brackets were included in previous studies (Kim, 2020, Kate James et al., 2022). The observations of which were comparable to the results observed in this experimentation. In 2005, Johnsen et al. (Johnson et al., 2005) included ICE brackets as part of their investigation, the steel ligature wire tends to fail at a mean of 198.65 MPa before tie-wings even fracture. Since these brackets did not fracture using the investigation protocol followed, therefore, they were considered as the highest in fracture strength. However, in a recent study conducted by Kim et al. (Kim, 2020) that included the same brackets, ICE came second to Radiance™ (AO, Sheboygan, Wis, USA) a MC bracket with minimal difference and failed at a load of 240.48 N.

According to Kim et al. (Kim, 2020), Symetri clear™ scored the highest among all the brackets tested both MC and PCR with a score of 301.03 N followed by Clarity™ (3 M, Monrovia, Clif, USA) a PCR bracket (Kim, 2020), however it came second to last with a score of 57.69 MPa following Clarity™ and Radiance™ (MC brackets) with no significant difference in Kate et al study (Kate James et al., 2022). As for this trial, Symetri clear™ failed at 59.25 MPa (111.99 N). These differences in the results could be related to sample handling and/or manufacturing process.

Standardization was maintained in this in-vitro study as much as possible during sample preparation and testing. One operator was assigned to do the ligation and to run the test. However, operator error and variations during the manufacturing process and other confounding variables could not be absolutely eliminated.

Despite being out the scope of this study, some interventions were included in previous studies to simulate the clinical variables such as sliding before testing, saliva immersion and fluoride effect on tie-wing fracture. However, further research is needed to imitate intraoral cyclic chewing forces, intraoral alternating temperature, and pH, as well as stresses created while brushing, whether separately or all together to closely replicate the clinical situation. Furthermore, a greater number of the currently available bracket type groups and larger sample sizes could provide stronger evidence and further help the clinician while selecting the appropriate appliance for his patients.

5. Conclusion

From the results of this in vitro investigation, it can be concluded that: 1) the highest tie-wing fracture resistance value was demonstrated by Inspire ICE™, which represented the monocrystalline bracket group, thus higher forces could be applied without risking failure; 2) the hybrid clear ceramic brackets (DISCREET™) had the lowest resistance to a tie-wing fracture; and 3) statistical differences were noted among all four groups, therefore, brand as well as the bracket structure seemed to influence tie-wing strength.

Data availability statement

The data shown are at the complete disposal and can be found through the corresponding author.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Author statement:

We confirm that the manuscript has been read and approved by all named authors and that there are no other person who satisfied the criteria for authorship but is not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

Ethical approval

This study was approved by King Saud University. According to the College of dentistry Research Center and the deanship of scientific research at King Saud University, neither approval from the ethics committee nor informed consent is required for this in-vitro research project CDRC (PR#0147).

Patient consent

Not applicable, as no patients were recruited in the trial.

Conflicts of interest

The authors declare no conflict of interest.

CRediT authorship contribution statement

Anisa H. AlBadr: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Nabeel F. Talic: Conceptualization, Writing – review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.