Introduction
In this clinical trial, we evaluated and compared bond failure rates of light-cured composite resin vs chemical-cured composite resin for 12 months.
Methods
Twenty-two subjects (356 stainless steel brackets) were included in this study, and a split-mouth design was used to randomly allocate diagonally opposite quadrants to either chemical-cured (178 brackets) or light-cured (178 brackets) composite resin; the brackets came from the same manufacturer. The survival and failure rates of the brackets were evaluated by the mode of polymerization. The overall bracket survival rates were estimated using the Kaplan-Meier product limit estimate.
Results
There were no significant differences in the bond failure rates between the chemical-cured and the light-cured composites ( P = 0.52). Bond failures were greater in posterior teeth (6.7%) than in anterior teeth (1.2%). The highest failure rate was observed in the second premolars (7.7%).
Conclusions
The overall failure rate of brackets with the 2 bonding systems was 2.8%, which is acceptable for clinical use. The polymerization mode did not influence the bracket survival rate significantly.
Highlights
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Bracket bond failure is a frustrating occurrence in orthodontic practice.
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Bond failure rates after chemical and light curing were compared in a split-mouth study.
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Survival and failure rates of the brackets were evaluated by mode of polymerization.
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Overall bracket survival rates were estimated with the Kaplan-Meier product limit estimate.
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Polymerization mode did not significantly affect the bracket survival rate.
Bonding of brackets to the tooth enamel has been an important issue since the introduction of direct bonding in orthodontics. Many new bonding agents have been developed, including composite resins, conventional glass ionomer cements, resin-modified glass-ionomer cements, and polyacid modified composites (compomers). Each bonding agent uses a different polymerization mechanism: chemical, light, or dual curing.
The success of fixed appliance therapy depends on the capability of the adhesive system to resist failure caused by many factors directed to the bracket-adhesive-enamel junction. These factors include stresses of mastication and stresses exerted by archwires as well as other factors particular to the oral cavity, including humidity, rapid changes in temperature, and pH. A good orthodontic adhesive material should enable the bracket to stay bonded to the enamel for the duration of treatment and permit easy removal of brackets when needed without damage to the enamel surface and with the least discomfort for the patient. In addition, orthodontic adhesives should be nonirritating to the oral mucosa, allow adequate working time for positioning brackets while setting quickly enough for patient comfort, provide a simple way of application and a convenient way of curing, and preferably have fluoride release potential.
Composite resins with different polymerization mechanisms such as chemical, light, or dual curing are the most frequently used adhesives in orthodontic bonding. Although composite resins provide sufficient bonding strength and are easy to handle, they adhere to the tooth enamel only by microretention, requiring a dry field. Traditionally, orthodontic adhesives were chemically cured. The use of light-cured adhesives has grown rapidly in popularity over the last years. The advantages of light-cured adhesive are the extended time for bracket positioning and the ease of cleanup around the bracket base before bonding.
Many factors can influence the bonding strength of orthodontic brackets. Etching the enamel surface is a critical variable that affects bond strength and bond failure location. Factors that influence acid etching of enamel include type of acid etch, concentration of acid etch, and etching time. Other factors affecting bond strength include the bonding agent, the filler level of orthodontic adhesives, the characteristics of the bracket base, and the status of the teeth if recently bleached. Operator and patient factors are likely to influence the failure rate of any bonding system. Care in the clinical technique, moisture control, choice of bonding material, and instructions given to the patient are all controlled by the operator, whereas the patient’s sex, age, malocclusion, and care of the appliance are patient-dependent variables.
Treating enamel with fluoridated prophylactic pastes of varying fluoride concentrations and use of pumice with or without fluoride before acid etching and bonding do not affect bond strength. Also, bond strength is not affected if chlorhexidine is applied after bonding or as a prophylactic paste on enamel before etching. Bond strength is reduced to an unacceptable level, however, if the chlorhexidine is applied as a layer on etched enamel or on the sealant before the adhesive is applied.
O’Brien et al investigated bracket failures in a clinical setting using light-cured vs chemical-cured orthodontic resin. In that study, bracket failure rates were recorded as cumulative percentages of failures that occurred by the end of the total observation period. Using this method of reporting bracket failure rates is problematic, especially if not all subjects were observed for the same period of time. Another potential problem with calculating the percentage of bracket failure at the end of the study is that it does not provide information about when the brackets actually failed. A more accurate approach is to use survival analysis to calculate the cumulative probability of bracket failures at various times during the study for each adhesive group.
Bond failure is one of the most frustrating occurrences in an orthodontic practice for both the practitioner and the patient. Consequences of bond failure can include increased treatment time, additional costs in materials and personnel, and unexpected additional visits by the patient. Thus, it is of great importance to determine the mode of polymerization of the bonding material with the lowest failure rate in orthodontic patients.
Material and methods
This was a clinical prospective study that included patients with complete permanent dentition treated with fixed orthodontic appliances using preadjusted edgewise appliance (22-in slot, MBT system; 3M Unitek, Monrovia, Calif). A list of patients seeking orthodontic treatment was obtained from the orthodontic clinics at the University of Khartoum, Faculty of Dentistry, in Khartoum, Sudan. Subjects were considered for the study according to the following inclusion criteria: treatment without extractions or with symmetrical extractions, no occlusal interferences, no facial restorations at the bonding area, and no gingival hyperplasia or congenital defects. All brackets had to be placed in both arches at the same appointment. All study protocols were approved, and permission was obtained from the ethical committee of the Faculty of Dentistry, University of Khartoum.
Patients satisfying the inclusion criteria were told about the study, and written informed consent forms were obtained from the 22 patients who agreed to participate. For these patients, all permanent teeth except molars were bonded, allowing up to 20 teeth per patient to be included. This provided a total of 356 teeth for the study. One hundred seventy-eight teeth were bonded with a light-cured composite resin, and 178 teeth were bonded with the chemical-cured composite resin from the same manufacturer (American Orthodontics, Sheboygan, Wis). We used the Fédération Dentaire Internationale numbering system to divide the teeth into 4 sets and a split-mouth design to allocate the type of adhesive. Accordingly, diagonally opposite quadrants were allocated randomly to either the light-cured composite resin system group or the chemical-cured composite resin group. This randomly alternated split-mouth design was used to eliminate any bias that could have been introduced because the clinician (R.E.M.) was right-handed. The light-cured and chemical-cured sides were switched from patient to patient to ensure randomization. All brackets were placed at the same appointment according to the manufacturer’s instructions. All bonding procedures were carried out by the same operator (R.E.M.) using the direct bonding technique. Enamel surfaces were cleaned with pumice and a rubber cup to remove the enamel pellicle, rinsed with water, and dried with an air syringe. The tooth surfaces were etched for 30 seconds using 37% phosphoric acid (acid etching gel; American Orthodontics). The etched enamel was rinsed with water for 15 seconds and dried with a 3-way oil-free air syringe to show the frosty, chalky white etched enamel surface. In the light-cured group, the composite’s primer was applied to the tooth surface and light cured for 10 seconds. Composite was placed on the bracket base, and the bracket was positioned firmly on the tooth. Excess composite material was removed from around the bracket with a sharp probe before curing the material with a light source for 40 seconds (20 seconds on each incisal and gingival aspect of the bracket). In the chemical-cured group (No-Mix adhesive; American Orthodontics), the composite’s primer was applied on the etched enamel. Composite was then placed on the bracket base with an applicator tip, and the bracket was positioned firmly on the tooth. The initial aligning archwire for each patient was selected to produce a light force to the brackets and the teeth. The usual choice of the initial archwire was 0.014-in nickel titanium, but a 0.012-in nickel-titanium archwire was used in patients with severe crowding. The initial archwire was tied into the bracket slots immediately after bracket bonding and curing. The patients were given verbal instructions about appliance care. They were instructed specifically to return if any bracket became loose or if they had any other problems. The patients were seen at 4-week intervals throughout treatment, and bond failures were accurately recorded in their case notes. The time of bond failure was recorded as the date of the appointment at which the bond failure was discovered.
Demographic data, malocclusion according to incisor classification, and the date of placement of the orthodontic brackets were recorded for each subject. The study continued for 12 months, during which all loose brackets were recorded. To ensure blinding of the operator, the data sheet that contained the adhesive allocation for each patient was kept with the study supervisor (S.A.). The failures were recorded on different data sheets. Matching was done between the 2 sheets after the observation period.
Statistical analysis
Demographic and clinical characteristics were analyzed using descriptive statistics. The clinical survival data were subjected to the Kaplan-Meier survival analysis, which enabled consideration of the time before failure. One advantage of survival analysis over other statistical methods is the ability to analyze a sample in which the event of interest is not observed in all subjects. This suits orthodontic bond performance in which the treatment duration varies, and most bonds survive until their planned removal. Accordingly, the overall failure rate and mean survival time for the study sample were calculated. Life tables and survival functions were generated from the survival data to enable overall analysis and within-sample comparisons. All statistical analyses were performed with Stata software (version 11; StataCorp, College Station, Tex).
Bond failure rates during a period of 12 months were estimated for each adhesive system, and the corresponding bracket survival curves were plotted using the Kaplan-Meier product limit estimate. The level of significance was set at 0.05.
Results
There were 22 patients in this study, providing a total of 356 teeth. Their minimum age was 13 years, and their maximum age was 32 years. The mean age was 20.24 ± 5.57 years. Ten bond failures were registered during the 12-month observation period. The overall failure rate was 2.8%. There were no significant differences in bond failures between chemical-cured composite and light-cured composite bonded teeth.
The right side of both the maxillary and mandibular arches had a higher failure rate (3.4%) than the left side (2.3%). The highest failure rate was observed for the second premolars (7.7%). The failure rate was lower in the maxillary teeth (2.3%) than in the mandibular teeth (3.3%) and was higher in the posterior teeth (6.7%) than in the anterior teeth (1.2%) ( Table ).
| Variable | Total bonded brackets (n) | 12-mo failures (n) | 12-mo bond failure rate (%) | MST (restricted mean) | MST (extended mean) | P value |
|---|---|---|---|---|---|---|
| Material | ||||||
| Light-cured composite | 178 | 4 | 2.3 | 11.8 | 528.0 | 0.521 |
| Chemical-cured composite | 178 | 6 | 3.4 | 11.8 | 349.9 | |
| Tooth position | ||||||
| Mandibular left | 90 | 2 | 2.2 | 11.9 | 534.0 | |
| Mandibular right | 90 | 4 | 4.4 | 11.6 | 263.8 | |
| Maxillary left | 88 | 2 | 2.3 | 11.9 | 522.0 | |
| Maxillary right | 88 | 2 | 2.3 | 11.9 | 522.0 | |
| Left | 178 | 4 | 2.3 | 11.9 | 528.0 | |
| Right | 178 | 6 | 3.4 | 11.7 | 349.9 | |
| Tooth type | ||||||
| Central incisor | 86 | 0 | 0 | — | — | |
| Lateral incisor | 88 | 3 | 3.9 | 11.7 | 305.9 | |
| Canine | 78 | 0 | 0 | — | — | |
| First premolar | 26 | 1 | 3.9 | 11.9 | 306.1 | |
| Second premolar | 78 | 6 | 7.7 | 11.4 | 149.8 | |
| Maxillary arch vs mandibular arch | ||||||
| Maxillary | 176 | 4 | 2.3 | 11.9 | 522.0 | |
| Mandibular | 180 | 6 | 3.3 | 11.7 | 353.9 | |
| Anterior vs posterior | ||||||
| Anterior | 252 | 3 | 1.2 | 11.9 | 1002.0 | |
| Posterior | 104 | 7 | 6.7 | 11.5 | 172.1 | |
| Overall | 356 | 10 | 2.8 | 11.8 | 421.1 | — |
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