Introduction
The aim of this in-vivo study study was to evaluate the effect of excluding the liquid resin component of a composite bonding product that is based on bisphenol A diglycidylmethacrylate when bonding lingual retainers.
Methods
The material comprised 40 metal multistrand lingual retainers bonded onto the lingual surfaces of maxillary and mandibular anterior teeth. Transbond LR composite paste and liquid resin (3M Unitek, Monrovia, Calif) were used to bond retainers in the control group (20 retainers). The same bonding material was used in the test group (20 retainers), but the liquid resin component was excluded. The durations (in months) of retainer survival were analyzed by using the Kaplan-Meier product limit method and the log rank test.
Results
Fifty percent of the retainers in the control group and 60% of those in the test group had no bond failures during the 5-year observation period; the difference was not statistically significant. Bond failures were recorded in 13.6% of the bonded tooth surfaces in the control group and in 14.9% in the test group; the difference was not statistically significant. On average, the retainers stayed intact in the control and test groups for 36 and 32 months, respectively. The median survival times of the control and test groups were 43 months and more than 47 months, respectively. Neither the survival analysis nor the 95% confidence intervals (24-49 months in the control group, 24-40 months in the test group) suggested any statistically significant difference between the groups.
Conclusions
Metal lingual retainers can be successfully bonded without liquid resin and serve the patient equally as well clinically as retainers bonded with the conventional bonding technique.
Composite resins based on bisphenol A diglycidylmethacrylate (bis-GMA) are used to bond orthodontic brackets and fixed retainers on patients’ teeth. Clinically lasting adhesion between the enamel surface and composite resins for therapeutic purposes was not possible until the introduction of the acid-etching technique on enamel in 1955. The original 85% phosphoric acid-etching technique was refined by a series of laboratory ultrastructural studies of acid-etched enamel surfaces. On the basis of the laboratory results, an optimized clinical protocol is now widely accepted to create microscopic troughs in the enamel sheaths and cores to mechanically interlock with liquid resin tags. In other words, sufficient wetting of the enamel surface by a fluid liquid resin was assumed to be critical to enable enamel adhesion between the composite resin and the etched enamel surface. This forms the foundation of the widely accepted mechanism of enamel adhesion.
However, in a calibrated in-vitro cell culture study, liquid resin had been identified as the primary irritant in bis-GMA–based composite. In addition, the degree of conversion of composite resin in vivo is always less than ideal. Chemical compounds, including bisphenol A (BPA), had been reported to leach out of cured composite resins. The annual report of the President’s Cancer Panel of the National Institutes of Health, 2008-2009, highlighted the public concern of the role of BPA in carcinogenesis and its hormonal effect as a weak estrogen. Although there is not yet any conclusive information on BPA in dental liquid resin used in vivo, it is logical to strive to minimize or, if possible, even eliminate the potential contribution of liquid resin on the overall BPA release from clinical materials by reducing the amount of polymer content of composite resins. One way to accomplish this goal is to avoid the clinical use of liquid resin.
Several studies have indicated that the exclusion of liquid resin seemed neither to reduce the in-vitro bond strength of bonded metal brackets to enamel nor to increase the failure rate of metal brackets in vivo up to a 2-year period of active orthodontic treatment. However, bonded fixed retainers sometimes must stay in place indefinitely, and it is therefore of interest to know how long fixed retainers bonded without liquid resin can last in vivo.
The aim of this investigation was to study whether the exclusion of liquid resin from a bis-GMA–based composite would impair the clinical performance of bonded fixed retainers.
Material and methods
The first author (A.T.H.T.) treated all the patients in this study and bonded all the retainers in the test and control groups between January 1998 and February 2000 at the Unit of Orthodontics, Department of Dental Medicine, Karolinska Institutet, Huddinge, Sweden, and the Specialist Orthodontic Clinic, Uppsala County Council, Uppsala, Sweden. The reviewing of the status of the retainers took place from June 2002 to August 2003 by 2 of the other authors (A.A.S. and C.M.F.), who were not aware of the test or control identity of the patients. The longest time interval between the bonding and the reviewing of the retainers exceeded 5 years.
Forty-five young patients treated with fixed appliances by the first author were randomly chosen for the study. The test group comprised 22 retainers in 18 patients, and the control group comprised 31 retainers in 27 patients. In the test group, 1 patient, with 2 retainers bonded from the maxillary right to the left lateral incisors, and from the mandibular right to the left canines, left the study. Consequently, 20 retainers in 17 patients were evaluated in the test group. In the control group, 2 patients, with 1 retainer each, both bonded from the mandibular right to the left canines, dropped out of the study. For the sake of statistical comparisons, the remaining 29 retainers in 25 patients in the control group were randomly reduced to 20 retainers in 18 patients ( Table I ).
| Test group | Control group | |
|---|---|---|
| Female:male ratio | 1:1 | 13:7 |
| Maxillary retainers bonded (n) | 7 | 9 |
| Mandibular retainers bonded (n) | 13 | 11 |
| Retainers bonded in university clinic (n) | 9 | 20 |
| Retainers bonded in public health clinic (n) | 11 | 0 |
In the control group, at the end of fixed appliance therapy, alginate impressions were taken to cast stone models. On the models, metal retainers were made by bending 0.0215-in multistrand wire (Penta-1; Masel, Carlsbad, Calif) to fit onto the lingual surfaces of the teeth. An index was made by adapting a self-curing silicon material onto the teeth and the retainer wire, leaving the 2 ends of the metal retainer wire free.
In the clinic, the teeth on which the retainers were to be bonded were cleaned and dried before the silicon index with the metal retainer wire was tried on to check for fit. Subsequently, the teeth were isolated from moisture contamination and then etched with 37% phosphoric acid on the lingual enamel surfaces for 1 minute followed by thorough rinsing and drying in an airstream. They were again isolated from moisture contamination and dried with compressed air until the enamel appeared frosty white.
Liquid resin (Transbond LR; 3M Unitek, Monrovia, Calif) was applied to the 2 teeth at the ends of the metal retainer. The liquid resin was light cured for 20 seconds with a Demetron VCL 400 (Kerr Corporation, Orange, Calif). The silicon index carrying the metal retainer was then adapted to the teeth, and Transbond LR composite paste (3M Unitek) was syringed onto the lingual surfaces of those 2 teeth and the ends of the metal retainer and light cured for 40 seconds. After removal of the silicon index, liquid resin was applied to the remaining teeth involved in the fixed metal retainer and light cured for 20 seconds. Composite paste (Transbond LR) was then syringed onto the tooth surfaces, and each surface was cured for 40 seconds. Finally, the composite was finished with a multifluted tungsten carbide bur in a high-speed hand piece and white stone, followed by a rubber cone in a low-speed hand piece.
In the test group, except that no liquid resin was used, the metal retainers were bonded according to the same protocol described for the control group.
Statistical analysis
The dates when the retainers were bonded and failed were retrieved from computerized (Ortus 2.0; RDT Soft, Uppsala, Sweden) and typewritten patient records.
Both the test and control groups consisted of complete and censored observations. For the complete observations, the duration of retainer survival was counted between the month of retainer placement and the month in which an accidentally debonded surface was recorded.
For the censored observations, retainers that remained intact throughout the observation period were counted from the month of their placement to the end of the reviewing period of each patient (June 2002-August 2003).
Survival analysis, the Kaplan-Meier product limit method, was used to analyze the data obtained. The log rank test was used for statistical differences between the test and control groups. All statistical procedures were carried out with SPSS software (version 10.0; SPSS, Chicago, Ill).
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