Introduction
In this in-vitro study, we compared the shear bond strengths of orthodontic brackets bonded to various commonly used esthetic pontic materials.
Methods
Prefabricated denture teeth (acrylic resin, Trubyte, Dentsply, York, Pa) and samples made from Integrity (bis-acryl composite resin, Dentsply Caulk, Milford, Del) and Alike (polymethylmethacrylate resin, GC America, Alsip, Ill) were used to represent the more common esthetic provisional materials. Each material group contained 30 samples; a total of 90 samples were bonded in the same fashion with APC PLUS maxillary lateral incisor brackets (3M Unitek, Monrovia, Calif). Each material group was then divided into 2 testing subgroups. One subgroup was tested for shear bond strength 24 hours after bonding, and the other subgroup was tested after bonding and storage in distilled water at 37°C for a week. Each bracket was loaded perpendicularly in a universal testing machine at a crosshead speed of 1.0 mm per minute until bonding failure. The mean shear bond strength and standard deviation were determined for each group. Analysis of variance (ANOVA, 2-factor and 1-factor) with Tukey HSD post-hoc tests, Student t tests, and Mann-Whitney U tests were used to test the main effects of pontic materials and time of loading (α = 0.05). The data were stratified, and 1-way ANOVA tests were performed with the Bonferroni adjustment (α = 0.01) to examine the effect of the pontic material on shear bond strength after either 1 day or 7 days of storage.
Results
Significant differences were found based on pontic material and time ( P <0.05), but there was a significant interaction ( P = 0.044), making the results uninterpretable. At 1 day, the Integrity material had a significantly higher mean shear bond strength than both Alike and the denture tooth materials ( P <0.001). However, at 7 days, both Integrity and Alike had significantly higher mean shear bond strengths compared with the prefabricated denture tooth ( P <0.001).
Conclusions
Although the use of Integrity or Alike requires an additional armamentarium, necessitating individual pontic fabrication by the dental practitioner, indications for clinical use are evident with direct applications to multi-disciplinary treatment modalities.
Direct bonding of metal orthodontic brackets to natural teeth has been an accepted technique since introduced by Newman in 1965, with reported in-vitro bond-strength values from 6 to 8 MPa for composite resin cements. The strength and dependability of this bond are important to ensure effective and efficient treatment with fixed appliances. According to Zachrisson, a low bond-failure rate should be a high priority, since replacing loose brackets is inefficient, time-consuming, and costly.
For patients undergoing orthodontic treatment involving management of anterior spacing associated with a missing tooth, maintenance of the edentulous space with an esthetic pontic is often required. Tooth loss can be a result of periodontal disease, extractions, trauma, or congenital causes. Anterior spacing is a major reason that adults seek esthetic orthodontic treatment, and provisional esthetic pontics are often required for space maintenance and desired esthetics.
Secure bonding of an orthodontic bracket to an esthetic pontic during treatment depends on the properties of both the adhesive and the pontic material. As stated by Chay et al, the “shear-peel bond strength of orthodontic brackets to provisional materials depends on material, surface treatment, and time.” Although various materials have been recommended for this purpose, clinicians now typically use a bracketed prefabricated plastic denture tooth (eg, interpenetrating polymer network acrylic, Trubyte, Dentsply, York, Pa), tied to the archwire for space maintenance. However, other esthetic materials such as bis-acryl composite resin (eg, Integrity, Dentsply Caulk, Milford, Del) and polymethylmethacrylate acrylic resin (eg, Alike, GC America, Alsip, Ill) are also used.
Few studies have been conducted on the bond strength of orthodontic brackets to provisional restorative materials. The objective of this in-vitro study was to evaluate the shear bond strength of an orthodontic bracket bonded to these commonly used esthetic pontic materials. The null hypothesis was that there is no difference in shear bond strength among the various pontic materials (Trubyte, Alike, Integrity) at either 1 or 7 days.
Material and methods
Prefabricated denture teeth (interpenetrating polymer network acrylic resin) and samples made from bis-acryl composite resin (Integrity) and polymethylmethacrylate acrylic resin (Alike) were used in this investigation ( Table I ). Each material group contained 30 samples. For all 90 samples, a section of schedule-40 polyvinyl chloride (PVC) pipe (length, 1.5 in; diameter, 0.5 in) was used as a holding device. In the first group, 30 identical denture teeth with mechanical undercuts were placed on adhesive tape, and a section of PVC pipe was placed around them. Acrylic resin (Alike) was poured into the ring and around the sample. In the second group, a section of PVC pipe was filled with Integrity. In the third group, a section of PVC pipe was filled with Alike. After all materials had set, the testing surfaces of all samples were flattened with 600-grit sandpaper and sandblasted (microetched) by using 50-μm aluminum oxide with a microblasting unit (Basic Quattro IS no. 2955-1000, Renfert GmbH, Hilzingen, Germany) to duplicate the clinical preparation procedures typically performed on prefabricated denture teeth.
