British Journal of Orthodontics
ISSN: 0301-228X (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/yjor19
The Effect of Adhesive Type and Thickness on Bond Strength of Orthodontic Brackets Frances Mackay To cite this article: Frances Mackay (1992) The Effect of Adhesive Type and Thickness on Bond Strength of Orthodontic Brackets, British Journal of Orthodontics, 19:1, 35-39, DOI: 10.1179/ bjo.19.1.35 To link to this article: http://dx.doi.org/10.1179/bjo.19.1.35
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Date: 27 August 2017, At: 01:49
British Journal of Orthodontics/ Vol. 19/1992/35-39
The Effect of Adhesive Type and Thickness on Bond Strength of Orthodontic Brackets FRANCEs MACKAY, M.Sc., B.D.S., F.D.S., M.ORTH*
Institute of Dental Surgery, Gray's Inn Road, London WCI X 8LD
Downloaded by [Australian Catholic University] at 01:49 27 August 2017
Received for publication February 1989
Abstract. Fine-mesh based brackets were bonded to plastic cylinders using four different adhesives. Adhesive thickness was controlled using a bonding jig. The bond was then tested to failure using a shear force. Each adhesive had its own minimum thickness, probably related to its viscosity. Increasing the thickness of the adhesives to 0·26 mm, using a stainless steel spacer had minimal effect on their mean shear bond strength. Index words: Adhesive Thickness, Shear Bond Strength
Thickness of Composite
Numerous investigations have been carried out on the bond strength of the bracket-adhesive-enamel bond. They are difficult to compare due to the many variables that have been introduced such as adhesive used, substrate, and design of test. The subcommittee on testing methods of the International Association for Dental Research in 1967 recommended a tensile test for measuring bond strength of resins to dental hard tissues. However, for the bracket-adhesive-enamel bond shear forces are likely to be most critical in vivo (Eden et a/., 1970).
The thickness of the adhesive layer under a bracket may be important for both final tooth position and bond strength. With increasing use of pre-adjusted brackets whose bases compensate for the differing thickness of teeth, it is important to ensure a consistently even layer of composite is placed under each bracket to take full advantage of the bracket design and to avoid the need for compensatory bends to be placed in the archwire. Pender et a/. ( 1988) felt that bracket form influenced bond strength. Molar brackets which had less anatomical detail and, hence, more adhesive under the bracket, failed at lower bond strengths. Increasing the thickness of the adhesive layer is reported (Buonocore, 1963) to give a weaker joint due to the introduction of imperfections leading to stress concentrations, increased polymerization shrinkage, and the fact that thicker layers are more readily deformed. Eden et a/. (1970) tested two thicknesses of adhesive with a tensile test. The 1-mm thick adhesive was stronger than the 2-mm thickness. However, these are not clinically relevant thicknesses. Schechter eta/. ( 1980) reported that with increasing thickness tensile bond strength was not affected whereas in shear, strength decreased as thickness increased. Evans and Powers (1985) reported the effect of increasing adhesive thickness on tensile bond strength. With both chemically cured and 'nomix' adhesives there was a decrease in bond strength
A variety of composite resins are available. Lutz and Phillips (1983) suggest that the best way to describe the filler type is to classify materials into: traditional, hybrid, heterogeneous microfilled, or homogeneous microfilled. The subtypes are well described in their paper and these subdivisions have been applied to the adhesives used in this study. Vankerckhoven et a/. ( 1981) using transmission electron microscopy noted that composites described as traditional, contained submicron particles and Lutz eta/. ( 1983) report that manufacturers commonly use submicron particles to control the viscosity of adhesives. Li eta/. (1985) felt that the volume of filler had a greater effect on physical and mechanical properties than filler size.
• Current Address: Senior Registrar, Department of Orthodontics, Dental Hospital of Manchester, Higher Cambridge Street, Manchester MIS 6FH. 030l-228X/92/002000+00S02.00
(!) 1992 British Society for the Study of Orthodontics
BJO Vol. 19 No. 1
as the adhesive thickness increased. Increasing the adhesive thickness of the 'no-mix' adhesives resulted in a lack of complete polymerization, leading to early bond failure. Aim of the Study
I. To measure the thickness of adhesive achieved between the substrate and the bracket when bonding, using various adhesives. 2. To investigate the effect, on the shear bond strength of increasing the thickness of each adhesive. Downloaded by [Australian Catholic University] at 01:49 27 August 2017
The jig used to control adhesive thickness.
Materials and Methods
The adhesives used are described in Table 1. They were classified according to criteria laid down by Lutz and Phillips ( 1983) using information on particle size obtained from the manufacturers. Adhesives were chosen to represent the different categories. The brackets used were 'A' Company (Slough, Berks) standard 0·022" slot upper lateral incisor brackets with fine mesh bases. These brackets have flat bases and their square shape enabled excess adhesive to be readily removed. The brackets were bonded to composite placed previously within plastic cylinders (Dickinson and Powers, 1980). Materials were dispensed or mixed according to the manufacturer's instructions, placed in an undercut machined into the top surface of a plastic cylinder and the composite wiped flat with the surface of the cylinder. The material was left to cure for I 0 minutes or, in the case of the lightcured materials, cured for 20 seconds using an Elipar light (Espe Premier, Germany). Following curing, the composite was ground flush with the cylinder and the height of the cylinder measured using a micrometer. Subsequently an 'A' Company cleaned bracket was placed on the centre of the cylinder and the total height measured. The measurements were repeated three times, correct to 0·0 I mm. If the discrepancy between the readings was more than 0·0 l mm further readings were taken until three consistent measurements were achieved. TABLE
The cylinder with the bracket on top was then transferred to the bonding jig designed to control adhesive thickness (Fig. 1). The jig consists of a plastic base into which the plastic cylinders were slotted, a moveable crossbar (B) and five steel posts which were individually adjustable. With the crossbar down and the posts raised, the cylinders with the brackets on top were placed in the jig. The steel posts which had double sided sticky tape on their lower end were then lowered to touch the brackets. Raising the crossbar also raised the brackets and allowed adhesive to be placed onto the bracket bases. The crossbar was then lowered and a weight used to produce constant pressure (150 g), the excess adhesive around the bracket carefully removed and the adhesive left for l 0 minutes to cure or cured for 20 seconds with the light directed from the front of the jig and the same period from behind to simulate the clinical situation. To increase the thickness of the adhesive under the bracket, a stainless steel spacer 0·26 mm thick was placed on top of the cylinder and the bracket attached to the post as described. The crossbar was raised, the spacer removed and the procedure repeated for the four other cylinders in the jig. This resulted in the brackets being suspended 0·26 mm above the cylinders when the crossbar was in the down position. The crossbar was raised, adhesive
Adhesives used in the study
Traditional Hybrid Hybrid Inhomogeneous microfilled
Delphic Aurafill Concise ·silar
Filler particle size (average) (J.tm)
15 15+submicron 8 + submicron 0·04
79 77 75 52
Effect of Adhesive Thickness on Bond Strength
BJO February 1992
Downloaded by [Australian Catholic University] at 01:49 27 August 2017
applied and the crossbar lowered under standard pressure. Excess adhesive was removed and the material left to set or cured as described previously. Following bonding of the bracket to the cylinder, the height of the bonded unit was measured using a micrometer. Measurements were taken correct to 0·01 mm. The adhesive thickness achieved when no spacer was used will be referred to as the minimum thickness, the thickness with the spacer will be referred to as standard thickness. Twenty brackets were bonded at each thickness for each of the four adhesives. All tests were carried out using an Instron Universal Testing Machine (Instron Corp, Canton, Mass, U.S.A.).
For the standard thickness group, the variation in thickness had no effect F1.7 5 = 0·68, P = 0·41, while the adhesive effect was significant F3.1s = 17·6, P