Evaluation of Tensile Bond Strength of Glass Ionomer Cements for Orthodontic Bonding

Abstract

Glass ionomer cements offer advantages over conventional composite resins when used for orthodontic bonding. These include chemical bonding to enamel, fluoride release, and ease of removal from enamel upon debonding. This study evaluated the tensile bond strengths of three Type I (luting) and three Type II (restorative) glass ionomer cements when used to bond a standard stainless steel orthodontic bracket to teeth. Bovine incisors were divided into six groups of 20 each. Each group had brackets bonded with one of the following glass ionomer cements: SHOFU ® Glasionomer Type I, Fuji I™, Ketac ®-cem, SHOFU ® Glasionomer Type II, Fuji II ™, and Ketac®-fil. A new regimen of specimen preparation was used in an attempt to align the brackets so that true tensile forces could be generated at testing. After the specimens were prepared, they were stored in room temperature water for two weeks, thermocycled, returned to water storage for two additional weeks, and then tested in tension on an lnstron Testing Machine until bond failure occurred. The type bond failure was determined with light microscopic analysis of the debonded bracket bases. The finite element method was used to further evaluate the bracket/adhesive/enamel complex with varying angulation of force application at debonding. The mean tensile bond strength value of 0.82 ± 0.36 MPa for SHOFU® Glasionomer Type I was significantly less (p<0.05) than the values of 1.13 ± 0.33 MPa for Ketac®-cem and 1.26 ± 0.40 MPa for Fuji I™. For the Type II glass ionomer cements, the mean tensile bond strengths of 2.00 ± 0.28 MPa for Ketac®-fil and 1.97 ± 0.29 MPa for Fuji II™ were significantly greater (p<0.05) than the 0.95 ± 0.35 MPa for SHOFU® Glasionomer Type II. The Ketac® and Fuji™ Type II cements developed bond strengths which were significantly greater (p<0.0001) than the Type I cements. Light microscopic analysis of bond failure revealed a high degree of within group variability. As a result, trends relating failure mode to tensile bond strength could not be established. The finite element method study showed that even a slight alteration in angulation of the force applied to the bracket greatly affected the maximum principal stress developed in the adhesive layer. An increase in maximum principal stress of ≈1,000 MPa was seen when a true tensile debonding force (0° offset) was adjusted to 12° of offset. The results of this study indicate that Type II glass ionomers have potential for clinical orthodontic use and that special attention should be given to the angulation of force applied to brackets in laboratory bonding studies.