Effect of simulated debracketing on enamel damage.

BACKGROUND/PURPOSE: A smooth enamel surface after the removal of a bracket from a tooth is essential for both esthetic demands and the prevention of plaque accumulation. The purpose of this study was to evaluate enamel damage caused by three standardized debracketing techniques. METHODS: We established three standardized test devices based on the principles of the squeezing, shearing, and tensile testing methods, which were simulated using a How Plier (TASK 60-306), a Direct Bond Bracket Remover (TASK 60-335 T), and a Lift-Off Debracketing Instrument (3 M-Unitek 444-761), respectively. Thirty teeth in each group were evaluated after debracketing. An optical stereomicroscope and a CCD camera with a computerized image analysis system were used to ascertain the proportion of remnant adhesive area (RAE) on the enamel surface. Fractography was analyzed using a scanning electron microscope. RESULTS: The squeezing debracketing method exhibited the highest debonding force (54.3 ± 7.0 N) and the least damage to the enamel surface (RAE = 99.5% ± 2.4%). The tensile debracketing method preserved most of the adhesive on the enamel surface (RAE = 98.7% ± 3.3%) and required the least debonding force (6.8 ± 1.2 N). However, the shearing debracketing method exhibited a significantly higher debonding force (32.0 ± 8.2 N) and smaller RAE (77.3% ± 33.5%) compared to the tensile debracketing method (p < 0.05). Three specimens appeared to have vertical fractures on their enamel prisms when using the shearing method. CONCLUSION: With the proposed method, we conclude that the squeezing and tensile methods are acceptable for clinical use when debracketing, whereas the Direct Bond Bracket Remover may cause shearing failure, leading to a risk for enamel damage.

Tension-compression viscoelastic behaviors of the periodontal ligament.

BACKGROUND/PURPOSE: Although exhaustively studied, the mechanism responsible for tooth support and the mechanical properties of the periodontal ligament (PDL) remain a subject of considerable controversy. In the past, various experimental techniques and theoretical analyses have been employed to tackle this intricate problem. The aim of this study was to investigate the viscoelastic behaviors of the PDL using three-dimensional finite element analysis. METHODS: Three dentoalveolar complex models were established to simulate the tissue behaviors of the PDL: (1) deviatoric viscoelastic model; (2) volumetric viscoelastic model; and (3) tension-compression volumetric viscoelastic model. These modified models took into consideration the presence of tension and compression along the PDL during both loading and unloading. The inverse parameter identification process was developed to determine the mechanical properties of the PDL from the results of previously reported in vitro and in vivo experiments. RESULTS: The results suggest that the tension-compression volumetric viscoelastic model is a good approximation of normal PDL behavior during the loading-unloading process, and the deviatoric viscoelastic model is a good representation of how a damaged PDL behaves under loading conditions. Moreover, fluid appears to be the main creep source in the PDL. CONCLUSION: We believe that the biomechanical properties of the PDL established via retrograde calculation in this study can lead to the construction of more accurate extra-oral models and a comprehensive understanding of the biomechanical behavior of the PDL.

Effects of different debonding techniques on the debonding forces and failure modes of ceramic brackets in simulated clinical set-ups.

INTRODUCTION: The objectives of this study were to evaluate the effects of different debonding techniques on the in-vitro mean debonding forces and failure modes of ceramic brackets bonded to enamel with clinically simulated setups. METHODS: Three kinds of ceramic brackets (Clarity; 3M Unitek, Monrovia, Calif; Inspire and Inspire Ice; Ormco, Orange, Calif) were bonded to extracted premolars with the same bonding system. Thirty ceramic brackets, 10 of each type, were removed by hand; 60 ceramic brackets, 20 of each type, were tested on a universal testing machine with the pliers according to the manufacturers' recommendations. To simulate clinical debonding conditions, specially designed setups were used to debond the ceramic brackets. Debonding forces and failure modes were investigated. Fractographic evaluations were performed by using scanning electron microscopy. RESULTS: Most brackets failed at the bracket-adhesive interface. Cohesive bracket fractures were noted in all 3 types of ceramic brackets (debonded by hand: 70% of Inspire, 20% of Inspire Ice, and 10% of Clarity; debonded by machine: 75% of Inspire, 30% of Inspire Ice, and 25% of Clarity). The cohesive ceramic fractures of the Clarity brackets were located at the junction between the wings and the body, and at the slot. However, for the Inspire and the Inspire Ice brackets, the cohesive ceramic fractures were located at the occlusal aspect of the base. The mean debonding forces of Inspire, Inspire Ice, and Clarity brackets were 25.72 +/- 11.98, 17.92 +/- 5.03, and 76.89 +/- 23.47 N, respectively. No enamel damage was found after the brackets were removed. CONCLUSIONS: The results of the failure modes showed that the new designs with a ball reduction band in the Inspire Ice bracket and the vertical debonding slot in the Clarity bracket significantly reduced the risk of ceramic bracket fracture during debonding. The force required to debond the Inspire Ice bracket was significantly lower than that of the Inspire bracket.