An Ex Vivo Study to Investigate Bond Strengths of Different Tooth Types

The materials involved in orthodontic bonding have been a field of constant change since the acid etch technique (Buonocore, 1955) permitted adhesion of acrylic filling materials to enamel surfaces. New bonding materials are tested to ascertain whether they represent an improvement on those previously used. The ideal system would be quick and easy to place,remain in situ with sufficient bond strength to resist dislodging forces during the active treatment period, and be debonded at the end of treatment without complication. A number of means of assessing the success of a bond are available, including measurement of bond strength ex vivo or the recording of bond failure in vivo.The simplest of these is ex vivo bond strength testing. Recording of bond strength can be achieved with a considerable degree of accuracy using modern testing systems and enables comparison of new materials with other materials already in use at the time of testing. The use of a universal-testing machine (e.g. Instron®; Zwick®) in orthodontic bond strength testing is widespread.These machines are capable of delivering a controlled and measured force to the bonded bracket via a moving crosshead (Figure 1). International Standards Organization (ISO, 1991) suggest testing to failure in shear, with the values of stress quoted in mega pascals (MPa). Fox et al. (1994) outlined the difficulties encountered in interpreting data published in support of bond strength claims due to a lack of standardization in the methodology. Comparison of results without standardization is of limited value. A number of suggestions arose from this critique in order to eliminate variables that were not under direct examination. These included recommendations about the nature of the storage media, the testing equipment, minimum sample sizes and the reporting of test findings. Since Newman (1965) first reported ex vivo bond strengths of epoxy adhesives in orthodontics, there has been a consistent tendency for research workers to report increased ex vivo bond strengths of new materials as indicating improved clinical performance. The validity of the long-held assumption that a high ex vivo bond strength is synonymous with low failure rates or enhanced clinical survival has been questioned. Sunna and Rock (1998) cast doubt over the applicability of this assumption when comparing ex vivo bond strength with in vivo clinical bond failure rates. Significant differences in ex vivo bond strength between various bracket/adhesive combinations did not correlate with clinical failure rates, which demonstrated no significant differences. Extracted premolar teeth have formed the basis of most bond strength tests to date (Fox et al., 1994), possibly due to the relative ease of obtaining test specimens following orthodontic extractions. Results obtained from premolar testing have been interpreted as being applicable to all teeth in both dental arches. The validity of this interpretation has, however, not been proven. To date, only one study has examined whether premolar bond strengths are representative of all tooth types. Hobson et al. (1999) examined variations in shear bond strength between different tooth types. Their findings suggest that significant differences in shear bond strength exist between different tooth types and opposing dental arches. Upper anterior teeth demonstrated higher shear bond strengths than upper posterior teeth and lower posterior teeth demonstrated higher shear bond strengths than lower anterior teeth. Recent evidence has suggested that different tooth types exhibit biological variation in their etch pattern after acid priming (Hobson and Mattick, 1997, 1998; Mattick and Hobson, 2000) and it has been proposed that these differences between tooth types may influence the bond strength An Ex Vivo Study to Investigate Bond Strengths of Different Tooth Types