Clinical Continued »
ceramic brackets rely solely on mechan- ical retention, using standard light or chemically cured adhesives, without the need for additional special bonding agents such as silane. Addition of a silane component has proved to increase the bond strength to an undesirably high level, risking enamel damage on debond. The bond strength achieved with mechanical retention alone favours a simple bonding technique. An initial pumicing of the teeth followed by self- etch primer (3M Unitek Transbond SEP) and orthodontic composite (3M Unitek Transbond APC or XT) ensures reliable bond strength when compared to cheaper quality brackets and acid-etch bonding methods. There are also significant advantages in time, labour and materials costs when compared to separate etch and bonding techniques.
Bracket bases The subject of bracket base is very important, yet often overlooked. Several developments have occurred in modi- fying the bracket base design in order to reduce bond failure and achieve reliable debonding at the end of a course of ortho- dontics. Predictable debonding is just as important (if not more so) than predict- able bonding of orthodontic brackets and should be considered/planned from the outset. The challenge is to achieve adequate
bond strength for clinical application that will not harm the enamel surface on bracket removal at the end of treatment. With this in mind, the type of bracket base is more relevant than base size. Bracket base area and bond strength have no direct relationship6
. Various types of mechanical
bracket base include: • microcrystalline • perforated • mesh • undercut channels • polymer coated • spherically photoetched • metal/ceramic • dovetail • dimpled. The features of the bracket base should
be undisturbed/unmodified for optimum performance (Fig 5).
Debonding aesthetic brackets The brittleness of cheap brackets can cause problems during treatment and at debond. Their hard nature can cause abrasion to the dentition and significant enamel wear has been found on the surfaces of upper incisors from contact
REFERENCES
1. Russell JS. Aesthetic Orthodontic Brackets. Journal of Orthodontics, Vol. 32, 2005, 146-163.
2. Arid JO, Durning P. Fractures of polycarbonate edgewise brackets. A clinical and SEM study. British Journal of Orthodontics, 14: 191-195.
3. Dobrin RJ, Kamel IL, Musich DR (1975). Load deformation characteristics of polycarbonate orthodontic brackets. American Journal of Orthodontics, 67: 24-33.
4. Swartz ML (1988). Ceramic brackets. Journal of Clinical Orthodontics, 22: 82-88.
5. Birnie D. Orthodontic material update. Ceramic brackets. British Journal of Orthodontics 1990;17:71-75.
6. Reynolds IR. A review of direct orthodontic bonding. British Journal of Orthodontics 1975;2:171-178.
7. Douglas JB (1989). Enamel wear caused by
ceramic brackets. American Journal of Orthodon- tics and Dentofacial Orthopedics, 95: 96-98.
8. Wang WN, Meng CL, Tarng TH (1997). Bond strength: a comparison between chemical coated and mechanical interlock bases of ceramic and metal brackets. American Journal of Orthodontics and Dentofacial Orthopedics, 111: 374-381.
9. Larmour C J, McCabe J F, Gordon P H 1998 An ex vivo investigation into the effects of chemical solvents on the bond behaviour of ceramic orthodontic bracket. British Journal of Orthodontics 25: 35–39
10. Bishara S E, Trulove T S 1990 Comparisons of different debonding techniques for ceramic brackets: an in vitro study. Part II. Findings and clinical implications. American Journal of Ortho- dontics and Dentofacial Orthopedics 98: 263-273
11. Winchester LJ et al., 1992. Methods of debonding ceramic brackets, BJO 19;233-7
Ireland’s Dental magazine 27
with the lower labially placed brackets after only six weeks7
. This can be reduced
by the use of bite stops placed on the posterior molars (generally), to avoid occlusal contacts whilst the teeth level and align, and checking for occlusal contacts between brackets and opposing enamel. These brackets should also be avoided in deep bite cases. A common cause of fractured brackets
is scratching them with a probe/instru- ment/ligatures unnecessarily as this introduces micro-crazing and introduc- tion of surface flaws. Metal ligatures should be used carefully and care taken with removal of these (unwind the metal ties). High quality ceramic brackets can
overcome the problems with fracture at debond with the use of specially designed bases to overcome these issues. The incor- poration of a vertical stress concentrator ensures reliable debonding at the bracket/ composite interface, leaving a layer of composite on the tooth surface. This composite can then be easily removed, protecting the enamel surface. The procedure for debonding these
brackets by “squeezing” in a mesiodistal plane is more reliable for the operator and comfortable for the patient. Uncon- trolled debonding on the other hand can be very unpleasant due to the high poten- tial of bracket fracture and teeth moving within their sockets. This can cause severe distress to the patient and under- mine a healthy dentist-patient relationship. Also, grinding of a layer of fragmented bracket on the tooth surface will often generate significant heat that can cause pulpal damage and is best avoided (Fig 6).
Fig 5
Bracket base features for mechanical retention: (a) compound contoured base, (b) microcrystalline bonding surface, (c) vertical stress concentrator
Fig 4
Microcrystalline Gemini Clear Ceramic Bracket
Fig 6
Gingival view of debonding a 3M ceramic bracket using a custom debonding instrument
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