The effect of thermal cycling on the integrity of the orthodontic band--cement--tooth interfaces.

Orthodontic bands are retained on the crowns o f teeth by the luting action o f the cement. Studies by several work­ ers have demonstrated the inability o f zinc phosphate cements to produce an effective marginal seal and have shown microleakage to occur along the walls o f cavities restored with zinc phosphate cements. Differences in the coefficients o f thermal expansion between enamel, dental cements and stainless steel suggest that temperature vari­ ations would increase the microleakage along the enamellcement/steel interfaces. Preformed stainless steel bands were cemented with a zinc phosphate cement to the crowns o f 60 extracted maxillary central incisor teeth. Thirty specimens were subjected to 13 000 cycles o f tem­ perature changes ranging between 6 and 70°C while 30 control specimens were retained at room temperature (20°C). AH 60 specimens were then immersed in col­ loidal graphite, embedded in resin and the crowns sec­ tioned in the sagittal plane. The cut surfaces were polished, mounted in DPX on glass slides and subjected to microscopic examination. The results showed that there was no significant difference in the extent o f the de­ fects observed between the experimental and control specimens at the bandlcementltooth interfaces. For the total sample, however, there was a significantly greater frequency o f severe defects occurring along the palatal interfaces compared with those occuring along the labial interfaces. OPSOMMING