**7. Last but not least**

The physical properties as well as rebonding and debonding of monocrystalline and polycrystalline alumina ceramic brackets were reviewed. Ceramic brackets fabricated from polycrystalline zirconium oxide (zirconia) were not mentioned. Thus, we would like to add a short paragraph about these brackets for the interested reader.

Zirconia ceramic brackets, manufactured in Japan and Australia, have attracted interest in the past [12]. Nevertheless, problems concerning color (yellowish tint) and opacity (nontranslucency) were reported approximately three decades ago [12, 70]. Furthermore, no significant advantage of zirconia brackets over alumina brackets with regard to their frictional characteristics were reported [71]. Limited published research on zirconia (zirconium oxide) brackets exists [72].
