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Mr. Weng, in his early 70s, is a heavy biter. Two of his old fillings (Fig.1,2 F) in the lower molars are pretty big and keep breaking down (*). The fillings are going to be replaced by a better material. It is a type of porcelain, called Zirconia. It is very strong. Basically it is cut from a stone in lab by a computer. It is reported never to be broken.
Since this material is so good, can we use it to cover the whole of our teeth so that they will never be bad again? No. The more teeth are covered by the material, the more tooth structure is removed. The teeth become weak after preparation. Furthermore, no material is better than our tooth structure.
For Mr. Weng, what we need to do is to remove broken fillings and use the good material (Zirconia) to replace broken portions of his teeth. So the repair is conservative. The restorations are called onlay (restoration on the top of the tooth, Fig.3,5 for the defect in Fig.1) and (mini) crown (Fig.4,6 for Fig.1's defect). For example when the tooth in Fig.2 needs the crown, the lingual (L, tongue side) portion of the tooth is chipped off (insert (upper right): grey area). The crown (Fig.4 C) covers more of the lingual aspect (Fig.6 arrowheads) than that in Fig.5. But the crown does not extend to the gums (G in Fig.6). In fact too much extension does not protect our teeth. It interferes with local cleaning.
On the buccal (cheek) side, the onlay does not cover the buccal (Fig.3 B) portion of the tooth. Therefore the onlay is more conservative than so-called mini crown. It should be made whenever possible. In all, our teeth are natural. They do not need to be covered completely by an artificial material no matter how good it is. Small is good.
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Xin Wei, DDS, PhD, MS 1st edition 09/30/2014, last revision 09/30/2014