Dental Education Lecture: What is Onlay?

When our cavity (caries) is small, simple filling (F in the tooth 31, Fig.1) can do the trick.  When the cavity is big, filling may not work.  For example, the filling in the tooth 30 in Fig.1 is not properly done.  There is a large step on the side, as pointed by red arrow.  You may have hard time removing food impacted between the teeth.  There is a new cavity underneath the filling (black arrow in Fig.1).  Anyway, we need a new restoration: onlay.

Onlay (O in Fig2) looks like filling.  It is a conservative way to fix the cavity when the latter is large. We try to save as much tooth (T in Fig.3) structure as possible.  Fig.3 shows biting surface of the tooth #30. The majority of the onlay does not contact the gums (G in Fig.4), reducing gum irritation.

When our tooth is badly broken down, crown is usually the first choice.  However, crown preparation is not so conservative.  We need to cut down a lot of tooth structure in order to make a crown. The more the cutting, the more irritating to our nerve inside the tooth and the more likely you experience pain after crown preparation.  The crown (C, Fig.5) is like a hat we wear for our head.  It covers our tooth in 360 degrees. Furthermore, the crown usually has close contact with our gums (Fig.6), increasing the possibility of gum irritation and bleeding later on, if we do not pay to attention to our home care.

How does the onlay have so many advantages?  Because it has sophisticated fabrication process.  After removing cavity, the doctor needs to take impression and send the latter to lab for onlay fabrication.  Then the doctor uses a special glue to stick the onlay to the tooth.  They form chemical bonding.  You may notice the smooth transition from the onlay to our tooth structure in Fig.3,4 and 5.

 Xin Wei, DDS, PhD, MS 1st edition 07/03/2009, last revision 07/04/2009