Dental Education Lecture: Why is my filling so big?

Sometimes we encounter unhappy customers.  "Why is my tooth more sensitive after filling?"  "My cavity was pretty small.  Why does my filling look so big now?"

To answer these questions, we need to know tooth anatomy and pattern of cavity formation 

Fig.1 is a cross section of our back tooth.  The outer layer is enamel (white), middle layer is dentin (yellow) and the red area represents pulp (nerve).  Early cavity is confined to enamel (black, arrow, Fig.2).  Due to alignment of enamel prism, the top of cavity is narrow, whereas the bottom of cavity is wide.  When a cavity enlarges and invades the underlying dentin, it also has features of a small opening and a large bottom (black in Fig.3).

To get rid of every bit of cavity in the bottom, the dentist should open up the cavity and the end result is a large filling (gray, Fig.4).  Not only does the filling looks wider, but also it is deeper, closer to the pulp (compare depth of cavity in Fig.3 and that of filling in Fig.4).  This may be the reason why sensitivity is apparently more severe than before.  But the discomfort may be temporary, since our tooth is able to make new dentin (arrow in Fig.5) to protect our nerve.  The sensitivity may gradually go away, since making new dentin takes time, probably one or two months.  To prevent or reduce after-filling sensitivity, the doctor may place a thin layer of medicine (white in Fig.6) underneath the filling.  The special medicine promotes our body to make dentin quicker and in larger amount (arrow in Fig.6).  

As patients, we should practice good oral hygiene and see the dentist on a regular basis.  Frequent exam can find the earliest cavities.  The chance of sensitivity after filling for small cavity (Fig.2) is less than that for a large one (Fig.3).  As long as the dentist does what he is supposed to do, you will be alright.  

Xin Wei, DDS, PhD, MS 01/28/2009, last revision 02/11/2009