There are two or three molars (Fig.1 M) and two premolars (P) in each of four quadrants in our mouth. A premolar is smaller than a molar. It is not rare to have one or two extra premolars, usually in a wrong place (Fig.1,4,7 P'). Extra premolars have different shapes and sizes, because they are sort of aborted or premature teeth. They may have normal appearance (Fig.2,3) and structures, such as enamel (Fig.2 E), dentin (D) and root (R). When the extra tooth have abortion late, it may have tooth appearance (Fig.5 C (crown), R), but no distinct tooth structures (such as enamel or dentin). If abortion (disruption of tooth formation) occurs early, the extra tooth has almost no tooth appearance (Fig.8,9). Instead the extra tooth has two portions: white mass (O) and dark area (L). In fact the white mass represents the hard portion of tooth structures (mixed enamel and dentin), whereas the dark area is the soft tissue of the tooth (pulp, nerve, normally inside the tooth).
When a tooth does not form normally, anything abnormal can happen (mess up). We need to keep an eye on the extra teeth. For example, the extra teeth with normal appearance and structures may have normal ability to erupt in spite of the fact that they are present inside the jaw. When they are erupting, they may press against the roots of normal neighboring teeth, possibly causing root resorption (2). Fig.6 is the same as Fig.3 except that the root of one of the neighboring teeth is outlined. The black line represents overlapping portion of the extra tooth and the root of the neighboring tooth. The root may undergo resorption (being eaten out).
When the extra tooth aborts quite early (Fig.8,9), it may have tendency to become a tumor, usually benign.
The orientation of extra teeth is various and fascinating. The extra tooth may be sideway (Fig.2), normal (Fig.3) and upside down (Fig.5).
Xin Wei, DDS, PhD, MS 1st edition 07/02/2012, last revision 07/15/2012