When Annie is 10 years old, the left bottom 2nd premolar has not come out (Fig.1: #5), because the space for this tooth is too narrow. The tooth #6 appears to have moved to the space (arrow). On the other side of the jaw, the corresponding tooth (#5') erupts normally.
By the time Annie reaches the age of 13, the tooth #5 is not visible from the front (Fig.2). When she opens the mouth, the tooth #5 is inside the dental arch (Fig.3). It is not in the right position, because it does not have enough space.
Three months after wearing braces, the bottom front teeth are straighter (Fig.4), as compared to Fig.2, 3. A spring is placed between braces of the teeth #4 and 6 (Fig.4: *). Under the pressure of the spring, these two teeth are going to be separated (double arrows).
In another two months, the tooth #5 has enough space: front and back (Fig.5: >). Under the tension of a flexible wire (*), the tooth #5 is free to move outward (arrow: --->).
Xin Wei, DDS, PhD, MS 1st edition 11/02/2013, last revision 05/29/2016