1. Crown: The labial surface is smooth, slightly trapezoidal, the mesial edge and incisal edge are straight, the distal edge and cervical edge are more protruding, the mesiodistal diameter is larger than the incisal diameter, and the width and shortness of the crown are important features of this tooth. . The mesial incisal angle is like a right angle, the distal incisal angle is rounded and blunt, and the cervical ridge is obviously protruding. The lingual surface and the labial surface are approximately equal in size, the mesial and distal marginal ridges are more prominent, the lingual protuberance is prominent, and the lingual fossa is obvious. The proximal surface is triangular in shape. Because the cervical ridge and lingual protuberance are obviously prominent, the crown neck is very thick and the crown and root are distinct.

2. Tooth root: It is a wide and flat single root, the labial surface is wider than the lingual surface, and the root length is about twice the crown length. The apical 1/3 of the root is labial and slightly distal. A wide crown and wide root are important anatomical features of this tooth.

1. Crown: The labial surface is smooth, the mesial and distal edges are symmetrical, the mesial incisal angle and the distal incisal angle are sharp and symmetrical, the incisal edge is straight, and the cervical ridge is prominent. The marginal ridge of the lingual surface is narrow and protruding, but the protuberance of the lingual surface is small and protruding, and the lingual fossa is obvious. The proximal surface is triangular in shape, and the incisal ridge is thin and located on the long axis of the tooth.

2. Tooth root: It is a relatively slender single root, and the root length is about twice the crown length. The root of the tooth is straight and the apex is labial.

(1) Buccal surface: Like a trapezoid, the width of the mesial and distal diameter is greater than the height of the neck, the mesial edge is long and straight, and the distal edge is short and protruding. The neck of the tooth is narrowed, so the cervical ridge is very prominent, especially the mesial part. The cheek tip is slightly protruding and slightly mesial.

(2) Lingual surface: smaller and more rounded than the buccal surface, and the tongue tip is rounder and more protruding than the buccal tip.

(3) Proximal surface: It can be seen that 1/3 of it is significantly narrowed, and the 1/3 of the buccal neck is very prominent.

(4) Facial surface: The shape is similar to the maxillary premolar, but the shape of the triangular ridge and groove of the buccal and lingual tips of the facial surface is not as clear as the maxillary premolar.

1.Dental crown

2. Tooth root: divided into mesial and distal roots. The root trunk is short and the root bifurcation is large.

1. The crown of the second deciduous molar is smaller than that of the first permanent molar and is milky white.

2. The proximal view of the second deciduous molar shows that the proximal cervical edge of the crown is significantly reduced, and the cervical ridge is prominent. The crown is gathered from the neck to the 𬌗 direction, and the proximal cervical surface is large and the 𬌗 surface is small.

3. The mesiobuccal cusp, distal buccal cusp and distal cusp of the mandibular second deciduous molar are approximately equal in size, and among the three cusps of the mandibular first permanent molar, the mesiobuccal cusp > the distal buccal cusp > the distal cusp

4. The maxillary second deciduous molar has three roots and the mandibular second deciduous molar has two roots. The root trunk is short and the root bifurcation is large.