|SECTION V||CLASS AND TYPE TRAITS OF PRIMARY MOLARS|
Now, consider the traits that apply to most primary molar crowns. Refer to Appendix page 10 while studying these traits.
- The prominent mesiobuccal cervical ridge (or mesial bulge on the buccal surface) is exaggerated by the curve of the cervical line apically (best seen when viewed from the buccal view [Fig. 6-24]), and by the constriction near the cervical line (best viewed from the proximal, Appendix 10e). This mesiobuccal cervical bulge makes it easy to distinguish right from left primary molars.
- Due to the taper of the crown from the cervical bulges toward the occlusal surface, primary molar crowns have a narrow occlusal table4 (Recall, the occlusal table is the chewing surface inside the line formed by the continuous mesial and distal cusp ridges for all cusps and the mesial and distal marginal ridges.) (Appendix 10c and proximal views Fig. 6-25 and especially in Fig. 6-26).
- As on permanent molars from the buccal view, all molar crowns are wider mesiodistally than cervico-occlusally (Appendix 10a).N
- In the primary dentition, primary second molars are decidedly larger than primary first molars. This is different in the permanent dentition where first molars are usually larger than seconds (Appendix page 10, compare firsts to seconds).
- Both maxillary and mandibular primary second molars are considerably wider than the second premolars that will replace them.J,K,O,P
- The primary molar occlusal anatomy is not pronounced. In other words, the cusps are short (not pointed or sharp, almost flat) (Appendix 10d and Fig. 6-27), occlusal ridges are minimal, and fossae and sulci are correspondingly not as deep as on secondary molars.
- There are few grooves or depressions in the crowns. (When studying the histology of teeth, you will also learn that the enamel rods at the cervix slope occlusally, unlike in permanent teeth where these rods slope cervically.)
The roots are thin mesiodistally and slender, have little or no root trunks, and spread out beyond the outlines of the crown, more widely on primary second molars than the first molars (the opposite of the adult molars)5 (Appendix 10g and f, Figs. 6-27 and 6-28). This root divergence makes room for the developing succedaneous premolars. Extraction of a primary molar when roots are complete and before they have started to resorb may cause the developing portion of the premolar to be removed along with the primary molar.7
Recall that primary first molars are located over the crowns of developing first premolars, just distal to primary canines and just mesial to primary second molars. Primary second molars form over the crowns of developing second premolars, just distal to primary first molars and, after age 6, just mesial to 6-year first molars.
When distinguishing maxillary from mandibular primary molars, first consider arch traits such as the number of roots. Like permanent molars, primary maxillary molars normally have three roots: the palatal (longest), mesiobuccal, and distobuccal (shortest), whereas mandibular molars have only two roots: the mesial (longest) and distal (shorter and narrower buccolingually). Also, like permanent molars, maxillary primary molars tend to be wider buccolingually than mesiodistally, whereas mandibular primary molars tend to be wider mesiodistally than buccolingually. Compare the occlusal outlines of primary molars in the Appendix on page 10.
Each type of primary molar will be discussed at this time to emphasize the traits that further differentiate each type. Discussion begins with primary second molars since they are so similar in shape to the permanent first molars that were described previously in Chapter 5.
1. Type Traits of Primary Second Molars
Both maxillary and mandibular primary second molars resemble the permanent first molars that erupt just distal to them, with cusp ridges and fossae corresponding to those of permanent first molars. Maxillary primary second molars may even have a cusp of Carabelli. Since these teeth are adjacent to one another during the time of mixed dentition, it is important to distinguish between a primary second molar and the permanent 6-year first molar that erupts just distal to it. First, consider the tooth position from the midline as an important clue for tooth identification. If there are no missing teeth, the primary second molar is the fifth tooth from the midline, and the permanent first molar is the sixth tooth from the midline. Also, a primary second molar is smaller in all dimensions than the 6-year first molar found just distal to it.L,M The differences in size and position between primary maxillary second molars and permanent maxillary first molars are evident in Figures 6-29A and B and 6-30.