Compare extracted maxillary and mandibular molars and/or tooth models while reading about these differentiating arch traits. Also refer to page 8 in the Appendix.

A. CROWN OUTLINE TO DIFFERENTIATE MAXILLARY FROM MANDIBULAR MOLARS

From the occlusal view, the crowns of mandibular molars are oblong: they are characteristically much wider mesiodistally than faciolingually.^B This is just the opposite of the maxillary molars, which have their greater dimension faciolingually (Appendix 8a).

B. NUMBERS OF CUSPS (AND NUMBER OF LOBES) THAT DIFFERENTIATE MAXILLARY FROM MANDIBULAR MOLARS

Both mandibular first and second molars have four cusps of similar size (two buccal and two lingual), but most mandibular first molars have an additional smaller fifth distal cusp located on the buccal surface just distal to the other two larger buccal cusps (see occlusal views of molars in Appendix page 8).

Most maxillary first and second molars also have four larger cusps (two buccal and two lingual), but the distolingual cusp varies in size: it is often considerably smaller, especially on maxillary second molars. On some maxillary second molars, this distolingual cusp is not present resulting in the only three-cusped molar. Further, maxillary first molars often have a functionless fifth cusp (cusp of Carabelli) on its lingual surface (the only tooth to have this sort of cusp) (Appendix 8i).

The number of lobes forming all molars is one per cusp, including the cusp of Carabelli if it is large. See Table 5-1 for a summary of the number of lobes forming first and second molars.

C. CROWN TILT THAT DISTINGUISHES MAXILLARY FROM MANDIBULAR MOLARS

When mandibular molar crowns are examined from the proximal views, the crowns appear to be tilted lingually on the root trunk (as is true for mandibular premolars), whereas the crowns and cusps of maxillary molars are centered over their roots (Appendix 8b). Also, when mandibular molars are viewed from the buccal, they may appear to tip distally relative to the midroot axis due to the considerable bulge of the distal crown outline beyond the cervix of the root, and the slope of the occlusal surface shorter on the distal. (The greater distal crown bulge and distal crown tilt can be seen on most mandibular molars in Fig. 5-4.)

D. NUMBER OF ROOTS DISTINGUISH MAXILLARY FROM MANDIBULAR MOLARS

Perhaps the most reliable trait to differentiate extracted maxillary from mandibular molars is the number of roots. Maxillary molars normally have three relatively long roots: two on the buccal called a mesiobuccal and distobuccal, and one lingual (palatal) root. Mandibular molars have only two roots: a mesial and distal root (see Appendix 8c). The root trunk on mandibular molars is shorter than on the maxillary molars since the furcation is closer to the cervical line, especially on mandibular first molars. Mandibular molars have the longest roots relative to crown length (greatest root-to-crown ratio) of any adult teeth.^C

Table 5-2 includes a summary of arch traits that can be used to differentiate maxillary from mandibular molars. Some of these traits will be discussede in more detail in Sections IV and V.

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