| SECTION III | ARCH TRAITS THAT DIFFERENTIATE MAXILLARY FROM MANDIBULAR PREMOLARS |
Refer to Appendix page 6 while reading about differences between maxillary and mandibular premolars.
In the same mouth, maxillary first and second premolars appear more alike than do first and second mandibular premolars.
A. LINGUAL CROWN TILT IN MANDIBULAR, NOT MAXILLARY PREMOLARS
Mandibular premolar crowns from the proximal view appear to be tilted lingually relative to their roots (the first premolar noticeably more than the second) (Appendix 6a and Fig. 4-8A). This lingual tilting of the crown is characteristic of all mandibular posterior teeth and enables their buccal cusps to fit and function into the sulcus of the opposing maxillary teeth. Due to the crown tilt to the lingual on both types of mandibular premolars, they are shaped like a rhomboid from the proximal view (Appendix 6b). A rhomboid is a four-sided figure with opposite sides parallel to one another, like a parallelogram.
Maxillary premolar crowns are aligned more directly over their roots and well within the boundary of the root outline, an important relationship imparting good functional support for a large chewing area (Fig. 4-8B). This difference is easy to recognize when comparing the proximal views of maxillary and mandibular premolars in Appendix 6a. Both types of maxillary premolars are shaped like a trapezoid from the proximal view (Appendix 6b). A trapezoid is a four-sided figure with two parallel sides (occlusal and cervical sides) and two nonparallel sides.
B. DISTAL CROWN TILT IN MANDIBULAR PREMOLARS
Viewed from the buccal, mandibular premolar crowns have more distal than mesial bulge beyond the root outline, which makes mandibular premolar crowns appear to tilt slightly to the distal relative to the root (Fig. 4-9). This is a very subtle tilt that is not always easy to discern.
FIGURE 4-9. A mandibular premolar with more crown bulge distal to the root giving the impression that the crown is slightly tilted distally relative to the root.
The buccal cusp is longer than the lingual cusp (or cusps) on all premolars, but the difference is minimal on maxillary premolars, especially on the maxillary seconds, whereas the difference is considerable on mandibular first premolars (compare Appendix 6c, maxillary, and 6p, mandibular).
Most premolar lingual cusp tips are positioned off center to the mesial (seen from lingual views in Appendix 6i and 6q), EXCEPT on mandibular first premolars where the lingual cusp tip may be centered or to the distal and on some mandibular second premolars where the lingual cusp tip may be centered. On three-cusp-type mandibular second premolars, the longer lingual cusp tip, the mesiolingual, is most often positioned more to the mesial.
The buccal ridge near the middle of the buccal surface of premolars is more prominent on maxillary than mandibular premolars.
From the occlusal view, maxillary premolars are more oblong or rectangular (considerably wider faciolingually than mesiodistally), whereas mandibular premolars, though usually wider faciolingually, are closer to equal dimension faciolingually compared to mesiodistally (Fig. 4-10). This difference may be even more apparent when comparing the outline of the occlusal table (the area bounded by a perimeter of ridges: mesial and distal cusp ridges of each cusp and mesial and distal marginal ridges). This difference is apparent when comparing the dimensions of the occlusal views of maxillary and mandibular premolars in Appendix 6d.
FIGURE 4-10. Arch differences in the outline of premolars. Notice that even though dimensions of the tooth outline (the outer boxes surrounding these teeth) for all maxillary and mandibular premolars are wider faciolingually than mesiodistally, maxillary premolars are obviously longer faciolingually compared to the nearly square shape of mandibular premolars. This is even more evident when considering the proportions of the occlusal table (the inner boxes).
LEARNING EXERCISE
Before learning about traits that distinguish first premolars from second premolars, make sure you can distinguish maxillary premolars from mandibular premolars. Looking at tooth models, extracted teeth or pictures consisting of a selection of both types of premolars from both arches, separate out the maxillary premolars from the mandibular premolars. To do this, review the major differences between maxillary and mandibular premolars highlighted in Table 4-2.
| TABLE 4-2 | Major Arch Traits That Distinguish Maxillary from Mandibular Premolars |
|
|
Maxillary Premolars |
Mandibular Premolars |
|
BUCCAL |
Buccal ridge is more prominent No distal crown tilt relative to root |
Buccal ridge is less prominent Crown exhibits slight distal tilt on root due to greater distal bulge |
|
LINGUAL |
Less difference between heights of buccal and lingual cusps (especially seconds) |
More difference between heights of buccal and lingual cusps (especially firsts) |
|
PROXIMAL |
Central groove centered over root Lingual cusp is just slightly shorter than buccal |
Crown tilts to lingual so buccal cusp tip almost centered over root Lingual cusp is much shorter than buccal |
|
OCCLUSAL |
Crown shape oval or rectangular Crown considerably wider faciolingually than mesiodistally |
Crown shape closer to square or round Crown less oblong faciolingually |
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