OBJECTIVES

While reading this section, examine several extracted mandibular premolars or premolar models. Hold these mandibular teeth with the crowns up and the roots down.

To appreciate differences in mandibular first and second premolars, it is first important to remember that there are two common types of mandibular second premolars³: a two-cusp type with one buccal and one lingual cusp and a slightly more common three-cusp type with one buccal and two lingual cusps (seen from the occlusal sketches in Fig. 4-22).

LEARNING EXERCISE

Look in your own mouth, and determine whether you have a first and second premolar on both sides of the mandibular arch. If you have each of your four mandibular second premolars, try to determine if each has two or three cusps. Then compare it to the data in Table 4-6 at the end of this chapter to see how common your findings are.

TABLE 4-6

Occurrence of Lingual Cusps on Mandibular Second Premolars (808 Females, 1532 Teeth)

A. TYPE TRAITS OF MANDIBULAR PREMOLARS FROM THE BUCCAL VIEW

Refer to views from the buccal of mandibular first and second premolars in Figure 4-23.

1. Pentagon Shape of Both Mandibular Premolars (Buccal View)

As with all other premolars and canines, the shape of mandibular premolar crowns (two-cusp or three-cusp type) from the buccal view is roughly a five-sided pentagon (Appendix 5g).

2. Relative Size of Mandibular Premolars (Buccal View)

The crown of the mandibular first premolar bears considerable resemblance from this aspect to the second premolar, but mandibular first premolars are slightly longer overall than second premolars with a noticeably longer crown (more like the adjacent canine), but a slightly shorter root.^U

3. Mesial Cusp Ridge Shorter Than Distal Cusp Ridge (Buccal View)

From this view, both types of mandibular premolars appear nearly symmetrical except for the shorter mesial than distal cusp ridge of the buccal cusp and a greater distal bulge of the crown. This greater distal bulge may give the appearance of a slight distal tilt of the crown relative to the midroot axis.

4. Buccal Cusp of Mandibular First Is Sharper Than Second (Buccal View)

Just like on maxillary premolars, the buccal cusp on a mandibular first premolar is relatively sharper (110°) than on a second (130°). The crown of the mandibular second premolar appears closer to square than the first premolar because it is somewhat wider in the cervical third (is less tapered), is shorter overall, and has a buccal cusp that is less pointed than on the mandibular first premolar, with cusp slopes meeting at an angle of about 130° (Appendix 6n and Fig. 4-24A and B).

5. Location of Proximal Contacts on Mandibular Premolars (Buccal View)

Because of the steeper angle formed by the cusp ridges of the buccal cusp, the contact areas on the mandibular first premolar appear more cervical from the cusp tip than they are on mandibular second premolars where cusp ridges are less steep. Mesial contacts of both types of mandibular premolars are near the junction of the occlusal and middle thirds (slightly more occlusal on second premolars). The distal contact of the mandibular second premolar follows the general rule: the distal contact is more cervical than the mesial contact area. The distal contact area of the mandibular first premolar is an EXCEPTION to most other teeth: the mandibular first premolar is the only adult tooth where the mesial contact is more cervical than the distal contact (Appendix 6o). A summary of the location of proximal contact areas in all types of premolars is presented in Table 4-3.

TABLE 4-3

Premolars: Location of Proximal Contacts (Proximal Height of Contour) in Premolars (Seen Best from Buccal View)a

^aGeneral learning guidelines:

1. For premolars, the mesial and distal contacts are closer to the middle of the tooth and are more nearly at the same level compared to anterior teeth.

2. Distal proximal contacts of premolars are more cervical than mesial contacts EXCEPT for mandibular first premolars where the mesial contact is more cervical than is the distal.

6. Cusp Ridge Notches and Vertical Depressions of Mandibular Premolars (Buccal View)

Vertical crown depressions in the occlusal third of the buccal surface on either side of the buccal ridge are not common on either type of mandibular premolars, but, when present, they are less discernible than on the maxillary premolars. However, shallow notches are commonly seen on both the mesial and distal cusp ridges of the buccal cusp of mandibular premolars. These notches (and vertical crown depressions when they are present) are more frequently located on the shorter mesial cusp ridge of mandibular first premolars and on the longer distal cusp ridge of mandibular second premolars^V,W as seen in Figure 4-25. Dr. Peter K. Thomas recommended developing notches (called Thomas notches) on certain cusp slopes when shaping large occlusal restorations and crowns because they permit smooth movement of teeth from side to side and are important as spillways for food during chewing.

7. Roots of Mandibular Premolars (Buccal View)

Both types of mandibular premolars have single roots that gradually taper to the apex. The roots apices are noticeably more blunt on mandibular second premolars than on first premolars. As with most roots, there is a tendency for the apical third of the root to bend distally, but note that as many as one fifth may bend mesially.^X

The roots of mandibular first premolars are almost as thick but slightly shorter than the roots of the second premolar.^Y The roots of mandibular second premolars (like maxillary second premolars) are nearly twice as long as the crowns. In both arches, second premolars have a larger root-to-crown ratio than on firsts.

B. TYPE TRAITS OF MANDIBULAR PREMOLARS FROM THE LINGUAL VIEW

For the lingual aspect, refer to lingual views of mandibular first and second premolars in Figure 4-26.

1. Lingual Cusp Width of Mandibular Premolars (Lingual View)

On mandibular first premolars, as on most teeth, the crown is much narrower mesiodistally on the lingual half than on the buccal half. This can also be seen on mandibular second premolars with one lingual cusp. However, the width of the lingual half of a 3-cusp type of second premolar with two lingual cusps is UNIQUE since it is the only type of premolar where the lingual half is likely to be wider mesiodistally than the buccal half.

2. Lingual Cusps Length on Mandibular Premolars (Lingual View)

The lingual cusp of a mandibular first premolar is quite small and very short but is often pointed at the tip. It is nonfunctional and could be considered a transition between the canine cingulum and more prominent lingual cusp (or cusps) of the second premolar (best appreciated from the proximal views). Much of the occlusal surface of this tooth can be seen from the lingual aspect because of the most obvious shortness of the lingual cusp. The location of the lingual cusp tip is variable: it may be mesial or distal to the middle of the tooth.

On mandibular second premolars with one lingual cusp, the single lingual cusp is still shorter and narrower than the buccal cusp, but it is relatively larger (longer and wider) than the lingual cusp of the first premolar. The single lingual cusp tip is most often just mesial to the center line of the root (Appendix 6q), which is similar to both types of maxillary premolars. In the three-cusp type, there is one large buccal and two smaller lingual cusps. The mesiolingual cusp is almost always larger and longer than the distolingual cusp, but this difference may be little or great.^AA The more prominent mesiolingual cusp tip is mesial to the midline of the root, similar to the lingual cusp of a two-cusp premolar.

3. Marginal Ridge and Contact Heights of Mandibular Premolars (Lingual View)

From the lingual view, differences in marginal ridge heights are apparent on handheld teeth when rotating the tooth first enough in one direction to see the mesial marginal ridge height and then enough in the opposite direction to compare the distal ridge height. As with most other posterior teeth, the mesial marginal ridges of the mandibular second premolars are slightly more occlusally located than the distal marginal ridges as is evident on all mandibular second premolars in Figure 4-26. This is also true of the proximal contacts on mandibular second premolars: mesial contacts are more occlusal than distal contacts. An EXCEPTION to all other posterior adult teeth is the mandibular first premolar, the only adult tooth where the mesial marginal ridge is more cervically located than the distal marginal ridge as is evident in Figure 4-26 for many mandibular first premolars. This is similar to the UNIQUE relative location of the proximal contacts on the mandibular first premolar where the mesial proximal contact is more cervical than the distal contact.

4. Mesiolingual Versus Lingual Grooves on Mandibular Premolars (Lingual View)

Unlike maxillary premolars, which have no grooves visible from the lingual view, both types of mandibular premolars may have a groove that could be visible from the lingual view (best appreciated on occlusal views in Fig. 4-27). On mandibular first premolars, there is frequently a mesiolingual groove separating the mesial marginal ridge from the mesial slope of the small lingual cusp.^BB (Rarely, a similar groove might also be present between the distal marginal ridge and the distal slope of the lingual cusp.) On mandibular second premolars with two lingual cusps, a lingual groove passes between the mesiolingual and distolingual cusps and may extend lingually slightly beyond the occlusal table.

5. Roots of Mandibular Premolars (Lingual View)

The single root of both types of mandibular premolars is tapered toward the apex, and the roots are close to the same length.^CC

C. TYPE TRAITS OF MANDIBULAR PREMOLARS FROM THE PROXIMAL VIEWS

When studying the proximal views of mandibular first and second premolars, refer to Figure 4-28.

1. Lingual Tilt and Shorter Lingual Cusps of Mandibular Premolars (Proximal Views)

Recall that the crowns of the mandibular first premolars tilt noticeably toward the lingual surface at the cervix (much more than any other premolar). This tilt places the tip of the buccal cusp almost over the midroot axis line, and the lingual cusp tip is so lingually positioned that it may be aligned directly over the lingual outline of the cervical portion of the root (Fig. 4-29A and on all mandibular first premolars in Fig. 4-28). As was also seen from the lingual aspect, the lingual cusp of the mandibular first premolar is considerably shorter than the buccal cusp, more obviously than on other premolars.^Z Since it is so short, it is considered a nonfunctioning cusp (Appendix 6p) with a very short, almost horizontal triangular ridge.

The mandibular second premolar crowns (both two-cusp and three-cusp types) also tip lingually, but not as much as on the mandibular first premolar. The tip of the buccal cusp of the mandibular second premolar is usually located at the junction of the buccal and middle thirds. As with the first premolar, the tip of the lingual cusp (or of the mesiolingual cusp) of this second premolar is usually about on a vertical line with the lingual surface of the root at the cementoenamel junction. A comparison of the lingual tilt of a mandibular first and second premolar is seen in Figure 4-29.

The lingual cusps (or mesiolingual cusps for three-cusp types) of mandibular second premolars are closer in length to the buccal cusp than on first premolars.^DD When the three-cusp type is viewed from the mesial, the longer mesiolingual cusp conceals the shorter distolingual cusp, but when it is viewed from the distal, part of the longer mesiolingual cusp is usually visible behind the shorter distolingual cusp (as seen on several mandibular second premolars viewed from the distal in Fig. 4-28).

2. Marginal Ridge Alignment of Mandibular Premolars (Proximal Views)

The mesial marginal ridge of the mandibular first premolar slopes cervically from the buccal toward the center of the occlusal surface at nearly a 45° angle and is nearly parallel to the triangular ridge of the buccal cusp (Fig. 4-30 and Appendix 6s). When comparing the height of mesial and distal marginal ridges from the lingual view, the distal marginal ridge is aligned more horizontally and therefore more occlusally than the sloped mesial marginal ridge, a trait UNIQUE to this tooth. The difference in marginal ridge angle and height is most helpful in differentiating right mandibular first premolars from lefts (by identifying the more downward sloping and more cervical mesial marginal ridge).

Similar to maxillary premolars, the more horizontal mesial marginal ridge of the mandibular second premolar is more occlusally located than the distal marginal ridge (compare mesial and distal views in Fig. 4-28).

3. Marginal Ridge Grooves and Mesiolingual Grooves on Mandibular Premolars (Proximal Views)

When viewed from the mesial, a mesiolingual groove on the mandibular first premolar^BB may be seen separating the mesial marginal ridge from the mesial slope of the lingual cusp (Appendix 6r). When viewed from the distal, it would be rare to see a groove between the distal marginal ridge and the distal slope of the lingual cusp.^BB

Mesiolingual grooves are not present on mandibular second premolars, but the marginal ridges on these teeth may infrequently be crossed by a marginal ridge groove (very rarely on the distal marginal ridge).^EE

4. Buccal and Lingual Crests of Curvature of Mandibular Premolars (Proximal Views)

As on the both types of maxillary premolars (and anterior teeth), the facial crest of curvature on both types of mandibular premolars is located in the cervical third. On most premolars, it is located close to the junction of the cervical and middle thirds, but on the mandibular first premolar, it is positioned more cervically than on other premolars: very close to the cervical line as on the adjacent mandibular canine (Fig. 4-30). The buccal contour in the occlusal two thirds of this crown is flatter than that on the mandibular second premolar (seen on most first premolars in Fig. 4-28).

For all mandibular premolars, the crests of curvature of the lingual surface of the crown are in the middle third, about in the center of the total crown length. On the mandibular first premolar, this is not far from the cusp tip of the very short lingual cusp (clearly seen on mandibular first premolars in Fig. 4-28). Because of the extreme lingual tilting of the crowns, the lingual surfaces of both types of mandibular premolar crowns bulge lingually, often well beyond the lingual surface of the root (Fig. 4-29).

5. Cervical Lines of Mandibular Premolars (Proximal Views)

Similar to other teeth, the occlusal curve of the cervical line on the proximal surfaces of all premolars is slightly greater on the mesial surface than on the distal.^FF The cervical line is also located more occlusally on the lingual than on the buccal (Fig. 4-30). This makes the crowns appear to be shorter on the lingual side.

6. Roots of Mandibular Premolars (Proximal Views)

The single roots of both types of mandibular premolars taper apically, with the least taper in the cervical third.

7. Root Depressions of Mandibular Premolars (Proximal Views)

Mandibular first premolars often have a shallow longitudinal depression in the apical and middle thirds of the mesial root surface but are more likely to have a longitudinal depression on the distal root surface, and this distal depression is most often deeper than on the mesial.^GG Most mandibular second premolars have no depression on the mesial root surface but are likely to have a longitudinal depression in the middle third of the distal root surface.

To summarize, all types of mandibular and maxillary premolars are, on average, likely to have a more prominent root depression on the distal root surfaces than on the mesial EXCEPT the maxillary first premolar, which is more likely to have its more prominent root depression (and unique mesial crown depression) on the mesial surface. See Table 4-4 for a summary of the location and relative depth of root depressions on all types of premolars.

TABLE 4-4

Occurrence and Relative Depth of Longitudinal Root Depressions (“Root Grooves”) in Premolarsa

^aGeneral learning guideline:

Premolars are likely to have deeper distal surface root depressions (EXCEPT maxillary first premolars).

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