|SECTION IV||TYPE TRAITS THAT DISTINGUISH MAXILLARY FIRST FROM SECOND PREMOLARS|
This section prepares the reader to perform the following:
- Describe the type traits that can be used to distinguish the permanent maxillary first from the second premolar.
- Describe and identify the buccal, lingual, mesial, distal, and occlusal surfaces for all maxillary premolars.
- Assign a Universal number to maxillary premolars present in a mouth (or on a model of the teeth) with complete dentition. If possible, repeat this on a model with one or more maxillary premolars missing.
- Holding any maxillary premolar, determine whether it is a first or a second, and right or left. Then, assign a Universal number to it.
From the buccal view, compare the maxillary first and second premolars in Figure 4-11. Compare tooth models and/or extracted maxillary premolars as you read the following characteristics, holding the crowns down and roots up, just as they are oriented in the mouth.
1. Pentagon-Shaped Crowns (Buccal View)
The pentagon-shaped crowns of maxillary first premolars are larger than on maxillary second premolars in all dimensions, but the root is shorter overall.E On maxillary first premolars, the mesial and distal sides of maxillary premolar crowns converge more noticeably from the contact areas to the cervical line than on maxillary second premolars, so the cervical portion of the crown of maxillary first premolars appears relatively narrower. This increased convergence of first premolar crowns is due in part to the more prominent bulk on first premolars where the cusp ridges join the proximal surfaces (called shoulders), especially on the mesial (Fig. 4-12). Observe the more angular mesio-occlusal bulk (shoulders) and increased crown taper on many maxillary first premolars when compared to second premolars in Figure 4-11.
2. Mesial Cusp Ridges Shorter Than Distal on Maxillary Premolars (Buccal View)
Maxillary second premolars, like canines and mandibular premolars, have the mesial cusp ridge of the buccal cusp shorter than the distal cusp ridge. The maxillary first premolar is UNIQUE because it is the only premolar (or canine) that has its mesial cusp ridge longer than the distal cusp ridge. This results in a cusp tip of the maxillary first premolar that is uniquely distal to the vertical midroot axis (Appendix 6e and highlighted with midroot axis lines on most maxillary first premolars in Fig. 4-11).
3. Buccal Cusp Sharper on Maxillary First Premolars (Buccal View)
The buccal cusp of the maxillary first premolar is relatively long and pointed or sharp (Appendix 6f), resembling the adjacent, pointed maxillary canine, with the mesial and distal slopes meeting at almost a right angle (average 105°), compared to the second premolar, which is less pointed and more obtuse (average over 120° to 125°), as seen when comparing first and second premolars in Figure 4-11.
3. Buccal Ridge More Prominent on Maxillary First Premolars (Buccal View)
The buccal ridge is more prominent on maxillary first premolars than on maxillary second premolars (Appendix 6g). Shallow vertical depressions adjacent to the buccal ridge in the occlusal third are most commonly located mesial to the buccal ridge on maxillary first premolars and distal to the buccal ridge on maxillary second premolars (depicted in the drawings on the top of the page in Figure 4-11).F
5. Roots of Maxillary Premolars (Buccal View)
The apical end of the root of all single-rooted premolars frequently bends distally, but these roots may also be straight or bend mesially.I,G Most of the time, the maxillary first premolar is divided into a buccal and lingual root splitting off of a common trunk in the apical thirdH (seen best from the proximal view in Appendix 6h). Even though the buccal root is normally longer than the lingual root, the buccal root frequently curves distally near the apex, so you might see the tip of the lingual root if it is straighter than the buccal root or curves mesially. Both roots are visible in several maxillary first premolars in Figure 4-11.
The single root of the maxillary second premolar is longer than on the first premolar.I The root is nearly twice as long as (1.8 times longer than) the crown, which gives this tooth the greatest root-to-crown ratio of any maxillary tooth.
Compare the lingual view of maxillary first and second premolars in Figure 4-13.
1. Lingual Cusps Slightly Shorter Than Buccal on Maxillary Premolars (Lingual View)
The lingual cusp is shorter than the buccal cusp on both types of maxillary premolars, but considerably more so on the maxillary first premolar. The buccal and lingual cusps of the maxillary second premolar are closer to the same length.J This difference in cusp height is seen when comparing almost all first and second premolars in Figure 4-13 and is evident on the lingual views of maxillary premolars on Appendix page 6.
2. Lingual Cusp Tip Mesial to Midline on Maxillary Premolars (Lingual View)
The mesial and distal ridges of the lingual cusp of the maxillary first premolar meet at the cusp tip at a somewhat rounded angle. The tip of the lingual cusp of the second premolar is relatively sharper. The tips of the unworn lingual cusps of both types of maxillary premolars are consistently positioned to the mesial of the midroot axis line (Appendix 6i). This trait is an excellent way to tell rights from lefts, especially for the maxillary second premolar, which is, in many other ways, nearly symmetrical.
3. Maxillary Premolar Mesial Marginal Ridge More Occlusal Than on Distal (Lingual View)
From the lingual view of maxillary premolars, differences in marginal ridge heights are apparent on handheld teeth when rotating the tooth just enough one way to see the mesial ridge height, then just enough in the opposite direction to compare the distal ridge height. The mesial marginal ridges of both types of maxillary premolars are more occlusal in position than the distal marginal ridge (recall Appendix 5j).
4. Roots of Maxillary Premolars (Lingual View)
The lingual root of a two-rooted maxillary first premolar is usually shorter, and narrower mesiodistally, than the buccal root.K Therefore, both the narrow lingual root and the broader and longer buccal root are visible from the lingual view (Fig. 4-14A). Both first and second single-rooted premolar roots taper narrower to the lingual.
Compare the proximal views of maxillary first and second premolars in Figure 4-15 and Appendix page 6.
1. Mesial Crown Concavity Unique on Maxillary First Premolars (Proximal Views)
Maxillary first premolars have a UNIQUE prominent mesial crown concavity just cervical to the contact area, whereas maxillary second premolars (and mandibular premolars) do not (Appendix 6j). This unique mesial crown concavity can be seen in almost all maxillary first premolars and not on any others, so it is a consistent and obvious trait that confirms the mesial surface of a maxillary first premolar. It is important to remember the presence of this unique mesial crown concavity when restoring correct restoration contours on this surface or when detecting and removing calcified deposits from this concave crown surface.
2. Buccal Cusp Slightly Longer on Maxillary Premolars (Proximal Views)
From this view, as from the lingual, the buccal cusp is noticeably longer than the lingual cusp on maxillary first premolars, compared to the second premolar, which has two cusps of nearly equal length (Appendix 6c). This difference is obvious when comparing first and second premolars in Figure 4-15. From the proximal view, it is often a challenge telling buccal from lingual on the maxillary second premolar based solely on the cusp heights, since the cusp heights are so similar. Differences in the crests of curvature, however (described next), will be useful for distinguishing buccal from lingual surfaces on these teeth.
The average distance between the buccal and lingual cusp tips of maxillary first and second premolars is about the same.L
3. Crest (Height) of Curvature on Maxillary Premolars (Proximal Views)
Like all teeth, the facial crest of curvature of maxillary premolars is located in the cervical third. Specifically, it is near the junction of the middle and cervical third. Lingually (like other posterior teeth), it is more occlusal, in the middle third (near the center of the crown) (Fig. 4-16). When holding the midroot axis exactly vertical, this trait helps distinguish the buccal from lingual surfaces on the majority of maxillary premolars from the proximal views. Try it for the teeth in Figure 4-15.
4. Marginal Ridge Grooves on Maxillary Premolars (Proximal Views)
Marginal ridge (developmental) groove serves as a spillway for food during mastication (seen extending across the marginal ridge and onto the mesial surface of a maxillary first premolar in Figure 4-17 and on an occlusal view in Appendix 6k). The mesial marginal ridge of the maxillary first premolar is almost always crossed by a mesial marginal ridge groove that may extend from the mesial fossa or pit onto the mesial crown surface.M The distal marginal ridge of this tooth, and the mesial and distal marginal ridges of the maxillary second premolars, are less likely to have marginal ridge grooves, and, when present, these grooves are less likely to extend onto the proximal surfaces.
5. Cervical Line Curve Larger on Mesial When Comparing Proximal Views
The cervical line on the mesial side of both types of maxillary premolars curves occlusally in a broad, but shallow, arc. As on anterior teeth, the mesial curvature is slightly greater than the distal curvature.N The cervical line on the lingual surface on a maxillary premolar may be in a more occlusal position than on the buccal surface, especially on maxillary first premolars. This would accentuate the appearance that the lingual cusp is shorter than the buccal cusp.
6. Roots and Root Depressions of Maxillary Premolars (Proximal Views)
The roots of both types of maxillary premolars are likely to have both mesial and distal root depressions of varying depths. Knowledge of the relative location and depth of these depressions can be helpful clinically when using dental instruments along the root surface into deep gingival sulci to detect and remove calcified deposits that contribute to periodontal disease and when identifying areas of decay on the root surfaces that are covered with gingiva.
Recall that maxillary first premolars most often have two roots with the buccal root slightly longer than the lingual root. The split into two roots (bifurcation) usually occurs in the middle or apical third of the root. As stated previously, this is the only premolar with a UNIQUE crown concavity on the mesial surface of its crown, and this depression continues across the CEJ to become a prominent mesial root surface depression whether there is one root or two rootsO (Fig. 4-17). This important type trait is seen clearly on the mesial views of all five maxillary first premolars in Figure 4-15. There is also a less distinct distal root depression found on both double- and single-rooted first premolars, but it does not normally extend up to the cervical line. The root of the maxillary first premolar is UNIQUE in that it is the only premolar (maxillary or mandibular) where the longitudinal root depression on the mesial surface is deeper than on the distal.
The maxillary second premolar usually has one root with longitudinal midroot depressions located in the middle third of the mesial and distal root surfaces. However, neither midroot depression on this tooth extends onto the crown.P The distal root depression is usually deeper than on the mesial root surface. This feature is the opposite from the maxillary first premolar, where the mesial root depression is deeper.
Compare occlusal views of maxillary first and second premolars. To follow this description, the teeth or tooth models you are using should be held as those displayed in Figure 4-18, so that the buccal surface is at the top and you are sighting directly down along the midroot axis.
1. Maxillary First Premolars Larger Than Second (Occlusal View)
In the same mouth, maxillary first premolars are most often larger in most dimensions than adjacent second premolars.Q
2. Central Groove Longer on Maxillary First Premolars (Occlusal View)
Characteristically, central developmental grooves run mesiodistally across the center of both types of maxillary premolars extending from a mesial pit to a distal pit. The length of the central groove of the maxillary first premolar is relatively longer (more than one third of the mesiodistal crown width) compared to the shorter groove on the maxillary second premolar (less than one third of the width). This is obvious when comparing maxillary first and second premolar grooves in Figure 4-18 and in Appendix 6l.R Because the central groove on the maxillary first premolar is longer, the pits are farther apart and are relatively closer to the marginal ridges than on maxillary second premolars. This longer central groove on the maxillary first premolar is accentuated by its continuation with the mesial marginal ridge groove that almost always crosses the mesial marginal ridge (Fig. 4-19). Marginal ridge grooves are much less common on the other marginal ridges of maxillary first and second premolars.
3. Triangular Fossae (Occlusal View)
Mesial and distal fossae on maxillary premolars are traditionally called triangular fossae. The base of the triangular shape is along the marginal ridge, and the other two sides roughly parallel the fossa grooves. On most maxillary premolars, the distal triangular fossa appears larger than, or equal in size to, the mesial triangular fossa.S
4. More Supplemental Grooves on Maxillary Second Premolars (Occlusal View)
There are more supplemental grooves on maxillary second premolars than on maxillary first premolars giving them a more wrinkled appearance. These supplementary grooves radiate buccally and lingually from the pits at the depth of each triangular fossa (Fig. 4-20B).
5. Maxillary Premolars Wider Buccolingually (Occlusal View)
The oblong (oval or rectangular) crown outlines of both types of maxillary premolars are noticeably greater buccolingually than mesiodistally.T This is obvious for all maxillary premolars in Figure 4-18.
6. Maxillary First Premolar Is More Asymmetrical (Occlusal View)
On both types of maxillary premolars, the lingual half of the tooth is narrower mesiodistally than the buccal half, more so on first premolars. From the occlusal aspect, the buccal outline of the maxillary first premolar is almost V shaped because of the prominent buccal ridge, but this ridge is less prominent on the second premolar as seen on some teeth in Figure 4-18. On both types, the tip of the lingual cusp is slightly mesial to the center of the tooth.
Seen from the occlusal aspect, the maxillary second premolar is typically quite symmetrical (similar shape for the mesial and distal halves). Its occlusal outline is smoother and less angular than that of the first premolar (Fig. 4-20B compared to Fig. 4-20A).
An asymmetrical occlusal outline is a distinguishing feature of the maxillary first premolar that distinguishes it from second premolars (compare outline shapes in Appendix 6m). This asymmetry is due, in part, to the location of the lingual cusp tip mesial to the midline, whereas the buccal cusp tip is actually located distal to the midline, a position UNIQUE to the maxillary first premolar with its mesial cusp ridge longer than its distal cusp ridge (Fig. 4-20A). Further, the mesial marginal ridge joins the mesial cusp ridge of the buccal cusp at an almost right angle (not so on second premolars), and the mesial surface forms a nearly straight or concave outline buccolingually rather than a more uniform convexity on the second premolar. This mesial outline concavity may be accentuated by the depression of the mesial marginal ridge groove and mesial crown concavity next to the root. This straighter mesial marginal ridge appears shorter buccolingually than the more convex distal marginal ridge. Note this characteristic on the occlusal surfaces of most maxillary first premolars in Fig. 4-18.
Compare the symmetrical outline of the maxillary second premolar to the asymmetrical outline of the maxillary second premolar in Figure 4-20C.
7. Contact Areas on Maxillary Premolars (Occlusal View)
Mesial contacts for both types of maxillary premolars are at or near the junction of the buccal and middle thirds (slightly more buccal on first premolars). Recall that one third of the tooth from this aspect means one third of the total buccolingual measurements of the crown outline, rather than one third of the occlusal surface measurement. Distal contacts are in the middle third on maxillary second premolars, located more lingually than mesial contacts. Just the opposite is true on first premolars with their asymmetry, where the distal contact is more buccal than the mesial contact (Fig. 4-21). Picture this asymmetry when viewing the hexagon outline presented in Appendix 6m for the maxillary first premolar and then to the examples of most maxillary first premolars in Figure 4-18.
See how many traits you can list that differentiate the maxillary first from second premolars and maxillary right from left premolars. When you have finished, compare your list with traits listed in the figures in this chapter.