Transcription
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Hey everyone, Dr. Ryan here, and welcome back to our Dental Anatomy series. In this video, we're going to talk about the maxillary first premolar.
So here we have a picture of that maxillary first premolar, which happens to be the largest of all the premolars. And using the universal tooth numbering system, this would include tooth number five and twelve.
So this is a nice look at both the first maxillary premolar as well as the second maxillary premolar. And maxillary premolars in general are more oval-shaped from this view, whereas mandibular premolars, when we get to them, are more circular or round.
Since we're talking about posterior teeth for the rest of the series, there's a lot more going on with the occlusal surfaces. So I'm going to generally spend less time on the facial and side views and more time on the occlusal views.
So like the canine next to it, this tooth has a pentagon shape from the facial view in terms of its crown. That cusp tip, though, is a little distal to the long axis. And the reason for that is because the mesial cusp bridge is slightly longer than the distal cusp bridge. This is opposite of the maxillary canine, where the distal cusp bridge was slightly longer.
For all premolars, the mesial height of contour is located between the junction of the occlusal and middle thirds, and the distal height of contour is in the middle third. There's also a notable mesial concavity between that height of contour and the CEJ that matches or mirrors the distal concavity that we talked about of the maxillary canine, and that makes that gingival embrasure symmetrical.
There are also two distinct developmental depressions with a dominant facial lobe, just like we saw again with that maxillary canine. The lingual aspect is narrower than the facial aspect, like we've seen with all the teeth so far. Even though it has two distinct cusps, it still only consists of one lingual lobe versus three facial lobes. Hence, why this lingual cusp that we're looking at is smaller than the facial cusp. The lingual cusp is classically about one millimeter shorter than the facial cusp, and it tends to point in a mesial direction.
This first bullet point is super duper important. The side view is a trapezoid, and this is an arch trait for all maxillary posterior teeth. Mandibular posterior teeth are going to take on a totally different shape, so stay tuned for that later in the series. But for now, remember that all maxillary posterior teeth are trapezoidal from the side view.
The cusp tips should line up with the root tips, so facial to facial and lingual to lingual. And the facial height of contour is in the cervical third, like it's always been. But the lingual height of contour should be in the middle third. Now, this diagram doesn't do the best job of showing that, but remember this pattern. These two bullet points right here are true for almost all of the posterior teeth.
Now, once again, looking at the mesial aspect without all those red lines drawn on it, you can notice there's a pretty flat mesial marginal ridge that's basically perpendicular to the long axis of the tooth. And then a very, very important landmark for this tooth, the mesial marginal ridge groove. And it's seen mostly and only on those maxillary first premolars. So that is a super duper important landmark and distinguishing feature of this tooth.
Now, a few more features to point out. Since we're now venturing into posterior teeth, we have multiple roots as the norm. And so we can finally show you what that bifurcation looks like, as well as the root trunk, that area between the bifurcation and the CEJ. Those are two terms we talked about in the very first video. Now finally coming to life.
Now, this tooth has a super strong, deep root flute that extends all the way into the cervical third of the crown. And the proximal contact here is located a bit more facially than what we're used to seeing. And that's also going to be the norm for posterior teeth. That makes the lingual embrasure larger than the facial embrasure, allowing food to be directed toward the palate and toward the tongue.
Now you can really appreciate what I'm talking about in that first picture, where that lingual embrasure is a lot deeper and larger than the facial embrasure, and that proximal contact is located a bit more facial to the center of the tooth.
Now to the distal aspect. Now, there are a couple of things that you're used to seeing here, and I'm about to flip them on their head. So the distal root flute is not deeper than the mesial one, as opposed to all the other teeth that we've seen so far. The distal root flute is actually flatter. There's really not much root fluting going on at all. The cervical line is flatter, that's a universal trait of all teeth, so we're used to seeing that. And then the distal marginal ridge groove is a lot smaller than that mesial marginal ridge groove was, or it's just absent completely, and you just see a nice flat marginal ridge without any grooves on it.
All right, as I promised, we're gonna spend most of the time on this occlusal view. So the facial lingual dimension greater than mesio distal. We've seen that quite a few times now. The tooth tapers from facial to lingual, just like the anterior teeth did, in order to fit into that U-shaped arch. I already mentioned that from the lingual view, but now you can really appreciate it. And there's this prominent disto facial line angle. It's a bit sharper than the more rounded mesio facial line angle. The facial cusp tip is slightly distal of center. The lingual cusp tip is slightly mesial of center. So the tooth is a bit, let's say, asymmetrical.
The mesial and distal height of Contours or proximal contacts are once again facial of center to allow for a larger lingual embrasure. Okay, here's another important term. The occlusal table is the area of the occlusal surface within the cusp tips that actually comes into contact with the opposing tooth. The best way I can explain it is the beloved game Connect the Dots, where the dots are the cusp tips as well as the extremities of the marginal ridges, and then all you have to do is connect all of those dots. So the mesial and the distal cusp bridges, as well as the mesial and distal marginal ridges, will form the boundaries of that occlusal table. And the surface formed by the occlusal table occupies a little bit more than half the total occlusal surface for this tooth.
And notice the acute distofacial line angle as opposed to the more 90 degree mesio facial line angle. The length of that distal marginal ridge is longer than the length of the mesial marginal ridge. Also, check out that cool mesial marginal ridge groove. By the way, another important landmark is the central groove. This is the developmental groove that connects the mesial and the distal pits, and it makes a slight smile with this slight lingual curvature to it. And the groove is always located lingual to the center of the tooth. Again, the pits are the very deepest part of a groove.
Here we see the triangular ridges highlighted. So it's called a triangular Ridge because the base of that Ridge is wider than the cusp tip where the ridge originates from. Then there are also these little triangular fossae that are formed by the inner part of the marginal Ridge and the Triangular ridges from both cusps. We have two cusps, we should expect two pulp horns, so that's not a surprise.
And this is the only non-molar tooth that typically has two roots. So most often we'll see two pulp canals, sometimes only one if those roots are fused, or we can even have three roots and three canals. Now, those root tips, if we go back to one of the side views, these are notorious for breaking off during extractions because that bifurcation is pretty close to the apices, and those are really tiny root tips. So just something to keep in mind. This tooth is notorious for breaking during an extraction.
And although the crown has a hexagonal shape from the occlusal view, it kind of has this again, asymmetrical hexagonal shape. If we cut the tooth in cross section at the middle of its root, we would see a kidney bean shape, and that's thanks to that strong root flute on the mesial surface.
So a summary of the maxillary first premolar: the occluso cervical height and the facial lingual dimension of the crown are very similar, so we'll just say those are equal, with the mesio distal dimension being the smallest. That mesial marginal ridge groove is a key landmark of this tooth. It looks like a pentagon from the facial view, a trapezoid from the side view, a hexagon from the occlusal view, and has a kidney bean cross section. And it normally consists of four lobes, two pulp horns, and two pulp canals.
That's it for this video. Thank you so much for watching. Please like this video if you enjoyed it and subscribe to this channel for much more on dentistry. If you'd like to support me, please check out my Patreon page and thank you to all of my patrons for their support. You can unlock access to my video slides to take notes on and practice questions for the board exams. So go check that out. The link is in the description. Thanks again for watching everyone. I'll see you in the next video.
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