First Premolar Tooth: Location, Function, and Anatomy

First premolars sit immediately behind your canine teeth, one on each side of both the upper and lower jaws, giving you four in total. They occupy the fourth position from the midline, bridging the gap between the pointed canines and the broader, flatter molars further back. Their anatomy is more variable and more clinically interesting than most people realize, with root configurations that differ between men and women, surface grooves that influence gum disease patterns, and a crown shape that serves a distinct biomechanical purpose no other tooth handles quite the same way.

Where First Premolars Sit in the Mouth

Dentists number teeth from the center of the mouth outward. Your upper (maxillary) first premolars are teeth number 5 and 12 in the universal numbering system used in the United States, while the lower (mandibular) first premolars are numbers 21 and 28. In everyday terms, if you run your tongue along your upper teeth starting from the very front, you pass the two central incisors, then the lateral incisor, then the canine (the pointy one), and the next tooth back is the first premolar. The same sequence applies on the bottom.

First premolars erupt around age 10 to 11, replacing the primary (baby) first molars. Because they appear in a transitional zone of the dental arch, they are sometimes called “bicuspids,” a reference to their two main cusps. That nickname, while common, is slightly misleading for the lower first premolar, which often has a very small lingual cusp that barely contributes to function. The upper first premolar, by contrast, has two well-developed cusps that share the workload more evenly.

Crown Shape and How the Upper Differs from the Lower

The maxillary first premolar has a broad, somewhat oval crown when viewed from above. Its buccal cusp (the one facing your cheek) is typically taller and more prominent than the lingual cusp (facing the tongue), and this asymmetry is actually more pronounced in first premolars than in second premolars. A three-dimensional modeling study comparing the two found that the buccal cusp is distinctly more developed in first premolars relative to second premolars, making the crown shape one of the reliable ways to tell them apart.¹ Between the two cusps runs a central groove that is an important landmark during dental procedures.

The mandibular first premolar looks quite different. Its lingual cusp is much smaller and rounder, sometimes barely visible, so the tooth can almost pass for a broad canine. This gives the lower first premolar a tilted appearance, with the buccal cusp dominating the occlusal (chewing) surface. The functional result is that the lower first premolar acts more like a shearing tool, while the upper first premolar can both shear and crush food between its two cusps.

Occasionally, first premolars deviate from the standard two-cusp pattern. A documented case report described a rare presentation in which both the maxillary and mandibular first premolars displayed three cusps, an unusual variation that can complicate restorative work if the clinician is not expecting it.²

What First Premolars Actually Do

First premolars handle the transition between tearing and grinding. When you bite into something firm, your incisors and canines do the initial cutting and gripping. As you move the food back toward the molars for thorough grinding, the first premolars are the first teeth equipped to crush it. Their cusps create a shearing action when the jaw moves side to side and a compressive action when it closes straight down. Think of them as a food processor’s pre-chopping stage before the heavy milling that the molars perform.

This transitional role also means first premolars absorb lateral forces that neither canines nor molars are designed to handle alone. When you chew something tough or fibrous, the first premolars stabilize the food bolus and break it into pieces small enough for the molars to finish. Losing a first premolar does not cripple your ability to eat, but it shifts extra work onto the canine and second premolar, which over time can accelerate wear on those neighboring teeth.

Root Anatomy of the Upper First Premolar

This is where the first premolar gets genuinely complicated. The maxillary first premolar is one of the most variable teeth in the mouth when it comes to root form. A cone-beam computed tomography (CBCT) study found that about 69% of upper first premolars had two roots, with the point where the root splits (the bifurcation) most often located in the upper third of the root.³ That same study noted a sex-based difference: men tended to have bifurcations higher up on the root, while women more often had them in the middle or toward the tip.

A separate CBCT and micro-CT study, however, found a different distribution, reporting that about 70% of maxillary first premolars had a single root and roughly 29% had two roots, with bifurcations most common in the middle third.⁴ The discrepancy between these two studies likely reflects differences in the study populations. Root anatomy varies among ethnic groups, which is something dentists keep in mind when planning treatment. The takeaway is that roughly a third to two-thirds of upper first premolars have two roots, and a dentist cannot assume one way or the other without imaging.

Adding another layer of complexity, a furcation groove runs along the palatal root surface in the vast majority of bifurcated upper first premolars. That same micro-CT study found the groove in about 96% of bifurcated teeth.⁴ This groove matters clinically because it creates a thin wall of dentin that is prone to fracture during root canal therapy and can harbor bacteria in periodontal disease.

Root Anatomy of the Lower First Premolar

The mandibular first premolar is almost always single-rooted, but its internal canal system can be surprisingly complex. A CBCT study of a South Indian population found that roughly 73% had the simplest canal pattern (a single canal running from crown to tip), while about 14% had a canal that divided and then rejoined, and smaller percentages had various other branching configurations.⁵

A particularly tricky variant is the C-shaped root canal, where instead of a round or oval canal cross-section, the canal looks like the letter C. Micro-CT mapping has shown that C-shaped canals can change dramatically in shape at different levels of the root, and the number of distinct canals at the tip of the root can jump to two in a large fraction of cases.⁶ For someone undergoing a root canal on a lower first premolar, this variability is the main reason the procedure can sometimes be more challenging than expected.

Why Root Canals on First Premolars Can Fail

Given the root and canal variability described above, it is no surprise that first premolars are among the teeth most likely to harbor hidden anatomy during endodontic treatment. A review of missed anatomy across tooth types noted that the likelihood of finding unexpected canal configurations is higher in premolars and molars than in front teeth, and that lateral branches off the main canal are common in all teeth but add particular risk in premolars.⁷

A retrospective study looking specifically at why initial root canal treatments fail in maxillary premolars identified four leading causes: inadequate filling of the canal, failure of the restoration placed on top, vertical root fracture, and missed canals. Missed canals accounted for about 11% of failures in first premolars, compared to about 22% in second premolars.⁸ The lower rate in first premolars may reflect the fact that clinicians already expect two canals in many of those teeth and look for them more diligently. Second premolars, which more often appear single-rooted on standard X-rays, catch people off guard.

Restoring a Damaged First Premolar

When a first premolar has had extensive decay or a root canal, how you rebuild it matters a lot. Because first premolars bear significant lateral chewing forces, a large filling that does not protect the cusps can leave the remaining tooth walls vulnerable to fracture. Research on endodontically treated maxillary premolars with wide cavities found that restorations designed to cover the cusps, with at least 2.5 millimeters of material thickness, produced both higher fracture resistance and fractures that could still be repaired rather than ones that destroyed the tooth entirely.⁹

Even when using a straightforward direct composite filling, cusp coverage makes a difference. A study comparing different restoration approaches for root-filled upper first premolars found that full-coverage composite designs outperformed partial ones in fracture resistance.¹⁰ On the other hand, bonded ceramic inlays milled by a computer, while excellent for fit and aesthetics, produced a significantly higher number of severe, non-repairable fractures compared to teeth without inlays when pushed to failure in the lab.¹¹ The practical lesson is that for a first premolar that has lost a lot of structure, a crown or an overlay that caps the cusps tends to be the safest long-term choice.

The Mesial Concavity and Gum Disease

One anatomical feature of the upper first premolar that rarely gets public attention is the mesial root concavity, a shallow trough running down the surface of the root that faces the tooth in front of it. This concavity is not a defect; it is a normal part of the tooth’s shape. One study found that 100% of maxillary first premolars had a mesial concavity, while about 39% also had one on the distal side.¹² Teeth with these concavities had significantly deeper periodontal pockets, greater attachment loss, and more plaque accumulation than those without. The bone loss pattern also differed: teeth with concavities tended to develop crater-shaped bone defects, while those without concavities showed a more gradual ramp-shaped loss.

A cross-sectional study confirmed that the mesial concavity specifically worsens bone loss on the buccal (cheek-facing) side of the tooth, with statistically significant differences across all age groups studied.¹³ A related study found that the deeper the concavity, the more bone loss occurred, with the deepest concavities averaging about 1.3 mm in patients who had already lost half their bone support.¹⁴

For you, this means the area between your upper canine and first premolar deserves extra attention when brushing and flossing. Plaque loves to settle into that concavity, and once it does, it is harder for both your toothbrush and your hygienist’s instruments to reach. If your dentist tells you that you have localized bone loss around an upper first premolar, the root’s anatomy may be a contributing factor, not just your hygiene habits.

Noncarious Cervical Lesions

First premolars, especially upper ones, are among the most common sites for noncarious cervical lesions (NCCLs). These are notches or depressions that form at the gum line without any involvement of decay-causing bacteria. They show up as smooth, scooped-out areas on the cheek-facing surface of the tooth, right where the crown meets the root. The causes are thought to be a combination of aggressive horizontal toothbrushing, acidic foods and drinks, and possibly mechanical stress from biting forces.¹⁵

The role of biting forces is debated. Some researchers believe that when the tooth flexes slightly under heavy or off-center chewing loads, stress concentrates at the cervical area and gradually chips away enamel, a process sometimes called abfraction. Finite element modeling has shown that eccentric (off-center) biting forces do generate high stress in the cervical zone, particularly in a thin layer just beneath the enamel surface.¹⁶ However, clinical evidence tying abfraction directly to NCCL formation remains mixed, and the concept is still somewhat controversial. What is well accepted is that the combination of acid exposure and abrasive brushing is the primary driver, and first premolars take the brunt because of their position right where most people’s toothbrush lands with the most pressure.

First Premolars in Orthodontic Treatment

If you have had braces or consulted an orthodontist about crowding, you have likely heard that premolar extraction is sometimes necessary. First premolars are the most commonly extracted teeth for orthodontic purposes because their location allows the orthodontist to create space and retract the front teeth effectively. The choice between pulling first or second premolars depends on factors like how severe the crowding is, what kind of bite correction is needed, and whether the patient’s facial profile would benefit from more or less retraction of the front teeth.¹⁷

A common worry among patients is that removing premolars will narrow the smile or create visible dark spaces in the corners of the mouth. A study evaluating smile aesthetics after orthodontic treatment with and without first premolar extraction found no predictable relationship between premolar extraction and smile aesthetics. Variables related to the “buccal corridor,” the dark space between the teeth and the cheek during a smile, showed no connection to whether premolars had been removed.¹⁸ In other words, a well-executed orthodontic plan that includes first premolar extraction should not leave you with a narrower or less attractive smile.

First Premolars in Forensic Identification

Teeth survive conditions that destroy most other identifying features of the body, making them valuable in forensic investigations. First premolars contribute to sex determination because their dimensions differ measurably between males and females. An odontometric study found significant sex-based differences in premolar measurements, with the mandibular left first premolar showing notable dimorphism in its mesiodistal width.¹⁹

Beyond size, the shape of the maxillary first premolar can also be used. A geometric morphometric study concluded that tooth shape, assessed objectively through the outline of the occlusal surface and contact areas, differs between males and females in ways that are driven by sex chromosomes and remain stable throughout life because enamel does not remodel once it forms.²⁰ Research using cross-sections of premolars has also shown that males tend to have greater crown-dentin area while females have greater enamel area and enamel thickness, providing additional clues when only fragmentary remains are available.²¹

Developmental Anomalies

While rare, first premolars can develop with structural oddities that affect treatment planning. One such anomaly is dens evaginatus, a small extra bump or tubercle that protrudes from the chewing surface. It consists of enamel on the outside, dentin underneath, and sometimes a thin finger of pulp tissue extending into it. Dens evaginatus most commonly affects mandibular second premolars, but it has been reported on maxillary first premolars as an extremely rare finding.²² The concern with this anomaly is that if the tubercle wears down or breaks off during normal chewing, it can expose the pulp directly, leading to infection or abscess in a tooth that otherwise looks perfectly healthy. Dentists who spot dens evaginatus early can protect the tubercle with a sealant or a small restoration to prevent that exposure.