How Rare Is a Tooth With Four Roots?

A tooth with four fully separate roots is genuinely rare, but exactly how rare depends entirely on which tooth you’re talking about. For a maxillary (upper) first molar, the global rate comes in around 0.05% based on a large meta-analysis, making it a roughly one-in-two-thousand finding. For a lower premolar, four roots are so uncommon that individual cases get written up in dental journals as the first of their kind. Wisdom teeth, on the other hand, sprout a fourth root a bit more often. The picture gets more interesting once you factor in population-level differences and the clinical headaches that surprise roots can cause.

What Counts as Normal Root Anatomy

Before talking about four roots, it helps to know how many roots teeth are supposed to have. Your front teeth and canines almost always have a single root. Premolars typically have one or two. Upper molars normally come with three roots, while lower molars usually have two. These are the textbook numbers, and most people’s teeth follow them closely. The interesting cases are the ones that don’t.

A systematic review of mandibular premolars, for example, found that about 97% of lower first premolars had one root, and more than 99% of lower second premolars did too.¹ In a large sample that classified tooth root phenotypes, anterior teeth were almost universally single-rooted, premolars ranged from one to three roots, and molars ranged from one to four.² So a fourth root on any tooth is already outside the normal range, but it’s a bigger anomaly on some teeth than others.

Four-Rooted Upper First Molars

The upper first molar is the tooth most commonly studied for four-root anatomy, probably because it’s already a three-rooted tooth and adding one more seems like a plausible developmental variation. A systematic review and meta-analysis that pooled 49 population-based imaging studies covering more than 26,600 upper first molars put the global incidence of a fourth root at 0.047%.³ That’s roughly 1 in every 2,100 teeth. The same study also compiled 48 case reports from around the world, confirming that the anomaly isn’t limited to any single region.

What makes these teeth especially tricky is their internal plumbing. About 62% of four-rooted upper first molars had four root canals, which is the configuration you’d expect, one canal per root. But the rest had more: five, six, or even seven canals packed into four roots.³ That kind of internal complexity turns what’s already an unusual tooth into a genuine puzzle for anyone performing a root canal.

Extra Roots on Lower Molars

Lower (mandibular) first molars normally have two roots, one toward the front of the mouth and one toward the back. But a well-documented variation called radix entomolaris adds a third root on the tongue-facing side of the tooth.⁴ This isn’t technically four roots, but it’s the most clinically relevant “extra root” scenario in everyday dental practice, and the frequency range is astonishing.

Depending on the population studied, radix entomolaris in mandibular first molars shows up in anywhere from 0.2% to 32% of people.⁵ That’s an enormous spread. Populations of East Asian descent tend to cluster at the higher end of that range, while European and African populations generally report lower frequencies. One phenotyping study recorded three-rooted mandibular first molars in about 18% of its sample.² By contrast, the same variation on mandibular second molars is much less common.⁵

A mandibular molar jumping from three roots to four would require yet another supernumerary root on top of the radix entomolaris. That situation is vanishingly rare and essentially confined to individual case reports rather than population-level data.

Four Roots in Premolars

This is where the term “rare” starts to feel like an understatement. Premolars are small teeth that almost always have a single root. A literature review of mandibular first premolars found four roots in fewer than 0.1% of cases.⁶ For the mandibular second premolar, the situation is even more extreme. A case report published in 2008 described a mandibular second premolar with four separate roots and four root canals, and at the time, the authors stated that this configuration had never been reported in the literature before.⁷

When a tooth variation warrants its own published case report as a first-ever discovery, you’re looking at something exceptionally uncommon. If you happen to have a four-rooted premolar, you’ve got anatomy that dental textbooks don’t cover and researchers have barely documented.

Wisdom Teeth Are the Exception

Wisdom teeth (third molars) are the wild cards of the human mouth. Their root morphology is far more variable than any other tooth, and they are the tooth type most likely to have four roots. A study of maxillary wisdom teeth in a Libyan subpopulation found that about 2.6% had four roots.⁸ That’s still unusual, but it’s roughly 50 times more common than a four-rooted upper first molar. The majority of maxillary wisdom teeth in that sample had three roots (about 80%), with two-rooted and four-rooted forms occurring at nearly equal rates across genders.

Lower wisdom teeth can show four roots as well, though the data is sparser. In the root phenotype classification study, mandibular third molars showed three-rooted forms in about 6% of cases and also displayed a separate buccal anomaly called paramolaris in roughly 3.6%.² Since many wisdom teeth are extracted before they’re fully erupted or imaged, the true population-level rates are probably underestimated.

Why Population Background Matters So Much

Tooth root number isn’t something that varies randomly from person to person. It follows strong population-level patterns. The 0.2%-to-32% range for radix entomolaris in lower first molars is a dramatic illustration: a dentist working in Seoul sees three-rooted lower molars routinely, while one in Stockholm rarely encounters them. These differences are genetic and heritable, meaning they track with ancestry rather than diet, climate, or individual health.

This matters practically because a dentist who trained or practices predominantly with one population may be less prepared to recognize root anomalies common in another. If you’re of East Asian descent and your dentist trained in a setting where three-rooted lower molars are uncommon, the risk of an overlooked root canal during treatment goes up. The reverse applies too: population-specific patterns mean that certain “rare” anomalies aren’t actually rare for you, depending on your background.

How Extra Roots Form During Development

Root formation happens after the crown of the tooth is essentially complete, driven by a tissue called Hertwig’s epithelial root sheath. This sheath acts like a biological template, guiding the shape and number of roots as they grow downward into the jawbone.⁹ When development goes according to the usual script, the sheath divides in a pattern that produces the expected number of roots for that tooth type. Extra roots appear when the sheath folds or branches in an unusual way, creating an additional compartment that mineralizes into a separate root.

Research on the molecular signals involved has identified a family of signaling proteins, including BMP2 and BMP4, that appear to regulate how root sheath cells behave differently from other dental tissues during this branching process.¹⁰ The exact genetic triggers that push a tooth from three roots to four remain poorly understood, though. Most of the research has focused on understanding normal root formation rather than cataloging the mutations or developmental disruptions that produce supernumerary roots. This is an area where the science is genuinely thin, partly because four-rooted teeth are too rare to study in large genetic cohorts.

Why Missed Roots Lead to Failed Root Canals

The clinical significance of extra roots comes down to root canal therapy. The entire point of a root canal is to clean and seal every canal inside a tooth. If a tooth has an unexpected root, it almost certainly has an unexpected canal, and if that canal goes untreated, the outcome suffers. A study modeling predictors of early endodontic failure found that untreated additional canals were significantly associated with treatment failure in the short term, with the association holding across one-, two-, three-, and four-year follow-up windows.¹¹

The issue isn’t that dentists don’t know extra roots exist. The problem is detection. Traditional two-dimensional dental X-rays compress a three-dimensional tooth into a flat image, and roots can overlap or hide behind one another. A study comparing conventional X-ray methods against cone-beam computed tomography (CBCT) found that experienced endodontists missed roughly one in five root canal systems when working from standard films alone.¹² CBCT produces a three-dimensional image and catches what flat X-rays miss. If you’re facing a root canal on a molar and your dentist suspects unusual anatomy, asking whether a CBCT scan is warranted is reasonable.

The C-Shaped Canal Complication

Not every root anomaly involves a clean, fully separated extra root. Sometimes roots partially fuse or fail to separate completely during development, creating what’s called a C-shaped canal. In a mandibular second molar, for instance, a groove running down the tongue-facing side of a root can indicate incomplete fusion between two roots, producing a single C-shaped canal system with multiple openings inside the tooth rather than distinct, round canals.¹³

C-shaped canals are relevant to the four-root question because they sit on the same developmental spectrum. The same root sheath irregularities that can produce a fully separate extra root can also produce a partially formed one, and the borderline cases are the hardest to treat. A root that looks single on an X-ray but actually has a deep groove or partial bifurcation may behave like two canals during cleaning, yet not respond to standard multi-root treatment protocols. These are teeth that can frustrate even experienced practitioners.

Four-Rooted Baby Teeth

Extra roots aren’t exclusive to permanent teeth. A case report described a primary (baby) upper second molar with four well-defined roots in a young male patient. What made it especially interesting was that the same tooth on the opposite side had a completely normal root pattern.¹⁴ This kind of left-right asymmetry suggests that whatever developmental event produces the extra root can happen independently on each side of the mouth, rather than being a whole-mouth genetic program.

Four-rooted primary teeth are clinically significant mainly because they can complicate extraction. Baby molar roots that splay out in unexpected directions increase the risk of root fragment retention, and fragments left in the jaw can interfere with the eruption of permanent teeth below. For pediatric dentists, a pre-extraction X-ray that reveals unusual root anatomy is the difference between a routine procedure and one that demands extra planning.

How Humans Compare to Other Primates

Humans aren’t the only primates with variable root counts. A study quantifying root number variation in a population of wild chimpanzees found that root number was highly stable for most tooth positions, with more than 97% of specimens showing the expected count. The main exceptions were certain premolars and first molars, where variation ran higher.¹⁵ This parallels the human pattern: premolars and first molars are the positions where extra roots are most likely to show up, while front teeth are boringly consistent.

The fact that both humans and chimpanzees show root number variation at similar tooth positions hints that the developmental vulnerability to extra roots is ancient, predating the split between the two lineages. It also reinforces the idea that root number isn’t a tightly locked feature of tooth identity. The developmental machinery that builds roots has enough flexibility that small perturbations, whether genetic or environmental, can nudge the count up or down, particularly at the positions where the root sheath has to execute the most complex folding patterns to create multi-rooted teeth.