A tooth with three roots represents a significant anatomical variation from the standard human dental structure. Most teeth typically develop with one or two main roots, which anchor them to the jawbone. For instance, front teeth and premolars usually have a single root, while lower molars have two, and upper molars usually have three. The appearance of an extra root is considered a developmental anomaly or natural polymorphism, forming during tooth development and changing the tooth’s structure.
Defining Rarity Prevalence and Geographic Variation
The rarity of a three-rooted tooth depends heavily on the specific population being studied, illustrating profound geographic and ancestral variation. Globally, the overall prevalence of a three-rooted permanent mandibular first molar—the most common tooth to exhibit this trait—is estimated to occur in 9% to 10% of the world’s population. This anatomical variant, known as a Radix Entomolaris, shows a striking difference across various ethnic groups.
The occurrence rate is notably high in populations of Mongoloid descent, including those from East Asia (Chinese, Taiwanese, and Korean groups). In these populations, prevalence can range widely, from 5% up to over 40% in some studies, suggesting the trait is almost a standard feature in certain ancestral lines. For instance, the prevalence in the Chinese population has been reported as high as 32%.
Conversely, the phenomenon is considerably rarer in people of Caucasian, African, and some Eurasian ancestry. For these groups, the prevalence of a three-rooted mandibular molar is consistently low, often falling below 5% of the population. This stark contrast highlights the genetic underpinnings of this anatomical difference.
Specific Teeth Affected by Extra Roots
While any tooth can theoretically exhibit an extra root, the mandibular first molar is overwhelmingly the most frequent and studied location for a three-rooted anatomy. This large back tooth typically develops with two roots: one mesial (towards the front) and one distal (towards the back). The third, supernumerary root most commonly appears as a distinct structure branching off the distal root.
This extra root is given a specific name based on its position, with the most frequent variation being the Radix Entomolaris. It is characteristically located on the distolingual aspect of the tooth, extending toward the back and the tongue side. This root is often smaller and more curved than the standard two roots and possesses its own independent root canal.
A much less common variation is the Radix Paramolaris, which is situated on the distobuccal side of the tooth, extending toward the cheek. The global prevalence for Radix Paramolaris is exceedingly low, at approximately 0.1%. Although the mandibular first molar is the main focus, the mandibular second molar can also rarely present with a third root, but its occurrence is significantly lower.
Clinical Implications for Dental Treatment
The presence of a three-rooted molar significantly increases the complexity of standard dental procedures, particularly root canal therapy. Endodontic treatment requires the dentist to locate and meticulously clean out all internal canals. The extra root often harbors a small, fine, and curved canal that can be easily overlooked, and failing to treat this additional canal leaves behind infected tissue, which is the primary cause of root canal failure.
For diagnosis, conventional two-dimensional X-rays may not clearly reveal the extra root because it can be superimposed over the main roots. To accurately identify the anatomy, dentists often rely on specialized imaging techniques, such as Cone Beam Computed Tomography (CBCT). A CBCT scan provides a three-dimensional view, confirming the number of roots and canals, and mapping the severe curvature of the extra root.
During treatment, the access cavity preparation must be modified to ensure the extra canal orifice can be found and instrumented. The sharp curvature of the Radix Entomolaris canal increases the risk of procedural errors, such as perforating the tooth wall or breaking an instrument inside the canal. The divergent nature and curvature of this extra root also complicate tooth extraction, as the unusual shape makes removal more challenging and increases the risk of root fracture.