Why Do Some People have a Fourth Molar?

In the past an article titled What to Do about the Fourth Molar: Similar Management Strategies as the Third Molar aka Wisdom Tooth? discussed the fourth molar which is an additional tooth beyond the wisdom tooth or third molar that some people have. In this article some images of the fourth molar were provided along with some discussion of management strategies. However, no real discussion was made surrounding some of the theories regarding the fourth molar. In an article titled “Presence of the fourth molar. Literature review” by Vázquez Mosquerira et al. appearing in Revista Odontológica Mexicana, (vol. 22, no. 2 April-June 2018, pp. 103-117) some discussion is made surrounding some of the theories of the existence of the fourth molar along with many X-rays of fourth molars.

In the article the authors call fourth molars, distomolars. The authors seem to lump these teeth together with supernumerary teeth which they call a rare development as a result of processes occurring during odontogenesis which is when teeth form in the bone. These supernumerary teeth can appear in all areas of dental arches. The authors say that historically archeological populations had very few published examples of supernumerary teeth and thus they believe they are a relatively new development and ancient man did not have any fourth molars. The authors state that the fourth molar can erupt into the arch in a position very similar to that of any other molar, or they can remain within the bone. In some cases the fourth molar can fuse with the third molar or wisdom teeth or even the third molar roots. The authors stated that when fourth molars appear on both sides in both jaws they can be associated with syndromes such as: cleidocranial dysplasia, Down’s syndrome, Ellis Van Creveld syndrome, Leopard’ s syndrome, Crouson’ s syndrome Gardner’ s syndrome, Sturge-Weber syndrome, facial orodigo disostosis, Hallerman Streiff syndrome, and cleft lip. Certain genes such as RUNX2 have been associated with supernumerary teeth.

The authors state the etiology of the fourth molar is not yet known. However there are different theories that can explain the fourth molar. The first theory is a hyperactivity of embryonic epithelial cells. The authors state that reasons for epithelial over-activity include local factors such as trauma or infection and genetic factors such as dysfunction of genes or certain hereditary traits. The second theory is hyperactivity of dental lamina or its remnants. The authors say that odontogenesis begins at the 6th week of intrauterine life and this is followed by dental bud formation. If alterations are present when dental buds are forming this can result in supernumerary teeth. If proliferation excess or prolonged survival of dental lamina epithelial cells are stimulated this can lead to the formation of a fourth molar. The third theory is an over-activity of the epithelial cord. During the eighth week of intrauterine life, the dental bud begins to separate from the dental lamina through an epithelial proliferation called the epithelial cord. If over-activity of the epithelial cord occurs a fourth molar can form. The fourth theory is an overactivity of the external layer of Hertwig’s sheath and Malassez’s epithelial remnants. Hertwig’s sheath is an epithelial prolongation of the enamel organ and when it completes the first dentin layer, the sheath fractures and degenerates forming Malassez epithelial remnants, which remain present in the periodontal ligament until adulthood. If an alteration of the processes involved in forming Malassez epithelial remnants occur a fourth molar can result. It is noted that the second, third, and fourth theories described above all deal with the various stages of tooth formation. The fifth theory is phylogenetic theory that puts forth the hypothesis of regression to ancestral dentition of primate dentition, with counting with a great number of teeth. The authors find this theory though to not really be supported in scientific works. The sixth theory is the dichotomy theory which proposes cleavage of the dental follicle which occurs from dental bud division. It puts forth that trauma or mutations can cause division of the dental follicle into two or more fragments and two teeth can form from one tooth bud. The authors provide some evidence for this theory from animal experiments and a few case reports. The seventh theory is the unified etiology which is from a combination of environmental and genetic factors. The authors state that supernumerary teeth tend to occur in families.

fourth molar xray teeth - Why Do Some People have a Fourth Molar?
This image is adapted from Fig. 18 of Vázquez Mosquerira et al.

In the article the authors also provide some of the incidence of supernumeary teeth which ranges form 0.1% to 3.8% across eight studies they looked at. The authors also provide some of the incidence of fourth molars across twelve studies and mention one particular study that found an incidence of 1.15% in the upper jaw and 0.021% in the lower jaw. The authors also mention some studies that explore the incident in various ethnicities of the fourth molar but these studies have some varying results. However it appears the fourth molar is more common to be found in males and in the upper jaw. The authors state that fourth molars are rarely found to be erupted and as such are found most often on radiographic exams. As far as treatment options the fourth molar can either be extracted or routinely watched by x-rays. The authors state fourth molars likely should only be extracted in cases where their presence might be responsible for eruption failure or poor alignment of permanent teeth, or if any complications occur such as cysts, tumors, periodontal disease, and repeated pain. The authors say that fourth molars are approached like wisdom teeth and special attention to complications including alveolar bone fracture or displacement in the maxillary sinus should be made if it is being extracted. They also provide a case where a patient had a fourth molar treated with a root canal. Ultimately in decisions being made regarding fourth molars without any active disease or problems, the patient should make a decision after carefully considering the risks and the benefits associated with extracting or retaining.

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