The third molar, commonly called wisdom tooth, is the most posterior of the three molars in each quadrant of the human dentition. The age at which wisdom teeth come through (erupt) is variable, but this generally occurs between late teens and early twenties. Most adults have four wisdom teeth, one in each of the four quadrants, but it is possible to have none, fewer, or more, in which case the extras are called supernumerary teeth. Wisdom teeth may become stuck (impacted) and not erupt fully, if there is not enough space for them to come through normally. Impacted wisdom teeth are still sometimes removed for orthodontic treatment, believing that they move the other teeth and cause crowding, though this is disputed. Impacted wisdom teeth may suffer from tooth decay if oral hygiene becomes more difficult. Wisdom teeth that are partially erupted through the gum may also cause inflammation and infection in the surrounding gum tissues, termed pericoronitis. More conservative treatments, such as operculectomies, may be appropriate for some cases. However, impacted wisdom teeth are commonly extracted to treat or prevent these problems. Some sources oppose the prophylactic removal of disease-free impacted wisdom teeth, including the National Institute for Health and Care Excellence in the UK.
History Although formally known as third molars, the common name is wisdom teeth because they appear so late – much later than the other teeth, at an age where people are presumably "wiser" than as a child, when the other teeth erupt. The term probably came as a translation of the Latin dens sapientiae. Their eruption has been known for millennia to cause dental issues; it was noted at least as far back as Aristotle:
The last teeth to come in man are molars called 'wisdom-teeth', which come at the age of twenty years, in the case of both sexes. Cases have been known in women upwards of eighty years old where at the very close of life the wisdom-teeth have come up, causing great pain in their coming; and cases have been known of the like phenomenon in men too. This happens, when it does happen, in the case of people where the wisdom-teeth have not come up in early years. The oldest known impacted wisdom tooth belonged to a European woman who lived between 13,000 and 11,000 BCE, in the Magdalenian period. Nonetheless, molar impaction was relatively rare prior to the modern era. With the Industrial Revolution, the affliction became ten times more common, owing to the new prevalence of soft, processed foods.
Structure
Tooth morphology The morphology of wisdom teeth can be variable. Maxillary (upper) third molars commonly have a triangular crown with a deep central fossa from which multiple irregular fissures originate. Their roots are commonly fused together and can be irregular in shape. Mandibular (lower) third molars are the smallest molar teeth in the permanent dentition. The crown usually takes on a rounded rectangular shape that features four or five cusps with an irregular fissure pattern. Roots are greatly reduced in size and can be fused together.
Dental notation There are several notation systems used in dentistry to identify teeth. Under the Palmer/Zsigmondy system, the right and left maxillary wisdom teeth are represented by 8⏌ and ⎿8, while 8⏋ and ⎾8 represent the right and left mandibular wisdom teeth. Under the FDI notational system, the right and left maxillary third molars are numbered 18 and 28, respectively, and the right and left mandibular third molars are numbered 48 and 38. According to the Universal Numbering System the right and left upper wisdom teeth are numbered 1 and 16 and the right and left lower wisdom teeth are 32 and 17.
Variation Agenesis of wisdom teeth differs by population, ranging from practically zero in Aboriginal Tasmanians to nearly 100% in indigenous Mexicans. The difference is related to the PAX9 and MSX1 genes (and perhaps other genes).
Age of eruption There is significant variation between the reported age of eruption of wisdom teeth between different populations. For example, wisdom teeth tend to erupt earlier in people with African heritage compared to people of Asian and European heritage. Generally, wisdom teeth erupt most commonly between ages 17 and 21. Eruption may start as early as age 13 in some groups and typically occurs before the age of 25. If they have not erupted by age 25, oral surgeons generally consider that the tooth will not erupt spontaneously. Root development can continue for up to three years after eruption occurs.
Function
Anthropologists believe human and other primate wisdom teeth may help with chewing tougher foods. After the advent of agriculture over 10,000 years ago, and especially with the industrial revolution in recent centuries, soft human diets became more common through the use of tools (cutting the food) and cooking to make food easier to chew. Compared to hunter-gatherer populations, post-industrial agriculturalist populations are thought to encounter less masticatory stress and consequently have shorter and wider mandibles, predisposing them to dental crowding and malocclusion.
Clinical significance
Wisdom teeth (often notated clinically as M3 for the third molar) have long been identified as a source of problems and continue to be the most commonly impacted teeth in the human mouth. Impaction of the wisdom teeth results in a risk of periodontal disease and dental cavities. Impacted wisdom teeth lead to pathology in 12% of cases.
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