# Parafunctional activity

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Use of a body part in an uncommon way

 Worn teeth with a deviated midline due to teeth grinding (bruxism)

A **para-functional habit** or **parafunctional habit** is the habitual exercise of a body part in a way that is other than the most common use of that body part. In [dentistry](/source/Dentistry), [orthodontics](/source/Orthodontics), and [oral and maxillofacial pathology](/source/Oral_and_maxillofacial_pathology), the body part in question is usually the [mouth](/source/Mouth), [tongue](/source/Tongue), or [jaw](/source/Jaw). Oral para-functional habits may include [bruxism](/source/Bruxism) (tooth-clenching, grinding, or both), tongue tension ("[tongue thrusting](/source/Tongue_thrust)"), [fingernail biting](/source/Fingernail_biting), pencil or pen chewing, [mouth breathing](/source/Mouth_breathing), and any other habitual use of the mouth unrelated to eating, drinking, or speaking.

[Crenated tongue](/source/Crenated_tongue) is when scalloping develops on the lateral margins of the tongue as a result of habitual forcing of the tongue against the teeth.

Contrary to common belief, functional activities such as chewing are not the main cause of [tooth wear](/source/Tooth_wear). Parafunctional habits are the most destructive forces for several reasons. Whereas teeth rarely come into contact during normal chewing, grinding of teeth may occur 1-4 hours in a 24-hour period, most often during sleep. The amount of [pressure](/source/Pressure) placed on teeth during functional habits is 140–550 kilopascals (20–80 psi), but the pressure can range from 2–20.7 megapascals (290–3,000 psi) during parafunctional habits.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*] The direction of forces during functional habits is placed vertically along the long axis of teeth, which is the least harmful because of the anatomical structure of the [attachment](/source/Periodontium) of teeth to the bone. On the other hand, parafunctional habits direct their forces horizontally. Normally, the [temporomandibular joint](/source/Temporomandibular_joint) (TMJ) acts as a class III [lever](/source/Lever), which helps to restrict the amount of force generated. Class I or class II levers may be created during bruxism[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*], which generates more force from the same amount of muscle activity and subsequently delivers more force to the teeth.

Extreme force upon the teeth can occur during some situations as a protective reflex. When a person senses the risk of an imminent car crash, for example, the teeth arches are normally firmly occluded. This overclenching is still considered parafunctional, although it serves a functional purpose; the maxillomandibular complex is much less vulnerable to harm and dislocation because it is bonded by muscles and interposed teeth. When this kind of reflex acts, having a good memory of one's "best bite" position helps avoid fractures.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*] It is one hypothesis for why military jet pilots crack more teeth than auxiliary crew.[1]

## References

1. **[^](#cite_ref-1)** Lurie, O; Zadik, Y; Tarrasch, R; Raviv, G; Goldstein, L (February 2007). ["Bruxism in Military Pilots and Non-Pilots: Tooth Wear and Psychological Stress"](http://www.ingentaconnect.com/content/asma/asem/2007/00000078/00000002/art00010). *Aviat Space Environ Med*. **78** (2): 137–9. [PMID](/source/PMID_(identifier)) [17310886](https://pubmed.ncbi.nlm.nih.gov/17310886). Retrieved 2008-07-16.

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Adapted from the Wikipedia article [Parafunctional activity](https://en.wikipedia.org/wiki/Parafunctional_activity) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Parafunctional_activity?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.