{{Short description|Use of a body part in an uncommon way}} [[File:Deviated_midline_2.JPG | thumb | right | 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]], [[orthodontics]], and [[oral and maxillofacial pathology]], the body part in question is usually the [[mouth]], [[tongue]], or [[jaw]]. Oral para-functional habits may include [[bruxism]] (tooth-clenching, grinding, or both), tongue tension ("[[tongue thrust]]ing"), [[fingernail biting]], pencil or pen chewing, [[mouth breathing]], and any other habitual use of the mouth unrelated to eating, drinking, or speaking.
[[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]]. 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]] placed on teeth during functional habits is {{convert|140|-|550|kPa}}, but the pressure can range from {{convert|2|-|20.7|MPa|abbr=}} during parafunctional habits.{{Citation needed|date=March 2026}} 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 [[periodontium|attachment]] of teeth to the bone. On the other hand, parafunctional habits direct their forces horizontally. Normally, the [[temporomandibular joint]] (TMJ) acts as a class III [[lever]], which helps to restrict the amount of force generated. Class I or class II levers may be created during bruxism{{Citation needed|date=March 2026}}, 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|date=December 2012}} It is one hypothesis for why military jet pilots crack more teeth than auxiliary crew.<ref>{{cite journal |author1=Lurie, O |author2=Zadik, Y |author3=Tarrasch, R |author4=Raviv, G |author5=Goldstein, L |title=Bruxism in Military Pilots and Non-Pilots: Tooth Wear and Psychological Stress |journal=Aviat Space Environ Med |volume=78 |issue=2 |pages=137–9 |date=February 2007 |pmid=17310886 |url=http://www.ingentaconnect.com/content/asma/asem/2007/00000078/00000002/art00010 |access-date=2008-07-16}}</ref>
==References== <references/>
[[Category:Pathology of temporomandibular joints, muscles of mastication and associated structures]]