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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. The term is most commonly used by dentists, orthodontists, or maxillofacial specialists to refer to para-functional uses of the mouth, tongue and jaw. Oral para-functional habits may include bruxism (tooth-clenching and/or grinding), tongue tension ("tongue thrusting"), 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 a descriptive term for 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 20–80 psi (0.14–0.55 MPa), but the pressure can range from 300 to 3000 psi (2.07 to 20.7 MPa) during parafunctional habits. 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 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, 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 over-clenching is still considered parafunctional, although is serves a functional purpose; the maxilo-mandibular complex is much less vulnerable to harm and dislocation because it is bonded by muscles and interposed teeth. When this kind of reflex acts, it is very important to have a good memory of one's "best bite" position in order to avoid fractures. It is one hypothesis for why military jet pilots crack more teeth than auxiliary crew. ==References== 〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Parafunctional activity」の詳細全文を読む スポンサード リンク
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