Parafunctional habit :

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 parafunctional uses of the mouth, tongue and jaw. Oral para-functional habits may include bruxism (tooth-clenching or grinding), tongue tension, mouth-breathing, and any other habitual use of the mouth unrelated to eating, drinking, or speaking.
Contrary to common belief, functional habits such as chewing are not the main cause of the wearing of teeth. 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 force placed on teeth during functional habits is 20 - 80 psi (0.14 - 0.55 MPa), but the force can range from 300 - 3000 psi (2.07 - 20.7 MPa) during parafunctional habits. The direction of forces during functional habits are 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.
The extreme force over teeth can occur during some situations, as a reflex of protection. For example, when a person feels a risk of an eminent car crash, normally the teeth arches are firmly occluded. It is a protective reflex, although it is considered parafunctional: over clenching. It allows the system to become solid. The maxilo-mandibular complex is bonded by muscles and interposed teeth and much less vulnerable to harm and dislocation. When this kind of reflex acts it is very important to have a good memory of "best bite" position. If not, during process, some tooth may be improperly overloaded causing fractures. It is one hypothesis why military jet pilots crack more teeth than auxiliary crew.[1]



References
1. ^ Orit Lurie, Yehuda Zadik, Shmuel Einy, Ricardo Tarrasch, Gil Raviv, and Liav Goldstein: Bruxism in Military Pilots and Non-Pilots: Tooth Wear and Psychological Stress Aviation, Space, and Environmental Medicine • Vol. 78, No. 2 • February 2007