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
Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the Human tooth, teeth, gums, and Human mouth, mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, dis ...
,
orthodontics
Orthodontics (also referred to as orthodontia) is a dentistry specialty that addresses the diagnosis, prevention, management, and correction of mal-positioned teeth and jaws, as well as misaligned bite patterns. It may also address the modificati ...
, and
oral and maxillofacial pathology, the body part in question is usually the
mouth
A mouth also referred to as the oral is the body orifice through which many animals ingest food and animal communication#Auditory, vocalize. The body cavity immediately behind the mouth opening, known as the oral cavity (or in Latin), is also t ...
,
tongue
The tongue is a Muscle, muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for chewing and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper s ...
, or
jaw. Oral para-functional habits may include
bruxism
Bruxism is excessive teeth grinding or jaw clenching. It is an oral Parafunctional habit, parafunctional activity; i.e., it is unrelated to normal function such as eating or talking. Bruxism is a common behavior; the global prevalence of brux ...
(tooth-clenching, grinding, or both), 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 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
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
placed on teeth during functional habits is , but the pressure can range from 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
In anatomy, the temporomandibular joints (TMJ) are the two joints connecting the jawbone to the skull. It is a bilateral Synovial joint, synovial articulation between the temporal bone of the skull above and the condylar process of mandible be ...
(TMJ) acts as a class III
lever
A lever is a simple machine consisting of a beam (structure), beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, l ...
, 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 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. It is one hypothesis for why military jet pilots crack more teeth than auxiliary crew.
[{{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]
References
Pathology of temporomandibular joints, muscles of mastication and associated structures