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 teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions o ...

orthodontics Orthodontics is a dentistry specialty that addresses the diagnosis, prevention, management, and correction of mal-positioned teeth and jaws, and misaligned bite patterns. It may also address the modification of facial growth, known as dentofacial ...

, and

oral and maxillofacial pathology Oral and maxillofacial pathology refers to the diseases of the mouth ("oral cavity" or "stoma"), jaws ("maxillae" or "gnath") and related structures such as salivary glands, temporomandibular joints, facial muscles and perioral skin (the skin aro ...

, the body part in question is usually the

mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...

tongue The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...

, or

jaw The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...

. Oral para-functional habits may include

bruxism Bruxism is excessive teeth grinding or jaw clenching. It is an oral parafunctional activity; i.e., it is unrelated to normal function such as eating or talking. Bruxism is a common behavior; reports of prevalence range from 8% to 31% in the gene ...

(tooth-clenching, grinding, or both), tongue tension ("

tongue thrust Tongue thrust (also called reverse swallow or immature swallow) is a pseudo-pathological name of what is either considered a normal adaptive lip seal mechanism, whereby normal nasal breathing or normal swallowing can occur. Or, it is seen as an o ...

ing"),

fingernail biting Nail biting, also known as onychophagy or onychophagia (or even erroneously onyhophagia), is an oral compulsive habit of biting one's fingernails. It is sometimes described as a parafunctional activity, the common use of the mouth for an activity ...

, pencil or pen chewing,

mouth breathing Mouth breathing, medically known as chronic oral ventilation, is long-term breathing through the mouth. It often is caused by an obstruction to breathing through the nose, the innate breathing organ in the human body. Chronic mouth breathing ma ...

, and any other habitual use of the mouth unrelated to eating, drinking, or speaking.

Crenated tongue Crenated tongue is a descriptive term for the appearance of the tongue when there are indentations along the lateral borders (the sides), as the result of compression of the tongue against the adjacent teeth. The oral mucosa in the area of crenat ...

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 Tooth wear refers to loss of tooth substance by means other than dental caries. Tooth wear is a very common condition that occurs in approximately 97% of the population. This is a normal physiological process occurring throughout life; but with in ...

. 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 e ...

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 articulation between the temporal bone of the skull above and the mandible below; it is from these bones that it ...

(TMJ) acts as a class III

lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or ''fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is div ...

, 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