Prognathism

Prognathism is a positional relationship of the mandible or maxilla to the skeletal base where either of the jaws protrudes beyond a predetermined imaginary line in the coronal plane of the skull. In general dentistry, oral and maxillofacial surgery, and orthodontics, this is assessed clinically or radiographically (cephalometrics). The word prognathism derives from Greek πρό (pro, meaning 'forward') and γνάθος (gnáthos, 'jaw'). One or more types of prognathism can result in the common condition of malocclusion, in which an individual's top teeth and lower teeth do not align properly.

Prognathism
Illustration of different types
SpecialtyOrthodontics 
Types
  • Alveolar
  • Maxillary
  • Mandibular, or progenism
CausesMultifactorial
TreatmentOrthodontics; oral and maxillofacial surgery
Frequency
  • Children: 0.5 to 2.0%
  • Adult: 2.0 to 4.0%[1]

Presentation
Mandibular prognathism, where teeth have almost reached their final, straight position by dental braces. This makes the prognathism more obvious, and it will take an operation, moving the jaw backwards, to give the ultimate result.

Prognathism in humans can occur due to normal variation among phenotypes. In human populations where prognathism is not the norm, it may be a malformation, the result of injury, a disease state or a hereditary condition.[2]

Prognathism is considered a disorder only if it affects chewing, speech or social function as a byproduct of severely affected aesthetics of the face.

Clinical determinants include soft tissue analysis where the clinician assesses nasolabial angle, the relationship of the soft tissue portion of the chin to the nose, and the relationship between the upper and lower lips; also used is dental arch relationship assessment such as Angle's classification.

Cephalometric analysis is the most accurate way of determining all types of prognathism, as it includes assessments of skeletal base, occlusal plane angulation, facial height, soft tissue assessment and anterior dental angulation. Various calculations and assessments of the information in a cephalometric radiograph allow the clinician to objectively determine dental and skeletal relationships and determine a treatment plan.

Prognathism should not be confused with micrognathism, although combinations of both are found. It affects the middle third of the face, causing it to jut out, thereby increasing the facial area, similar to the phenotype of archaic hominids and apes. Mandibular prognathism is a protrusion of the mandible, affecting the lower third of the face. Alveolar prognathism is a protrusion of that portion of the maxilla where the teeth are located, in the dental lining of the upper jaw.

Prognathism can also be used to describe ways that the maxillary and mandibular dental arches relate to one another, including malocclusion (where the upper and lower teeth do not align). When there is maxillary or alveolar prognathism which causes an alignment of the maxillary incisors significantly anterior to the lower teeth, the condition is called an overjet. When the reverse is the case, and the lower jaw extends forward beyond the upper, the condition is referred to as retrognathia (reverse overjet).

Classification

Alveolar prognathism
Alveolar prognathism, caused by thumb sucking and tongue thrusting in a 7-year-old girl.

Not all alveolar prognathism is anomalous, and significant differences can be observed among different ethnic groups.[3]

Harmful habits such as thumb sucking or tongue thrusting can result in or exaggerate an alveolar prognathism, causing teeth to misalign. Functional appliances can be used in growing children to help modify bad habits and neuro-muscular function, with the aim of correcting this condition.

Alveolar prognathism can also easily be corrected with fixed orthodontic therapy. However, relapse is quite common, unless the cause is removed or a long-term retention is used.

Maxillary prognathism

In disease states, maxillary prognathism is associated with Cornelia de Lange syndrome;[4] however, so-called false maxillary prognathism, or more accurately, retrognathism, where there is a lack of growth of the mandible, is by far a more common condition.

Prognathism, if not extremely severe, can be treated in growing patients with orthodontic functional or orthopaedic appliances. In adult patients this condition can be corrected by means of a combined surgical/orthodontic treatment, where most of the time a mandibular advancement is performed. The same can be said for mandibular prognathism.

Mandibular prognathism (progenism)
Charles II of Spain showing Habsburg jaw, a severe type of mandibular prognathism.
121st. Emperor of Japan Kōmei, showing progenism

Pathologic mandibular prognathism is a potentially disfiguring genetic disorder where the lower jaw outgrows the upper, resulting in an extended chin and a crossbite. In both humans and animals, it can be the result of inbreeding.[5] In brachycephalic or flat-faced dogs, like shih tzus and boxers, it can lead to issues, such as underbite.[6]

In humans, it results in a condition sometimes called lantern jaw, reportedly derived from 15th century horn lanterns, which had concave sides.[7][lower-alpha 1] Traits such as these were often exaggerated by inbreeding, and can be traced within specific families.[5]

Although more common than appreciated, the best known historical example is 'Habsburg jaw', or Habsburg or Austrian lip, due to its prevalence in members of the house of Habsburg, which can be traced in their portraits.[9] The process of portrait-mapping has provided tools for geneticists and pedigree analysis; most instances are considered polygenic,[10] but a number of researchers believe that this trait is transmitted through an autosomal recessive type of inheritance.[11][12]

Leopold I, showing progenism

Allegedly introduced into the family by a member of the Piast dynasty, it is clearly visible on family tomb sculptures in St. John's Cathedral, Warsaw. A high propensity for politically motivated intermarriage among Habsburgs meant the dynasty was virtually unparalleled in the degree of its inbreeding. Charles II of Spain, who lived 1661 to 1700, is said to have had the most pronounced case of the Habsburg jaw on record.,[13] due to eight incest in the dynasty preceding his birth.[14][15]

Treatment

Prior to the development of modern dentistry, there was no treatment for this condition: those who had it simply endured the condition. Today, the most common treatment for mandibular prognathism is a combination of orthodontics and orthognathic surgery. The orthodontics can involve braces, removal of teeth, or a mouthguard. None of this, however, removes the predisposing genetic condition.

The surgery required has led, in some cases, to identity crisis in patients, whereby the new facial structure has a negative impact mentally on how the patients perceive themselves.

In insects

In entomology, prognathous means that the mouthparts face forwards, being at the front of the head, rather than facing downwards as in some insects.[16]

See also

Notes
  1. The OED uses a slightly different definition, "long thin jaws, giving a hollow appearance to the cheek".[8]

References
  1. Wolff, Wienker & Sander 1993, p. 112.
  2. Fotek, Ilona (2018-02-05). "Prognathism: MedlinePlus Medical Encyclopedia". www.nlm.nih.gov.
  3. Vioarsdóttir, O'Higgins & Stringer 2002, pp. 211–229.
  4. "de Lange syndrome definition - Medical Dictionary definitions of popular medical terms easily defined on MedTerms". medterms.com.
  5. Vilas et al. 2019, pp. 563-571.
  6. Beuchat 2015.
  7. Online Mendelian Inheritance in Man (OMIM): PROGNATHISM, MANDIBULAR - 176700
  8. "lantern jaw". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  9. Vilas et al. 2019.
  10. Wolff, Wienker & Sander 1993, pp. 112–116.
  11. https://www.elibrary.ru/item.asp?id=44270977
  12. https://pubmed.ncbi.nlm.nih.gov/31786955/
  13. Mitchell 2013, pp. 303-308.
  14. https://www.elibrary.ru/item.asp?id=44270977
  15. https://pubmed.ncbi.nlm.nih.gov/31786955/
  16. "Prognathous". A Glossary of Entomological Terms. Retrieved 11 September 2019.

Sources
Classification
External resources
  • The dictionary definition of prognathism at Wiktionary
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