Jaw disproportion is a term that specifically summarises a number of developmental conditions where disproportionate growth of the jaw bones results in malocclusion, the misalignment of teeth (malocclusion can also occur purely from dental misalignment, without disproportionate growth of the jaw bones).
Jaw disproportion conditions include
- mandibular hypoplasia (the lower jaw is too small) results in overbite;
- maxillary hypoplasia (the upper jaw is too small) results in underbite;
- mandibular hyperplasia (the lower jaw is too big) results in underbite;
- anterior open bite (the front teeth of the upper and lower jaws have no contact);
- vertical discrepancies (the upper jaw bone has too much downward growth);
- bimaxillary retrusion (both upper and lower jaw are disproportionately small relative to the rest of the facial skeleton).
The severity of these conditions varies from very minor (with no need for treatment), to medium (where orthodontic treatment alone, to better align the teeth but leave the jaw bones untreated, may be the best option), to severe (where surgical correction of the jaw bones is required; this branch of surgery is called orthognathic surgery). Orthodontics, non-surgical methods to align teeth by braces and similar, is not further discussed on this website as it is a completely separate although closely related dental specialty. However, orthognathic surgery almost always includes orthodontic treatment before and/or after surgery.
Depending on the severity of jaw disproportion conditions, problems with biting and chewing, speech and breathing (for example, sleep apnoea) may be encountered in addition to issues with facial imbalance and appearance.
Assessment of these conditions requires not only an in-depth medical history and examination. In addition, photography and special X-ray images, careful measurements of the facial proportions and study models (made from dental impressions using alginate or silicone moulds) are all necessary for treatment planning. It is important that an informed decision is made about treatment because a successful outcome of orthognathic surgery / orthodontics treatments requires long-term commitment and engagement by the patient.
Disproportionate jaw conditions that lead to asymmetry of the facial skeleton are not discussed in this section but are included in our section about craniofacial anomalies. Strictly speaking, all jaw disproportions are a sub-category of craniofacial (developmental) anomalies.
It is not uncommon that people with chronic jaw joint (TMJ, temperomandibular joint) pain are referred to a maxillofacial clinic for consideration of orthognathic surgical interventions. There is little evidence that surgical correction of malocclusion alleviates jaw joint pain and dysfunction.
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