Mandibular prognathism

Mandibular prognathism is a forward position of the lower jaw relative to the upper jaw. It makes the lower teeth sit in front of the upper teeth when the mouth is closed (underbite or anterior crossbite) and gives the profile a concave look with a prominent chin. In orthodontics it is classified as a class III malocclusion.

Causes

Prognathism is a disorder of facial bone growth. It can be caused by:

• Overgrowth of the lower jaw (true prognathism).
• Underdevelopment of the upper jaw, which makes the lower jaw look forward even if it is normal in size.
• A combination of both, which is the most common situation.

It has a strong hereditary component and often runs in families. The best-known historical example is the so-called Habsburg jaw, seen in several members of the Habsburg dynasty. Less often it is linked to hormonal disorders such as acromegaly.

Symptoms

• Lower teeth in front of the upper teeth.
• Prominent chin and lower lip, with a concave profile.
• Difficulty biting and chewing certain foods.
• Trouble pronouncing some sounds.
• Uneven tooth wear and, in some cases, discomfort in the temporomandibular joint.

Beyond function, prognathism often has a significant impact on self-esteem.

Diagnosis

It is carried out by the maxillofacial surgeon and the orthodontist through clinical examination of the face and bite, together with imaging: panoramic X-ray, lateral cephalometric X-ray and, increasingly, cone-beam CT (CBCT) and intraoral scanning. These tests show whether the problem lies in the lower jaw, the upper jaw or both, and allow treatment to be planned in 3D.

Treatment

It depends on age and severity:

• In growing children, when the upper jaw is underdeveloped, orthopaedic appliances (such as a facemask) can stimulate its growth.
• In mild cases with a small skeletal discrepancy, orthodontics alone can compensate the bite by moving the teeth.
• In adults with a skeletal discrepancy, the reference treatment is orthognathic surgery combined with orthodontics. It is performed once facial growth has finished, usually in late adolescence. Depending on the case, the lower jaw is set back, the upper jaw is advanced, or both are treated (bimaxillary surgery). It corrects the bite and the profile at the same time.

Orthognathic surgery is a core procedure of oral and maxillofacial surgery.