Ideally, top incisor teeth should overlap bottom incisors vertically and horizontally. In some individuals, one or more of the top teeth are “behind” the bottom teeth upon teeth closure. This is referred to as anterior crossbite or underjet (if all incisors are affected).

Anterior crossbite, even on a single tooth can be very destructive and should be corrected as soon as they are detected.  If left untreated, they may result in damage to the structures supporting the tooth causing looseness of the tooth, bone loss, tooth fractures and/or gum recession.  Anterior as well as posterior crossbites usually result of a “functional shift” of the bottom jaw (the bottom jaw slides either sideways or to the front once the front teeth meet). This functional shift may result in a change in growth direction in growing children causing facial asymmetries. Jaw asymmetries are very hard to treat especially if severe in a non growing individual, where jaw surgery may be necessary to correct the problem.

If all top incisor teeth are behind bottom incisors this is referred to as negative overjet or underjet. This is usually caused by a skeletal disharmony between an over grown top jaw and/or over developed bottom jaw. This is referred to as class III malocclusion. This problem manifests itself in people with a “strong jaw”. It usually runs in families but is luckily rare. Class III malocclusions should be detected very soon since early treatment in these cases is more effective and strongly recommended. Treatment may be commenced as soon as 5-6 years of age and aims to encourage the top jaw to grow forward to match the over developing bottom jaw. This involves the use of palatal expanders, facemasks, chin cups, and/or headgears. The problem is still correctible at a later age (11-14 years) but treatment can be more challenging and may require tooth extractions or even surgery in some cases.  Non-surgical treatment aims at correcting the dental occlusion while accepting the facial appearance of a strong bottom jaw. If the problem is severe or if the correction of facial appearance is a priority to the individual then surgery is the best option. Jaw surgery should be delayed to an older age (late teens to early twenties) to avoid relapse by continuation of growth after surgery.

Anterior crossbites and underjets can be easily diagnosed by dentists or orthodontists at an early age. Those problems, especially with a skeletal component, should be preferably managed by specialized orthodontists who are dentists who received additional 2-3 year of training to treat various malocclusions with all degrees of difficulty.