Overjet is a specific measurement used in orthodontics to describe the horizontal relationship between the upper and lower front teeth. It quantifies how far the upper incisors protrude forward past the lower incisors when the back teeth are closed together. This measurement is a common component of a comprehensive dental evaluation, helping practitioners assess the alignment of the bite. Understanding the nature and degree of this horizontal overlap is important for diagnosing a type of malocclusion, or “bad bite,” which can affect both oral function and facial aesthetics.
Defining Overjet and Overbite
Overjet refers to the horizontal distance, measured in millimeters, from the back surface of the upper front teeth to the front surface of the lower front teeth. This measurement describes the extent of the forward protrusion of the upper teeth. A normal overjet is typically between 2 and 4 millimeters. When the measurement exceeds this range, it is classified as an excessive overjet or protrusion.
It is important to distinguish overjet from overbite, as the terms are often confused. Overbite refers to the vertical overlap, or how much the upper front teeth cover the lower front teeth when biting down. While overjet describes the front-to-back relationship, overbite describes the top-to-bottom relationship of the teeth. Both measurements are used to determine the overall alignment of the bite.
Factors Contributing to Excessive Overjet
An excessive overjet, often called “buck teeth,” arises from causes categorized as skeletal or dental/habitual. Skeletal factors involve disproportionate jaw growth, such as an underdeveloped or backward-positioned lower jaw (mandible), or an overgrown or forward-positioned upper jaw (maxilla). These conditions create a mismatch in the foundation upon which the teeth sit.
Dental and habitual factors involve forces that push the teeth out of alignment. Common causes include prolonged oral habits during childhood, such as persistent thumb sucking or extended use of a pacifier past the age of four. These actions apply continuous, outward pressure on the developing upper front teeth. Another factor is tongue thrusting, where the tongue pushes against the back of the upper front teeth during swallowing.
Functional and Aesthetic Impact of Overjet
The consequences of an increased overjet affect several aspects of oral health and function. A primary functional risk is increased susceptibility to dental trauma, as the protruding upper teeth are unprotected and exposed to accidental injury. Studies suggest that an overjet greater than 4 millimeters can double the risk of traumatic damage to the incisors.
Excessive overjet can also lead to difficulties in achieving lip competence, the ability to comfortably seal the lips together. The inability to form a lip seal can result in chronic mouth breathing and dryness, contributing to gum inflammation and an increased risk of tooth decay. Furthermore, a significant horizontal overlap can interfere with proper chewing (mastication) and may cause speech impediments. Aesthetically, the protrusion can significantly impact the facial profile and a person’s self-perception.
Treatment Options for Overjet Correction
The approach to correcting an overjet depends on the patient’s age, the severity of the protrusion, and whether the cause is primarily skeletal or dental.
Treatment for Growing Patients
In children and adolescents who are still growing, early intervention (Phase I treatment) is highly effective. This involves growth modification appliances, such as functional appliances like the Herbst or Twin Block, or headgear. These devices gently encourage the lower jaw to grow forward, correcting the underlying skeletal discrepancy.
Treatment for Dental Misalignment
For moderate overjet cases or those with primarily dental misalignment, comprehensive orthodontic treatment is typically used. This often involves fixed braces or clear aligners, sometimes combined with elastic bands to retract the upper teeth. In some instances, the orthodontist may recommend the extraction of select teeth, such as the upper first premolars, to create space to pull the front teeth back into alignment.
Treatment for Adults
In adult patients with a severe overjet rooted in a major skeletal discrepancy, conventional orthodontics alone is usually not sufficient since jaw growth has stopped. The most predictable solution is often a combination of orthodontics and orthognathic surgery (jaw surgery). The surgery physically repositions the upper or lower jaw, or both, to achieve a balanced and functional bite relationship.