Occlusion (dentistry)

In dentistry, occlusion is the way the teeth of the upper jaw and lower jaw meet when the two parts of the jaw come together. Occlusion occurs when the jaw is closed or during chewing. In normal occlusion, the teeth of the upper jaw—the maxilla—are aligned with the teeth from the lower jaw—the mandible. This allows for proper jaw function, aids in preventing disease, and leads to healthy facial esthetics and speech. A misalignment of the teeth and bite in the upper and lower jaws is known as malocclusion. Dental experts have divided this misalignment into several types. The condition can be caused by hereditary factors, facial trauma, oral or gum disease, or bad habits such as thumb sucking.

Background

The earliest signs of teeth actually form in humans just a few weeks after conception. By about three to four months, the hard surface around the teeth begins to form. While some infants can be born with teeth, typically, these first teeth are below the gumline and do not begin to protrude until a few months after birth.

The first teeth to develop are usually the incisors, the narrow teeth in the front of the mouth. In most cases, incisors on the lower jaw emerge first, followed by the teeth on the upper jaw. The two frontmost molars—known as first molars—typically emerge next. Molars are large, powerful teeth located in the back of the mouth. They are followed by the canines, the pointed teeth between the molars and incisors. Finally, by about two years old, the backmost, or second molars, come in. By this age, most children have a total of twenty baby teeth. These teeth are typically spaced farther apart to allow for growth and eventual replacement by the permanent teeth.

Children lose their first baby teeth by about age six and lose their last by about age twelve. The twenty baby teeth are replaced by thirty-two permanent teeth. These consist of eight incisors—four on top and four on bottom—four canine teeth, and twelve molars. Permanent teeth also include eight premolars, or bicuspids, which are located between the canine teeth and the molars. The twelve molars include a third set of molars, which are located in the very back of the mouth. These third molars, which typically emerge between the ages of seventeen and twenty-five, are often referred to as wisdom teeth.

Overview

As the permanent teeth emerge, the teeth on the top and bottom jaws should match up with one another, and the jaws themselves should align. In the late nineteenth century, dentists classified the interaction between the two sets of teeth when the jaw is closed as occlusion. In the early twentieth century, pioneering American dentist Edward Angle categorized occlusion based on the position of the top and bottom jaws’ first molars in relation to each other.

According to Angle, normal occlusion occurs when the raised points on the upper first molar align with the grooves on the lower. He further divided misalignment, or malocclusion, of the teeth into three classifications. A class I malocclusion has normal alignment between the first molars, but the other teeth in the mouth may be overcrowded, improperly rotated, or irregularly positioned.

In a class II malocclusion, the raised points on the upper first molar are misaligned to the front of the bottom molar. Class II malocclusions can be further broken down into two types. A class II division 1 malocclusion occurs when the upper incisors are tilted outward over the bottom incisors. In a class II division 2 malocclusion, the upper incisors are tiled outward and toward the lips. A class III malocclusion occurs when the points on the upper molar are misaligned to the back of the bottom molar.

Examples of class II malocclusions include an overjet and overbite. An overjet occurs when the top teeth extend outward horizontally over the bottom teeth. In an overbite, the top teeth excessively protrude over the bottom teeth to the point where they overlap. In the most severe overbite cases, the bottom teeth can make contact with the upper gums. In general, overjet is more difficult to treat than overbite, but both can be remedied with dental braces, cosmetic bonding or veneers, or the use of dental crowns.

Other malocclusions can include conditions such as open bite, crossbite, and anterior crossbite, also known as an underbite. An open bite occurs when the front top teeth do not overlap the bottom teeth, leaving a space between the teeth when the jaw is closed. While it can be caused by jaws that do not grow evenly, it can also develop from childhood habits such as thumb sucking.

A crossbite is usually the result of a person having a narrow upper jaw. It is characterized by the upper teeth biting inside the lower teeth. It can occur with one tooth or a group of teeth and take place in the front or side of the mouth. An underbite is an example of a class III malocclusion and occurs when the lower teeth protrude out over the teeth of the upper jaw. It can be caused by an undergrowth of the upper jaw or an overgrowth of the lower one. Treatment methods for underbite and crossbite vary be the severity of the condition, but they typically include braces, corrective headgear, tooth extraction, or dental surgery.

The most apparent problems associated with dental malocclusions are physical, as jaw misalignment can range from slight cosmetic issues to more pronounced abnormalities. However, dental misalignment can also lead to medical problems such as jaw pain, earaches, or headaches. Cracked or worn tooth enamel caused by the issue are more prone to tooth decay and bacterial infections. The conditions can also result in problems eating, breathing, and sleeping.

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