Chapter 10: Overbite Correction and Functional Appliances

Understanding Overbite and Overjet

An overbite refers to vertical overlap between the upper and lower front teeth; overjet refers to horizontal protrusion of the upper front teeth beyond the lower ones. A pronounced overjet — sometimes visible as noticeably protruding upper front teeth — often reflects a lower jaw that is growing more slowly than the upper jaw, and is one of the most common reasons for functional appliance therapy during the interceptive stage.

How Functional Appliances Work

Functional appliances work by holding the lower jaw in a slightly forward position for extended periods each day, which appears to stimulate more favorable growth at the lower jaw's growth centers during a child's natural growth window. They are most effective when timed close to a child's individual growth spurt, which is one reason an orthodontist may track growth indicators over several visits before starting active functional treatment.

Twin Block Appliance

The Twin Block is a two-piece removable appliance — an upper and lower component with interlocking bite blocks — that guides the lower jaw forward each time the child closes their mouth. It is one of the most widely used functional appliances because it is generally well tolerated and allows relatively normal eating once the child adjusts.

Fränkel Appliance

The Fränkel appliance takes a somewhat different approach, using shields and pads that hold the cheeks and lips away from the teeth and jaws, creating a favorable space for growth while also encouraging forward positioning of the lower jaw. It's bulkier than a Twin Block but achieves broadly similar underlying goals.

Bionator

The Bionator is a simpler, single-piece removable appliance that also encourages forward lower jaw positioning, generally used for milder to moderate overjet cases where a bulkier two-piece design isn't necessary.

Activator

The Activator is one of the earliest functional appliance designs still in occasional use today, working on similar principles to the Bionator, typically for select overbite cases in a similarly growing child.

What Daily Life Looks Like With a Functional Appliance

Functional appliance therapy asks a lot of a child's cooperation. Involving the child in understanding why they're wearing it — not just that they must — tends to improve consistency significantly.