Plain-Language Meaning

Crossbite

When upper and lower teeth don't align correctly when biting down

Crowding

Not enough space in the jaw for teeth to sit straight

Expander

A fixed appliance that gradually widens a narrow upper jaw

Functional appliance

A device (e.g., Twin Block, Bionator) that guides jaw growth rather than just moving teeth

Mixed dentition

The stage when a child has both baby and permanent teeth present

Overbite

Vertical overlap of the upper front teeth over the lower front teeth

Overjet

Horizontal protrusion of the upper front teeth beyond the lower front teeth

Phase One / Phase Two

Common names for interceptive and comprehensive treatment, respectively

Relapse

Teeth shifting back toward their original position after treatment, usually from skipping retainers

Retainer

An appliance worn after active treatment to hold teeth in their corrected position

Space maintainer

A small appliance that holds space open after an early baby-tooth loss

Tongue-thrust

A swallowing pattern where the tongue pushes against or between the front teeth, which can contribute to an open bite

Ankylosis

When a tooth fuses directly to the underlying bone and stops moving/erupting normally

Cephalometric X-ray

A standardized side-profile X-ray used to measure jaw and skull relationships

Class III

A growth pattern where the lower jaw is positioned ahead of the upper jaw (underbite tendency)

Interceptive orthodontics

Active early treatment (roughly ages 7–11) that guides jaw growth or corrects an emerging problem

Open bite

When the front teeth don't touch at all when biting down, even with the back teeth closed

Palatal expander

A fixed appliance that gradually widens a narrow upper jaw using a small screw mechanism

Preventive orthodontics

Monitoring and small interventions (ages roughly 6–10) aimed at catching problems before they start

Protraction facemask

An external appliance used to encourage forward growth of the upper jaw in Class III cases

TMJ

The temporomandibular joints connecting the lower jaw to the skull, just in front of each ear