It is a fixed custom-made appliance in metal, cemented in the child oral cavity. It keeps the space arising due to premature loss of milk teeth (for any reason) open and preserved. This allow the permanent tooth erupt in correct place and position, which otherwise might contribute to crowding the permanent teeth.
Child needs a space maintainer if the milk tooth removed earlier due to dental decay or any reasons. As the child’s jaw is undergoing constant growth, the premature loss of a primary tooth will be utilized the erupted or erupting teeth. This would cause loss of space and crowding in the permanent set of teeth.