How much space is created from expansion or premolar extraction?

The aim of this study was first to investigate the relationship between maxillary arch expansion and change in arch depth (overjet), and secondly to quantify the reduction in maxillary arch depth following extraction of 4\4 with complete space closure. A model of maxillary typodont teeth was constructed to allow expansion and premolar removal. Arch dimensions were recorded using a reflex microscope. A linear relationship was found between arch expansion and reduction of the arch depth. When the premolars were removed, there was a greater reduction in arch depth than the mesio-distal width of these teeth.