An interdental laterally expansive wedge includes a prismatic wedge member having a furled bottom side expansible from a first position to an expanded position when fluid is forcibly directed into the wedge member through a proximal interconnect. Lateral expansion of the wedge member is thereby effected when each of a pair of basal ridges is forced apart to impress consistent and even pressure in contact with interproximal structure, such as approximal teeth and a matrix band inserted to gird a cavosurface for approximal filling preparation and finishing. Importantly, the present interdental laterally expansive wedge is laterally expansive without effecting deformation or ballooning between and around the teeth, and prevents invasion into the cavity proper.