A pre-formed tooth insert is provided for insertion in a prepared cavity in a tooth, particularly suitable for cavities accurately prepared to match pre-formed inserts. In this context it is provided that, in addition, using a model, a model value is determined respectively for the voltage in the longitudinal direction andor the transverse direction, the difference between the setpoint voltage value and the model value is ascertained, and in response to the exceeding of a specifiable maximum difference, an error signal is triggered. The light transmissive region includes a first edge extending along a first direction and a second edge extending along a second direction.