Use of extracted teeth in research

A researcher wanting to use enamel from extracted teeth in his work on dental enamel recently rang to ask me whether teeth extracted at a dental hospital during a therapeutic procedure may be later used in research without consent from the people from whom they came. The research ethics committee in his institution wanted advice on whether it is legally or ethically necessary to obtain consent, either at the time when the teeth are extracted or by re-contacting people afterwards and asking for their consent at that time.