An instrument in particular suitable for being inserted into the cavity within a vertebra, which instrument features a collapsed position and an expanded position, in which collapsed position the instrument can be inserted into the cavity through an opening in the vertebral wall. The instrument includes a first upper elongated contact element and a second lower elongated contact element, and member for moving said elements apart in a direction substantially transversely to their contact surfaces to a particular end position, in which the elements are locked in position relative to each other.