Retreiving Item Number

HI Experts

In my requirement I am getting the change document number passing Order number (VBELN) as object id to FM CHANGEDOCUMENT_READ_HEADERS.Then based on the change number in the returning table of above FM I am reading the change document items from table CDPOS.But in this method I am unable to get the item number .Can anyone please suggest any FMs or table where to get the item no (POSNR) against the selected change items.

Thanks In Advance