A bone anchor for a tongue manipulation device which has a tissue anchor for attachment to the tongue, the bone anchor, for example for attachment to the mandible and a tether line which fixes the tissue anchor to the bone anchor. The bone anchor has a spool arrangement with a indexing part and a spool part. In a first configuration the spool part is operable to spool the tether line without operation of the indexing part, and in a second configuration the spool part is operable to spool the tether line with operation of the indexing part. This means the spooling to take up slack in the tether line does not cause wear to the indexing springs, and it also means the user can feel when the slack has been taken up.
