An implantable electrode arrangement for a cochlear implant system comprising: an electrode lead configured for carrying one or more cochlear stimulation signals and configured to fit through a mastoid tunnel (401, 402) extending along a longitudinal tunnel axis from a lateral outer surface of patient mastoid bone through the patient mastoid bone into the middle ear; an electrode array connected to a distal end of the electrode lead and configured for insertion into a patient cochlea, wherein the electrode array has an outer surface with a plurality of stimulation contacts configured for applying the cochlear stimulation signals to target neural tissue within the patient cochlea; characterized by a lead structure control element within the electrode lead configured to control a lead shape of the electrode lead between two stable states: i. an insertion state wherein the electrode lead within the mastoid tunnel (401, 402) has a longitudinal lead axis lying entirely along the tunnel axis; and ii. a post-insertion state wherein: a facial recess portion of the electrode lead fitting within a limited diameter portion of the mastoid tunnel associated with a facial recess portion of the mastoid bone has a longitudinal lead axis along the tunnel axis, and a storage portion of the electrode lead fitting within an enlarged diameter portion of the mastoid tunnel lateral to the facial recess portion has a longitudinal lead axis coiled in a helical shape radially around the tunnel axis. an electrode lead configured for carrying one or more cochlear stimulation signals and configured to fit through a mastoid tunnel (401, 402) extending along a longitudinal tunnel axis from a lateral outer surface of patient mastoid bone through the patient mastoid bone into the middle ear; an electrode array connected to a distal end of the electrode lead and configured for insertion into a patient cochlea, wherein the electrode array has an outer surface with a plurality of stimulation contacts configured for applying the cochlear stimulation signals to target neural tissue within the patient cochlea; characterized by a lead structure control element within the electrode lead configured to control a lead shape of the electrode lead between two stable states: i. an insertion state wherein the electrode lead within the mastoid tunnel (401, 402) has a longitudinal lead axis lying entirely along the tunnel axis; and ii. a post-insertion state wherein: a facial recess portion of the electrode lead fitting within a limited diameter portion of the mastoid tunnel associated with a facial recess portion of the mastoid bone has a longitudinal lead axis along the tunnel axis, and a storage portion of the electrode lead fitting within an enlarged diameter portion of the mastoid tunnel lateral to the facial recess portion has a longitudinal lead axis coiled in a helical shape radially around the tunnel axis. i. an insertion state wherein the electrode lead within the mastoid tunnel (401, 402) has a longitudinal lead axis lying entirely along the tunnel axis; and ii. a post-insertion state wherein: a facial recess portion of the electrode lead fitting within a limited diameter portion of the mastoid tunnel associated with a facial recess portion of the mastoid bone has a longitudinal lead axis along the tunnel axis, and a storage portion of the electrode lead fitting within an enlarged diameter portion of the mastoid tunnel lateral to the facial recess portion has a longitudinal lead axis coiled in a helical shape radially around the tunnel axis. a facial recess portion of the electrode lead fitting within a limited diameter portion of the mastoid tunnel associated with a facial recess portion of the mastoid bone has a longitudinal lead axis along the tunnel axis, and a storage portion of the electrode lead fitting within an enlarged diameter portion of the mastoid tunnel lateral to the facial recess portion has a longitudinal lead axis coiled in a helical shape radially around the tunnel axis.