A flexible prosthetic cover shaped to form a replacement nucleus pulposis for an intervetebral disc with an aperture for the introduction of filling material therein, and an elongate introducer member configured to pass into the aperture is provided. The cover has a strengthened portion substantially opposite the aperture for engaging the distal end of the member, the strengthened portion and the distal end being arranged to interlock, for facilitating orientation of the cover, for example so as to prevent relative rotation there between.
