A device (100) for treating a stiffened blood vessel comprises a self-supporting, surgically implantable, elongate casing (101) adapted to extend about and restrain a stiffened portion (2) of the blood vessel. The casing (101) defines an elongate cavity (103) for receipt of the stiffened portion (2) of the blood vessel. The cavity (103) has a transverse cross-section with an aspect ratio of greater than one in an unloaded state. The casing (103) is configured such that, upon application of increasing pressure to the casing (101) from within said cavity (103), the casing (103) elastically deforms such that the aspect ratio decreases towards one and the area of the cross-section increases.