A spinal disc nucleus replacement including an elastomeric sheath (12) assembled around a rod (14), a portion of the sheath being arranged for sliding along the rod, and a sheath compactor (18) adapted to slide a portion of the sheath along the rod from a first position to a second position, wherein in the first position the sheath is in a non-expanded orientation and in the second position the sheath is in an expanded orientation wherein folds of the sheath expand radially outwards from the rod.