A self-locking bone screw for attaching an implant to bone (V1, V2, V3), the bone screw comprising: a screw body having a head and an elongated shank extending from the head, the head comprising a plurality of radially expandable petals that surround a hollow passage into the head, the passage comprising a recess extending between the petals and a socket that extends from the recess into the shank, the recess comprising a mouth section that forms an entrance into the passage from outside the head, and a chamber having a seat section, the mouth section having a first diameter, the chamber having a second diameter larger than the first diameter of the mouth section so as to form a lip between the mouth section and chamber, the lip projecting radially inwardly into the passage; and a spring held captive in the recess, the spring comprising a ring shaped body portion and at least one deformable extension extending from the body portion, the body portion comprising a smaller diameter section, a larger diameter section, and a shoulder between the smaller and larger diameter sections (206, 208), wherein the spring is moveable in the recess between a first operative position, in which the shoulder on the spring abuts the lip in the passage to expand the petals radially outwardly and lock them in the radially expanded position, and a second operative position, in which the shoulder is displaced out of abutment with the lip to unlock the petals and allow them to retract radially inwardly, characterized in that the at least one deformable extension engages the seat section in the chamber and creates a spring bias that holds the shoulder in abutment with the lip. a screw body having a head and an elongated shank extending from the head, the head comprising a plurality of radially expandable petals that surround a hollow passage into the head, the passage comprising a recess extending between the petals and a socket that extends from the recess into the shank, the recess comprising a mouth section that forms an entrance into the passage from outside the head, and a chamber having a seat section, the mouth section having a first diameter, the chamber having a second diameter larger than the first diameter of the mouth section so as to form a lip between the mouth section and chamber, the lip projecting radially inwardly into the passage; and a spring held captive in the recess, the spring comprising a ring shaped body portion and at least one deformable extension extending from the body portion, the body portion comprising a smaller diameter section, a larger diameter section, and a shoulder between the smaller and larger diameter sections (206, 208), wherein the spring is moveable in the recess between a first operative position, in which the shoulder on the spring abuts the lip in the passage to expand the petals radially outwardly and lock them in the radially expanded position, and a second operative position, in which the shoulder is displaced out of abutment with the lip to unlock the petals and allow them to retract radially inwardly, characterized in that the at least one deformable extension engages the seat section in the chamber and creates a spring bias that holds the shoulder in abutment with the lip.