An expansible cannula

Claim 1

CLAIMS: An expansible cannula comprising a stem and a holder, the stem taking the form of a conical coiled foil retained at its larger diameter proximal end in the holder in a circluar shape; biased by its own resilience into the conical coiled "free" condition and adapted to uncoil from the conical shape into a substantially cylindrical shape when a cylindrical expander is forced into and along it from the proximal end, characterised in that the foil stem comprises a tapering caudex anchored to the holder at its proximal end in a manner which will permit rocking on its anchorage in outward radial motion only, as required to permit the caudex to move from its closed location where it forms part of the conical shape of the stem to the fully open location where it forms part of the cylindrical shape of the stem, and a leaf portion attached to one longitudinal edge of the caudex along substantially the entire length of its parallel edges, and arranged so that when in the "free" condition its distal edge forms a helix subtending at least 3600 around the longitudinal axis of the stem the helix angle being such that the proximal extremity of the distal edge is spaced a substantial part of the total length of the stem from its distal extremity; the proximal end of the leaf being constrained by the holder in a manner. which gives it freedom for both circumferantial motion (uncoiling) and longitudinal motion, so that the helix angle of the distal edge of the leaf portion reduces to zero or near zero when the stem is expanded into the open condition, to provide the k_ - is necessary leaf material at the distal extremity of the stem to form the substantially cylindrical form in the open condition, but the holder preventing movement of the proximal end of the leaf portion outsi.de the cylindrical form dictated by the holder.