Surgical assembly, and bone anchor screw and device for extending one such screw that belong to said surgical assembly

A surgical assembly, comprising:

• a bone anchorage screw having an anchorage portion for anchoring in a vertebral column, and a head comprising a socket;
• and
• an extension device adapted to fit over the socket of the bone anchorage screw for forming a longitudinal extension thereto;
• wherein:
• said extension device comprises a first inner tubular element comprising an inside diameter greater than or equal to an inside diameter of the socket of the bone anchorage screw, and extended longitudinally by two diametrically opposite branches, said branches having resilience allowing the branches to be moved apart from each other and towards each other, and free ends of the two branches forming jaws for clamping onto an outside surface of the socket so as to lock onto the socket;
• the extension device further comprises a second outer tubular element mounted around the first inner tubular, and element comprising an inside diameter matching an outside diameter of said first inner tubular element to an extent such that said second outer tubular element slides along said first inner tubular element between a non-locking position and a locking position, in which the second outer tubular element does not lock onto and locks onto the bone anchorage screw, respectively, the second outer tubular element being extended longitudinally by two diametrically opposite branches superposed on the branches of the first inner tubular element, free ends of the two branches of the second outer tubular element being in register with the free ends of the two branches of the first inner tubular element, and each of said free ends of the second outer tubular element being provided with first retaining means adapted, when the extension device is fitted over the socket of the bone anchorage screw and when said second outer tubular element is slid into the locking position, for co-operating with complementary second retaining means arranged on the socket to prevent said branches of the second outer tubular element from moving apart from each other, thereby preventing the branches of the first inner tubular element from moving apart from each other so as to maintain the extension device locked onto the socket of the bone anchorage screw;
• and
• wherein the first retaining means comprises contact surfaces provided at the ends of the branches of the second outer tubular element that are inclined relative to a transverse plane of the second outer tubular element and face outwards of each branch, and the complementary second retaining means comprises contact surfaces provided on the socket that are inclined relative to a transverse plane of said socket and face towards the inside of the socket, the contact surfaces of the first retaining means and the contact surfaces of the complementary second retaining means being adapted to come to bear against one another when the second outer tubular element is slid into the locking position for preventing the branches of the second outer tubular element from moving apart from each other, wherein contact surface of the first retaining means of each branch of the two diametrically opposite branches are angled away from each other as they extend longitudinally from the ends of the branches.