In such systems a bone screw, such as for example a pedicle screw, is screwed into the vertebral arch and into the pedicle adjoining it, wherein the anchoring is controlled via a particular design of the pedicle screw thread and/or of the core cross section of the pedicle screw.
Pedicle screws are used chiefly for posterior thoraco-lumbar stabilization systems, whereby a fixation of the spine is sought with a screw-rod system. The pedicle screws used here are exposed to considerable stress, such that a very great variety of approaches have been disclosed with which the anchoring in the pedicle channel is meant to be improved.
Thus for example an arrangement is known from document U.S. Pat. No. 8,388,660 B1, whereby a screw shaft extends through a threaded sleeve which is expanded outward for anchoring in the pedicle in the transition to the vertebral body.
In document US 2012/0116465 A1 a screw is also used, which has a radially expandable distal threaded portion.
According to U.S. Pat. No. 8,343,200 B2 a pedicle screw is used which is screwed into a screw sleeve. According to U.S. Pat. No. 7,935,138 B1 the outer screw sleeve, in a similar construction, is executed in slotted fashion.
Finally a system is known from document US 2008/0086131 A1 whereby the additional fixing of the pedicle screw occurs by means of a ring lying beneath the screw head, which embeds itself with thorn-like extensions into the vertebral arch.
All known solutions have either the disadvantage that the radial expanding occurs with little control, whereby an increased risk of fracture results, or otherwise the disadvantage (in the case of using the spikes according to US 2008/0086131 A1) a more or less uncontrolled influence on the bone substance which may lead to bone damage.