Mechanical Properties of a New Device for Internal Fixation of Spine Fractures: The “Fixateur Interne”

The cammon implants for internal fixation of thoracic and lumbar spine fractures act as a four point bending system. As they have a mobile link to one vertebra and need a second bony support on the lamina of the next vertebra on each side of the afflicted area, they work only if at least 5 vertebrae (better 7) are included in the fixation. It has been shown that such a long immobile segment in the spine can cause problems for rehabilitation. To overcome this and other disadvantages of the common implants a new, fully implantable device has been developed with a different biomechanical base. The “Fixateur interne” (F.I.) provides stability against flexion and rotation by itself and can be used independent of the condition of all ligaments (the anterior longitudinal ligament included) and of the posterior wall of the fractured vertebra: The implant (1) consists of Schanz screws of 5 mm diameter for transpedi-cular fixation in the vertebrae and threaded longitudinal rods of 7 mm diameter to connect the Schanz screws. They carry chucks and jaws which are first mobile in every direction and permit application of kyphosing, lordosing, distracting, compressing and rotating forces. Finally they can be fixed by nuts in any wanted position. A system of flattenings on the threaded rods, non-turning washers, grooves and bars secures a firm hold of the position of the Schanz screws towards eachother, including rotational stability. As long as the hinges stay firm, no redislocation will occur.