BIOMECHANICAL ASPECTS OF TRANSPEDICULAR FIXATION IN THE THORACOLUMBAR JUNCTION AREA: THE INFLUENCE OF LATEROFLEXION

The paradigm shift in surgery of the injured spine during the last few decades is characterized by the active implementation of the principle of stabilization without fusion. This approach significantly expands the possibilities of surgical interventions in terms of the completeness of decompression and spinal axis restoration, but also it determines higher requirements for the reliability of the fixation systems and the uniformity of load distribution on both metal systems and bone structures. Objective. To determine the features of load distribution in the area of the thoracolumbar junction after resection of one vertebra, as well as the effect of the transpedicular screw length and cross-links of the stabilization system. Methods. Mathematical finite-element model of the thoracolumbar human spine was developed. The model simulated the state after surgical treatment of a traumatic injury to the thoracolumbar junction with significant damage to the body of the ThXII vertebra. We studied 4 variants of transpedicular fixation (using monocortical screws and long bicortical screws, as well as two cross-links and without them). Results. When analyzing the stress-stain state of the model, we found that the most loaded bone structures during lateroflexion are the vertebral bodies. For the LII vertebral body, the load values were 17.2, 16.2, 16.3, and 15.5 MPa, respectively, for models with monocortical screws without cross-links, bicortical screws without cross-links, monocortical screws and cross-links, and bicortical screws and cross-links. The peak loads on the transpedicular screws were recorded on those implanted in the body of the ThXI vertebra (24.8, 25.7, 22.8 and 24.3 MPa, respectively, for the considered models) and in the body of the LII vertebra (20.2, 24.6, 19, 7 and 23.7 MPa). Conclusions. The use of long transpedicular screws causes less stress on the bony elements than the short screws. At that time stresses on the screws themselves and the bone tissue around them increase. Сross-links help to reduce stress at all control points on models with both short and long transpedicular screws.