The biomechanical state of the thoracolumbar junction with various options of transpedicular fixation under flexion load

Abstract

Introduction. Morphological and biomechanical features of the thoracolumbar junction determine the large number of cases of traumatic bone injuries. Reconstructive and stabilizing surgeries performed in this area, due to the significant load on both the elements of hardware and bony structures, require high reliability of fixation. Objective. To study the stress-strain state of the model of the thoracolumbar section of the spine after the Th12-L1 vertebrae resection with various options of transpedicular fixation under the influence of flexion load. Materials and methods. The stress-strain state of the mathematical finite-element model of the thoracolumbar section of the human spine under the influence of flexion load was studied. The model simulated the condition after surgery for a significant traumatic lesion of the thoracolumbar junction with laminectomy, facetectomy, and corpectomy of the Th12 and L1 vertebrae. Four variants of transpedicular fixation were studied (using short or long bicortical fixation screws, two crosslinks and without them). Control points of the model characterizing the load distribution both in bony structures and on metal elements of fusion and body replacement systems were studied. Results. Crosslinks have the greatest effect on reducing the level of stress both in the bony elements of the models and in the metal elements. When comparing the length of the screws, the use of monocortical screws was determined to have minor biomechanical advantages. The stress analysis of the area of the screw entry into the pedicle of the arch of the fixed vertebrae (clinically significant zone) revealed that in the model with short screws and without crosslinks, the stress for the vertebrae Th10, Th11, L2 and L3 is 5.0, 1.9, 7.8 and 13.6 MPa, respectively, while the presence of crosslinks reduces the corresponding values to 4.6, 1.9, 7.3 and 12.7 MPa. In models with bicortical screws, the corresponding values are 5.1, 2.3, 10.2, and 12.7 MPa in the absence of crosslinks and 4.7, 1.8, 9.9, and 12.2 MPa with the presence. A similar trend is observed in other control points. When comparing the results with the compression load in the models studied earlier, it was established that flexion causes an increase in the stress of the models with monocortical screws by an average of 33.7%, with bicortical screws by 39.6%. Conclusions. In case of flexion load, the use of crosslinks makes it possible to reduce the level of stress in all control points of the models, regardless of the length of the used transpedicular screws, while the length of the screws does not have a fundamental effect on the stress distribution.