The lumbar spine is an important component that transmits the upper body weight to the lower limbs. The lumbar spine has a high incidence of diseases such as spondylolisthesis, fractures and disc degeneration due to a large range of motion and stress concentration [17,18,19,20]. Spinal cord decompression combined with dual screw-rod fixation through a single posterior approach has achieved satisfactory curative effects in the treatment of the above diseases and has been widely used in clinical practice . However, diseases such as lumbosacral spinal tuberculosis and tumors often cause damage to the anterior vertebral columns and intervertebral discs, and it is difficult to thoroughly clear the lesions through the posterior approach alone because of the complex anatomy of the region. To solve the above deficiencies, some scholars have proposed an anteroposterior approach. Although this kind of operation has the advantages of sufficient clearing and solid fixation, the anteroposterior approach is still controversial because of the substantial trauma and high cost. Thus, a single anterior approach is favored by some surgeons, which has the advantage of clearing the lesions under direct vision. However, some previous studies have shown that the stability of anterior fixation was inferior to that of posterior fixation. Some anterior lumbosacral internal fixation systems have been designed to improve the strength of internal fixation, but these fixation systems cannot be applied to spinal tuberculosis patients who have irregular bony destruction due to defects of their own construction. To remedy the above deficiencies, we have developed a new surgical procedure, one-stage anterior debridement and cage implantation combined with anterior-lateral fixation by a dual screw-rod construct underneath the iliac vessel. After our long-term follow-up study, the therapeutic effect was deemed satisfactory, and no complications such as vascular rupture occurred. However, some surgeons question the fixation; therefore, we conducted an in vitro biomechanical study with human specimens to assuage these doubts.
The main purpose of this study was to observe the effects of anterior and posterior fixation on lumbosacral spine stability in the flexion, extension, lateral bending, rotation and axial compression directions. According to the results of the preliminary experiments and related literature reports, the maximum loading displacement of the bending test and axial compression test was set to 5 mm, while the maximum loading angle of the axial torsion test was set to 5 °. Prostheses yielding and vertebral fracturing were not observed throughout the whole experiment.
The results of the bending test showed that the loads of the specimens in the three groups increased as the displacement increased, which was consistent with the larger displacement caused by a larger load. The loads in the AF group and PF group were significantly higher than that of the I group in terms of unit-loading displacement in the flexion, extension and lateral bending directions, which may be related to the buffering effect of the intervertebral disc in the I group. Moreover, the results also showed that the two fixation methods could achieve better stability in the four movement directions. Under the maximum load displacement, the load of the AF group was significantly greater (P < 0.05) than that of the PF group in the flexion and left and right lateral bending directions, indicating that one-stage anterior debridement and cage implantation combined with anterior-lateral fixation by a dual screw-rod construct could achieve better stability than posterior dual screw-rod fixation in flexion and lateral bending. However, in the extension direction, the load required by the specimens of the PF group was significantly greater (P < 0.05) than that of the specimens of AF group. This finding suggested that the immediate stability of the AF group was slightly inferior to that of the PF group in extension. A possible reason for this phenomenon was that the titanium cage was located in the anterior column of the spine, and the cage provided only support and could not provide tension for the spine when it was not fused with adjacent vertebrae.
The axial torsion test showed that the load of the I group was smaller than those of the AF and PF groups under the maximum rotation angle (P < 0.05), while the loads of the latter two groups in the above direction were very similar (P > 0.05). These results showed that the disc acted as a buffer in the rotation of the spine and that both anterior fixation and posterior fixation could provide immediate stability for the lumbosacral spine in the rotation direction. The results of the axial compression test showed that the loads of the specimens in the AF and PF groups were significantly higher (P < 0.05) than that in the I group, which indicated that both fixation methods could meet the axial compression requirements of the lumbosacral spine. However, the load of the AF group was significantly higher than that of the PF group under the maximum loading displacement, indicating that the anterior fixation could better meet the axial compression requirements. The reason for this finding is that 80 to 90% of the pressure of the spine is mainly transmitted through the anterior column. The anterior fixation played a very good supporting role through the titanium cage, which enabled anterior fixation to exhibit better performance than posterior fixation under axial compression.
The limitations of this study must be acknowledged. First, although our results suggest that one-stage anterior debridement and cage implantation combined with anterior-lateral fixation by dual screw-rod could provide better immediate stability in the directions of flexion, extension, lateral bending, rotation and axial compression, the results should be cautiously interpreted because of the study’s low power. The use of a larger sample set should be considered in future studies. Second, the experiment only assessed the mechanical properties in vitro; the in vivo bio-safety shall be further studied.