4.1. The significance of pedicle screw bicortical insertion
How to improve the pullout force and internal fixation stability of pedicle screws has always been the focus of scholars. Clinicians wisely use bone cement augmentation, cortical bone channels, expandable pedicle and new screw fixation. These new technologies have achieved good clinical results.[8, 13-16] For traditional pedicle screws, surgical techniques also enhance the internal fixation strength, especially the diameter, required insertion depth and insertion angle of the pedicle screw. [17, 18]
Studies have shown that when the screw is inserted in the anterior cortex of the vertebral body but not penetrated, the fixation strength can be increased by 16%, and the anterior cortex can be broken through, which can increase the pedicle screw pullout force by 60% and the fixation strength by 20%-25%.[19-22] Bicortical anchorage increases the length of screw insertion and the stress is dispersed between the two cortical bones so that the fixation strength is significantly higher than that of cancellous bone.[4, 5]
However, the technique also requires precise screw placement, as protruding screw tips may damage blood vessels.[23, 24] In this study, it is considered that the safety range of the protruding tips of the screw should be kept within the 3mm. At the same time, in order to avoid contact with large blood vessels, the direction of screw implantation of screws should be considered in combination with the CT images of the prone and the supine positions.
4.2. The effect of body positions on the distance between vertebral body and the great vessel
Huitema et. al[25] proved a substantial difference in the position of the aorta relative to the spine in the prone and in the supine position(range,T4-L2), while Vaccaro et al.[26] demonstrated substantial mobility of the great vessels in different positions (range,L4-S1). We also verified the results of the aorta, and we found that the AVD of T12 is the shortest distance both in the prone and the supine positions. Considering anatomical factors, the thoracic aorta extends into the abdominal aorta from the aortic sac of the diaphragm, which is mostly located at the T12-L1 positions and is close to the vertebral body. Thus, the aorta and vertebral bodies are fixed at T12 and L1 level and will not change due to changes in body positions. Compared with the aorta, we believe that the mobility of IVC has no obvious changes in different body positions.
4.3. Discussion on the safety distance between vertebral body and blood vessel
Sarwahi et al.[27] claimed that anterior/anterolateral protrusion is less than or equal to 4mm on CT poses no significant risk of impingement and can be considered safe. In gross anatomy, 23 misplaced screws do not endanger any structures and the distance they protruded are less than 4mm on CT scan.[27] Because of the large distance between the large vessels and the vertebral body in the prone position, we evaluated and measured the safe distance in the supine position with CT. The shortest distance between the aorta and the vertebra is 3.18±0.68mm at T12, and the shortest distance between the IVA and the vertebra is 5.50±2.17mm at the L3 level. Considering the large distance in the prone position and the poor visualization of soft tissue on CT images, we conservatively believe that it is safe for the protruding tip of the screw to be less than that of 3mm. Due to individual differences, we recommend that the actual safe distance between the great vessels and the vertebral body can be measured according to our method before surgery. In addition, imaging examination shows that labial hyperosteogeny may occur on the upper and lower margin of the vertebral body in elderly patients, pushing the anterior vertebral vessels to the front of the vertebral body (Fig. 2A), increasing the safe distance between the blood vessel and vertebral body.
When evaluating misplaced screws in contact with blood vessels, the protruding tips of some of the screws are too long, which puts more degrees of impingement on the blood vessels and is more likely to cause chronic vascular injury.[28] There will also be contact between the screw and the blood vessel after the bicortical fixation. However, patients with pedicle screws in contact with major vessels may not necessarily suffer adverse sequelae.[29-31] We believe that if the tip of the screw can be controlled within the safe range, it will cause a lesser degree of screw impingement into the blood vessels and is unlikely to cause vascular damage.[28]
4.4. The effect of the angle between the vertebral body and the blood vessel on bicortical fixation of the pedicle screw
Due to individual differences, some segmental vertebrae are close to blood vessels, and there is no obvious safe distance. However, simulating screw placement on preoperative supine and prone CT images can find the appropriate transverse screw angle (TSA), and the screw direction can avoid great vessels completely. As shown in Fig. 8, postoperative CT image of the patients with bicortical fixation presented that the great vessels of the L2 vertebral body can avoid the direction of the screw axis. Liu et.al[32] summarized the appropriate TSA of each pedicle of L1-L4. However, there is still an error in the insertion point of pedicle screw between the preoperative evaluation and the actual operation, which leads to a greater error in TSA. This requires surgeons to build their own preoperative models according to habits and to plan the correct TSA range to reduce errors.
4.5. Limitations of this experiment
This study also had certain limitations, such as a small sample size and inevitable measurement error, although we have chosen angiography CT to minimize errors. We observed that at the level of T8-T12, the boundary between the IVC and liver tissue passing through the hepatic vena cava sulcus is not obvious. However, there is a sufficient safe distance between it and the vertebral body, which will not affect the bicortical anchorage at all.(Fig.2) In addition, we also agree that bicortical anchorage is not suitable for implantation of L4-L5 vertebrae[32], and the above relevant data were not collected in the experiment.