4.1Pedicle screw bi-cortical insertion significance
As the population ages, the number of elderly patients who have osteoporosis with thoracolumbar spine disease is increasing,[4, 8–10] and pedicle screw fixation is widely used to treat such patients. However, because osteoporosis involves decreases in bone density, it severely affects the holding power of the vertebral body. Loosening and dislodging of the screw directly affect internal fixation strength, which in turn determines the success of the operation.[3, 4, 10, 11]
To increase the internal fixation strength of the pedicle screw in patients with osteoporosis, clinicians wisely use bone cement augmentation, cortical bone channels, and expandable pedicle screw fixation. However, these techniques significantly increase the risk of nerve injury and the operation time, as well as the amount of bleeding.[2, 9, 12, 13]Surgical techniques also enhance the internal fixation strength, particularly increasing the diameter of the pedicle screw, the depth necessary for insertion, and the insertion angle show an advantage.
Studies have shown that, when the screw is placed in the anterior cortex of the vertebral body but not penetrated, the fixation strength can be increased by 16%, and the anterior cortex be broken through, which can increase the pedicle screw pull-out force by 60%, increasing 20%-25% fixed strength. [2, 15]Based on this biomechanical theory, the pedicle screw bi-cortical fixation technique is applied to patients with osteoporosis.
Double cortical fixation has two mechanical advantages over single cortical fixation: it increases the length of screw insertion and it uses cortical bone rather than cancellous bone for screw fixation. The stress is dispersed between the two cortical bones so that the fixation strength in the cancellous bone is significantly increased. [3, 4]The bi-cortical fixation technique involves significantly shorter operation time and lower risk of nerve damage than other placement techniques. However, the technique also requires precise screw placement, as protruding screw tips can damage blood vessels.[16, 17] As such, more stringent requirements are placed on the surgeon, because the screw placed in the anterior cortex of the vertebral body must not be too long. Despite this, no previous studies have investigated safe lengths for screw extrusion. Thus, the present study addressed this question and provided theoretical guidance for the accurate implementation of bi-cortical fixation.
4.2 The effect of body position on the distance between vertebral body and blood vessel
The data showed that the AVD differed significantly among the T9, T10, and L2 vertebral bodies (P < 0.05), that the AVD in the prone position was larger than that in the supine position, and the∠AOY of T9–T12, L2, and L3 vertebral bodies differed. There was a statistically significant difference (P < 0.05), and the prone position was closer to the anterior vertebral center line than the supine position. This indicates that the positions of the artery and the vertebral body are relativeactivity, except in some segments. The overall position of the aorta was changed relative to the vertebral body. The lower aorte of the prone position is farther from the vertebral body and closer to the anterior vertebral center line. Vaccaro et al. obtained similar conclusions. They found that prone positioning resulted in greater distances between the disc and iliac vessels at L4-L5 and L5-S1 by an average of 3 mm, probably because, in the supine position, magnetic resonance imaging (MRI) underestimates the distance between the blood vessels and the intervertebral disc. They also found changes to the active bifurcation point and IVC junction. It was found that the position in the prone position moved up from the supine position to the head side, confirming the vascular activity. Similar results were reported by Sucato et al., as well as by Huitema.
The present study showed that the relative positions of the IVC and vertebral body in the L1–L3 segment were relatively fixed. In different postures, no obvious changes occurred: there was no significant difference in VVD between the L1–L3 vertebral body and the IVC (P ≥ 0.05). Furthermore, in the supine and prone position, there was no significant difference in ∠VOY between the vertebral body and IVC (P ≥ 0.05). In the position of the IVC and vertebral body in the L1–L3 segment.
4.3 Influence of anatomical factors on the distance between vertebral bodies and blood vessel
The relative vascular distance of vertebral bodies T9–L3 decreased first and then increased (Fig. 4). The AVD was the smallest at T12; similar results were observed by Sarlak et al. in 12 patients with scoliosis. There was no significant difference in the T12 AVD and∠AOY data (P ≥ 0.05), indicating that the relative positional distance between the T12 vertebral body and the aorta does not change with changes in body position, and that the same is true for the adjacent segments T11 and L1. Considering anatomical factors, the thoracic aorta continues 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 this point and do not change due to changes in body position.
This study did not obtain anterior venous data at the level of the T9–T12 vertebrae. However, anatomical studies have shown that the IVC flows into the left and right common iliac veins before L5, and that it ascends to the right side of the vertebral body, entering the thoracic cavity through the vena cava, and the vena cava was at the center of the T8 vertebral body, close to the central aponeurosis, away from the vertebral body, so the blood vessels gradually move away from the vertebral body during the running process; the data shows that the distance VVD of the IVC and vertebral body of L3-L1 gradually increases, and L1, L2VVD > 10 mm, T8-T12 The distance between the anterior vertebral veins is farther, and there is no research significance. Furthermore, the IVC passes through the hepatic vena cava through the liver, and the boundary of some liver tissues is not obvious. As shown in Fig. 2A, no relevant data were obtained.
In addition, imaging studies have shown that, in elderly patients, lip-like bone hyperplasia can occur in the upper and lower margins of the vertebral body, pushing the anterior vertebral vessels to the front of the vertebral body (Fig. 2A). Right anterior border bone hyperplasia pushes the blood vessel forward, increasing the safe distance between the blood vessel and vertebral body.
4.4 Discussion on the safety distance between vertebral body and blood vessel
To ensure that the pedicle screw bi-cortex insertion is safe, distance must be placed between the vertebral body and the blood vessel. The present study found that postural changes alter the distance between the blood vessels and the vertebral body. The prone position may confer a greater safe distance. In general, three-dimensional CT reconstruction in the supine position underestimates the safe distance between the vertebral body and the blood vessel.[20, 22, 23]It Provided a basis for greater safety margins during the surgical procedures.
The data showed that the vertebral body was closest to the aorta at T12 (> 3.2 mm), while the IVC was the closest to the vertebral body at L3 (> 5.4 mm). In theory, when the thoracolumbar segment (T9–L3) undergoes bi-cortical insertion, it is safe to use the left vertebral screw to break the anterior cortex length of (> 3.2 mm). The right pedicle screw has a greater safe range before rupture (≤ 5.4 mm).The distance of some segments may be below average distance due to individual differences, and we need to be carefully observed before surgery to determine whether or not the intraoperative breakthrough and the breakthrough length. At T12, due to anatomical factors, the distance from the vertebral body is the closest, and the position is relatively fixed. Special attention must be paid to the safe distance range before surgery.
When discussing safety distance, Sarwahi et al. claimed that the anterior/anterior lateral cortex of the screw can be safely broken at ≤ 4 mm; the study found that 33 screws that broke the anterior cortex did not damage the vascular organs under anatomical direct view. The undeveloped CT scan of the anterior soft tissue of the vertebral body underestimates the safe distance between the blood vessel and the vertebral body. The soft tissue within 4 mm of the anterior vertebral body can completely embed the tip of the protruding screw, rendering it safe. In patients with bi-cortical fixation, CT scans have shown prominent screws in contact with blood vessels, but most studies have found that patients with screw-to-vascular contact had no clinical symptoms.[24, 25] Foxx et al. reviewed 107 patients with spinal internal fixation and found that 33 of the 680 screws were in contact with large vessels, with an average follow-up of 44 months. Postoperative imaging follow-up showed no vascular abnormalities at the site of contact. Therefore, screws that are in contact with blood vessels are not considered to cause vascular damage.
4.5 Effect of the angle between thevertebral body and the blood vessel on bi-cortical fixation of the pedicle screw
Relative distance is an important basis for assessing the safety of pedicle screw placement. However, the present study also revealed changes to the angle of the vessel relative to the vertebrae. Considering that the distance between the aorta and the vertebral body can also change with body position, the position of the aorta may fluctuate within a range.
Using three-dimensional CT reconstruction measurement data, we simulated screw placement in the ideal pedicle plane (positioning map 3A) and found that the screws were placed within the appropriate and safe screw inclination transverse screw angle (TSA), and the protruding length of the screw from the anterior cortex of the vertebral body may partially overlap or avoid the anterior vertebral vessels. the anterior vertebral vessels of the few segments can completely avoid the screws. As shown in Fig. 7, postoperative resection of the patients with bi-cortical fixation revealed that the anterior vessel of the L2 vertebral body avoided the direction of the screw axis. In such cases, even if the blood vessel is close to the vertebral body, screws that breaks through the anterior wall of the vertebral body are less likely to damage the blood vessels. Therefore, the risk of vessel injury due to bi-cortical fixation cannot be evaluated based on relative distance alone. Three-dimensional CT scanning in the supine position measures the position and angle of the blood vessel, predicts whether the blood vessel can avoid the screw placement range, and reveals the TSA range of the blood vessel to guide the surgeon during the operation. This is important for more precise and safe bi-cortical placement. However, it also puts higher demands on the surgeon with regards to hand-stitching.