Although PKP/PVP is the main surgical intervention for KD, both of them can significantly relieve the pain of patients[26], some studies[10, 27] have found that bone cement displacement occurred after PVP or PKP, which caused serious problems and required treatment. Gao et al.[10] have found that the thoracolumbar fracture, stage Ⅱ KD, anterior cortex defect, uneven cement distribution, cement leakage, and high restoration of the local Cobb angle were risk factors for bone cement displacement after percutaneous vertebral augmentation in KD. Gao et al.[9] demonstrated that when the anterior vertebral cortex was defective, it would not only lead to cement leakage, but also increase the risk of cement displacement to a certain extent. Although there is no consensus on the optimal volume of bone cement to inject, Kim et al.[15] believe that the volume of bone cement slightly more than that of the IVC space should be injected to prevent intravertebral instability caused by non-union in percutaneous vertebroplasty for KD. Some scholars[14, 28, 29] mainly suggested that polymethylmethacrylate cement in vertebroplasty is merely a space-occupying material without mechanical interlocking ability and biocompatibility, meaning that the bone cement and the surrounding bone tissue did not joined, which was another important cause of cement dislodgement. Some scholars believe that once bone cement displacement occurs, patients often need to undergo open posterior, anterior, or even anterior and posterior revision surgery to remove the displaced bone cement, reconstruct spinal stability, and restore the spinal sequence and fusion[14, 29, 30].
The hollow pedicle screw system was developed to interlock the bone cement and the surrounding bone tissue to avoid bone cement displacement during or after surgery. Compared with PKP alone, it serves as a bridge, seamlessly linking bone tissue with bone cement mass. This innovative concept aligned with Professor Yonezawa[16, 17], who inserted hollow pedicle screws into the cement block through the pedicle[17] to prevent cement dislodgement. In addition, Wang et al.[18] also confirmed the excellent short and medium-term therapeutic effect of a novel bone cement screw system combined with vertebroplasty for KD in preventing bone cement displacement. Xu et al.[31] demonstrated that the bone cement screw combined with PVP could effectively reduce the relative displacement of the bone cement through finite element analysis. Unilateral cement screw placement can obtain a better fixation effect and has a less biomechanical effect on the vertebral body than bilateral bone cement screw placement. However, the design of hollow pedicle screws in this study was different from that of Xu et al.[31]. The tailless design of the screw caused less damage to the surrounding muscles and blood vessels, while Xu's screw with a long tail. In addition, in contrast to Wang et al.[18] and Xu et al.[31], this study advocates that bone cement injection should be performed before hollow pedicle screw placement so that the bone cement can be fully diffused and evenly distributed. Besides, the threaded part of the hollow pedicle screw was designed as a double thread, which enhanced the anchoring force of the screw and prevented the potential risk of internal fixation failure in patients with osteoporosis. Therefore, the novel hollow pedicle screw used in this study combined with bone cement block to form a stable and firm complex, which has great potential in preventing complications of bone cement displacement.
As an important cushioning device of the spine, the greater the stress on the intervertebral disc, the more likely it is to cause deformation. The study found that: in 7 directions, the stresses of T11/T12 and T12/L1 discs in M1 and M2 models were significantly higher than those in other models. The stress of T11/T12 and T12/L1 discs in M3 and M5 models with unilateral operation was higher than that in M4 and M6 models with bilateral operation, and the stress of M3 was higher than that of M5, and the stress of M4 model was greater than that of M6, indicating that compared with the preoperative M1 model, the postoperative bone cement filling could help to change the stability of the affected vertebrae and reduce the degeneration of the adjacent intervertebral disc. Moreover, the stress of PKP combined with a hollow pedicle screw on the intervertebral disc was significantly less than that of PKP combined with pedicleplasty or PKP alone. Bilateral hollow pedicle screw placement is superior to unilateral placement. Besides, the stress distribution of T11/T12 and T12/L1 discs in the M1 and M2 models was more uneven than that in other models. Although PKP alone can relieve the clinical symptoms of KD patients, there is a potential risk that it cannot effectively prevent the degeneration of adjacent intervertebral discs. In addition, the cement distribution of bilateral injection was more uniform than that of unilateral injection, indicating that the cement was only distributed in one side of the vertebral body, which was strengthened, but not the other side, which may cause instability of the spine and aggravate the degeneration of the adjacent intervertebral disc.
In this study, we compared the biomechanical effects of three procedures and pre-operation on adjacent vertebrae (T11 and L1). The effect on adjacent vertebrae is similar to that on adjacent intervertebral discs. PKP combined with hollow pedicle screws can reduce the risk of adjacent vertebrae fracture more effectively, but the difference is not as obvious as that of the adjacent intervertebral discs, which may be because the intervertebral discs have a buffer effect on reducing the stress of adjacent vertebrae. In addition, since the hollow pedicle screw runs through the anterior, middle, and posterior columns of the vertebra, the elastic modulus of the screw is also larger than that of other components, and the hollow pedicle screw also effectively shares the stress of the spine. It has also been shown that the load can be transmitted to the adjacent vertebrae along the longitudinal axis of the spine, which is one of the important reasons affecting the risk of adjacent vertebral fracture[32].
According to the results, we found that compared with M1, the von Mises stress on T12 from M2 to M6 was significantly reduced, which indicated that the vertebral fracture was significantly improved after surgery, the vertebra was more stable, and the stress on the vertebra was less prone to re-fracture. The stress of M6 on T12 and bone cement was the smallest, while M2 was the largest. In addition, under the same conditions and the direction of bone cement injection, the stresses on T12 and bone cement in PKP combined with pediculoplasty were significantly greater than that in PKP-HPS surgery, and the stresses in bilateral surgeries were also less than that in unilateral surgeries. Some studies have shown that cement distribution and vertebral stiffness are important factors affecting the stress balance of the vertebral body for recompression fractures of the vertebral body after PVP[33]. This asymmetric cement distribution may reduce the stiffness of the non-reinforcing side, leading to fracture recurrence[34]. Therefore, PKP combined with hollow pedicle screws is superior to PKP combined with pediculoplasty and superior to PKP alone in reducing the risk of adjacent vertebral fracture, adjacent disc degeneration, and vertebral refracture, and bilateral surgery is superior to unilateral surgery.
In this study, the cement displacement of M2 and M3 was significantly greater than that of M4, M5, and M6, suggesting that cement augmentation alone is more likely to cause cement loosening or displacement. This result is consistent with the biomechanical results of the finite element analysis by Wang et al. [13]. Therefore, patients should be aware of cement displacement or loosening after PKP alone or unilateral PKP combined with pediculoplasty for KD. This may be due to the changes in the center of gravity and stress of the vertebra during flexion and rotation, which is more likely to cause the micromovement of bone cement in the vertebra. The distance of bone cement movement was the smallest in the M6 model, which indicated that the hollow pedicle screw system combined with kyphoplasty has better stability and safety than the c conventional PKP and PKP combined with pediculoplasty, indicating that this treatment has the best ability to resist bone cement loosening and displacement.
The limitations of this study include: (1) Only one KD patient's CT image was used as the basis for constructing the finite element model, and those models just included a single type of vertebral fracture; (2) The mechanical test under different loading conditions was not comprehensive enough, only the experimental data of 500N and 7.5Nm were analyzed. (3) Due to the complexity of spinal anatomy, the preprocessing content also increased, which further enlarged the gap between the finite element model and the real spinal structure. In the future, realistic biomechanical tests are needed to further verify the stability of this operation.