With the aging of the world population, the incidence of osteoporotic vertebral compression fractures is increasing year by year. Conservative treatment and vertebroplasty are the two most common methods for the treatment of osteoporotic vertebral compression fracture. At present, conservative treatment of osteoporotic vertebral compression fracture will further lose bone mass, and osteoporosis will be more serious, which lead to a vicious circle of more prone to fracture and seriously affect the quality of life of patients[17]. Some studies had pointed out that surgical treatment may be a better way to solve osteoporotic vertebral compression fractures[18].However, unilateral vertebroplasty will cause uneven distribution of bone cement due to the influence of surgeons, and if it intend to achieve a more ideal distribution pattern in the process of injection, it is bound to increase the angle of puncture needle, which will inevitably increase the risk of operation. Bilateral puncture will also lead to corresponding puncture injury and operation cost. Therefore, how to obtain better dispersion of bone cement will greatly affect the curative effect of vertebroplasty.
Through this study, we concluded that there was no significant difference in the operation time, the number of intraoperative fluoroscopy and the amount of bone cement between the two groups. That was, PCVP would not increase the prone time of the patients during the operation, and could ensure the smooth operation. It also shown that the performance of the improved method of PCVP is no less than that of PVP which has been skillfully mastered in clinic. The postoperative VAS score and ODI score of the two groups were significantly improved. Although it could not be ruled out whether it was caused by the operator, it was sufficient to show that PCVP was not inferior to PVP in postoperative pain relief. Our study also collected the changes of the height of the anterior edge of the injured vertebra before and after operation. The height of the anterior edge of the cone increased significantly in both methods, and the degree of recovery of vertebral height obtained by the two methods was similar, which suggested that the effect of PCVP in restoring vertebral height was similar to that of PVP.
The biggest difference in this study was the distribution of bone cement, which was in line with our previous conjecture. Including unilateral distribution that did not cross the midline of vertebral body 1 case, unilateral filling that reached the midline of vertebral body 1 case, bilateral filling that crossed the midline of vertebral body 28 case, bilateral filling that crossed the midline of vertebral body and reached bilateral filling 3/5of vertebral body 20 case, and in the PVP group, they were respectively 11 case , 12 case, 21 case, 6 case. There was no doubt that the reason for this difference lied in the different working channels adopted. PCVP used the curved channel to realize unilateral puncture and inject bone cement into the opposite side of the vertebral body, which could realize the filling of the whole vertebral body. Due to the characteristics of this channel, when the rigid needle tube or working casing is held straight into the vertebral body, the memory alloy returns to the original angle. In this way, the uneven dispersion of bone cement in the vertebral body caused by various uncertain factors such as puncture position, vertebral bone condition and bone cement dispersion can be avoided. Thus,the distribution of bone cement is one of the important factors affecting the prognosis in many studies, and it is more likely to affect the complications after vertebroplasty.
Firstly was bone cement leakage. Previous studies had shown that cortical rupture, excessive cement volume and low bone mineral density were the three major predictors of cement leakage[19]. Thus bone cement leakage was a strong evidence of risk factors for new OVCFs[20]. Clinically, complex osteoporotic vertebral compression fractures such as incomplete posterior wall of vertebral body and vertebral compression more than 75% are contraindications for vertebroplasty. In vertebroplasty, because it can not cross the sagittal midline of the vertebral body, and whether the bone cement approaches the middle of the vertebral body is closely related to the internal inclination angle. In order to obtain a good effect of bone cement diffusion, clinicians are bound to increase the introversion angle or increase the amount of bone cement perfusion or even choose bilateral vertebroplasty. Which a study had shown that the maximum distance between the safety line of the vertebral body between the C3-L5 and the posterior vertebral line on the lateral X-ray film was (5.22 ±0.62) mm, which gradually decreases from L1 to L5 to (1.05 ±0.64) mm. The leakage rate of bone cement in the experimental group without crossing the safety line was significantly lower than that in the control group[21]. Therefore, it is undoubtedly difficult to control bone cement in clinic, not only to achieve good diffusion effect, but also to reduce the occurrence of bone cement leakage. In this study, we believed that the difference of bone cement leakage between the PVP group and PCVP group was due to the difference in the distribution of bone cement between the two groups. And there was study had confirmed our conjecture. HeCJ et al confirmed that the inadequate distribution of bone cement was related to the leakage of bone cement, because in the process of bone cement injection, the pressure in the vertebral body will gradually increase with the increase of the amount of bone cement injection, which may increase the risk of leakage during the injection process, especially in unilateral vertebroplasty[22]. However, the bendable working channel used in PCVP could squeeze the cancellous bone when entering the vertebral body, achieve the effect of low pressure injection, and achieve the effect of multi-point injection when reversing the working channel, so that the bone cement can be better dispersed.
The second was the occurrence of recurrent vertebral fracture. There was no recurrent vertebral fracture in both groups in our study. This was different from some recent studies. The effect of the distribution of bone cement on recurrent vertebral fractures had also been described in a number of studies. Among them, Liebschner et al found that symmetrically distributed bone cement could obtain greater stiffness. In addition, there was a close relationship between bone cement leakage and vertebral re-fracture. This may be reflected in (1) the uneven distribution of bone cement in the diseased vertebral body, resulting in an uneven increase in the strength and stiffness of various parts of the vertebral body; (2) the relative strength and stiffness of the injured vertebral body is significantly higher than that of the adjacent vertebral body after injection of bone cement[23]. Rho YJ et al also confirmed that the distribution pattern of bone cement was related to the occurrence of new compression fractures in adjacent vertebrae[24]. Subsequently, many experts had discussed the distribution pattern of bone cement. According to the study of BaekSW et al., theoretically, the circular distribution of bone cement along the edge of the vertebral body could reduce the uneven stress of the adjacent vertebral body, thus reducing the risk of re-fracture[25]. And another study had shown that no matter whether the bone cement was in full contact with the upper and lower endplate, it could have a good immediate analgesic effect. However, if the bone cement was in full contact with the upper and lower endplate, it could better restore the vertebral body strength. Thus better maintain the vertebral body height and reduce the risk of vertebral recompression, and its long-term effect is better[26].
The impact of a better distribution pattern of bone cement on clinical efficacy is also obvious. In Liu's study, the fused bilateral bone cement distribution group achieved a better short-term analgesic effect than the separated bilateral bone cement distribution group. It was believed that due to the uneven distribution of bone cement, the vertebral body was still stimulated by nerve endings under unilateral load, and the instability of the vertebral body will still lead to low back pain after operation[27]. In our study, PCVP was able to obtain better cement distribution due to the design of bendable channels. It could easily cross the midline of the vertebral body and achieved the effect of bilateral distribution. However, in our study, PCVP did not show better short-term postoperative pain relief and a reduction in the incidence of adjacent vertebral fractures. The possible reasons we consider were:(1) In order to obtain better results during the operation, the surgeon artificially increased the introverted angle during the PVP operation; (2) The use of postoperative analgesics and braces affects the patient's judgment of postoperative treatment;(3)1-year follow-up period was not enough to reflect the long-term low back pain and adjacent vertebral fractures.
In our study, PCVP already had clinical advantages for PVP, but it still had its shortcomings. Although some studies had shown that PVP and PKP as minimally invasive surgery for the treatment of OVCF will not increase the incidence of new vertebral fractures[28]. However, some studies had shown that vertebroplasty itself became a risk factor for new fractures when vertebroplasty was chosen[29]. In percutaneous balloon kyphoplasty (PKP) surgery, bone cement is easy to spread to non-fracture area and endplate bone tissue through trabecular space, which will make bone cement spread to non-fracture area and endplate bone tissue more easily. The wider the range of bone cement support, the closer the fusion with the surrounding cancellous bone. Balloon dilatation in kyphosis can better expand the cavity to infuse bone cement, and the dispersion of bone cement will be disturbed by the tight barrier formed by the formation of cancellous bone driven by balloon. Compared with the patients treated with PVP, to a large extent, the destruction of the microstructure of cancellous trabeculae by PKPleads to secondary fractures of adjacent vertebrae due to their uneven stress[30]. Due to the rotation of the conveying catheter during the PCVP, the cancellous bone is broken and squeezed through the curved segment in front of it, although it can obtain a better diffusion effect of bone cement. It may increase the risk of new adjacent vertebral fractures in patients with osteoporotic vertebral compression fractures. Although our study shown that PCVP does not increase the risk of adjacent vertebral fractures, we believed that longer follow-up was needed to verify this conclusion.
Meanwhile, studies had noted the relationship between bone cement injection and the clinical outcome of OVCFs treatment. Previous study had shown that the amount of bone cement injection was the key factor to improve mechanical stability by filling bone trabecula. The ideal value of bone cement significantly affects the relief of pain and the prevention of cement-related complications[31]. The appropriate amount of bone cement is helpful to directly improve the stiffness and strength of the surgical vertebral body, and stabilize the vertebral body. Increasing the amount of bone cement perfusion helps to maximize the recovery of compressed vertebrae and effectively reduce the degree of pain; low-dose bone cement may not provide enough support to restore compressed vertebrae and correct kyphosis. Therefore, a reasonable range of high-dose bone cement can reduce the risk of vertebral re-fracture, but it is not the more cement the better. Many clinicians have found that too much bone cement can cause cement leakage and damage the nerve root of the spinal cord. Liebschner et al believed that 3.5mL (15% of the fracture vertebral volume) bone cement can fully restore the strength of the fractured vertebral body[32]. More detailed reports indicated that the upper thoracic vertebrae need 2.5-3.0mL bone cement, the thoracolumbar vertebrae need 3.0-4.0mL bone cement, and the lumbar vertebrae need 6.0-8.0mL bone cement[33]. More study had established a reliable biomechanical model of lumbar fracture through the numerical simulation of CT scan data. In that model, spinal fractures and vertebroplasty may cause biomechanical changes in adjacent vertebrae. With the increased of the amount of bone cement injection, the influence of biomechanical changes may increase significantly. In which study, the best injection amount for PVP was 4mL[34]. However, in our study, the amount of bone cement perfusion was not strictly controlled according to the different injured vertebrae of the patients, so it was impossible to analyze the effects of the two treatment methods after accurate infusion of bone cement in different vertebral bodies. this may also be one of the reasons that affect bone cement leakage or long-term complications such as adjacent vertebral fractures. But, it was certain that PCVP can theoretically obtain a larger injection volume of bone cement than PVP, but its safe range of injection volume still needs more in-depth study.
There are several limitations in our study. Firstly, the main deficiency of the study was that the number of cases is insufficient, the bone cement leakage can not be classified, and the relationship between bone cement perfusion mode and various types of bone cement leakage could not be analyzed. Secondly, the follow-up to only 1 year after operation might not be sufficient to inquiry the long-term complications of PCVP. Finally, although the area of bone cement observed by X-ray is always measured by the same experienced radiologist, the distribution of bone cement in the fracture vertebral body is not calculated accurately, because it does not seem to be rigorous to rely solely on X-rays to evaluate the distribution of bone cement. A new evaluation methods and calculation methods need be applied to our further clinical exploration.