We found that follow-up time, incidence of bone cement leakage, refracture rate of adjacent vertebra and intraoperative amount of bone cement injection between the two groups was not statistical difference. Both groups significantly relieved patients' pain of low back, recovered the height of vertebral body and kyphosis angle and improved their quality of life, but PVP was associated with less surgical time, blood loss, and radiation exposure than those of PKP. The rate of vertebral compression and kyphosis correction between the two groups was not found significantly difference, but decreased significantly with time, which suggests that the correction was gradually lost.
Since 1984, Galiber et al. [12] applied PVP in the treatment of 1 case of C 2 vertebra invasive hemangioma to find its advantages of simple operation and definite effect. PVP and PKP have gradually become one of the effective methods to treat vertebral tumor and OVCF [8, 13, 14]. Kümmell's disease is a rare type in OVCF after minor trauma, which is progressive occurrence of vertebral collapse and kyphosis due to vertebral ischemia and necrosis. Kümmell's disease is often manifested as intractable pain of low back, pseudojoint formation, and may be accompanied by nerve injury in severe cases [15, 16]. In plain radiographs and CT imaging of vertebral body pseudoarthrosis, the fracture sites often present gas accumulation, which is termed the vacuum phenomenon or IVC sign. Vacuum phenomena often appear as intravertebral radiolucent shadows that are typically band like or linear in shape and are often accompanied by peripheral sclerosis [17]. MRI imaging can find limited fluid filling in the vertebral body. So Kümmell's disease is also referred to as delayed post-traumatic vertebral collapse disease, nonunion of vertebral fractures, and vertebral ischemic necrosis [1, 2, 18]. Previous studies have suggested that IVC is mainly located in the thoracolumbar region [4, 19], which is similar to the segmental distribution of the two groups of patients in this study. 87.5% of the fractured vertebral bodies were located in the thoracolumbar segment, most of which were wedge fractures. And the fissures mostly occurred near the upper or lower endplate of the vertebral body. These suggests that the occurrence of the disease may be related to the repeated stress activity and high mobility in the thoracolumbar segment, leading to vertebral nonunion and ischemic necrosis.
Previous studies have found that a small dose of bone cement can restore the mechanical properties of the fractured vertebral body, and the injection amount of bone cement has no significant correlation with the analgesic effect, even believed that 1.5 ml of bone cement is injected into each vertebral body to obtain satisfactory analgesic effect [20, 21]. Biomechanics studies in vitro have confirmed that the strength of the vertebral body can be restored by injecting about 2 ml bone cement or 16% of the vertebral volume with bone cement, and the stiffness of the vertebral body can be restored by injecting about 4 ml bone cement or 24% of the vertebral volume with bone cement [21]. In this study, the average injection amount of bone cement was 4.2±1.15 ml in the PVP group and 4.6±1.55 ml in the PKP group, both of which met the requirements of restoring the strength and stiffness of the vertebral body without significant difference between the two groups.
Significant pain relief was observed after surgery in both groups, and maintained until the last follow-up, However, no significant difference was seen between the groups. After filling the fissures in the vertebral body with bone cement, the height and kyphosis deformity of the vertebral body were partially restored and corrected, and the abnormal activity of the injuried vertebral body was eliminated, which was an important reason for pain relief [11]. Previous studies have found that both PVP and PKP can effectively relieve the lower back pain of Kümmell's disease, achieve satisfactory clinical effect, and partially restore the height of the vertebra and correct the kyphosis [11, 22-29]. During spinal flexion and extension, due to the presence of IVC and the formation of pseudojoint, the injured vertebra of Kümmell's disease can stretch and expand, which can widen the fractured vertebral body. Therefore, the height of the collapsed vertebral body can be retracted and kyphosis has been partially corrected during spinal hyperextension. In PVP treatment, bone cement is usually limited to diffusion in the fissure, which can maintain the effect of hyperextension kyphotic correction without the help of balloon dilation for reduction in PKP treatment. Previous studies have also found that patients with Kümmell's disease can achieve spontaneous reduction in the postextensor position without further balloon expansion reduction [30, 31]. Heo et al. [32] reported that excessive reduction tends to accelerate the process of vertebral ischemia and necrosis, leading to severe re-collapse. Therefore, excessive reduction of intraoperative injured vertebra should be avoided. We found that the rates of vertebral compression and kyphosis correction both in PVP and PKP groups obtained obvious correction, but no statistical difference of the rates between the two groups was observed, which further confirmed the point of view " patients of Kümmell ’s disease can have spontaneous reduction in the posterior extension without further balloon expansion ".
We found that the rates of vertebral compression and kyphosis correction in the two groups gradually decreased significantly with time, suggesting that vertebral height and kyphosis angle gradually lost after surgery, which was consistent with previous findings [7, 14]. In the treatment of OVCF without IVC by PVP and PKP, the injected bone cement of the former is mainly embedded in cancellous bone, while the bone cement of the latter is mainly filled with clumps, so stress occlusion is more likely to occur after PKP and lead to recollapse [33]. However, in this study, there was no significant difference of the vertebral recollapse between the two groups after 2 years of post-operation. The reason was that the distribution of bone cement in the IVC region was inconsistent with that in the non-IVC region. Due to the low pressure in the IVC region and the obstruction of the surrounding fibrous membrane, IVC was used as a "reservoir" during intraoperative injection of bone cement in both groups, and bone cement was filled in the IVC region in the form of solid masses. The limited bone cement mass cannot connect with the adjacent endplates of the upper and lower levels and strengthen cancellous bone of the vertebrae, thus failing to support the normal physiological stress from the body and causing the collapse again [34].
The most common complications of PVP and PKP are bone cement leakage and adjacent vertebral fractures [35, 36]. The incidences of cement leakage of PVP and PKP for OVCF were 54.7% and 18.4%, respectively [36]. Krauss et al. [37] reported that in the treatment of OVCF with IVC by PVP, the bone cement leakage rate was 18.2%. Wang et al. [38] reported that the bone cement leakage rate of OVCF with IVC was 7.4% in PKP treatment. This study found that the leakage rates of bone cement were 17.9% and 10.7% in PVP and PKP group respectively, and both are similar to previous studies, which may be related to accurate preoperatively measurement of surgical approach, careful operation by the operator and not pursuing the maximum amount of bone cement. Besides, kümmell's disease exists obvious hardened zone and "crack" of vertebral body. A closed space is formed by the fissure, the surfaces of hardened zone, and the anterior longitudinal ligament. The injection of bone cement is confined in the closed space, and less prone to spread to other parts of the vertebral body. Therefore, the abnormal leakage of bone cement both in PKP group and PVP group was lower than that in the treatment of OVCF. In this study, although there was no significant difference of the bone cement leakage rate between the PVP group and the PKP group, the bone cement leakage rate of the PKP group was lower than that of the PVP group. The reason was mainly related to the fact that the PKP group could squeeze the surrounding cancellous bone during balloon expansion and reduce the bone cement leakage. In the other study, among the 219 patients of OVCF with single thoracolumbar fractures, 29 cases (13.2%) occurred non-surgical vertebral fractures in the PKP treatment [39]. Eleven patients (14.1%) in the early PVP group (n = 78) and 18 patients (39.1%) in the late PVP group (n = 46) experienced an adjacent vertebral fractures during the first year following PVP [40]. In this study, the secondary fracture rates of PVP and PKP were 7.1% and 10.7%, respectively, which were both lower than previous studies, and may be related to postoperative restriction of premature activity, thoracolumbar protection and continuous anti-osteoporosis treatment.
However, this study also had the following limitations. Despite the use of the blinded and random fashion, researchers failed to fully implement the principle of blindness. This was a prospective study with a small number of patients in each group. Furthermore, there is a lack of time biomechanical study on cement distribution in vertebrae to support the results. The current findings require further validation in multicenter, randomized, double-blind clinical trials.