Since 1984, Galiber et al. [14] applied PVP to treat a case of C2 vertebral invasive hemangioma, PVP has gradually become one of the effective methods to treat vertebral tumors and OVCFs due to its advantages of simple operation and definite effect [11, 15, 16]. Kümmell's disease is a special type of OVCFs. After minor trauma, vertebral collapse and kyphosis are gradually aggravated, which is related to vertebral ischemia and necrosis. Patients often suffer intractable lumbar and back pain, pseudarthrosis, and even nerve injury [17, 18]. Imaging examination shows IVC or localized fluid filling in the vertebral body [1, 3, 19]. IVC is mainly located in the thoracolumbar region, most of which are wedge fractures, and near the upper and lower endplates of the vertebral body [10, 20]. It suggests that the occurrence of the disease may be related to the repeated stress activity and high activity of thoracolumbar segments. In this study, patients with thoracolumbar segments were selected in this group. We found that the course of Kümmell's disease was significantly longer than the course of OVCFs. Kümmell's disease mostly has no obvious trauma, but OVCFs mostly has a history of trauma. The degree of osteoporosis and vertebral compression rate were more serious in Kümmell's disease than in OVCFs.
A small dose of bone cement can restore the mechanical properties of the fractured vertebrae, and the injection amount of bone cement has no significant correlation with the analgesic effect [21, 22]. 1.5 ml of bone cement injected into each vertebrae can obtain satisfactory analgesic effect [21, 22]. Biomechanical studies have confirmed that the strength of vertebral body can be restored by injecting about 2 ml bone cement and the stiffness of the vertebrae can be restored by injecting about 4 ml bone cement [22]. In this study, the average injection amount of bone cement in the K and O group were 4.2 ± 1.15 ml and 3.6 ± 1.35 ml, respectively. All of them met the requirements of restoring vertebral strength, and the K group met the requirements of restoring vertebral stiffness.
After filling the fissures with bone cement in the fractured vertebrae, the height of the vertebrae was partially restored and kyphosis deformity was partially corrected, and the abnormal activity of the fractured vertebrae was eliminated, which was an important reason for pain relief [2]. Previous studies have found that PVP can effectively relieve low back pain of Kümmell's disease and OVCFs, and partially restore vertebral height and correct kyphosis [2, 23–30]. The ODI and VAS scores in both Kümmell's disease and OVCFs groups decreased after PVP treatment, but the mean score in Kümmell's disease group was higher than that in OVCFs groups [31]. Our study is consistent with the above studies, and the ODI and VAS scores in both groups can be maintained until the last follow-up.
Kümmell's disease is caused by the presence of IVC and pseudarthrosis due to vertebral collapse, ischemia and necrosis. When the spine flexes and bends, the injured vertebra can stretch and widen the cracks in the fractured vertebra. The height of the collapsed vertebrae can be reduced and kyphosis can be partially corrected in posterior extension of spine. In PVP treatment, bone cement is usually limited to diffusion in the fissures, which can maintain the effect of extension correction, without the further help of balloon dilation in PKP treatment. Patients with Kümmell's disease can achieve spontaneous reduction in the posterior extension position without further balloon expansion reduction [32, 33]. Heo et al. [34] reported that excessive reduction tends to accelerate the process of vertebral ischemia and necrosis, leading to severe recollapse. Therefore, excessive reduction intraoperatively of the injured vertebra should be avoided. Our study found that the correction rates of vertebral compression and kyphosis after PVP were significantly corrected in both groups, but significant correction was achieved at each time point in the K group compared with the O group, further confirming the view that "spontaneous reduction can occur in Kümmell's patients with posterior extension".
We found that the correction rates of vertebral compression rate and kyphosis in the two groups gradually decreased with time, suggesting that the vertebral height and kyphosis angle gradually lost after surgery, which was consistent with previous findings [16, 35]. In the treatment of OVCFs by PVP and PKP, the bone cement injected during the operation 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 leads to recollapse [36]. In this study, it was found that local solid lump distribution pattern of bone cement was dominant in the K group (65.71%), while embedded trabeculae distribution pattern was dominant in the O group (74.29%). In the two groups, two years after the surgery, re-collapse and intervertebral height loss occurred in the enhanced vertebra, and the loss in the K group was more significant than that in the O group. As a "reservoir" of IVC region, bone cement was filled in the IVC region in the form of solid mass. The limited bone cement mass cannot connect with the adjacent endplates of the upper and lower levels to strengthen cancellous bone of the vertebrae, thus failing to support the normal physiological stress from the body and prone to collapse again [37].
The most common complications of PVP are bone cement leakage and adjacent vertebral fracture [38, 39]. Krauss et al. [40] reported that in OVCFs treated with IVC by PVP, the bone cement leakage rate was 18.2%. Wang et al. [41] reported that for OVCFs treated with IVC by PKP, the bone cement leakage rate was 7.4%. This study found that the leakage rate of bone cement in the K and O groups were 14.29% and 8.57%, respectively, which may be related to accurate preoperative surgical approach measurement, careful intraoperative operation and not pursue the maximum amount of bone cement. There was no significant difference in cement leakage rate between the two groups, which was consistent with previous research results [42, 43]. Among the 219 patients with single thoracolumbar OVCFs, 29 cases(13.22%) occurred non-surgical vertebral fractures [44]. Eleven patients (14.1%) of OVCFs 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 [45]. In this study, the new adjacent vertebral fractures of the K group and O group were 17.14% and 11.43%, respectively, suggesting that postoperative restriction of premature activity, lumbar protection and sustained anti-osteoporosis treatment are very important for the prevention of re-fractures.
This study has some limitations. Firstly, the number of cases is small, which needs to be further analyzed and clarified by expanding sample size. Moreover, there is a lack of time biomechanical study on cement distribution in vertebrae to support the results. Patients and data collection researchers were not completely randomized double-blind. So, the current findings require further validation in multicenter clinical trials.