Nowadays PVP and PKP become more and more popular in the treatment of osteoporotic vertebral compression fractures (OVCF), osteolytic vertebral tumor[1–3]. Though they are minimally invasive procedure, complications still exist. According to the literature[2, 3, 7], cement leakage is the most common complication. Once the PMMA leaks into the paravertebral venous plexus, then migrate via segmental spinal veins, Inferior vena cava or azygos vein, eventually to pulmonary artery and induce the PCE[20–22].
PCE is a rare complication, most PCE cases during PVP or PKP procedure were reported in the form of case report[11–14]. In the previous literature, The incidence of PCE ranged from 0.3%-23% [14, 19, 23]. However, many scholars[1, 11, 14] still believe that the actual incidence of PCE has been ignored and underestimated due to lack of routine pulmonary imaging screening after operation. In our study, the incidence of PCE was 1.9% based on the postoperative pulmonary CT scan. We believe that in addition to postoperative imaging screening, there are other risk factors closely related to the occurrence of PCE.
There was no significant difference in age, gender and BMD between PCE group and non PCE group. Although theoretically speaking, the more bone cement is injected, the higher the probability of PCE will appear, but according to our statistics, the total bone cement filling amount of PCE group is not significantly increased than that of NPCE group (p = 0.068).
From the data of logistic regression analysis, when two vertebral segments were involved, compared with one segment, there was no significantly statistical difference in the occurrence of PCE (p = 0.560), but when three or more segments were involved, The significantly statistical difference was observed compared with one segment (p = 0.046). The more vertebral segments involved, the greater the probability of PCE. First of all, more involved segments suggest that more bone cement should be injected during the PVP or PKP procedure, and the probability of leakage is significantly increased. Secondly, long-time prone position leads to the decrease of cardiopulmonary compensatory function in elderly patients with osteoporosis, which can easily cause hemodynamic instability and increase the risk of surgery. Therefore, it’s not recommended that more than three vertebral segments were filled with bone cement at a time.
In terms of fracture location, thoracic vertebrae occupied 20 patients, thoracolumbar spine accounted for 13 patients, lumbar spine occupied only 1 patient, the risk of PCE gradually decreased from throcic spine to lumbar spine. This is mainly related to the anatomical characteristics of the spine. First of all, compared with lumbar vertebrae, thoracic vertebrae is more smaller and pedicle is more thinner, so surgical puncture is difficult and bone cement leakage is more common. Secondly, thoracic vertebrae is close to cardiopulmonary vessels. Once paravertebral cement leakage occurs, unpolymerized bone cement can quickly enter the heart, and then reach the pulmonary artery to cause PCE, while lumbar bone cement leakage needs to go through longer paravertebral vein or lumbar ascending vein to the inferior vena cava or azygos vein, sometimes bone cement have polymerized in the vertebral venous system before reaching the larger vein.
Our statistical results confirm that the operation timing of OVCFs is an independent risk factor for PCE. Although some studies have shown that early surgical treatment of OVCFs has advantages in relieving pain symptoms, restoring the height of fractured vertebral body and reducing the risk of long-term adjacent segment fracture[24], Guan et al.[25] found that both early (≤ 2 weeks) and delayed (> 2 weeks) operations of kyphoplasty can achieve satisfactory outcomes for OVCFs, but the risk of cement leakage during kyphoplasty will decrease obviously in delayed operation. Our study also suggests that if operation is performed more than 2 weeks after fracture, the risk of PCE will be significantly reduced. After osteoporotic fracture, the integrity of basivertebral veins is damaged. If PVP or PKP is performed early, bone cement will easily enter the injured basivertebral veins and its connections, then cause paravertebral vascular embolism or even PCE.
In PVP group, the incidence of PCE was 3.30% (28/846), while in PKP group, the incidence of PCE was only 0.67% (6/902). As an improved surgical procedure, PKP have been widely mentioned in the previous literature[25–28]. The balloon inflation can not only restore the height of the vertebral body to a certain extent, but also compact the surrounding cancellous bone in the process of balloon expansion, form a cavity in the vertebral body, reduce the perfusion pressure of bone cement, seal the osseous defects and make the leakage rate of bone cement significantly lower than that of PVP.
In addition to the risk factors mentioned above, the other operation details may also be the potential risk factors of PCE that we can not ignore, including the viscosity of bone cement and the amount of cement per single time. Although the thin cement is conducive to the dispersion in the vertebral body, it is also easy to cause the leakage of cement. At present, it is generally recommended to use cement in toothpaste-like state[17, 21, 29], which can not only give consideration to dispersion, but also have relatively lower leakage of cement. Although in our study, there was no significant difference in the total injected amount of cement between the PCE group and the NPCE group, it is suggested that the total amount of cement should be divided into multiple injections under X-ray fluoroscopy during the operation. It is recommended that the single injected volume should not exceed 0.3ml, so that even if there is leakage of cement, patients will not have serious complications. In order to prevent the uninjected cement from coagulating at room temperature, it can be stored in iced water to prolong its coagulation time. Perhaps it is because of these operational details that the incidence of PCE in our study is relatively low, and there is no fatal pulmonary embolism.
At present, there is no standard guideline for the treatment of PCE patients[1, 17, 29, 30]. Optimal treatment depends on the severity of symptoms and the location of the cement embolus. For PCE patients with severe symptoms, most scholars still recommend surgical removal of embolus, followed by anticoagulant therapy[17, 21]. For patients with mild symptoms, anticoagulation and observation are the main method. However, there is no consensus on the specific anticoagulant time. Janssen et al.[17] suggests that patients with mild or asymptomatic PCE were all treated with anticoagulation for at least half a year. In our study, there were 34 patients with PCE, and only 6 patients had discomfort. After supportive treatment, we continued to give oral anticoagulant (rivaroxaban) for 1 month. If the patient had no any discomfort, the anticoagulant drugs would be stopped. Other 28 asymptomatic PCE patients were found 1 month after operation when they returned to the hospital for intravenous zoledronic acid to delay the progress of osteoporosis, no further anticoagulants were given due to no any discomfort. Up to now, there are no patients who need to return to the hospital for further treatment due to PCE. The specific anticoagulant time of PCE is still questionable[17, 28].
Our study also has its own limitations. First of all, this is a retrospective study, and there is a certain selective bias. We only retrospectively analyzed the patients who underwent pulmonary CT examination after surgery, and the detection time of postoperative CT varies according to whether the patients have discomfort or not. Secondly, because the OVCF patients often have calcification of cardiopulmonary vessels, sometimes it is difficult to distinguish them from bone cement embolism images under CT scan, which has a certain impact on our diagnosis. If pulmonary CT scan can be performed before operation, the influence of vascular calcification can be eliminated by preoperative and postoperative comparison.