Percutaneous vertebroplasty is one of the most commonly used methods for the treatment of osteoporotic vertebral compression fractures, but there are still complications, including cement leakage, adjacent vertebral fractures, and inadequate postoperative pain relief. Postoperative residual back pain greatly decreased patient satisfaction with surgery and significantly affected the quality of the patients' daily life.
Several studies have reported the postoperative residual back pain after PVP. Barr et al. performed a retrospective analysis of 38 patients with 70 vertebrae treated with PVP and reported significant pain relief in 24 patients (63%), moderate pain relief in 12 patients (32%), and inadequate pain relief in 2 patients (5%). Dohm et al. compared PVP and PKP for osteoporotic vertebral compression fractures, reporting procedural pain (12/191 vs 9/190) and back pain (14/191 vs 28/190) as the most common adverse events at 30 days postoperatively. We found that the incidence of residual back pain at 30 days after PVP was 13.81%(37/268), which is in line with the results of past findings[15-17]. We selected one month as our post-operation study period, which aims to reduce the variation in risk factors associated with residual back pain due to the follow-up time varies. Multiple previous studies have shown that low bone mineral density, bone cement volume, bone cement distribution, preoperative posterior fascia oedema, and IVC may be associated with residual back pain after PVP[5, 7-10, 18]. In contrast, our findings found that bone cement distribution, preoperative posterior fascia oedema, and IVC were independent risk factors for residual back pain after PVP, which wa consistent with those reported in previous studies.
In our study, we found that bone mineral density and bone cement volume were not risk factors for residual back pain after PVP. Osteoporosis is an important factor in the occurrence of vertebral fractures, as well as a risk factor for the collapse of fractured vertebral bodies after PVP and even secondary fractures of adjacent vertebral bodies. When multiple vertebral bodies collapse after surgery, the sagittal balance of the patient changes and the body compensates for the increased damage to the low back muscles in order to regulate the balance, leading to chronic low back pain and the same is true for adjacent vertebral body fractures secondary to PVP[19, 20]. Therefore, low bone density is an important risk factor for residual back pain after PVP. However, the results of this study found that the preoperative bone mineral density was lower than -2.5 SD in both groups, but the difference was not statistically significant. Analysis of the reason may be related to our time point of 1 month after PVP surgery. Collapse of the fractured vertebrae was not significant at the observation point of 1 month postoperatively and was much less likely to occur consecutively in multiple vertebrae. An important factor contributing to residual back pain after PVP is primarily sagittal imbalance, and patients do not experience significant sagittal imbalance 1 month after PVP[7, 21]. Therefore, low bone density should probably be a risk factor for long-term residual back pain after PVP, rather than short-term (1 month postoperatively).
The injection of bone cement can enhance the vertebral strength and stiffness and effectively prevent the progressive vertebral collapse, thus relieving the back pain to some extent[22, 23]. However, related studies have shown that there is no significant correlation between the amount of bone cement injected and the degree of pain relief[18, 24, 25]. The possible reason is that small doses of bone cement are already enough to enhance the strength of fractured vertebrae. A biomechanical study has found that stiffness of the fractured vertebrae can be restored when the volume of bone cement reaches approximately 15% of the vertebral body. Few significant benefits have been shown when the volume of cement used exceeds 24% of the vertebral body, at which point effective pain relief is already achieved[27, 28]. The results of some studies have shown that satisfactory pain relief can be achieved by 1.5 ml bone cement[24, 25]. The results of the present study are consistent with the results of previous studies that the amount of bone cement is not a risk factor for residual back pain after PVP. The possible reason for this is that the average bone cement injection volume in this study was greater than 3 ml, which is already the minimum dose for pain relief. We found that the rate of blocky bone cement distribution was higher in group R than group N, and multivariate logistic regression analysis showed that blocky cement distribution was an independent risk predictor of residual back pain after PVP. The spongy bone cement distribution can achieve tight binding between bone cement and cancellous bone, effectively reducing the risk of postoperative re-collapse of the injured vertebra[29, 30]. Meanwhile, more adequate bone cement distribution can effectively fix the fracture fragments, thus better relieving back pain[31, 32].
Although the majority of patients with OVCFs are due to low-energy injury, there are still 18 patients with posterior fascia oedema. A prospective observational study by Yan et al. found that the VAS and ODI scores were significantly lower in patients without posterior fascia oedema than in those with posterior fascia oedema. And a previous retrospective study has confirmed the correlation between posterior fascia oedema and residual back pain after PVP[7, 8, 33]. The results of our study are consistent with the above study that the posterior fascia oedema is an important risk factor for residual back pain after PVP.
Although, PVP or percutaneous kyphoplasty(PKP) has been recommended to treat patients with OVCFs with IVC and achieved an ideal outcome. However, some studies still report that the presence of preoperative IVC during long-term follow-up will more or less affect the clinical outcome after PVP, such as vertebral body re-collapse, severe over residual back pain, or even compression fractures of adjacent vertebrae[14, 35]. The presence of IVC sign in OVCFs has been reported to be a major risk factor for postoperative re-collapse of the injured vertebra, progressive kyphosis, and chronic back pain[34, 36, 37]. Some scholars have analyzed the possible reason for this is an insufficient amount of bone cement filling the fibrocartilage membrane around the IVC, which prevents a tight bonding between the cement and the surrounding cancellous bone, thus causing instability of the fractured vertebrae[36, 38]. Consistent with these studies, our results show that preoperative IVC is a risk factor for residual back pain after PVP.
Paraspinal muscle degeneration was not mentioned in previous studies as a risk factor for residual pain after PVP. However, the relationship between the paraspinal muscles and degenerative disc disease has become of great interest in recent years[12, 39, 40]. Several studies have found muscle atrophy of the paraspinal muscles in patients with lumbar spinal stenosis who have low back pain[41, 42]. Meanwhile, strengthening functional exercises of the back muscles has been shown to be effective in reducing nonspecific pain[43, 44]. Therefore, it can be speculated that there is a certain connection between paraspinal muscle degeneration and low back pain. The results of our study showed that the degree of paraspinal muscle degeneration was significantly higher in group R than group N, and multivariate regression analysis proved that paraspinal muscle degeneration was an independent risk factor for postoperative residual back pain. However, the Goutalier grade classification method was used to qualitatively assess paraspinal muscle degeneration in this study, and a more precise quantitative analysis of paraspinal muscle fat infiltration was lacking. Therefore, quantitative analysis of paraspinal muscle fat infiltration is needed in the future and may be more helpful in clarifying the risk factors for residual back pain after PVP. Nomogram is a graphical model in which the probability of the outcome can be calculated. It has been im-proved to be a feasible model in risk prediction. So, we depicted and validated the nomogram based on a postoperative imaging parameters. Our nomogram could provide a precise predictionability for the residual back pain with a c-index of 0.774(0.696~0.852), and was validated to be 0.752 through bootstrapping validation.
Limitations:This study currently has some limitations. First, our study is a retrospective study with a relatively small sample size, which may result in some bias.
Second, we selected one month as our post-operation study period, the follow-up period was too short, therefore it was difficult to identify more risk factor, including BMD values, infection, re-collapse of the injured vertebra and adjacent vertebral body fractures, et al.Thus, multicenter, large sample, and a long follow-up period studies are needed to further establish the risk factors for residual back pain after PVP in patients with OVCFs.What’s more, our nomogram had a good performance in internal validation, but it still needs to be assessed externally in wider populations.
Conclusion: Our findings suggest that the IVC sign, posterior fascia oedema, blocky cement distribution, and severe paraspinal muscle degeneration are important risk factors for residual back pain after PVP in patients with OVCFs. We provide a nomogram that accurately predicts the risk of residual back pain after PVP.