Screening risk factors for residual low back pain after PVP
Approximately 5.4% of OVCF patients in this study had residual low back pain after PVP, which is lower than that reported by Gaughen and Sayed; these differences may be due to the different implementation standards of residual low back pain after PVP. Our study is based on the fact that when the VAS score was>3, patients needed oral analgesics to sleep; otherwise, it seriously affected quality of life. Therefore, a VAS score > 3 was used as the threshold to assess residual low back pain after PVP. We analyzed the current literature on residual low back pain after PVP and found that many factors affect residual low back pain after PVP. The main factors include preoperative TFI, cement leakage, postoperative vertebral infection, recurrent vertebral fracture, secondary fracture of an adjacent vertebral body, amount of bone cement injected, poor distribution of bone cement, the number of responsible vertebral bodies, increased intravertebral pressure and an inflammatory reaction caused by cement aggregation[13-16]. In clinical practice, we have found that most OVCF patients do not accept standard conservative treatment, eventually leading to nonunion or delayed union of the vertebrae and show signs of intravertebral liquefaction on MRI; thus, such patients usually have residual low back pain after surgery. Of course, this difference may also be related to the difficulty of surgery, the amount of intraoperative bone cement injection, the operation time, etc. Therefore, in this study, we analyzed whether a liquefaction signal on MRI of the preoperative vertebrae affected residual low back pain after PVP. Because factors such as pain or spinal cord injury in the nerve root innervation area, increased intravertebral pressure, and inflammatory reaction caused by cement aggregation are difficult to quantify, we excluded these factors from statistical analysis. In addition, the longer the follow-up time was, the more complex the factors that affected postoperative pain, and the greater the difference. To avoid potential differences caused by different follow-up times, we included only patients with residual low back pain in the early postoperative period (the second day to 1 month after PVP). Postoperative vertebral infection, secondary vertebral fracture, nonunion of bone cement and contact surface fracture occur approximately 3 months after surgery and do not belong to the category of early residual low back pain after surgery. Finally, we included age, sex, BMI, course of disease, preoperative BMD, preoperative VAS score, TFI, a liquefaction signal on MRI of the affected vertebrae, the number of responsible vertebral bodies, surgical approach, operation time, the volume of single vertebral bone cement injection, and the distribution of bone cement in the univariate analysis. The above factors were analyzed, and we found significant differences in preoperative TFI, a liquefaction signal on MRI of the affected vertebrae, the number of responsible vertebrae and the distribution of bone cement (Table 1, P < 0.05).
Effect of TFI on residual low back pain after PVP
Yan et al. reported an association between TFI and residual low back pain after PVP. They also showed that the VAS score of the low back in patients with TFI was significantly lower than that in patients without TFI. However, our study reached the opposite conclusion, which indicated that TFI was a high-risk factor for residual low back pain after PVP, with an OR of 5.378. We believe this finding may be explained by the following reasons. First, after osteoporotic thoracolumbar fracture, especially one caused by trauma, and after fracture because of bone-derived pain, TFI immediately exists but may be masked by bone-derived pain, which is also why the patient fell on his/her buttock and landing conscious with intense thoracolumbar pain. However, when PVP was performed, patients felt more lumbosacral pain. The main reason for this observation is that the injection of bone cement in a short time makes bone-derived pain disappear or significantly weaken, while pain caused by TFI via trauma appears more prominent at this time. However, it is worth noting that with prolonged follow-up time, MRI hyperintensity resembling lumbar and dorsal soft tissue injury will gradually subside, mainly because the self-repair of local soft tissue alleviates inflammatory reactions such as edema. This also suggests that low back pain due to TFI can be improved by conservative treatment and explains why the difference in pain between the two groups occurred only in the early postoperative period.
Effect of a liquefaction signal on MRI on residual low back pain after PVP
Because OVCF is classified as a fragile fracture, most patients have no obvious history of trauma and differences in the regional level of diagnosis. Some patients have no obvious clinical symptoms after onset and usually miss the best time for diagnosis and treatment (resulting in a missed diagnosis and/or misdiagnosis), and some patients have a liquefaction signal on vertebral MRI when confirmed. These phenomenon may be explained by the failure to undergo timely and correct treatment after the fracture, resulting in delayed or nonunion of the fracture, coupled with the pathological basis of osteoporosis, trabecular bone resorption at the fracture end, and liquefaction signals in the fracture area that mimic those observed in Kümmell's disease[18, 19]. When PVP is performed in such patients, it is usually difficult for the cement to diffuse. In most cases, the cement is confined to the liquefied area of the fracture and distributed in a mass. The presence of liquefaction signals on MRI of the affected vertebrae may effect cement leakage, cement distribution, and cement volume, eventually leading to residual low back pain after surgery. This study found that a liquefaction signal on vertebral MRI was a risk factor for postoperative residual low back pain, with an OR of 6.111, and should be considered. Of course, although there was no significant difference in the course of disease between the two groups, some patients could not accurately describe the time of the initial injury or fracture. Theoretically speaking, the presence of a liquefaction signal on MRI of the affected vertebrae generally indicates that the patient has a long disease course. Whether the disease duration is related to a liquefaction signal on MRI still needs further study.
Effect of the number of fractured vertebral bodies on residual low back pain after PVP
The more fractured the vertebrae, the more collapsed the vertebrae are; this can easily lead to kyphosis and secondary sagittal imbalance. For a long period of sagittal imbalance of the spine, patients will compensate for the increasing thoracic kyphosis deformity by flexing the hip and increasing pelvic retroversion and the lumbar lordosis angle, thus maintaining the balance of the spine in the sagittal plane. This nonphysiological compensation can easily lead to thoracodorsal muscle strain and intermittent low back pain. With late sagittal decompensation of the spine, there will eventually be persistent low back pain, which may be one of the causes of residual low back pain after PVP in multiple vertebral bodies. In addition, the patients included in this study had multiple vertebral bodies with osteoporotic compression fractures and underwent PVP surgery at the same time. Excessive puncture in the lumbar and dorsal surgical area may lead to local soft tissue injury, and certain patients may experience local hematoma formation or increased hidden blood loss, which ultimately leads to lumbar and dorsal pain shortly after surgery. However, with the prolonged follow-up time, the soft tissues in the surgical area were gradually repaired, and residual low back pain was relieved.
Effect of the distribution pattern of bone cement on residual low back pain after PVP
The distribution of cement depends on the degree of fracture, the course of the fracture line and the surgical method. In general, it is easy to obtain a satisfactory distribution of cement by injecting cement via a bilateral pedicle puncture. However, this study showed that a unilateral or bilateral puncture does not affect the clinical outcome, but obtaining satisfactory cement distribution can effectively reduce the occurrence of postoperative residual low back pain. This study also found that the OR of the distribution pattern of bone cement was less than 1, which explains the satisfactory distribution pattern of bone cement and indicates its role as a protective factor in the relief of low back pain. For patients with poor cement distribution and residual low back pain, He et al.and Gaughen et al. used secondary bone cement injection after PVP, which effectively alleviated residual low back pain.
Deficiencies of this study
Residual low back pain after PVP in OVCF patients is affected by many factors. Only patients with residual low back pain occurring within 1 month after surgery were included in this study, and the relevant factors during this period were analyzed. Late- and midterm influencing factors, such as delayed union, nonunion and secondary adjacent vertebral fractures, were excluded. These factors tend to become more important after 3 months. As a result, the risk of related factors in the early postoperative period was gradually weakened. This is a shortcoming of the present study, as was the lack of an analysis of long-term risk factors for residual low back pain after PVP.