As society ages, the incidence of osteoporosis is increasing, with decreased bone strength increasing susceptibility to fracture (1). In 2000, there were 1.4 million vertebral fractures worldwide, however, it is projected that the number of cases will reach 3 million by 2050 in China alone (2). It usually lacks a clear trauma history or occurs after low-energy injury neglected by patients, resulting in persistent severe pain, local vertebral kyphosis, respiratory dysfunction, increased risk and mortality of new fractures, and severe decline in quality of life (3). Currently, the clinical treatment of OVCF is categorized into conservative and surgical treatments (4). Conservative treatment includes bed rest, or wearing a brace, along with medication such as painkillers and muscle relaxants. In contrast, minimally invasive surgery is typically used for surgical treatment, including percutaneous vertebroplasty (PVP) and percutaneous kyphoplasty (PKP). By injecting polymethylmethacrylate (PMMA) bone cement, vertebral bone strength can be increased and mechanical stability can be achieved by increasing the space between vertebrae, thus providing immediate pain relief (5). Compared to conservative treatment, minimally invasive surgery has advantages in improving patients' quality of life and prolonging survival time (6). Nonetheless, some patients still suffer from residual back pain after PVP and PKP, which hinders early ambulation and functional recovery due to moderate or severe pain (7, 8).

Currently, the risk factors for RBP after PVP have been well-studied, which include low bone mineral density (BMD), presence of intravertebral cleft, paravertebral muscle degeneration, sarcopenia, multilevel OVCFs, unsatisfactory bone cement distribution, insufficient bone cement filling, unrecoverable vertebral height, large pelvic angle of C7 vertical sagittal axis (SVA) T1 (TPA), and lumbar lordosis (LL)-pelvic incidence (PI) mismatch, etc (9–12). However, few studies have described risk factors for RBP after PKP. In the clinical setting, many patients have high expectations for their prognosis after minimally invasive surgery. Once RBP occurs in the postoperative period, patient dissatisfaction or hostility can easily be aroused. Meanwhile, there is a lack of early and accurate prediction and diagnosis of postoperative RBP, relying mainly on patient self-reporting during postoperative rounds. Therefore, it is crucial to identify risk factors for postoperative RBP in patients with OVCF. On the one hand, it allows for early intervention in patients with RBP to improve prognosis. On the other hand, relatively accurate prediction enables clinicians to identify high-risk patients and control their expectations. In this study, we hope to establish a nomogram model that can predict residual back pain after PKP by comprehensively analyzing the risk factors, the diagnostic effects of the independent risk factors, and their respective and comorbid factors, so as to provide a basis and guidance for effective clinical prevention and treatment.

The aging population has made OVCF a significant global health issue. Approximately 20% of the world's population is over the age of 70, and at least one in five patients over the age of 50 has one or more vertebral fractures (14, 15). PKP, a modified technique based on PVP, corrects kyphosis caused by vertebral collapse with a balloon. Compared to PVP, PKP contributes to pain relief, restoration of vertebral height, reduction of the average difference in kyphosis wedge angle, and risk of cement leakage (16, 17). Although it has the advantages of small trauma, fast pain relief, early mobilization, and effectively improving the prognosis of patients, some patients still experience postoperative RBP (18). RBP is the most common complication of PKP, with approximately 9%–35% of patients having residual pain after treatment, which is a significant concern for patients (19, 20). However, insufficient attention has been placed on RBP after PKP, with few relevant studies addressing this issue. This study identified several independent risk factors for postoperative RBP in OVCF patients, including young age, high BMD,trauma history, low back fascia edema, high PDW, low serum chloride levels, and no recovery of middle vertebral height.

Previous studies have suggested no statistical difference in the age of patients between the postoperative pain and pain-free groups, indicating that age is not a risk factor for the occurrence of RBP (7). However, this study obtained different results where the younger the age, the higher incidence of RBP in this sample of people over 50 years old. Chen et al. (21) reported that low BMD was an independent risk factor for RBP after PKP. Conversely, the results of this study showed that the incidence of RBP was higher when BMD was 2.5 ≤ |t| ≤ 3.5, while patients with preoperative |t| > 3.5 were less likely to experience RBP after PKP. For this difference in results, we consider that patients with low BMD have more severe osteoporosis where low energy injuries can cause vertebral compression fractures, while patients with higher BMD have relatively less severe osteoporosis and require greater violence to cause fractures. Thus, patients with higher BMD may have experienced more severe injuries, increasing the likelihood of RBP after PKP.

Moreover, a clear trauma history is also an independent risk factor for RBP after PKP. Osteoporotic vertebral compression fractures usually occur in older adults. Impaired physical coordination, along with a decline in the strength of muscles, ligaments, and other structures, increases their susceptibility to trauma. At the same time, fractures often cause varying degrees of soft tissue damage. The pain from the preoperative fracture may overshadow this, but as the fracture heals, postoperative soft tissue pain becomes more pronounced, leading to RBP. According to Yan et al. (22), the prevalence of fasciopathy in patients with OVCF is as high as 42.1%. Our study found that posterior fascia edema is a strong risk factor (OR = 6.076) for residual pain after PKP. Other studies have also proposed fasciopathy may contribute to residual pain (23, 24). This may be due to the following reasons: (1) lumbar soft tissue injuries, such as superficial fascia and muscle, leads to local pain; (2) pain can be induced by fascial injury and soft tissue edema, which compresses the branches of the dorsal root of the spinal nerve and stimulation of inflammatory factors. PKP is used exclusively for the treatment of vertebral fractures and cannot relieve pain caused by fascia and soft tissue injury. However, it is challenging to determine whether fasciopathy exists preoperatively, with fracture pain masking its presence, or if the pain is a result of intraoperative injury radiating along the sacrospinal muscle.

This study analyzed the results of the preoperative laboratory findings of patients. A high PDW and a low serum chlorine value were independent risk factors for residual pain after PKP. PDW is a parameter that represents the variation of platelet volumes in the blood, which is used to express the degree of homogeneity of platelet. It is also a determinant of platelet activation and is considered a marker of inflammatory disease. During platelet activation, the release of various inflammatory cytokines leads to the formation of platelet clots and the formation of a thrombus (25). A decrease in complement Cl concentrations can enhance the inflammatory response of endothelial cells (26, 27). Cl- reduction is key to foam cell formation and inflammation (28). When the patient was in such a prethrombotic state of inflammatory intensity, an increase in inflammatory factors and poor blood circulation can increase pain. Such pain interferes with early functional exercise activity and reduces muscle strength, leading to further pain. PDW and serum chloride can be used as biomarkers to predict residual pain after PKP in patients with OVCF.

Our study found a statistical difference between the two groups regarding AVH restored and MVH restored, but only the middle height of the vertebral body not recovered was identified as an independent risk factor for the occurrence of residual pain. Denis' three-column theory suggests that the anterior and middle columns of the vertebrae bear 80% of the stress, and the middle column plays a key role in maintaining spinal stability. Balloon dilatation during PKP restores the middle height of the vertebrae, with sufficient cement diffusion, and the chemical and thermal neurolytic effects of PMMA effectively relieve pain at the fracture site (29, 30). The biomechanical environment is restored to equilibrium which brings about an improvement in the compliance of the soft tissues of the back to reduce the possibility of residual pain in the postoperative period (31). Conversely, if the middle vertebral height is not fully restored, the incidence of RBP can be increased. Furthermore, previous studies have shown that facet joint violations, a large number of fracture segments, fracture location, refractive fracture of adjacent or operated vertebral bodies, insufficient dispersion of bone cement in the fracture line area, leakage of bone cement, bone cement volume, inflammatory reaction or local ischemia caused by bone cement, and segmental kyphosis are also independent risk factors for residual pain (19, 32).

The ROC curve shows that the predictive ability of the combined model was stronger than that of each independent factor, indicating that a complete assessment of multiple factors is required to improve accuracy when predicting the risk of RBP. The study established a nomogram that can calculate the risk of RBP in OVCF patients after PKP, which can help to screen high-risk patients and provide a basis for early clinical prevention and treatment. Sample calculations for scores using different variables for an 80-year-old patient can be estimated as follows: patient with OVCF [score 0.95 × (100–80) = 19], lumbar BMD t of −2.0 (score 16.32), with a cleartrauma history (score 19.58), posterior fascia edema (score 44.21), and a measured PDW value of 12.80 [3.47 × (12.80–6) = 23.60], and with a serum chlorine value of 105 [score 2.55 × (115–105) = 25.5], if the height of 1/3 of the vertebral body is not recovered (score 35.68), the total score is indicated by the sum of the above scores (183.89) and the corresponding risk value is close to 80%. Therefore, the clinician should pay greater attention to the possibility of RBP after surgery and provide timely intervention.

This study analyzed the risk factors for RBP after PKP in patients with OVCF. The results showed that young age, high BMD,trauma history, posterior fascia edema, high PDW, low serum chlorine, and no recovery of middle vertebral height were independent risk factors for RBP after PKP in patients with OVCF, and the predictive value of combined factors was higher. Thus, a visual nomogram model was established to predict the risk of residual pain that is simple and easy to read. The nomogram model demonstrated strong predictive ability, good calibration, and high clinical applicability for predicting postoperative RBP in OVCF patients undergoing PKP. The model's practical utility is further supported by its performance in both internal and external validations and decision curve analysis. Therefore, this model can guide clinical risk assessment and take measures against high-risk factors for the timely prevention of RBP.

There are some limitations to this study. Firstly, this study was a retrospective case-control study using a visual analog scale in the medical chart and follow-up records to determine whether a patient experienced RBP, which may be subject to selection bias. Secondly, this study was a single-center study, and the model may not be generalizable to the settings outside the study site. Therefore, prospective studies with large samples, multicenter and long-term follow-up are still needed to verify the validity of the model.