In 1987, Galibert first used vertebroplasty to treat C2 vertebral hemangioma. Studies have been reported on PVP for OVCFs in 1988 and 1994, respectively. Since then, the technique has been developed and refined, and it has gradually become the main methods for treating OVCFs due to its simplicity and efficacy. PVP strengthens the vertebral body by injecting bone cement into the fractured vertebral body to restore the height, strength, and stiffness of the vertebral body, while correct the local kyphosis and produce a thermal effect on the nociceptive nerves around the vertebral body to rapidly relieve pain symptoms. However, this method is affected by many factors, such as BMD, the amount, distribution, and leakage of bone cement, etc.
A biomechanical study of 120 vertebrae from 10 osteoporotic female cadavers found, on average 16.2% and 29.8% of the vertebral cement filling is required to restore strength and stiffness, respectively, and it was no correlation between the recovery of vertebral strength and stiffness and the percentage volume of bone cement filling. Liebschner  et al. performed a single lumbar PVP finite element analysis study, which found that only a small amount of bone cement (14% volume) was required to restore the stiffness of the fractured vertebrae to pre-injury levels, and that a larger volume of bone cement did not provide greater benefit, as the increase in vertebral strength with bone cement resulted in asymmetric distribution of bone cement and strength imbalance on both sides of the vertebral body. Related studies also confirm the above-mentioned view [20, 21]. In addition, the correlation between bone cement volume and surgical outcome is small, and an increase in cement volume may increase the risk of cement leakage . Bone cement injection volume is a one-sided indicator of the benefit of bone cement and does not reflect the distribution of bone cement within the vertebral body. Therefore, it is important to study the distribution of bone cement and the clinical outcome and prognosis of PVP. However, few studies have reported the effect of bone cement distribution on radiographic and functional recovery after PVP treatment.
Compared to PVP, PKP results in poorer cement distribution and a greater likelihood of postoperative vertebral height loss . The main reason is that the expansion of the balloon compresses the more lax cancellous bone in the cone, thus creating a "cavity" at the balloon site, and the injected bone cement tends to be distributed in this low-pressure cavity without dispersing into the surrounding bone, making it difficult to bind tightly to the cancellous bone. Therefore, this blocky distribution of bone cement has been shown to be an important factor in vertebral body height loss. The subjects selected in this study were all post-PVP patients, excluding the influence of the surgical approach on the results.
A study by Chen  et al. reported that the symmetric distribution of bone cement is closely related to the stiffness of the vertebral body, and in unilateral vertebroplasty, the bone cement is often confined to the ipsilateral side of the vertebral body and cannot effectively diffuse across the midline; therefore, the end result can be a significant reduction in the stiffness of the vertebral body on the unreinforced side of the bone cement compared to the reinforced side. The biomechanical imbalance can exacerbate the pressure load on the spine, resulting in effects that are difficult to reverse, such as loss of height of the fractured vertebral body, disc degeneration in adjacent segments, and even fracture of the vertebral body in adjacent segments [25, 26]. Therefore, all subjects included in this study received bilateral arch root PVP surgery, further reducing the detrimental effects of asymmetric distribution of bone cement in the coronal plane. The majority of females in this study indicated that postmenopausal women are more likely to have osteoporosis in patients.
Spinal imaging changes such as vertebral body height and Cobb angle are often used as indicators to assess the efficacy of PVP. Yan et al. reported that PVP was able to restore vertebral body height and correct kyphosis. The results of the present study showed that PVP was able to significantly restore anterior vertebral body height and reduce the Cobb angle without considering the cement distribution, and if the cement was spongy in the vertebral body, it could better maintain the height of the vertebral body and reduce the risk of postoperative vertebral body height loss as well as local kyphosis. There are numerous factors that contribute to enhanced postoperative vertebral body height loss and increased kyphosis that do not require exposure to a traumatic event and may be related to the severity of osteoporosis, daily activity level, and cement distribution. However, bone cement distribution is an important factor contributing to vertebral height loss. He et al. reported that the incidence of vertebral height loss was higher in the uninterlocked solid pattern of bone cement distribution than in the interlocked solid pattern. Furthermore, Yu et al. also confirmed that the comparatively diffused pattern of bone cement distribution has better medium and long-term clinical outcomes, compared to the solid lump distribution pattern. The spongy bone cement distribution allows cancellous bone and bone cement to more fully interlock and increases vertebral strength and stiffness with greater homogenization, thus reducing the risk of vertebral height loss after PVP. Our study also found a loss of vertebral body height and local kyphosis over time in the blocky and spongy group postoperatively, which is consistent with previous findings[30, 31]. In contrast, the blocky group showed more pronounced changes on imaging than the spongy group, which was related to the distribution of bone cement.
Our study is consistent with previous studies reporting[32, 33] that PVP significantly relieves short-term pain and restores function, regardless of the bone cement distribution pattern. In long-term follow-up, it was also found that VAS and ODI scores were significantly lower in the spongy group than in the blocky group, suggesting that spongy bone cement distribution has better analgesic and functional recovery effects. The cause of persistent lower back pain is mainly related to insufficient filling of the bone cement. The bone cement does not bind effectively to the fractured vertebrae, and the low strength and stiffness of the vertebrae are not sufficient to provide effective support, resulting in a continuous loss of height[35, 36]. Therefore, we speculate that the spongy distribution of the bone cement allows greater contact with the cancellous bone within the vertebral body, which can adequately immobilize the fractured fragment, increasing spinal stability and reducing micromovement of the trabeculae, thus reducing pain and achieving functional recovery[34, 37].
Liebschner et al. reported that the recovery of vertebral body strength was closely related to the distribution of bone cement. The strength and stiffness of the vertebral body after bone cement strengthening are significantly higher than the adjacent vertebrae, and the inhomogeneous distribution of bone cement makes the strength and stiffness of the vertebral body asymmetrical in all areas, and all of these factors tend to increase the risk of fracture of the adjacent vertebral body after PVP[24, 38, 39]. Therefore, numerous studies have confirmed that homogeneous distribution of bone cement within cancellous bone can reduce stress concentration and thus reduce the risk of fracture in adjacent vertebrae[29, 40]. The spongy group has a spongy and homogeneous distribution of bone cement, which can fill the cancellous bone better and reduce the concentrated stress between adjacent vertebrae. Our results confirmed that the incidence of adjacent vertebral fractures was significantly lower in the blocky group than spongy. Therefore, achieving good distribution of bone cement within cancellous bone is crucial in reducing the risk of adjacent vertebral fractures.
There is an association between the distribution of bone cement and cement leakage. The overall bone cement leakage rate in our findings was consistent with the results reported in previous studies, and most patients were clinically asymptomatic. The rate of bone cement leakage was lower in the blocky group than spongy, but the difference was not statistically significant. This may be due to the diffuse distribution of the spongy cement, which is more widely distributed than the blocky group and more likely to leak through the broken bone cortex or endplate to the intervertebral disc or paravertebral area. Therefore, the surgeon should carefully analyze the imaging data preoperatively and should suspend the procedure in case of intraoperative cement leakage.
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, the grouping method in our study differs from previous studies, in which it may lead to subjective bias in the results. Therefore, multicenter, prospective studies with larger samples are needed to further elucidate the relationship between intravertebral bone cement distribution and clinical outcomes of PVP.