OVCF is commonly seen in elderly patients. In a multicenter prospective study involving 2451 elderly women, 32% of the patients had at least one vertebral compression fracture , which may occur with or without slight trauma. The bone cement can restore the strength of the fractured vertebral body and produce thermal necrosis effect on the pain nerves in the vertebral body. It is an effective way to treat OVCF [4,5] and affected by many factors such as the patient's BMD, the volume and distribution of bone cement . Biomechanical tests have shown that restoration of strength and stiffness required vertebral body cement fills of 16.2% and 29.8%, respectively . It is not only the volume of bone cement, but also the distribution of bone cement in the vertebral body has an important influence on the effect of operation and the long-term maintenance of the vertebral body height.
Some literatures classified the bone cement distribution according to its diffusion on anteroposterior X-ray film, and then their influence on the long-term outcome was studied . However, this classification method was mostly applicable for unilateral puncture cases, because bilateral puncture injection of bone cement can often achieve a uniform distribution on both sides of the vertebral body, effectively avoiding the uneven stress caused by asymmetric distribution on the coronal plane. Many previous literatures have also confirmed that bilateral puncture does not significantly increase the risk of complications such as cement leakage and nerve injury as long as the puncture route is strictly followed [9,10]. Therefore, studying the distribution of bone cement in the sagittal plane on lateral radiographs may be more meaningful for the analysis of surgical efficacy.
Our study found that if bone cement can fully contact with the upper and lower endplates, it can better maintain the height of the vertebral body and reduce the risk of vertebral recompression. In our study, the surgical vertebral recompression rate in Group A was 5.2%, which was significantly lower than that in group B (16.5%). Previous literatures reported a surgical vertebral recompression rate of 3.2%~27.6% with different criteria of recompression and follow-up time [11,12,13]. This kind of surgical vertebral recompression is multifactorial, often without a clear traumatic event, may be related to the degree of osteoporosis, daily activities and the distribution of bone cement . Insufficient filling of bone cement is an important cause of recompression, especially the uneven distribution in the sagittal plane . If the cement is in full contact with the upper and lower endplates, it will fill the whole vertebral body and play a full role of "bonding" for the cancellous bone and the endplate, which can better restore the strength of the vertebral body . When the cement only touches the upper or lower endplates, vertebral strength only increases about 2 times. If the cement touches the upper and lower endplates at the same time, it will prompt 8-12 times and significantly improve the stress transmission . In Kim’s study, 46.7% (7/15) of the patients have a recompression at an average of 3.4 months post operation if the cement had no contact with the endplate . Our finding is consistent with Liang, after an average follow-up of 29.6 months, 8.16% or 37.4% of the patients will have surgical vertebral recompression if the cement is in full contact with the upper and lower endplates or not in their study .
In subgroup analysis, we found the recompression rate was higher in group B3 when bone cement contacted none of the endplate, but there is no statistical difference, which may due to the relatively small sample size. The upper and lower endplates are equally important. The lack of contact between the bone cement and any endplate will result in an unfilled vulnerable area, where recompression usually happened.
In our study, regardless of the type of cement distribution, it has obvious benefits for short term pain relief and recovery of vertebral height. And this is consistent with previous clinical experience . However, in the long-term follow up, the degree of pain in group A were significantly lower than group B. Ye revealed insufficient filling of bone cement was associated with chronic lower back pain . Recompression may lead to changes in spinal balance, local kyphosis, and consequently chronic pain. Especially when there is a fracture in vertebral endplate, if the cement is not well connected with the endplate, it will provide insufficient support and lead to the continuous compression of the fracture vertebral body, which is the reason for the persistence of postoperative pain .He found that the long-term effect of H-type distribution of bone cement is better than O-type distribution, which is related to the closer contact between bone cement, endplates and cancellous bone in H-type distribution . Our study and previous literatures show that if the bone cement is evenly distributed and closely contacted with the upper and lower endplates, it can better maintain the strength and height of the vertebral body, reduce recompression risk and eventually improve the patients’ chronic back pain.
Much care need to be taken to obtain a satisfactory distribution by increasing the bone cement volume. Bone cement volume is only weakly related to the effect of the operation . Biomechanical study found that the strength of injured vertebral body can be restored when the volume of bone cement reaches 15% of the vertebral body. If the injection continues, there is no significant benefit, and it may also cause asymmetric distribution of bone cement and excessive rigidity of the vertebral body . The volume of bone cement required for different segment to recover strength is different , but when the volume of bone cement reaches 24% of the vertebral body, it can effectively improve the pain of patients, no need to increase the volume . In our study, no significantly difference of bone cement volume was found between groups. Additionally, an increase in the cement volume may increase the risk of cement leakage [28,29,30]. Compared with percutaneous kyphoplasty (PKP), PVP may achieve better cement distribution. Loss of vertebral height was more likely after PKP than PVP [31, 32]. Because balloons squeeze cancellous bone around during expansion in PKP, creating a "cavity". Cement tends to distribute in this low-pressure cavity without infiltrating into the surrounding bone, making it difficult for cancellous bone to bond tightly, and this mass-like cement distribution has also been proved to be risk factor of recompression .
In addition, when puncturing bilaterally, the angle of puncture needle can be adjusted so that the two puncture needles point to the upper or lower endplate respectively , using high-viscosity cement , using hydraulic assistant device to inject bone cement slowly and uniformly may be more beneficial for a better distribution of bone cement . High-viscosity cement has the advantages of fast bonding with bone, long working time window and low polymerization temperature . A meta-analysis found that high-viscosity cement has significant advantages in pain improvement, recovery of cobb angle and cement leakage comparing with low-viscosity cement .
There are some shortcomings in this study, the retrospective study and relatively small sample size may produce some bias. Prospective studies with large numbers of cases are needed to further clarify the relationship between cement distribution and surgical outcome.