Our study reported that using decompressed kyphoplasty resulted in a lower rate of intradiscal cement leakage, but did not lead to a lower adjacent segment fracture rate. Intradiscal cement leakage is highly associated with the absence of a radiographic cleft sign when using high viscosity cement.
Both vertebroplasty or kyphoplasty were used to treat osteoporotic compression fractures for decades, with good results27. During performance of the procedure, cement leakage is a common complication, and can cause devastating consequences. According to previous studies, the leakage rate is heavily underestimated, and may be higher than 70%9. Preventing cement leakage has always been an important issue. Recently, high-viscosity cement was applied to treat compression fractures, in an attempt to reduce the cement leakage rate16,28. In this present work, the leakage rate using high viscosity cement is 18.6%, which is lower than previous reports regarding traditional vertebroplasty. However, we observed a significantly higher intradiscal leakage rate in the high viscosity cement group as compared to the decompressed kyphoplasty technique25. This phenomenon can be explained by the different cement driving forces of the two techniques. Cement in decompressed kyphoplasty follows the guide of the pressure gradient created by a suction force25. Once the balloon is inflated and creates a cavity in the vertebral body, it forms a corridor for the cement to reside in. After applying negative pressure from one trocar, cement installed from another trocar was drawn to the contralateral side passively under the guidance of the negative pressure. Conversely, the distribution of the high-viscosity cement is mainly dependent on active pressure provided manually by the surgeon. The cement often diffused profoundly from the tip of the trocar, and aggregated at the site with lowest resistance29. When the endplate defect existed, the cement can easily flow into the relative empty disc space 30. This phenomenon might be more evident with the high-viscosity cement, which has a poorer ability to penetrate into cancellous bone31, causing a higher risk of intradiscal leakage. We also discovered an interesting finding in the subgroup analysis, in that the intradiscal leakage is highly correlated to whether the “cleft sign” is observed on x-ray. The risk of intradiscal leakage is higher when the pre-operative radiography does not have a “cleft sign” visible (p = 0.02). This “cleft sign” or “vacuum phenomenon” represented a pre-existing space inside the vertebral body, either occupied by gas or fluid, indicating a non-healing vertebral collapse or pseudoarthrosis32. This space could be enlarged when the patient was put into a prone position and postural reduction was performed, forming an ideal space for cement to fill 33. The risk of intradiscal leakage is lower when this space presented. We speculate that it is because the cement can accumulate in the space inside vertebral body, instead of infiltrated randomly out of fracture fissures. In our study, all nine patients with intradiscal leakage occurred on the fractures which did not demonstrate a pre-existing vacuum phenomenon, with a concomitant upper endplate fracture. This suggests that patients without the vacuum phenomenon on their pre-operative x-ray are under a higher risk of intradiscal disk leakage. Historically, the relationship between the cleft sign and cement leakage in vertebroplasty had been widely discussed. Some studies indicated that a cleft sign did not affect the risk of cement leakage, but rather influencing the leakage pattern34,35. Other studies revealed a significantly lower cement leakage rate in conjunction with the presence of a cleft sign 36,37. There were also papers which demonstrated the opposite results, where the cleft sign may be a risk factor for increased cement leakage38. Currently, the meta-analysis of Zhan et al reviewed the evidence regarding cleft sign and concluded that the phenomenon is a risk factor for cement leakage39. This result is contrary to our findings for high viscosity cement vertebroplasty. Since the technique for vertebroplasty is similar, we presumed that unique physical properties of high-viscosity cement may lead to a different influence of a cleft sign with vertebroplasty.
Currently, the long-term effect of intradiscal cement leakage is still not well known. This type of cement leakage does not lead to neurologic injury, but it raises the concern of adjacent segment fracture because of uneven pressure distribution40. While some studies reported that the leakage did not induce adjacent vertebral body collapse41,42, others obtained opposite results43. In the meantime, many studies revealed that bone cement may be toxic to the intervertebral disc and that it accelerates degeneration44–46. In our study, we did not observe a higher rate of adjacent segment fracture for the high-viscosity cement group, despite its higher intradiscal leakage rate. However, considering bone cement's potential damage to disc tissue, we should put all effort into preventing this complication, including the selection of an appropriate technique for patients without a cleft sign on pre-operative x-rays.
The study was limited by its retrospective nature, and a well-designed prospective randomized controlled trial is necessary in order to confirm our conclusions. The patients were evaluated with plain-film radiography following their surgery, which may under-estimated the leakage rate. For adjacent segment fracture, the patients only underwent a 12 month follow-up. Although previous studies reported that adjacent segment fracture following vertebroplasty or kyphoplasty often happened within three months of the procedure47–49, a longer follow-up period may be required in order to confirm the long-term effect of the different bone cements.