Osteoporotic vertebral compression fracture is a common spinal injury which the elderly patients are more likely to get the disease, PKP has a significant effect of small trauma and rapid pain relief and has now become an important method for the treatment of OVCF [12]. These days with the advancement of technology and the skilled operation of doctors, the leakage rate of bone cement is getting less and less, but it is inevitable. Once the bone cement leaks, the bone cement leakage around the vertebral body can stimulate the sympathetic nerves, the leakage in the spinal canal can compress the nerves, the intervertebral disc leakage can accelerate the degeneration of the intervertebral disc in the adjacent vertebral body segments, and the leakage of bone cement into the lungs may be life-threatening. Although more and more studies have been done on bone cement leakage, little has been said about endplate and disc leakage.
Previously, some scholars who mainly study the influence of the classification of bone cement leakage on the treatment effect [13–16]. However, we can see that the classification criteria used in different studies are not the same and the conclusions are still unable to reach a consensus accounting for the bone cement leaking into the intervertebral space has different anatomical distributions and its different dosages, directions, solidification.
In this paper, a new classification based on the nutritional relationship between the intervertebral disc and the upper and lower endplates was developed to explore the impact of bone cement leakage on patient outcomes and intervertebral discs. Adequate bone cement contact with the unilateral endplate can provide good immediate analgesia, but there may be no specific discomfort in the short term once leakage occurs. However, under the action of long-term leakage, bone cement may play a certain role in the degeneration of the intervertebral disc [17]. On the one hand, the force of the vertebral body is uneven, and on the other hand, the nutrient supply relationship between the bilateral endplates to the intervertebral disc is broken.
In our study, we can see that the CV of Group C is more than other groups. In a sense, the increase in the amount of bone cement leads to the severity of leakage. There is intense debate about the optimal injection volume of bone cement. The main function of bone cement is to restore the stiffness and strength of the vertebrae, and the risk of bone cement leakage increases as the amount of bone cement increases. Most scholars pointed out that only 2ml of bone cement can restore the strength of the vertebral body, but more bone cement can restore the stiffness of the vertebral body. Studies have also shown that 15% bone cement filling rate in the vertebral body can restore the stiffness of the vertebral body, and when the bone cement volume fraction reaches 24% or higher, bone cement leakage or new fractures will occur [18, 19]. According to the leakage, we can explore the effect of bone cement leakage to the upper and lower endplates of the intervertebral disc on the prognosis of patients to some degree.
The main components of the vertebral body endplates are proteoglycan, collagen and water, which are consistent with the composition of the intervertebral disc. These nutrients enter the intervertebral disc through passive diffusion. The endplate of the vertebral body plays an important role in nourishing the intervertebral disc and transferring stress. The endplate has a significant influence on the normal physiological state and pathological changes of the lumbar intervertebral disc. Injury and degenerative changes of the vertebral body endplates can significantly affect the biomechanics and nutrient supply status of the intervertebral disc, which in turn leads to degeneration of the lumbar spine [20–22]. When bone cement leaks into the bilateral end plates of the disc, it inevitably accelerates disc degeneration and instability. There was no difference among groups in short-term postoperative VAS when bone cement penetrated into the disc and bilateral endplates. However, with bilateral endplate involvement after long-term operation, VAS and ODI at the last follow-up were significantly worse than the other two groups. It can be seen from this that intervertebral disc nutrition can change due to leakage of bone cement into the intervertebral disc or bilateral endplates, which can cause dysfunction of intervertebral disc cells and even death of intervertebral disc cells. The dehydration of the intervertebral disc loses its normal elasticity and tension, which accelerates the degeneration of the intervertebral disc. On this basis, the annulus fibrosus is weakened or ruptured due to severe trauma or repeated indistinct injuries, and the nucleus pulposus is prolapsed from there, compressing the nerve root, resulting in signs of nerve root compression. This may also be the reason why patients with bone cement leaking into the bilateral endplates have recurrent intractable low back pain. Therefore, the effect of simple leakage of one endplate of the intervertebral disc and the simultaneous involvement of both endplates on the intervertebral disc and the postoperative curative effect are worthy of our study.
In our study, regardless of the type of cement distribution, there was a clear benefit for short-term pain relief and restoration of vertebral height. This may be because there is another endplate that provides nutrition and water to the intervertebral disc and this is consistent with previous clinical experience. However, at long-term follow-up, pain in group C was significantly higher than in the other two groups. When bone cement leaks to the other endplate, recompression of the vertebral body on the one hand may lead to spinal balance, local kyphosis, and thus chronic pain [23, 24]. However, our study found no significant difference in AVHR and LKA among the groups, only statistically significant relative to preoperative. On the other hand, the nutritional supply of the intervertebral disc is unbalanced, which accelerates the degeneration and may cause discogenic neuralgia. In addition, when the vertebral body endplate is fractured, if the connection between the bone cement and the endplate is not good, it will provide insufficient support, resulting in vertebral body fracture. The fractured vertebral body is continuously compressed, which is why the pain persists after surgery.
There are different opinions on the causes of AVF after surgery. It is still controversial for the reasons for re-fractures such as the natural development of osteoporosis, biomechanical changes and excessive injection of bone cement and the leakage of bone cement into the intervertebral disc. However, with the in-depth study of the OVCFs, many scholars have found that recurring vertebral fractures after PKP. They are mostly in the adjacent segment vertebral body and the incidence is relatively high. At present, most scholars believe that low BMD, fracture surface and the number of the vertebral body, amount of bone cement, the degree of leakage of bone cement intervertebral space and the compression of fracture vertebral body, postoperative height recovery, ISH, etc. They all may be related to recurrence of vertebral fractures after PKP.
Compared with the previous two groups, the disc height of the C group was lower, which is similar to the previous study [25], and at the last follow-up, the disc height of the cemented disc leakage group was significantly reduced compared with the disc height of the non-leakage group, indicating that bone cement intervertebral disc leakage will accelerate the progress of intervertebral disc degeneration to compress the height of dis. However, the mechanism by which the leakage of bone cement aggravates the degeneration of the intervertebral disc is still unclear. It has been noted in previous studies that bone cement leakage into the disc increases pressure on the disc and accelerates disc degeneration procedures [26, 27]. When the leakage of the upper and lower endplates of the same intervertebral disc is involved at the same time, whether it is the force on the intervertebral disc or the nutritional supply of the intervertebral disc, the degeneration of the intervertebral disc is promoted to a certain extent [28, 29]. This may also be why AVF and recurrent low back pain are more likely to occur. To explore the relationship between intervertebral disc degeneration and AVF caused by bone cement leakage to the intervertebral disc and endplate, more researches are needed on bone cement leakage into the intervertebral discs and endplates in the future.
Limitations
Because this study is a retrospective study, and due to the limitations of the small sample size, it still needs to be further explored in the later stage of prospective wins, large samples, and multiple randomized controlled studies. The best indicator of intervertebral disc degeneration is not the height of the intervertebral disc in X-ray examination, but MRI scan. Because X-ray examination has the advantages of low price and fast imaging compared with MRI examination, X-ray examination is often selected for regular review after PKP not MRI scans. All indicators are measured manually and there are certain measurement errors. In addition, there may be some deviations in the frontal and lateral projection angles of the spine taken at various time points after surgery and the resulting measurement errors are also included in this study.