The incidence of OVCFs has been increasing with an increasingly aging society, resulting in 1.4 million new cases reported worldwide each year (14). Another study examining 27 countries in the EU reported an OVCF incidence of 1.9% (15), and OVCF is more prevalent among women, occurring approximately 2–3.5 times more frequently in women than in men. OVCF occurred almost 8 times more frequently in women aged 85–89 years than in women aged 60–64 years. OVCF can be associated with other complications, including back pain, kyphotic deformity, and drop pneumonia, which can negatively affect the quality of life of patients and increase the risk of death (16, 17).
At present, the treatment options for OVCF primarily consist of conservative treatment and surgical treatment. Conservative treatment typically includes bed rest, the application of analgesic drugs, and wearing braces. If conservative treatment does not relieve pain, surgical treatment may be considered. Vertebroplasty and kyphoplasty are minimally invasive surgical modalities for the treatment of osteoporotic compression fractures (18, 19). The use of a device to provide distraction to the compressed vertebral body can restore the vertebral body height, and the intraoperative injection of bone cement can stabilize the fractured vertebral body. Numerous studies have shown that PVP is more effective for relieving pain than conservative treatment (20). However, with the increasing clinical application of PVP surgeries, some complications associated with the procedure have also begun to be gradually appreciated.
One of the primary complications associated with kyphoplasty is the development of new vertebral fractures, and the incidence of adjacent vertebral fractures accounts for 58.8%–67% of new vertebral fractures (4, 21). Current studies have identified numerous causes for adjacent vertebral fractures, including differences in cement volume, leakage, and vertebral vacuum fissures. David’s study concluded that no significant increase in compressive stiffness or intradiscal pressure occurs when the amount of cement-filler exceeds 15%, equal to approximately 4–6 ml (3). Jin suggested that the effective cut-off value for avoiding adjacent fractures was approximately 5.05ml. A cement injection fraction of more than 20% will produce an increasing number of subsequent fractures, and an injection volume greater than 9.2 ml will certainly result in subsequent and adjacent fractures (22). According to previous studies and our own surgical experience, we used approximately 4 ml of bone cement to fill the fracture, which was better able to restore the strength of the vertebral body (23, 24); moreover, based on surgical steps used to simulate balloon distraction, we applied an ellipsoid shape to the bone cement and performed the relevant measurements.
Biomechanical studies have shown that upper vertebral advancement is an important cause of adjacent vertebral fractures. Lee et al. performed a study of 402 patients and concluded that the offset generated by bone cement along the superior and inferior axis was a risk factor for the development of compression fractures in the adjacent vertebral bodies (25). Our study found that after the central injection of bone cement, the lower endplate stress of the upper vertebral body increased by 10.08%, 13.83%, and 8.56% during flexion, hyperextension, and rotation, respectively, whereas stress decreased by 9.8% during flexion. The changes in stress in response to different movements changed further when the bone cement was offset, causing an increase in the VMS for certain movements, such as the maximum stress applied during the hyperextension of the lower-left model, which was 31.3 mPa. Following the central injection of bone cement, the upper endplate stress of the lower vertebral body also changed, increasing by 11.19%, 14.58%, 3.58%, and 1.12% during flexion, hyperextension, rotation, and lateral bending, respectively. Similarly, when the bone cement position was offset, some VMS stresses increased further, with a maximum stress level of 31.64 mPa estimated during lateral bending movements for the lower-left model. This increase in the stress value occurred in the endplate adjacent to the repaired vertebral body, which may explain fractures that occur in adjacent structures.
Some studies have analyzed the uneven distribution of bone cement, which can cause strut-like effects. When bone cement is injected into the affected vertebra, the inward bulging of the endplate of the reinforced vertebral body decreases, resulting in the increased stiffness of the intervertebral disc and an increase in the inward bulging of the endplate of the adjacent vertebral body, which can result in the development of adjacent vertebral fractures (26, 27). Ottardi’s study found that the superior endplate stress of the vertebral body increased by 15%–40% after the injection of bone cement, which may indirectly verify the “strut-like effect”(28). Dabirrahmani et al. suggested that the inferior endplate of the L2 and the superior endplate of the L4 vertebrae were the most prone to secondary fractures using a model of cement injection into the L3 vertebral body (29). Our study identified a similar situation, in which the maximum VMS of the endplate was often concentrated in the corresponding position of the cement, whereas the VMS associated with other positions was relatively low, which could cause the upper and lower endplates of the adjacent vertebral bodies to fracture first at the position corresponding with cement placement, resulting in the adjacent vertebral fracture.
Previous trials have typically examined a single vertebral body or an adjacent vertebral body as the object of biomechanical analysis and, therefore, could not analyze stress changes throughout the entire thoracolumbar region. We performed a finite element analysis of the entire thoracolumbar segment, which more fully reflected the stress changes associated with the entire thoracolumbar segment after cement injection. In addition, we simulated the distribution of various cement positions and found that when the cement was offset along the coronal or sagittal positions, the stress values applied to the endplates of adjacent vertebral bodies also changed, and for some movements, the endplate stress was significantly higher than that for the model when cement was injected in the middle of the vertebral body. This finding suggests that during clinical surgery, to improve the biomechanical function of the column, bone cement injections should be distributed along the middle of the vertebra to the greatest extent possible, which can reduce the change in stress applied to the upper and lower vertebral endplates. When the bone cement distribution is offset, either coronally or sagittally, the stress applied to the adjacent vertebral endplates can further increase, resulting in the fracture of adjacent vertebral segments.
This study has some limitations. First, the finite element model and the material properties of the bone cement applied to this model represent simplifications of the real situation, as is the stress loading. Second, this model only used a single individual, and individual differences, such as variations in vertebral bodies, ligaments, and other structural variations, may make these findings less generalizable. Third, the test did not construct a wedge model and only simulated the ideal result of complete recovery of vertebral body height following PVP. In practice, however, the vertebral body height is only partially restored in most patients. In future research, we hope these factors can be addressed to provide a more realistic model.