VP and BKP are common interventions for the treatment of osteoporotic compression fracture of the spine vertebrae. VS can achieve controlled anatomical restoration before bone cement augmentation and has gained popularity recently. Although the height restoration and augment material property may vary, these interventions use bone cement to stabilize fracture cracks. The aim of this study was to investigate the biomechanical effects on osteoporotic compression fracture of these different interventions.
Clinical studies of different cement augmentation procedures have been encouraging. Several studies have suggested both VP and BKP not only improved quality of life, pain relief, improved functionality, and restored vertebral body height.12 A meta-analysis by Zhao et al. showed patients treated with BKP are more effective for long term VAS, ODI, improved kyphosis angle, mean vertebral body height, and significantly reduced risk of cement leakage.13 Clinical outcomes of VKP and VS (SpineJack) cement augmentations were also compared in previous studies. Noriega et al. demonstrated both procedures were safe and led to significant clinical improvement for patients with osteoporotic vertebral compression fractures.5 In 2019, a prospective, international, randomized study compared an implantable titanium vertebral augmentation device versus BKP in the reduction of vertebral compression fractures and found non-inferiority of the titanium augmentation device with an excellent risk/benefit profile for results up to 12 months.14 Furthermore, in vitro biomechanical cadaver studies demonstrated height restoration was significantly better in the VS (SpineJack) group compared with the BKP group.15 The clinical implications include a better restoration of the sagittal balance of the spine and a reduction of the kyphotic deformity. From our results, the VS group has the highest von Mises stress among the three surgical interventions of 5.94 MPa. The usual maximum compressive strength of PMMA is 93MPa, which is far more than the measured maximum stress in the VS group.16 Therefore, it is reasonable that the compressive strength of PMMA is well within the safe range for all three surgical procedures.
The effect of cement augmentation on the adjacent vertebral body has been debated. Many studies have reported the incidence of adjacent level fractures, but the results were not consistent. Ma et al. conducted a Meta-analysis which encompasses 12 studies with 1,081 patients.17 They concluded that BKP and VP are both safe and effective procedures for treating osteoporotic vertebral compression fractures. There were no statistical differences in visual analog scale, Oswestry Disability Index, cement leakage rates, and adjacent vertebral fracture rates. Similarly, Zhao et al. found both VP and BKP had a similar effect on short-term pain relief, posterior vertebral body weight, and adjacent-level fractures.13 In this study, the stress distributions of the adjacent lower T11 and upper L1 endplate also showed no significant difference among the surgical intervention groups, implying no significant effect on the risks of adjacent fracture.
This study also showed that compression fracture treated with VS have the highest stiffness of 4690.34 N/mm. According to a study on the stress burden of the spine after VP and BKP by Rohlmann et al., the stiffness and strength of the vertebrae increase as the bone cement volume increases.8 Their probabilistic and sensitivity study suggested that in cement augmentation procedures, the maximum stresses moderately depend on the injected cement volume. The VS group has the most amount of cement augmentation and VP has the least amount. According to Rotter et al., during intraoperative period, VS (SpineJack) could preserve the maximum height gain significantly better than BKP, which creates cavity absence of load-bearing after balloon deflation and before cement injection.18 Furthermore, the additional support of titanium alloy implant also contributes to a higher stiffness for the VS group. Interestingly, the results of this biomechanical study also suggest a superior body height maintenance ratio for the VS surgical model due to a higher stiffness.
However, several previous articles have reported the possible risk factors for recollapse of cemented vertebrae after percutaneous cement augmentation. In 2011, Chen et al. retrospectively reviewed 1,800 patients with a 2-year follow-up. The incidence of refracture of the same vertebra after VP with an incidence rate of 0.56%.19 Osteonecrosis, greater anterior vertebral height restoration, lesser kyphosis angle correction and cystic filling pattern were found to be significant risk factors for recollapse. As for BKP, Lavelle et al. reported a 10% incidence recurrent fracture after BKP of a previously operated vertebra primarily within the first 90 days after surgery.20 The study by Kim et al. demonstrated the presence of intervertebral cleft and non-PMMA-endplate-contact may contribute to future recompression of BKP treated vertebrae.21 Li et al. conducted a risk factor analysis for re-collapse of cemented vertebrae after percutaneous VP or BKP.22 Low bone mineral density, percutaneous BKP, and low volume cement injection were found to be associated with high risk of recollapse. Therefore, cautious interpretation of patients with the above risk factors is imperial to prevent deterioration of fracture condition.
There are several limitations to this study that need to be considered. The creation of a finite element model requires several simplifications and assumptions, including a homogeneous cement filling model and orthotropic material simulation. Physiological boundary conditions, cortical and trabecular bone region distinction, subsequent bone remodeling and pre-existing damage may be neglected. Furthermore, as of many other biomechanical studies, our finite element analysis was limited to a single thoracic vertebral segment and may not be generalized to other vertebrae. Despite the shortcomings mentioned above, this is the first study comparing the stress distribution and maximum von Mises stresses on the treated and adjacent vertebrae among VP, BKP, and VS using finite element analysis. This biomechanical study can improve the understanding of cement augmentation on vertebral compression fracture. To make the results from this finite element analysis clinically applicable, we encourage future multicenter studies with long term follow up to confirm these findings.