Previous studies that have examined the use of PVA and PCD in DLBP with Schmorl’s node or Modic change have consistently demonstrated superior results, relative to other methods. He et al. followed 11 patients with a symptomatic Schmorl’s node treated by PVP for an average of 58 months, and reported satisfactory long-term pain relief with no evidence of surgery-related complications [8]. Zhi-Yong et al. reported 32 patients treated with PKP for whom VAS, ODI, and SF-36 scores exhibited significant improvements after surgery and were well maintained over a 5-year follow-up period [10]. Of these patients, only 3 cases (6.98%) of cement disc leakage and 2 cases (4.65%) of adjacent vertebral fracture (AVF) were observed. In addition, Tian et al. reported that PCD treatment for patients with DBLP with Modic changes provided satisfactory outcomes, with no obvious surgical-related complications [13,14]. Based on these observations, the promise of these two minimally invasive surgical treatments have been proven. However, the evidence each of these therapies is based on a small number of case series reports, with no randomized controlled trials used to validate these outcomes. Due to the lack of high-level evidence, theoretical demonstration of these two kinds of therapies is still necessary.
The mechanism underlying DLBP pathology includes a combination of disc pressure [24], instability [25], nerve fiber ingrowth [25,26], and release of cytokines [25,27,28]. The mechanism of pain relief achieved by PVA is thought to be mediated by a combination of strengthening and stabilization of the edema area of subchondral bone marrow, along with suspension of the processes underlying trabecular injury, and potential thermal effects [29]. On the other hand, PCD is usually used in combination with percutaneous lumbar discectomy (PLD) [13,14] in which as much of the nucleus as possible needs to be removed, which allows PCD to achieve satisfactory intervertebral stabilization and expansion. Furthermore, with the removal of the nucleus, the release of the intradiscal cytokines such as IL-1, IL-6, IL-8, PGE-2, NO, and phospholipid A2 was also resolved.
Regardless of the method used, achieving any of the pain relief described above requires an ideal distribution of bone cement. Both the PVA-ideal and PCD-ideal models facilitate a more stable parapodular biomechanical microenvironment; however, achieving this degree of bone cement diffusion is almost impossible achieve on in practice. Of the many factors that can affect bone cement distribution, the only controllable factors are injection speed and injection direction. Therefore, we also developed both PVA-nonideal and PCD-nonideal models as a means of simulating the suboptimal distribution that is more likely to occur during a PVA or PCD procedure.
In this study, a control model was used to simulate the biomechanical characteristics of the human spine under pathological conditions. Under load, the increased intradiscal pressure forces the nucleus to move toward any defect in the endplate or cortical bone. The centrifugal stress and strain in the edematous bone marrow around the Schmorl`s node in the control model is used to describe the instability of the local microstructure and the diffusion of pain causing factors. In the PVA-ideal model, this centrifugal stress and strain is blocked by the strengthened PMMA-cancellous bone complex which has a much higher elastic modulus than the edematous bone marrow. On other hand, in the PCD-ideal model, we also observed that centrifugal stress and strain were significantly diminished due to the stress-shielding effects of the PMMA in the intervertebral space, suggesting that the an elimination of the intradiscal pressure could be attributed to a combination of stress-shielding and the removal of the nucleus. The elimination of centrifugal stress and strain in the perinodal structure means that the stimulation of intradiscal cytokines to bone marrow is reduced, and the trabecular microinjury of cancellous bone is resolved. However, in the PVA-nonideal and PCD-nonideal models, the above effects are weakened. Obviously, it is impractical to expect an ideal distribution of bone cement in every surgical practice. Therefore, orthopedists need to weigh the potential risks of reduced efficacy caused by unsatisfactory distribution before the PVA/PCD surgery.
For segmental stability, the PCD-ideal model showed superior intervertebral stability, with significantly reduced ROM in all five motion directions relative to other models. By contrast, the PCD-nonideal model exhibited significantly lower stability improvement due to the incomplete filling of the intervertebral space. Neither the PVA-ideal or PVA-nonideal models showed significant improvements in intervertebral stability relative to the control. Therefore, we consider PCD to be an effective treatment for DLBP patients with severe disc degeneration or vertical instability, consistent with previous studies [12–15].
For the stress distribution of the endplates, PVA surgery did not significantly affect either the affected endplate or the adjacent vertebral endplate. In both the PVA-ideal and PVA-nonideal models, we found only a mild effects of bone cement on endplate stress, consistent with a previous study [30]. However, the biomechanical mechanism of PCD has not been reported. In our simulation, when the high elastic modulus PMMA material completely fills the intervertebral space, the annulus fibrosus is no longer the main structure to transmit stress, which means the vast majority of stress is directly transferred to the adjacent vertebral endplate through the PMMA, resulting in significant stress-shielding. By contrast, we observed a sharp increase in endplate stress in the PCD-ideal model. As is widely known, significant intradiscal cement leakage can lead to the disc degeneration [31] and increase the risk for AVF [32,33] and should therefore be avoided. When PCD was first reported, it immediately attracted attention, and triggered extensive discussion regarding the merits of this technique. One of the core concerns is the boundary of PCD for patients with osteoporosis or spinal deformity [34,35]. The exact boundary of contraindications for PCD remains unclear. Generally, T-scores ≤ − 2.5 DS in the hip, or a history of fragility fracture of the hip, wrist, or vertebrae, were considered contraindications for PCD. Moreover, despite PCD-mediated improvements in scoliotic curves, it is not recommended for severe spinal deformity [35]. Our results support the above views, with the stress of the adjacent vertebral endplate having increased 40.5% after PCD. When an additional 10 N·m bending moment is applied, the stress increase in the motion direction will reach 220.8%. This sharp increase is expected to significantly enhance the “direct pillar effect” [36] of bone cement, leading to the induction or exacerbation of AVF symptoms. Although the stress seen in the PCD-nonideal model is not as large as that seen in the PCD-ideal model, the uneven stiffness of the intervertebral disc forces the adjacent endplates to undergo uneven strain (Fig. 5), which also increases the risk for endplate collapse. Therefore, we suggest that the potential risk for PCD should be carefully considered prior to use in patients with osteoporosis.
Limitations
Our research had the following limitations. Due to the limitations of computer modeling, the cancellous bone, edema zone, PMMA, and compound were all considered homogeneous linear materials, thus the process of Schmorl’s node content entering cancellous bone under pressure could not be fully recreated in the macro model. The changes in stress and strain around Schmorl’s nodes represent the trends in cytokines release in various models. In addition, the scope of modeling was limited, with ROM results described in terms of changes in segment stability, which means this model could not be used to predict the effects on global spinal stability. We speculate that limited treatment would have only minor effects on the stability of other uninvolved segments. Finally, it was a purely theoretical study, which will require validation in clinical studies.