In this study, five important biomechanical parameters (ROM, ESPC, ESPFI, ESPC, and ESAVCB) were measured to determine the biomechanical differences obtained with OLIF surgery augmented or not with different fixation instruments. ROM and EPSFI reflect the stability of the lumbar spine after fusion, as demonstrated in previous studies [32–33, 40–42, 44], and ESPC, ESPCE, and ESAVCB the stress environment of the intervertebral disc as well as the resistance of cage subsidence and disc height maintenance [32–33, 45–47].
The stability of the surgical segment is a crucial indicator of successful LIF surgery, as instability may be accompanied by complications such as cage subsidence and pseudoarthrosis, resulting in pain and possibly work disability for the patient [8, 10]. Our study showed that OLIF reduces the ROM of the L4-L5 surgical segment and thus provides a high degree of stability at this segment. In all postures, the ROM of the surgical segment in the five OLIF surgery models decreased by > 80% (86.26–98.97%) compared with the intact model, consistent with the trend reported in previous studies [32–33, 40–42]. Lu et al. [32] conducted a FE analysis to explore the biomechanical performances of four types of LIF surgery (PLIF, TLIF, XLIF, and OLIF); the ROM of the surgical segment was reduced by 75.3–92.6% when surgery was combined with bilateral pedicle screw fixation. The ROM of the surgical segment in their OLIF model decreased by > 80% (86.9–89.7%) in all loading directions [32]. Chen et al. [33] developed single-segment crenel lateral interbody fusion (CLIF) surgery models and showed that the respective ROM values decreased by 76.84–97.97% compared with the intact model. Oxland et al. [40] published a literature review to evaluate the biomechanical characteristics of different LIF surgeries. The maximum reduction of the ROM at the index level was 90%, achieved using LIF procedures. Hector et al. used cadaver specimens to simulate OLIF and direct lateral interbody fusion (DLIF) procedures while testing intersegmental rotation [41]. They found that the addition of posterior instrumentations (bilateral pedicle screws) to the interbody spacer increased the stability of the construct significantly, regardless of cage insertion trajectory or screw type. These studies demonstrate that OLIF surgery can provide good stability for the surgical segment.
The advantage of our study was that it evaluated five OLIF models of the L3-S1 segment, including the fixation systems most commonly used in OLIF. As seen in Fig. 8, the ROM was larger in the stand-alone OLIF model than in the other four OLIF surgical models; among the latter, the differences were not statistically significant. Among the four OLIF models that included fixation instruments, the OLIF + BPSF model had the smallest ROM. Thus, the stand-alone OLIF model offered the worst stability, and the OLIF + BPSF model the greatest stability of the surgical segment. These results also demonstrated the different impacts of the different instrumentation systems with respect to the stability of the surgery segment, as they increased the stiffness of the surgical segment to different degrees. The larger the increase in the stiffness of the surgical segment, the larger the decrease in the ROM of that segment. Lai et al. conducted a cadaveric biomechanical analysis of multilevel lateral lumbar interbody fusion (LLIF) with and without supplemental instrumentation [42]. They found that, even in multi-segment LLIF surgery, the bilateral pedicle screw fixation system provided greater stability than that obtained with other fixation systems (unilateral pedicle screw and lateral plate). Since the stability of the spine as a whole hinders excessive deviation of each spinal motion segment, the middle area between the vertebrae is kept within the physiological limit. The results of our FE study together with those of in vitro cadaver experiments [32–33, 40–42] indicate that bilateral pedicle screw fixation for OLIF provides greater stability than that obtained with other fixation instruments.
The changes in the ROM of the segments adjacent to the L4-L5 surgical segment in the OLIF models with different instrumentations were very small compared with the intact model (Fig. 7). From a biomechanical point of view, fusion surgery unites an originally movable joint, which greatly increase the rigidity of the surgical segment. Compensatory efforts to maintain the overall ROM of the spine may decrease the stiffness and increase the ROM of other segments. However, this scenario was not the case in our study, perhaps due to its simulation of the timely effect of spinal fusion, as our FE simulation represented the immediate post-operative period and a change in the ROM of adjacent segments is a progressive process that may not appear in the early postoperative stage. In previous studies [23, 43] we showed that both disc degeneration and OLIF can cause the degeneration of adjacent segments. Wang et al. compared the ROM of adjacent segments after OLIF and TLIF and found that the two surgeries similarly increased the potential risk of adjacent segment degeneration [44]. Therefore, our model can be improved, with the use of more advanced software needed to simulate this progressive change.
Under all loading conditions, the ESPFI was significantly higher in the OLIF + LPF model than in the other models (Fig. 9). In a previous FE study of CLIF surgery, a larger peak stress of the plate occurred with lateral plate fixation instrumentation than with other fixation systems, with a peak stress of the plate in the range of 39.6–145.8MPa [33]. This range is slightly larger than that determined in our study (35.27–116.01 MPa), most likely because, in the former, only the L4-L5 segment was evaluated. In contrast, we constructed an L3-S1 segment model, which allowed stresses of the surgical segment to be assigned to adjacent segments. The greater stress loading observed in the lateral plate could result in instrumentation failure, which would explain why in clinical practice the lateral plate is not often used as an independent fixation system [33, 45]. In addition, in osteoporotic patients there may be a greater risk of mechanical failure due to the high stress on this type of device. For this reason, the lateral plate is mostly used for supplementary fixation in combination with other instruments [10, 48]. Among the surgical models tested in our study, the OLIF + BPSF model had the smallest ESPFI. Bilateral pedicle screws provide strong fixation for the surgical segment, as they result in sharing of most of the load from the cephalad direction and significantly decrease the load transferred to the anterior column [32]. Thus, in terms of a reduction in the peak stress of the fixation instrument, a screw-rod may be better than a screw-plate. From this perspective, the BPSF system provides the most stability and LPF the least stability in OLIF, which would account for the prevalence of BPSF systems in clinical practice [49].
The ESPC was larger in the stand-alone OLIF model than in the other OLIF surgical models (Fig. 10). A comparison of the ESPC and ESPFI values suggested that the absence of additional fixation in the stand-alone OLIF could explain this result. A large ESPC will damage the adjacent endplates, leading to an abnormal increase in endplate stress and possibly also to degeneration. Destruction of the physiological environment and of the endplate structure increases the risk of cage subsidence and disc height collapse [17–18]. Cage subsidence is a frequent complication in LIF surgery, and the reduction in disc height is often associated with adjacent segment degeneration [8, 10, 17, 19, 43]. Once the cage subsides and the disc height decreases, the endplate and cancellous bone in the surgical segment may be further damaged, which may lead to changes in the biomechanics of adjacent segments [32]. The ESPC values of the OLIF + LPF and OLIF + UPSF models were second only to the ESPC of the stand-alone OLIF model whereas the ESPC values of the OLIF + BPSF and OLIF + TFJF + UPSF models were similar and much smaller than the values of the other surgical models. Thus, in these two models the ability to resist cage subsidence and maintain disc height was greater. In addition, in terms of restricting segment motion and preventing instrumentation failure, the OLIF + BPSF and OLIF + TFJF + UPSF models had smaller ROM and ESPFI values, indicative of greater stability. The reason for the relationship between ESPC and the stability in the OLIF models was suggested in previous in vitro and FE studies, which demonstrated the greater stability of the surgical segment conferred by bilateral pedicle screws [33, 41–42, 44]. In our study, the OLIF + BPSF model had the smallest ESPC; it can thus be expected to provide the strongest resistance to cage subsidence and to maintain disc height.
The ESPCE of each OLIF surgical model was much higher than the ESPCE of the intact model (Fig. 11). Excessive peak stress may cause damage and rupture of the endplate or even risk accelerating endplate degeneration, which in turn could accelerate disc degeneration [50]. Because the endplate is a bridge for transferring nutrients, endplate degeneration may also interrupt nutrient transport, impairing the vitality of intervertebral disc cells [51]. Therefore, cage design should be improved to increase the contact area between the cage and the endplate, to reduce endplate stress after OLIF surgery. A better understanding of the changes in the endplate stress of the surgical segment after OLIF surgery, and therefore improved clinical treatment, requires further analysis of the mechanism underlying lumbar degenerative diseases.
The ESAVCB in the stand-alone OLIF model was very close to that of the intact model (Fig. 12), perhaps because the micro-environment of cancellous bone stress in the stand-alone model was similar to that of the intact model. According to our study, different fixation instruments influence OLIF surgery to a varying extent. All of the fixation systems provided strong support, such that the ESAVCB values obtained in the OLIF with fixation models were higher than the ESAVCB of the intact model. In the different OLIF models of this study, the stress on the proximal fixation system increased, causing it to concentrate on the contact interface between the cancellous bone and the fixation instrument. We therefore measured the ESAVCB to avoid both stress concentration and inaccurate ESAVCB trends. A previous study that analyzed the biomechanical performance of different lumbar interbody fusion surgeries found that larger cancellous and endplate stress peaks can lead to cage subsidence and a reduction of disc height [32]. In general, in most postures the ESAVCB and ESPCE values were largest in the OLIF + LPF model and smallest in the OLIF + BPSF model. Therefore, compared to the other models, cage subsidence resistance and disc height maintenance may be better in the OLIF + BPSF model and worse in the OLIF + LPF model.
Since the physiological structure of the lumbar spine is much more complicated than in our FE models, our study had several limitations. Firstly, the paravertebral muscles were not simulated in our FE model, which may have misrepresented the true motion of the lumbar spine and the stress distribution of some spinal components. Secondly, the ligaments were simulated as one-dimensional nonlinear spring elements, as their complicated actual structure and the difficulty in reproducing three-dimensional structures hindered their more accurate representation. Both of these limitations affected the FE analysis of the lumbar spine. Thirdly, the geometric morphology of the lumbar spine, including disc height, disc degeneration, and facet joint degeneration, varies from person to person. Our intact model was developed based on the geometric information of the lumbar spine obtained from a single person. Therefore, to a certain extent, our model could only reflect the changes in the biomechanical trends of the lumbar spine under different loads. Fourthly, although the bilateral pedicle screw fixation system is better from a biomechanical point of view, its corresponding disadvantages, such as its higher cost, longer operation time, and longer recovery time, cannot be ignored. Finally, relying solely on the ROM as the validation target is a common defect in spinal research using FE models. Nonetheless, despite these limitations and simplifications, the data obtained with our FE model of the lumbar spine showed good consistency with published experimental data and can be used to explore the biomechanical effects of different instruments in OLIF surgery.