Subsidence is a common complication following various lumbar interbody fusion surgeries and a matter of great concern for OLIF in particular, as the surgery relies primarily on indirect decompression achieved by restoration of disc height and sagittal alignment. The reported incidence of subsidence with OLIF has ranged from 4.4–21.6%. The development of subsidence is considered to be a multifactorial process, and studies have investigated several potential risk factors, including low BMD, disc space overdistraction, insufficient cage width, construct length, endplate violation, use of osteobiologics, and supplemental fixation[9, 26–32]. Inconsistent results likely resulting, at least in part, from a lack of a uniformity in the methodology used to measure and report subsidence have made it difficult to understand this radiological phenomenon[7, 10, 33, 34]. In the present study, subsidence was defined as any compromise of either endplate due to the cage and was recorded based on specific numeric measurements of cage settling, which can easily be discerned and measured radiographically. Based on the conclusion of previous studies that subsidence is an early postoperative event that does not progress significantly beyond 3 months postoperatively[10, 35], we used postoperative 6 months as the last follow-up date. Over-distraction is an important factor for subsidence; however, to the best of our knowledge, there has been no standard for this issue until now. In the present study, we compared the changes in disc height before and immediately after the operation, which showed that there was no statistical difference, so as to make cage subsidence comparable between the three groups.
Among the patients in the present study, the OLIF procedure was an effective method for restoring disc height, regardless of the fixation pattern used, which was consistent with the results of previous studies[36–38]. In all three groups, loss of disc height had occurred by 6 months after surgery. Notably, in one previous study, subsidence was reported as an expected occurrence rather than a complication. However, the subsidence in the BPS group was significantly less than that in either of the other two groups, indicating that supplementary fixation and a specific pattern can help to avoid a loss in disc height. In recent decades, the use of supplemental fixation in OLIF has been the subject of ongoing debate. Previous studies showed that a sufficiently distracted intervertebral space following discectomy can be stabilized for multidirectional movement by tension forces of the residual annulus and ligaments[14, 39, 40]. Also, the authors supporting stand-alone OLIF have stated that it avoids the morbidity associated with supplemental screw fixation and is less expensive. Conversely, Fogel et al. reported that a stand-alone cage decreased the ROM by only about 23% compared with the normal spine and significantly increased the anterior-posterior (interbody) displacement in a L4-L5 spondylolisthesis cadaver model. Liu et al. used a finite element model to evaluate the biomechanics of three-level lateral interbody fusion with and without supplementary instrumentation and found that stand-alone lateral interbody fusion could not provide adequate ROM restriction, whereas lateral cages with bilateral pedicle screw and rod fixation provided favorable biomechanical stability. Additionally, stand-alone OLIF generated significantly higher endplate stress than did OLIF with supplemental instrumentation, which may increase the risk of cage subsidence. Nevertheless, their results can only be applied to multilevel fusion. Whether supplemental instrumentation should be applied to prevent subsidence in single-level OLIF remains controversial. Malham et al. proposed that patients with one- or two-level disease, normal bone density, and no obvious instability were candidates for stand-alone OLIF. In contrast, in the present study, the subsidence in the SA group was significantly greater than that in the BPS group at the 6-month follow-up, indicating that stand-alone OLIF may not be sufficient for maintaining disc height even in single-level disease, which is consistent with the result of Choi and Sung.
Bilateral pedicle screws represent the biomechanical gold standard for adjunct stabilization[30, 42]. However, studies have reported greater blood loss and longer operative time for OLIF with supplementary BPS compared with stand-alone OLIF due to the requirement of additional posterior surgery[39, 43]. These results were consistent with our findings. As a traditional method for posterior fixation, BTS has several advantages over BPS, such as fewer incisions, being minimally invasive, having less influence on the adjacent segments, and less cost, as only two screws are used at each spinal segment. In the present study, the operative time tended to be shorter when the operation was performed with patients in the lateral decubitus position versus the prone position, but the difference was not significant, probably due to the small sample sizes of the subgroups. Previous studies have demonstrated that BTS provides equivalent stiffness against segmental movements compared with BPS[20, 44]. Chin et al. even showed that BTS offers greater stability than pedicle screws in the motion of flexion. However, in the current study, the subsidence rates of the BTS and SA groups were similar and significantly higher than that of the BPS group, indicating that although additional BTS can make the overall construct stiffer, this construct is unlikely to achieve more resistance to axial compression than stand-alone OLIF. The screw placement was done under the guidance of the TiRobot system. Previous studies at our center have demonstrated a high accuracy of BPS and BTS with assistance by this robotic system[45, 46], and thus, accuracy was not a concern in this study.
It is well established that improper sagittal alignment of the spine results in inefficient energy use and maximizes muscle tension due to exhaustive bracing and spine instability, contributing to adjacent-level disease[47, 48]. In a review of 12 retrospective and 2 prospective studies including 1266 levels in 476 patients, Costanzo et al. showed that lateral interbody fusion is effective when the lumbar lordosis and sagittal balance correction goals were less than 10° and 5 cm, respectively, but the results for sagittal balance restoration using the technique were inconsistent. Chen et al. compared the alteration and maintenance of SL and LL in lateral interbody fusion with and without supplementary fixation. They found that lateral interbody fusion alone and with supplementary fixation could increase the post-operative SL and LL angles, but BPS could maintain the LL angle but not the SL angle for 2 years after surgery. In the present study, OLIF with or without supplemental fixation improved the SL angle to 0.3°~2.3° on average, with no significant difference among the groups. At the 6-month follow-up though, the reduction in the SL angle was significant only in the SA group. Overall, research to date has produced inconsistent findings regarding local and global sagittal balance restoration after OLIF, and further studies are needed to address this issue.
The questionnaire-based clinical outcomes in our cohort showed significant improvements in symptoms and function at 6 months after surgery compared with preoperative values, regardless of the fixation pattern, which was consistent with previous studies[6, 50, 51]. The change in the VAS score for leg pain did not differ among the three groups, indicating that indirect decompression can be achieved and maintained by the stand-alone technique. However, the improvements in the VAS score for back pain, the JOA score, and the ODI score were worse in the SA group compared with the other two groups. Back pain and related disability may have resulted due to the instability caused by the lack of posterior column support. In a finite element analysis, stand-alone OLIF generates significantly higher endplate stress compared with that of supplemental instrumentation, which results in an increase in the risk of cage migration and development of clinical symptoms. Our results confirm this limitation of the SA technique. Interestingly, although BTS cannot offer more resistance to subsidence compared with stand-alone OLIF, the symptomatic and functional improvement with BTS were better. These findings were consistent with other studies that investigated the clinical outcomes with BTS as supplementary fixation for lumbar interbody fusion and observed promising results[22, 52]. This can be explained by the additional stability provided by the transfacet screws. In a study of the effect of supplemental BTS on the stability of stand-alone ALIF under physiologic compressive preloads, Phillips et al. found that the stand-alone cage is likely to be less stable and supplemental BTS can enhance the stability of the motion segment, particularly during conditions of low compressive preloads. Moreover, the results of our study also indicated no obvious correlation between subsidence and clinical outcomes. This is consistent with the results of previous studies comparing subsidence to final clinical outcomes in which no clear relationship was observed[12, 54, 55]. Marchi et al. graded subsidence and proposed that high-grade subsidence (> 50% into the vertebral endplates) could lead to persistent back pain or radiculopathy and a need for revision surgery. Similar results were reported by Tempel et al., who reported a strong correlation between subsidence grade and the risk of revision surgery. Because the sample size of the current study was small, we did not grade the subsidence. A further prospective, randomized, and controlled study is needed to clarify the correlation between subsidence grade and clinical outcomes.
Because of the novelty of OLIF, there is a paucity of data regarding its complications. In a multicenter study including 155 patients, Abe et al. reported a complication incidence of 29.5%, and a systematic review reported 1.5% intraoperative and 9.9% postoperative complications among 1453 patients treated with OLIF. A recent analysis of OLIF-related complications identified only 20 research studies of sufficient quality and relevance, and reported overall combined rates of 5.7% for postoperative psoas weakness, 8.7% for thigh numbness, and 3.3% for paralytic ileus. These rates were similar to those in the present study (psoas weakness 3.3%, thigh numbness 13.3%, and paralytic ileus 1.7%), in which the overall complication rate was 23.3%. These complication rates did not differ significantly among the groups, but these findings should be interpreted with caution due to the relatively small sample sizes.
A major limitation of this study was its retrospective design. Additionally, the study was a single-center study with a small sample size. Although we matched the patients according to main parameters, complex baseline differences may have been confounding factors for clinical and radiographic outcomes among the groups. Differences between surgeons could be a potential confounding factor in this study, because the surgeries were performed by three senior spine surgeons. Although all three surgeons had received standardized training for the OLIF procedure and implemented standardized surgical maneuvers, and there was no statistical difference between the numbers of cases in the different treatment groups for the three surgeons, this confounding factor cannot be completely ruled out. Another limitation was the lack of dynamic radiographic evaluation at the 6-month follow-up. Thus, no conclusions regarding instability or union can be made. Ideally, fusion status should be assessed by CT scan at one year after surgery. Future studies should focus on the stability status after OLIF rather than subsidence only.