Standalone oblique lateral interbody fusion vs. combined with percutaneous pedicle screw in spondylolisthesis

Degenerative lumbar spine disease (DSD) is an important cause of disability, affecting 3.6% of the world population. The prevalence of DSD is four times higher in low-and middle-income countries compared with high-income countries [1].

Fusionis the corners to neither treatment of an unstable degenerative lumbar spinal disease, but various techniques are available [2, 3]. Oblique lateral interbody fusion (OLIF) is are gently introduced surgical technique [4] that evolved from lateral lumbar interbody fusion (LLIF) and anterior lumbar interbody fusion (ALIF) [5]. To avoid femoral nerve plexus injury during the transpsoas approach in LLIF, OLIF use a natural space between the left lateral border of the abdominal aorta (AA) and the anterior medial border of the left psoas muscle (PM), all owing access to the lesion discs from L2/3 to L4/5 without splitting the left PM [6]. OLIF has advantages over LLIF in that the larger lateral cage can achieve greater fusion rate and angular correction. This surgical procedure alleviate spost operative back pain, shortens the time of operation and hospitalization, and also reduces bleeding compared with open surgery [5, 6].

It remains controversial as whether internal fixation is required for OLIF [7]. The standalone OLIF approach is associated with small trauma and short operation time and hospital stay [8‐11]. Zeng et al. [7] suggested that the rate of complications was lower with the use of combined screw fixation. Posterior pedicle screw fixation can be indicated inpatients with endplate damage [7]. Ohtori et al. [12] used posterior screws in all their patients and reported good outcomes. Studies comparing standalone OLIF and OLIF combined with pedicle screws are lacking inpatients with spondylolisthesis.

Considering the emerging interest for OLIF, the present study aimed to analyze the clinical experience with standalone OLIF and OLIF combined with pedicle screws, and to compare their clinical and radiological outcomes in the management of primary spondylolisthesis.

OLIF has been found to result in a 30.2% median increase in the cross-sectional area of the dural sac [16] and a 30.0% average increase in the neural foramen area [17]. The results suggest that the standalone OLIF group achieved better clinical outcomes in the early postoperative term (1 week and 3 months), but the differences disappeared by 2 years. The probable reason is that standalone OLIF does not invade the muscle groups on both sides of the spine and does not cause intra muscular hematoma, leading to better early recovery [18]. The difference disappeared by 2 years once the muscles and hematoma healed in the combined OLIF group.

In the present study, the rate of cage subsidence in the standalone OLIF was 15.6% while it was 7.3% in the combined OLIF group. Zeng et al. [7] reported a rate of cage subsidence with standalone OLIF of 36.3%, higher than in the combined OLIF group (29.9%). Good clinical results of the OLIF without additional posterior pedicle screw fixation have been reported, but an indication was the absence of endplate damage [8‐11]. Although the sample size was small in the present study, our results are supported by Zeng et al. [7] and the main reason for cage subsidence is probably endplate damage during surgery.

In total, there were three cases of cage subsidence in the standalone OLIF group and five in the combined OLIF group. All of them had a T score < − 2.5 on DEXA. Standalone OLIF inpatients with mild lumbar spondylolisthesis could be associated with a concern about complications, especially cage subsidence. Based on the present study, a T score warning line of <− 2.5 or body mass index (BMI) ≥30 kg/m² could be associated with worse outcomes of standalone OLIF [19]. Of course, cage subsidence may occur after OLIF combined with pedicle screw fixation as well, but theoretically pedicle screw fixation could add more protection. Patients who are osteoporotic or obese might be better undergoing combined rather than standalone OLIF. On the other hand, for patients without those risk factors, standalone OLIF could be enough. Formal analyses of T values and BMI should be performed in future studies. In addition, the three patients with cage subsidence in the standalone OLIF group were elderly (88, 81, and 77 years of age). Of course, osteoporosis is associated with age, and we cannot rule out which factor is more important based on three cases. Nevertheless, other authors reported similar results. Li et al. [20] and Lee et al. [21] showed that lower BMD could result in cage migration. Pan et al. [22] and Lee et al. [23] reported that BMI of patients with cage retropulsion was higher than that in those without. Future studies could examine whether additional posterior pedicle screw fixation could be applied in the operation.

Lin et al. [24] evaluated 52 patients who underwent standalone OLIF without posterior instrumentation and reported a fusion rate of 81.9% at > 12 months after surgery when assessed by CT. Kim et al. [25] reported a 12-month fusion rate of 92.9% in 29 OLIF patients with posterior pedicle screw fixation when assessed with CT. In this study, we found that the low fusion rate in both OLIF group was related to cage subsidence, and the poor clinical effect was mostly inpatients who suffered from cage subsidence. The pedicle screw and the rod system are widely accepted and used to achieve stable and rigid fixation in patients undergoing fusion surgery, but there was no difference in fusion rate between the two groups, at least within 2 years of follow-up. The reason could be that despite the fact that standalone OLIF is more unstable compared with OLIF combined with screw implantation, the bilateral muscles remain intact with standalone OLIF, which could help stabilize the spine. Therefore, both methods achieved a similar fusion rate.

The results suggest that cage subsidence and adjacent vertebral fractures could be mutually causal. Endplate damage further causes trabecular bone fracture in the vertebral body, causing the vertebral body to be weakly supported, resulting in cage subsidence. Before calcification of vertebral hematoma, vertebral trabecular fracture and cage subsidence form a vicious circle. The intervertebral endplate and the cage are slightly moving, resulting in a decrease in the fusion rate (Fig. 5). Until the cancellous bone of the vertebral body is compressed to a certain density and reaches the support cage strength, a stable state is reached and the artificial bone in the cage ensures fusion when the growth of the upper and lower vertebral bodies starts. Micro-fractures in the vertebral body can produce hematoma, and the patient can feel pain and discomfort [18].

Despite the fact that fusion has been shown to achieve similar outcomes to those of conservative approaches [26], surgery was attempted in the patients included in the present study because those patients had failure to at least 6 months of conservative treatments. In addition, direct decompressions were not performed. Indeed, all patients included here suffered from grade I lumbar spondylolisthesis, and the instability of the lumbar spine mainly causes low back pain and only mild lower limb symptoms. For this kind of condition, OLIF can be used, and the clinical results are generally satisfactory. But for patients with concomitant conditions such as lumbar spinal stenosis or lumbar disc herniation, the lower limb symptoms may be more significant, therefore, and standalone OLIF cannot be used. The reason for using OLIF combined with pedicle screw fixation is that for osteoporosis patients, adding posterior internal fixation can reduce cage subsidence and increase the early fusion rate. Of course, the addition of posterior internal fixation will increase the economic burden on the patients. Therefore, for patients without osteoporosis or obesity, standalone OLIF is probably enough and should avoid damage to the back muscles and improves recovery, but additional studies are necessary to confirm those results. In addition, whether direct decompression could provide additional benefits should be examined.

Demineralized bone matrix (DBM) is a key factor in promoting fusion, and both groups received this growth factor, limiting the intergroup variability. In elderly patients with osteoporosis who received posterior pedicle screw fixation, the use of DBM increases the local stability and avoids compression fracture of the endplate caused by the cage, leading to a higher fusion rate. In this study, no allograft, autograft, or other bone substitute was used, and it should be examined in the future whether their use result in similar outcomes.

The present study has limitations. There trospective design introduces a degree of uncertainty due to missing and erroneous data. Especially, imaging examinations were performed routinely and the exact parameters were at the radiologists’ discretion at the time of examination, possibly introducing some bias in the measurements. Second, the small sample size likely affects the power of the statistical analysis of the demographics and radiological parameters. Third, the follow-up was short because OLIF has been introduced only a short time ago. Finally, even if the surgeons share a common education and training and regularly assist each other with cases and have a long history of cooperation, the patients were from two hospitals, possibly introducing bias. The possibilty of proof lies within a future prospective study, preferably an RCT.

Standalone OLIF may achieve equivalent clinical and radiological outcomes compared to OLIF combined with fixation for spondylolisthesis. The rate of complication was similar between the two groups. Standalone OLIF does not invade the paraspinal muscle groups, possibly leading to better clinical effect and faster recovery, saving social and personal medical costs. Patients who are osteoporotic might be better undergoing combined rather than standalone OLIF.