Due to advantages in reducing soft tissue damage, improving lumbar lordosis, and reducing nerve tissue disturbance, OLIF has attracted recent attention from spine surgeons. OLIF has been found to increase the cross-sectional area of the dural sac by a median of 30.2%, and increase the neural foramen area by an average of 30.0% [11–14]. Compared with traditional posterior TLIF surgery, the probability of nerve root injury was about 1.3% in OLIF the procedure [12, 14]. No patients in this study experienced post-operative nerve root edema or direct nerve root injury. This indicated that OLIF was a superior technique in protecting nerve tissue.
Since OLIF does not involve iatrogenic damage to the posterior structure, the impairment of spinal stability is limited. There has been some controversy about whether additional fixation is necessary [6, 14]. However, due to the different elasticity modulus between the cage and the endplate, there is a risk of subsidence when the cage is used alone [14, 15]. Therefore, the insertion of pedicle screws is necessary for patients with endplate damage, osteoporosis, or post-operative residual radicular symptoms that require posterior surgeries [13, 15]. Lin et al.  evaluated 52 patients who underwent OLIF without posterior instrumentation, and reported a fusion rate of 81.9% at 24 months after surgery as assessed by CT scan imaging. Kim et al.  reported a 12month fusion rate of 92.9% in 29 OLIF patients with posterior pedicle screw fixation as assessed with CT. In the present study, no cage subsidence nor nonfusion were found within 24 months post-surgery, which further confirmed that limited internal fixation can reduce the risk of cage subsidence and promote intervertebral space fusion.
In this study, the intervertebral space fusion rate at 12 months post-surgery was 72% in group A and 78.6% in group B, which were relatively low. We presume the major negative factor for the inferior fusion rate was the material of the bone graft. Compared with iliac crest or bone morphogenic protein, using allogeneic bone could result in a lower fusion rate [11, 13].
Posterior percutaneous screw fixation could reduce paraspinal muscle damage where iatrogenic impairment to the paravertebral soft tissues is still unavoidable.This could lead to muscular atrophy and low back pain. Secondarily, given the lack of anatomical reference markers, percutaneous pedicle screw implantation has a relatively higher incidence of screw misplacement compared with open surgery [7, 8]. Utilizing a corridor of OLIF for segmental fixation can effectively reduce the risk of screw misplacement for the direct procedure of implantation and the massive docking area for screws.
In this study, when compared with PPSF, ASRSF resulted in lower back pain at 1 week and 3 months after surgery. The ODI index was also lower in the ASRSF group at 24 months post-surgery, which confirmed the superiority of ASRSF in relieving pain and improving lumbar function. This may be due to evidence of paraspinal muscle protection.
The height of the intervertebral foramen was comparatively better maintained at 3 months post-surgery in the ASRSF group compared with the PPSF group. It was presumed this might originate from the relatively close position to the central axis of the spine in the anterior fixation group, which diminishes the "self-locking" phenomenon in the posterior fixation that could result in a reduction of the intervertebral foramen area . However, this superiority disappeared at 12 months post-surgery, which might be related to a possible increase in lumbar lordosis.
ASRSF showed acceptable stability in the debridement of intervertebral space and anterior vertebrectomy, which confirmed the single screw rod could provide stability given that the posterior column was intact [19–21]. In this study, there was no internal fixation migration in the anterior group, supporting the stability for anterior fixation solely. The possible reason comes first from the inaction of the posterior column in the OLIF procedure. Second, the average age of the patients in this study was 43.3 years and the pre-operative BMD was around − 1.3, which equally contributed to maintaining stability and the excellent skeletal condition of the participants.
In summary, ASRSF combined with OLIF for lumbar spondylolisthes could reduced early post-operative low back pain more effectively, increased the area of foramen, and improved post-surgerylumbar function compared with the clinical manifestation of PPSF with OLIF. In addition, it provided sufficient stability for intervertebral space fusion.
The present study had some limitations. First, this study was a retrospective one. Due to the lack of case selection, there might be a selection bias. Second, this study was a single-center study with a relatively small sample size, therefore it was impossible to evaluate independently based on surgical segments. Finally, since patients in this study were relatively young with ideal bone quality, further research is needed on whether the conclusions made here are appropriate for elderly patients. Large-scale randomized controlled studies are needed to draw definite conclusions.