Lumbar spondylolisthesis is defined as the forward slippage of the superior vertebrae relative to the inferior vertebrae (1). In addition, according to the degree of slippage assessed by radiology, Meyerding proposed that 1%−25% slippage represents Grade 1, 26%−50% represents Grade 2, 51%−75% represents Grade 3, and 76%−100% slippage represents Grade 4; low-grade spondylolisthesis represents from 1% up to 50% slippage (2). In addition, based on the etiology of spondylolisthesis, Wiltse classified it into five types, namely, congenital, isthmic, degenerative, traumatic, and pathologic (3). For patients with symptoms of degenerative or isthmic spondylolisthesis, such as persistent mechanical pain or low back pain and/or overt neurological deficits, surgical management has been confirmed to have a greater advantage than conservative treatment strategies (4–6). To date, various surgical techniques have been performed to deal with symptoms of spondylolisthesis. Furthermore, the main aim is to stabilize the spinal segment and decompress the neural elements (7–9). In 1982, Harms first described the transforaminal lumbar interbody fusion (TLIF) technique (10). Since then, it has gained popularity as an effective management strategy for lumbar spondylolisthesis (11). TLIF reduces the retraction of nerve roots and the thecal sac and preserves the structural integrity of the posterior column (12, 13). The clinical indications for TLIF surgery are as follows: (1) Grade 1 or 2 degenerative or isthmus lumbar spondylolisthesis without accompanying neurological symptoms or with only unilateral neurological symptoms; (2) discogenic lower back pain with ineffective conservative treatment; (3) intervertebral disc herniation accompanied by lumbar instability (including extreme lateral disc herniation); (4) patients with single-segment lumbar disc herniation who require revision after surgery; and (5) formation of false joints between vertebrae. Moreover, with the development of operative instruments, the TLIF technique has developed into a minimally invasive procedure, which was first described by Foley in 2002 (14). With the potential advantages of smaller incisions, less soft tissue trauma, and faster recovery, the minimally invasive TLIF (MIS-TLIF) technique has become more and more popular (15, 16). In addition, the indications for MIS-TLIF are also constantly expanding, while the contraindications are comparatively shrinking. However, no consensus has been reached on the correlation between reduction and clinical outcomes in patients with low-grade spondylolisthesis. Does intraoperative reduction result in better outcomes after TLIF? This study was performed to estimate the clinical efficacy and safety between reduction and arthrodesis in situ with MIS/open-TLIF for low-grade spondylolisthesis.

The initial surgical strategy for lumbar spondylolisthesis was laminectomy, followed by lumbar interbody fusion. The implementation of fusion surgery can effectively improve the clinical prognosis in patients with symptomatic lumbar spondylolisthesis. Compared with PLIF, the TLIF technique reduces the retraction of the thecal sac and nerve roots and preserves the structural integrity of the posterior column, which reduces the risk of various complications, including neurological tissue damage. In addition, interbody fusion can maintain the height of the intervertebral space and restore the height of the intervertebral foramen, thereby improving or relieving the narrowing of the intervertebral foramen, and the normal anatomical curvature of the corresponding segment can be reconstructed to restore or maintain the overall physiological curvature of the spine. However, whether intentional reduction with TLIF is necessary for the surgical treatment of low-grade lumbar spondylolisthesis remains controversial. Only one meta-analysis study (25) has been performed on this subject, which concluded that no statistical difference was observed in terms of operation time, estimated blood loss, patient-reported outcomes, and fusion rate and complication rate. However, in the aforementioned study, patients in one included study underwent posterolateral fusion (PLF) (26), patients in three studies underwent PLIF (27–29), and patients in the remaining three studies underwent MIS/open-TLIF (18, 21, 22). The uncontrolled confounding factors for surgical operations might exert an effect on their results. To our knowledge, several surgical techniques have been performed to treat symptomatic spondylolisthesis, which could not be treated by conservative treatment strategies. Moreover, the advent of minimally invasive technologies for spine surgery led to the logical progression of open-TLIF to MIS-TLIF (30). The TLIF technique has been proven to be an effective management strategy for spondylolisthesis as it preserves the structural integrity of the posterior column and reduces the retraction of thecal sac and nerve roots (12, 13). In addition, our study aims to compare the clinical efficacy and safety of reduction vs. arthrodesis in situ with only the MIS/open-TLIF technique for low-grade spondylolisthesis, as, currently, there is still a lack of powerful clinical evidence. The pooled results in our study revealed that both techniques had similar patient-reported outcomes (VAS, ODI, and good and excellent rate) during short- and long-term follow-up, and no significant difference was observed in the fusion and complication rate, as well as the segmental lordosis. Although the reduction group did achieve better slippage correction, it was associated with increased operation time and intraoperative blood loss compared with the arthrodesis in situ group.

Intentional reduction of the slipped vertebrae may be a useful procedure, and the idea remains attractive in theory. It restores the spinal column into a more anatomic alignment. Fan reported that significant advantages in slippage decrease were observed in the reduction group (20). And, as shown in our study, the reduction group did achieve better slippage correction. In addition, some studies considered that intraoperative reduction might relieve early muscular fatigue and hypolordosis-induced back pain and prevent disc degeneration of the adjacent segment (31, 32). However, no evidence-based conclusion was reached to support the above viewpoint (29, 33). The pooled plot in our study indicated that the reduction group had increased operation time and blood loss, which might seem reasonable, especially in MIS-TLIF technology.

Patient-reported outcomes such as VAS-BP and ODI are of paramount importance when estimating the clinical effect of a certain technique (34). We predefined a follow-up that ranged from 3 to 6 months after surgery as a short-term follow-up and a follow-up of at least 2 years as a long-term follow-up. In our study, both techniques had similar VAS-BP and ODI during short- and long-term follow-up. The primary goal of intervention for spinal diseases is to relieve the pain and restore function (35, 36), and the excessive pursuit of anatomic restoration in MIS/open-TLIF procedures may not help in improving pain and function in patients (20). In addition, for the complication and fusion rate, Bai et al.'s (25) paper of meta-analysis demonstrated no statistical difference, which was again confirmed by our findings. In a word, intraoperative reduction in MIS/open-TLIF procedures, while safe, did not result in better clinical outcomes in patients with low-grade lumbar spondylolisthesis.

Moreover, bone quality is an important factor in the success of the reduction of slippage in the vertebrae. It is not easy to obtain the reduction, especially in patients with osteoporosis. Surgeons may fail in intraoperative reduction due to screw loosening in low-bone mass patients. However, the index of bone density was not mentioned in all five included studies in our meta-analysis. For intraoperative screw loosening after reduction, bone cement could be used to reinforce the pedicle screws.

We recognize the limitations of our study. First, all studies included in this meta-analysis were retrospective cohort studies and were inherently prone to methodology defects. Although all studies were rated with a total score of more than 5 according to the NOS (23), which represented relatively moderate to high quality, the validity of the data available might be weakened due to selection bias and other biases. In addition, the quality of evidence based on GRADE (37) was very low (Table 4), and the strength of the recommendations was relatively weak. Second, a relatively small number of participants and various definitions of complications among the studies might exert an effect on the results because of the limited statistical power and homogeneity. Third, the decision for intraoperative reduction was made primarily based on surgeon preference, and diverse MIS/open-TLIF technical specifications and postoperative management applied by diverse surgeons in different treatment centers might also have influenced the results. Moreover, data about health-related quality of life (HRQOL) were not available, which would be necessary to know if patients undergoing intraoperative reduction had an improvement in the HRQOL score over the patients who underwent arthrodesis in situ.

Based on the available evidence, intraoperative reduction does not result in better clinical outcomes in low-grade spondylolisthesis after MIS/open-TLIF, and the arthrodesis in situ technique could be an alternative. In view of the limitations of this study, a multicenter, large sample, well-designed randomized controlled study is essential to draw a more convincing conclusion.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

RQ: Validation, Writing – original draft, Writing – review & editing. MZ: Data curation, Investigation, Methodology, Software, Writing – review & editing. PZ: Software, Supervision, Validation, Writing – review & editing. AG: Software, Supervision, Validation, Writing – review & editing.